




31 thoughts on “UPDATE: Roll your own GPS can now track twice as many satellites

    1. There are quite a few different methods of resolving position from GPS signals. The most common is SPS – standard positioning system – which uses a single frequency from multiple satellites. This is what most of your phone and car GPS units use, and it is accurate within about 10m. There is a military/government grade version, called PPS – precise positioning service – which uses two frequencies to perform ionospheric correction and gives an accuracy of around 1m when alone.

      You also have additional features such as Wide-Area Augmentation Service(WAAS) and ground based Differential GPS(DGPS) which uses a known-position receiver to broadcast in real time the correction to the satellites’ signals. This works within about 100km of the broadcasting correction station. Using the phase of the radio signal as a measurement of its own, you can get down to 4-10cm resolutions with these in higher-end kinematic and survey receivers.

      Contrary to popular belief and urban legend, the government is not trying to prevent you from accurately determining position using GPS by limiting the methods and technology you can use. Ever since 2000 the SPS and PPS have been basically identical for accuracy.

    1. COCOM is a US law. This guy appears to be in the UK. Maybe they have their own version? If it’s like the US one it just says he can’t export it. Then again, putting the design on a website probably qualifies as exporting. But are they even looking for that? I think this just shows how out of touch and unenforceable such laws are. Politicians are always trying to put the genies back in the bottle!

        1. I don’t know what being salaried has to do with enforcement of law. Besides, this is trivial to implement:


          Making your own gps device will be low on the priority list if you can hack together (don’t forget to post) something that can go 60k up and still have the energy to be going 1,200 miles an hour.

    2. “CoCom ceased to function on March 31, 1994″

      No men in black are going to bust down your doors for not limiting your homebrew GPS receiver. Toshiba directly violated CoCom in the 80s and it netted them right around ZERO punishment/retaliation/sanctions.

    1. Only if you happen to know exactly what time it is locally. Two satellites only constrain you to a 2D space, since all you can say is that you are x numbers of meters further away from one satellite than the other. Three then puts you on a 1D line. The fourth satellite lets you resolve true time, so that you can calculate absolute distance from the other three satellites, instead of only relative difference between satellites.

      So no, you need four satellites.

      1. To add accuracy, use 4 satellites and do the math with each of the 3 pairs you have to create a ” zone ” you are in ( including orientation of pitch, roll, yaw vs sat position ) and the error between the 4 locations is your error rate. Being in 25 meters is pretty good for ” comercial ” units ( have an iWay 250 that i got for $15Cdn due to charge issues ) and I have to hack that one day to find out why it takes soooo long to get under 95k error for position. 15 minuets on in a field on top of the car should get the error down to less than the 20m I get with it tracking 4 solid and ‘ knowing ‘ another 5 on the graph ( but not indicating that it’s using them to track ). Good work on this build in getting the error down!

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