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Old Nov 05, 2006, 04:34 AM
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AndyOne's Avatar
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GPS and Video Overlay Discussion Thread

Please discuss the subject of GPS and video overlay in this new thread we should split away from the "Video goggles what do you use and your opinion" thread, to see the early discussion on GPS etc please read what's there...

http://www.rcgroups.com/forums/showthread.php?t=583886

We have dragged that thread a bit off topic and I think this subject needs its own dedicated thread here.

Andy.
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Old Nov 05, 2006, 04:53 AM
Oxford Panic
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This may be of interest to anyone writing a PIC program to extract GPS data system an NMEA 0183 device. NMEA 0183 seems to be the standard protocol for the data output from GPS devices and the document shows all the codes needed. The usual physical form of the interface is RS232 at 4800 baud although there are some that have USB interfaces.

http://vancouver-webpages.com/peter/nmeafaq.txt

Andy.
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Old Nov 05, 2006, 06:40 AM
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I've been looking at hand-held GPS devices and I notice that some of them derive the altitude barometrically rather than from the received satellite signal.
The question I have to ask is...
If the altitude is not derived from GPS, is it included in the data set sent out on the serial interface?

Andy.
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Old Nov 05, 2006, 07:15 AM
W4UAV
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Quote:
Originally Posted by AndyOne
I've been looking at hand-held GPS devices and I notice that some of them derive the altitude barometrically rather than from the received satellite signal.
The question I have to ask is...
If the altitude is not derived from GPS, is it included in the data set sent out on the serial interface?

Andy.
Thats a good question. I have a Geko 301 it has baro. altitude, but Im not sure if it still sends out the GPS alt it gets in the RMA string!
Mike
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Old Nov 05, 2006, 08:18 AM
Pull out early!
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I have the Etrex that's one step under barometric, but from reading the website it uses the barometric alt to "calibrate" the GPS alt at regular intervals. There's a chance this calibration only shows to the user interface and not the data stream, interesting question, have wondered it myself.

Dave
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Old Nov 05, 2006, 09:04 AM
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Years ago I bought one of the early hand held GPS's to use on my mountain bike. (text only in those days) The altitude information was wildly unreliable under the best of conditions. Errors of like - 30,000 feet!

Pete
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Old Nov 05, 2006, 10:17 AM
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I think your best bet for Altitude is going to be something on the lines of the zLog. Then somehow bring that data into the OSD. Currently MX put out a Way Point Sequencer in the UAV forum. This was for use with an OEM GPS unit and the RCAP. There has been some debate over baro altitude hold, but not much conversation moving forward.

The RCAP 2.5 was supposed to become a all in one solution for UAV navigation and Altitude hold. I'm not sure where that project stands anymore. Based off of that it does seem likely you could blend together GPS data and baro data into an OSD. If I remember right MrRC-Cam's OSD does something like this.
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Old Nov 05, 2006, 02:34 PM
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Andy , great idea !

I like this thread idea you come up with "GPS and video overlay".

It leads toward the direction I would like to go . It would be heipful to be able to fly A pattern with the use of GPS thru going to the different waypoints , but still be able to fly manualy , then automatically back to the next or nearest waypoint . (don't know if thats possible , technically or within my $$$ to buy it) Then be able to see all the information overlayed on A monitor .

This would be A good thread to see the equipment that people are using and where I can get it !

Jim
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Old Nov 05, 2006, 05:51 PM
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Quote:
Originally Posted by typicalaimster
I think your best bet for Altitude is going to be something on the lines of the zLog. Then somehow bring that data into the OSD. Currently MX put out a Way Point Sequencer in the UAV forum. This was for use with an OEM GPS unit and the RCAP. There has been some debate over baro altitude hold, but not much conversation moving forward.

The RCAP 2.5 was supposed to become a all in one solution for UAV navigation and Altitude hold. I'm not sure where that project stands anymore. Based off of that it does seem likely you could blend together GPS data and baro data into an OSD. If I remember right MrRC-Cam's OSD does something like this.

Assuming there will be a problem with altitude read-out is there a bare baro sensor that could be used to sense altitude which could be read independently and fed into the OSD.

Andy.
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Old Nov 05, 2006, 06:27 PM
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I e-mailed Garmin tech support a year or so ago asking about the barometric altitude and whether or not the corrected altitude was put out in the string. They said it indeed was. I donít have their reply anymore though.

I have a high end Garmin GPS (500.00+) at work that uses barometric sensing for greater altitude accuracy. You have to calibrate it at a known altitude prior to use. I can watch it change altitude +/- 10 to 15 feet while sitting still.

That's about all you can expect from a BP unit at our cost. There are manufacturing variations and tolerances in parts that cause any of them to have +/- percentage of accuracy. If your BP driven unit tells you it's at 348.3 feet high, you can be pretty sure youíre between 340 to 355 feet. GPS with WAAS is right in there with those numbers. The bigger issue may be refresh rates of handheld units.

My Garmin geko 201 using WAAS enabling provides me with 3 to 5 meters of altitude accuracy. The joys of WAAS.


Dan
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Old Nov 05, 2006, 06:51 PM
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I was thinking of getting the Garmin Geko 301, It seems to be the best one I can find for this application. 3 to 5m altitude accuracy will be OK as I wasn't planning to use it as an ILS. The update rate is 1s according to the manual on the Garmin web site. Until the technology allows a better spec for a reasonable price, I guess I can work with it as is. I assume there is enough calibration allowance to set the altitude you are at so you can read it to AGL which is far more use than ASL. It would though, be possible to build a zeroing system into the processor between the GPS and the video overlay device.

Andy.
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Old Nov 05, 2006, 07:11 PM
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Something to be aware of guys is that GPS units output altitude in the $GPGGA sentence. This is altitude in Meters above Sea level. Even if you select the handheld unit to read out in feet, the GGA still outputs it in Meters and thats MSL.

Some OSD units, like the one I use only read that out. We should probably get guys in here that know of other units. Ones that allow you to set it to 0 feet at your field, and read out in feet above that.

Dan
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Old Nov 05, 2006, 07:21 PM
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I think I will have to do some computation between the GPS and the overlay so doing a zero and units conversion should be easy.

Andy.
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Old Nov 05, 2006, 08:14 PM
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I use the Garmin propriarity NMEA sentence $PGRMZ (in feet).
I know where to find the sentance on baro vs GPS, I just have to find it... :-)

Hey Jet, here is some code for the BS2p
http://www.nutsvolts.com/~downloads/simple%20gps.bsp
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Old Nov 05, 2006, 08:21 PM
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The Garmin proprietary sentence $PGRMZ gives altitude in feet. But I can't figure out if it's Baro altitude or GPS altitude or both. Some places I read say Baro but then what does the number mean in the "Position fix dimensions" field? 2 = user altitude 3 = GPS altitude

http://home.mira.net/~gnb/gps/nmea.html#pgrmz

Jeff
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