The Status of Aviation Related Improvements
By Melvin J. Zeltser
This article provides a status report of the key technical issues associated with the implementation of improvements to GPS for aviation use, and includes coverage of the Wide Area and Local Area Augmentation Systems (WAAS and LAAS), as well as the new civil signals supported by the White House. Before these subjects are addressed, a few comments are offered regarding the need for WAAS and LAAS given that the White House just announced plans to set selective availability (SA) to zero.
There is little doubt that, without SA, the accuracy of GPS will improve to about 1020 m (95 percent probability). Most existing receivers would have a reduction in the integrity function’s false alarm rate, and new receivers could be designed to increase availability of the receiver’s autonomous integrity monitoring function. However, this accuracy is still not adequate for precision approaches, and more important, setting SA to zero does not address aviation’s safety/integrity requirement (i.e., timely notification to the user within a few seconds when the GPS signals should not be used for a particular operation). The bottom line is that, even though setting SA to zero reduces measurement errors, it has a small impact on the need for and design of WAAS and LAAS ground systems and avionics.
Status: WAAS Accuracy and Integrity
WAAS provides the following augmentations to GPS: differential corrections, an estimate of the error bounds in the differential correction, and a ranging signal. This data is broadcast via a geostationary communications satellite in a form that can be received by a GPS receiver capable of decoding the WAAS message. The differential corrections are transmitted as two components: fast corrections to overcome clock errors and slow corrections to overcome ephemeris errors and map the propagation delay through the ionosphere. The rate of the fast corrections can be reduced when SA is set to zero. The ionospheric mapping is transmitted as vertical delay corrections at the vertices of an imaginary latitude/longitude grid (5° by 5° at an altitude of 350 km). All corrections are generated in a WAAS Master Station (WMS) using range measurements at L1 and L1L2 from WAAS Reference Stations (WRSs) distributed throughout the service volume. In the WAAS Phase I system, the 2 s accuracy currently achieved is about 1.52 m horizontally and 23 m vertically. This accuracy exceeds the WAAS accuracy requirements by a significant margin.
The challenging issue in the aviation application is the need to “guarantee” that the errors in the corrections are bounded to levels that would cause the probability of hazardous misleading information (HMI) to be less than 10 7 during any landing (150 s interval). This is the WAAS integrity requirement. It is achieved in part by the monitor design in the WMS with corresponding fault detection monitors and an algorithm (in the avionics) that is a function of two parameters measured and transmitted by the WMS: User Differential Range Error (UDRE) and Grid Ionospheric Vertical Error (GIVE). UDRE is a bound on the error in the clock and ephemeris corrections, and GIVE is a bound on the corrected ionospheric delay error. The receiver algorithm generates Horizontal and Vertical Protection Levels (HPL and VPL) that are compared with a threshold preset in the receiver. For example, Vertical Alert Limit (VAL) values of 50, 20, or 12 m are used as thresholds depending upon the classification of the approach.
Figure 1 is a block diagram of the WAAS Phase I showing the 25 WRSs feeding 2 redundant WMSs. Each WMS contains a corrections processor (CP) and a safety processor (SP). The CP uses carriersmoothed data from each WRS as an input to the algorithm that estimates the corrections and error bounds. The error bounds estimated by the CP are derived from measurements having low level noise inputs. However, these error bounds cannot be used to determine integrity because the CP and its software are certified to level D (and not level B, which is required for landing system integrity). The SP contains an operating system and software certified to level B, and is intended to ensure that error bounds for UDRE and GIVE adequately reflect all “fault” conditions (including rapid changes in the ionosphere). In the initial WAAS Phase I implementation, the SP used minimally smoothed data; this resulted in large values for the UDRE and GIVE parameters, which in turn led to low availability and continuity of service. Also, an initial set of analyses that calculated probability of HMI did not show that the GIVE bounded true errors in the event of an ionospheric trough (i.e., rapid changes in the ionospheric delay in a relatively small region), and some cases of HMI were not detected by the monitor.
WAAS Issues to be Resolved
To expedite the certification of WAAS for operational use, FAA formed two panels: a WAAS Integrity Performance Panel (WIPP) and an Independent WAAS Integrity Panel (IWIP). The role of the WIPP is to define a feasible incremental implementation of the WAAS integrity function. WIPP members include FAA, Stanford University, Ohio University, Jet Propulsion Laboratory, Zeta Associates, MITRE Center for Advanced Aviation System Development (CAASD), and Raytheon. Initial WIPP efforts are focused on providing a precision approach with a VAL of 50 m by retaining the current architecture and making changes to the algorithms in the SP based on new ideas and prototype results. The second WIPP effort will determine the SP’s architecture and algorithms that can support a VAL of 12 and 20 m. In addition to the architecture and algorithm effort, the WIPP will provide the analytic justification for all assumptions and validate the completeness of the monitor. The IWIP members will use the documentation to verify that the integrity requirements are being satisfied. The IWIP comprises experts from FAA, MITRE/CAASD, Stanford University, Ohio University, BoozAllen & Hamilton, and the Naval Air Warfare Center. The individuals on this panel have had no direct involvement with the WAAS project, but have LAAS integrity expertise.
The key technical activities that need to be done are the following:
• Define the trade space for the correction bounding parameters.
• Develop techniques to reduce measurement noise.
• Modify the SP algorithms that estimate clock/ephemeris residuals (i.e., ensure that UDRE is a suitable upper bound on clock/ephemeris in the presence of measurement noise).
• Modify the SP algorithms that estimate GIVE residuals under quiet and severe ionospheric storm conditions.
• Conduct a comprehensive fault detection analysis.
• Develop and prototype a new architecture and algorithms that will improve UDRE and GIVE residuals for a VAL of 20 and 12 m.
• Identify the schedule and cost impacts of achieving a VAL of 20 and 12 m.
• Compare the performance and cost with user expectations.
Status: LAAS Developments
LAAS provides these augmentations to GPS: differential corrections, an estimate of the error bounds in the differential correction, and an additional ranging source, if needed. These functions are the same as those of WAAS, but the implementation is different. In LAAS, the differential corrections are transmitted over a VHF communications channel as a single value correcting for all errors impacting the pseudorange and range rate for each satellite. The corrections are generated from inputs from 3 to 4 reference stations. The accuracy of the differentially corrected position is about 1 m. Prototype LAAS ground stations have demonstrated the ability to satisfy CAT I integrity requirements. The challenging LAAS issue is to “guarantee” that the errors are bounded during allweather landing (i.e., including zero ceiling conditions).
The additional ranging source, if needed to improve availability, is provided by an airport pseudolite (APL), which transmits a pulsed signal on the GPS frequency. The APL has been prototyped, and suitable standards have been developed by RTCA. A LAAS CAT I specification has been developed and is being validated by the FAA.
The governmentindustry partnership between FAA, Honeywell, and Raytheon is producing CAT I LAASs that are planned to be type certified next year. FAA plans a LAAS procurement starting in 2003.
Resolving Key LAAS Issues
The key technical activities that still need to be done are the following :
• Validate the CAT I LAAS specification.
• Type certificate industry’s CAT I LAAS ground facilities.
• Start FAA procurement of CAT I LAAS systems.
• Develop and validate CAT III LAAS integrity concept.
• Develop a CAT III LAAS specification.
