GPS WAAS Update (Magellan Sportrak)

Today, we manipulate bits to resurrect a hobbled 15 year old GPS receiver. In the process, I learned about WAAS and s-records to successfully update a client’s  Magellan Sportrak GPS unit.

calc

waas_sats

GPS and WAAS

The first part of this project requires an understanding of the global positioning system (GPS) and the Earth’s atmosphere.

http://www.nstb.tc.faa.gov/RTData_WaasSatelliteData.htm

https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System

Magellan GPS

The Magellan GPS receiver used older WAAS data. Specifically, INMARSAT 3-F2 (aka. PRN 120) and INMARSAT 3-F4 (aka. PRN 122 AOR-W) were hard coded into the GPS unit. These ceased WAAS service and were not replicated.

You can verify which WAAS satellites are being used by entering the secret [menu][left][right][left][right] 03 menu.

S-Records

Directly updating the GPS firmware is required.

https://en.wikipedia.org/wiki/SREC_(file_format)

I created an Excel document to assist the firmware modification process: STPS-v1

A 7z compressed update file for only the Magellan SportTrak is available at: spor534-WAASco

 

Client’s Response

The client responded with the following

You did it!!  Your update works just fine.  SporTrak now shows WAAS when moving and WAAS AVERAGING when stopped.  I misquoted the # of satellites it is capable of receiving:  the screen shows space for 12 and I got 11 at one time this AM!!   Apparently this version does not show EPE except when it cannot view a WAAS satellite.  But I can tell from the displayed elevation that the precision is greatly improved.

 

Update: 29 August 2020. In response to an emailed question about line checksums, I revisited this project. Enclosed is a portion of my response.


I only modified the 6 GPS numbers. The GPS locations were kept the same, as an updated ephemeris should be (and was) received on startup.

Original:  7A00FFF1  78000040  830000B2  8600008C  8100008C  89000000
Updated: 8700FFF1  8A000040  850000B2  0A00008C  0D00008C  1E000000

Once you find the right spot in the hex file, this is an almost embarrassingly trivial change. But it’s worth the cost and use of the GPS unit.

It turns out that the Magellan does not use the checksums. Surprising but true.