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GPS and Battery Changes

Dodge DeBoulet

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Dec 27, 2018
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222
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Maine
I noticed the other day that when changing a battery right after a prior flight and powering up the Spark again, I have essentially the same number of GPS satellites immediately locked as I did at the end of the previous flight. No need to wait for enough satellites to be "Ready to Fly."

That's obviously quite different from the initial flight of the day, where I usually waste 5-7% of the battery just waiting for 10 satellites so I can at least take off and have reasonable confidence of a RTH location.

I'm not sure why this is, but it makes me wonder if, in the case I might need to maximize my flight time for each run, having a "GPS lock" battery set aside for the initial GPS lock might make sense. This could be one of the earlier batteries with the retention issue that occasionally pop up on sale.

Thoughts?
 
I too have noticed that with my flights.

I don't know the inner workings of the Spark but perhaps there's a flash memory chip or something that retains the GPS information between batteries?

Now if it could remember the Histogram display between batteries, that would be a plus. One less thing to fiddle with.
 
My handheld GPS always takes longer after it isn't used for awhile. I believe that the information from the satellites needs to be downloaded and a brief lapse of connection requires little information transfer.
 
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Also, since satellite positions move a "cold" or new start will be acquiring sat positions that have moved since last sat acquisition. Same day subsequent starts will experience less sat movement than "cold" start.
 
I noticed the other day that when changing a battery right after a prior flight and powering up the Spark again, I have essentially the same number of GPS satellites immediately locked as I did at the end of the previous flight. No need to wait for enough satellites to be "Ready to Fly."

That's obviously quite different from the initial flight of the day, where I usually waste 5-7% of the battery just waiting for 10 satellites so I can at least take off and have reasonable confidence of a RTH location.

I'm not sure why this is, but it makes me wonder if, in the case I might need to maximize my flight time for each run, having a "GPS lock" battery set aside for the initial GPS lock might make sense. This could be one of the earlier batteries with the retention issue that occasionally pop up on sale.

Thoughts?

Time elapsed for GPS lock has nothing to do with a particular intelligent battery.

The flash memory on the aircraft itself saves setting information including last known GPS positions, camera options, etc.

If you pack up a GPS unit and start it up across the country the satellite acquisition could take even longer -- several minutes. It has to search the whole sky again starting from scratch because the satellite positions it saved in memory are invalid.

My boat and car GPS have the same behavior. The satellites are in geostationary equitorial orbit but if the Spark is moved far enough or powered off long enough it's gonna take longer to find enough satellite signals.
 
see Time to first fix - Wikipedia

The mentioned flash memory is integrated in the GPS receiver chip and is managed by the receiver chip itself.

PS: GPS satellites are not geostationary. They are in a 12540mls orbit with an orbital period of one half day. So the satellites are moving around you. This explains why you will see the number of visible (locked on is the wrong wording btw) satellites is changing even if you are stationary in a wide open field with unobstructed visibility horizon to horizon. Satellites vanish and show up in the receivers field of view. It also explains why it’s possible to have low accuracy even with a high number of (visible) satellites. Some might be close to the horizion and others in a bad relative position to each other in order to obtain a acceptable solution of the navigation equations. Google delution of precision (DOP). I am 100% sure this is what DJI is using to determine the availability of GPS. The number of satellites is nearly meaningless as 4 satellites in ideal positions are better than 12 in bad positions. Probably also explains why it sometimes drops out of GPS into ATTI with a seemingly good number of satellites. A high DOP tells you to not rely on the data. In navigation you‘d revert to dead reckoning. DJI obviously chose to degrade to a semi-manual mode until reliable GPS data (low DOP) is available again.
 
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Time elapsed for GPS lock has nothing to do with a particular intelligent battery.

The flash memory on the aircraft itself saves setting information including last known GPS positions, camera options, etc.

If you pack up a GPS unit and start it up across the country the satellite acquisition could take even longer -- several minutes. It has to search the whole sky again starting from scratch because the satellite positions it saved in memory are invalid.