Status of Additional Civil Signals
In March 1996, a presidential decision directive made GPS a dualuse, dualservice system. The White House announced (during 1998 and 1999) that two civil signals will be provided: a C/A code will be added to L2 (signal at 1227 MHz), and a new L5 signal will be provided at 1176 MHz. The C/A code on L2 is intended for surface applications able to tolerate occasional interference from the many radars operating in the 12151385 MHz band. The L5 signal is be located in the Aeronautical Radio Navigation Service band (i.e., 9601215 MHz), where all emitters are managed by civil aviation authorities for safetyoflife applications. To minimize the impact on existing systems, the new L5 civil signal will have 6 dB more power than the L1 and L2 C/A signal and a higher chipping rate (i.e., 10 vs. 1 MHz), and the receiver will be required to incorporate a “pulse blanker,” and improved selectivity. In addition, current systems in the band (i.e., DME, TACAN, and JTIDS/MIDS) may be “rechanneled” to assure proper L5 reception.
An incremental implementation plan is being developed for these civil signals under the oversight of the Interagency GPS Executive Board. The implementation plan being considered by the GPS Joint Program Office is to add the C/A code on L2 starting with some Block IIR satellites, add both the L2 and L5 signals to the Block IIF satellites, and include both signals in the GPS III modernization. A constellation of at least 24 space vehicles having the L5 signal is not expected before about 2014.
L5, when operational, will provide two primary benefits to safetyoflife applications. First, the L5 signal removes the singlefrequency vulnerability by duplicating the WAAS and LAAS service on L5. (Will it be duplicated at L2 also? — Editor) In addition, L1/L5 avionics could use both frequencies to measure ionospheric delays thereby improving the availability of precision approach service.
What Needs To Be Done?
Significant progress has been made in defining the L5 signal characteristics, conducting theoretical analyses and simulations of receiver operation in the anticipated interference environment, and initiating an L5 standards development activity in RTCA. The L5 signal specification has been completed and is scheduled for RTCA SC159 plenary review in June.
The key technical activities that still need to be done are the following:
• Confirm the characterization of the RFI environment by direct measurement of DME, TACAN, JTIDS/MIDS, and radars.
• Confirm the operation of a receiver with pulse blanking in the RFI and L5 pulsed pseudolite environments.
• Confirm the ability to reassign DME and TACAN frequencies.
• Confirm the performance of WAAS and LAAS in the presence of the new military (M) code on L1 and L2 (e.g., validate the performance of the WAAS reference stations when the M code is added to L1 and L2).
• Complete the development of avionics standards for L5 and L1/L5 operation.
• Develop international standards for L5 and L1/L5 operation.
• Develop a plan to upgrade WAAS and LAAS to use the L2 and L5 civil signals.
—Melvin J. Zeltser, with The MITRE Corp.’s
Center for Advanced Aviation System Development,
is head of the department responsible for navigation
support to the FAA.
SA Fades Away
With the press of a button at midnight GMT (8 p.m. EDT) the first day of May, the government shut down selective availability (SA) and opened up what many feel will be a boom market in GPS applications. As one government spokesman dramatized it, a person with a Kmart receiver woke up the next morning finding that “they’re suddenly 10 times more accurate.”
The military, by intensive work on countermeasures, beat the inevitable 2006 deadline for turning SA to zero, by almost six years. At a White House briefing announcing President Clinton’s decision, Dr. Arthur L. Money, assistant secretary of Defense, said the pacing item for the decision was the ability to jam or deny the more accurate GPS signal to an adversary in a selected region. Once that was achieved, he said — final definitive Navwar tests were completed only last February — “we went forth with the recommendation to the President.”
The decision, surprising in its speed, came the week before the opening of the World Radiocommunications Conference (WRC) in Istanbul, where U.S. delegates are trying to safeguard new spectrum allocations for GPS modernization purposes, and rising interest among other countries in rival satellite navigation systems, such as Europe’s Galileo system.
SA had come to be a Cold War legacy of declining value for national security purposes, officials felt, especially when weighed against the benefits that could accrue to the growing majority of civil GPS users world wide. The widespread use of differential (D) techniques largely negated the effects of the military’s deliberate degrading of the civil signal (called selective availability) that limited accuracies to about 100 meters; civil users routinely achieved lessthan10meters accuracy with DGPS. Differential services still are essential, of course, to provide integrity, as well as for certain safetyoflife applications in transport, for precision survey and geodetic work and other applications.
In hailing the removal of SA, the White House noted in a fact sheet that a backpacker with a single lowcost receiver now “will find that the accuracy of GPS exceeds the resolution of U.S. Geological Survey topographical quad maps.” Industry leader Charles Trimble said the commercial impact will be secondary, that “the real beneficiary is the consumer, and GPS applications in information technology. GPS is a fundamental technology, the basis for all sorts of systems that affect the economy.”
Without SA, GPS in cell phones may become the preferred choice for location services. The FCC has mandated that mobile phones using Enhanced911 service must provide location accuracy of 50 meters 67% of the time, and 150 meters 95% of calls — a requirement now easily surpassed by civil GPS capability. Another burgeoning mass market is in vehicle navigation systems; their are than 40 million vehicles in the U.S. alone.
Timing data broadcast by GPS — widely used to time stamp financial transactions, synchronize utility networks, TV broadcasts, etc. — improves to within 40 billionths of a second. This could mean telecommunications companies could tighten the spacing between data packets, loading more information on existing optical cables, among other infrastructure benefits.
General aviation pilots, who now sometimes wave handheld receivers out the window, enjoy significantly better positioning/ navigation. “GPS without SA would yield aviation safety benefits, including better position information to help runway incursions,” says Phil Boyer, President of the 360,000member Aircraft Owners and Pilots Assn. Principal aviation benefits will be improvements in Receiver Autonomous Integrity Monitoring (RAIM), used for navigation, and in future use of the FAA’s WAAS/LAAS systems, used for precision approaches. SA had been the main cause error in this system.
Gene Conti, assistant secretary of the Department of Transportation, wouldn’t hazard a dollar estimate of increased receiver sales by U.S. manufacturers as the result of the demise of SA. But he noted that the government has forecast a robust $8 billion GPS industry in 2000, doubling in three years to $16 billion, before the announcement.
Prelude to SA Decision
A massive, coordinated push within the government, backed by those intent on halting SA sooner rather than later, resulted in the landmark announcement by Clinton. At the Pentagon, the Joint Staff formed a working group in December 1998, comprised of all four services, the U.S. Space Command, GPS Joint Program Office, and associated agencies such as the CIA, National Air Intelligence Center, Defense Intelligence Agency, Defense Information Systems, National Imagery and Mapping, National Reconnaissance Office and the National Security Agency. After a 14month thorough review by the working group, the Joint Staff submitted its recommendation Feb. 17 this year. U.S. military allies were notified in April; none protested, officials said. The recommendation was coordinated with the Interagency GPS Executive Board (IGEB), chaired by Defense and the Transportation Department.
Defense Secretary William Cohen notified the White House the week of April 24, President Clinton approved it Friday April 28, and the public announcement was made Monday May 1. That day, Dr. Money told a White House press briefing: “Defense, I believe, has demonstrated the capability to negate GPS signals in a threat area, consistent with military needs and the President’s policy; thus, we can set selective availability to zero. Given the widespread use of GPS for peaceful purposes, we believe this approach is (more) effective than world wide degradation.”
Defense has declined to specify the methods it will employ to deny the GPS signal in a theater of threat, nor the size of an area affected. It is believed that directional groundbased jammers will be the principal element, essentially requiring no satellite or system modifications, according to experts familiar with the military planning.
Perhaps the defining statement at the May 1 White House briefing was by Dr. D. James Baker, head of the National Oceanic and Atmospheric Administration (NOAA). “Now we have one system,” Baker declared. “All of these benefits that have been talked about will come without any receiver upgrades or fees whatsoever.”
