I recently purchased a Globalsat ET-102 GPS receiver board off of eBay. It was a good deal at $3 for the receiver and $3 for the antenna. With shipping it came out to $10.80. When the package arrived there were two boards and two antennas. Guess he was just trying to get rid of them because he later offered a board and antenna for $3 total so I bought another!
A Google search turned up a product sheet on the ET-102. It’s a 12 channel SiRF receiver, whatever that means. The serial output is at TTL levels so it can’t be connected directly to a computer’s RS-232 port. I was planning to use it with a Byonics TinyTrak3 which will accept TTL levels. However, I wanted to test the receiver with my computer so I ordered an RS-232 to TTL level converter. These are available from many sources.
When the GPS boards arrived I connected one to my computer through the level converter. I like to use VisualGPS to check the GPS NMEA output. The ET-102 was only sending out gibberish. I reversed the TXD and RXD connections between the GPS and the level converter and started getting NMEA sentences on the monitor. It works! However, the GPS was only sending out the $GPRMC sentence. While the TinyTrak3 will accept $GPRMC sentences VisualGPS will not display any information with only that sentence. I wanted $GPGGA, $GPGSV, and $GPGSA sentences to get a good test on the GPS.
More Google searching turned up a product sheet for a Globalsat ET-212 GPS board. That datasheet had some NMEA input commands as well as the output sentences. So, I decided to try those commands to turn on the other NMEA sentences I wanted.
I have previous experience sending NMEA sentences to a Furuno GN-80 GPS. The only tricky part is you have to send a checksum at the end of each command. In working with the GN-80 I found a website that had a script to calculate the checksum for NMEA sentences. How convenient! The website was public domain so I copied it, modified it slightly and set it up at nmeachecksum.eqth.net so I wouldn’t lose the site just in case it went away sometime.
With the input commands and the checksum determined I just needed a terminal program to send the data to the GPS. I found a program called Realterm that worked great for this purpose.
So to turn on the $GPGGA sentences I sent “$PSRF103,00,00,01,01*25” to the unit and it immediately starting sending the $GPGGA sentences as well as the $GPRMC sentences. So, the ET-212 commands work! I sent the appropriate commands for the other sentences and I was in business.
(Since originally writing this up I found a program called SiRFDemo that makes it easy to set the NMEA sentences as well as tinker with the GPS.)
The GPS works great! I was able to receive 7 satellites with the antenna in the window. When I took it outside I was receiving 9 of the 10 satellites in view. Next was to try it with the TinyTrak3.
To use TTL levels you have to check a box in the TinyTrak configuration software. I set that up in the configuration but when I connected the ET-102 the TinyTrak didn’t show it was receiving GPS data. I thought I was just going to have to use a level converter but I read the TinyTrak documentation (helps sometimes!) and found the TinyTrak checks the GPS for TTL data when it is first powered up. I had not turned the TinyTrak off when I connected the GPS but when I did that the TinyTrak started receiving GPS data!