The topic for the week is the Automatic Packet Report System (APRS). However, we can’t jump off into a deep topic about APRS without covering some basic information about digipeaters and packet data.
While digipeaters and APRS go together like shoo-wop-shoo-waddle-waddle-dippity-dingy-dong, digipeaters and packet data are a stand-alone. Packet radio is digital data transferred by radio frequency. It is transferred by an originating station to a destination station using a digipeater. The originating station is connected directly to the receiving station.
Digipeaters differ from duplex repeaters in two ways. One, they can operate on any frequency the digipeater operator chooses, though there is an ARRL suggested band plan. And two, they receive data, store it internally, and then retransmit on the same frequency, sequentially.
APRS is a tactical communications network that transfers packet radio data using a combination of digipeaters and the internet to get the job done. It was invented by Bob Bruniga, WB4APR, in the 80’s. For a little perspective, the first version of APRS was invented on the Apple II in 1982. A more "advanced" version came along in 1984 for the Commodore 64.
APRS is a large network. Packets move through the network from digipeater to digipeater. All stations in range of the frequency receive the packet, but it will only be transmitted the number of times the originating station specified in the original path. The packet’s path specifies the type of station able to re-transmit the packet and the number of times it can be digipeated. As each digipeater receives and re-transmits a package, it inserts it’s callsign into the path. The significance of the previous statement is: you can tell how many times a packet has been transferred by seeing how many callsigns are in the header information. This is considered local area coverage.
The true worldwide capabilities come into view when the packets are transferred to APRS Internet Servers and Internet Gateways. The APRS Internet Service (APRS-IS) is a network of worldwide servers also connected to APRS stations called Internet Gateways (iGates). Eventually, most packets are heard on iGates and from there to the APRS-IS, where the data is collected, stripped of duplicates and extemporaneous information and redistributed to other servers in the network. Computer programs connect to the APR-IS to retrieve display information such as station position or objects. This invisible internet backbone is what makes APRS the worldwide conglomerate it is.
Of course, in order to see worldwide data, you need software. Want to take a test run? Take a look at www.APRS.fi and take a world tour. Here you can see mobile hams, click on objects to see their information, turn layers on and off, or take a mini-vacation to Hawaii, Europe, or South America from the comfort of your ham shack. No mosquitoes involved!
Software can be downloaded to work on laptops, cell phones, notebooks or any other type of device. These interfaces work on Windows, Macs, Droids and Linux. Some are free, some are not, and some are open-source – meaning, if you’re one of those Super Geeky Programmer Sorts, you can tweak their program with you own code. Here’s a few to check out: APRS.FI, APRSISCE/32, OPEN APRS, and APRS DROID.
You can transmit any number of things via the airwaves. Location information is what APRS is known for. And for good reason – it’s a slick process. APRS uses GPS satellites to ascertain position and telemetry information and then uses the radio to broadcast it to the masses where it can be displayed using APRS software.
The software itself can also be used to create objects. Objects are created by users and are the representation of a geographical object such as club meeting locations, repeaters, weather stations, etc. When other users click on the object, they are greeted with a small amount of text information pertaining to the object, such as an address and meeting time, or the repeater frequency and tone.
Also part of the APRS packages is two-way text messaging and emails. Unlike beaconing information that is broadcast out to anyone tuned to the appropriate frequency, texts and emails are addressed to specific stations. This is standard packet radio -– there is an error checking method involved. The receiving station will acknowledge receipt or the sending station will re-transmit until it receives confirmation or times out.
Anyone who has played around with Google Earth knows internet mapping products have become more and more sophisticated over the years. APRS mapping software is no different. National Weather Service Radar layers are available at the click of a button. Created layers can be exported from other mapping software packages such as ESRI ArcGIS, Delorme XMap and Google Earth. Some high-end GPS units will export way point files. YOUR MILEAGE MAY VARY! Each software has it's own set of acceptable file formats, so read the fine print.
Well, that’s pretty much the technology and software side of things. Let’s talk hardware. Even if you’ve decided your goal is to run APRS on your mobile rig, start by setting up your home station internet gateway.
Home APRS stations require a display device (computer), client software, Terminal Node Controller (TNC) or sound card software, and an interface to your two-meter radio. TNCs encode and decode packet information. If you own a radio with built-in APRS support, it already contains a TNC. Just connect the radio directly to your computer. If you’re radio does not have built-in APRS support, there are other options that work with any mobile radio. You can buy an aftermarket TNC or you can purchase software, such as AX25 Soundmodem , that uses your existing sound card as a modem.
For mobile APRS, you need GPS, a radio, the internet and display capabilities. Again, a radio with APRS support is the easiest option. The TNC and GPS are wrapped up into one tidy, compact, slightly more expensive package. However, if you would prefer to use an existing transceiver, there are some cheaper work-arounds. You can buy an add-on unit specifically for use with existing radios. These gadgets combine the TNC and GPS portion of APRS and plug-in to the radio through the microphone and speaker sockets. All you need is your phone or a laptop in your vehicle to display.
I hope this helps you along your way to setting up your APRS ham station and iGates. For more information, check out Bruniga’s website at www.aprs.org To do a little snooping and playing, checkout APRS.fi.