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!

Monday, October 4, 2010

2010 AfterBurn Report, Part 1 - Voice

The Black Rock Amateur Radio Association is a year-around resource for Amateur Radio operators in the Black Rock desert. But we'd be kidding ourselves if we didn't admit that Burning Man is the peak of "high season" for us. After all, what other event brings so many people -- and so much interest -- to the Black Rock desert?

So, in the spirit of the AfterBurn reports that Burning Man does after each year's event, we thought we'd write up a BRARA AfterBurn report. We'll do it in two parts: this part deals with our UHF voice repeater while part two will cover digital (APRS) stuff.

Great News: BRARA Gets a Mountaintop!

The great news for BRARA late this summer was that the Bureau of Land Management (BLM) approved our lease application for space on a mountaintop overlooking the Black Rock desert! This meant we were able to move our UHF voice repeater (444.175 MHz, PL 100.0, IRLP node 7249) from beautiful downtown Gerlach to a desolate (and RF quiet!) peak high above our beloved desert playground. I won't bore you with tales of humping a lead-acid battery and solar panel up the hill, but I will say that we now have a solar- and battery-powered mountaintop repeater ... and several very tired and sore BRARA volunteers who moved it there over the course of a very long weekend.

The reason the move up the hill was such great news for us is simple: altitude = range. UHF radio waves travel more or less in a straight line and get cranky when stuff gets between them and their destination -- in other words, they mostly work where there's line-of-sight. As any mountain climber will tell you, your line of sight is a heck of a lot better on top of a 1,000-ft peak than on top of a 50-foot tower.

Indeed, when we tried the repeater out right before Burning Man, it rocked -- it was handheld-friendly (think: less than 5 watts) from locations as far south as Gerlach as as far north as Black Rock Hot Springs (think: 40 miles north of Gerlach).

There Goes the Neighborhood

It rocked, that is, until the circus -- Burning Man -- came to town. To provide communications for the event, Burning Man set up a bunch of (legitimate, licensed, legal) commercial radio repeaters less than five miles away from our quiet mountaintop. Even worse, the Burning Man radio frequencies started a mere 2 MHz away from ours (the input to our repeater is at 449.175 MHz and the Burning Man commercial radios start around 451 MHz.) All that meant that our RF-quiet mountaintop was now a RF-noisy mountaintop. A very RF-noisy mountaintop.

The result of this noisy environment was our repeater started "kerchunking" the Friday before Burning Man. By "kerchunking" I mean that the repeater started randomly keying up but transmitting dead air -- that is, no voice signal -- for anywhere from a few seconds to a few tens of seconds. But of course, it wasn't random: it was caused by our repeater seeing interference near its input frequency and mistaking it for an input signal.

Ain't Nobody's Fault But Our Own

To be clear, this wasn't anybody's fault but our own. Our solar-powered repeater was somewhat hastily designed and pressed into service. It lacked two key things: (1) CTCSS ("PL") tone protection on the input and (2) proper RF input filtering. We actually designed it to have PL input protection but for reasons that are too embarrassing to go into here, it didn't quite work out the way we wanted it to. The result was that, if you keyed up on our repeater input with the proper PL code (100.0 Hz), everything worked great. If you keyed up with no PL code, or the wrong PL code, our repeater still triggered ... but then transmitted dead air. Doh!

We weren't able to add proper PL protection to the input in the 48 hours before the event. But thanks to the support and extraordinary generosity of Bob Nagel, WA6TLW (owner/operator of numerous Nevada voice repeaters and APRS digipeaters), we were able to get some high-quality RF filtering installed on the input to the repeater. This necessitated one more climb up the hill (on the first day of Burning Man, no less) to install the filter. The filtering definitely helped but alas, it did not eliminate the kerchunking problem. Thanks to BC, K7BRD, for hiking up the mountain with me!

But It Still Worked, And We Learned Some Valuable Lessons

Even with the intermittent kerchunking the repeater was still quite usable and got a lot of use during Burning Man. In addition to local users within Black Rock City we heard numerous IRLP calls come in and go out during the event. The phone patch even came in handy a few times as well.

Rest assured, we've learned our lesson, and we will be re-engineering the repeater to have proper PL protection and RF filtering before Burning Man 2011!

73 de Phil, N6TCT

Did you end up making use of the BRARA UHF repeater at Burning Man? Leave us some a comment telling how you used it and what your experience was if so.