Tutorial Mode S uplink

Tutorial: Mode S uplink (interrogations on 1030 MHz)

What it is

Secondary surveillance radars and collision-avoidance systems (ACAS or TCAS) ask aircraft transponders questions on 1030 MHz; the aircraft answer on 1090 MHz, which is ADS-B and Mode S replies. Decoding the questions shows which radars are around, how fast they rotate, which aircraft they ask, and which transponder registers they request.

What you need

• A radio that samples at 8 Msps or more. The mode asks for 10 Msps, the lowest working rate is 8 Msps (channel width 8 MHz). An RTL-SDR (about 2.4 Msps) cannot run this mode. HackRF, Airspy and other wideband radios can. See Supported radios.
• An antenna for 1030 MHz, with a view in the direction of the radars; interrogations are strong close to a radar head.
• Frequency: 1030 MHz (mode table 1025 to 1035 MHz).

Before you start

• Gain: radar signals are strong. Start low to avoid overload and raise it only to see weaker radars.
• The panel tells you "Start the receiver on 1030 MHz (8 Msps or more)" if the rate is too low.

Step by step

1. Choose the group Aviation, entry Interrogations, 1030 MHz (mode Mode S uplink).
2. Check the sample rate is 8 Msps or more and press Start.
3. Open the views Interrogators, Aircraft, TCAS and Log.

What you should see

Each radar shows up as an interrogator with a code, a rotation period in seconds, a pulse rate and a level. You can read a radar's rotation, often 4 to 12 seconds, from its repeated hits.

Reading the screen

• Interrogators: Interrogator code, Rotation, Rate, Level, Heard, Last and the kinds of interrogation. "All-call / roll-call" interrogations and "Mode A / C, sidelobe" pulses are counted separately; an interrogator seen only once is marked "Heard once" because a bit error gives a wrong address, so OnAir waits for a second interrogation.
• Aircraft: addresses that were addressed, with the Registers the radars asked for.
• TCAS: ACAS interrogations (UF0 and UF16) used for coordination between aircraft; the panel says "no ACAS interrogations yet" when none are heard.
• Log: Time, Format, Address and What (decoded content) per interrogation.

Options and settings

The panel also keeps a pulse count and the intermode (Mode A/C) rates; check the panel for the exact counters.

Troubleshooting

• Nothing: sample rate below 8 Msps, wrong frequency, or no radar within reach.
• Interrogators with odd codes: single-hit errors; wait for the second interrogation.
• Overload: reduce gain.
• Few aircraft addressed: most ground radar interrogations are all-call; selective ones need more time.

Tips

Run ADS-B at the same time on another receiver slot to see both directions of the exchange.

Legal and privacy notes

Interrogations are not encrypted but they are radar signals; receiving is lawful in many countries, not all. See Legal.

Under the hood

Pulse position and DPSK-style modulation on 1030 MHz: Mode A/C interrogations are pulse pairs (P1, P3), Mode S uses a P6 DPSK block with 56 or 112 data bits (uplink formats UF) and a 24-bit parity overlaid with the address. ICAO Annex 10 vol. IV.

Related

Mode S uplink mode page, Tutorial: ADS-B, Tutorial: UAT.