One GPS Receiver for multiple transmitters

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Digit
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One GPS Receiver for multiple transmitters

Postby Digit » Sun Aug 06, 2017 5:05 pm

I'm new to the rocketry electronics scene and was wondering if there's a GPS receiver (BRB etc) capable of receiving data from multiple transmitters simultaneously or would I need multiple receivers? I.E. tracking multiple parts of a non-tethered rocket.

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Re: One GPS Receiver for multiple transmitters

Postby drew » Tue Aug 08, 2017 10:10 am

You'd need multiple receivers. Also BRB only makes transmitters, not receivers. That's if you want simultaneous tracking.
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Re: One GPS Receiver for multiple transmitters

Postby Viking » Tue Aug 08, 2017 12:28 pm

As Drew said you would need multiple receivers.
It is technically possible, but I've not seen anything commercially available that can do that.
If you're a DIY type then you could home-brew your own with Mesh networking using the RadioHead library. It's something I've been tinkering with for a while.... at a snails pace.
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Re: One GPS Receiver for multiple transmitters

Postby drew » Tue Aug 08, 2017 2:14 pm

This thread looks like it could be of use as well.

http://www.rocketryforum.com/showthread.php?141685-25-GPS-Solution
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Re: One GPS Receiver for multiple transmitters

Postby Happy Heyoka » Tue Aug 08, 2017 5:21 pm

Digit wrote:I'm new to the rocketry electronics scene and was wondering if there's a GPS receiver (BRB etc) capable of receiving data from multiple transmitters simultaneously or would I need multiple receivers? I.E. tracking multiple parts of a non-tethered rocket.


As Viking says "technically possible"; I am working on something of my own today just like that, so I'll blather away with the boring technical stuff and you can ask questions...

Most of the GPS stuff I have seen folks fly uses a GPS unit and a data transmitter that just sends the raw GPS data (NMEA 0183 format); that doesn't usually (from a standalone GPS) include anything to identify the actual instance of the GPS (eg: vs the one in the other piece of rocket). So you need an extra level of "protocol handling" - that might be to put the GPS transmitters on different radio channels (...) or to use a microcontroller attached to that GPS to "wrap" the data with extra details identifying that unit.

The radio thing is not trivial - some radio types handle multiple transmitters/one receiver ok (I'm using LoRa radios and they have a simple numbered addressing scheme). You still have to deal with things like not transmitting continuously so that the other GPS units can get some "air time". You could in theory use several simple transmitters and receivers all individually tuned (and a bunch of USB serial adapters).
Or, for bonus points, you could use a software defined radio receiver to decode all the different transmitters simultaneously - I will buy you a beer if you pull that off (i'll be impressed, it's in theory totally doable).

It all depends a lot on what kind of radios you want to use and what kind of hardware you want on the rocket and how much messing around in the shed you're prepared to do :D
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Re: One GPS Receiver for multiple transmitters

Postby Digit » Tue Aug 08, 2017 8:48 pm

Thanks for the lead Drew! And it sounds like you are up to some awesome things HH!

What you say with the instance of the GPS and not knowing which half of the rocket is which, what would happen to your radio if for arguments sake both transmitters were on the same channel?

This stuff really interests me, will you be bringing any of it to MARS by chance??
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Re: One GPS Receiver for multiple transmitters

Postby Happy Heyoka » Wed Aug 09, 2017 1:54 am

Digit wrote:What you say with the instance of the GPS and not knowing which half of the rocket is which, what would happen to your radio if for arguments sake both transmitters were on the same channel?


that depends. For example the radios I'm using send packets of data about 60 bytes long; so they buffer the incoming data (like GPS) and then sit and wait for a full buffer or a timeout (~500ms) to send the next one.
From the remote perspective, in the one-to-one scenario, it looks like a continuous stream - less dropouts which most of the GPS decoding stuff handles ok. Thing to note about packets from these radios is they're (squint a bit) kind of like the packets on the internet and have an address for the "group"; by default mine are all set to group zero and they can "hear" radios in the same group.

Other radios are less sophisticated and send characters immediately as their produced by the GPS.

Adding in two transmitters in this two scenarios will likely result in different kinds of effects: the simple radios, if they happen to interfere (transmit at the same time) will likely just result in garbage out the other end.