My boat and car GPS have the same behavior. The satellites are in geostationary equitorial orbit but if the Spark is moved far enough or powered off long enough it's gonna take longer to find enough satellite signals.

Thanks, Andre. I was quite sure that the specific battery had nothing to do with getting and retaining satellite lock, and multiple responses here bear that out.

My thinking with regard to obtaining lock using a specific battery was to be able to prep the Spark for as much contiguous flight time as possible per multi-battery flight session by using a dedicated battery just for acquiring GPS lock. Said battery might be sub-optimal for normal flying (such as the earlier batteries that would occasionally fall off the Spark mid-flight, or one that no longer has a capacity useful for flight).
 
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the djo logs the locations of the satellites and uses the gps on board small memory for location saving...it has a small rechargable battery in the gps module that keeps the information... if you dont move your craft much from the potision then it still has most of the info and the lock on time cuts from 30 seconds or 10 for the newer gps modules to just 3 or less...

the gps modules that you can get from ebay for fpv drones that use aftermarket flight controllers do the same thing.. so if you dissconnect your drone for say a day or so that battery will fade and loose the lock on that specific location.. or its a month cant remember so power on your fpv drone once a month to charge the battery... the dji does the same thing so if you dont fly for a month youll notice that it takes 30 seconds to 1 minute to lock on.... this is normal... dji lovvvvvvvvvvvvvesss 12 too 13 sats when it only takes 6.. to 9 to have a decent lock... you used to be able to tell it how many but not anymore... dji slowly taking away features that let us control it..

3dr's mission planner if you google it and look at it it has tons of parameters then again its open sourced or it used to be if you use a old controller....and it did all this dji did back 10 years ago... maybe dji copied them and just packaged it in a nice little box like apple does aka steal ideas and just put them together... lol
 
I’m having the same problem I’ll get a lock on the home point an as soon as I take off it will say no home point satellite to week fly safe i was about to take it in for service
 
the djo logs the locations of the satellites and uses the gps on board small memory for location saving...it has a small rechargable battery in the gps module that keeps the information... if you dont move your craft much from the potision then it still has most of the info and the lock on time cuts from 30 seconds or 10 for the newer gps modules to just 3 or less...

the gps modules that you can get from ebay for fpv drones that use aftermarket flight controllers do the same thing.. so if you dissconnect your drone for say a day or so that battery will fade and loose the lock on that specific location.. or its a month cant remember so power on your fpv drone once a month to charge the battery... the dji does the same thing so if you dont fly for a month youll notice that it takes 30 seconds to 1 minute to lock on.... this is normal... dji lovvvvvvvvvvvvvesss 12 too 13 sats when it only takes 6.. to 9 to have a decent lock... you used to be able to tell it how many but not anymore... dji slowly taking away features that let us control it..

3dr's mission planner if you google it and look at it it has tons of parameters then again its open sourced or it used to be if you use a old controller....and it did all this dji did back 10 years ago... maybe dji copied them and just packaged it in a nice little box like apple does aka steal ideas and just put them together... lol
Do you have a link, about this rechargeable battery of DJI GPS modules?

Also, location has nothing to do with number of satellites. So, even if you store a location, number of GPS satellites is changing all the time.
 
Please try and get a basic idea of how the GPS system works to understand your observations. You might want to start here:
Global Positioning System - Wikipedia
Dilution of precision (navigation) - Wikipedia

This is a good basic read to understand why it sometimes takes more time to aquire a first fix: The Almanac, Time to First Fix and Satellite Health | GEOG 862: GPS and GNSS for Geospatial Professionals

To the question of recharchable battery: all modern GPS receivers have some means on storing dowloaded GPS system data in non-volatile oder battery buffered memory to be able to do acquisition warm starts even if the main system (your drone) is taken off power. But it is in the nature of the GPS system that it might take up to 12 minutes worst case to obtain a first fix. So all your observations about varying times to first fix are perfectly within the to be expected range.
 

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