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GPS quality drop

Flags a stretch where the receiver's horizontal accuracy or satellite count crosses the threshold for two seconds. It observes a weaker fix, not a wrong position and not the cause of the drop.

Rule referenceMarkdown
On this page8
gps_quality_dropv1.0On by default
Detects
Horizontal accuracy or satellite count crosses the vehicle threshold.
Threshold
eph above 2.5 m or fewer than 8 satellites for 2 s
Reads
Flight log
Report wording
Observed: GPS quality degraded

What it reads#

The rule reads two fields of sensor_gps in the flight log, and both come from the GNSS receiver:

SensorGps.msgPX4 v1.16.0, the two fields this rule readsText
float32 eph                     # GPS horizontal position accuracy (metres)
uint8 satellites_used           # Number of satellites used

eph is the receiver's own estimate of its horizontal accuracy, in meters. It is not an error measured against a known position.

In INC-0141 the timeline draws both as lanes. Satellites plots sensor_gps.satellites_used against the line Below 8, and Horizontal accuracy plots sensor_gps.eph against Above 2.5 m.

When it fires#

A finding is produced when, for 2 s, eph stays above 2.5 m or fewer than 8 satellites are in use. The event sits on the first sample that crosses, and the finding is confirmed 2 s later. In the INC-0141 flight log a sensor_gps sample arrives every 200 ms, so 2 s spans 11 samples.

ValueExample from INC-0141
First sample below11:06:23.802, eph 2.61 m, 7 satellites
Confirmed at11:06:25.802, eph 3.30 m, 7 satellites
Thresholdeph above 2.5 m or fewer than 8 satellites for 2 s

The sample before, at 11:06:23.602, read 2.48 m with 11 satellites, inside both limits. The finding's title is GPS quality below threshold: eph 2.61 m, 7 satellites.

Wording and evidence level#

Every finding reads Observed: GPS quality degraded. The flight log shows the receiver's figures directly, so the level is observed.

What the finding does not mean#

  • It does not say the position was wrong. The rule reads the receiver's own accuracy estimate and satellite count. It cannot check a position against the ground truth.
  • It does not give the cause. A weaker fix has several possible causes, and the rule reads none of them. In INC-0141 the receiver reported jamming 0.8 s after this finding's first sample, which gnss_jamming records as a finding of its own.
  • It is not PX4's position failsafe. PX4 judges its estimator's position error, not the receiver's eph, as the section below explains.

Threshold and overrides#

The two limits and the 2 s duration share one threshold text: eph above 2.5 m or fewer than 8 satellites for 2 s. The scope is All vehicles. A change under Rules with Edit saves as gps_quality_drop 1.1, and findings already produced keep version 1.0 and its threshold.

For reference, the PX4 Flight Review guide looks for satellites used "around 12 or higher" and a horizontal accuracy "below 1 meter". The rule's 8 satellites and 2.5 m mark a clear drop, not a fix short of ideal.

The closest PX4 parameter is COM_POS_FS_EPH, the horizontal position error threshold for hovering systems, 5 m by default. PX4 compares it with its estimator's horizontal error in vehicle_global_position and vehicle_local_position, not with sensor_gps.eph. Once a position is valid, PX4 declares it invalid only above 2.5 times the value: 12.5 m at the default.

The example Q4 vehicles run COM_POS_FS_EPH at 5 m, and the vehicle page lists it under Failsafe parameters. This rule reads a different number and sits well below that line.

Example finding#

In INC-0141 UAS-10 surveyed the north fence, and this rule produced the incident's first finding. Satellites in use fell from 18 to 11 at 11:06:22.402, and to 7 at 11:06:23.802, when eph read 2.61 m. The finding was confirmed at 11:06:25.802, with eph at 3.30 m.

The fix kept weakening: 6 satellites from 11:06:30.002, and eph at 4.8 m from 11:06:33.002 to 11:06:58.002. It recovered at 11:07:05.202, with 15 satellites and 0.9 m. The Findings row labels the finding GPS quality drop.