With the "packet" stuff, you get pieces of GPS conversation interleaved in weird ways (because GPS reports fit unevenly in the 60 byte buffer) so you get an odd looking GPS stream with reasonable looking but irretrievably mangled data.

You could do something like set the GPS units to report minimally and on different time intervals so there is less chance of a collision.

BUT you still can't tell which report belongs to which piece of the rocket with the plain GPS which is where some kind of higher level protocol comes into it... in your case that's not so much a big deal; you'd have several fixes and if you were just using a serial terminal you could go to each fix and find your rocket bits.
Bad news is that if you fed that to some tracking program expecting just a regular single GPS fix you'd get something odd, possibly undecipherable.

Several of the GIS apps I've seen do support several GPS streams simultaneously but through separate serial ports.

I'm using just a simple riff on the NMEA style:

Code: Select all

[23:20:22:323] $GPS-36.3233,143.4089␍␊
[23:20:22:323] $ALT116.9,0␍␊
[23:20:22:323] $SPD0.57,0␍␊
[23:20:22:323] $DAT2017,8,8␍␊
[23:20:22:323] $TIM13,20,21.00␍␊

that's after decoding on the receiver end and less the header - a "start of message" marker and a short rocket ID and then a checksum on the end - with as many of those lines as I can fit in the 60 chars.
Idea being that it's human decipherable worst case scenario.

Anyway it's that "rocket ID" that makes the difference with the interleaved data scenario (as opposed to the whole bunch of receivers scenario).
I wasn't so much thinking about multiple pieces of rocket as multiple rockets but it'd work for that.


This stuff really interests me, will you be bringing any of it to MARS by chance??

yes, hopefully. That's what I am aiming for.
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Re: One GPS Receiver for multiple transmitters

Postby SpaceManMat » Wed Aug 09, 2017 9:19 am

If you mapped the 2 mixed signals as a single path it would create a zig zag pattern going back and forth between the 2 locations. Would be messy but I'm pretty sure you could find both stages and could even separate out the data for the 2 paths back to where staging occurred. Not that I'm recommending you do that.

If you where to use the one reciever then at some stage you need to split the singal back into its parts based on an identifier of some sort. Not sure but something like GPS Splitter may be able to do the filtering, if not you are probably going to need to write your own custom application to filter the data. Then you would be able to feed the 2 separate signals into a suitable GIS program.
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Re: One GPS Receiver for multiple transmitters

Postby drew » Wed Aug 09, 2017 11:01 am

Actually thinking on this a bit more last night I think it's appropriate to mention that this IS possible with off the shelf hardware. IIRC Nic Lottering uses two BRB GPS transmitters for his high altitude attempts. Now I'm venturing into speculation here somewhat but IIRC he configures them to transmit once every few seconds on the same frequency. As long as you don't power them simultaneously they should transmit independent of one another and not "stomp" on the other transmitter's transmissions. And by configuring the ID string field on the BRB transmitter you could have one transmitting as TOP and the other as BOTTOM. A handheld with an inbuilt APRS TNC would then process the transmissions as two separate stations.

Also remember that transmit interval and store interval are two separate entities in BRB GPS transmitters. So you could still log your GPS position on board every second whilst transmitting your GPS position every 5 seconds.
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Re: One GPS Receiver for multiple transmitters

Postby Happy Heyoka » Wed Aug 09, 2017 9:31 pm

drew wrote: A handheld with an inbuilt APRS TNC would then process the transmissions as two separate stations.

Been a while since I played with amateur packet radio, but APRS is interesting. Reading the protocol description it seems that while it drops the connection oriented X25 stuff (=reliable transmission, like tcp sockets) it retains some of the other features - which IIRC includes some kind of collision detection back off algorithm (to stop stomping each other). Provided you didn't aggravate the situation by using really short transmit intervals you'd be fine.

Also remember that transmit interval and store interval are two separate entities in BRB GPS transmitters

I think that would be all you need.

