minimum amount of locked satellites to fly?

sar104 · Jan 16, 2018

BlakPhoenix said:
Barometers are not precise enough to give altitude tracking down to the precision the Mavic is able to obtain on their own. The Mavic uses all of its sensors in combination to get super precise measurements of it's position, including the barometer, GPS, accelerometers, thermometers and (when close to the ground) VPS.

When you take out part of that puzzle (GPS) you greatly reduce the accuracy of all positioning data, including height. Depending on the relative location of the satellites available, this will determine which dimensions are lacking precision.

The IMU estimates of position and velocity don't, primarily, use GPS or barometric data. In P-GPS mode, the IMU initializes position with GPS data and heading with compass data. It records both home point GPS altitude and barometric pressure (and, by extension, barometric altitude assuming a standard atmosphere) but it doesn't need those directly to estimate flight altitude.

Once moving, the primary update to location, velocity and attitude comes from the 200 Hz inertial data from the 3-axis accelerometers and rate gyros via quaternion algorithms, with the barometer (200 Hz), compass (50 Hz) and GPS (5 Hz) data providing low-gain adjustments to counteract bias and drift in the inertial data (aka sensor fusion).

Mossiback · Jan 17, 2018

okw said:
...Once this distance is accurately calculated, it is said to have locked onto that satellite.

Additionally, when an accurate location has been calculated by the receiver, many GPS users say that they have a good satellite lock.

I understand how GPS works, I was just hung up on the word "locked". I tend to overthink things. Thanks for the clarification.

Flyin2low · Jan 17, 2018

So the final answer to all of this is 7

sar104 · Jan 17, 2018

Flyin2low said:
So the final answer to all of this is 7

That's the minimum number that the FC requires to switch to P-GPS mode. It's not user-selectable. You can fly with fewer, including none, but it will be in OPTI or ATTI mode.

However, flying with only 7 risks the aircraft dropping out of P-GPS. I prefer to see 12 or more to provide a reasonable margin.

BerndM · Jan 20, 2018

I have always wondered why the MP needs such a large number of satellites for safe flight.
I own several Garmin and DeLorme GPS units and ALL of them will give me very accurate positioning info (within 10-20 feet) AND altitude after acquiring only 4 satellites.
Also is there any way to know how many of the satellites the MP receives are US or Glosnass satellites?

HighlandsVideo · Jan 21, 2018

sar104 said:
none of the DJI FCs will enter P-GPS mode with fewer than 7, and GPS health drops to zero at or below 6.

That's the answer for me. My Mavic gives me a yellow light on the rear and will not give me a green light until it has acquired the minimum number of satellites for maximum stability. I would simply not launch without a green light.

GlenT · Jan 21, 2018

Does the Mavic use both GPS and GLONASS simultaniously? I didn't think it could.

Dragonfly said:
India has exactly the same amount off satalites as the rest of the world.
The Mavic uses US and GLONASS system which for US has a total 24 sats (6 on 4 axis planes) that rotate around the globe. This puts at least a minimum of 8 sats in view at any one time at any point on earth of which only 3 are required for a fix (4 for 3D height) now add in the Russian GLONASS sats and you have 48 plus a couple of spares. Rotating on their axis plains giving your Mavic access upto 20 at any one time that may in view from any point on earth.

India that you refer to is sending up its own GPS satalite system called IRNSS and at moment only 7 are accessible or in view for India along with 2 ground stations.

There are also other GPS sat systems up there as well.

Does the Mavic use both GPS and GLONASS simultaneously? I didn't think it could.
On my big hexa which has the DJI Wookong FC it'll happily fly in GPS mode with 5 sats. But 90% of the time with that rig I fly in Atti. Less chance of errors, and it keeps you on your toes.

sar104 · Jan 21, 2018

GlenT said:
Does the Mavic use both GPS and GLONASS simultaniously? I didn't think it could.

Does the Mavic use both GPS and GLONASS simultaneously? I didn't think it could.
On my big hexa which has the DJI Wookong FC it'll happily fly in GPS mode with 5 sats. But 90% of the time with that rig I fly in Atti. Less chance of errors, and it keeps you on your toes.

Yes - the Mavic uses GPS and GLONASS. The aircraft DAT files include data on how many of each are locked.

