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 (see above)
Garmin GPS 18x LVC GPS receiver with built-in antenna GPS Receiver
OpenTracker+ SMT APRS modem APRS Encoder (TNC)
SRB MX146LV Embeddable VHF FM transmitter FM Transmitter
Kenwood TM-D710E VHF/UHF mobile transceiver FM Receiver, APRS Decoder (TNC)
AvMap G5 Personal navigator with APRs support Tactical Navigator
Telemetry Recorder

Garmin GPS 18x LVC

The GPS 18x LVC is a small, highly accurate GPS sensor featuring a 12-parallel-channel, WAAS-enabled GPS receiver and an integrated magnetic base. The receiver is 2.4 inches in diameter and weighs just 115 grams (depending on cable length), and it can operate at extremely high altitudes.

The unit defaults to output data in the industry standard NMEA 0183 data format, but may also be user programmed to output data in the GARMIN proprietary format. It also provides a pulse-per-second logic level output whose rising edge is aligned to the UTC second within 1 microsecond.

Acq times
  • Reacquisition: less than 2 sec
  • Warm: 15 sec
  • Cold: 45 sec (initial time, position and almanac known, ephemeris known)
  • AutoLocateTM: 5 min (almanac known, initial position and time unknown)
  • SkySearch: 5 minutes (no data known)
Accuracy less than 15 m standard, less than 3 m WAAS
Supply voltage 4.0 to 5.5 VDC
Current consumption 60 mA at 5.0 V
Size 61 mm diameter, 19.5 mm height
  • 160 g with 5m cable
  • 110 g with 3m cable
Operating temperature -30°C to +80°C
Data interface NMEA or Garmin
Electrical interface Wire (RS232)
Special features
Pin Color Function
1 Yellow Timing pulse output
2 Red Power in
3 Black Ground
4 White Data out
5 Black Ground
6 Green Data in


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

TM-D710E standard display.
TM-D710E APRS display.

The Kenwood TM-D710E is a VHF/UHF transceiver featuring a dual receiver (receive to frequencies at the same time), built in packet TNC and APRS support.

The APRS support is extremely well suited for tactical tracking when combined with the AvMap G5 navigation unit.

The TM-D710E can perform the following functions in balloon tracking (some are TBC):

  • Direct reception of APRS packets from the balloon
  • Send received balloon position to the AvMap G5 tactical navigator (or other unit that supports the Kenwood format)
  • Digipeat received balloon packets
  • Get own location from AvMap G5 GPS (or other compatible GPS receiver)
  • Send own location via APRS network.

With the optional VGS-1 voice unit installed the TM-D710E can even read out loud the received APRS packets.

Receiver 118-524 and 800-1300 MHz
Transmitter 2m and 70cm amateur bands
TX power 5 / 10 / 50W
Modes F1D, F2D, F3E
Voltage 13.8 V DC ±15%
RX Current < 1.2A at 2W audio out
TX Current 5.0 / 6.5 / 13.0A
Operating temperature -20°C to +60°C
Weight 1.2 + 0.3 kg

AvMap G5

The AvMap G5 is a personal navigator with vuilt-in 20-channel GPS receiver. What distinguishes the AvMap G5 from other personal navigators is that it has special firmware that supports APRS and it can be interfaced to the Kenwood TM-D710E (or other APRS TNCs). Thus the G5 + TM-D710 provide the desired tactical navigation functions out of the box without any hacking!

The built in APRS functionality allows the AvMap G5 to receive APRS data from the TM-D710 and plot it on the map using the APRS symbols. In tactical mode, the AvMap G5 can navigate to the position of an APRS station even if the station is moving, see video below.

At the same time, the AvMap G5 can send GPS data to the TM-D710, which then can use this to report it's own location via APRS. In practice this means that those following the event via the web, e.g. http://aprs.fi/ can follow both the balloon and the chasing car at the same time. (TBC since I don't yet have the TM-D710 so I don't know if this is actually possible)

IPH4mwn4uko}} G5.jpg


Telemetry Recorder

Not yet decided... This will most likely be a lightweight laptop or netbook that can connect to the TM-D710 and record the received packets. Also TBC if this is at all necessary – maybe the AvMap G5 can perform this function.

Electrical Wiring

To be written...

Money Budget

On-board APRS Transmitter

The prices for the on-board units have been derived from US $ assuming a 1:1 $-to-€ conversion ratio, which would be the case in Denmark (import tax + 25% VAT). This is probably the worst case in Europe.

Component Price
Garmin GPS 18x LVC 70 €
OpenTracker+ SMT 40 €
SRB MX146LV 70 €
Antenna 50 €
Cables & connectors 100 €
Sum 330 €

Mobile Tracking Station

Component Price
Kenwood TM-D710E 540 €
AvMap G5 350 €
Telemetry Recorder 0 €
VHF antenna 50 €
Cables 100 €
Sum 1040 €

Although the mobile tracking station consisting of the TM-D710E radio and AvMap G5 navigator is rather expensive, it is important to emphasize that they have many other uses than APRS tracking. Obviously, the AvMap G5 can be used as standard car navigator, while the TM-D710E can be used for standard VHF/UHF amateur radio operations, including satellite communications. So this setup is very feasible if dual-use can be exploited.

If this is not the case, cheaper alternatives can be considered consisting of a simple VHF FM receiver and 1200 bps TNC + laptop. One could even ditch the hardware modem and use soundcard modem, though this could be unreliable under difficult and weak signal conditions. This could be improved by switching to 300 bps operating mode.

Mass and Power Budget

Component Weight Power (@ 5V DC)
Garmin GPS 18x LVC 110 g 60 mA
OpenTracker+ SMT 50 g (TBC) 20 mA
SRB MX146LV 50 g (TBC) 300 mA (TBC)
Antenna 100g (TBC) 0 mA
Cables & connectors 150 g (TBC) 0 mA
Sum 460 g (TBC) 380 mA (TBC)

Shall be able to operate at least 5 hours → we need 5V 2Ah.

Link Budget

Using a roof mounted car antenna as well as MGA


To be written