M3 & Sat Lock

[millbah]

I have read many complaints about the M3 & time to achieve enough stats to fly. I am located close to the equator and generally sat lock for me is instantaneous. Generally I fire up the M3, then the controller & I am ready to fly. So I have no issue with the performance.

 

[shb]

I was watch a show on the disappearance of that Malaysian airliner, and they said you only need 3 satellites to determine your position. So I wonder why a drone needs 10.

 

[Not A Speck Of Cereal]

I have read many complaints about the M3 & time to achieve enough stats to fly. I am located close to the equator and generally sat lock for me is instantaneous. Generally I fire up the M3, then the controller & I am ready to fly. So I have no issue with the performance.

And you have updated to the latest firmware?

Chris

 

[Meta4]

I was watch a show on the disappearance of that Malaysian airliner, and they said you only need 3 satellites to determine your position. So I wonder why a drone needs 10.

With only 3 sats, the best you can get is an unreliable 2D fix.
You need a minimum of 4 sats to get a 3D fix.
DJI makes their drones wait for a minimum of 6 sats just for additional safety.

But the minimum number of sats doesn't guarantee accurate position information.
As well as the minimum number you need a good spread of sats to provide good geometry.
Sats that are close together in the sky won't provide good sat geometry.

But as DJI use more satellite constellations (3 different constellations with more recent models), you will have more sats in your sky, but with more sats, you will also have more that are bunched and not providing the reliable GPS that you'd assume just looking at sat numbers.

 

[millbah]

And you have updated to the latest firmware?

Chris

Yes 1.5.10

 

[Starz]

I fire up the M3, then the controller

Following this procedure won’t speed the satellite acquisition process. By the time the controller boots up, the aircraft is already part way through the satellite hunting adventure. By this method, the aircraft gets a head start in doing what it’s designed to do. That design is controlled by whatever firmware(and bugs)were installed courtesy of DJI.

 

[Deleted member 154956]

I was watch a show on the disappearance of that Malaysian airliner, and they said you only need 3 satellites to determine your position. So I wonder why a drone needs 10.

3 satellites are required for 2D positioning (lat-long), and 4 satellites are required for 3D positioning (lat-long-altitude). These are absolute minimums. As the earth rotates and satellites (geosynchronously positioned) rise and fall above the horizons, more than 3 or 4 satellites are required for a stable continuous lock, as would be needed for navigational or positioning use. Drone manufacturers obviously have written their firmware to require more than the bare minimum for stable operation of their drones. Dilution of precision (DOP) is EVERYTHING in satellite GPS navigation and depending where the gps satellites are in relation to each other it could significantly degrade signal quality to the point where you would lose one or more signals. An example would be if 2 out of your 4 signals were coming from the same azimuth in the sky due to those satellites only being separated by a degree or two. That would be a considered a poor positioning signal. And we all know very well how our beloved drones act when they don't know exactly where they are at in the world. I will include a short wiki article on exactly what DOP is, and for most it is probably a dry read. But since DJI Drones are GPS dependent, it might help explain problems that users experience when flying.

WIKI
The concept of dilution of precision (DOP) originated with users of the Loran-C navigation system.[1] The idea of Geometric DOP is to state how errors in the measurement will affect the final state estimation. This can be defined as:[2]

Conceptually you can geometrically imagine errors on a measurement resulting in the

term changing. Ideally small changes in the measured data will not result in large changes in output location. The opposite of this ideal is the situation where the solution is very sensitive to measurement errors. The interpretation of this formula is shown in the figure to the right, showing two possible scenarios with acceptable and poor GDOP.

More recently, the term has come into much wider usage with the development and adoption of GPS. Neglecting ionospheric [3] and tropospheric[4] effects, the signal from navigation satellites has a fixed precision. Therefore, the relative satellite-receiver geometry plays a major role in determining the precision of estimated positions and times. Due to the relative geometry of any given satellite to a receiver, the precision in the pseudorange of the satellite translates to a corresponding component in each of the four dimensions of position measured by the receiver (i.e.,

,

,

, and

). The precision of multiple satellites in view of a receiver combine according to the relative position of the satellites to determine the level of precision in each dimension of the receiver measurement. When visible navigation satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when far apart, the geometry is strong and the DOP value is low. Consider two overlapping rings, or annuli, of different centres. If they overlap at right angles, the greatest extent of the overlap is much smaller than if they overlap in near parallel. Thus a low DOP value represents a better positional precision due to the wider angular separation between the satellites used to calculate a unit's position. Other factors that can increase the effective DOP are obstructions such as nearby mountains or buildings.

DOP can be expressed as a number of separate measurements:

• HDOP – horizontal dilution of precision
• VDOP – vertical dilution of precision
• PDOP – position (3D) dilution of precision
• TDOP – time dilution of precision
• GDOP – geometric dilution of precision

These values follow mathematically from the positions of the usable satellites. Signal receivers allow the display of these positions (skyplot) as well as the DOP values.

The term can also be applied to other location systems that employ several geographical spaced sites. It can occur in electronic-counter-counter-measures (electronic warfare) when computing the location of enemy emitters (radar jammers and radio communications devices). Using such an interferometry technique can provide certain geometric layout where there are degrees of freedom that cannot be accounted for due to inadequate configurations.

The effect of geometry of the satellites on position error is called geometric dilution of precision (GDOP) and it is roughly interpreted as ratio of position error to the range error. Imagine that a square pyramid is formed by lines joining four satellites with the receiver at the tip of the pyramid. The larger the volume of the pyramid, the better (lower) the value of GDOP; the smaller its volume, the worse (higher) the value of GDOP will be. Similarly, the greater the number of satellites, the better the value of GDOP.

 

[bogdi1988]

With only 3 sats, the best you can get is an unreliable 2D fix.
You need a minimum of 4 sats to get a 3D fix.
DJI makes their drones wait for a minimum of 6 sats just for additional safety.

But the minimum number of sats doesn't guarantee accurate position information.
As well as the minimum number you need a good spread of sats to provide good geometry.
Sats that are close together in the sky won't provide good sat geometry.

But as DJI use more satellite constellations (3 different constellations with more recent models), you will have more sats in your sky, but with more sats, you will also have more that are bunched and not providing the reliable GPS that you'd assume just looking at sat numbers.

That is not the experience I have. The app waits until I get 12 solid satellites to get homepoint etc. Otherwise the drone is limited to 30 ft or so height and so it is literally unusable. A cold start (battery swap), takes me up to 10 minutes or sometimes more to get a solid 12 satellite fix. If the app allowed for 6 sats only, I'd have a fix almost instantly.

 

[Meta4]

That is not the experience I have. The app waits until I get 12 solid satellites to get homepoint etc. Otherwise the drone is limited to 30 ft or so height and so it is literally unusable. A cold start (battery swap), takes me up to 10 minutes or sometimes more to get a solid 12 satellite fix. If the app allowed for 6 sats only, I'd have a fix almost instantly.

You need to reread what I wrote in post #4.
I didn't say what you think I did.
And note the underlined words.

 

[ianwood]

To put it as simply as possible, the more satellites used to plot the location coordinates, generally the more accurate those coordinates are. DJI chooses to wait until the accuracy is above a threshold they define before giving you the green light (white icon).