BlakPhoenix · Jan 21, 2018

sar104 said:
The IMU estimates of position and velocity don't, primarily, use GPS or barometric data. In P-GPS mode, the IMU initializes position with GPS data and heading with compass data. It records both home point GPS altitude and barometric pressure (and, by extension, barometric altitude assuming a standard atmosphere) but it doesn't need those directly to estimate flight altitude.

Once moving, the primary update to location, velocity and attitude comes from the 200 Hz inertial data from the 3-axis accelerometers and rate gyros via quaternion algorithms, with the barometer (200 Hz), compass (50 Hz) and GPS (5 Hz) data providing low-gain adjustments to counteract bias and drift in the inertial data (aka sensor fusion).

Thanks for the super detailed reply! Great to read

Youser · Jan 22, 2018

My Hubsan drone needed a minimum of 6 satellites before you could turn the motors on while in GPS mode.

I normally don't look at how many satellites I'm locked on to. When the app says 'Ready to go', I'm ready to go.

Youser · Jan 22, 2018

Dragonfly said:
India has exactly the same amount off satalites as the rest of the world.
The Mavic uses US and GLONASS system which for US has a total 24 sats (6 on 4 axis planes) that rotate around the globe. This puts at least a minimum of 8 sats in view at any one time at any point on earth of which only 3 are required for a fix (4 for 3D height) now add in the Russian GLONASS sats and you have 48 plus a couple of spares. Rotating on their axis plains giving your Mavic access upto 20 at any one time that may in view from any point on earth.

India that you refer to is sending up its own GPS satalite system called IRNSS and at moment only 7 are accessible or in view for India along with 2 ground stations.

There are also other GPS sat systems up there as well.

Good post. Was just talking to a friend about satellite signals in that region. He said he couldn't get a signal on his sat-nav. UAV app was showing an estimated 12+

TrayBoz · Jan 22, 2018

Where exactly in the DJI GO app do you set the "minimum satellites to fly" threshold?

I don't remember seeing this setting anywhere before.

EDDY · Jan 22, 2018

Big Fil said:
How many satellites you need to safely fly all depends on the pilot. Some people have the skills to safely fly a mavic with zero satellites. Because you are asking i'm going to assume you will be relying on GPS assistance to fly. In that case i'd say wait for DJI go to give you the green Safe to Fly message in GPS screen.

This is a dumb question, but I am totally new so i am allow to ask. If GPS ok, and satellites Ok too, why do we need those offline maps ? i am assuming those maps are needed for more details of the flying area ?

Mossiback · Jan 22, 2018

With the offline maps loaded you do not need any internet connection to use the map while flying. Using Android.

sar104 · Jan 22, 2018

EDDY said:
This is a dumb question, but I am totally new so i am allow to ask. If GPS ok, and satellites Ok too, why do we need those offline maps ? i am assuming those maps are needed for more details of the flying area ?

Satellites and GPS are the same thing for the purposes of this discussion, and only provide location (coordinates) and velocity (speed and track). GPS data do not include any map data, so if you want to see the aircraft actually on a map then you have to get the map data onto the mobile device. That can be done in real time, via a cellular data connection or wifi, or offline, where you download in advance the maps for the location where you will be flying.

beachcombing · Jan 22, 2018

TrayBoz said:
Where exactly in the DJI GO app do you set the "minimum satellites to fly" threshold?

I don't remember seeing this setting anywhere before.

You don't.

The OP was talking about the app called UAV Forecast.

swooshdave · Apr 12, 2018

BerndM said:
I have always wondered why the MP needs such a large number of satellites for safe flight.
I own several Garmin and DeLorme GPS units and ALL of them will give me very accurate positioning info (within 10-20 feet) AND altitude after acquiring only 4 satellites.
Also is there any way to know how many of the satellites the MP receives are US or Glosnass satellites?

How fast and high is your Garmin going?

BerndM · Apr 13, 2018

Well it's not going very high but it is often traveling at 75 miles per hour.
My point is that a GPS only requires 4 satellites to get a 3D Lock including altitude.
So I don't get why DJI needs so many. Is it simply for redundancy?
That I would get

swooshdave · Apr 13, 2018

Yes, when you’re in the air redundancies are appreciated.

Afremont · Apr 13, 2018

It's all about desired accuracy. 4 locked sats, even with good spacing isn't going to give you a very accurate fix. It would be wrong by about 15m on a good day. There are a ton of factors that can make the accuracy even less. By averaging the readings over time and using data from as many satellites as possible (including glonas), you can get a much better idea of the position (like within a few feet).