GPS near transmitters?

[MX]

I'm playing around with using a US GlobalSat EM-406 Sirf-III GPS near a Maxstream XStream 900 MHz data transceiver. What are people's experience with using a GPS module near a transmitter? Mine seems to be unable to lock if I start it up near the transmitter, but if I let it start and lock before turning on the transmitter, it seems to work ok. I'm still wondering if it is working sub-par though. How far away should it be before it no longer would swamp the GPS receiver? I also want to use a 2.4 GHz video downlink at some point.

MX

[hugo_vincent]

I'm playing around with using a US GlobalSat EM-406 Sirf-III GPS near a Maxstream XStream 900 MHz data transceiver. What are people's experience with using a GPS module near a transmitter? Mine seems to be unable to lock if I start it up near the transmitter, but if I let it start and lock before turning on the transmitter, it seems to work ok. I'm still wondering if it is working sub-par though. How far away should it be before it no longer would swamp the GPS receiver? I also want to use a 2.4 GHz video downlink at some point.

MX
I used a u-blox TIM-LA gps module next to an Aerocomm AC4490 1W data transceiver without any visible problems. They were mounted on opposite sides of a 4-layer PCB with lots of ground planes though... I think where you mount the GPS antenna matters more - at first we used a small helix one directly mounted to the GPS module, but then we switched to a external one about 20 cm away.

We did have a problem with interference between our microcontroller and the GPS though - our micro was clocked at 400 MHz (398 MHz actually) which has a harmonic almost exactly on the GPS L1 frequency. We tried a bunch of things like shielding the GPS receiver, filtering its power supply though a ferrite bead, and using the external antenna, all without success, so we had to under-clock out micro to 300 MHz (still plenty fast enough).

Hugo.

[MX]

[QUOTE=hugo_vincent]I used a u-blox TIM-LA gps module next to an Aerocomm AC4490 1W data transceiver without any visible problems. They were mounted on opposite sides of a 4-layer PCB with lots of ground planes though... I think where you mount the GPS antenna matters more - at first we used a small helix one directly mounted to the GPS module, but then we switched to a external one about 20 cm away...QUOTE]

Mine has an integral patch antenna. I'll be making a short ribbon so I can move the gps module a few inches away from the xstream module to see if that helps.

MX

[poynting]

I don't have any experience with the gps rx or the tx you're working with, but most GPS receivers, especially when doing a cold or warm start, generally take about 6dB more signal-to-noise ratio to download Ephemeris data and the Almanac than they do to track satellites once the data is downloaded. So, setting up your power system to turn on your gps (only) and allow it to lock up before you turn on close transmitters/other noise sources may help. Depending on the actual frequency of the interferer, moving the GPS antenna away may or may not help.

[MX]

...most GPS receivers, especially when doing a cold or warm start, generally take about 6dB more signal-to-noise ratio to download Ephemeris data and the Almanac than they do to track satellites once the data is downloaded...

Ah, that makes sense. I didn't realize there was any difference between startup and normal operation. I'll still move the GPS away from the 900Mhz transmitter some, since if it's affecting the startup, then it's probably reducing all of the performance.

Thanks,

MX

[Vindication]

We have an aerocomm modem and gps pratically on top of each other.. no problems.

[bbstriker]

Hi
Can anyone provide an indication of how to use an Aerocomm to transmitt GPS data?

I have two Aerocomm AC4868 transceivers and a Garmin GPS.

Thanks

Tony