How many locked satellites do I need?

[cameraz]

The other night I took my Mavic Mini downtown to try to get some photos of the Blue Moon, but when I opened my UAV Forecast app on my phone, it said "Not Good to Fly." The reason was that there were only 10.9 Sats Locked. Now I know I can go into the settings and change that parameter (it's currently set at 12 Sats Locked), but was I in danger of flying my drone with only 10.8 satellites locked? I didn't want to take a chance so I didn't fly.

 

[anotherlab]

The other night I took my Mavic Mini downtown to try to get some photos of the Blue Moon, but when I opened my UAV Forecast app on my phone, it said "Not Good to Fly." The reason was that there were only 10.9 Sats Locked. Now I know I can go into the settings and change that parameter (it's currently set at 12 Sats Locked), but was I in danger of flying my drone with only 10.8 satellites locked? I didn't want to take a chance so I didn't fly.

To get a GPS location, you need a minimum of 4 satellites. Three of them to determine the position, and the fourth for error checking.

I don't use that app, so I don't know how they count satellites. Or how they counted a partial one.

 

[SWTSU]

I always take with a minimum of #12, but I am in rural central Texas with no obstructions and most of time it’s #15+.

I think I read that #12 was a good rule of thumb here.

 

[hansenn]

I only fly with at least 10 satellites.

 

[cameraz]

I always take with a minimum of #12, but I am in rural central Texas with no obstructions and most of time it’s #15+.

I think I read that #12 was a good rule of thumb here.

Yes, I also heard that 12 was a good rule.

 

[MS Coast]

How many satellites do you need?

Enough for the drone to establish its position and change the color of the GPS satellite icon in the top right corner of the screen from red to orange to white.

Only then is the home point set. Focus on the icon color, not the satellite count.

 

[cameraz]

How many satellites do you need?

Enough for the drone to establish its position and change the color of the GPS satellite icon in the top right corner of the screen from red to orange to white.

Only then is the home point set. Focus on the icon color, not the satellite count.

Thank you. That makes sense!

 

[Meta4]

Despite the suggestions to the contrary, there is no magic number of sats.
Post #6 has the correct answer.
The actual number will vary.

when I opened my UAV Forecast app on my phone, it said "Not Good to Fly." The reason was that there were only 10.9 Sats Locked. Now I know I can go into the settings and change that parameter (it's currently set at 12 Sats Locked),

Your drone uses sats from two separate satellite constellations. (GPS + Glonass)
The default settings in UAV Forecast are only showing their estimate for one (GPS).
So your drone is always going to find many more sats that UAV forecast is suggesting.

but was I in danger of flying my drone with only 10.8 satellites locked?

Next time you fly, watch the number of sats and you'll see that the drone always picks up signals from may more sats than UAV Forecast is telling you.
Even if you change the settings to account for GPS + Glonass sats, you can just ignore UAV Forecast's GPS section anyway because it's not giving any useful information.
And as long as your drone has a clear, unobstructed view of more than half of the sky, you will always have more sats than you need.

 

[Dave Maine]

The additional thing to consider is that the accuracy of the location can vary with the actual geometry of the satellites when the measurement is made. There are times when a small number of satellites are so close to each other that the location is poorly triangulated. The drone will automatically change over to the satellites that give the best geometric fit if they are present, but more than 4 satellites will increase the chances of a good fix.

 

[rjwmorrell]

To get a GPS location, you need a minimum of 4 satellites. Three of them to determine the position, and the fourth for error checking.

I don't use that app, so I don't know how they count satellites. Or how they counted a partial one.

4 is fine if you want accuracy to about 100m. I think for flying a drone you may want better accuracy.

 

[Meta4]

The drone will automatically change over to the satellites that give the best geometric fit if they are present, but more than 4 satellites will increase the chances of a good fix.

4 is fine if you want accuracy to about 100m. I think for flying a drone you may want better accuracy.

This doesn't apply to your drone.
It doesn't "swap over" to satellites, but will use all the sats it is receiving.
And old DJI drones that only received sat signals from just one GPS constellation needed a minimum of 6 sats.
However all modern DJI drones receive signals from 2 or 3 different constellations, so the numbers they will receive in open areas are 20-30 and the minimum you'll have to record a home point is roughly 12-15.
But if you rely on the number of sats, you are missing the point that it's not the number of sats.
You need to wait for the drone to let you know that it has what it thinks is enough and it records a home point and the sat icon turns white - regardless of the number shown.

 

[OdysseyShane]

4 is fine if you want accuracy to about 100m. I think for flying a drone you may want better accuracy.

No, 4 is technically correct. You can get a horizontal position with 3 satellites, but need 4 to get vertical accuracy and you can get positions to the centimetre with 4 satellites. It's just rare these days to have so few. I wouldn't want to fly with 4, but as a surveyor, I am just clarifying the GPS theory.

 

[Meta4]

No, 4 is technically correct. You can get a horizontal position with 3 satellites, but need 4 to get vertical accuracy and you can get positions to the centimetre with 4 satellites. It's just rare these days to have so few. I wouldn't want to fly with 4, but as a surveyor, I am just clarifying the GPS theory.

DJI never let their old drones go to P-GPS mode with less than 6 sats locked.
Any talk about 4 sats relates to handheld units, not DJI drones.
And those numbers are a hangover from years ago and not relevant nowdays since DJI drones made since 2015 have used sats from at least two different constellations and are easily locking onto 20+ or 30+ sats.

And you are never going to get centimetre level accuracy with consumer GPS and just 4 sats.

 

[OdysseyShane]

Yeah, I went off topic a bit. I'm just talking GPS theory, not necessarily DJI GPS requirements, and the theoretical centimetre accuracy is achieved with professional GPS equipment and longer observations. But 4 is the absolute minimum for a 3D position.
Trust me, I'm thrilled that we have access to multiple satellites now!

 

[CrystKen]

Very useful and informative discussions. Thanks everyone.

 

[Yaros]

It doesn't really matter, what matters is the HDOP - Horizontal Dilution of Precision DJI doesn't show this for some reason, but you can kind of know it by looking at the GPS icon, more precisely the color of it. If it is red then don't take off, if it's orange then wait a bit more, if you can't lock more sats for it to become white, take off carefully and rise the drone, then wait for GPS up there. If it's white then you're good to go.

If you're interested to read about HDOP, here's a Wikipedia article: Dilution of precision (navigation) - Wikipedia