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!

So, look for N5NA-9 on findu.com!

6 Comments

Threaded

  • KC8OZA  
    I have 23 of them sitting here. :-) My first 3 were like $5.00 each with antenna, I made a best-offer at 2.75 a unit on the last 14, but by the time they made it to my house I was the proud owner of 23.
    I was banging my head against the wall trying to interface these things until I stumbled across your posting - I had totally missed the fact that these were TTL. So an hour later (thanks to a MAX232 chip from the parts bin) I'm getting NMEA into my computer, and lat and long that seem correct. So I think its working.

    That was one heck of a deal! Now that I've successfully gotten NMEA outta these, I'm going to pass around the good deal to some other local hams and maybe drum up some more APRS activity as a result. :-)
    #1
  • KC8OZA  
    Ok I've been playing with this more. Sirfdemo is totally cool. It made turning on and off various modes trivial (tell it how often to send, you're done). I've got everything I want (I think)coming out of the unit. This looks ready to go for APRS duty. Now I guess I've gotta drag it upstairs and connect it to my digipeater and see if it works. Not that my digipeater is mobile or anything (although that's another project).
    #2
  • N5NA  
    Yeah, SiRFDemo lets you get into the GPS and set some parameters you couldn't otherwise. I tried to figure out if it was possible to upgrade the firmware in the GPS but I don't know enough to do it.

    I just hooked one up to a TT3+, powering the GPS through the TT3+ and it works great. At first I thought I had a problem it was RF getting into the TT3+. After fixing that it worked perfect.
    #3
  • KC8OZA  
    I just finished testing all 23 of mine. 22 of them passed, one gives me nothing on the output. Maybe DOA maybe a baud rate issue. Either way I'm tickled. I passed 10 of these along to fellow hams, kept 6 for myself, and have a couple left that will find their way into the hands of more local hams. Sirfdemo by far was the easiest way to enable the NMEA sentences, and these things (most of them anyway) seem to get a GPS fix relatively fast, although a few of them took some time.

    Mine came with 2 kinds of antennas, some OEM thing with no name and some Trimble's. I kept the trimbles for myself. :-)
    #4
  • KD8IAY  
    I just picked up a couple of these off ebay also, what a great CHEAP little gps. I was tinkering with it, and through SiRFDemo you can enable the 2nd serial port on it. First I had to put the gps in Sirf mode, then the option to select Uart shows up. In there you can assign what protocols you want to go to each Uart. It shows 4, but I'm betting they only have 2 built on the board. :-P
    #5
  • titi  
    Ok I've been playing with this more. Sirfdemo is totally cool. It made turning on and off various modes trivial (tell it how often to send, you're done). I've got everything I want (I think)coming out of the unit. This looks ready to go for APRS duty. Now I guess I've gotta drag it upstairs and connect it to my digipeater and see if it works. Not that my digipeater is mobile or anything (although that's another project).
    #6

The author does not allow comments to this entry