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

Geo April 16, 2009 at 9:02am

In the C code it notes that the RMC head contains the following: UTC, UTC status ,Lat, N/S indicator, LON, Lon, E/W indicator, speed, course, date, mode, checksum>>> But in the datasheet for the 5Hz Locosys (Mediatek chipset) GPS it is actually:$GPRMC,UTC Time,Status,Latitude,N/S Indicator,Longitude,E/W Indicator,SpeedOverGround,CourseOverGround,Date,MagneticVariation,VariationSense,Mode,Checksum

the code is missing magnetic variation and variation sense.... Is there a reason for this or am I reading it incorrectly?

Geovanie
CrashingDutchman April 14, 2009 at 6:05am

does anyone know if a Parallax propellor plug is basically the same as a FTDI cable? If yes, how would I hook up my Locosys LS20033 to this cable?
The pin config for this GPS is:
pin1 - pwr (3.3V)
pin2 - gnd
pin3 - tx
pin4 - rx

propeller plug:
vss
res
tx
rx.

I am a little confused of what VSS and RES mean. The prop plug should be able to work with 5v and 3.3v. Do I still need to power my gps from an external source?

I appreciate your suggestions!
Tom in NOVA April 8, 2009 at 10:38am

Anybody has experience with using and/or interfacing the 14 Channel 10Hz GPS Receiver - Venus634FLPx (at Sparkfun) with this or any other project? Is the difficulty of interfacing compare with the LS20031? Thanks for any input.
Noth666 March 17, 2009 at 6:30am

@Dan: yes if it has an uart, but you need to check the voltages also as LS20031 etc are 3.3V.
It will output the position in standard gps sentences over the serial out so you can make/use a parser for this like done in the ArduPilot project.
For more info I'd suggest looking at the mainpage of the ArduPilot which is a uC controlling an airplane based on GPS data. You can get there by clicking on ArduPilot in the top bar of the site.
Dan March 17, 2009 at 12:19am

hi..I'm new in this. Can I connect 5Hz Locosys GPS to a microcontroller? I'm trying to detect the position of my small airplane and tell it where to go from the position received by the GPS. Like program it in a microcontroller. I hope this is not confusing.
Geo March 13, 2009 at 8:52am

Thanks for the feedback. Just in case others are wondering about the intermediate component between the RS-232 and the TTL logic coming out of the LS20031 GPS I used a MAX232 chip from Texas Instruments. This chip will convert the TTL logic into workable RS232 voltage.
Geo March 10, 2009 at 9:04am

LS20031My apologies it's actually LS 20031. I will look at my baudrate setting again, and perhaps try the real terminal software you used. Any other suggestions (now that I gave you accurate information) would be greatly appreciated. Thanks
3D Robotics

Chris Anderson March 9, 2009 at 7:07pm

Geo: LS3001? I don't know that one. Are you sure you're using the right baud rate?
Geo March 9, 2009 at 2:31pm

I bought this Lycosys LS3001 GPS and hooked the 3 pins in the back (TX, RX, GND) to a RS232 connector and I have a RS232 to USB connector (CP-US-03) from CP technologies and the fourth pin to a 3.3v power source so that I can use it on my laptop (laptop lacks RS232 connect). Does this configuration work? I mean the RS232 to USB because I have been having trouble trying to see anything out of the GPS. Although, admittedly, I am using the mini GPS software from sparkfun's website. I am new at this, any suggestions?
JimEli February 18, 2009 at 11:28am

For those having problems getting the Locosys LS20031 GPS programmed, here’s how I did it:

(1) Purchased a 3.3V USB TTL-232R cable (available from many sources).
(2) Powered the GPS via the 3.3V output on an arduino board.
(3) Plugged the TX/RX lines into the FTDI cable and followed the above directions, except my GPS was initially running at a default speed of 9600 baud.
(4) I got nothing but garbage on the terminal program until I ran a common ground between GPS, arduino and cable.

Worked like a charm. Thanks guys!

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