Thursday, October 21, 2010

2010 AfterBurn Report, Part 2 - APRS

Welcome to the second of two parts of our 2010 AfterBurn report. Part 1 dealt with our UHF voice repeater. This part deals with our digital system, APRS.

APRS: A Quick Intro for the Uninitiated

APRS -- the Automatic Packet Reporting System -- is ham radio's answer to cell phone text messaging. APRS allows you to send brief text (digital) messages over your ham radio. APRS messages can be relayed from one radio to the next or they can be gatewayed onto the Internet, where everybody can see them. You send these messages by hooking up a little widget called a "TNC" -- basically an old-skool modem -- to your radio.

APRS is mostly used for reporting your position. To do this, you hook up your GPS to your TNC/radio, and every so often it broadcasts your position as an APRS message. That way you and your friends can all know where you are -- a handy thing out in the Black Rock. These position reports can also be seen by non-hams over the Internet, at web sites like You can also use APRS to send messages -- brief emails, for example, or weather data, or even tweets -- but mostly it's used for position reporting.

Just like you use a voice repeater to extend the range of your HT, a digipeater is a radio/TNC that extends the range of APRS stations. It does this by listening to APRS transmissions and then rebroadcasting them.


For quite a while the Black Rock Amateur Radio Association has maintained an APRS digipeater and APRS Internet gateway in Gerlach, APRS call sign "GERLCH," on the standard APRS frequency of 144.39 MHz. Like our old voice repeater (back before we moved it up to the high ground -- see Part 1), GERLCH's range was limited: if you had a reasonably high powered mobile APRS rig, GERLCH could probably hear you from Black Rock City, but it wasn't a sure thing.

So, when we got our mountaintop spot from the Bureau of Land Management, we also put up a digipeater up there -- APRS call sign RAZOR. RAZOR is well positioned to hear most APRS packets in the Black Rock desert and retransmit them so that everyone else can hear them. RAZOR and GERLCH can hear each other just great, so when RAZOR retransmits an APRS packet, GERLCH can then reliably forward it out to the Internet.

Enough Already, How'd We Do at Burning Man?

We rocked it! :-)

During the week of Burning Man GERLCH received about 55,000 APRS packets total -- way above average. The chart to the right (click on it to see a larger version) shows the number of packets received by GERLCH for the week before, during, and after the event. Nice spike, huh?

Of those 55,000 packets, GERLCH gatewayed about 70% of them to the Internet. Two-thirds of the packets arriving at GERLCH came came from the Black Rock desert; the remainder came from APRS digipeaters on other mountain tops in Nevada, like VIRGPK or SLIDE. Of the packets from the Black Rock, GERLCH heard about 40% directly over the radio and the rest it heard via digipeats from RAZOR or "BRC" -- a digipeater set up in Black Rock City by one of BRARA's Founders, Tzara, K1BRC. The chart at right shows the number of packets received by GERLCH broken down by call sign for the top 50 APRS transmitters.

Several people ran experiments with APRS this year. One of the most interesting was a weather station set up by Ted, KE6ROS, that broadcast its weather data via APRS. Watch this space for more technical details about Ted's weather station setup.

All in all, a very good year.

73 de Phil, N6TCT

If you have questions, comments, or ideas about how APRS could be used in the Black Rock -- either at the event or during the other 51 weeks of the year -- please leave us a comment!

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