A WHITE HOUSE PRESS RELEASE
Statement by the president regarding the United States’ decision to stop degrading global positioning system accuracy
Today, I am pleased to announce that the United States will stop the intentional degradation of the Global Positioning System (GPS) signals available to the public beginning at midnight tonight. We call this degradation feature Selective Availability (SA). This will mean that civilian users of GPS will be able to pinpoint locations up to ten times more accurately than they do now. GPS is a dualuse, satellitebased system that provides accurate location and timing data to users worldwide. My March 1996 Presidential Decision Directive included in the goals for GPS to: “encourage acceptance and integration of GPS into peaceful civil, commercial and scientific applications worldwide; and to encourage private sector investment in and use of U.S. GPS technologies and services.” To meet these goals, I committed the U.S. to discontinuing the use of SA by 2006 with an annual assessment of its continued use beginning this year.
The decision to discontinue SA is the latest measure in an on going effort to make GPS more responsive to civil and commercial users worldwide. Last year, Vice President Gore announced our plans to modernize GPS by adding two new civilian signals to enhance the civil and commercial service. This initiative is ontrack and the budget further advances modernization by incorporating some of the new features on up to 18 additional satellites that are already awaiting launch or are in production. We will continue to provide all of these capabilities to worldwide users free of charge.
My decision to discontinue SA was based upon a recommendation by the Secretary of Defense in coordination with the Departments of State, Transportation, Commerce, the Director of Central Intelligence, and other Executive Branch Departments and Agencies. They realized that worldwide transportation safety, scientific, and commercial interests could best be served by discontinuation of SA. Along with our commitment to enhance GPS for peaceful applications, my administration is committed to preserving fully the military utility of GPS. The decision to discontinue SA is coupled with our continuing efforts to upgrade the military utility of our systems that use GPS, and is supported by threat assessments which conclude that setting SA to zero at this time would have minimal impact on national security. Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened. This regional approach to denying navigation services is consistent with the 1996 plan to discontinue the degradation of civil and commercial GPS service globally through the SA technique.
Originally developed by the Department of Defense as a military system, GPS has become a global utility. It benefits users around the world in many different applications, including air, road, marine, and rail navigation, telecommunications, emergency response, oil exploration, mining, and many more. Civilian users will realize a dramatic improvement in GPS accuracy with the discontinuation of SA. For example, emergency teams responding to a cry for help can now determine what side of the highway they must respond to, thereby saving precious minutes. This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world.
Life After Selective Availablity
By Rob Conley
Shortly after Selective Availability was discontinued, the GPS Standard Positioning Service (SPS) changed dramatically. The transition as seen from Colorado Springs, Colorado, USA at the GPS Support Center is shown in figure 1. The data were taken using a Trimble SVeeSix receiver positioned at a surveyed location. The position solution errors were generated from foursatellite solutions, using singlefrequency measurements.
Can It Be That Good?
The first reaction from many people in viewing the transition data has been, “I don’t believe it … it can’t be that good!”. Well … yes it can. The basis for Position/Navigation/ Timing (PNT) performance experienced by the user is a combination of User Range Error (URE) and constellation geometry. The GPS Control Segment is currently maintaining constellation signalinspace User Range Errors (UREs) at a consistent 1.7 meters (1s). From a constellation management perspective, sustaining an average of 25 operational satellites is providing Root Mean Square (RMS) Horizontal Dilutions of Precision (HDOP) values of about 1.4, and Vertical DOP (VDOP) values around 1.9, for foursatellite solutions. Obviously, allinview solutions will generally provide even better geometry statistics. When we put these components together into a global assessment of base GPS performance without considering propagation or receiver contributions, it usually looks like the plot in figure 1. The example is for 5 May 2000, and the plot indicates global horizontal performance at a 95% error threshold level, for foursatellite solutions. Below the plot is a table containing key summary statistics associated with the plot.
With the basic GPS service providing such performance, the most significant SPS error source becomes the singlefrequency ionosphere model. In an analysis we conducted at the GPS Support Center last year covering the month of March, we examined singlefrequency model results against instantaneous dualfrequency measurements taken from the National Satellite Test Bed (NSTB). The bottom line to the analysis was that the single frequency model contributed approximately 3 meters (RMS) to satellite UREs.
When we examined singlefrequency effects on the foursatellite position solution, vertical error 95% statistics jumped to between 10 and 15 meters.
As the dust settles from this landmark event, users will inevitably wonder what it means to their particular community. In other words, “what does removing SA mean to me?” The answer to this question depends a great deal on who you are. I have provided below some thoughts for several different user groups.
The Aviation User
Aviation applications will in all likelihood be the most visible user group to benefit from the discontinuance of SA, at least in the short term. The primary effect of the improved accuracy will be to improve Receiver Autonomous Integrity Monitor (RAIM) availability to greater than 99.999% for all phases of flight except precision approach, particularly if the avionics suite aids GPS with a barometric altimeter. Even with SA discontinued, precision approach will still require some form of augmentation to ensure integrity requirements are met while providing a sufficient level of availability, and to reduce the probability of Hazardously Misleading Information (HMI) to the lowest possible level.
The Time and Frequency User
Most precise time users may not see significant benefit in the short term after SA is discontinued. This is because most users that depend on precise time transfer currently use techniques such as commonview observations to eliminate almost all the SA effect. Users with time synchronization requirements below the fivenanosecond level will still probably use such techniques. Many users may however find that direct access to UTC to within 1020 nanoseconds 95% of the time will be sufficient for their needs. The most likely longterm time/frequency application beneficiaries of setting SA to zero will be communication systems that can realize significant future increases in effective bandwidth use due to tighter synchronization tolerances.
The Vehicle Tracking User
Vehicle tracking system needs for precise positioning vary. Tracking an interstate trucker often needs only an accuracy good enough to locate which city the truck is in, whereas, public safety applications can require knowing the precise address of the vehicle. Thus, elimination of SA may have little effect on the trucking application, but will indeed significantly affect public safety’s use of GPS. First, current users of Differential GPS (DGPS) for vehicle tracking will in all likelihood drop the need for such. This eliminates the need for sophisticated and costly differential GPS systems, and results in decreased hardware costs at the vehicle (no DGPS required) and software costs at the base station (no DGPS reference stations or special DGPS processing). The loss of these systems will drop the demands on data networks for the transmission of differential correction data. Second, users who are not currently using DGPS will see a marked improvement in performance. Vehicles that used to be reported off the road will now be correctly represented as being on the highway.
The Maritime User
The removal of SA has the potential for significant benefits to maritime use of GPS. These benefits will be seen most directly for navigation in congested waterways or in very poor visibility conditions. The new SPS also provides a great deal more flexibility to waterway management authorities in overseeing and heading off possible navigation problems as commercial traffic increases. A major benefit will accrue to maritime users that need to navigate to a previous location, since repeatable accuracy has also increased by an order of magnitude. As in aviation, the use of differential services may diminish but will in all likelihood continue for many safetycritical operations due to the increased level of integrity available via a differential broadcast.
The Personal Navigation User
Today, consumers have a variety of options for using GPS for personal navigation. These include invehicle display units, handheld GPS receivers, cellular telephones with GPS and maps, and laptop and palmtop computers with GPS and mapping programs. Without some form of aiding, GPS with SA enabled reported positions with errors up to 100 meters (95%). Several companies have developed a number of techniques to correct for this error, including the use of inertial sensors and mapmatching algorithms, all of which have resulted in increased cost to consumers. Few personal navigation users take advantage of the benefits of DGPS, primarily due to cost and availability. Elimination of SA will in most cases provide an immediate improvement to personal navigation users with no extra effort or cost. The most obvious change to a handheld user will be the fact that the receiver’s altitude and velocity values will no longer change dramatically while standing still. Terminating SA should allow manufacturers to produce simpler and less expensive products, resulting in decreased consumer cost and a corresponding proliferation of use.
