APRS, or Automatic Packet Reporting System is a realtime tactical information system used over Amateur Radio. See aprs.fi for a view of the data that goes through this system.
Most APRS data packets contain GPS positions of vehicles, boats, or (outside the UK) balloons! APRS is also used for Weather Station Telemtetry, as it allows for relaying data to the internet from remote locations (through RF=>Internet iGates) via a many-mile RF path.
For mobile APRS I use a netbook running Linux AX.25 soundmodem software along with a couple of beacon scripts that I have written, these interface with a USB GPS dongle through gpsd. The netbook is connected to a 5W/25W FM transceiver, with a 50W Amplifier often used as well.
I also have the capability to use the GPS beacon scripts with the Aprx digipeater software, allowing full mobile digipeater+igate capability with an accurate and realtime position on the vehicle, interfaced with both RF and APRS-IS transmission paths (APRS-IS through a tethered mobile phone).
- Beacon Scripts - github.com/thecraag/aprs-scripts
Using a Raspberry Pi in place of the netbook is a better fit of computing power. The Pi will still run the full soundmodem + aprx stack, but with massively reduced power consumption. Also we can interface a GPS directly to the UART, bypassing the USB interface that has been problematic for my mobile setup.
Compact APRS trackers tend to be rather expensive. I'm looking for a cheap way to build a very compact tx-only device that I can stick in the car and forget about. So far my idea is to use a uBlox MAX-6 GPS like I have in the Raspberry Pi HAB Payload, along with an Atmel AVR microprocessor.
Great work has been done by Darkside VK5QI, in his APRS HAB Payload Micronut. By substituting a 144.800 MHz TX Module, we'd have a high gps performance, low-power, compact UK APRS tracker. I'd really like to use the ATMega32u4 for this, as used on the Arduino Leonardo boards, as it has an integrated USB interface that would allow easy runtime configuration from a laptop or PC over USB (for setting callsign, tx interval, path, etc), without disconnecting the GPS.
For slightly more than the price of the Radiometrix TX modules usually used, we can get a cheap 5W Baofeng handheld off ebay. This'll give us 5W of TX power, as well as the opportunity of RX capability at a later stage. For now a simple wire from the receiver could be used to know if the channel is free before transmitting the beacon.
I use APRSdroid on my phone (running Android 2.3.7). The App supports TCP, HTTP or UDP internet connections as well as AFSK TX through the phone speaker or headphones.
AFSK TX through a Handheld
I have successfully used APRSdroid in AFSK TX mode with the Audio Output of the phone wired to Mic In on my Wouxon handheld. With VOX enabled on the handheld, 200ms of TX delay was enough to open the TX and successfully transmit the packet at 5W. This was often digipeated by MB7UW but sometimes picked up by G0PEB-1 on the Isle Of Wight directly.
Unfortunately after using this setup for a few days with the phone and handheld separated, I put them both in the same pocket on my rucksack. The setup transmitted one packet and then lost GPS lock. Since then the GPS performance of the phone has been severely diminished, often failing to get a GPS lock even with a great view of the sky. I believe the 5W transmit from the antenna on the handheld was enough to fry the LNA on the GPS, drastically reducing the sensitivity of the GPS receiver.