Section 7: Tracking the Tracker
Any amateur-radio based APRS beacons will transmit short bursts of digital data across amateur radio frequencies to a receiver. It is the job of the receiver (specifically a Terminal Node Controller or TNC) connected to that receiver to actually decode the position and telemetry data from the beacon. These short bursts are data are typically sent out on 144.39MHz in North America, and sound like "braappp". They are not human-decodable and will take specialized receiving equipment.
While on the ground, the ArduinoTrack typically is only capable of being tracked by a radio receiver, and has a range of 1-4km depending on the terrain and the height of the antennas. There are many possible configurations to receive the data, and one of the simplest is an all-in-one data radio such as the Kenwood TH-D72A, or the Yaesu FTM-400DR. Other solutions capable of receiving AX.25/APRS data can be assembled for far less cost, but are outside of the scope of this document.
Once the ArduinoTrack is airborne (or happens to be in near-proximity to an APRS Internet Gateway, known as an iGate), the data packets are received by thousands of volunteer amateur radio operators around the world, and are "gateway'ed" onto the Internet. Once gateway'ed, the tracker position can be found on websites such as http://aprs.fi/ by searching for your beacon’s callsign.
The beacon's position will be plotted on a map, and you can click on the beacon's icon to get extra data such as speed and altitude. You can also use the http://aprs.fi/ website to view and download the raw data packets, which will look something like this:
W0ZC-11>APRS:/192155h3724.74N/10008.47WO180/016/A=016391 3D6Sats Batt=7.7 IAT=6.7 OAT=0.3 Press=561.3 Burst=091895 Project Traveler flight 2015f
- The packets can be read as follows:
W0ZC-11 - APRS Beacon Callsign
- 192155h - Time the packet was sent, in UTC time, according to the GPS
- 37 24.74N - Latitude in degrees, minutes, decimal minutes format
- 100 08.47W - Longitude in degrees, minutes, decimal minutes format
- 180 - Current course direction in degrees (000 is north, 090 is east, 180 is south, 270 is west)
- 016 - Current speed in knots.
- A=016391 - Current altitude in feet above mean sea level
- 3D6Sats - The GPS has a 3D lock, and is tracking 6 satellites
- Batt=7.7 - Current battery voltage in volts
- IAT=6.7 - Current Inside Air Temp in degrees Celsius.
- OAT=0.3 - Optional external sensor wired into the I2C bus, consisting of a TMP102 temperature sensor
- Press=561.3 - Barrometric pressure in millibars
- Burst=091895 - The measured maximum altitude that the balloon obtained before bursting
- Project Traveler flight 2015f - The user-definable Status Message