I need someone to explain this to me.

Blasting off with GPS

Launching model rockets is a good time, but more often than not, it’s hard to tell how high the rocket went or how fast it moved – both essential facts when bragging about your latest flight. [Chris] recently built a GPS-based altimeter for the USC Rocket Propulsion Lab, so that they could track the performance of their latest project. The circuit is based off a Picaxe 18x and uses a GPS module to obtain NMEA altitude data. Once the data is obtained, it is stored on an external EEPROM to be read back after the rocket has been recovered.

[Chris] unfortunately does not have any pictures of the board he built, but he has made his circuit diagram and source code available. He reports that the logger worked perfectly aside from a small bit of time where the GPS module temporarily lost its satellite lock.

If you are interested in reading more about flight data recording and telemetry, be sure to check this out.


  1. mrbrt says:

    That’s interesting. At first glance I thought the sattelite lock would have been lost due to the speed reached, as some modules are programmed to stop providing accurate data past certain altitudes/speeds so that commercial gps unit’s can’t be used for guided missile platforms.
    Looks like it was just a glitch on the way down though.

  2. Gummi says:

    Somewhere I heard there was a limit on the highest speed a civilian-bought GPS receiver was able to work with. Anybody that can confirm or deny this for me?

  3. Was the loss of sat-lock due to exceeding the altitude limit on some GPS devices? My impression was that there was an intentional limit on civilian devices to keep them from being used in guided missiles and the like.

  4. Ah, nope! It looks like it lost signal on the way down.

  5. Hugo says:

    This is amateur compared to this:

    Badger2 is a hobby project of some students at Cambridge University – a rocket control board that includes a software defined GPS receiver, to overcome the ITAR limitations (can’t go faster than 1000 knots or 60,000 feet, as well as some acceleration and jerk limits) of commercial GPS receivers. They basically sample the raw GPS signal (after amplification and downconversion) and run all the signal processing on a Gumstix module.

    Super hard core project!

  6. MS3FGX says:

    What is the point of using GPS if all you wanted was altitude? In my experience, the altitude returned via GPS is not terribly accurate, and surely the added complexity of establishing and maintaining a satellite lock is less than ideal (indeed, it gave them a little hiccup here).

    Unless it is that difficult to source small, high altitude, altimeter modules?

  7. meh says:

    If you rest your GPS mouse on your windowsill here in Europe your altitude data will be jumping +/- 20 m about every 5 sec. IMHO commercial GPS modules are useless for altitude tracking and even for positioning it’s not really useful if you need an accuracy better than 10 m. The public signal isn’t called coarse acquisition without a reason. If you need better accuracy you’ve got to build your own differential GPS, since in Europe you’ve got to pay extra for DGPS via geostationary satellites if things haven’t changed lately. Stupid EU should get going on funding Galileo. The costs sound immense but compared to 100 km highway it’s cheap as sh*t.

  8. o says:

    I have a GPS with no altitude limit. You jelly?

  9. Cultrocker says:

    Take a look at Altus Metrum project by Bdale Garbee. The project produces a control module that deploys a parachute for high power rockets. Chute deployment is timed by integrating acceleration and backed up by a barometric sensor and GPS. Its a tested and proven controller. It logs acceleration and air pressure at a high sample rate. It is designed as open hardware and uses open source software. The code is available under GPL v2 using a GPL toolchain. The code was written by Keith Packard of the X11 project.

  10. Koolguy007 says:

    @o pics or it didn’t happen.

  11. Colecoman1982 says:

    I’ve done a pretty good amount of reading on the altitude/speed issues being faced by amateur rocket and weather balloon enthusiasts. The way I understand it, the FAA regs state that a commercial GPS unit isn’t allowed to operate if it is both over the altitude limit and traveling over the maximum allowed speed AT THE SAME TIME. Unfortunately, the lower cost GPS units (ex. sirf-II and surf-III chipset based units) have their firmware written to stop working if EITHER situation occurs. No one seems to know if this was just in inaccurate implementation of the FAA regs by the manufacturers or an intentional handicapping of the lower cost GPS units but it doesn’t change from generation to generation so it doesn’t seem to be a programming bug.

    In the case of this particular project, none of this really matters. If you check out the telemetry graph on the project web page, you’ll see that the test rocket never even broke 20,000 feet much less 60,000. Also, a rough calculation of the speed based on the same graph values seems to suggest that it was only ever traveling at well under the 1000 knots (1687.8 feet per second) mentioned above.

  12. MattieShoes says:

    If you’re curious about the commercially available alternatives, yes they exist.

    Barometric altimeter units can be had for $75ish


    The MAWD actually records altitude readings several times per second the whole way up/down and runs around $100. I own one and I’ve got flight data for a few flights as well.


    The Raven is highly thought of among amateur rocketry folk — it has a barometric sensor and accelerometers

    For tracking/GPS stuff with rockets, I think BigRedBee is the best known.


  13. kraken says:

    just out of curiosity – isnt something that fast classified as munition under US law? That would mean, putting a public band gps receiver on it illegal.

  14. biozz says:

    i thought there was cap on how high GPS works for civilian use?

  15. TopFarker says:

    Uh, guys, uh, what about the, uh, limit, uh, on commercial GPS, uh, units?

    Can’t, uh, people read more than, uh, the post above, uh, them?

  16. Jonathan Wilson says:

    @Hugo, since it still has a blackbox GPS chip on board, there is no way to know if that GPS chip contains any restrictions.

    What I want to see is a GPS receiver built using NO GPS-specific chips at all.

  17. Whatnot says:

    I for one have 0 trust in GPS altitude data of a rocket.

  18. yetihehe says:

    I have -0.99999 trust in GPS altitude data ;)

  19. 1000100 1000001 1010110 1000101 says:

    While I was in the Aerospace Engineering program at Penn State, I had the oportunity to use the R-DAS flight computer by AED Electronics. While not perfect, it was very easy to use for someone who generally has no clue what is going on.


  20. Swankie says:

    makes no sense to use gps for height data. its inaccurate as hell. barometric pressure is perfect for this. especially since you can easily find out the starting height and the flight is of short durateion (so weather changes dont affect it).

  21. macegr says:

    They might not have had access to all the good GPS stuff we have now, since this project was done in 2007.

  22. biozz says:

    i did some research … the cap on all civy GPS units is 11 miles or 18K but limited to 515 metres per second …. still plausible for height of most model rockets

  23. Roscoe Rizzi says:

    If a hobbyist in the US wanted to do this with fuming nitric acid where could I find some? I understand the stuff needs to be handled very carefully, with proper safety and ventilation and not disposed of in an unsafe way.

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