Random Loss of GPS Sync

I currently have a site that has 4 90 degree sectors that are using ABAB frequency reuse that are NOT currently live with customers. Periodically the radios lose GPS sync at random times. If the North radio loses sync, the South doesn't necessary lose sync at the same time. Same with the East/West radios. Additionally, I have a Force 110 PTP w/ sync at this site as well that is on another channel. I had to turn off  GPS on that radio because it was dropping the customer for up to several hours while sync was restored. I've replaced the GPS pucks, and tried installing them both in the hidden compartment on the Cambium sectors, or on top of the mast. The building is 22 floors and we are on the roof with nothing blocking the sky. I don't think something is interfering with the GPS signal since they all don't drop at once. Below is a syslog of the events occuring. All radios are running 2.6.1 and have had similar issues with any other firmware version.

Sep  1 00:00:17 pmp1w-balt kernel: GPS Sync Lost. (05:00:09:380365)
Sep  1 00:00:17 pmp1w-balt kernel: GPS Sync Restored. (05:00:15:21366)
Sep  1 00:00:20 pmp1w-balt snmpd[1835]: DFS status: N/A
Feb 29 17:24:48 pmp1w-balt kernel: GPS Sync Lost. (22:24:48:464420)
Feb 29 17:26:25 pmp1w-balt kernel: GPS Sync Restored. (22:26:25:969066)
Feb 29 17:52:44 pmp1w-balt kernel: GPS Sync Lost. (22:52:44:464428)
Feb 29 17:53:42 pmp1w-balt kernel: GPS Sync Restored. (22:53:42:969191)
Mar  2 14:13:26 pmp1w-balt kernel: GPS Sync Lost. (19:13:26:504421)
Mar  2 15:55:32 pmp1w-balt kernel: GPS Sync Restored. (20:55:32:11231)
Mar  2 16:15:06 pmp1w-balt kernel: GPS Sync Lost. (21:15:06:504420)
Mar  2 16:17:02 pmp1w-balt kernel: GPS Sync Restored. (21:17:02:12204)

 Additionally, when there is good GPS sync, the radios display the following. When there isn't good GPS sync, it only shows that it has 1-3 visable satellites.

On-board GPS Height 74.7 meters
GPS Time (Greenwich Mean Time) 16:02:53
GPS Firmware Version AXN_1.51_2838
Satellites Tracked 6
Satellites Visible 13

Any insight would be greatly appreciated. Thanks!

We've had issues with GPS signals on sites where we co-locate with mobile phone operators.

GPS signal would be fine and just randomly drop off or just not be able to hold a steady signal at all.

Are you co-locating with any mobile operators?

1 Like

This is in a downtown urban environment, and there are cell carriers on builings everywhere. There are no cell providers on our building, but the building next to ours does have a cell carrier or two on it, as well as a few buildings within a few block radius. Have you found a way to combat the issue yet? I'm assuming the answer will be a CMM/Sync injector like PacketFlux, but I'd really like the GPS sync to work as intended. 

We only really have this issue on a Water Tower so as you can imagine there's not much distance between our equipment and the mobile radios.  If the nearest provider to you is the next building it may be something else causing your problems.

We're hoping to deploy a packetflux soon.

Would have liked to try a uGPS receiver and get it up as high as possible but they won't work with ePMP.

a few things for this issue,

set your GPS hold off time to 1200 seconds, this will keep the pulse alive for quite awhile. the hold off time keeps the radio transmitting without GPS pulse and keeps the timing together. you don't want to let them go long without the pulse because they will eventually get out of snyc.

2nd, how are you placing your GPS antennas? if you've got them flat, you need to stand them up so the symbol is upright facing the sky, you;ll see a huge jump in GPS sat RSSI...

Almost everyone i've talked to with this issue had the gps antennas flat and have fixed it by standing them up. the cell operators are dependent on the same GPS timing we are. Attached is a photo of how we place the antennas, and the RSSI for the snyc pulse. try and have 5 or 6 of the GPS sats above 32 SNR. the more the better of course.

the GPS screen shot is from a cell tower we are installed as position 3, with 2 more carriers above our gear getting those SNRs. 


Thanks for the tip Chris. This is interesting considering they're now currently installed in the hidden bay that Cambium has designed on their sector antennas and that is only to be done in a flat orientation. I believe that my PTP link is also installed flat right now. I'll have to make a trip to the site and change the orientation of the pucks to match yours and see if my SNR increases. As you can see, I don't have nearly the signal that you carry.

we figured this out by accident, one of the force 110s who we planned to not use GPS had its gps antenna attached and just tossed on the side so it was lost, and i just happened to see the SNR on the unit and had a regular sync AP on the same tower having miror GPS issues, moved the GPS ap to the same orentation and gained 10 db on many of the sats. 

1 Like