The ULTIMATE GPS rocket locator

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Viking
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Re: The ULTIMATE GPS rocket locator

Postby Viking » Tue May 30, 2017 1:08 am

I nearly bought some of those new PW modules but realised the 915 versions only have 27dBm PA and therefore 'only' 163dB max link budget compared to the 168dB(claimed) of the 95W's I'm already playing with.
Might still get some to see if they will work with a 95W in simplex... +27dBm tx with a -148 dBm rx.... 175 dB link budget ;)
Simon
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Re: The ULTIMATE GPS rocket locator

Postby strud » Tue May 30, 2017 8:32 am

Thinking about this a little more, the main issue is getting a packet from the rocket that indicates it's location within the radius you are likely to start getting packets again when approaching during the search.

So maybe the issue is that we should start increasing the rate of packet transmissions (and GPS updates) as the rocket gets close to the ground to increase the likelihood of getting the most representative location ?

eg increase rate of GPS updates and transmissions to say 5Hz below 2000ft, then back down to say once every 15s when on the ground to save battery power.

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OverTheTop
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Re: The ULTIMATE GPS rocket locator

Postby OverTheTop » Tue May 30, 2017 8:49 am

Good ideas Strud. I particularly like the lower rate on landing to conserve battery life.

The faster rate during descent is a little more contentious. You may get similar/better results by keeping the 1Hz GPS rate and dropping the baud rate on the telemetry. Not sure which would have the overall better outcome. Faster packets give more packets with the same probability of being received correctly, giving a better overall probability. Lower baud rate makes each symbol bigger (in time) and gives the decoder a better chance of not being affected by noise. Lots of variables to consider :(
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drew
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Re: The ULTIMATE GPS rocket locator

Postby drew » Tue May 30, 2017 12:35 pm

strud wrote:Thinking about this a little more, the main issue is getting a packet from the rocket that indicates it's location within the radius you are likely to start getting packets again when approaching during the search.

So maybe the issue is that we should start increasing the rate of packet transmissions (and GPS updates) as the rocket gets close to the ground to increase the likelihood of getting the most representative location ?

eg increase rate of GPS updates and transmissions to say 5Hz below 2000ft, then back down to say once every 15s when on the ground to save battery power.


In theory that makes sense but in practice from my understanding you only get a once per 10 minute slot to transmit data via the satellite modem. So I'm not sure how you'd be able to increase your packet transmission as you've only got a once per 10 minute window to transmit location anyway. Given that my expectation is that the ability to get a usable antenna orientation to ensure on the ground location transmission is crucial.

I guess as a backup a 70cm HopeRF radio could be used in tandem to continually transmit location data at say a 1Hz or 5Hz rate dependent on user preferences as there's no overhead other than power to limit transmission rate. That said I'd expect with a project that could use something like this having two or three 70cm GPS tracker transmitters for tracking would be smart. Considering the AeroPac 2012 flight reached 104,000ft and yet the sustainer was recovered 35 km from the launch site redundant tracking makes a heck of a lot of sense imho.
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Re: The ULTIMATE GPS rocket locator

Postby strud » Tue May 30, 2017 1:20 pm

Hi Drew

My comment on making more rapid transmissions as altitude approaches ground applies to a non-satellite system ie try and make the last packet you will get until within range again (ie close when walking towards it) more representative of final location.


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