—Rob Conley of Overlook Systems Technologies, Inc.,
is Program Director for the DoD’s GPS Support Center.
Tycho Brahe Award
The next major breakthrough in precision navigation
may well be the brainchild of astronomytrained contemporaries.
The ION Council is considering the establish ment of a new annual ION award, the Tycho Brahe Award, named after the Danish astronomer, for achievements space navigation.
The eligibility requirements are to be reviewed at the June ION Annual Meeting in San Diego. In supporting the award, Len Sugerman, chairman of the Tycho Brahe Award Committee, wrote a refulgent endorsement. “The ION,” Sugerman wrote, “can surely become a major player in bringing the frontiers of outer space and discovery to the people.”
Sugerman said that for 50 years, ION awards have honored seminal achievements in navigation, and the Tycho Brahe Award contin ues that tradition, and expands it to space. “Astronomy distinguishes itself from most dis ciplines in many ways but especially in the mindstretching scales it invokes to describe space, time and the size of objects in the uni verse,” Sugerman said.
“On the horizon is interplanetary naviga tion, opening up space beyond the limit of earth orbits and travel to the Moon, Mars and other celestial bodies.
“The next major breakthrough in precision navigation may well be the brainchild of astronomytrained contemporaries,” Sugerman declared. “A whole new century is dawning. New forces and new players are waiting in the wings to take up the challenges. With determi nation, imagination, confidence and faith, new space navigation initiatives will surely be developed and expanded.”
Courtesy of Litton G&C Systems
President Thomas Jefferson whose intellect, scientific prowess and foresight led him to realize in 1803 that our continent needed to be explored beyond its border at the Mississippi River. With the acquisition of the Louisiana Purchase, this need became more compelling. Jefferson believed that there was a Northwest Passage, a waterway of adjoining rivers that could connect the Missouri River to the Pacific. Congress approved $2500 for this expedition. Jefferson selected his secretary, Captain Meriwether Lewis, to organize the “Corps of Discovery” to establish this route and bring back specimens from nature, maps and navigation data. Lewis then chose his close friend Lieutenant William Clark to be his coleader of the “Corps of Discovery.”
Lewis received three weeks of studies in celestial observations under Andrew Ellicott eminent astronomersurveyor. He also received tutoring in botany, fossils and more lessons from Robert Patterson in the determination of latitude and longitude. Lewis and Clark carried the necessary instruments for measuring the altitude of celestial bodies (sextant and quadrant), a chronometer for determining longitude and compasses to determine course. They also carried the best available maps of the region, an Astronomical Ephemeris and Nautical Almanac, Practical Introduction to Spherics and Nautical Astronomy and tables for finding latitude and longitude. It is to be noted that Lewis and Clark used an artificial horizon in their celestial observations as the clear horizon was not always available and the terrain elevation above sea level was not generally known. At sea the sextant measured angle to a celestial body which was then corrected for height above sea level (dip angle). Clark maintained a daily record of courses and distances traveled and frequently mapped the regions encountered by taking bearings and estimating distances to references.
The team traveled on a 55 foot masted keel boat and canoes when on the waterways. When they reached their Pacific Coast destination in Oregon territory which they named Fort Clatsop (for the neighboring Indian tribe), Clark estimated that they had traveled 4,162 miles from the mouth of the Missouri to the Pacific. This estimate has been cited to be in error by ~ 1% (40 miles) of the actual distance traveled. It is claimed that Lewis’ celestial observations at Fort Clatsop, when reduced to latitude and longitude, would locate the site within 4 miles of its actual position.
How were these navigational accomplishments achieved?
A. Clark used dead reckoning and Lewis used meridian transits of the Sun for latitude and lunar distances to establish
Greenwich time and longitude.
B. Lewis used eclipse tables to establish longitude and Polaris to establish latitude; Clark used inductive reckoning.
C. Lewis used Viking tables for longitude and meridian transits to establish latitude; Clark used dead reckoning.
D. All of the above.
Clark, versed in surveying and map making, maintained a daily log of courses and distances traveled and transferred the information to his map. Courses were determined from his compass. He could determine the magnetic variation by comparing compass magnetic north to true north. True north could be obtained by taking a bearing of Polaris (which traced a circle approximately 1° in radius around the celestial pole). Knowing magnetic variation he could plot his dead reckoning position relative to true north. He could determine the speed of the boat by timing a log chip dropped in the river along the side of the boat. If the log chip traversed the boat’s length in 71/3 seconds for example, he would know that he was traveling about 5 miles per hour. However, it is doubtful that Clark could achieve a dead reckoning error of 1% without compensatory errors. He relied upon a compass with an inherent error of at least a degree; his estimate of speed and distances was about 5% to 10% if not more.
Determining Longitude and Latitude
Both Lewis and Clark obtained the data for determining latitude and longitude by making equal altitude measurements (before and after noon) of the Sun using the sextant or quadrant and chronometer and were capable of reducing the data. The actual reduction of the data (which was recorded on tabular forms) to establish longitude by lunar distances en route was accomplished at West Point by mathematicians after the expedition was completed. Lewis and Clark were instructed to measure the altitude of the Sun at least two hours before noon, set the instrument down, and wait until the altitude would return to the same altitude verified by observation. The two times were then averaged to establish the time the Sun was on their meridian. This was the local apparent noon. Subtracting the time of noon at Greenwich (obtained from the Nautical Almanac) from the time recorded for the local noon (in Greenwich time) would yield the difference in time of the two locations. Multiplying the time difference by 15°/hr would yield longitude. If the altitude of the Sun were taken and plotted periodically between the initial and final observations, one could determine latitude which would be calculated at the midpoint between the initial and final observations of the Sun when the Sun reached its highest ascension and was on the observer’s meridian. This whole procedure was known as determining local apparent noon (Figure 1 and 2). One of the watches could be reset to local time on the basis of this procedure (allowing the chronometer to maintain Greenwich time). The chronometer was regulated prior to the expedition which meant that its error rate was known and could be acknowledged in the computations.
Longitude was to be obtained by performing measurements of lunar distances by Lewis and Clark. Lunar distances was a technique for determining Greenwich time and longitude by measuring the horizontal angle of the Moon to the Sun or one of the selected stars and measuring their altitudes using the sextant. A tedious calculation using a spherical triangle was employed to clear the distance of refraction and parallax effects for each measured altitude and other errors. This information was compared to tabular data in a table to obtain Greenwich time and longitude. This technique was conceived in the 15th century and underwent perfection over the centuries. was a very difficult procedure for most navigators.
Establishing Local Apparent Noon
Assume Lewis and Clark conducted this observation of the Sun and established their position (Figure 1):
|Steps in Obtaining Longitude: Conversion |
Factors Arc time 15°= 1 hour 1° = 4 minutes
15’ = 1 minute 1’ = 4 seconds Arc Equivalents ° degree of arc ‘ minute of arc “ second of arc
1°= 60’ 1’ = 60”
Add initial to final time of observation: 21hr. 31min. Greenwich time final observation 17hr. 31min. Greenwich time initial observation = 39hr. 02min.
