Just started using and going through the MODGPS library, is there a straight forward method to obtain velocity data in the same way as lat,long,alt?

I'm flight testing my autonomous parafoil out of a plane. The first day of testing was to verify that the parafoil deployment system was functional. The deployment system consisted of an mbed that the plane's pilot activated with just an on switch. The program just waited 10 seconds for the pilot to throw the structure out of the plane. Than the system retracted the Firgelli PQ12 linear actuator with an H-Bridge Driver (SN754410 Quad Half H-Bridge). When the actuator retracts, it releases the support line loop that pulls the parafoil out of its chute bag using the drogue/deployment chute.

Now that the deployment system is proven, I can start putting some of my expensive flight electronics onboard. I would really like to get some velocity data.

Modgps currently only handles GGA and RMC sentences. It should be fairly trival task to add the VTG sentence to it. The only reason I didn't include it at the time was a) I didn't need it and b) my module was setup to only send GGA and RMC.

[edit: I just noticed RMC does contain velocity, but it's in knots rather than kph or kps. I could modify it to incluide that velocity data.]

I know it should be trivial but I'm having trouble figuring out how MODGPS reads the serial char stream. I searched every file for a getc() and obviously it's not used. So where does it actually read the characters output from the gps?

RMC is processed in nmea_rmc() withinn GPS_Time.cpp

It would be fairly easy to add both velocity (in knots) and track data (in degrees true) in that class.

in fact, I will add it into the library today as it's pretty trival and obviously of use. Just need to go get my GPS module out of the cupboard and hook it up. If you can wait that it :)

I'll see what I can do. Oh yea, I got a fourth UART working with the MAX3100 and your library without any problems. Thanks.

Thanks, I kind of wanted to ask if you could just throw it in for me, but you've been so helpful already that I didn't want you to think I wasn't willing to put in my own work. I really got a million things to do, like get the pitot sensor working with the mbed and calibrating it in the wind tunnel. And also adding as much to the LabView base station VI as possible before the next flight testing on Saturday.

Hi there.

OK, I started on looking at adding the extra data in the RMC sentence. However, I found that my GPS module sent null values in this sentence, so it wasn't much use. So I have now added the NMEA VTG vector speed and direction sentence handling. In the library, look at example3.cpp to see how to access this extra data.

Also, I have a slight problem. My GPS antenna is hanging out of the window going no where. So when I run my tests all the vector data is zero (as you would expect). So I have no real way of testing that I actually got this correct. So if you could test it for me and let me know all is well that would be rather useful feedback! Thanks.

Ok, I'll test it in a few hours after my exams. Thanks

Ok, it's been 14hrs and I haven't got it tested. My GM862-GPS doesn't output VTG by default. I have to send a command to the GPS serial port to enable it. And another problem develops.... The coolcomponents board for some reason that I don't know has pin 9 connected to 3.3v, and before I knew this I connected the rx pin on the gps serial port to pin 9 (mbed tx). So I have to hook up my pololu usb to serial to the GM862-GPS serial port and enable VTG.

I've also battled some problems with my RF modem today that "fixed itself", and now I'm also trying to figure out why my UM6 IMU/AHRS is drifting on the yaw angle.

Note, I just published a new version so you'll need to update (v1.14). I found that some NMEA sentences that contain null/empty fields weren't processed properly.

It's working. Going to drive around with it and log it to sd card.

Alright, here's the data I got thanks to your help from inside the plane during flight testing. My GPS doesn't support magnetic declination so I couldn't test it.

Looks good! Thanks for your feedback :)

Amazing !! Can you tell me what software you did that 3D mapping with please ?

Matlab Student Edition. I have never messed with kml Google Earth stuff, but the plot3 function in Matlab gets the point across. Just a warning before you look into Matlab, you could spend the rest of your life learning all its uses. Check out the realtime workshop and automatic c/c++ code generation.