Stel Satellite Signal Generator (SSG)
   Front Panel of STEL SSG showing Two Controllers and An Upconverter 0.75

Model number: Model 7200

Period/Dates when in use: 1989-2000

Description: The SSG generates real-time GPS satellite signal environment on L1 (1575.42 MHz), and L2 (1227.6 MHz) and can output the Precise (P) and Course Acquisition (C/A) ranging codes. The SSG accepts input from a cesium clock, and can simulate platform trajectory and satellite control via IEEE 488 by outputting an RF satellite signal. The SSG accepts input from a host vehicle interface emulator and monitoring station. Also a second alternate simulator can be daisy chained as well to increase the number of simulated satellite Radio Frequency (RF) signals, or satellites.

The SSG allows the user to modify simulated satellite vehicle (SV) constellation to be able to re-create any observed GPS constellation anywhere on earth at any day and time for GPS receiver performance evaluation. This allows the user to define the SV constellation simulation with respect to fidelity on total message content, power level and RF shading considerations.

The SSG can accept specific receiver ephemeris files and create a simulated satellite constellation. The SSG allows commands from input files to simulate platform scenario motions of various military vehicles such as aircraft, missile, surface vehicle or surface ship. The SSG allows the user to modify a simulated SV constellation scenario to be able to re-create vehicle platform dynamics for the purposes of testing a GPS UE in a jamming or spoofing environment (i.e. counterfeit GPS signals).

The GPS Satellite Signal Generator is needed to test the dynamic response and navigation accuracy of the GPS UE and help verify host vehicle emulation algorithms in a laboratory environment and to extend the available laboratory test time beyond periods of satellite availability. The SSG is used to generate/simulate the radio signals as they would appear at the antenna of a NAVSTAR GPS User Equipment. The simulated radio signals are then used by the UE in the same way as the actual signals from real satellites. The difference is that the operator of the SSG will be able to select the parameters (position, velocity, etc) that determine the exact characteristics of the signal sent to the UE. The SSG consists of the GPS signal generators, digital frequency synthesizers, an RF combing network, a jammer, a computer, a master oscillator, and an internal GPS UE. The inputs to the SSG are derived from a reference model of position velocity and acceleration. The reference model is basically a navigation reference position within the mission scenario. If no reference model is established then the SSG has the capability of generating its own trajectories and providing them as an output. There is an internal GPS UE (a calibration receiver) which is used for a continuous self check of the GPS SSG.

The actual signal generator consists of two 5-channel continuous signal sources capable of simulating up to 10 satellites transmitting L1 C/A and P code and L2 P code simultaneously. The Digital Frequency Synthesizer supplies a precise clock standard to the signal generator. It is phase locked to the master oscillator. The output is stepped in frequency and/or phase under control of the computer.

The RF network contains the individual attenuators for the output of each signal. These outputs are inputs to the UE.

The computer translates commands from the operator or from the host vehicle for each channel and directs coordinated frequency slewing of the ten synthesizers. The computer that was used (a DIGITAL Equipment VAX 1100 series) includes its peripherals can also use a pre-stored position or trajectory to direct the synthesizers. This pre-stored position or trajectory in the computer can be sent to the host vehicle as a reference model for other equipment under test in the laboratory.

The technical features of the GPS SSG are:

  1. The GPS SSG generates pseudo-noise code signals exactly like those from actual GPS satellites in orbit.
  2. The satellite signals generated by the SSG are under computer control.
  3. The computer executes commands that have been programmed to simulate position, velocity, etc. from a host vehicle.
  4. In an alternate configuration, the computer can send commands such as position, velocity, etc back to the host vehicle.
  5. Using the output signals from the GPS SSG, the GPS UE calculates a navigation solution (i.e. position, velocity, time, etc.)
  6. The GPS SSG is able to simulate fixed position locations selectable by the operator.
  7. The GPS SSG has the capability to create situations in which the host vehicle has dynamics that can move along a pre-planned trajectory.
  8. The SSG can vary the RF signal power of each satellite signal generated from a range of -90dBw to -180dBw.
  9. The internal calibration receiver tracks the combined output going to the GPS UE and the host vehicle without jamming. This is primarily for the self check of the GPS SSG but can also be used for feedback to the host vehicle.
  10. Each signal generator has an output port (connector) plus additional port for the purpose of observing the simulated GPS signals in the laboratory environment.
  11. The NAVSTAR GPS L1 and L2 signals can be controllable and time delays can be introduced on the C/A and P ranging codes to simulate multipath conditions (i.e. RF signal reflections).
  12. The GPS SSG has a time base corrector for synchronizing time in the signal generators.
  13. The SSG allows you to combine signals from an external jammer for GPS UE testing in hostile RF environments.

In 1993, the SSG was programmed by NRaD personnel to be the first simulator to demonstrate integer carrier phase ambiguity simulation capability. This is documented in a paper which won Best in Session award: May, M.B., Weiss, J., Haiges, G., “Testing Differential GPS Using Satellite Signal Generators” ION Satellite Division Symposium, 21 September 1994, Salt Lake City. In 1995, another pioneering application employing the SSG to test Embedded GPS Inertials was demonstrated by NRaD personnel and documented in: May, M.B., Mullen, R., “Simulated Inertial GPS Navigation Laboratory (SIGNaL)” , Proceedings of the 51st Annual Meeting of The Institute of Navigation, June 5 - 7, 1995

Item History: The Stel Satellite Signal Generator (SSG) model 7200 was procured by NADC sometime between 1988 and 1989 by the Navigation and Communications Department, Global Positioning System (GPS) Branch Division Code 4013, in support of the Navigation Satellite Tracking and Ranging (NAVSTAR) GPS Program to test and evaluate military GPS receivers, also called GPS User Equipment (UE). The Naval Air Development Center (NADC) in the 1980’s and 1990’s was the principal DoD activity for the testing and integration of GPS receivers. Prior to the procurement of the SSG, GPS receiver testing was accomplished using a very limited live sky satellite constellation of approximately 6 satellites. Live sky satellite testing sometimes had to be scheduled at odd hours when satellites were in view overhead in the sky. Testing sometimes would begin at midnight and end at 4:00 AM when the satellites were no longer in view. This limited “test window” prompted NADC to partner with Stanford Telecommunications with developing and fabricating the SSG. Satellite simulator testing enabled NADC to test anytime while exactly simulating the live sky satellite constellation, and it was also a useful tool in being able to re-run a test by recreating the exact same test conditions as the previous test. This feature was extremely useful in troubleshooting GPS receiver anomalies. It was the first of its kind, and also was delivered with source code, enabling the NADC engineers to improvise new simulator applications. In 1995, the SSG as well as the entire GPS Central Engineering Activity (CEA) that was located at NADC went through a transitional period. NADC’s navigation functions and duties were combined with the NRaD laboratory facility located in San Diego, CA under the Base Realignment and Closure (BRAC) decision, and in 1996 NADC’s lab was reconstituted in San Diego where it still operates today supporting the NAVSTAR GPS Modernization Program and Platform Integration. On March 23, 2011, Mr. Ted Reed, who is the lead GPS UE Test engineer and current GPS CEA Laboratory Manager for the Space and Naval Warfare (SPAWAR) Systems Center Pacific (SSCPAC) in San Diego, donated to the NADC museum the first built GPS satellite generator (Serial No. 1) manufactured by the Stel Inc. under contract to the U.S. Navy. This equipment is now outdated and non-operational and has been replaced in the CEA Lab by the Advanced GPS Navigation Simulator or AGNS.


Submission authored by:
Ted Reed
SPAWAR Systems Center Pacific