As you know by now, AutoQuad relies on GPS for accurate naviagtion and even in stabilized flight.
A great tool to determin health and accuracy of your Ublox GPS modules and antennas is the Ublox
u-center GNSS evaluation software for Windows. You can download it here. Use a USB/UART connection at a baud rate of 1200 in the tool´s “Receiver” menu.
Download and install uBLOX uCENTER GNNS software.
Connect to the USB COM port at 1200 baud (tells the M4 that you want to speak directly to the GPS.) Turn on the NAV_SAT messages and you will get a graphic representation of satellites received, strengths and positions. GPS type is ublox8 for the M4r6 V2 production boards and uBlox7 for the BETA boards.
During startup the ublox GPS will acquire satellite fixes and if enough it will report the 3D lock. But for very precise location (both altitude as lon/lat) another fix is needed, the SBAS fix.
If you need ultimate accuracy, you should let the machine sit on the ground for 5 or so minutes soaking up all the sat info it can get before flying. If your backup battery is working, subsequent flights should have most of the ephemeral info stored from previous flights. One exception to this is the SBAS ephemerals, which for some reason are not stored. This means that the SBAS shift will happen at some point every time. Also note that this shift is not limited to vertical position, it will happen to a lesser degree for the horizontal. This is why you sometimes come back home from a mission to find you are 1.5m away from where you started.
US and EU SBAS
The system used in EU is EGNOS and is currently broadcasted on 120 (AOR-E), 124 (Artemis) with 126 and 131 coming online at some point in the future.
The system most used in the US is broadcasting on 135 & 138
The UBLOX LEA recievers can track 3 SBAS sats and will choose the best available
u-blox receivers are capable of receiving multiple SBAS satellites in parallel, even from different SBAS systems (WAAS, EGNOS, MSAS, etc.). They can be tracked and used for navigation simultaneously. At least three SBAS
satellites can be tracked in parallel. Every SBAS satellite tracked utilizes one vacant GPS receiver tracking channel. Only the number of receiver channels limits the total number of satellites used. Each SBAS satellite,which broadcasts ephemeris or almanac information, can be used for navigation, just like a normal GPS satellite.
For receiving correction data, the u-blox GPS receiver automatically chooses the best SBAS satellite as its primary source. It will select only one since the information received from other SBAS GEOs is redundant and/or could be inconsistent. The selection strategy is determined by the proximity of the GEOs, the services offered by
the GEO, the configuration of the receiver (Testmode allowed/disallowed, Integrity enabled/disabled) and the signal link quality to the GEO.
References for this highly technical subject:
GPS Predictor Tool
Note: beware of solar flares!
It has been reported numerous times that solar flares (sun storms) might affect your GPS reception and the accuracy of your automomous features that heavily depend on precise GPS data.
Here’s a graph displaying the actual solar flares. Be careful when the days are marked as yellow or red!