GPS/HDOP.. Not directly related to Arm or switch to Manual..

[Jubalr]

After a morning of great flying, I decided to do some testing. So what I did was take the Solo to a marginal GPS area at the edge of some very tall trees. I got a ready to Arm with 6 sats and HDOP of 2.1. I then went into the very tall trees to purposely lose GPS so that it would switch to manual from Fly mode. I continued to fly while it switched back and forth 4 or 5 times. I did not go into any other modes, I let it stay in Fly and go into manual at loss of GPS. It would briefly obtain GPS again and go back to Fly mode. I let it do that a few times and landed. I then went through the log and found some interesting data. And it actually points back to something one of the developers said about the firmware change and the use of the GPS module model. Here is what I learned:

1) When the system switches you from Fly to Manual and back to Fly it has nothing to do with # of sats OR HDOP. The actual reading that is used for determining minimum accuracy is called ekfposhor( horizontal position accuracy).
When this reading gets above aprox 0.5 for more than a sec or 2 it will switch you to manual. When it stays below that again for a few sec it will switch you back.

First picture below is the readings when it first switched from Fly to Manual. The green line in the graph on the Right is tracking this reading. In the HUD on the left you can see the actual numerical value of 1.96. Also note that the Sat count is 10 and the HDOP is 1.74

The image below shows the point when it switched back to Fly(loiter) mode. Reading is now 0.16, but Sat count is still 10 and HDOP is still 1.74

Below is when it again switched to Manual. Reading is 0.5 with a sat count of 8 now and HDOP of 1.98 (still higher than when I took off) Notice the spike in the green line again.

And another time when it switched back to Fly. Reading of 0.05, Sat count of 9 and HDOP of 1.8. Green line back down to the bottom.

2) The 2nd issue is determining the point that you get the OK to Arm the first time after you power up. I noticed several time the difference from 1 flight to the next. 1 time it will arm with 6 sats and 2.2HDOP and the next time might be 8 sats with 1.8HDOP. So I created a log where I turned it on, waited for Arm, Armed then turned it off. It appears that once again, the Sat count and HDOP s not the determining factor (or at least by them selves). Just prior to getting Arm, showed the following values. Note that the Sat count is 6 with an HDOP of 2.1.

Here is the point where the Solo gave the OK to Arm: Same HDOP and Sat count..(Shows Disarmed as it is the point directly after the OK to arm is given but before pressing and holding Fly).

I have went through the #s and the only reading that appears to have changed is the Alt. Prior to being able to Arm, this reading stays flat at 0 the entire time. At the point it finally has a reading(0.032808), we get the OK to arm. It is possible that this reading is used in conjunction with other readings, but also possible that the it is a direct indicator that will not give a reading until a minimum satellite condition is met. I'm not sure.

But I am fairly certain of the readings at the top regarding switching of fly/manual and back. And it is also possible that this could be fine tuned in firmware upgrades.

Of course, it is always a possibility that I am full of sh*t. That's why I posted the images too!

 

[Raybro]

Is it just me or what.....no pics

 

[FlexFanatic]

Is it just me or what.....no pics

I see embedded pics.

 

[Sarah B]

In case nobody else says it, kudos for being able to look at those numbers, extrapolate data from them, and draw conclusions based upon the data. Impressive!

 

[Ian [P13]]

@Jubalr HDOP is now only a secondary check for the pre-arm, it's not used in flight, the most important measure is now sAcc (speed accuracy), it's a much better indicator of when the GPS is about to go FUBAR.

I don't believe you can view or map that indicator.

This change was done in the last firmware, this means even if your HDOP is below 2.5 you might still be waiting some time before the Solo will allow a take off, as the sAcc might be triggering a warning too.

 

[Jubalr]

@Jubalr HDOP is now only a secondary check for the pre-arm, it's not used in flight, the most important measure is now sAcc (speed accuracy), it's a much better indicator of when the GPS is about to go FUBAR.

I don't believe you can view or map that indicator.

This change was done in the last firmware, this means even if your HDOP is below 2.5 you might still be waiting some time before the Solo will allow a take off, as the sAcc might be triggering a warning too.View attachment 645

Well, that certainly makes sense. I see so many posts that tie it exclusively to HDOP and a magic 2.0 number. But I remember a developer saying that wasn't true and so I wanted to run some #s to see what came up.

Your right too about it not being a # you can track in the Tlogs. I wonder if it shows up in the more inclusive DF logs?

 

[Ian [P13]]

It's code that's so new I doubt it'll even be in the dataflash logs yet, I am going to do a video tutorial on dataflash logs this week, so I'll post it here too.

HDOP of 2 is true of Arducopter in general but on the Solo and Iris is 2.5, but was made a pre-arm only check in the last firmware, with sAcc instead being the preferred test now.