GPS unit. What to buy?

[twinturbostang]

Hey guys. New here. I'm just getting into AP and UAV operation. I'm wondering what's the best GPS device to purchase. I see a lot of talk about the Garmin Geko 201. Looks to be a nice compact, lightweight unit that is relatively inexpensive as well. Is there something better out there though? What about the Geko 301? It has a couple of added features that look like they might be worthwhile. It has a built in compass, which I'm assuming gives you instantaneous bearing indication, whereas the 201 has to infer bearing based on tracking movements. It also has a barometric pressure sensor for measuring altitude, but I'm not sure why this is included since you get altitude from the GPS data. Unless it's used for tracking altitude more precisely than the GPS signals can provide. Or maybe it gives you quicker altitude changes, where the GPS signal may not refresh as quick. *shrug*

Anyway, if you guys could give me some ideas of what would be a good unit to purchase, that'd be great. Might be handy for other people as well. :)

Brian

[Mr.RC-CAM]

If the choice is between the 201 and the 301, and the budget is open, go for the 301. The compass and barometric alitimeter are useful for our app. However, depending on your needs, the Garmin smart routing feature could be an issue. Or so I have heard.

[CrashingDutchman]

Considering value for money, what would the choice be between:
- Geko 201
- Geko 301
- Etrex
- Other?

Don't want to hijack this thread, but I now have an Garmin GPS12, would this one work with a RCAP2? The GPS12 is a twice as heavy as the others mentioned above.

My modified Slowstick would carry it without any problems, but as weight is always an important factor, I might consider to sell this one and get a different model.

CD

[Mr.RC-CAM]

For basic GPS data, the 201 looks to be a good value. If you don't need waypoints, then one of the low cost OEM GPS modules from Sparkfun.com might be of interest to you. Keep in mind that the RCAP needs waypoints, so the little OEM modules will not work in that app.

kd7ost has mentioned that the GPS12 does not have Garmin's goofy smart routing feature. If you intend to follow a complex waypoint route, then not having this feature is a good thing.

Maybe you could just remove the GPS12 from its case to save weight? I don't know of anyone that has done that with the GPS12, but the hack was popular witht the Garmin Etrex. With care, the Etrex would go from 5.5 ounces to 1.9 ounces. The GPS12 may also accommodate the weight reduction program. Some old rc-cam forum details: Photo (http://groups.msn.com/rccam/rccamprojectphotos.msnw?action=ShowPhoto&PhotoID=124). Case Removal Discussion (http://groups.msn.com/rccam/general.msnw?action=get_message&mview=0&ID_Message=5094&LastModified=4675402214521535963)

[twinturbostang]

The goofy "smart" routing "feature" you speak of has something to do with it skipping over way points if it finds one closer to your current location, correct? I'm still totally new to GPS units. But I would think that if you can manually enter way point coordinates, then just a little bit of time adding additional way points so the mentioned "smart feature" does not kick in, would be an easy solution.

[kd7ost]

The goofy "smart" routing "feature" you speak of has something to do with it skipping over way points if it finds one closer to your current location, correct? I'm still totally new to GPS units. But I would think that if you can manually enter way point coordinates, then just a little bit of time adding additional way points so the mentioned "smart feature" does not kick in, would be an easy solution.

You are correct. Here's a thread that describes it better. http://www.rcgroups.com/forums/showthread.php?t=475531

The GPS you chose will depend on it's application for you. If you're just taking data and transmitting it via video overlay, recording in flight or tracking via radio modem like APRS or Maxstream applications, a small OEM type unit will work fine.

If you intend to use it for navigation as well, there are other options. At the low end is to use an RCAP type device which reads the NMEA 0183 data from the GPS. It translates tracking error into PPM signals for your rudder. You have to use the waypoint management feature in the GPS for that. Lots of guys are starting to use these on video planes where you aren't flying autonomously. They get employed with a GPS that is performing a "goto" your location. If the RCAP is enabled, it will cause the plane to fly back to you and loiter overhead. Kinda takes the sting out of a fly off or saves your bacon if you get turned around and try to fly back to yourself while "under the hood" and don't realize till failsafe kicks in that you're really flying away. :eek: Now you really know how far your range is. You have to use a PCM Radio or a receiver like the FMA DSR type to keep enable pulses going to the units. A standard AM or FM PPM radio won't do it by itself. You need the failsafe function.

A lot of guys are using the geko 201 now. I have several. There is a mod listed on the RCAPA forums area that can help you optimize it's use aboard a plane. I can't post a link right now because the site moderators blocked my IP address from here for some reason. Just look in the UAV section for a thread about a geko hack. At the very least, you need to hot glue that little battery in there to keep it on the PC board.

The geko 101 has no serial output. It won't work.

The geko 301 has the additional BP altitude lock and compass but those aren't used in the RCAP anyway. BP altitude information does help accuracy in altitude with the device and the corrections are included in the strings. But, it's a BP sensor and due to local atmospheric BP changes, you have to calibrate it to local pressure before use if you really are concerned about the right height. I don't use it and get to watch altitude information in meters on my screen and it's not all over the map. It's pretty good. You get a lot of satellites and a clear sky view in a plane above the trees. Altitude is reconciled quite well IMHO especially with WAAS enabled in the geko.

You can purchase a higher end Pico Pilot type unit from UNAV and go back to a little OEM GPS. They built a waypoint manager into the little unit and it holds 32 waypoints. It won't do the "smart routefinding" thing that a handheld unit will.

