Friday, July 29, 2011

GPS Signal Reception

While I was working on Data Bus I ran across a few interesting behaviors in GPS signal sensitivity.

source: geocaching.com
You may recall that Data Bus V1.0 uses a Pharos iGPS-500 GPS module, based on a SiRF StarIII chipset. I got it for under $20 through a vendor on Amazon thanks to a tip from a forum on LetsMakeRobots.

First, I noticed that the closer the GPS device was to the chassis electronics, particularly the mbed microcontroller, the worse the signal reception. So I put it up on a mast about 30cm from the main electronics.

Second, I discovered that whenever I placed the GPS on a small sheet of metal, signal reception increased by 5 to 10dB for all satellites in view. I am guessing that the metal acts as some sort of ground plane enhancement to the metal underneath the ceramic antenna on the module. Feel free to offer explanations in the comments.

On Data Bus, I placed the module on a square sheet of tin about 4" square and consistently got really good reception: 7-9 satellites in the basement, for example. Of course the accuracy still suffered in adverse conditions. Such as, oh, I don't know... sitting on the starting line at the Sparkfun AVC?!

Third, I attempted to add a CMUcam V1 with an Arduino acting as a bridge between TTL serial and I2C and whenever I turned on these devices, the GPS reception tanked. Half the satellites could no longer be seen, the other half were so weak I could hardly get a 2d lock let alone 3d. The cause? I don't know. RFI/EMI? Or dirty power supply? Haven't researched that yet.

What I do know is that mounting a GPS sitting atop a 30cm mast on an 1/10th RC truck makes for some craptastic handling.

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