Aeronautical hand0-held gps data output?

[Thalass]

Hey guys. I've got a question about the hand-held gps units used with ultralights. One of my coworkers recently finished a vans rv-6a, and apparently his efis/ahrs system interfaces with his gps via a two-wire connection, and the gps just sends raw NMEA data down that, which the aircraft uses to position the flight director bars and display other information.

 

Now I haven't seen this myself, but the coworker who mentioned it is reliable with that sort of thing (He's my boss, avionics LAME, and he installed the avionics on the rv, so he ought to know). He also mentioned that one of the pilots flys some kind of ultralight, and that he wants a minimalist FD system for his aircraft. I think I can do it, maybe, but it all hangs on what kind of data the gps unit can send. I've only ever seen hand held gps units with PC serial or USB ports, not a NMEA port.

 

The vague idea I have in my head is a picaxe chip monitering the data stream coming from the gps (specifically the data from $GPXTE), and convert the distance and L or R components of that sentence into a binary figure, which can be sent to a binary decoder, to drive a series of LEDs. The middle LED means you're on track, and the LEDs on either side mean fly left or right. Possibly I could also have it sniff for $GPAAM, which is a waypoint arrival alarm, to flash a separate light too.

 

Of course I have no experience with picaxe, so my chances are slim - and it doesn't help that the picaxe forum apparently doesn't like me even before I sign up...

 

But anyway, have you guys, who have used aeronautical handheld gps units, seen such a data port?

 

Thanks :)

 

 

[Guest Andys@coffs]

Most Garmin units, at least the older ones Im familiar with have a setup option where you can choose if you want the output to be Garmin propriatory (default) or NMEA. My 295 drives my Autopilot wing leveler / Route follower and it uses NMEA sentences. It can be done

 

Andy

 

P.S the even older tiny handheld etrex legen map unit could speak NMEA (at a slower sentence rate), Most units update via a serial port (albeit RS232 or USB) so have the necessary underlying serial port so making it speak NMEA wasnt technically challenging for the manufacturer. if you go to the garmin support web site and download the pdf manuals for the gps('s) your considering you should be able to track down the NMEA settings and how you get to the raw data lines.

 

P.P.S the AP unit I have displays all those things you were looking to include in your design, in my case they do so via an alphanumeric display, but in any event the fact that it displays all that means its picking it up from the NMEA data because the AP doesnt have anything in it to generate that sort of data.

 

P.P.P.S playing with these small single chip micros with inbuilt interpreters are heaps of fun. I did similar things with the much earlier predecessors with radio control stuff, the best thing is that it encompasses both H/W and S/w design, the reality is that the H/W generally doesnt take that long but the software does but in incremental steps...As I said heaps of fun.

 

 

[Thalass]

Ah thanks! I don't have one myself, so I can't fiddle with the settings or anything. But I'll see if I can find the model from work and maybe I can have a go.

 

Mostly I've gotten about as far as I can go with hardware only electronics. Well, not as such but suddenly the old evil of software is starting to sound interesting. Pretty much anything can be done nowdays for little money if you have a decent chipset and the right code. And besides, I can use this knowledge for my own ultralight one day! haha

 

Its good to know it is possible. At least then I know its me when it doesn't work, and I can try again from another angle or something. Rather than it being physically impossible :P

 

 

[bushcaddy105]

A Lowrance Airmap 500 (and I presume all its siblings) quite happily outputs (via a single wire) an NMEA signal to drive a TruTrak ADI

 

Unfortunately, as of July 2009, Lowrance have given up the aviation scene to concentrate on marine GPS. A pity - their Airmaps are a good unit, simple to drive

 

 

[wanabigaplane]

GPS output

 

I know my Etrex and most Garmin products output the NMEA string in a serial format at 4800 baud, typically at 0-5 volt levels. I suspect just about all GPS receivers do. With the Garmin products, you can choose which string you want to output. Typically you get one string every one or two seconds, depending on the length of the string. All strings include the lat, long, and time. The difference between the remaining strings is just a function of their remaining content.

 

Now, if you want to play with this data, you almost automatically have to use a picaxe, or other microprocessor to parse what you want out of this string. If you want to send this data by radio, you need a modem (or tone encoder) to convert the ones and zeroes to tones. At the receiving station you need a similar modem (or tone decoder) to convert the tones back into ones and zeroes. Then you need another microprocessor to process this data and send it to your display device.

 

So, if you are still interested, and need further info, I suggest you get in touch with some radio hams who do this sort of thing for a hobby. They track mobile stations using exactly these techniques. Some accurate clocks just use the time part of the string. Floating buoys use the lat and long to record whre they are, and transmit their position by satellite so we know where to pick them up.

 

Jack. :):)

 

 

[Thalass]

I wasn't planning on transmitting any data for this, but as a geek and an avionics AME I am interested in that sort of thing in general. Getting my amateur radio license is on my list of things to do, up there with ultralight pilot's license and converting a bike to electric power.

 

Sadly the interested party doesn't seem to be interested any more. So there isn't much reason to do this until I've got my own ultralight (which may not be for years). But it has gotten me interested in picaxe so it's not all bad! hah

 

 

[Dieselten]

Most autopilots and "glass-screens" seem to prefer NMEA 0183 as their standard NMEA input. Various sentence-lengths are available with NMEA data output frm GPS. Try to put too much in a sentence and autopilots get very stroppy. We found that out with a TruTrak autopilot and an Avmap GPS. The reason for the preference is NMEA 0183 is the data protocol for transfer of information between marine electronic navigational devices such as auto-pilots, GPS, speed-log, echo-sounder, gyrocompass etc and, by extension, aeronautical equivalent equipment.

 

FWIW, my Garmin 296 won't output the correct NMEA sentence for an Aspen Pilot PFD, preventing the GPS info from being overlaid on the DG section of the Aspen. I'd probably need to go to a Garmin GNS430W to get the right NMEA output. Now a Garmin GNS430W and Aspen Pilot PFD total a tad over $US15K if you buy both new. Not in a hurry on this one.

 

My point is don't expect a handheld GPS to offer too much in the way of NMEA output. Only so much code gets written into the ROMs for any particular price-point. You get what you pay for - if you're lucky.