Using the 5Hz Locosys GPS with Arduino/ArduPilot

Sparkfun is now selling the excellent 5Hz Locosys (Mediatek chipset) GPS that Dean Goedde uses in ArduPilot. We're developing a daughterboard that will allow this 3.3v module to be a plug-and-play replacement for the 5v, 1Hz EM406 GPS that we're currently using for ArduPilot. In the meantime, if you want to start playing with a 5Hz GPS with Arduino here's some code and instructions to get you started.
The code is Jordi's new NMEA parser for the Locosys GPS. It's checksum verified, rocksolid, continuous, no delays, loop rate is about 32000hz. It uses pointers, so it's efficient.

You have to configure the Locosys for the first time you use it. It's very easy, you just need an FTDI cable. Be sure to connect the GPS to a 3.3 volt source. (We'll do this setup for you in the commercial version)

Instructions (please use the picture above as reference):

1-Run Realterminal and open the port @4800bps, you should see all the NMEA stuff, is not check the connections..
2-Now make sure you have checked the options marked with the green arrow... +CR +LF
3-Copy and paste this code: $PMTK251,38400*27 to the box indicated with the red arrow.
4-Then click the "Send ASCII" button indicated with the blue arrow..
5-Then the NMEA should disappear, and starting showing strange values. Go back to the "Port tab" (yellow arrow),
close the port, and choose the Baud "38400", and open the port again. You should the NMEA data again... =)
6- Then go back to the "Send" tab, and copy&paste this code: $PMTK314,0,1,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*28 then click Send, this should turn Off all sentences except GGA and RMC
7-Copy&paste now this code: $PMTK220,200*2C This should increase the updates to 5hz... (WOW!!!)..
8- Now you are able to connect the GPS module to Arduino, (only connect the RX pin to the TX of the GPS)
9-Then upload the code above, and open the port monitor, you should see Lat, Lon, Course, Speed, Altitude, Fix Position... like this:
46024504 8050998 312 0 519 1 (and yes you now know the exact position of my Swiss Chalet =P)...

If you want other setup codes for this GPS, you can find them in this datasheet.

Code explanation

If you'd like to understand the NMEA parser code a bit better, here are some of the library functions that it calls (all from the standard C++ library):

First of all pointers, which are very easy. They just give you the memory address where a variable is allocated in the ram: info
Then the strncmp(), better know as String Comparator. We use it to compare the header of the NMEA string "$GPGGA": info
Then strtl(), which means String to Long Variable. It converts a string number into a long variable: info
Then the strtok(). This is the string tokens, which looks for tokens in a string, for example commas ",", the ones used to separate values in the NMEA sentence: info
The atoi() will convert strings to integers: info
If you want to know all the available functions, see the library called "strings.h", "stdlib.h": info and info

(All these libraries are already installed in the Arduino IDE.)

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Comments

3D Robotics

Chris Anderson January 19, 2009 at 7:47am

We get lat, lon and bearing from RMC and altitude from GGA. All the sentences are described here.

NMEA data

Explanation of NMEA sentences as used in GPS receivers.
Howard Gordon January 19, 2009 at 7:02am

Chris / Jordi -

I was wondering about your choice of processing just the GGA and RMC messages. Does GGA provide everything you would otherwise get from GLL, and RMC provide everything you would get from VTG ? If this has been discussed elsewhere, perhaps you could point me to that resource.

Thanks
Howard
bGatti January 17, 2009 at 11:04am

The Arduino includes the FTDI Device.
You should be able to include this programming code into a simple arduino send command. (You /may/ still need the level shifting etc...) I've found some 3.3 device work fine on 5v, but I'm certainly not going to warranty the results - ;)
Claudio January 9, 2009 at 4:48pm

Hello Chris,

I bought the Locosys for my current thesis work on a tracker device for a vehicle, but I don`t have an FTDI cable, can I hook it up to the Arduino with some modification to the excellent code you have or do I have to configure it and there's no other way?, what type of FTDI cable did you use? I did some google on it and found several types, thank you very much in advance for any help given...

Claudio from Caracas, Venezuela
Ando Commando January 8, 2009 at 3:25pm

GPS lat/long resolution is only accurate down to around 3 to 5 meters generally speaking. It is possible for the GPS system to be more accurate, using DGPS, WAAS or LAAS, but you still pretty much can't guarantee any type of resolution lower than the 3 - 5 meters. Any receiver that was produced within the last 5 or so years should be as accurate as you can get for right now.
3D Robotics

Chris Anderson December 30, 2008 at 1:00pm

I'm afraid I don't know whether it would offer increased lat/lon resolution. I was only referring to course (directional) data.
Nathan December 30, 2008 at 12:47pm

So in your opinion do you think this GPS unit will be worth the money and will provides at least some increased Lat/Long resolution to make a good enough difference? and thank you Chris for your timely responses
3D Robotics

Chris Anderson December 30, 2008 at 12:32pm

I haven't compared them head-to-head, but the 5Hz updating would be a big advantage in determining true course.
Nathan December 30, 2008 at 12:26pm

Lat/Long resolution. I am making GPS navigation for an RC car and I want it to get very close to where I want it to be (one idea is running the bases on a baseball diamond). Also, if possible, a comparison between "true course" between the two because I will be using that to drive the car instead of a digital compass.
3D Robotics

Chris Anderson December 30, 2008 at 12:03pm

By "accuracy" what do you mean? Sat acquisition speed/number? Updating speed? Altitude resolution? Lat/lon resolution?

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