Establish midpoint of observations for meridian transit: 39 hr. 02 min. /2 = 19 hr. 31 min. GT (Sun is on your meridian) Noon at Greenwich (0° longitude) from the tables was at 1204 1 / 2 . Subtract the Greenwich noon time from Greenwich time of the local noon yields 7 hr 26 1 / 2 min (use conversion factors above) convert the difference in time components to degrees and arc minutes and add them:
15°/hr X(7 hrs.) = 105° 0.25°/min. X
Longitude = 111° 38’ W
How latitude is obtained by meridian
Steps in Obtaining Latitude
Lewis determines that at meridian transit of the Sun, its elevation was 60° 02.4’ (after corrections for refraction and semidiameter) Latitude = (90° h) + d Given declination “d” of Sun is 15° 15.4’ N h = 60° 02.4’
Therefore Lewis finds his latitude as 45° 13’N and longitude as 111° 38’ W In reality Lewis and Clark fully calculated longitude by lunar distances only once early in their journey up the Missouri River. It was left for the mathematicians at West Point to reduce the extensive data recorded on the expedition. So vexing was the task to reduce the data for longitude by lunar distances that F.R. Hassler, a West Point mathematics instructor, never succeeded in completing the calculations casting doubt as to whether Fort Catslop was located by lunar distances. If it were located with the fourmile accuracy claimed, it may have been achieved by meridian transit calculations. A lunar positions table (extracted from the British Almanac issued in 1766 for the year 1767) is shown in Figure 4. A diagram of the lunar distances spherical triangle showing the angles to be measured and the angle to be calculated is shown in Figure 5. The meticulous preparation for the expedition, the use of the finest available maps of the region, the creation of maps and charts en route and the recorded data made it possible for Lewis and Clark to accomplish their goal and preserve the Northwest region beyond the Louisiana Purchase for later claim by the United States.
Afterword on Lunar Distances
Captain Vancouver (earlier a midshipman under Captain Cook), an experienced navigator, utilized lunar distances in establishing the longitude of Nootka a port on the west coast of Vancouver Island in 1792. He and his sailing master Lt. Whidbey made 13 sets of observations with an average difference from the mean value of 8.7 minutes of longitude (5.7 nmi at his latitude) and a standard deviation of the sets of 10.18 minutes of longitude. Each set was an average of from two to eight sets of lunar distances. The average number of observations in one set was 7.5. Since the scatter of a number of operations is reduced by the square root of the number of observations averaged, the standard deviation should be multiplied by the square root of 7.5 or 2.7. Therefore for a single lunar distance observation, the expected random error should have been about 30 minutes of longitude (19.5 nmi for latitude 49.5°). At sea one could expect not to obtain better than one degree accuracy from a single lunar distances observation.
Duncan, Dayton and Ken Burns. Lewis & Clark An Illustrated History. New York: Alfred A. Knopf 1999.
Emmott, Norman W. “Captain Vancouver and the Lunar Distances.” Litton Avionics Newsletter, Vol. One No. Three ( March 1970): 2739.
Jones, Landon Y. The Essential Lewis and Clark. New York: The ECCO Press An Imprint of HarperCollins Publishers, 2000.
IAIN (International Association of Institutes of Navigation) World Congress in association with the U.S. ION Annual Meeting;
Catamaran Hotel San Diego, CA, USA
Federal Radionavigation Plan (FRP) Users Group Meeting;
Catamaran Hotel, San Diego, CA
CarolAnn Courtney, DOT,Volpe
Civil GPS Service Interface Committee (CGSIC),
Hilton Salt Lake City, Salt Lake City, Utah
USCG Navigation Center
The Satellite Division of the Institute of Navigation,
13th International Technical Meeting; Salt Palace Convention
Center Salt Lake City, Utah, USA
ION, Tel: 703-683-7101
The Future for Satellite Navigation; Royal Institute of
Tel: 44(0) 20 7-591-3130
International Loran Association (ILA)
29th Annual Convention and Symposium;
Holiday Inn on the Hill, Washington, DC
GPS The Next Generation GPS III and Modernization
Capping an ambitious modernization plan for GPS, the Department of Defense (DoD) is now seeking funds to begin studies for the next generation of GPS satellites, called GPS III, that would carry the global system into 2030 and beyond.
The current modernization program will add new military signals (Mcode) to Block IIR and IIF satellites, as well as new civil capabilities. That schedule now looks like this:
• Last 12 Block IIR satellites: Add Mcode to L1 and L2 with increased power, add present civil C/A code to L2. First scheduled launch, 2003.
• First 12 Block IIF satellites: Add civil signal to third frequency, called L5, increase power. Add Mcode. Improve ground control system. First scheduled launch, in 2006 or later.
Rather than exercising contract options with Boeing Corp. for up to 21 additional IIFs, and trying to negotiate modifications, the Air Force plans now to halt any further buys of IIFs. Instead, it has announced a decision to launch a new competition for a new generation of spacecraft, or GPS III, expected to cost more than $1 billion. Military officials estimate this may be a cheaper and more effective way of adding spot beam capability and other modifications than trying to heavyup the remaining IIF satellites in the Boeing contract.
Although very much in the planning stage, the initial definition study for GPS III would be performed by the military’s two Federal Funding Research & Development Centers (FFRDC), the MITER Corp. and the Aerospace Corp. The FFRDC’s would develop data on such issues as realistic options, system capabilities, reasonable cost factors, etc., according to Pentagon managers familiar with the program.
Definitive Studies Needed
This phase would be followed by contracts, in FY2001, with three industry groups to conduct more definitive studies on requirements, system architecture, competitive trade issues, and other matters. Both civilian and military requirements would be solicited. Boeing, Lockheed Martin and several companies are expected to bid on this architecture study contract. DoD then sees a down selection to one contractor by 2003 for the design and production of the new series of GPS spacecraft.
Some preliminary information relevant to a new generation of satellites already exists in a draft Operations Requirement document prepared by the U.S. Space Command. Civilian input as now envisioned would come through coordination with the existing Interagency GPS Executive Board (IGEB), jointly chaired by Defense and the Department of Transportation.
Reprinted from Air Force Print News
Laserguided munitions came to prominence during the Gulf War when television screens across America showed the accuracy of the weapons. Iraqi bunkers, buildings and armored columns were routinely destroyed with unprecedented accuracy, helping make the Gulf War one of the most decisive military victories in global history. In the desert, only a sandstorm or, in some cases, the smoke created by burning targets nearby, would prevent the delivery of laserguided munitions.
Kosovo, on the other hand, reinforced the limitation that laserguided weapons systems are only effective if the target is not obscured. There, the poor weather and low visibility made laserguided munitions undeliverable or inaccurate and, in a handful of cases, dangerously so. Northrop Grumman’s Joint Direct Attack Munition, a GPSguided munition, was instrumental in the campaign during Kosovo’s consistently poor weather conditions.
“GPS guidance will still direct the munitions to target regardless of the weather,” according to AF Col. Robert George, commander of the AF AirtoSurface Munitions Directorate. “If you lose your laser spot while the weapon is guiding, chances are you’ll miss your target.” A small number of GBU24, GBU27 and GBU28 laserguided bombs currently in the Air Force inventory will be upgraded, but the majority of the inventory of such weapons will be purchased from the manufacturer with GPS already installed.
“There were reports that we’re doing GPS (modifications) on all our laserguided bombs,” George said. “We’re doing some, but we don’t plan to retrofit the entire inventory.”
While the conversion will end the laserguided munitions era and relegate the laser guidance system to secondary status, according to George, contrary to published reports, the GPS conversion doesn’t mean that the Air Force’s inventory of laserguided weaponry will be mothballed or no longer used in future warfare. It merely gives Air Force pilots options and flexibility.
“The GPS will guide the weapon to within a few meters. Add laser guidance to that, and, in theory, you should be able to guide it through an open window,” George said.
| ||Can We Share? |
GPS Spectrum Issues Pending Before WRC
By Lawrence Chesto
A number of important issues affecting the Global Positioning System are before the World Radiocommunication Conference 2000 (WRC2000), which began May 8 in Instanbul, Turkey, and runs to June 2. The outcome of the conference will be revised International Telecommunication Union (ITU) Radio Regulations (RR). GPS related issues for WRC2000 are listed below and are addressed by their related agenda item. GPS operates under the RR allocation of radionavigationsatellite (RNSS). A Conference Preparatory Meeting (CPM) was held in November 1999 and its report addresses the technical, operational and regulatory/ procedural matters to be considered by WRC2000.
