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 navigation unit that is able to dynamically update the target coordinates.

The tracking system is based on APRS and has many years of heritage. When used on standard amateur radio APRS frequencies it can take advantage of the existing global APRS network of digipeaters and IGates, but it can also be used in point-to-point links on any suitable frequency including unlicensed bands.

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 IGates and publishes them on the Itnernet, see http://aprs.fi/ – this can be used to follow the balloon trajectory from anywhere over the Internet.
  3. A mobile APRS receiver station that is used for real time tactical navigation in the field where no Internet is available. This station 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.


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)
The APRS encoder takes the input digital data (position and telemetry) and converts it into 1200bps AFSK audio signal. In other words, this is where the conversion from digital data to analog signal takes place.
FM Transmitter
The FM transmitter converts the audio signal containing the 1200bps AFSK data to VHF or UHF radio signal.
FM Receiver
The FM Receiver receives the VHF/UHF radio signals and converts it into audio signal that contains the 1200bps AFSK data.
APRS Decoder (TNC)
The APRS decoder decodes the received AFSK data into APRS packets from where the balloon position and telemetry can be extracted. Technically, this is where the conversion from analog signal to digital data occurs.
Tactical Navigator
Car navigation unit that supports tactical navigation (chasing). It receives balloon position data from the TNC and plots it on the map. In tactical mode it updates the route from the current location to the target location (balloon position) in real time.
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.
Digipeaters and IGates
These are automatically part of the system if using amateur radio APRS frequencies. This is an existing infrastructure and will not be described further here.


This section presents the physical components of the tracking system and provides a mapping for each component to the functions they provide.

Component Description Functions

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.

Ot1plus-smt-coin.jpg Ot1plus-smt-1 5-schematic.png Ot1plus-smt-1 5.png

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



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.

The MX146 comes in two versions:

  • +8VDC version that gives > 500mW RF out
  • +5VDC version that gives > 350mW RF out

We chose the 5V version because its lower power consumption and to allow operation of all components from a single 5VDC supply. In addition to the lower power consumption, the 5V version is capable of higher duty cycle than the 8V version.


Specifications (5V version)
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 min 350mW into 50Ω (400mW typ)
  • SPI® and I2C® interface or
  • 16 preprogrammed frequencies, pin selectable. 3.3V CMOS level
Supply voltage +5VDC (4.7V to 6V)
Current consumption 1mA stdby, TBD mA TX
Size 50x25x2.5mm
Operating temperature -40°C to +85°C
Mechanical interfaces Pin-header or wire
Special features


Kenwood TM-D710E

AvMap G5

Telemetry Recorder


Link Budget

Using a roof mounted car antenna as well as MGA