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Old 07 August 2008, 08:35   #1
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Chartplotter for rocket recovery vessel

Hello collective wisdom

Shortly our rocket society will be launching scientific rockets (all-metal, 6-10 foot long, 8 inch diameter) from a Danish military shooting range on the coast and out over the North Sea. For recovery we have purchased a Zodiac Hurricane 440 to sail the necessary 10-20 miles offshore. But once there we also need to find the rockets, and for this we have a built-in gps-reciever hooked up to a radio transmitter in the rocket.

Now we need advice on which chartplotter to buy which best utilizes the data from the rocket. The plotter must be able to accept radiotransmitted gps-data and mark the spot with an "X" on the map. (We can build an electronic device to change the data from the radio reciever into any format demanded by the plotter).

The plotter should also update the mark as the target moves in the water and should of course be able to dynamically plot a course towards the object.

I would prefer it to be able to read owner┤s own maps also. And it should be detachable and usable on land as well since we also launch to lower altitudes at landlocked shooting ranges.

Any ideas/suggestions?

Thanks

RocketDane
www.mainstage.dk

PS Oh - and don't worry about safety. The ranges are of course closed for all air and sea traffic during launches.
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Old 07 August 2008, 11:38   #2
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Sounds rather interesting, I dont know of any marine gps that uses this kind of technology, although stolen vehicle tracking systems and surveillance are areas that might be worth a look at.
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Old 07 August 2008, 12:26   #3
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The easiest way to get GPS data into a chartplotter is using NMEA. Many GPS systems will already use this by default - what does your rocket GPS have?

The NMEA data input on chartplotters is just 4800 baud serial with a signal cable and ground. You will need to submit the data as waypoint NMEA sentences into the system. If you're not familiar with NMEA there's lots of web material on the subject but the actual tech standards books are expensive (but cheap compared to a rocket). The sentence only contains lat,long, name and checksum.

Within the NMEA waypoint sentence ($GPWPL) there is space for a waypoint name. On the chartplotter, I would guess that the previous position with the same name would be overwritten but you'd need to confirm that with manufacturer/testing.

You'll need to use the chartplotter facilities to navigate to the last point presented by the transmitter. If the waypoint is being updated while you are under way, I don't know if the chartplotter would dynamically update - you would have to test that.

For a suitable product, I would suggest the Garmin GPSMAP 276. It is portable with it's own rechargeable battery pack (5-10 hrs+) but still has full chartplotter functionality. Because it's portable, screen size is modest but it is clear and bright. You would have to use the Garmin charts though, but there isn't one on the market that you can load your own charts onto (except PC software type). This is because the chartplotters work from vector data, not raster based images,
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Old 07 August 2008, 12:57   #4
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Hmmm

Yes, I see that using the NMEA input to change the waypoint would force the chartplotter to recalculate. We could set it to refresh only every other minute or so, I suppose.

But can we use the serial port to input GPS-data this way? I would presume that the input was for the antenna, ie the chartplotters own position.

I would love to test this with some equipment before committing to buy. We are after all paying for this out of our own pockets or - when we're lucky - with sponsorships.

Thanks for the input.

RocketDane
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Old 07 August 2008, 14:09   #5
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Quote:
Originally Posted by RocketDane View Post
Shortly our rocket society will be launching scientific rockets (all-metal, 6-10 foot long, 8 inch diameter) from a Danish military shooting range on the coast and out over the North Sea.
fuk mee. wots de bettin dat wan ov theez fkin rokkits wil lannd onn dat nobbur rowg waivs hed

gud fing reely. dat twatt neads blowin upp. saiv mee a jobb.

garF
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Old 07 August 2008, 14:34   #6
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Quote:
Originally Posted by RocketDane View Post
Yes, I see that using the NMEA input to change the waypoint would force the chartplotter to recalculate. We could set it to refresh only every other minute or so, I suppose.

But can we use the serial port to input GPS-data this way? I would presume that the input was for the antenna, ie the chartplotters own position.

I would love to test this with some equipment before committing to buy. We are after all paying for this out of our own pockets or - when we're lucky - with sponsorships.

Thanks for the input.

RocketDane
/DSC
You will not be able to dynamically input waypoint data this way. You would have to reprogram the gps and that code is generally "locked". You should check with manufacturer's to be sure though.

