Hello Everyone, we all know our GPS board (the one that comes with the APM1) has no battery to backup data.
This has the disadvantage to make the GPS not very precise in the first minutes after power up and that every startup is a cold startup (at least one minute to get a 3D fix).
This is because the GPS needs find the satellites without a previous almanac (that is information on position of the satellites visible in that location at a specific time).
As you understand almanac is VERY important. Also considrer that the almanac is sent by every satellite and requires 12.5 minutes to transmit (or so).
From Wikipedia:
"Finally, the almanac allows a single-frequency receiver to correct for ionospheric error by using a global ionospheric model. The corrections are not as accurate as augmentation systems like WAAS or dual-frequency receivers. However, it is often better than no correction, since ionospheric error is the largest error source for a single-frequency GPS receiver. Each satellite transmits not only its own ephemeris, but transmits an almanac for all satellites"
Of course this is not sufficient to have a perfect position: sky visibility, surrounding objects and number of satellites are the key factors.
This is why after a FIX (fixed blue led) number of satellites is very low (5 to 8) and this get higher (10 or 12) after some time of flying.
So, because of this, I decided to bring up an old hack made by Dcaspi, an old friend who worked also on the former Ardurpirates code.
The hack requires some soldering skills, so I suggest you to do it only if you are confident with it.
Beware if you fry the board or GPS unit, I cannot (nor Dcaspi) be responsible for it...
This mod works and has been used for a while.
This is the original location:
Have fun!
Emile
Introduction
If you use the DIYDrones MediaTek? GPS, you're probably aware of the fact that, on each activation, it make take a very long time to get proper satellite lock. The reason is that it always starts from scratch; it is not aware of it approximate geographic location, not of the time.
The reason for this is lack of backup battery. Data is completely erased once you disconnect the main battery.
The GPS module itself has a backup battery input. Here we show how to connect an external battery to that input.
What You Need
You will need a a battery holder and a small, 3V, coin-shaped lithium battery. These can be purchased at any DIY electronics store.
I also used a piece of perforated prototyping board, but this is for convenience and not a must.
How It's Done
Below, you can see the GPS module board layout. Cut one trace as shown, and connect the two leads from the battery holder to the marked places.
Here is how the setup looks:
This is how it looks from below. Note the place where the trace is cut:
Comments
Jani, of course I know that. The question is, why didn't my original backup battery work correctly with ACM, and what is different in the board you provide? The design itself is very simple, as the GPS chip has a dedicated backup voltage input. I assumed at the time that this was a software problem, e.g., the GPS-to-APM communication protocol not able to synchronize if not reset at the same time, but I did not have a chance to debug this. So what has changed since?
Dror.. well if you don't know/remember, jDrones is one of the core developers for ArduCopters (Original ArduCopter is our creation) and others. So we know exact requirements for the software and naturally they are 100% compatible with all APM electronics.
Jani, the same question applies to your solution too. Have you tested it with APM1 (and ACM s/w)?
Dror
Drotek, the DIYdrones GPS has its own firmware version custom-made for them. Does the one you sell have this firmware?
Also, my original battery had problems, which were NOT because the battery wasn't connected correctly (I did it according to the manufacturer's spec). I assume the same problem would happen with similar devices such as yours. So, have you tested it with APM1? Can you state that it works correctly?
Dror
Jani would you sell this lower battery board separate? Can I use it to add to the diydrones GPS so I don't have to hack a super cap or 2032 on it. Would it also help reception to put an aluminum square about 1"sq under it and connect to ground pin?
We have even easier way to do it. If you don't want or cannot to modify your GPS board. We at jDrones made new board for it and this board have even bigger groundplane compared to original MediaTEK GPS
There will be an additional board soon for this GPS too that will be mounted under it. On additional batteryboard we have place for CR2032 battery and also keep alive charging circuits for it.
On board there are small solder jumper that chooses if you have internal backup battery (eg. none) or external battery. External battery feed is connected on trough holes seen on topleft/topright corners of GPS module
you can change the configuration (baud and frequency) with a FTDI and the GPS tool available on our site.
So is this Drotek GPS flashed with the same firmware as the one sold by DIYdrones? What is the difference between it and my battery hack - I mean what makes it work correctly with APM?
If you want to do more simple and does not change your GPS there is this solution
-> Small PA6C GPS + LLC
It's the same thing I have on my zephyr and as long as both batteries are of equal amps and volts then it's ok.
But if you plug in two different amp batteries or different voltage then yes there will be a quick exchange of energy :)
Really though it's not the solution I would prefer. I would prefer a backup battery or super cap on the GPS. For now I just keep extra batteries around and plug one in to get started and then usually change it out or just plan on a short first flight and then battery changes after that are as fast as I can change them which usually results in the APM2 getting a lock in a minute.
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