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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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.

What code did you use to test your gps ?

I tried the one on arduino.cc that you pasted but it seems the gps is not connecting to any satellites, all I get is blank lines (after the lables).  

The picture in the thread showing a comparison of a few units was just a captured data stream saved to a file. GPSVisualizer.com will generate maps from that. In my own personal Arduino projects, I am using TinyGPS and I have used newsoftserial on a Pro Mini and on a Fio, as well as one of the hardware serial ports on both MEGA models (the now outdated 1280 and the the 2560). And I am absolutely getting good position data out of them. As noted in another thread or two, I am having some issues with getting 2 units to match close enough to bring a vehicle within a couple of feet of a beacon reliably, but within 15' is no problem. In the video I posted in the thread on getting fresh data, it was looping around nearby pretty widely but crossed within 6' or so in front of me twice in the short time I let it run. They work.

Did you confirm it was working on the USB adapter before cracking it open? You can test it with an FTDI cable by wiring it like I have it on the picture with the little breadboard. Just run your favorite terminal emulator at 4800-8-N-1 on the COM port (or whatever happens with USB on other systems; something similar). If you don't have an FTDI, you should be able to make the adapter that came with it work, either by soldering leads to it or simply taping your pins to it for a temporary test. I can try to get a picture of how to do that tonight; it's just a matter of thinking about what met up to what and making sure you have the adapter pointed the right way, pressing the pins against the adapter contacts and binding them in place. Because of the type of connectors and shape of the adapter, this works better that it sounds; good enough for a two minute test.

Also, please post a picture of your soldered leads.

I just got in another one of these a few days ago and to double check myself, I came back to this thread instead of looking at one of my other units. It's working fine, so my instructions are correct.

Oh yeah - I assume you do get the blue light?


It turns out that under a truly open sky, after being given a couple of minutes to lock in, relative accuracy within a couple of feet is doable if you are using one in a mobile bot to find another. Let me explain the "if"...

You will get some "chatter" that will make it appear that you are moving around in a small area. I have not ever had it return only one exact location over and over. But under open sky, that small area it will report is about 6'x 6' and if the unit is completely still and has no obstructions moving around it, the majority of the positions reported will be a single location. Anyway, within a 6'x6' area, just circling is likely to be adequate to physically encounter the unit with the other GPS.

As far as absolute accuracy, I can't say for sure that GPSVisualizer (which uses Google mapping) is 100% valid, but the tests I have done (like the parking lot picture in this thread) are pretty much dead on.


Sorry about this but it seems that the receiver works. The blue light comes on, when I tested it in the car I got data immediately, and when I tried it on Gpsvisualizer the dot was right where I was parked, as Borat would say "Great success!" 

I tested it 4 times at home for the first time when I got it wired up to the seeduino, twice inside next to the window but got no results, then I tried two times again holding the receiver on the edge of the balcony and still nothing. I thought that it was just my garmin gps that had bad reception from the balcony, turns out it effect all of them. 

Btw thanks for the GPSVisualizer.com !


They are pretty much the gold standard. You can equal the performance for less money, which is what this is about. But beat it? Maybe certain low end products but most of their stuff is as accurate as you can get. I can beat their price, but if a Garmin fails to register you are going to have trouble with just about anything.

It may never pick anything if it can't get satellites, but I have been surprised a few times by it deciding to go with what it had after a few minutes. When it does that, it will be reasonably accurate but jump around a lot.

It arrived !!!!! Ended up buying the one from amazon and getting a friend to send it over to Ireland. 

Just have to ask one thing, one pin on my receiver is split into 2, I know it is not used from your picture of the 500 but still wonder if it changes anything? 

The macro on the camera doesn't work great but you can see that the pin on the left is split into 2. 

No, it changes nothing. My picture was actually slightly edited because I had started to solder that pin (those pins?) to do a header and then decided to go with wires only to stuff I would actually use. When you hook power and ground up, you should see the blue light.

If you are interested, you probably found it anyway, but...


This won't always be good, of course.