GPS receiver condition and noise of the GPS signal

Hi all,

evaluating GPS performance is more than looking at the P/H/DOP values and the number of satellites.

the UBX messages in the log file contains information such as:

  • jamInd
  • noiseperms
  • agcCnt

I would like to start a discussion about how to interpret these values - to be sure the GPS is healthy.

From the "u-blox M8 Receiver Description" we learn that:

  • the jamInd is the CW Jamming indicator and that it is scaled from 0 to 255 where 0= no CW jamming, 255 = strong CW jamming),
  • the noiseperms is the noise Level as measured by the GPS Core,
  • and that the agcCnt is the Automatic Gain Control (AGC) Monitor (counts SIGHI xor SIGLO) and ranges from 0 to 8191.
But: what is the healthy range and when do we have to think about changing our setups to reduce interference?
Looking forward to your logs, replies, suggestions, experiences, knowledge...
Thanks in advance and best regards,
Thorsten

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Hi Thorsten,

On the general principles , with devices dealing with radio waves, the best situation is the one that maximizes the ratio of the useful signal divided by the noise signals. That is thus a relative measurement.

-jamInd .I do not think absolute values of jamming (CW) would mean a lot without realtively comparing it to the measured useful signal (a high jamming is probably ok if you also have a strong useful signal)

-Same logic I would guess with "noiseperms" Ias this measures a noise level.

-agcCnt : a high value means a high gain, meaning the useful signal is measured low. Thus a low gain means a strong enough GPS signal is received, not requiring amplification. I think the lower the better in all cases. Because an amplifier will not only boost your signal but also the noise. (and we come back on the ratio between signal and noise that must be as high as possible)

I'd also like to know what is normal. 

I have a ublox 8.
I usually quickly receive 6 or 8 satellites, even inside.  In my back yard I get 10-12.  In the air, 15-19.
HDOP is around 2 in my back yard and 1.2 in the air.

When I have the copter sitting in my back yard (on the ground) near trees and the house, it drifts around in mission planner a lot, 10 or 20 meters, ~.5 meters per second.
In the air above the trees and buildings, it holds its position very well.

jamInd is between 16 and 18 (It was less w/ FPV gear off)

noisePerMS bounces between 85 and 95

agcCnt is ~1300 on the ground, and ~900 in the air.

Hi Hugues,

What I have in mind is finding a healthy range of these values - or better the upper limit - similar to vibrations.

So I think first we need to collect some data from different setups.

Maybe the EKF GPS innovation (IVN,IVE,IVD,IPN,IPE) can be used to determine the effect of the strength of the interference seen in the UBX logs.

Hi Scott.

thanks for sharing!

Maybe it is a good idea to differentiate between environments where multipathing effects occur and where not. This will help differentiating the effect of the UAV setup and other external effects.

This is from a recent log flown with master (similar to 3.3rc7) and with no multipathing problems. 

I am flying with xbee telemetry S1 Pro (2.4GHz) and Immersion RC 5.8GHz 25mW Video transmitter. 

The GPS is a small DroTek M8N.

It seems the values could be better. 

  • noisePerMS = 110
  • jamInd = 30 (11.8%)
  • agcCnt = 2300

However, EKF innovations look pretty good.

Will be interesting to see how it compares to the larger versions with ground plane. 

Here is my graph, showing the same info as yours.  I don't know how to interpret the EKF data.  Other than take off and landing, I'm hovering in a single spot w/ clear sky.  ~19 satellites.

I have a large ground plane on mine.  I don't have logs from before I added it, but I do know that after adding it, I always received about 2 more satellites than w/out it.   The log results I posted above are with the ground plane.  I don't intend to leave it this large, but this is my setup w/ the results I posted above.

The graph is with everything turned on..  GoPro, FPV security cam and 600mw 5.8ghz transmitter.  FrSky x8r.  915mhz telemetry.

There is some information in the wiki about all the EKF parameters and their ranges:

EKF3
IVN,IVE,IVD: Innovations for the North,East,Down GPS velocity measurements (m/s). These are an important measure of health for the navigation filter. If you have god quality IMU and GPS data they will be small and around zero.

IPN,IPE: Innovations in the North, East GPS position measurements (m). Similarly to the velocity innovations, they should be small and centred on zero.


EKF4 (these might be interesting as well)
SV: ratio of the combined GPS velocity inconsistency to the limit set by the EKF_VEL_GATE parameter.

SP: ratio of the GPS total position inconsistency to the limit set by the EKF_POS_GATE parameter.

So from the ranges your values seem ok but IVN and IVE are not centered around zero. But I cannot tell you if this is problematic. 

At least when comparing our values it is somewhat different to what I expected: Your GPS noise/interference level is a lower whereas my EKF innovations seem better. Hence, there is maybe no obvious relationship.

After looking at a few things in the logs, I think I need to redo my compass calibration.  I think that accounts for my IPN and IVN being below zero. 

Some notes from Craig Elder:

We need to have the agcCnt value to properly interpret the NoisePerMS and jamInd values.
For example if you are evaluating the placement of a GPS antenna on a vehicle and you move it closer to a noise source the increase in noise can cause the automatic gain control to back off and then the jamming indicator and the NoisePeMS will decrease because of the reduction in the receiver gain.  You have to track the AGC value to evaluate the other figures.
Also the real and imaginary impedance values listed in UBX2 ofsl, magl, ofsQ, and magQ values are very useful for tuning an antenna and ground plane to correct antenna input impedance and the best signal to noise ratio.

The signal to noise ratio and jamming indicator could help you setting up your drone to make sure there isn't interference from something internally. Take a baseline measurement with the GPS antenna in open sky and away from everything, then place it where it will be mounted on the drone and compare the values. 

Beyond doing this I don't think that the jamming indicator and signal to noise ratios will be very useful. You'll (probably) never be in real GPS jamming scenario. A lower signal to noise ratio on your GPS measurements doesn't necessarily mean worse accuracy.

For knowing if the GPS is healthy the best indications are number of satellites, DOPs, and the estimated accuracy. When the number of satellites drop off and DOPs grow then view of the sky is becoming limited. Estimated accuracy would become worse from this, but it also would grow if there was multipath or partial sky blockage (like trees). 

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