What frequencies are you transmitting gps data on? What is the transmitter height? A half floating rocket's signal will have a horizon of about 1km or less. What is your receiving antenna for these signals?

You would be better off skipping the gps entirely and using something like a 121.5 Mhz homing signal with a yagi directional antenna. Range won't be great from a floating log to a boat no matter what.
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Old 07 August 2008, 16:35   #7
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You will not be able to dynamically input waypoint data this way. You would have to reprogram the gps and that code is generally "locked". You should check with manufacturer's to be sure though.
There was a thread about this using a Garmin GPS 152 and I think the outcome was that you could send waypoints to the unit (although in that case they were coming from another chartplotter). They might have to manually select the latest one for route though. Don't know if it would be different on a more advanced unit.

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Range won't be great from a floating log to a boat no matter what.
Good point. Perhaps they could use something like the SPOT satellite messenger which can be set up to send position signals via satellite every 10 mins or so. Might be a problem if it's stuck in a tin tube though.
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Old 07 August 2008, 17:01   #8
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GPS/beacon, frequencies etc

Quote:
Originally Posted by captnjack View Post
What frequencies are you transmitting gps data on? What is the transmitter height? A half floating rocket's signal will have a horizon of about 1km or less. What is your receiving antenna for these signals?

You would be better off skipping the gps entirely and using something like a 121.5 Mhz homing signal with a yagi directional antenna. Range won't be great from a floating log to a boat no matter what.
We use a 433 MHz data transmitter and I am sure we will get a fine signal all the way down from 30.000 ft ;-D After splashdown it will be a problem, no doubt, but movement in the water will be negilible. If we go to the last reported position before impact it should be possible to catch the signal from there for the last leg. I do not yet know which antenna the reciever will have.

We already have a beacon onboard for homing and triangulation. It is transmitting at 140 MHz, and the antenna is currently just a wire taped to the outside of the rocket but we are considering alternatives including balloons for flotation and visibility. If the "log" ;-) doesn't float then it would be hanging in the water in a string from the balloon thus righting it. An antenna at the top would be easy to pickup.

But all this still needs a chartplotter to work properly. I cannot be bothered doing a search pattern in the North Sea for something small and not very visible... But thank you all for your informed suggestions. I appreciate it and hope they keep coming.

Best
RocketDane
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Old 07 August 2008, 17:32   #9
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A straightforward laptop program would be sufficient, most would accept either WPL or TTM/TTL, ARPA type data.

Maybe try something like SOB @ www.digiboat.com.au, or Seapro @ www.euronav.co.uk

Try having a search for a few and pick one that suits best

Or if you want something waterproof, then a small Garmin/Simrad?raymarine plotter would all do the job. Download the spec sheets and double check that they will take the NMEA data, most will.

Good luck.

PM if you need help.
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Old 07 August 2008, 20:06   #10
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A straightforward laptop program would be sufficient, most would accept either WPL or TTM/TTL, ARPA type data.
I agree. As you've described you're only going to have 1 position to work from and that's the coordinates transmitted just prior to splashdown. The format of that data string is going to depend on your antenna/receiver combo. That is, no matter what, a chartplotter is not going to accept a 433Mhz analog signal. Why not just output to a text file and manually enter it into the chartplotter? You'll have plenty of time to enter those 2 numbers as you begin transiting the landing range. Not a sexy solution I know.

You might want to refresh a little on horizons and how the height of the antenna makes a HUGE difference in the distance you can transmit.
http://en.wikipedia.org/wiki/Horizon

Its not a question of movement of the rocket by currents. Unless you get your homing transmitter higher or get a much bigger boat with a tall receiving antenna, your 433 MhZ and 140 Mhz ranges are going to be almost uselessly short. You are trying to transmit on line of sight frequencies from a peanut to maybe a person. Even a tiny wave will block both signals. Guessing that your receiver is 2m tall, your maximum practical reception distance is ~4km. And if the sea is not nearly flat <1km is plausible.

You should have bought a helicopter in addition to the RIB

One possible solution is to use a vertical dipole antenna for receiving and "hang" it from a ballon or kite. This would give you alot more range. Would lose the directionality of a yagi unfortunately. If you could somehow do the same on the transmitter side (even just a few meters) the wave induced loses would drop dramatically and you'd have a prayer of actually having these signals be useful.

The chartplotter is not your biggest problem here IMHO. Its actually generating, propogating and receiving a useful signal.
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