ePMP – GPS Notice

Problem Description
ePMP Access Point loosing GPS synchronization or GPS satellites.

Products affected

GPS synchronization is one of the most effective ways to limit self-interference. GPS synchronization allows all APs to share an accurate timing reference so that all can transmit and receive at the same time. With GPS synchronization the network can scale to significantly larger sizes without any AP’s transmission interfering with the reception of the others and more efficient frequency reuse across sectors and towers can be employed. By deploying all sites in a network using the same framing parameters (frame size, duty cycle, max range), the downlink and uplink times of all sectors never overlap.
It was reported that in several installations Access Point may lose the GPS sync.

GPS receiver modules
ePMP Access Points are using three different generations of the GPS receiver modules. Please find the detailed information about them in the following table.

It is crucial to use the latest GPS firmware on the 1st and 2nd generations of the GPS modules. Please refer to the following knowledge base article on how to upgrade the GPS firmware:

GPS Pucks that were shipped with ePMP radios

GPS Signal Quality Evaluation

A GPS receiver processes the GPS signals received on its antenna to determine position, velocity, and timing. The signal at the antenna is amplified and filtered (to remove frequencies outside the intended frequency range for the digital signal that would alias into it) and digitalized.
For each satellite used by the receiver, the receiver must first acquire the signal and then track it as long as that satellite is in use.

ePMP web user interface(MonitorGPS page) provides the following GPS related information:

  • Number of Tracked Satellites
  • Number of Visible Satellites
  • Satellite ID (aka PRN)/tracking SNR for each Satellite

The SNR of visible, but not tracked satellites is 0 which means it cannot be used to calculate GPS coordinates and precise time.

For reliable GPS synchronization, it is required to have at least 3 satellites with sufficient SNR level for the satellite acquisition.

SNR above 33 dB can be considered as “Good”, SNR in the interval 27-33 dBm can be considered as “Low” and SNR below 27 dB can be considered as “Very Low”.

To have good signal levels it is highly recommended to use an external GPS antenna puck in case the system is working in the mode requiring GPS synchronization.
Cambium Networks sector antennas have a special place to install the GPS antenna puck. Please also refer to the User Guide on the antenna installation guidelines so the GPS antenna puck can have an open sky view and not be obstructed by the other equipment or metal constructions.

External GPS antenna vs Internal built-in GPS antenna
GPS receiver modules used in ePMP can work using an external GPS antenna puck or using a built-in GPS antenna. The system is automatically switching to an External antenna in case the GPS antenna puck is connected.
In the 4.6.1 Firmware Release, it is possible to check if an external or internal antenna is connected. Please refer to the Monitor->GPS page. This feature is available only for the systems using a 3rd generation GPS receiver.


>>My ePMP AP is still losing GPS sync. What shall I do?
Please use the following steps for troubleshooting

  1. Upgrade the ePMP firmware to the latest recommended version.
  2. Upgrade GPS receiver firmware to the latest version.
  3. Check if an AP can track a minimum required number of satellites with good SNR.
  4. Check if the system is using an internal GPS antenna and install an external GPS antenna puck.
  5. If all the listed recommendations did not solve the issue for you, please open a ticket with the Cambium Support Team (Contact Support - Technical, Sales, Orders, Inquires | Cambium Networks).
    Please try to capture and attach the Tech Support File collected not later than 2 hours after the GPS sync loss event has been detected.

>> I bought ePMP3000L and there was no GPS puck in the box, but I do have low GPS signals. What should I do?
Please open a ticket with the Cambium Support Team (Contact Support - Technical, Sales, Orders, Inquires | Cambium Networks) and we would be happy to send a GPS puck to you.

>> Cambium Support Team suggested removing external GPS puck. Why should I put an external GPS puck now?
In some cases where ePMP equipment is collocated at the same tower with LTE equipment, it was observed LTE has a negative impact on the performance of the GPS receiver. It was confirmed in such cases internal GPS antenna provides better filtering and performs better than an external GPS antenna. It applies only to ePMP1000/ePMP2000 with 1st and 2nd generation of GPS receivers. It is not recommended to disconnect the GPS antenna puck on ePMP3000 or ePMP3000L.


For a stable GPS signal, 6 dB SNR is enough to hear 3 or 4 satellites
So, all GPS devices in the world work, only in your Cambium the threshold is 33 dB SNR,
You predicted something.
And that’s the reason we’ve been arguing for years.
Best Regards

Good information. I went through and checked out epmp 1000’s and we look good (haven’t had any GPS issues at all) however I noticed on 2 of our 2.4 AP’s, the GPS firmware shows AXM_5.1_8513, which would indicate 3rd gen, which according to the above chart should not exist on ePMP 1000’s. Is that an error?

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Thank you for the feedback.
You are right, ePMP1000s were equipped with 3rd gen GPS chip as well.
I’m editing the original post.

Thank you.

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Mar 16 2021 Sakid Ahmed Cambium

In a couple of days, I will post images and specs of all the pucks that are out there.

Has this been done and I’m just not seeing it ?

Hi Zica49,

For a stable GPS signal, 6 dB SNR is enough to hear 3 or 4 satellites
So, all GPS devices in the world work, only in your Cambium the threshold is 33 dB SNR,

GPS specifications gives a receive signal on the ground station around -130dBm/Hz(-160dBW). Thermal noise of 2MHz bandwidth is -111dBm at the room temperature. The signal is already below the thermal noise, but thanks to spread spectrum modulation techniques we can get about 43dB( calculated as 10*log(10^6/50)) of “processing gain”(Process gain - Wikipedia) to have a “tracking threshold” SNR around 24dB(-130+111+43).

Saying that unfortunately SNR of 6dB is impossibly low figure for any existing equipment.


PS the calculation includes only thermal noise for the ideal non-existent receiver system and doesn’t include any sources of interferences or real life RF component characteristics.


Why was my post removed?
Did I offend anyone?

Zica49 - I believe your post was “flagged by the community”, which means by community members who are tired of reading the same unhelpful rant. Most of the community members are interested in helping Cambium solve the GPS problems by submitting their Tech Support Files, so that Cambium can try to find and squash all the bugs.

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The original article has been updated with GPS pucks information.

Thank you.

You appear to be missing the most recent “new and improved” pucks and the model numbers so we can order them.