After moving the antenna connected to a u-blox NEO-M9N GNSS board I started receiving two SBAS satellites but only GPS satellites were used–no other GNSS constellations were used. The satellite reception bars in u-center showed green (in use) for GPS but blue (ready but not used) for all other constellation satellites. This resulted in an SBAS PDOP of 2 to 3 versus 0.8 to 1.1 when not using SBAS and receiving all four GNSS constellations. In other words, multi-band GNSS is more accurate than GPS+SBAS. Is this normal?
To clarify my question, is it normal for multi-band GNSS receivers to only receive one type of GNSS (US based GPS in my case) when using SBAS? When I disable SBAS all four GNSS satellite types are received and the PDOP drops from 2 to 3 down to 0.9 to 1.1.
That’s a good question. It’s my understanding that the SBAS constellations are very regional. WAAS signals are available in US/North America, EGNOS in Europe, etc. So any one given GNSS receiver is probably only going to receive signals from one SBAS constellation.
Now, which GNSS constellations do the various SBAS constellations augment?
Does WAAS only augment GPS?
Does EGNOS only augment Galileo? Or perhaps it augments GPS and Galileo?
etc. I don’t know.
And if the uBlox NEO-M9N is configured to use SBAS, does it prefer an SBAS solution with fewer constellations over a four-constellation autonomous solution? Perhaps if the user asks for SBAS solutions it does.
And if the chip is only receiving WAAS SBAS, and WAAS only augments GPS, well then Robert is your mother’s brother.
The above is hypothesis on my part, you probably need to ask uBlox if no one else here knows.
The uBlox specs for the NEO-M9N Horizontal position accuracy are
1.5 m CEP (with SBAS)
2.0 m CEP (without SBAS)
This makes sense to me, as the SBAS solutions will have better corrections for more errors (iono, ephermis, clocks, etc.) But the difference isn’t that great. Note PDOP is not the only indicator of accuracy; it’s a (to my understanding) independent factor based on the satellite constellation geometry that doesn’t take into account many, indeed most even, other sources of errors.
The problem is that when I get an SBAS satellite I lose the other 3 constellations so I only have GPS with SBAS which reports worse PDOP and HDOP than when all 4 constellations are used. This seems like a bug in the receiver’s firmware.
Despite what the PDOP says, GPS+SBAS might be more accurate than four constellations without SBAS. uBlox’s spec sheet implies that it is. If this is true, the behavior you describe would be a feature, not a bug.
You’d have to do a fair bit of testing to refute uBlox’s spec sheet. You also could contact uBlox or get on the uBlox forums and ask there. Good chance there are folks on the uBlox forums who can definitively answer your questions.
You need to look at other numbers in addition to the PDOP to know how the receiver is performing at any given moment.
Thanks, toeknee.