GPS choice

[MilaP]

Hi everybody,

what are the parameters for a good choice of the GPS?

Thanks!

MilaP

[moon-dawg]

How much are you willing to spend? That is pretty much the determining factor.

Important considerations are the number of channels, WAAS capability, NMEA output on serial interface, waypoint capability, external antenna, map feature is nice to have.

I use a Garmin Etrex Legend (lowest price handheld with WAAS) as well as a Garmin GPS V for my model that uses an external antenna.

[MilaP]

Hi moon-dowg

WAAS is available only in North America?

[direwolf]

In Europe, you have EGNOS (European Geostationary Navigation Overlay Service), and it is compatible with the WAAS system in the United States. EGNOS is brand new, and is not yet fully operational. In fact, they just completed testing this year, and they started the initial run-up to full operation on July 28th.
http://www.essp.be/

-- Brian

[MilaP]

Hi Brian,

What can you tell me abaut the WAAS capability? I'm completely out of information about GPS :(


Mila

[kd7ost]

Here's some basic info on WAAS.

http://www.garmin.com/aboutGPS/waas.html

You really have to think about how needed this is for hobby applications. It's not really a large boon to us. It can gain you position accuracy of up to 5 times better accuracy than without it. Let’s think about that for a moment though. Without WAAS you might only get within 15 feet of a waypoint before position error is a factor. With WAAS, you can get "up to" within 3 feet of that waypoint before position accuracy is an issue. Now, think of winds, velocity etc. Most 200 mph or less UAV's are going to have a small track error due to corrections; GPS refresh rates, winds, airframe dynamics and control surface response times. You don't want radical corrections to keep you with inches of your course or you'll be porposing up down, left, right etc. Using WAAS or not isn't likely going to be seen in position accuracy in a flying UAV that isn't in a vacuum. Once you approach a way point and start to pass it 10, 25, 75 feet away or whatever, your GPS waypoint sequencer is going to call it made and cycle to the next waypoint. Imagine if you had your software set till you had to hit a waypoint within 3 feet before it would allow you to move on to the next point. Your plane might spend all of it's fuel circling right there before the waypoint is made good. With WAAS you can find the back of a truck. Without it, you can find the side of a truck. WAAS may be important for auto take off and landing UAV's where approach accuracy has to be tight. But if you're taking off and landing manually it's not needed. (I do have it in my GPS and use it though. Just because it's there) It doesn't make any difference when I fly if I use it or not in my 25 pound 70 mph plane.

Put a GPS in your car. Program in waypoints along a road you follow. Set it up as a route then go drive it. You'll see it sequencing to the next waypoint even if you’re passing one 100 feet away or farther. (Depends on velocity) Drive 60 mph to replicate your UAV flight. You will also see that unlike the human driver, the GPS does not anticipate corrections to keep you on course. It lags and can't tell you to turn until it sees track error. (When cornering)

This all having been said, GPS with WAAS does have much better altitude information. The AP4 and other devices use altitude from the GPS to keep about at the right altitude. It's better to use BP sensing for altitude hold though. WAAS isn't at all worthless. It is very viable. But I would like to underscore some of the viability for us. We don't need to make runway approaches in the fog. (FAA won't allow WAAS for that anyway) It's still not dialed in well enough for commercial aviation.

Dan

[shedao]

Here is a paper created by Ublox (PDF) that has a ton of details on the GPS system, WAAS, EGNOS and more:

http://shai.enemy.org/papers/Embedded/GPS-X-02007.pdf

You can obtain this paper from the ublox (http://www.u-blox.com/) website but it requires site registration so I thought I'd post it for those that wish not to.

[LukeZ]

Ian, that's a pretty useful document. Nice of you to post it for us, thanks.

[MilaP]

Well, the activity of the UAV I'm working for will be emplyed for demining, so the speed fly will be in the order of 10 m/s.

[kd7ost]

Wow, did I figure that right? thats only about 22.5 mph. Motor glider speed with pretty light wing loading. Very slow.

I guess we should back up and ask you what kind of budget you have? There are different means of doing this. They vary considerably in cost.

Dan

[MilaP]

Hi Dan,
I'm working for a firm that is employed in Aerodynamics of Road Vehicles, I'm a business engineering and I'm completely out of informations about fly and aeromodelism. They asked me to follow a project regarding an UAV that will be used for the demining, that's the reason of the slow speed (image processing). These kind of problems (autopilot, GPS and image processing ) is out of knowledge for us, so they asked me to looking for low-cost systems (the UAV will be a gift for poor contry)..

Thanks

Mila

[kd7ost]

That helps to know. I don't know what demining is though? Maybe that’s not important.

You need low speed and low cost. I would check out this company, but I don't think they export very far. You'll have to check. http://www.u-nav.com/

Are you familiar with RC systems used in the hobby industry? Futaba, JR, Airtronics etc?

I have a unit called a PDC-10. It follows the waypoints that have been put into a handheld GPS and set in a "route" to follow. You use a GPS unit with a NMEA output at 4800 baud and of course it has to have a serial output. The unit connects between your RC receiver and the rudder servo. If you have the unit disabled, it is transparent to your rudder input from ground station. If you enable it from a spare channel, it starts to follow your route. It won't do the job of flying the platform by itself though. It only induces yaw commands to steer. When you induce yaw, you will also get roll. You can use a device called a Co-pilot to get rid of the roll. https://www.fmadirect.com/site/fma.htm?body=Search_Results

If you use this unit and employ it on your ailerons, you can have rock solid roll control.

Altitude needs to be a little different. You can use Throttle as in conventional aircraft, or Elevator which generally works better in RC planes due to thrust stability built into engine mounting angles. But, It's best to use a Barometric sensing altitude lock. Devices like the AP4 found on e-bay will work and are by far the cheapest devices. But, the handheld GPS puts out new position data as slow as once a second, and has to take turns dealing with yaw and pitch. It won't keep your plane at a real tight altitude.

The BP device is dedicated to the task and responds all the time to keep your altitude position much tighter. That’s important when your mapping. U-nav (above link) has units that will do that job.

These three types of devices in unison are the cheapest way to auto fly and stabilize a camera platform in very few parts for your application. It doesn't fly the plane in coordinated flight. It fly's it in big flat turns keeping the Camera pointed pretty straight down. Instead of flying coordinated turns and then having to develop a stabilized camera platform. It keeps it simple, and big slow high wing planes adapt well to this type of flight.

There is a shortcoming to doing this if using Garmin GPS devices. They have a software routine called "Smart Route finding". It prevents your GPS from flying the waypoints as you put in exactly. The Older GPS12 will fly it correctly but it's big, old and heavy by today’s standards. You can work around "smart route finding" but with limitations. I don't know how the Magellan units work with routes though.

If you use the Pico nav, again from the above U-nav link, it uses a blind GPS receiver with a fast refresh rate. It will follow the waypoints exactly as you put them in. U-nav developed this device because of the "smart route finding" concept. (Something I consider not to smart by the way) If you use a Pico pilot with a co-pilot from FMA, I believe this is the most "bang for the buck" in your application. There are cheaper ways, but they won't give you the position accuracy and will be all over the place as far as camera placement.

You will still have winds effecting position accuracy and it doesn't matter if you use WAAS or any other position accuracy GPS system when flying at that speed. You will get blown off track a little, (and cross winds will do that no matter what) your plane will still steer to the waypoint, it just won't do it in a straight line. It will arc in depending on cross wind component and aerodynamics and speed of your plane. It will move on from the waypoint as soon as the software says your close enough to move on. The calmer the air and more quality of build in your plane, the closer and straighter it will fly its course.

Dan