Jumper(s) for APM2 onboard GPS?

UPDATE: See the manual.

Hey guys, this is mainly a question for our friends at 3DR, but anyone who has looked at the traces on the APM2 daughterboard, and knows the GPS module, could offer ideas.

I wonder what you'd think about (as a hack, as a revision to the APM2 daughterboard, etc) installing some jumpers or switches to "disconnect" the onboard GPS, as needed. We might also add a battery socket to maintain the almanac, for those who want to accept the extra weight of the small battery, but that is another topic.

I have a small but growing fleet of APMs, some APM1 and some APM2s. However, I never have enough, so I find myself swapping them around as needed. I leave the magnetometers, air speed sensors in place and connect them as needed. Some airframes are better served with an external GPS, but I would prefer to have all my APM2s be, physically, the same. That is, I'm willing to buy all my APM2 with the extra cost of the onboard GPS whether or not I need it, in order to ease my "fleet management" if you will. If the daughterboard were not soldered in place, I might instead get extras, but this approach might work best.

However, I would also like to buy or use uBox and other external GPS units, as needed. I think this represents opportunities for the 3DR and affiliate stores. Many users will purchase the APM2 w/GPS, and some will later decide they want to use an external. Some will buy a separate unit w/o the GPS, but most will not. Most of the population simply live with the existing GPS. However, if there were a switch or jumper(s) to kill the onboard GPS, then casually swapping to an external GPS (and, casually purchasing additional GPS units) is easier. It also allows for subsequent upgrades (opportunities for sales) to future GPS units. 

As a hack, are there recommendations on which trace(s) are best to cut and/or install a jumper for an existing APM2? I imagine there are a couple of ways to do this, not all are ideal. Is there any merit to a revision of the APM2 where there is a jumper or switch for the GPS? 

Thoughts? 

Tags: APM2, External, GPS, Jumper

Views: 4518

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Replies to This Discussion

Permalink Reply by Colin on June 18, 2012 at 8:02am

Thanks for doing these tests Mike. I have an idea for the mechanical solution, but I doubt the APM will be so pretty after ;-)

My idea is to take one of the pins out of a two pin male header. The hole in header (from the removed pin) would slide over the GND daughter board mounting pin next to the 3D fix writing- the other pin can float outside the daughter board. This leaves two pins next to each other ready for the jumper. Now solder pin 2 of the GPS to the new header (the one floating). All of this would then be secured tightly in place with hot glue or Shoe Goo (really good stuff).

Then the GPS can be disabled by sliding the jumper into place.

My advice for soldering the pin #2 is to heat it up and add a small amount of additional solder. Then with a small ball of solder on the end of the iron (to conduct heat better), push the resistor into pin #2 while heating the leg of the resistor. This means you don't have to touch the pad while adding the resistor. I'll make a video when I do it later this week if it helps.

If you cut the power line, I believe you will still have the 3D fix light illuminated no matter what. It takes the GPS to be powered up and pulling the transistor high for it to go out. I think it being illuminated could be a good indication that the jumper is correctly seated when you power up- when was the last time you instantly got a 3D fix ;-)

Also, the datasheet says in the disabled state the current draw is about 15uA, so nothing.

The resistor value, I don't know. On pull-up resistors and the like with Arduino, 10k is pretty common. So I would guess something similar here. The 4K seems to have worked well for you, I'd be inclined to just stick with that. You need to make sure that pin #2 is below 0.25v by a reasonable margin.

I think cutting will also work fine, so do that if you're more comfortable. I just don't like cutting tracks :-)

Permalink Reply by Michael Pursifull on June 18, 2012 at 8:12am

Excellent; I think it would make an excellent blog post, what do you think? I'd love to see some photos and/or a video, if you find time to do this. I would like to give it a go also, but I've packed up my soldering iron and workspace already, and will not have access to them for two or three weeks. If you don't get to it within the month, I can do a hack job of my own. While I believe a board revision from 3DR might add a feature like this at some point, there are a lot of folks who will still have APM2 boards in play. And if that feature is not yet on the table, a blog entry would maybe get the right folks thinking about it. 

Permalink Reply by Colin on June 18, 2012 at 8:20am

I totally agree. I'll go ahead and do that, I expect to post a blog in a week or so :)

Permalink Reply by Colin on June 19, 2012 at 12:54pm

Hey guys, I liked the idea of the jumper, but decided it was going to be messy and I wasn't going to need the onboard GPS for a while (or I was lazy?). Instead I just soldered a 10k 0402 SMD resistor between pins #2 and #3 on the GPS module. The 3D fix light on the daughter board is always illuminated, but this is meaningless. The external GPS works flawlessly. Here's a picture:

I figure when I want it back this is going to be really easy to remove

Colin

Permalink Reply by Jonathan M on June 21, 2012 at 10:46pm

I am thinking of trying this.  I believe my current GPS is toast.  It no longer gets a 3dfix and is pretty much useless.  Rather than buy a new APM it would be nice to just spend the money on a GPS.


Are you using this https://store.diydrones.com/MediaTek_MT3329_GPS_10Hz_Adapter_Basic_... as a replacement?

Permalink Reply by jbabio on June 22, 2012 at 3:32am

Yes, and don't forget the EM-406/uBlox/MTK Adapter Cable to connect it to the APM

Permalink Reply by Michael Pursifull on June 22, 2012 at 6:40am
You probably just have radio interference affecting your GPS. Take the APM2 outside, with only the esc and a battery and hold the esc away from the APM. See how long it takes to get a fix in clear sky's and without your rx or any telemetry or any other electronics nearby.
Permalink Reply by Jonathan M on June 22, 2012 at 10:31am

You would think.  But prior to this occuring, I was getting solid lock no issues.  Loiter was even getting decent.  No hardware or configuration changes.  Just one day it stopped getting a fix.

And yes, I have done that test with no luck :(

Permalink Reply by Michael Pursifull on June 22, 2012 at 10:51am

Well, I hope it is a bad GPS unit, perhaps a weak solder joint between the GPS and the antenna. Because if you have ruled out your gear, then it could be environmental factors, like a harmonic from a local, high power transmitter. There are quite a few possibilities, some are discussed in http://blockyourid.com/~gbpprorg/mil/gps/Papjam_St_Peter97.PDF and similar documents. 

And of course if you live deep in a valley or are surrounded by a nearby, high mountain ridge, you can have issues with LOS to enough satellites, which may come and go as your picture of the constellation changes. But you are probably right, a damaged or problematic GPS unit is more likely. But smile, it is also the second cheapest thing to fix, after addressing interference sources in your own gear.  ^_^

Permalink Reply by DC on June 22, 2012 at 1:11pm

Just a thought. But I warn you I do not have my APM2 yet so I have nothing to look at to see if its feasible.

What would be the issue with coming up with a another small PCB that had what ever was required to disable the GPS and fix the the 3d Fix led that could be soldered in place of the GPS module and had the GPS module mounted to the top of it.

It would be sandwiched between the GPS module and the APM2 daughter board. 

Permalink Reply by Steven Harsanyi on July 5, 2012 at 4:08pm

I installed a battery and now I get s gps lock almost immediately on power up (8 to 13 sats in view in a minute).  I also have no JP1 installed and only power the apm2 from the input rail using a ubec that outputs 5.03 volts.  Anything much higher than 5.0 volts leads to gps problems I noticed (it is also in the wiki).

Permalink Reply by Michael Müller on July 6, 2012 at 1:45pm

Wopuld you please explain (or provide a link) how you have installed the battery. I think this is a solution for many people having this GPS problem.

Thank you

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