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
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.
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.
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.
@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....
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?
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
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
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.
@emile: Sure! I'll see if it works first, then put the full story up here.
The intention is to do something like this (using lousy ASCII-art for the diagram)
5v(+)--->|--->|---\/\/\/\---(A)---|(-----(O) Gnd
This forms a string of components from 5V to GND, in order
1N4148 Signal diode
1N4148 Signal Diode
150Ohm 1/4W Resistor
1F "SuperCap" Battery backup capacitor 5V (any big 5V supercap should work, like 1.5F, 2F etc).
Point A is connected to the GPS unit's VBACKUP pin. Previous connections to VBACKUP are removed/cut; for APMv1, this is the same cut as above, for APMv2, this means removing R3.
Power retention won't be brilliant; several days from a quick back-of-envelope calculation. It would be very adequate for a day's flying. You could have multiple battery-pack changes, without having to go through the bother of a full cold boot on any startup other than the first, or if you fly often enough, very seldom have cold boots.
Hope this helps!
M
@Mark: use = us ... typo.
@Nick this is a good start point the Doc says the Backup pin should always be connected in order to work, so probably to attach a battery one should remove the actual trace giving the 3.0V
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