Ups and downs

Along with many other freelancers, I’ve been using the bank holiday season to concentrate on getting some work done. One of the major tasks on hand at the moment is to further develop my Colony project ready for the Platinum sharing event.

My first attempt at reading two GPS datastreams into an Arduino microcontroller over serial ran into some strange issues, so I decided to investigate I2C just to make sure I didn’t spend an incredibly long time barking up the wrong tree trying to make the serial version work.

Well, it’s been a steep learning curve and I’m truly indebted to the likes of GB, Ben and Adrian, but I we have now got the basics of some code working!

My coffee table currently looks like this:

3 RBBB microcontrollers, 4 breadboards, 2 GPS modules and lots of wire

Each of the GPS modules has a slave Real Bare Bones Board reading its data and sending longitude and latitude values over to the master RBBB when requested. The master RBBB reassemble then does a quick comparison between the two values to give a measure of difference.

I left this set-up logging the difference between the two GPS positions whilst I went out for an hour or so, and this is what I returned to:

The measure of difference (y-axis) plotted against time (x-axis, very approximately in seconds)

The kit is on a table not particularly close to the window in my attic flat, so there’s GPS reception, but I wouldn’t expect it to be particularly stable. That said, the graph above struck me as being very regular somehow: not as random as I had expected.

I’m wondering if the peaks correspond to the ebb and flow of the satellites overhead. Is that likely?

edit:Scratch that; wrongheaded. What would affect one GPS module and not the other?