Tactical Tracking System

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This page describes a Tactical Tracking System for high altitude balloons – or any other object – based on the Automatic Position Reporting System (APRS). The term Tactical Tracking here refers to real-time tracking and chasing, where the coordinates of the tracked object are received in real time and sent to a car navigation unit that is able to dynamically update the target coordinates.

The tracking system is based on APRS. Therefore it has many years of heritage and can take advantage of an existing global APRS network. It does not require any R&D since all components are available off-the shelf and even complete turn-key solutions are available.

In addition to position reporting, the system can also transmit telemetry read via analog and/or digital inputs.

Functional Overview

The tracking system consists of the following subsystems:

  1. The transmitter placed on-board the balloon. It transmits GPS coordinates and other telemetry at regular intervals.
  2. The global APRS network that receives the APRS packets from the balloon via digipeaters and publishes them on the global APRS network, see http://aprs.fi/
  3. A mobile APRS receiver station that is used for real time tactical navigation in the field where no Internet is available. This station also receives the APRS packets directly from the balloon.


System Design


GPS Receiver

OpenTracker+ SMT

The OpenTracker+ is an open source APRS tracker that receives NMEA data from a GPS receiver encodes it into APRS packets and generates AFSK signal suitable as audio input to an FM transmitter. It can also transmit telemetry data that is read from the built-in temperature sensor as well as the available ADC channels.

OpenTracker+ can be purchased assembled and tested or as a kit. Additionally, there is an SMT version, which is the one we are using.


Size 31x18x5 mm (24-pin DIP footprint)

Pin Name Function Utilisation
1 5VIN Regulated 5-volt input
2 RXD RS-232 data in (from GPS or computer)
3 1WIRE Dallas 1-wire bus for external sensors
4 AOUT Audio out to radio
5 ADC5 Analog input
6 T1CH1 Timer channel
7 ADC6 Analog input
8 RST Reset (active low)
9 ADC7 Analog input
10 IRQ Transmit-now, profile select, or counter
11 ADC8 Analog input
12 ADC9 Analog input
13 AIN Audio input from radio
14 ADC4 Analog input
15 ADC3 Analog input
16 TXD RS-232 data out
17 RED Red LED output
18 GREEN Green LED output
19 ADC2 Analog input
20 ADC1 Analog input
21 PTT Push-to-talk signal to radio
22 REGOUT Regulator output - 5 volts
23 GND Ground
24 REGIN Regulator input - 6.5 to 28 volts

Ot1plus-smt-1 5-schematic.png

Ot1plus-smt-1 5.png


FM Transmitter

The Mobile Receiver Station

FM Receiver

Car Navigator

Telemetry Recorder


Risks Register