Let's Make Robots!

very cheap GPS

I posted this as a comment on the cheap FTDI cable idea and it occurs to me that a lot of folks who might be interested will miss it there...

My cheap suggestion costs 4 times as much, but includes a free GPS. Currently (Dec 2010) there is a vendor on Amazon selling the Pharos GPS-500 with Microsoft branding (was packaged with Streets) for $19 delivered, including the USB adapter and a USB extension cord. It will come in a plastic case that says Microsoft on it. Carefully crack that open to expose the GPS. Lay it chip down with the connector toward you. The 2nd lead from the left is for 3.3v or 5v (I have had success with both), followed by GND, GPS Tx and GND. Wire those 4 and it will start spewing NMEA strings at 4800 baud. I am pretty sure that GPS Rx is 1st lead on the left. I don't know that it listens for commands, but that would be important if you want to use the USB adapter that came with it to make an FTDI cable. Just match up which would connect to what using the GPS pinouts and you are good to go!

Another poster on that project thread noted that using adapters that make power providing serial cables is not the same as true FTDI.

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This is fantastic information!  Thank you !!!

I wished I'd known this before ordering a $60 LS20031 :(  Which ended up DOA :(  Could've saved a lot of money...

Well, I have one of these on order now. I will try to remember to post up if I am able to send data / configs to it on the remaining pin. That *has* to be the RX pin, right?  It's a SirfStar III chipset, so when it arrives, I'll try sending known commands for the chipset...

Thanks again,


I have not used the Rx pin. The easy/safe way to test it would be to loop back on the USB adapter without the GPS attached - just clip a lead running from the one we know is going to Rx on the PC to the one we are pretty sure is going to Tx. Bring up a terminal program , type a character and see if it is echoed. Obviously I have not tried sending commands, but yes the GPS-500 is a SIRFstar-III. The GPS-360 is a SIRFstar-II. My most recent tests with the GPS-500 on open water went amazingly well. Given an open sky, I don't think you will find a more reliable and accurate GPS without spending a lot more money to get a high end Garmin or Trimble.

Anyway, I might get a chance to try the loopback tonight.

Sounds great, thanks!  Mine shipped today so should get here in 3-5 days.  I'm planning to using mine on a UGV for a robomagellan type contest.

I verified that the pin in question (leftmost, next to GND) is the PC's TX pin. Used an oscilloscope (fewer hands required vs loopback). Have not yet been able to get the GPS to respond to proprietary sirf III commands.

Which one? Beats me; that's what makes reverse engineering interesting. :) I don't know whether that will work or not, but it certainly can't truly need 2 ground pins, so it may be that one of them is a mode pin and tying it to ground puts it in "spit out data" mode. I tested another GPS that had that sort of pin.

I tried to find more information about the other pins the Pharos 500 but there isn't much online. I think that the Pharos Tech Support would likely email the pinout if you told them that it was for a hobby project and not a competing product. Anyways, here's the two links I did manage to find. [Link 1] [Link 2]

Hello, i'am new here and to electronics also ;-) I'am now searching for GPS receiver for less than 60$, and i would like to ask if someone compared mentioned here gps360/500 with FMP04-TLP GPS (25$ http://www.ohararp.com/products) based on MediaTek Single Chip Architecture (MT3329) and LS20031. I was thinking also bout spending little extra on FV-M8 (http://www.sparkfun.com/products/8266) but is it really that good as they mention on sparkfun? (9 sats indoors) Thanks for your time and help :-)

I saw it said 7 sats indoors.

I got a 7-8 sat 3d fix in the garage last night with the iGPS-500. I am able to get fixes not just inside but in the basement.

The San Jose module looks like the same chipset as the Locosys and the same software (MiniGPS from MediaTek) works on it, but the Locosys does up to 10Hz if that matters.

Although I've had bad luck with mine... the first one was bad, the second worked but never all that great, and I broke off the RF shield and have tried repair but never got it working as well as the iGPS-500.

Depending on what you want to do with it, the iGPS could do the trick fine. Plus if you can find the $18 deal on amazon that is hard to beat.  To me the big downsides are the fixed 4800bps comm and lack of configurability.

I haven't had a chance to test the other two pins that we suspect might be serial RX to receive commands. Even if one works as an RX, there isn't a lot that you can do, as far as I can tell, with the SiRF StarIII chips. The MTK chip on the Locosys is extremely configurable. Quite nice.

So no luck with having it respond to commands? Another possible avenue for investigation would be to buy it on eBay with the software (if you look hard, you can find pretty much the same price) and hook up someting to spy on the pin for data to see if they send anything to adjust set up. Sometimes proprietary protocols are very thin wrappers for the chipset, like a byte or two as a command to pass through the nexte few bytes as a chipset command. For my purposes, a reliable position updated every second or so is fine.

Haven't gotten that far yet. Just soldered two wires onto the half-pins and measured voltages (both show a voltage, incidentally). I had previously measured voltages on the usb serial adapter and found one pin grounded and the other not.  Have been up to my ears in trying to get my autonomous robot navigating, but will revisit this question in several more weeks after the competition is over and I've begun sleeping again. :)  I'm with you, a good position update every second at 4800bps seems to be entirely adequate.