GNSS and Fixed Service (FS)
Agenda items 1.1 (deletion of country footnotes to the RR) and 1.15.3 (status of allocations to services other than the RNSS) are related because RR country footnotes S5.355 and S5.359 exist in the 15591610 MHz band where RNSS is allocated. S5.355 allows FS to operate on a secondary basis in 25 countries in the 15401645.5 MHz and 1646.5–1660 MHz bands. S5.359 allows FS to operate in 44 other countries in the 1550–1645.5 MHz and 1646.5–1660 MHz frequency bands.
Studies have shown that FS stations can cause harmful interference in large areas to GPS operations because of their high power (up to 39 dBW) and wide bandwidth of several MHz. The CPM report stated that “On the basis of these studies, it is concluded that RNSS receivers would be unable to support cofrequency interference from FS transmissions within radio lineofsight.” It was also noted that “FS stations registered in the ITU MIFR with higher power and greater bandwidths were not considered in the current studies.” The CPM also stated, “In order to protect present and future RNSS applications, sharing of the band 15591610 MHz between RNSS and FS is not recommended.”
Only those countries that are listed in the footnotes can remove themselves from the footnotes. Removal of these footnotes in the 1559–1610 MHz band is required for GPS operations to exist without potential harmful interference within or near these countries when an FS station is in operation in the RNSS band.
GPS, and Mobile Satellite Service (MSS)
Agenda item 1.9 (feasibility of an allocation to MSS in a portion of the 1559–1667 MHz band) addresses the need for more spectrum for MSS in the 15591567 MHz band. The issue is can a MSS allocation exist with an RNSS allocation in the 15591610 MHz band. There are some that think that sharing with GPS is possible at a certain power flux density but there is no agreement what this power flux density should be. The CPM stated, “It is therefore concluded that sharing between ARNS/RNSS and MSS (spacetoEarth) is not feasible in any portion of the 15591567 MHz band.” It is important that no allocation to MSS in the RR be made at WRC2000 in order to protect current and future RNSS operations as there is no way to have MSS meet the stringent RNSS safety requirements (e.g., control of power flux density to ensure no harmful interference).
New Allocations for RNSS
Agenda item 1.15.1 (consider new RNSS allocations in the 1 to 6 GHz band) addresses several potentially new radio frequency bands for the addition of an allocation to RNSS to support new navigation services such as the GPS L5 signal at 1176.45 MHz and other new RNSS systems. The CPM report addresses the 9601215 MHz, 1260 1300 MHz, 13001350 MHz, 50005030 MHz and 50915150 MHz bands as a potential for new RNSS allocations. The critical band for the new GPS L5 (1176.45 MHz ±12 MHz) signal is the 9601215 MHz band.
The 9601215 MHz band is currently allocated to the aeronautical radionavigation service (ARNS) on an exclusive, worldwide basis. It is used for ICAO standardized radionavigation and surveillance systems that include the Secondary Surveillance Radar (SSR), the Airborne Collision Avoidance System (ACAS) and the Distance Measuring Equipment system (DME) and is also used for the Tactical Air Navigation system (TACAN). A portion of the band is used by ground DME and TACAN transponders.
The CPM study concluded that RNSS signals could be designed so they do not cause interference to ARNS receivers. However, ARNS transmitters can cause harmful interference to RNSS receivers. Radar systems in the 12151400 MHz band are also a potential problem. Several potential solutions to these problems have been investigated including design of the RNSS receiver, reassignment of certain DME/TACAN frequencies, and use of a dualbandwidth RNSS signal. Interference to RNSS receivers at high altitudes can be reduced by use of a RNSS narrowband signal component and at lower altitudes, use of a wideband signal component can be used for precision applications.
The CPM report stated “It was recognized that, if an allocation to RNSS were to be made in any part of the band 9601215 MHz, further study effort will be required by ITU, ICAO and others to develop recommendations to ensure compatibility between the various systems in the band.”
Agenda item 1.15.2 (addition of RNSS spaceto space direction allocations) addresses new RNSS allocations for spacetospace operations in the 1215–1260 MHz and 1559–1610 MHz bands. This service is being used on satellites and spacecraft for navigation and other related uses. While the CPM text supports an allocation to RNSS spacetospace direction in these bands there are several options that could restrict the amount of protection this new allocation would receive at WRC2000. It is important that an allocation is made for spacetospace RNSS and that any footnotes that limit protection should not be approved at WRC2000.
The CPM report addressed three options. All options propose the addition of an allocation to RNSS, spacetospace direction, in the 12151260 MHz and 15591610 MHz bands with a provision indicating that no protection should be given to spaceborne RNSS receivers from RNSS systems already operating in these bands or for which complete advance publication information has been received prior to the end of WRC2000.
The first option is as above. The second option includes the statement that spaceborne receivers operating in the 15591610 MHz band should not request protection from unwanted emissions of stations of the MSS (Earthtospace) operating in the band 16101660.5 MHz. The third option includes an additional statement that spaceborne RNSS receivers should be deployed and operated in a manner that they either avoid or accept possible interference at levels equivalent to those caused by current MSS (spacetoEarth) systems in the bands 15251559 MHz or those for which a request for coordination has been received prior to the end of WRC2000.
Summary of WRC2000 Potential Impact on GPS
WRC2000 will make changes to the RR and is not required to adhere to CPM recommendations. Decisions at the WRC are based more on political and economic issues than on technical and operational considerations.
Country footnotes for FS are a potential problem for worldwide GPS operations. Harmful interference will most likely be caused to GPS operations within radio lineofsight of FS stations operating at or near the GPS frequency. Some countries have changed their frequency assignments to avoid harmful interference to GPS operations. All countries should do this as a minimum.
An allocation to the MSS in the 1559–1567 MHz band could lead to harmful interference to current GNSS receivers and restrict new applications of GNSS in the 1559–1610 MHz band. An allocation to MSS must be avoided to ensure protection of GPS safety operations.
The GPS L5 (1176.45 MHz) frequency (including sufficient bandwidth) needs to be protected by a RNSS allocation. Failure to obtain the RNSS allocation means no international protection for this GPS L5 signal and thus can limit worldwide operational use. Spacetospace RNSS allocations are essential to protect the many new applications now being implemented for GPS receivers on satellites and other spacecraft. The spacetospace allocation should not have too many restrictions or there will be little protection achieved with a new allocation.
—Lawrence Chesto is an aviation consultant. He is chairman of RTCA Special Committee 159 on GPS standards for aviation. He also is an advisor to the U.S. representative of ICAO’s GNSSP.
The Fourth User Domain GPS Space Use Evolving
By John Rush
T errestrial, maritime, and aviation uses of GPS are all well known. However, space is a fourth user domain that has quietly developed over the past few years and is rapidly moving from research to every day application. More and more satellites are being launched that use GPS in a variety of applications. The Figure 1 chart was developed from data collected by Dr. George Davis, Orbital Sciences Corporation, for the Goddard Space Flight Center and it depicts the dramatic increase in the number of satellites launched each year with GPS receivers onboard. A major reason for the increase in recent years has been the introduction of GPS receivers onboard operational commercial satellites.
For the most part, current applications involve position determination and spacecraft time synchronization for satellites in Low Earth Orbit (LEO). In some applications GPS is also used to determine spacecraft attitude (orientation about its axis) as a backup to the sensors traditionally used for this purpose.
