School me on APRS (1 Viewer)

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Oct 1, 2008
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Okay, I think I have APRS down, but feel free to explain it in 1st grade terms just in case I don't. From what I gather, I can get a TinyTrak, configure it using a null-modem cable to my computer, then disconnect, hook it up to my GPS, and run a cable to my handheld. From that point, the GPS transmits the coordinates to the TinyTrak, where they are coded and sent to the radio. The radio broadcasts the signal, which is then picked up by, decoded, and someone can see where you are.

For the decoding, I understand that you need a scanner or receiver to monitor the freq, and then a patch code to deliver the sounds/codes from the scanner to the laptop. Software on the laptop then decodes the message into usable grid coordinates. If you're lucky enough to be transmitting your packets through a gateway, the tracking party doesn't need the scanner, just an internet connection.

My question (one of many) is that how does this work while I'm traveling cross country? The wife is planning a trip back to the States and I want to monitor her progress using APRS. When she's broadcasting the signal, I just don't get how the repeater knows how to pick it up or how the radio automatically moves to the next repeater. Basically, how does this work if she's too far away for my scanner/receiver to hear her transmission?

Also, is there a way to see which repeaters cover which routes so I can "tweak" her route based on where there's coverage?

Thanks in advance!
 
Joined
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You need to do more reading on APRS - LOTS on the internet to read.

Basically APRS uses a designated frequency and transmission protocol. It is mostly used on the 2m band - google and you'll find the frequency. Being 2m the range is somewhat limited - you won't be picking it up in another country (unless you are just across the border from each other). Due to the range, it relies on digipeaters to receive the transmitted packet and repeat it on. Digipeaters are often on top of mountains/peaks etc to provide long range receiving and repeating capability. Once the packet is digipeated to a gateway it will get onto the internet and end up in the APRS database from which several web based and pc based programs can query it to display the position on a map etc.

This is not a cell based system, possibly more than one digipeater hears your message and will try to forward it on. There is no guarantee your packet will get anywhere. If someone sends a packet near you at the same time, likely both will be lost or the more powerful signal will make it through.

Basically there are digipeaters in many areas and IGATES (that put the packet onto the internet). Your packets may or may not be heard by a digipeater or IGATE. If your packet is heard and successfully put onto the internet it will likely get into the APRS database. It might be received twice and the APRS software will toss duplicates (packets coming in too close together). There is a scheme to prevent packets being digipeated too many times or going around in circles etc.

Also, it will take more than a HT to get your packets out (in a semi reliable fashion) - you will need an external antenna on the vehicle you plan to install the APRS gear. Some areas have no digipeaters so there will be no APRS reception possible. Do also note that the packet header has your Call Sign - so you do need to be licensed...

There is a lot to read and it takes more than just buying a few bits and plugging them together and having a working system - this is Ham equipment, not a turnkey solution and we're all impressed when it actually works. i.e. I wouldn't rely/expect it to work - just happy when it does.

If you want turnkey and ease of use - you are better off buying a Spot unit and paying for the service to send tracking information. If your wife is mostly in cell phone areas, then you could get a cell phone based tracker that likely will use the gprs system.

cheers,
george.
 

e9999

Gotta get outta here...
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these digipeaters (no idea what that is), they all use the same frequency and same or no access code then?
 
Joined
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these digipeaters (no idea what that is), they all use the same frequency and same or no access code then?
Digipeater is basically a receiver/transmitter whose sole purpose is to receive you packet and then retransmit it with higher power and from higher elevation.

If you listen with a 2nd radio when you send an APRS packet out of the first radio you will hear the packet go out (modem sounding squack for a second or so and then quite and then you'll hear the packet again from the digipeater (digital repeater).

There is no access code - it is a free system. It relies on your packet having the correct protocol and in your packet header is your callsign - and that is used to locate your position on the APRS database maps etc.

Here's an example APRS webserver Login - Google Maps APRS

Type in KI6IBX (my callsign) and then select 24 hours and you'll see me appear on the map (I sent some packets out yesterday).

Enter in a callsign and it will locate the general position on a map from your registered callsign home location. If you have sent a packet recently with a new position then the map will center there and show the icon you assigned yourself (comes from your transmitted packet). Compressed packets are pretty short, about 15 bytes and that has your callsign, icon, lat/long/altitude/speed, checksum etc

Google is your friend to learn more :)

Edit: ok, I'm just sending a few more packets and will be heading out this morning for some errands and you'll see my track as I go (in the next 1/2 hour)

cheers,
george.
 
Joined
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Here at work we've been doing the local APRS for a few months now just as I described (usually with no external antenna on the HT). My main question was to how to get it up and running on the highway, I think we've got it down for local use. I like info though so I'll read whatever you have time to post:) Sounds like highway use is a crap shoot at best as far as reliability and getting your packets on the radio.

I have a satellite system that tracks the car and my family members, I was just interested in branching out and finding a solution that I can mess around with, test, try stuff, etc. I'll keep reading up and see if any new developments change the long range applicability. Thanks for the input.
 

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