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GPS trackers and large events

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Schuyler

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I'm fairly new to GPS tracking so please excuse my ignorance. I have a Big Red Bee 900mhz tracker that I will be bringing to LDRS this month. If someone else is using a 900mhz transmitter will they interfere with each other? How do we prevent this from happening? I plan to put the transmitter in the nosecone and I'm using shear pins which means I will have to turn it on before installing the pins. I don't want to transmit any longer than needed if its going to cause problems though. Thanks for any info.
 

Crash-n-Burn

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Your BRB has a three-digit key listed on it that pairs transmitter with receiver. This greatly reduces the chances of what you described from happening.
 

OverTheTop

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If they are both on similar frequencies then they can interfere with each other. If they are both on the same frequency then they could stomp on each others packets which might ruin someone's day. APRS or other protocols may be able to handle two different packet IDs if the repetition rates are different enough to get them out cleanly. I don't have detailed knowledge of the protocols unfortunately.

At Tx near somebody else's Rx (say back in the flight line area) may cause the Rx sensitivity to drop due to front end overload, effectively blocking its mates weaker Rx signal (perhaps from a more distant racked of flying rocket).

Down here in Oz we had a clash of frequencies at a launch that involved a couple of Altus Metrum devices on 435MHz. Since that incident we have instigated a frequency board at the RSO table. No problems since.

Also, don't leave your telemetry on the default frequencies and ID codes :)
 

Steve Shannon

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I'm fairly new to GPS tracking so please excuse my ignorance. I have a Big Red Bee 900mhz tracker that I will be bringing to LDRS this month. If someone else is using a 900mhz transmitter will they interfere with each other? How do we prevent this from happening? I plan to put the transmitter in the nosecone and I'm using shear pins which means I will have to turn it on before installing the pins. I don't want to transmit any longer than needed if its going to cause problems though. Thanks for any info.
Ask the launch organizers if they are coordinating tracker frequencies. Some do; some don't.
 

bigredbee

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As mentioned earlier in this thread, each pair of BRB900 TX and RX has a unique ID which prevents my receiver from picking up your transmitter. This is done by setting the Destination ID (ATDT) parameter to a unique value. Additionally, the hopping sequence (ATHP) is set to 5, and the identifier (ATID) is set to 3BEE. Lastly, each transmitter only sends a packet once every 5 seconds, at an offset from 0 determined by the "transmit slot" The default for that is 3, so you'll transmit at 3, 8, 13, 18 seconds after the top of the minute. All of these should help mitigate interference from any other BRB900 users, or anyone else using a similar transmit module (assuming they use a different hopping channel and Identifier)

I've never heard of any complaints of transmitter interference using these 900 Mhz models. However, I often hear complaints when multiple people use the same frequency on the 70cm version (hint -- don't use the default 433.920 frequency!)

Greg
 

ksaves2

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Ask the launch organizers if they are coordinating tracker frequencies. Some do; some don't.
I've seen a lot of large launches where there is a open sign-up sheet for volunteers to fill-in to help out. If that is straight-forward to do, should be easy to have a frequency list where one can post what type of units, frequencies etc.
Could be on the honor system. Person could state what day or days they are attending. Could include all trackers, RDF, 2 meter and 70cm Ham Bands/APRS and the ISM trackers. Just a thought to keep in mind if the Rf spectrum starts to
get crowded on the 33cm/900Mhz ISM bands. Would be easy to take a look at the list in advance and re-tune ones devices before arriving.

The only potential problem with an open list is someone surreptitiously messes with it to their advantage but me thinks the folks who attend major launches are a helpful bunch and could police themselves. Kurt
 

Schuyler

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Thanks for the replies. I feel much better about this now and I learned something new. :)
 
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