Although many of these applications are experimental, there are some instances where GPS is being used in critical spacecraft operations. As we gain more experience with its use aboard satellites and become more confident in its reliability, GPS may become the standard way of accomplishing the satellite tracking function that has been traditionally done using ground communication antennas. The maturity of GPS use on satellites for the tracking function has the potential of reducing operational costs, and can provide more precise positioning information than is typically available through the use of ground antennas. But, perhaps most importantly, satellitebased GPS use is an enabling technology that will lead to the ability to operate satellites more autonomously and enable new capabilities.
A few examples of what satellitebased GPS use can enable in the future.
• Satellites that maintain their own orbits as ordered with a few simple commands from ground controllers: Today we have to provide tracking information from ground antennas and build detailed commands to adjust and maintain satellite orbits. In the future a GPS receiver combined with onboard orbit determination software and a closed loop propulsion system will be able to autonomously maintain the desired orbit. The commanding needed to do this will be simple descriptions of the desired orbit.
• Formation flying of small satellites clusters through the use of differential GPS: Groups of small satellites will be able to conduct coordinated Earth observations while maintaining precise relative location with respect to each other. The satellites in a cluster will be able to work together implementing coordinated observations, for example, enabling new levels of stereoscopic imaging. This same relative navigation technique will be of benefit in future space operations involving orbital rendezvous of autonomous vehicles bringing supplies to the International Space Station or in other autonomous Earth orbit rendezvous operations.
• GPS receivers onboard satellites can double as a science instrument: Two applications in particular that show great promise are the use of GPS signals to investigate atmospheric composition, and the use of GPS signals reflected off of the ocean surface to determine sea state. In the first application dual frequency signals are observed for GPS satellites that are low on the horizon such that their signals are partially occulted by the Earth’s atmosphere. Measuring the relative distortion in the signal path can provide information concerning the density of the atmosphere at various altitudes which can further be related to atmospheric conditions such as water vapor content. In the second application very weak GPS signals would be detected by GPS antennas that look toward the Earth. Scatter measured in the reflected signal would be a function of the roughness of the water surface. The above examples are just a few instances where GPS may be used to advance the stateoftheart in satellite capabilities and operations. All of the above examples are being actively pursued today in research and development efforts at NASA and international space agencies around the world. But there are a few recent and future events and initiatives of note that will have a significant bearing on GPS space users:
Elimination of Selective Availability
Of course, the recent termination of Selective Availability provided greatly improved precision potential for single frequency space users of GPS. Estimates from the Goddard Space Flight Center indicate that the improvement will allow about 50m in instantaneous orbit positioning precision for a satellite in a 450 km orbit, and about 10m for a satellite in a 700 km high orbit. This is down from about 100m. The difference in the level of precision estimate between the two is attributable to the greater amount of signal distortion encountered by the lower satellite due to the ionosphere.
The next major enhancement for space users will come from the GPS Modernization program. Many LEO satellites operate in a region where they encounter ionospheric distortion. The addition of the second civil signal to GPS satellites beginning in 2003 will be a major step toward improving GPS precision in the use of GPS for satellite positioning. With the second civil signal at L2 being broadcast with the same C/A code as on the L1 signal, dual frequency spaceborne receivers that correct for ionospheric distortion can be provided at little added cost.
Expanding the Space User’s Envelope — Higher and Higher
As mentioned earlier, space use of GPS today is pretty much limited to Low Earth Orbit. However, there is research ongoing to determine the degree to which GPS can be used by satellites in orbits near and above the GPS constellation. Two early experiments, the EquatorS and FalconGold missions, confirmed that GPS signals can be detected above the GPS constellation. Shortly a number of new experiments will be conducted using satellites in highly elliptical orbits with apogees above the GPS satellite orbits. These experiments will measure the characteristics of the signal near and above the constellation. One such experiment will be the GPS At Geostationary Transfer Orbit Experiment (GAGE). GAGE is a jointly sponsored NASA / GPS JPO / British Defence Research Agency (DERA) experiment that will carry a NASA / JPL BitGrabber GPS receiver into a highly elliptical orbit aboard the British Space Technology Research Vehicle (STRV) 1c. The mission is presently scheduled to be launched in October of this year aboard an Ariane.
Onboard the same launcher will be an amateur radio satellite with a GPS receiver supplied by Goddard Space Flight Center that will enter a similar orbit and conduct a similar experiment. Between the two experiments and together with data already collected from EquatorS and FalconGold, we will be able to better understand the GPS signal characteristics at higher orbit altitudes in order to build GPS receiver systems that will be able to extend the benefits of GPS out to Geostationary Orbits around 20,000 km above the GPS constellation.
Regulatory Protection — The SpacetoSpace Allocation Initiative
Fundamental to the use of GPS onboard satellites is the confidence that the GPS signals can be received free of interference from other radio emissions. The first step in assuring this level of confidence is being addressed in the next few days at the World Radio Conference 2000 (WRC00) in Istanbul, Turkey. In addition to other key considerations affecting surface and aviation GPS users, the Conference will be considering whether to grant a global radio spectrum allocation in the spacetospace direction to protect spaceborne users of radionavigation signals in the bands 1215 – 1260 and 1559 –1610 MHz. These bands include the GPS L1 and L2 bands. Currently the international allocation only protects the use of GPS signals in a spacetoEarth direction, meaning that spacecraft using GPS have no regulatory protection.
The proposed spacetospace allocation was brought forward at WRC97 where it was put on the agenda for the next Conference (WRC00). It was directed that the proposal undergo technical studies to assure compatibility with the existing electromagnetic environment. During the past two years an NTIAled technical study team has conducted numerous studies on space use compatibility of GPS that have been presented at international study group meetings preparing for WRC00. All of these studies have demonstrated space use compatibility with other uses of the spectrum. At the time of writing we know of no major issues that should stand in the way of achieving the allocation.
During the coming years it is anticipated that the use of GPS aboard Earth orbiting satellites will move from the research to the operational stages in a number of different application areas. As discussed above, many of these applications go well beyond navigation alone and many hold the promise of enabling new ways of doing business in space.
NASA Headquarters, Space Navigation and Communications System Architect.
Satellite Division News
Satellite Division Nominations
Nominations were submitted by the 2000 Nominating Committee for officers of the Institute of Navigation’s Satellite Division. The Satellite Division’s Nominations Com mittee was chaired by Mr. Gaylord Green.
Pursuant to Article V of The Institute of Navigation Satellite Division Bylaws, “additional nominations may be made by petition, signed by at least 25 mem bers entitled to vote for the office for which the candidate is nominated.”
All additional nominees must fulfill nomination requirements as indicated in the ION Satellite Division Bylaws and the nomination must be received at the ION National Office by June 26, 2000.
Ballots will be mailed in July. Election results will be announced during the 13th International Technical Meeting of the Satellite Division of the ION, being held Sept. 19–22, 2000, in Salt Lake City, Utah.
The newly elected officers will take office on Sept. 22, 2000, at the conclu sion of the ION GPS Meeting and will serve for two years. Election results will be reported in the ION Newsletter.
Kepler Award Nominations
The Satellite Division is seeking nomina tions for the esteemed Johannes Kepler Award. The Award will be presented Sept. 22, in Salt Lake City at the ION GPS 2000 Awards ceremony.
The winner of the Award is determined by a special nominating committee. The primary purpose of the award is to honor an individual for sustained and significant contributions to the development of satel lite navigation. All members of the ION are eligible for nomination, but the award is bestowed only when deemed appropriate.
Individuals are encouraged to submit names for consideration. Please provide a supporting letter to Dr. Jim Spilker, Satellite Division Awards Committee chair. Fax to 1 7036837105, or email: firstname.lastname@example.org before Aug. 1, 2000.
