GPS Telemetry

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This comes about as a project I've had in my mind for several years now. While this isn't for rockets (at this time) it is for something that flies. I purchased several large weather balloons over a year ago, and have been wanting to get a high altitude balloon up since the purchase, the only real hold up was a telemetry system to recover the payload or "capsule" as the ballooners call it. I could get a ham radio license and go that route, but there is a cheaper, easier and more versatile way to go. I've been aware of the Maxstream RF modems for some years. A post maybe 4 years ago on aRocket indicated a real range of over 40 miles line of sight. Hey, that should do the trick. The nice thing about these RF modems is that you can input RS 232 data at one end and receive it at the other end, just about like a long RS 232 cable. For a rocket, I think you could set up sensors to monitor about whatever you wanted, run them into a A/D convertor, then to the modem and gather real time data on the ground. Sweet... but I'm getting ahead of myself. This is for a balloon project!

GPS has some limitations for security reasons. An off the shelf GPS has one or both of two limitations.

1) Altitude limit of 60,000'

2) Speed limit of 999 knots

The reason for these limitations from the Department of Defense should be obvious. But they don't have to have both limitations, just one. As it turns out, the Garmin eTrex engine has the speed limitation but not the altitude limit. Of course a balloon isn't going to set any speed records, so I'm in business with my eTrex Legend. While browsing the DARK site the other day, I noticed Hans Olaf Toft had written software called GPSView to acquire, display and record real time NMEA serial data. So I had to patch my GPS into my laptop and see if the software would work. It worked, and very well at that. Now I just need that long RS 232 extension cable, the MaxStream RF modem.

I looked at two models, both on the 900 MHz frequency. The first was 100 mW, the other was adjustable from 1 mW to 1 Watt. The higher power unit was $100 more for the development kit, but I decided to go that way from the start rather than regret it later. Here's a link to the 1 Watt development kit.

The MaxStream RF modems seem to have almost endless uses for me, now that I've made the initial purchase of the development kit, no doubt I'll be adding more modems to my equipment locker in the near future. Mike Bennett has been working on a GPS/flight computer utilizing the MaxStream modems, with luck I'll be able to get some pointers from Mike on future work integrating the modem into a flight computer.

My MaxStream modems arrived the other day, so I went about setting them up for a test. For now at least, all I need to do is get GPS data software running on my laptop PC. I tried the GPS View software I mentioned earlier as well as Visual GPS. Try as I may, I couldn't get the software working with the eTrex GPS set on NMEA 0183 output. Odd, as they both worked when the GPS was directly connected to the PC. But I found some better software, and it would accept Garmin's output protocol, it's called GPS Trackmaker. What I like about GPS Trackmaker is that you can import image files of scanned maps or aerial photographs and calibrate them to the software.

Here is one of the radio modems connected to the eTrex GPS.

Connecting the GPS to the radio was very easy, the RS 232 output of the eTrex connects to a null modem adapter supplied in the development kit, and the null modem adapter directly into the radio modem. With everything hooked up and the GPS Trackmaker software running, I quickly had a basic system running that is able to real time track and record data from a high altitude balloon.

MaxStream includes a battery clip power supply. While this clip will plug directly on a 9 volt battery, I don't think a 9 volt battery will last very long transmitting at 1 watt. I had a battery holder for 8 AA batteries, so I hooked it up to the power supply adapter, now I had a mobile transmitter.

I had to test the system, so I threw the mobile radio modem into a box, started recording and hit the road in my car. I was only using the small whip antennas included in the kit, the stationary radio was indoors and the mobile unit inside a car, so I wasn't sure if I'd get much range out of it that way. I went out 2 miles, then headed back. Once home, I saved the track in the software and replayed the saved file. Very cool! You could even see what lane I was in on the highway! Nice straight tracks with no drop out of the signal.

Here is a screen capture of real time GPS position using GPSTracker and the radio modems.

In the above capture I used a portion of a digital map image and imported it into GPSTracker. The little blue pointer in the middle of the map is the current position. In the future I'll use a gray scale map so the pointer and track lines are easier to see. All I would really need to track is the information from the yellow box in the upper left corner, but it will sure make tracking easier with a real time track over a map.

I ordered a high gain Yagi antenna today. It's an 11 dB gain antenna, which is the most I can go and keep it legal, considering the antenna wire loss... There are higher gain antennas, but the power limit is an issue, as well as size of the antenna. Someone will need to hold the antenna out a window I suppose, so I needed to keep it fairly short and light weight. I'll need to get someone to help me test the system, one person can drive on a random course and I'll try to track him. Once the new antenna comes in I'll give that a try.

I'll need to make some boxes to hold the radios, and I'll need to come up with a power supply for the laptop computer and the base radio power supply. I have an uninteruptable power supply I had used in my old data acquisition system, I think I'll set that up in my Blazer for AC power.

I mounted the radio in a small project box. This will be the "flight" radio connected to the GPS. I should get about 4 or 5 hours transmit time from the battery pack, but when I fly this in a balloon I'll use lithium batteries for several reasons. First, lithium batteries perform very well at low temperatures, next they are very light weight and lastly they have 2 to 4 times the Watt hour capacity of alkalines.

