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To evaluate the repeatability of LabSat, two tests were performed. The first was carried out by generating an ideal scenario from scratch using the SatGen software package which would contain theroetically perfect data. The second was a test drive along a typical road.
Artificial scenario test
An artifical scenario was created using SatGen which consisted of the following commands:
Each circle was within 1cm of each other in all of the tests, and in a straight line, the maximum deviation between all three tests was 1.1cm. This shows that once the errors associated with real world GPS signals have been removed, LabSat produces a very stable, highly repeatable signal, which can be used to test the calibration and tolerances of a GPS engine under test.
The GPS engines used in these tests were a 100Hz Javad Survery Grade GPS engine for the artificial scenario and a 20Hz Hemisphere Survery Grade GPS engine for the drive test.
Drive repeatability test
In this test there was a VBOX data-logger recording the output from a GPS engine and a LabSat recording the RF GPS data. They were both connected to the same antenna via an RF splitter. The test was then repeated on the bench using the LabSat to replay the test drive. This test was then repeated a total of three times.
A typical section of the position information from all four tests is shown on the right, the Live antenna test is shown in Red, and the three replay tests are shown in Blue, Green and Purple. The X&Y scale is in metres, and the portion of the test shown was driving around a small roundabout. Comparing the individual X&Y positions in all four tests, shows a deviation of less than 1.5m throughout the entire test. It most cases the difference in position was around 0.5m.
No WAAS or DGPS was switched on during this test. This is about the same kind of deviation you can expect if you ran three similar GPS engines from the same source antenna.
The velocity information from all four tests is shown to the right, the Live antenna test is in Red and the three replays are in Blue, Green and Purple. As you can see, it is very difficult to make out individual traces as they are so similar, in fact the average velocities of all four runs over the period shown are all within ±0.02 km/h.
If the velocity is integrated to get the total distance travelled, then over the entire length of test, which was 2.96km, all four tests are within 88cm of each other which equates to 0.03%