Add a backup battery on the MTK GPS module.
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:
http://code.google.com/p/ardupirates/wiki/MTK_GPS_backup_battery?ts...
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:
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Comment by Nick Arsov on May 31, 2012 at 9:04am -
Emile, exactly! When not using battery backup, you do not need to power this pin. I'm using this approach in my own GPS modules based on 3339.
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Comment by Dror Caspi on May 31, 2012 at 12:02pm -
Well, I'm the author of this hack...
The sad fact is, I never got it to work well. I believe it has something to do with the GPS not being reset properly and/or communication with the APM not getting synchronized. The s/w at the time (more than a year ago) was ArduPirates.
Anyhow, soon after doing this hack I lost my quad, with the APM and GPS inside :( A lot has changed since, with a completely revised ACM. I have an ACM-based quad on my bench waiting forever to be finished.
So take this with a grain of salt and be ready to do some debugging.
Dror
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Moderator Comment by Roberto Navoni on May 31, 2012 at 3:38pm -
Dear Friends,
in any design that i developed where i put a gps i use the battery backup . You need to put only a resistor in series with battery so you don't have any kind of problem on gps. If you don't put it could be have problem.
Normally in 10 second after power on i have a great and stable gps signal without problem , mantain almanac and the quality of signal is fine :)
in this picture you see my standard configuration :
http://code.google.com/p/multipilot32/
best
Roberto
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Comment by steve F11music.com on May 31, 2012 at 4:51pm -
Do you know if it's possible to use this super cap instead?
http://www.sparkfun.com/products/10317
Can I install it the same way?
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Comment by emile on June 1, 2012 at 1:40am -
@Roberto, this is the reason of posting this blog. I use your board with a same GPS as APM and backup battery and never had any issues. Considering we are using a 1:1 port of the Arducopter code I can say there are no known issues of having a backup battery.
@Dror, there is no reason why a battery should not work a part from poor design, bad components or bad SW.
At that time I remember there were quite a few problems with the components and the v-reg that was not supplying a stable 3.3v. I had to change two of thiose before I could get good v-reg.
In our situation where flight times often are in the order of 6-10 minutes, waiting 1 or two minutes before we get a 3d fix is quite annoying. You have to be lucky to get a reliable position in the first 2-3 minutes after lock.
Believe me, when you fly with a GPS that gets a fix in a few seconds is joy....
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Comment by emile on June 1, 2012 at 1:43am
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For the sake of users, we should try to get a good solution to solve this and post some results.
From my side I will try to get some useful data and show the difference.
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Comment by Nick Arsov on June 1, 2012 at 2:22am
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I'd suppose you'd better place a question of the problem to the manufacturer - http://www.gtop-tech.com/jsf/index.jsf
They are supper correct guys and will answer immediately.
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Comment by steve F11music.com on June 1, 2012 at 6:22am
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What about the super cap option? Is it possible to install? Do I install it just like the battery hack? I know on the remzibi it uses the same GPS and the most I have to wait is 1 minute but usually it's 10 seconds. I installed a CR2302 by carefully soldering wires directly to the battery and then wiring it up like this:
https://sites.google.com/site/remzibiosd/2-remzibiosd/4-gps-module
Before I did that, it literally would take up to 30 MINUTES to get 5-6 satellites! Now it's 1 minute max.
I use remzibi on a plane but it's more important on the quad that eats too much % of the battery before I can fly. I'm thinking to put a Y cable to plug in 2 same mA batteries so that before I fly I can just unplug the secondary. I just need to make sure the voltages and mA are equal.
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Comment by emile on June 1, 2012 at 6:40am
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@F11Music.com Don't do that thing of Y cable with two batteries. You will drain the charged one onto the discharged one at a rate that the discharged one will not tolerate!
When a battery says you can charge max at 2C you will charge it at a rete of at least 10-20C depending on your battery!
And this is not good!
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Developer Comment by R_Lefebvre on June 1, 2012 at 7:54am -
Yeah, I remembered seeing that the battery backup caused a problem. What was the problem, and has it been fixed?
I always use a separate battery to power the APM. So my proceedure is simply to take the aircraft out first, and immediately power up the APM. By the time I'm finished unpacking everything else, including the ground station, the GPS is locked on and good to go.
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