HX1 APRS Transmitter Hookup Guide
MikeGrusin 
What is APRS?
APRS was conceived in the 1980s by Bob Bruninga (WB4APR), currently a senior research engineer at the United States Naval Academy and still very active in the APRS community. It filled a need for a simple way to transfer small amounts of textual data between multiple amateur radio stations without having to manually copy voice messages. APRS has since evolved into a rich, global system for transmitting location and sensor values that others can receive and use.
Receiving APRS Messages
Speaking of receiving, if the HX1 is just a transmitter, how does one receive APRS messages?
You can easily receive APRS packets with the proper equipment. An inexpensive 2-meter receiver tuned to 144.390MHz will receive the audio tones that make up APRS packets. You can then pipe the audio to a computer running the proper software to decode the packets. You can then use the information in the packets to display locations on a map, graph other data included in the packets, etc.
But you don't necessarily have to set up your own receiver. Many people in the APRS community have already set up receiving stations, called "gateways," that receive APRS packets and pipe the data onto the internet for aggregation by various websites. One of the largest websites is APRS.fi. It receives live APRS data from all over the world, displaying station locations and data on a map. (If you become an enthusiast of APRS, you're encouraged to set up your own gateway and join the network.)
What is a Packet?
An APRS "packet" is a short (25 to 200 characters), self-contained text message. APRS has strict formatting rules, with fields for various types of data (station callsign, GPS location, time, weather data, etc.). Because the formatting is standard, packets generated with one software package can be received and decoded by everyone else. Formatting details are available in the APRS Spec available at APRS.org.
Because APRS uses a single shared frequency (in North America it's 144.390MHz), everyone can see what everyone else is doing. By keeping the packets short and turning off transmitters between sends, many people can share the same frequency. Packets are often sent at slightly random times to avoid packet collisions (two people transmitting at the same time). If there are collisions the data for both packets is garbled. However, APRS systems are normally set to resend new packets every few minutes, guaranteeing fresh data before too long.
What Can I Do With APRS?
If you go to APRS.fi and look at the live map, you'll see the current locations of vehicles, aircraft, and sometimes high-altitude balloons. You'll also see fixed weather stations and other environmental sensors. These transmissions are all being made by people who have a personal or hobbyist use for various data (such as vehicle or balloon locations), or who want to provide useful data to the larger community (such as a weather station at their house).
What CAN'T I Do With APRS?
It's worth noting that since APRS uses amateur radio frequencies, the transmissions must abide by the rules and spirit of the amateur radio community. Specifically:
Amateur radio frequencies are solely for non-commercial use. You can use APRS for your own hobby or scientific use, but you can't use APRS for any part of a business. (Amateurs are very serious about this - radio bands are a valuable commodity, and amateurs regularly have to fight to keep what they have from being sold to commercial bidders.)
The data you send must be open. Since you're transmitting on an open system everyone can see your data, but you're not allowed to hide it by encrypting or obscuring it. Along these lines, it's encouraged that the data you send be of interest to the community - weather stations and other public-service data sources are always appreciated.