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.

The page is intended to present a reliable tracking solution for those not already familiar with APRS. If you are already using APRS, you may find the information on this page trivial.

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/ – this can be used to follow the balloon
  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.

A functional diagram of the complete tracking system is shown below followed by a description of each functional block.

Functional Diagram

GPS Receiver
The GPS receiver tracks GPS satellites and calculates the position of the balloon in real time. It sends standard NMEA formatted data to the APRS packet encoder. Besides the obvious low mass and power requirements, it is very important that the GPS receiver works above the 18km limit.
APRS encoder (TNC)
FM Transmitter
can be part of the system if using standard APRS frequency
FM Receiver
APRS Decoder (TNC)
Tactical Navigator
Car navigation that supports tactical navigation (chasing)
Telemetry Logger
This functional unit is used to log the received APRS packets from the balloon, both position report and telemetry packets. This can be a simple laptop or netbook connected to the TNC.

Electrical Architecture

On-Board Transmitter

The complete on-board APRS transmitter consists of the following components

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 for audio input to an FM transmitter. It can also transmit telemetry data that is read from the built-in temperature sensor (non-SMT version) 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. The SMT version does not include a built-in temperature sensor or LEDs.


Specifications for SMT version
Operating modes
  • 1200 bps AFSK (RX and TX)
  • 300 bps AFSK, PSK31 (TX only)
Supply voltage DC 6.5 to 28V unregulated or 5V DC regulated
Current consumption 8 mA idle, 20 mA transmitting
Size 31x18x5 mm (24-pin DIP footprint)
Operating temperature
GPS interface NMEA in/out
Mechanical interfaces Pin-header or wire
Special features
  • Max 200 mA @ 5 V available for GPS
  • Source code released under Modified BSD license
  • 8-bit analog TM from ADC1-5
  • digital TM for T1CH1 and ADC6-9

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


SRB MX146 FM Transmitter

TBC whether we use 5V or 8V version

The MX146 is an embeddable VHF transmitter module from SRB Electronics. It's programmable for any frequency from 144-148 MHz in 2.5 kHz steps, or it can be used on one of 16 pre-programmed frequencies.

Power output is 500 mW minimum, and typically closer to 850 mW. It draws under 1 mA in standby, has a fast wake-up time (< 25 ms) and is suitable for voice or data with bitrates up to 10 kbps or more.


Frequency range 144MHz to 148MHz
Channel spacing 2.5kHz
Modulation Digital injection modulation
Modulation Bandwidth >20kHz
Modulation Sensitivity 23kHz/V (typ)
Input Impedance ~600Ω
Spurious suppression > 80dB (channel spacing > 10kHz, typ)
Harmonic suppression 45dB
Frequency stability +/5ppm (typ)
Turn On delay (after PTT) 25msec (typ)
Output power > 500mW (nominal) into 50Ω
  • SPI® and I2C® interface or
  • 16 preprogrammed frequencies, pin selectable. 3.3V CMOS level
Supply voltage +8VDC (7.5V to 8.5V)
or 5VDC
Current consumption < 5mA RX
< 400mA TX
Size 50x25x2.5mm
Operating temperature -40°C to +85°C
Mechanical interfaces Pin-header or wire
Special features


The Mobile Receiver Station

FM Receiver

Car Navigator

Telemetry Recorder


Risks Register