GPS sync failing

I noted that someone else has a similar GPS issue recently reported, but here is ours.

We have two ePMP APs in a test setup.  The aim of the tests is to establish stable system configuration prior to a 10-mast build that will be the pilot for a subsequent 500+ mast large-scale roll-out.

It is indoor but we have very strong GPS through windows above (11 satellites between 10-30dB SNR, and other equipment has no problem maintaining consistent GPS).  APs have s/w version 2.3 in both banks.  GPS was also flashed (once) on the APs. CPE also at 2.3.

AP GPS antenna is the stock device fitted according to instructions in the top of the standard Cambium 90 degree antenna.

Generally both APs have come up and established GPS sync without problem.

However, on at least one occasion (6 days ago), GPS sync went down on both APs and stayed down for several hours.  A soft reboot led to reestablishment of GPS on both APs within 5mins, suggesting this was a software issue.

It happened again about 1 hour ago.  After soft reboot then hard reboot, one AP has established sync but the other has not even after 20mins.

GPS sync is core functionality for our network, and if we can't get it working we are seriously considering two other vendors.  Any help?

Hello Anthony,

The description of your problem is similar to what happens during a GPS chip lockup event. GPS firmware AXN_1.51_2838 should take care of this.

Could you please verify the firmware version of the GPS chip from the Monitor->GPS page (see attachment)?



Hello Luis,

I'm a colleague of Antony, and replying on his behalf.  Our units are both already running AXN_1.51_2838.  Is there anything else we can check to help your diagnosis?



Hello Joe,

If you can arrange remote access to the APs when you run into this issue again, that would help. I will send you guys  a PM.


Hi.  I'm just currious how this worked out, and what the outcome was?

Hi Luis,

the network has been continuing to fail in the way we described before - it's been working very little of the time over the last two weeks.

I've now arranged remove access and will PM you the details.


Hello Anthony,

Got your email. Please let me know if Cambium Support cannot help.



Hi Luis,

Just to let you know that I've sent details of the system via PM.  I couldn't get a dmesg output, perhaps you can provide details of how to do that.

The GPS on the two APs are continuing to go up and down.

To check for possible interference I've just run a spectrum analyser with a GPS antenna across the entire GPS band - there is nothing, even at fine resolution bandwidth.

Looking forward to your response.


So here is a solution with thanks to Luis.

It seems that the GPS antennas shipped to us (in July 2014) with the ePMP GPS Sync radios may have poor performance.  Using a GPS antenna from a more recent ePMP shipment has fixed one of our units, and a Trimble GPS antenna P/N 66800-52 has fixed the other.  So as I've seen noted elsewhere, a replacement GPS antenna may help fix this issue.

We are working indoors, and moving the antennas a foot or so has given a better view of the sky.  I don't think this was the root cause of the issue, but it means that the devices comfortably get about 8-10 satellites tracked, where before we were on typically 5-6 even when working.

Setting GPS hold-off time will also help - we might try ~5min as that's a typical time to regain GPS if there has been some failure, rather than the default 30s.

We will now monitor our devices long term and will post if more issues are found.

Kind regards,


I know this is an old post, but I've got an 2.4GHz AP with the same symptoms. This one is on a tower.  It shows something like 10 satellites,but will only track 6.  GPS goes up and down and stops transmitting.  I have 2.5.2 and the latest GPS firmware installed.

Any help is appreciated!



The AP needs to be tracking at least 4  satellites to maintain sync. In most cases, the device will be "seeing" more satellites than it can "track". This is normal and the device will continue to operate as long as it can still track at least 4 satellites. Couple suggestions:

- Make sure the GPS antenna is properly placed (magnetic side is facing down) and have an unobstructed view of the sky.

- Try a different GPS antenna.

- To minimize service interruptions, adjust the "Synchronization Holdoff Timer" to a larger value (300 secs for example) to allow the device to continue operating during short GPS outages.