GPS Failsafe


GPS Failsafe

A GPS failsafe event will occur when a GPS 3D fix is lost. This event will be reported to the log file and to the Ground Control Station.

At the same time, a recovery action decision will occur on the flight controller based on current flight mode.

If the current mode is requiring a 3D GPS fix (RTL, AUTO, LOITER, CIRCLE, POSITION, GUIDED) then auto Landing will occur after a 5 seconds delay.

When a 3D Fix is restored, this is reported to the log file.

Without GPS 3D fix, the inertial sensors do allow approximately 10 seconds of fly with a reduced drift. After this delay, horizontal drift will probably rise more and more.

In this regard if you still have RC radio control it is recommended to take back control using a manual mode (stabilize or acro) as soon as possible.

Technical background and safety recommendations :

The GPS system is not designed to be used as a primary navigation system. So you should be aware that GPS signal dropouts or integrity problems events can occur from time to time, even if predicted satellite elevations, dilution of precision, and visible satellite counts are favorable during all the duration of your automated fly.

It is particularly important to avoid auto mission flying inside canyons, between buildings, mountains or inside backyards because of heavy multipathing producing large GPS errors. More fix probability will drop dramatically in those conditions from about 99% to 38 % and position errors can rise to hundred meters or more.

Only a clear sky view will give good results.

For better results and safety, it is recommanded to check for GPS signal quality and GPS advisories for your region before to fly using GPS prediction and information sites :

Link to the NavComTech Satellite Position Predictor

Link o the US Coast Guard Navigation Site

SBAS (Satellite-based augmentation system) :

If one or more SBAS satellites can be received in your location and you are in a covered region (WAAS, EGNOS, MSAS…) then your GPS receiver will get more precision, typically around 1 meter in good conditions, with a guaranteed precision of 7.6 meters. Another function of SBAS is integrity report : your GPS receiver should be warned about a GPS system integrity defect within 6.2 seconds. For this to work the SBAS integrity detection needs to be enabled in your GPS receiver.

Getting SBAS acquisition completed (DGPS fix) needs a few minutes after getting a 3D fix. So if you want to get the benefit of it (better precision and integrity detection), you should wait between 3 to 10 minutes before to fly. GPS Hotstart function (get a fast fix in a couple seconds) does not work for SBAS corrections. So you’ll need to wait minutes after each GPS power off to get a 3D + DGPS fix where a 3D fix only will typically ask only for a couple seconds.

SBAS geostationary satellites are often low on the horizon, so they are not always visible before take off. This can explain why RTL precision can be affected by a SBAS correction (DGPS fix) applied only a few minutes after take off.

In locations where SBAS satellites are not available or very distant, it is better to fully disable SBAS corrections. This will save GPS CPU resources, save a couple correlators, and avoid eventual position jumps when trying to receive weak or conflicting SBAS satellites.

SBAS will not help for :

– very localized atmospheric perturbations causing localized signal delays

– multipathing causing signal delays by reflexion against mountains or buildings.

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