Calibration of Garmin GPS48 signal strength bars.

Recently, while comparing some GPS antennas at work we decided it would be helpful to check the calibration of the signal strength bars on the GPS48 so we could quickly swap the antennas and then take pictures of the screen showing the signal loss relative to the Garmin quad helix type antenna.

The reference signal source is a Spirent STR4760 GPS simulator. The GPS unit was connected directly to the front panel via a stack of connector adaptors

Picture of typical bars with a quad helix antenna, sky obscured by gantries and buildings.

Bar Number (counting from the bottom)	Signal strength for bar aligned with line
5	-122 dBm
4	-127 dBm
3	-132 dBm
2	-135 dBm : starts going a bit noisy
1	-139 dBm : signals bouncing on and off

It looks like the bars are basically calibrated as one pixel = 1 dB of relative signal strength, with a line every 5dB. At low signal strengths the integration time for signal strength estimation is not long enough to give a stable signal strength.

At -140dBm the GPS48 will no longer indicate a position 100% of the time but will continue to occasionally output position fixes.

This performance figure does not represent a particularly sensitive reciever. However the signal processing and filtering downstream of the satellite tracking software is very good at getting position fixes when simple calculations would not have yielded a position fix at all.

NB How to get Power Drain on Backup Battery

Note If a GPS 48 is accidentally connected to a current limited external power supply which does not blow the supply fuse when connected the wrong way around , it will drain both the replaceable batteries and the non-replaceable backup battery.

mike@hamble.demon.co.uk