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.)

You need to be a member of diydrones to add comments!

Join diydrones

Comments

Michal B February 16, 2009 at 6:23am

Yann: check NMEA reference

NMEA data

Explanation of NMEA sentences as used in GPS receivers.
Yann February 16, 2009 at 6:15am

Thanks Reto, I got it. It's just an XOR on received data...
Yann February 15, 2009 at 10:04am

Still about the 5Hz Locosys (Mediatek chipset) GPS...
Could anyone please explain to me how the checksum at the end of every NMEA sentense is calculated ?
This point is not covered in the data sheet.
Thanks,

Yann
Yann February 14, 2009 at 3:48am

Thanks for your answer. I did a cable that works for 5v. I'll have to modify it for 3.3v. I soldered the small PCB at one end and sunk it in epoxy. No big deal.
3D Robotics

Chris Anderson February 13, 2009 at 2:17pm

Realterm

If you can build your own TTL-level serial cable that works, my hat's off to you.
Yann February 13, 2009 at 1:59pm

Hi guys,
I just bought a 5Hz Locosys (Mediatek chipset) GPS.
Where do I get Realterminal to properly set the GPS module ?
Do I really need an FTDI cable or can I use a regular serial cable with level shifter ?
Thanks,

Yann
Ben February 10, 2009 at 4:49pm

Hi there. Fairly new to GPS and microcontrollers. When you mention the FTDI cable to initially program this what actually is this? Is it http://www.sparkfun.com/commerce/product_info.php?products_id=8772 or something else?

Thanks
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.
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.

of 5

This reply was deleted.