Meanwhile, there's soldering to be done... ah, Wednesday, where'd the time go :?
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Re: One GPS Receiver for multiple transmitters

Postby vance2loud » Fri Aug 11, 2017 11:43 pm

depending on size requirements and budget, gps dog trackers could be an option.
http://www.dusupply.com/dog-tracking/garmin-alpha-dog-tracking/multiple-dog-systems.html I'm not sure if there is something similar available in Australia but a bit of google work should find out
Failing that it shouldn't be that hard to design a modular system that could do the job.
I thought about trying to make a club style system with a single display and 4 or so transmitters . I was aiming to get a compass bearing to each transmitter and the coordinates of the last received signal.
That system would have the display stationary so size wasn't originally a concern, how big (physical dimensions and weight) would people consider too big for a system like this?
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Re: One GPS Receiver for multiple transmitters

Postby Digit » Mon Aug 14, 2017 10:46 am

That's not a bad looking setup with the dog trackers Vance, I could look at using something like that. Size shouldn't be an issue. I could always strip the casing off and mount innards to a sled.
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Re: One GPS Receiver for multiple transmitters

Postby drew » Mon Aug 14, 2017 11:19 am

vance2loud wrote:I thought about trying to make a club style system with a single display and 4 or so transmitters . I was aiming to get a compass bearing to each transmitter and the coordinates of the last received signal.
That system would have the display stationary so size wasn't originally a concern, how big (physical dimensions and weight) would people consider too big for a system like this?


I think the one downside of a system like that is the high probability that a stationary base station for receiving telemetry data would most likely not get the final packet with the rocket location on the ground due to RF interference. I routinely find that when I fly GPS I rarely get the "final packet" from the transmitter until I get closer to it. The higher you go the less likely you'd get that final packet unless you're lucky enough for the rocket to land reasonably close to the launch site. Given that I think it makes much more sense to have your receiver portable.
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Re: One GPS Receiver for multiple transmitters

Postby Happy Heyoka » Mon Aug 14, 2017 3:48 pm

vance2loud wrote:depending on size requirements and budget, gps dog trackers could be an option.


those are very cool.

Failing that it shouldn't be that hard to design a modular system that could do the job.
I thought about trying to make a club style system with a single display and 4 or so transmitters . I was aiming to get a compass bearing to each transmitter and the coordinates of the last received signal.


I haven't forgotten that I said I'd send you the Android one I'm working on - life got in the way for a while there but I'm working on it again (flew it one the weekend). I started out with something like that dog tracker very much in mind.

vance2loud wrote:That system would have the display stationary so size wasn't originally a concern, how big (physical dimensions and weight) would people consider too big for a system like this?

drew wrote:I think the one downside of a system like that is the high probability that a stationary base station for receiving telemetry data would most likely not get the final packet with the rocket location on the ground due to RF interference. I routinely find that when I fly GPS I rarely get the "final packet" from the transmitter until I get closer to it. The higher you go the less likely you'd get that final packet unless you're lucky enough for the rocket to land reasonably close to the launch site. Given that I think it makes much more sense to have your receiver portable.


Coming around to the subject again: there's no reason why you can't have multiple receivers - I'd have to refer back to that APRS stuff again but I got the impression that was almost the point of it.

However you do it, you're not going to get that last packet unless you're really lucky with terrain... from "human height" you're just not tall enough (Fresnel zones etc). However with portable unit and an good idea of the general direction you can close the distance and that seems to be what people do (ie: sit and watch the computer while the flight is on and then jump in the car and go and get it later).

I like Vance's idea of a "club system"; at what level exactly you integrate that strikes me as the problem. At the basic telemetry level is problematic because if I'm flying 29mm and you're flying 75mm then we have pretty different space, height & range requirements.
So maybe the answer is a simple wifi mesh at the launch site - my lappy sends telemetry to the "big board" during my flight, your computer sends it during yours...?
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Re: One GPS Receiver for multiple transmitters

Postby vance2loud » Mon Aug 14, 2017 11:58 pm

I know what you mean about life getting in the way, I haven't had a chance to even look at flying at all this year.
I was thinking of a system with multiple receivers so the display would be stationary and a smaller single receiver unit for recovery.
I have the single receiver down to about 80x80x25mm so it will just fit in a shirt pocket.
How did your system perform on the weekend?
wish I could have made it there for MARS to catch up with you all.
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