Here's my new 11 dB gain antenna and the ground/receive radio now mounted in a project box.

At some point I'll get a friend to drive around in his car with the transmitter/GPS unit and I'll do some practicing trying to track him. But I wanted to get an idea of the ground range of the system first. So I set my GPS/radio outside in a box, loaded up the receive system in my car and went for a drive. First I went East, through and out of town, I lost the signal about 1/2 mile out. While it's not a big town, the building next to the radio has heavy metal siding, and there are grain elevators between the transmitter and where I was. So I wasn't overly surprised I lost the signal. At one mile out I stopped and tried pointing the antenna towards the transmitter, still no luck. So I went the other direction which doesn't have the building and town in the way, one mile out and I still had a good signal with the antenna just laying sideways on the dash of my car.

I stopped at each mile farther out, pointed the antenna out my window, at 5 miles I lost the signal. Anything beyond about 1/2 mile was not line sight, although the terrain isn't terribly hilly, but still not line of sight.

A few things I learned:

A good thing is that the receiver picks up a lost signal very quickly, once the antenna is pointed the right direction it only takes a second or two to have a location fix.

The Yagi is very directional, not only for what direction it's pointed, but also for polarization. While that's good for radio direction finding, I'm not sure it's going to be the best for this application. I can see it's going to be a real PITA holding the Yagi antenna out a window to track a weather balloon for 2 to 3 hours. While the high gain of the Yagi antenna is nice for maximum range, I'm not sure I'll need it until the payload has landed. While the balloon is in flight, I have a feeling most any antenna would work. Keep in mind my little 20 mW tracking transmitter in the Defiance rocket. I had the signal attenuated from 6 miles away, and this transmitter is 1000 mW. So I may buy a magnet mount unidirectional 900 MHz antenna for general tracking while the balloon is at altitude, then switch to the Yagi if/when the signal is lost. Ok, just ordered a mag mount 7dBi antenna... Next week I'll have to try that out.


I've made two balloon flights since I last posted and I wanted to fill in here what I've learned. The magnet mount uni-directional antenna arrived, and it works great. I used the roof top antenna on both my balloon flights, and it worked great from over 14 miles up! I did lose the signal a couple of times at about 60,000' when we drove the chase vehicle directly under the transmitter. That's to be expected though, as the antenna has a null area directly above and below it. Simply backing off a couple of miles cured the problem, and I never lost the signal again. On the second balloon flight we were able to actually drive close enough to the payload to see it land. Very nice I must admit.

While the balloon flights are easy and fun, I really want to develop this system for use in a rocket. The hand held Garmin eTrex is a great unit, but maybe not the best for integrating into a rocket payload. So I've ordered a new Garmin GPS 15 along with a Garmin remote antenna. This GPS module is an OEM unit, designed to be integrated into a manufactures system. There are some limitations placed on GPS units by the DoD. But the GPS 15 has only two limitations, first is a 6 g limit. A rocket will quickly exceed 6 g's at lift off, but as soon as the motor burns out the GPS should send data again. The next limit is speed at 999 knots (which is about 1,150 mph). But that limit only kicks in above 60,000'. So the unit should work well for most amateur rocket flights. Even if you did exceed 60,000', so long as you were under 1,150 mph the GPS would continue sending data. So I wouldn't think you'd have any problems up to 70,000' or so. And if you went higher above 1,150 mph, well you would simply lose data again until the rocket slowed. So in most flight profiles the only lose of data should be those first X seconds until motor burnout.

So that will lead me to my next series of tests. I'm making a new 5" nose cone to mount the new GPS and the MaxStream transmitter in. I intend to fly the nose cone as a payload in a future balloon flight. Then this Fall, I would like to fly a modest 5" rocket to test the system in. Perhaps targeting 15,000' or so with the flight.


Update: June 2008

I've flown a standard hand held GPS in two flights over the past year, but I wanted to get my Garmin GPS 15 working with the MaxStream radios. Here's what I found.

Both MaxStream radios must be programmed to 4800 bps.

Using the Maxstream development board, an RS232 shifter must be used. I used a Spark Fun shifter board, since the GPS 15 requires regulated 3.3 volts, I powered the shifter board from the same power supply as the GPS 15. My power supply is an LM 317 regulated to 3.3 volts. The MaxStream development board needs the null modem adapter plugged into it, then the shifter board plugs into the null modem adapter. I haven't looked at the MaxStream specs, but my guess is it might be easier to not use the development board, the GPS 15 would probably communicate with the radio directly. But I'm going with what is working...

From the GPS 15 only three wires need to be connected. The yellow wire from the GPS 15 goes to the shifter board RX-I. The red wire from the GPS 15 goes to a 3.3 volt regulated power supply positive, and the black wire to negative.

On the shifter board. VCC goes to the same 3.3 volt positive power supply as the GPS 15. The GND goes to the power supply ground. Again, the RX-I on the shifter board connects to the yellow wire on the GPS 15.

The version of the GPS 15 that uses a cable, rather than a mini flat connector doesn't have an antenna terminal that connects to anything I've found. I have no idea what's up with that. So I unsoldered the little antenna terminal and soldered my active GPS antenna wire directly to the PCB. It seems to working well and getting a good signal that way.