Item History: The TI 4100 was first offered for sale circa 1981 when there was only a test constellation of GPS Block I space vehicles (SVs). The TI 4100 was a third generation GPS receiver. The design was based on custom large scale integrated (LSI) components that were originally intended for military applications. The LSI technology was integrated injection logic (I2L). I2L was the highest speed low-power bipolar digital technology of that time period. A geodetic version of the TI 4100 evolved from a tri-agency contract administered by Applied Research Laboratory of the University of Texas and funded by the Defense Mapping Agency, National Geodetic Survey and the U.S. Geological Survey. The geodetic versions were delivered in pairs with phase matched antennas plus calibrated receiver characteristics. The basic TI 4100 configuration included an antenna/preamp assembly, the receiver, and a control display unit (CDU). There was also an optional dual-drive cassette recorder. Another option supported customer reprogrammable navigation. Navigation error (typical 4 SV): 14 m P code, 47 m C/A code, 0.015 m/s velocity.
Geodetic error: 5 mm + 1 PPM with surface weather tropo compensation.
Noise figure: 5 dB. Reference oscillator: Ovenized w/1X10-11Allan deviation (1s gate).
External time standard: Built-in phase lock provision for 5 MHz atomic standard.
Time to first fix budget: 20 minutes sky search; 4 minutes initialized.
Tracking modes: 4 SV (initial); 3 SV (altitude hold); 2 SV (time bias rate hold).
Dynamic modes: Geodetic-2nd order PLL, Normal-3rd order PLL; High-4th order PLL, all with FLL assist at one order lower.
Anti-jamming: Virtually immune to low duty cycle pulse jamming plus a proprietary front-end spreading code to minimize CW leak-through. Operating environment: Harsh wind, rain, sleet, snow and sand at -20o C to 50o C.
Shock and Vibration: 2g vibration load, 5g sustained load, 15g half sinewave shock load.
Size/ Weight/ Power: Receiver: 14.7 in (37.3 cm) L x 7.5 in (44.5 cm) W x 8.3 in (21.1 cm) H, 53.0 lb (24.0 kg); Antenna: 6.6 in (16.8 cm) D x 11.0 in (27.9 cm) H, 3.8 lb (1.7 kg); CDU: 7.0 in (17.8 cm) L x 4.2 (10.9 cm) W x 2.1 in (5.3 cm) H, 1.0 lb (0.5 kg); 22 to 32 VDC input; 93 W operate, 15 W standby.
Measurements: L1 P code state (converted to pseudorange using receive time tag); L2 P code state; time rate of change of L1 P code state (pseudorange rate); time rate of change of L2 P code state; L1 Doppler induced carrier phase (whole cycles and fractions); L2 Doppler induced carrier phase; instantaneous L1 Doppler frequency shift; instantaneous L2 Doppler frequency shift; averaged line-of-sight SV acceleration; L1 C/A code state; time rate of change of L1 C/A code state (pseudorange rate); signal-to-noise ratio and other quality indicators. Unique attributes: Single channel hardware with multiple-channel re-entrant software capable of multiplexed tracking of both the L1 and L2 signals of four (4) SVs in P code and L1 C/A code. Thus, in a sampled data sense, the TI 4100 tracking and data demodulation were continuous. Since all signals passed through the same receiver hardware path, there was essentially no interchannel bias. The same architectural features in the TI 4100 that supported multiplexing also provided precise replica code and carrier Doppler phase measurements with negligible quantization noise and only a few nanoseconds of epoch timing error. The normal geodetic mode of the TI 4100 provided all raw measurements on identical GPS receive time epochs rather than on set time epochs. Thus, even though there was unavoidable time skew between the set times of multiple GPS receivers, there was no time skew between TI 4100 differential measurements no matter how many receivers were involved.
Some unique contributions to the art and science of navigation and related technology: Originally, GPS was developed to provide an all-weather, worldwide, continuous navigation system for absolute, three-dimensional, real-time navigation with accuracy in the 10 meter range. But the design intent of the TI 4100 was to achieve the ultimate relative navigation precision using carrier interferometric techniques as well as precise time transfer. Prior to the TI 4100, “pseudorange” was (and continues to be) described in most specifications as the GPS receiver code ranging observable. Very little was published about the carrier ranging receiver observable when the carrier tracking loop is in phase lock or the intrinsic precision time transfer capability of GPS. The TI 4100 was unique in that it did not use counters to synthesize its measurements. Instead, the TI 4100 synthesized its code states and carrier states as natural by-products of its digital code and carrier tracking loops phase states. These were the uncorrected SV transmit times and Doppler induced carrier phases, respectively. The uncorrected SV transmit times were in the form of the prompt replica code phase offset measurements with respect to the GPS start-of-week time epoch. The Doppler induced carrier phase was actually the uncorrected, ambiguous integer count and exact fraction of carrier wavelengths between the antenna phase centers of the SV and the TI 4100. The differential integer ambiguity solution problem was and continues to be the subject of numerous technical papers.
The TI 4100 measured its replica code and carrier states with quantization precisions of 2-32 P-chip and 2-32 cycle, respectively. In other words the quantization noise of the TI 4100 was effectively zero. This and other design attributes helped the TI 4100 to achieve very high GPS receiver accuracy when using relative navigation techniques to remove all common mode errors while contributing minimal GPS receiver error to the end solutions. Primarily because of the availability of these natural raw measurements and the customer’s freedom to modify the way these measurements were incorporated, the TI 4100 was the basis for numerous pioneering advances of ultra-high precision GPS technology using various types of relative navigation and time transfer techniques. The basic principle of operation usually involved positioning with respect to a known and fixed reference location, but there were also precise relative navigation techniques developed using moving reference points. A single TI 4100 could navigate precisely relative to a designated starting point for the time duration that the same four SVs could be tracked. For example, centimeter accuracy hover-hold techniques were demonstrated for helicopter applications. Typically, two TI 4100s were used simultaneously.
The most remarkable differential GPS technology breakthroughs were achieved by the geodetic versions of the TI 4100 delivered under contract to the National Geodetic Survey (NGS now under the National Oceanic and Atmospheric Administration or NOAA), the U.S. Geological Survey, and the Naval Surface Weapons Center at Dahlgren, VA. The phase-matched conical spiral antennas contained precise indexes on the tripod-mounted preamp bases. In the field, both indexes were pointed toward magnetic north to minimize differential phase center migration. The L1 and L2 phase centers were also inscribed on the antenna radomes. These systems were used in the 1980s and early 1990s with the Block I SVs as the GPS standard for establishing geodetic control. Using the intrinsic accuracy of the TI 4100 two-frequency carrier interferometry measurements, these systems exceeded previous first order geodetic surveying standards. The typical baseline error was less than the width of a stack of 5 dimes (5 mm). Also permitted were very long geodetic baselines whose end-points were previously required to be within optical (line-of-sight) view. Even though the GPS Block I test constellation typically provided only about 2 hours of 4 SV observations, careful field planning provided highly cost-effective first order geodetic surveying.