In the landscape of modern cooperative aviation surveillance, 1090 MHz ADS-B (Automatic Dependent Surveillance-Broadcast) has revolutionized flight tracking. However, a significant percentage of military aircraft, rescue helicopters, and General Aviation (GA) planes still operate with legacy Mode S transponders or SSR (Secondary Surveillance Radar) units lacking GPS position broadcasting capabilities. This is where Multilateration (MLAT) comes into play: by leveraging the Time Difference of Arrival (TDOA) of interrogated or spontaneous packets captured by a dense network of ground receivers synchronized to the nanosecond via GPS Pulse-Per-Second (PPS) signals, it is possible to calculate the exact three-dimensional position of any radio target.
â¡ Technical Sheet & Operational Data
1. Physical Principles of TDOA and Arrival Time Geometry
The operating principle of passive multilateration relies on hyperboloids of position. When an aircraft equipped with an ICAO 24-bit Hex ID (e.g., 48406F) transmits a Mode S pulse (Short Squitter DF11 or Surveillance Reply DF4/DF16), the electromagnetic wavefront propagates at the speed of light ($c \approx 299,792,458 \text{ m/s}$). If at least four ground stations receive the exact same binary packet, each station records an arrival timestamp $t_i$.
The time difference between station $A$ and station $B$ defines a geometric hyperbola:
\Delta t_{AB} = t_A - t_B = \frac{||R_A - P|| - ||R_B - P||}{c}By intersecting at least three baselines (four coordinated receivers), the system solves the non-linear equation system determining latitude, longitude, and barometric altitude of the aircraft. The required millimeter-level accuracy makes the use of atomic or GPS Pulse-Per-Second (PPS) time references with precision under 20 nanoseconds indispensable.
2. Hardware Architecture and PPS Synchronization for ADSBItalia Feeders
To actively participate in the ADSBItalia multilateration network, a standard SDR receiver tuned to 1090 MHz is not enough; flawless clock stability is required. The use of cheap crystal oscillators (TCXO) is insufficient due to thermal drift which would introduce position errors of tens of meters in TDOA calculations.
The recommended hardware setup for an advanced feeder node includes:
- SDR Receiver: RTL-SDR v4 (with improved shielding and optimized Q-branch) or Airspy Mini / Airspy R2 to handle high sample rates (20 MSPS).
- 1090 MHz Antenna: 5dBd fiberglass vertical collinear antenna tuned precisely to 1090 MHz, connected via very low-loss LMR-400 or RG-213 coaxial cable.
- SAW / Bandpass Filter: Ceramic filter centered at 1090 MHz with a 12-24 MHz passband to eliminate strong GSM/LTE and TV broadcast interference.
- PPS Synchronization Module: Dedicated GNSS receivers such as the u-blox NEO-M8T or LEA-M8T capable of outputting a hardware PPS pulse directly connected to a Raspberry Pi GPIO pin or via FPGA timestamping interface.
*028c42b8e0104f;
-- CRC: OK
-- Downlink Format: 11 (Acquisition Squitter)
-- Capability: 4 (Permanent alert / SPI)
-- ICAO Address: 48406F (ITA - Italian Air Force / Coast Guard)
-- Timestamp PPS: 1718374928.109284719 s3. Decoder Software Configuration and Server Forwarding
The standard decoding and forwarding software for MLAT data is readsb or dump1090-fa, properly compiled with support for external clock and network synchronization. Below are the installation and configuration commands for Debian/Raspberry Pi OS systems:
sudo apt update && sudo apt install -y git build-essential debhelper libusb-1.0-0-dev pkg-config rtl-sdr librtlsdr-dev
git clone https://github.com/wiedehopf/readsb.git
cd readsb
make -j$(nproc) AIRCRAFT_HASH_BITS=16
sudo make installTo launch the acquisition daemon setting the optimal gain of the RTL-SDR v4 and enabling raw MLAT data feed to the central collector, use the following systemd service command:
readsb --device-type rtlsdr --net --net-connector feed.adsbitalia.it,30004,beast --gain 49.6 --ppm 0 --net-bi-port 30005 --lat 41.9028 --lon 12.4964 --fix --net-heartbeat 60 --net-ro-size 1280 --net-ro-interval 0.054. Reference Table: Frequencies and Parameters of Surveillance Signals
The following table summarizes the main parameters of radio frequency aeronautical transmissions used for ADS-B, Mode S, and MLAT tracking compared to traditional ground radar systems:
| Service / Protocol | Operating Frequency | Modulation | Surveillance Purpose |
|---|---|---|---|
| ADS-B / Mode S Downlink | 1090 MHz ± 1 MHz | PPM (Pulse Position Modulation) | Dependent autonomous surveillance (GPS position included) |
| SSR Radar Interrogation | 1030 MHz | DPSK / Pulse Pairs (P1, P3, P4) | Active ground interrogation for Mode A/C/S |
| UAT (Universal Access Transceiver) | 978 MHz | FSK (Frequency Shift Keying) | UAT data surveillance (mainly USA / FAA) |
| MLAT TDOA Mode S | 1090 MHz | PPM (Passive Squitters / Replies) | Passive geolocalization via time difference of arrival |
5. Share Your Data with ADSBItalia (Feeding HFDL/ACARS & MLAT)
Contributing to the growth of ADSBItalia's open-source monitoring network helps improve radar coverage across national and European territory, integrating advanced MLAT tracks and ACARS/HFDL communications. If you own an SDR receiver and want to configure your setup as a certified ground station, visit our Official ADSBItalia Feeder Guide for detailed step-by-step instructions.
All data collected by our feeder stations flows in real-time onto our Live Radar Map map.adsbitalia.it, where you can immediately verify reception quality, proper PPS clock operation, and the position of aircraft tracked via MLAT multilateration.
