GPS Part 2 - Success!

A project log for MagiLog: Open Automotive Datalogging

Developing a full featured datalogger on the cheap

nigelNigel 04/10/2018 at 06:430 Comments
I’m still not sure what the issue was but everything seems to be working now. I’m able to send configuration messages and have them change parameters. Now that the problem is behind me I can start digging into the workings of the module.

For use in MagiLog there are a few settings that we’ll need to set. We need to set the update rate to 20Hz, align the messages to UTC seconds, and increase the baud rate. App note AN0003 covers the binary format used to communicate with the Venus modules.

This is the overall structure of the message, it’s fairly straightforward and won’t be too difficult to work with. One thing to note is that each message needs a checksum number generated to be sent with the message. Since there’s only a small number of messages I’ll ever need to send it will be faster for me to just extract the messages with my Logic and export the values from the software.

Once I had everything configured how I wanted I set up the antenna and let it sit for a few hours. Here are the results I got.

As you can see the overall spread is sizeable. That said, the calculated accuracy is actually better than what is advertised, 1.77m vs 2.5m. This was in less than ideal conditions, so I’m impressed. By combining the GPS data with the accelerometer data, I think I can get that error down even more.

The last feature that’s worth covering is the 1 pulse-per-second (1PPS) output. It’s accurate to around 20ns and will be stable throughout a wide temperature range. I’m looking into using it as a reference for the other trigger clocks to keep all the data aligned. From what I can tell it should work and will make a lot of timing issues much simpler.