Dan

[CrashingDutchman]

Maybe you could just remove the GPS12 from its case to save weight? I don't know of anyone that has done that with the GPS12, but the hack was popular witht the Garmin Etrex. With care, the Etrex would go from 5.5 ounces to 1.9 ounces. The GPS12 may also accommodate the weight reduction program.

The GPS12 is waterproof, I think that removing the case is a problem, but I will see if I can find something about it.

The GPS12 alone weighs about 156 gram without batteries. Just the 4 x AA batteries add so much that total weight is 165 gram. Maybe I can power it through a BEC? Other suggestions maybe?

CD

[twinturbostang]

KD7OST: Thanks for the info. Yeah, the plan is to purchase a PCM receiver for my Futaba 9C, and get a RCAP2 and GPS device. I'm building an AP/AV rig, but I may want to experiment w/ some UAV stuff as well. Looks too cool to pass up! :) But for the AP/AV stuff, I definitely want to have the "go home" function if I exceed the RC transmission range or get lost (via HMD), so that will mean I need the RCAP2 and waypoint functionality.

As for the Geko 301 features... the built in compass definitely looks useful for me. I plan on using it also when I go on trips and stuff, so having an instantaneous bearing indicator will be handy for that. The BP sensor seems a little bit redundant, since you get altitude from the GPS data. I guess it's just another check mark they can put on the package. :) I would think, though, that they use the GPS information as the absolute altitude reading, and then use the BP for minute/quick changes in elevation beyond the resolution of the GPS. Atmospheric changes will drive a BP sensor way off, so I wouldn't want to rely exclusively on that. My guess is they correct for weather changes by calibrating the BP sensor off the GPS data.

So... I guess it's a Geko 201 or 301 for me. Will come down to how much $$'s I decide to fork out for it. :)

[kd7ost]

BTW, now that I'm at home here's that link to the mod for the geko. http://www.rcapa.net/forums/viewtopic.php?t=503

If you're going to use it for other out of plane navigation you won't want to put in the wires. But I highly recommend you plunk some hot glue around that little battery. I almost lost a plane because it broke loose and shorted out something inside. I was very lucky though because the plane was still overhead. When I flew autonomous flights I would always enable the plane systems while flying away from or at least 90 degrees off heading for my first waypoint. Part of my test that it was underway was to watch it start turning as soon as I turned off the transmitter. If all was working well, it would turn right away and head out. This one day it wouldn't do that. It just kept flying off in a straight line. I brought it in and checked the GPS and it was off. I turned it back on but it went right back off. I went home and took it out and heard something rattling inside. I opened it and was able to solder it back in place then hot glue it to make sure those two tiny little leads didn't have to absorb all the vibration. I was lucky. If it came loose after it was out a mile or so I would never have gotten it back. I dodged the bullet but be aware of it. I now take every new one apart and do that glue on there and have had no other issues.

I've never been inside a 301 though. If you get one and do that, if you report back to one of us we'll be able to pass it along too. You'll need a number 6 torx bit to open it up. Make sure the two side center screws get in fairly tight. Power it up after you get it back together and tap sharply on its sides with your palm. The back has the battery holder built in. There are little metal tabs that contact the PCB to bring power to it from the AAA cells. If you don't get those two little screws tight enough it might lose power. If you tap on the sides and it stay running, you'll be fine. I also bring in aux power from outside the unit. 3 or 3.3 VDC are fine. A single LiPo cell is too much. I use a single LiIon cell and a fixed 3vdc LDO regulator. The AAA batteries go pretty fast by themselves. You can't run it in battery save mode because it refreshes data at a very slow rate. Only once every 10 seconds.

You can get after market OEM connectors from pfrank. http://www.pfranc.com/ Look for the flat one. There are little quirks to doing it that way. You have to make a GPS mount that accommodates that connector and helps cradle it to the GPS in the plane. That's why I like the servo wire mod so well. It takes the strain off the little connector. A factory OEM cable and plug is a lot more robust than the pfrank. But it's very high cost when you're going to chop most of the wire off anyway.

There's always a few good ways to skin a cat and nothing says you can't improve on all this info though.

Good luck.

Dan

[twinturbostang]

Thanks for the info Dan. I'm assuming the little watch battery is to keep waypoints and trip info in memory when the AAA batteries need replacing?

I will definitely want to ditch the AAA batteries though, and power it via a LiPo or something else lightweight, with voltage regulator of course. Do you know how much it weighs in this configuration (Geko 201)? And does anyone ever fly with these things naked (no case)? :)

[kd7ost]

I would have to weigh mine and don't have time this AM. Check out this link though. Here's a Slow Stick UAV from Bill Strong of Black Widow fame. Naked geko. :eek:

Dan

http://yb2normal.com/UAV_SlowStick.html

[CrashingDutchman]

Nice. I saw the wing leveler in one of the pictures. Can the RCAP2 work without this?

[kd7ost]

The RCAP2 is designed to steer the plane with rudder. If you use a plane with built in stability about the roll axis, (Polyhedral) you don't need roll control. There has been talk that the unit can also be used to turn the plane by connecting it to ailerons instead of rudder. Then you need an altitude control device of some sort. (A little more obscure at this point)

Dan

[Hovertime]

BTW I think one can use "track back" feature of Garmin units to avoid its "smart" way point selection. First you would need to create a track though, and am not sure if all units have this feature.

[twinturbostang]

I saw some GPS units on Sports Authority's website so I decided to go check them out in the store. Well, they didn't have as many in the store. I found the Garmin etrex but they didn't have any of the Geko's there. The etrex was pretty heavy too, something like 5 ounces. Not sure of it's capabilities either. Anyway, just a little FYI. The online store has more to choose from.