LabSat

In order to evaluate the sensitivity of the receiver, a test was carried out to monitor the signal strength of satellites which were close to the horizon. At low elevations, the signals from the satellites have to pass through a greater amount of atmosphere than a signal coming from a satellite directly overhead. This causes distortion and a decrease in signal strength.

A GPS engine was connected to the same antenna as a LabSat using an RF splitter, and data from both was recorded over a 50 minute period. During this period a number of satellites appeared above the horizon, and a number descended below the horizon.

The test was repeated, substituting the GPS antenna with the RF output of the LabSat. The RF GPS data recorded during this test was replayed, and the output from the GPS engine was monitored.

Click on image to enlarge

During the test, the average signal strength of the replayed signal was observed to be within 1dB-Hz of the original signal, even when the satellites were showing 0 degrees elevation (see snapshot on the right taken at identical receiver times).

Also, The time it took for the GPS engine connected to a Live antenna to drop the satellite going below the horizon was exactly the same for both the live and replayed tests.

This shows that there is no significant attenuation in signal strength in a replayed LabSat scenario.

Other points of interest in this test are the values of 'Position Dilution of Precision' or PDOP which show exactly the same value for both tests, and also of note is that both tests are tracking three WAAS satellites and showing 'Code differential' or DGPS.