DJI Mavic, Air and Mini Drones
Friendly, Helpful & Knowledgeable Community
Join Us Now

GPS status (rant)

AlanTheBeast

Well-Known Member
Joined
Jan 18, 2017
Messages
1,918
Reactions
864
Age
63
Location
Montreal
This rant applies to most appliances that display GPS "status" whether cell phones, car GPS, and so on ... and the MP RC and GO displays.

This rant is prompted by a comment by another poster trying to figure out why he had "No GPS" state despite 15 sats and "5 bars of signal (max)".

I'm not addressing his issue, at least not directly (there seems to be various comments about healthy appearing GPS but the MP is in ATTI mode...)

GPS does not have a cell phone like "signal" level on a receiver. It (internally) has signal level for each satellite. The displayed "5 bars" is a mashup to give the user some indication of what the GPS is providing. It is misleading in some cases as a "poor" 5 bar can display when there is more than adequate coverage - and v-v.

Generally if you have as few as 4 GPS satellites in a geometrically dispersed pattern (in 3 directions, low elevation about you and one reasonably high above you), then you'll get a very accurate fix. Today's constellation of GPS guarantees that (in open areas) you will likely have 6 - 10 satellites. In most of the continental US and southern Canada you'll also receive 2 or 3 SBAS sats. Same in Europe and Japan. These not only give GPS ranging functions but corrections for all the other sats and health status of them as well. And then GLONASS.

Our devices dumb all this down into location and a "status" bar. Indeed on the MP controller we're lucky to have the satellite count at all!

I really wish they would show the scatter plot of where the sats are relative to the receivers along with a colour grade for the signal from each satellite. That would help diagnose issue related to the system reverting to ATTI when it should stay locked in GPS. Or maybe not - the issue at present may be some s/w or f/w glitch that DJI may in its good time resolve... having that plot of individual GPS (and GLONASS) sats would go a long way to sorting that out.

Rant over.
 
It's a lot more complicated than that, and the DJI controller helpfully says "READY TO GO" a long time before you are really ready to go with proper GPS lock. I will partially repeat an earlier post of mine.

Generally, anything under the receiver won't affect GPS, but metal or leaves or rock or wood between the GPS and the sky will.

The screen displays for Litchi and DJI GO may be slightly different but the way GPS works internally is the same for both.

The satellite count is how many satellites the GPS/GLONASS receiver has seen however briefly in the sky. Once you've seen a satellite, you need to capture a set amount of data from that satellite to learn its detailed orbit information (they call this 'ephemeris'). Capturing the ephemeris takes about 30 seconds for each satellite found, but the receiver can process multiple satellites in parallel.

GPS needs to know the full ephemeris for 4+ satellites, so about 30 seconds after the fourth satellite is seen, as long as those four are still visible in the sky, you should be able to calculate your worldly coordinates. I know less about GLONASS but I think it also needs to know similar data. This is where I think we shift from 7 or less (YELLOW GPS MODE) to 8 or more (GREEN GPS MODE).

The problem is the software is showing satellites in the number on the screen before the receiver has full ephemeris. I have definitely seen 14 satellites quickly (maybe 9 GPS and 5 GLONASS, maybe 7 and 7) but still had a YELLOW GPS MODE. This is not ready for flight.

I always wait for 8 satellites plus at least 45 seconds. This gives a nice GREEN GPS MODE and you're likely to see 9 or 10 or 15 satellites in a few seconds. Only then will I take off.
 
It's a lot more complicated than that, and the DJI controller helpfully says "READY TO GO" a long time before you are really ready to go with proper GPS lock. I will partially repeat an earlier post of mine.

Generally, anything under the receiver won't affect GPS, but metal or leaves or rock or wood between the GPS and the sky will.

The screen displays for Litchi and DJI GO may be slightly different but the way GPS works internally is the same for both.

The satellite count is how many satellites the GPS/GLONASS receiver has seen however briefly in the sky. Once you've seen a satellite, you need to capture a set amount of data from that satellite to learn its detailed orbit information (they call this 'ephemeris'). Capturing the ephemeris takes about 30 seconds for each satellite found, but the receiver can process multiple satellites in parallel.

GPS needs to know the full ephemeris for 4+ satellites, so about 30 seconds after the fourth satellite is seen, as long as those four are still visible in the sky, you should be able to calculate your worldly coordinates. I know less about GLONASS but I think it also needs to know similar data. This is where I think we shift from 7 or less (YELLOW GPS MODE) to 8 or more (GREEN GPS MODE).

The problem is the software is showing satellites in the number on the screen before the receiver has full ephemeris. I have definitely seen 14 satellites quickly (maybe 9 GPS and 5 GLONASS, maybe 7 and 7) but still had a YELLOW GPS MODE. This is not ready for flight.

I always wait for 8 satellites plus at least 45 seconds. This gives a nice GREEN GPS MODE and you're likely to see 9 or 10 or 15 satellites in a few seconds. Only then will I take off.

Thanks for the brief. I worked intimately with GPS for many, many years and with people who designed, built and certified the first airline compliant receiver for production Boeing 777 back in the 90's.

To clarify, 4 sats is not necessarily enough if they are all in the same area in the sky (why what you say about blocking is so true - also don't stand near or over the drone at this time - you can block quite a bit).

I wait until the "position update" has completed - that means a pretty good fix. Also, as the little voice says: check your position on the app map to be sure.

As an improvement a really cool update would be for the Android phone to copy its GPS ephemeris data to the drone via the RC. (Can't do this with iOS as Apple obfuscate low level data such as ephemeris - Apple are turds this way (I am certifiably an Apple Fan Boy, but in some respects ...)).

What Really bothers me are the reports of in flight MP's suddenly going to ATTI from GPS. Unless deep in the city streets or jungle or canyons this should not happen. And it is happening to people out there in unobstructed space. Definitely something DJI need to look into. Very likely a communications glitch between the GPS module and the navigation "service" in the firmware.
 
Lycus Tech Mavic Air 3 Case

DJI Drone Deals

New Threads

Forum statistics

Threads
130,410
Messages
1,552,341
Members
159,418
Latest member
jamoke