Tactical Tracking System
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.
The tracking system consists of the following subsystems:
- The transmitter placed on-board the balloon. It transmits GPS coordinates and other telemetry at regular intervals.
- 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.
- 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)
- FM Transmitter
- 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.
- 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.
- Garmin GPS 18x LVC: https://www.argentdata.com/catalog/product_info.php?cPath=23&products_id=37
- Trimble Copernicus GPS Receiver: https://www.argentdata.com/catalog/product_info.php?cPath=23&products_id=95
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|
|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)|
|GPS interface||NMEA in/out|
|Mechanical interfaces||Pin-header or wire|
|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|
|8||RST||Reset (active low)|
|10||IRQ||Transmit-now, profile select, or counter|
|13||AIN||Audio input from radio|
|16||TXD||RS-232 data out|
|17||RED||Red LED output|
|18||GREEN||Green LED output|
|21||PTT||Push-to-talk signal to radio|
|22||REGOUT||Regulator output - 5 volts|
|24||REGIN||Regulator input - 6.5 to 28 volts|
TBC whether we use 5V or 8V version. 5V version gives less output...
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|
|Modulation||Digital injection modulation|
|Modulation Sensitivity||23kHz/V (typ)|
|Spurious suppression||> 80dB (channel spacing > 10kHz, typ)|
|Frequency stability||+/5ppm (typ)|
|Turn On delay (after PTT)||25msec (typ)|
|Output power||min 350mW into 50Ω (400mW typ)|
|Supply voltage||+5VDC (4.7V to 6V)|
|Current consumption||1mA stdby, TBD mA TX|
|Operating temperature||-40°C to +85°C|
|Mechanical interfaces||Pin-header or wire|
Using a roof mounted car antenna as well as MGA