From a past thread I found the Mavic Pro uses some variant of the ublox 7P or M8P receivers. These are purported to perform high accuracy missions in a wide swath of applications. A recent thread comment from @JLane trying to block signal to the GPS in order to do some ATTI flying got me wondering about the sensitivity of the ublox modules. JLane could not manage to block the signal easily with aluminum sheets.
From that I'm guessing that signal leaking in through the side or bouncing off of other components is getting around JLane's masking.
Indeed the ublox GPS spec comes in at a very sensitive -161 dB tracking level.
Airborne GPS (the certified ones in airliners) are nowhere near that sensitive (to my recollection). I think they bottom out at about -145 dB - I may be wrong - but I doubt it's less than -150 dB. That is so deep in the noise that it takes beautifully designed correlators toeek eke out the signal. At -161 dB the noise is incredibly high - but that sensitivity will allow the correlators to (noisily) eek eke out the signal. The correlation will be very jittery however.
Aviation receivers have the benefit of a very benign signal operating environment. First off the antenna is on top of the fuselage with nothing but the vertical stabilizer in the way. Signal easily gets around that so it's no issue and most (99.9999%) of the rest of the sky has nothing in the way. Even if the airplane rolls to a high bank angle, "skin effect" will carry the signal over the skin to the antenna fairly well. So these receivers don't need to be too sensitive. Even on the ground at the airport gate, most of the sky is clear and by the time they taxi to takeoff, all available sats are easily tracked.
Further, it's not desirable to be too sensitive for three more reasons: 1) multipath and 2) correlator saturation. If the front end is too sensitive then multipath signals may get through and be tracked resulting in error (or just barf up the legitimate signal) and if the correlator sees too much energy then the correlation function, which should be a sharp pointed shape, becomes topped out. The amount of "top" on the function is a direct contributor to error (the pseudo-range to the satellite will have a large error). If all the satellites have saturated correlators, then the contribution to error increases.
The 3) reason is simple interference rejection. GPS is pretty vulnerable because of its required sensitivity - making the receiver even more sensitive just increases its vulnerabilty to interference. Not a good thing for aircraft.
If these errors from sensitivity result in enough error it's even plausible that the navigation solution fails - some GPS algorithms do not "publish" if the solution has too high an error probability. I don't know if the ubloc GPS receivers do such.
Since the MP flies in a very benign GPS reception environment I wonder if the above could be a reason for the dropouts from GPS to ATTI.
Sorry for the long post. I don't have a drone to fly. Damnit.
From that I'm guessing that signal leaking in through the side or bouncing off of other components is getting around JLane's masking.
Indeed the ublox GPS spec comes in at a very sensitive -161 dB tracking level.
Airborne GPS (the certified ones in airliners) are nowhere near that sensitive (to my recollection). I think they bottom out at about -145 dB - I may be wrong - but I doubt it's less than -150 dB. That is so deep in the noise that it takes beautifully designed correlators to
Aviation receivers have the benefit of a very benign signal operating environment. First off the antenna is on top of the fuselage with nothing but the vertical stabilizer in the way. Signal easily gets around that so it's no issue and most (99.9999%) of the rest of the sky has nothing in the way. Even if the airplane rolls to a high bank angle, "skin effect" will carry the signal over the skin to the antenna fairly well. So these receivers don't need to be too sensitive. Even on the ground at the airport gate, most of the sky is clear and by the time they taxi to takeoff, all available sats are easily tracked.
Further, it's not desirable to be too sensitive for three more reasons: 1) multipath and 2) correlator saturation. If the front end is too sensitive then multipath signals may get through and be tracked resulting in error (or just barf up the legitimate signal) and if the correlator sees too much energy then the correlation function, which should be a sharp pointed shape, becomes topped out. The amount of "top" on the function is a direct contributor to error (the pseudo-range to the satellite will have a large error). If all the satellites have saturated correlators, then the contribution to error increases.
The 3) reason is simple interference rejection. GPS is pretty vulnerable because of its required sensitivity - making the receiver even more sensitive just increases its vulnerabilty to interference. Not a good thing for aircraft.
If these errors from sensitivity result in enough error it's even plausible that the navigation solution fails - some GPS algorithms do not "publish" if the solution has too high an error probability. I don't know if the ubloc GPS receivers do such.
Since the MP flies in a very benign GPS reception environment I wonder if the above could be a reason for the dropouts from GPS to ATTI.
Sorry for the long post. I don't have a drone to fly. Damnit.
Last edited:
