Beefchicken Industries


Obsolete GPS

Trimble Resources

Story Time

1995: My older brother, an avid outdoorsman, received a GPS receiver for Christmas. It fascinated me that this little gizmo knew where it was.

2001: I bought a Garmin eTrex Venture, which I used for geocaching and generally nerding out about my location. My obsession then moved to increasing accuracy, so I started to investigate DGPS, and internet DGPS feeds, eventually creating a 300 baud modem out of an XR2211 so that I could abuse a GMRS radio to send internet DGPS corrections to my GPS receiver in the back yard. With DGPS corrections, I was able to achieve 1 m accuracy, and I shelved the project at that point. I had reached the limits of my equipment and set the project aside.

2017: I learned about a project that used an undocumented debug mode on Garmin GPS receivers to extract pseudorange data from of the receiver in a RINEX file. Using that tool I was able to collect data for a long period of time, then submit that data to the CSRS-PPP service, and locate my couch within a 15 cm error ellipse. That was cool, but to get better accuracy, I apparently needed a dual frequency receiver. I didn't know what that meant, and my priorities shifted, so the project was shelved again.

2020: On a whim I searched eBay for a survey grade GPS receiver. I discovered that Trimble's GPS receivers from the mid-90s, the 4000 series, could be had for a very low price, so I bought one.