Prior Kepler Award Winners:
1999: Dr. James J. Spilker, Jr.
1998: Dr. Peter Daly
1997: Dr. Gerard Lachapelle
1996: Dr. Frank van Graas
1995: Dr. Richard J. Anderle
1994: Ron Hatch
1993: Dr. A.J. Van Dierendonck
1992: Dr. Rudy Kalafus
1991: Dr. Bradford Parkinson
The University of Calgary, Alberta, Canada, is awarding Dr. Bradford Parkinson an Honorary Degree at its Con vocation June 16 in honor of the recipient’s leadership as program director, 1973 - 78, while a Colonel in the U.S. Air Force, of the GPS program. Parkinson presently is a pro fessor in the Department of Aeronautics and Astronautics, Stanford University.
GREATER PHILADELPHIA SECTION.
Bylaws were adopted and a slate of officers submitted at an organizational meeting to revitalize the dormant section at Penn State University (PSU) facilities on April 24. The following officers and chairpersons were designated: Marvin B. May, ARL/PSU, chair; John War burton, FAA, vice chair; Ray Filler, CECOM, secretary, and Victor Wullschleger, FAA,, treasurer. Chairpersons included Victor Wullschleger, FAA, special activi ties; Phil Holmer, FAA, programs; John Phanos, Galaxy Scientific, membership; Lou Naglak, NAVMAR, corporate affairs, and Neil Weinman, ARL/PSU, student awards.
NEW ENGLAND SECTION
The Section's 15th meeting was held at the Volpe Center on March 29th and featured a presentation on the Joint Precision Approach and Landing System (JPALS) by Maj. Dan Uribe of the GATO/MC2 Program Office at ESC, Hanscom AFB. Also short presentations were given by Ilir Progri (graduate student at WPI) on an Evaluation of CATIII Landing with Pseudolites and Bette Winer of MITRE on an overview of MITRE activities in the navigation area.
Third Block IIR GPS Satellite Orbited
After a threeweek launch delay, the third in the Block IIR series of GPS satellites lifted into space aboard a Delta II rocket from Cape Kennedy at 9:48 p.m. on May 10.
The $42 million satellite, built by Lockheed Martin (LM), replaces one in the constellation launched 11 years ago. LM has a contract to sup ply 17 more Block IIRs over the next five years. Pending approval of funding by Congress, LM will be tasked to make further improve ments in the last 12 Block IIRs, including additional military and civilian signals, increased power and other ‘modernization’ enhancements.
The Delta series of rockets, now made by Boeing, have racked up a record for launching 31 GPS space craft, including the most recent. Boeing has a launch contract on Delta IIs for the next 17 Block IIRs. It also is producing the next gener ation of GPS satellites, the Block IIFs, which are to be launched on the new Boeing Delta IV rockets that are part of the Air Force’s Evolved Expendable Launch Vehicle (EELV) program.
Global Marketing of GPSBased Aviation Services
With the acquisition of Comsat Corp., Lockheed Martin seeks to strengthen its bid to operate a global satellite system for aviation navigation and offer a full range of WAAS and LAASlike services.
The acquisition of Comsat, largest owner of the global Intelsat and Inmarsat satellite communications networks, is expected to be completed later this sum mer. Comsat currently leases capacity on two Inmarsat satellites to the FAA for WAAS broadcasts; it recently signed a five year extension of that contract. A LM spokesman said the price is about $2.4 million per year per satellite.
A new company called Synchronetics, formed by LM, is taking over, and expanding, the company’s range of navigation services offerings. Part of the ser vices include the Regional Positioning System (RPS), a plan announced by LM in June of last year to own and operate three geosynchronous satellites to broad cast GPS augmented signals for improved accuracy and integrity. An application for this system is pending before the FCC.
Mating With Ground Segment
Beyond that, Synchronetics is now seeking partners to supply the ground segments for both WAAS and LAASlike systems to coun tries in Latin America and elsewhere that lack ground augmentation networks. “We’re looking for partners for the both the ground and the space segments,” explains Daniel Brophy, director of navigation services for LM air traffic control management.
Synchronetics is forming three region al subsidiaries to provide seamless satel lite coverage worldwide: the Americas, AsiaPacific and EuropeMiddle East Africa. Each region is to secure separate financing and seek inregion partners to develop the system.
“This structure provides regional par ticipation and control,” LM declares in a March 20 statement, “while capturing the efficiencies of a single global archi tecture and facilitating a single standard for user equipment.”
LM says the space system will be compatible with the U.S. WAAS and LAAS and the European EGNOS. Capital costs for a dedicated threesatellite global system are about $450 million for the space segment, Brophy estimated.
LM is trying to line up partners, find internal financing, push its application for a dedicated system (not one hosted on other communications satellites) through the FCC, work with Honeywell in building a technical competence in ground segment engineering, all the while keeping an eye on proceedings at the World Radionavigation Conference in Istanbul. It is backing vigorously the U.S. effort to designate frequencies for the GPS L5 carrier.
|From the Editor |
What a Difference A Day Makes
The White House reports that more than 700 U.S. households connect for the first time to the Internet every hour. They are part of the legends joining the rush to the wired world: “More than half of all American households now use the Internet,” according to the White House statement. “More than half of U.S. classrooms are connected to the Internet today, compared to less than 3 percent in 1993.”
The above examples served to buttress President Clinton’s budget requests for record federal spending in FY2001 on science and technology. Overall, the Administration is asking for a whooping $43 billion in Research & Development (R&D) across all agencies, including Defense, up $2.8 billion over the previous year. Federally funded basic research tangentially benefits the sciences of navigation/ positioning and timing.
The proposals are now before Congress. Changes in the Administration’s requests are inevitable, but science and technology generally enjoy the bipartisan support of both Republicans and Democrats.
The Administration repeated the supporting theme that R&D programs form the bedrock of America’s prosperity in the 21st century. Even Alan Greenspan — the world listens when the Federal Reserve Board chairman speaks — said at one time that 70 percent of the U.S. economic boom is due to technological progress.
One of the biggest beneficiaries of the Administration’s R&D largess is the National Science Foundation (NSF). The NSF budget would rise to $4.6 billion, a proposed 17 percent increase and the largest oneyear dollar increase in its history. Although the NSF accounts for a small portion of overall government R&D spending, it supports about 50 percent of all basic research in science and engineering disciplines (nonmedical fields) at universities and colleges.
Among all the science budget initiatives, perhaps one with an obvious impact on navigation/positioning is new proposed information technology (IT) programs. Like GPS, IT delivers tools and capabilities that benefit almost every R&D effort. An example: research to ensure that mobile and wireless systems can be integral parts of the Internet. These inventions will permit devices embedded in equipment or vehicles such as GPS receivers, or even wearable devices such as navigation boxes for the blind, that identify themselves to networks automatically and operate with appropriate levels of privacy and security.
The column in the previous issue of this Newsletter applauded the Air Force’s Independent Review Team (IRT) for its work in establishing the new GPS L2 and L5 civil codes/signals. It was not meant to give the IRT undue weight, or ignore other contributors. Brian Mahoney, FAA, reminds us in a note that the RTCA made critical inputs in defining the L5 signal. He mentions the work of RTCA SC159, chaired by Larry Chesto. Specifically the work done by A.J. Van Dierendonck and Chris Hegarty on the L5 Working Group within SC159 in “establishing the baseline and trying to find an equitable solution to the coexistence of L5 with the FAA’s DME and DoD’s JTIDs systems.” From the ION standpoint, we’ll add Keith McDonald who early helped stage conferences and ION working groups to identify and define civil improvements.
To others, so many others in industry and government, too numerous to mention here, who contributed to the GPS modernization package, the nation thanks you.
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