Why the Interest in SVN 49?
SVN 49 was launched on 24 March 2009 as a standard replenishment satellite with L1 and L2 signals and a L5 demonstration payload. Observations of the L1 and L2 signals exhibited larger pseudorange errors than expected, larger than other satellites in the constellation. This additional pseudorange error grows with elevation angle with respect to the receiver antenna.

SVN 49 has been allocated PRN-01 and, to date, has its health status set to unhealthy. At some time in the future the status may be changed to healthy, and since there is no straightforward way to correct the anomalous L1 and L2 behavior, assessment of the impact of this anomaly on receiver equipment is desirable ahead of the status adjustment.

To this end, Spirent Federal has produced test scenarios to allow the generation of the anomalous L1/L2 signals with a healthy status.

Spirent Federal SVN 49 Test Scenarios Description
The test scenario should allow the user to assess the impact of SVN 49 being set healthy with no other modifications to its signal other than the health bit being changed. At present SVN 49 is assigned to PRN-01. A real date/time/almanac combination was selected as a test case. During the nearly 4 hours of the scenario run, PRN-01 crosses the sky from 90 degree elevation to the horizon. Using the full range of elevations will allow the user to assess the multipath anomaly characteristics at every elevation angle. These characteristics have been plotted against elevation angle and published in a number of places.

A real YUMA almanac file was selected to allow the simulator to position the simulated GPS satellites in the correct locations for the date/time selected for the test case. Two modifications were made to this YUMA almanac. First, and most important, the health of PRN-01 was set to healthy. Secondly, PRN-30 was moved.

PRN-30's Role
PRN-30 was selected as a "calibration satellite." For the duration of the scenario, PRN-30 would normally not be visible to a receiver in the test case location. We plucked PRN-30 from its orbit and put it in an identical orbit to PRN-01. The satellite clock characteristics from PRN-01 were also transplanted to PRN-30. This means that the measured code and carrier for PRN-30 should be identical to those for PRN-01 — except for the anomaly. This provides a mechanism by which we can log/plot the magnitude of the anomaly by differencing PRN-01 and PRN-30.

This is the difference between Scenario A and Scenario B. Scenario A has this "calibration satellite," PRN-30 present. This should be useful for logging/plotting truth data, but it may be extremely confusing for a receiver. Hence, Scenario B is likely to be the one most users will use for testing a receiver's performance in the presence of an anomalous PRN-01.

15 Minutes of Settling
During testing, it was discovered that the anomaly was most accurately reflected by the receiver if it had 15 minutes of settling time at the beginning of the scenario. This allows the receiver to acquire all the visible satellites, calculate a position solution and calibrate the receiver clock bias. Hence the duration of the scenario allows PRN-01 to cover 90 degree elevation to 0 degree plus a 15 minute period before the satellite reaches zenith for settling.

38 Nanosecond Delay
Observation of the real-world satellite leads us to a delay of 38 ns for the reflected signal relative to the direct signal for SVN 49. This is easily created in SimGEN using the fixed offset multipath capabilities.

The story is not quite as simple as this, however. The magnitude of the reflected signal is different at L1 and L2 and varies with elevation angle. This led to the creation of a static scenario to keep things simple and a .UCD file, a User Command file was included. This allows the introduction of MOD (Signal Modification) commands at specific times into the scenario. These modifications allow automated adjustments to the signal level of the reflected signal at specific times—these adjustments are different at each frequency. The modifications also allow the introduction of carrier phase modifications as required.

MOD commands are introduced when the satellite makes a one degree change in elevation angle. From 40.02 to 39.98 degrees, for example, triggers a new MOD command at a certain number of seconds into the scenario. That modification stays in effect until the elevation angle dips below 39.00 degrees before a new MOD command is issued. This granularity has not shown any negative impact during testing. If required, SimGEN would allow a nice smooth MOD file profile.

Results
Extensive simulation testing has resulted in receiver performance which accurately reflects the expected live-sky performance.

To Access SVN 49 Anomaly Scenario Files
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The paper "Methodology for Modeling the SVN 49 Anomaly for Static Scenarios" was presented at the Institute of Navigation — International Technical Meeting, 24-26 January 2011 in San Diego, California.

Click here to access the paper.

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