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Sunday, February 24, 2008

Some toys to play with

For the last two weeks I have been doing a lot of research on GPS chips and modules and antennas. There are lot of GPS vendors out there and each of them has its own flavor of GPS chips and antennas. So I have looked around and around and around.

I consider a small footprint very important and that is mainly what I have been selecting on since the current GPS technology is already so far advanced that there are not really any bad GPS units out there. Here are some of the main specs for most of them:

  • 12-20 channel position engine (>12 channels should be plenty)
  • Sensitivity: -155dBm to -160dBm
  • Typical start-up times
    1. Cold start: ~30sec-1min (that is from turn-on to GPS-lock without initial position data)
    2. Hot start: a few seconds
  • Most units have WAAS which is nice to have for improved positioning accuracy up to <2.5meters>
  • Interface: most units support both UART and USB. The iPhone uses 3V level UART (Rx/Tx lines)
  • Power supply: varies from module to module
Now, in order to keep our package small, we need a small module and we have to make sure that not too many external PCB board components are needed to operate the module. So the iPhone supply voltage of 3.3V pretty much determines what voltage the module should be (typically ~2.7V-3.6V)

Besides the GPS module, we need an antenna, and also here there is plenty to choose from. Most modules have an LNA (low noise amplifier) built in so we should be able to get away with a passive patch antenna only, as long as it is properly matched and not too lossy. Most patch antennas are about 25x25mm in size which in my opinion is just a little too big for the package I have in mind. So I have set my eyes on a smaller one. Experiments will have to prove if it is good enough or not.

In the meantime, I have ordered myself some toys to play with for the time being. I thought it might be a good idea to order a good module with an active antenna and an evaluation board to set a baseline I can measure the actual plug-in against later. See the picture for what I got (sorry for the quality, it was a close-up taken with the iPhone....): the GPS module at 30x30x8.5mm on top of its evaluation board. This module takes about a minute to start up and give a GPS-lock. I further adapted some code for reading out the NMEA data through the UART (COM-port) on my PC. The code should run on the iPhone too and my next goal will be to have the iPhone log some position data while I take it together with the GPS module on a trip.

B.T.W. This is of course not the module that will eventually be used for the plug-in. Its size of 30x30mm is simply too big. I have selected a more compact unit for that...

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