How it works...

Aircraft Tracking Modes Explained

This page explains the three core surveillance modes commonly seen on an aircraft tracking station: ADS-B, Mode S and MLAT. Use the buttons above to switch between them and see how each one works, what data it provides, and how it appears inside an Airradar-style receiver setup.

Main Frequency

1090 MHz

Display Style

Airradar UI

Core Inputs

SDR + Decoder

Coverage Logic

RF Line of Sight

What ADS-B Is

ADS-B stands for Automatic Dependent Surveillance–Broadcast. It is a surveillance technology where the aircraft calculates its own position from onboard satellite navigation and then broadcasts that information repeatedly for anyone with a compatible receiver to decode.

In practical terms, this means the aircraft is not waiting to be asked where it is. It is proactively telling the world: “this is my position, altitude, speed, track and identity.”

Why it is so useful

It provides precise position updates directly from the aircraft.
It can be received by ATC systems, airline infrastructure and hobby stations.
It is ideal for real-time map displays such as Airradar.
It usually gives the richest information of the three modes on this page.

Signal Flow

STEP 01

GNSS position is computed

The aircraft navigation system determines position, time reference, speed and track.

STEP 02

ADS-B messages are built

The transponder encodes callsign, ICAO address, altitude, velocity and location data.

STEP 03

Broadcast on 1090ES

Data is transmitted continuously on 1090 MHz extended squitter frames.

STEP 04

Ground station displays traffic

Your receiver decodes the bursts and software paints the aircraft on the map.

What You Can See

ICAO hexadecimal address
Callsign or flight number
Latitude and longitude
Barometric and sometimes geometric altitude
Ground speed and vertical rate
Track / heading information
Status fields depending on avionics capability

How It Fits Into Airradar

For a typical home station, the chain looks like this:

1090 MHz antenna → SDR receiver → decoder (Dump1090 / similar) → aircraft data feed → Airradar web interface

That is why ADS-B is the backbone of most hobby radar pages: it is direct, rich in information, and easy to visualize.

What Mode S Is

Mode S is a transponder system used by aircraft to respond to interrogations from radar and other surveillance systems. The “S” stands for Select, because the aircraft can be selectively addressed using its unique 24-bit ICAO address.

Unlike ADS-B, plain Mode S does not automatically broadcast full position data by itself. It mainly sends identity and other transponder-related replies when interrogated, although some messages can still reveal useful information to tracking software.

Typical data available

ICAO hex address
Squawk code
Pressure altitude in many cases
Sometimes callsign or limited status data
No direct latitude/longitude unless ADS-B is also present

How It Works

STEP 01

Radar or system interrogation

A ground system sends a query to aircraft transponders.

STEP 02

The aircraft replies

The transponder transmits a selective response using its unique address.

STEP 03

Receiver captures the reply

Your station decodes those replies on 1090 MHz.

STEP 04

Software extracts partial data

You may see aircraft identity-related information, but not always a map position.

Why Mode S Matters

Mode S is important because it lets your station detect aircraft that are visible on the radio side but are not providing full ADS-B positional broadcasts. In other words, it expands awareness beyond “only aircraft with direct GPS coordinates.”

On an Airradar-style interface, Mode S traffic may appear with reduced detail compared with ADS-B aircraft. You might see the ICAO address, altitude or callsign, but not a precise plotted route unless another method—such as MLAT—derives position.

Mode S in a Home Station

Antenna → SDR → decoder receives replies → partial aircraft data → Airradar list / alert / metadata view

This is especially useful for military, business or legacy traffic where position messages may be absent, restricted or intermittent.

What MLAT Is

MLAT stands for Multilateration. It is a technique used to estimate an aircraft position by comparing the exact arrival time of the same transponder signal at multiple receiving stations.

This means the aircraft does not have to broadcast its own coordinates. Instead, the network figures out the position from timing differences between geographically separated receivers.

Why MLAT exists

It can provide a position for aircraft that transmit Mode S replies but no ADS-B coordinates.
It turns a network of synchronized receivers into a location engine.
It fills some gaps where direct ADS-B position is unavailable.

How MLAT Works

STEP 01

One aircraft signal is heard by many stations

Several receivers capture the same transponder burst across different locations.

STEP 02

Precise timing is compared

The network measures the tiny differences in arrival time between stations.

STEP 03

Position is mathematically solved

Hyperbolic geometry is used to estimate where the aircraft must be.

STEP 04

The derived track is displayed

The resulting calculated coordinates can then be shown on the map.

MLAT Strengths and Limits

Strength: gives positions for some non-ADS-B aircraft.
Strength: uses the power of a shared receiver network.
Limit: needs several synchronized stations hearing the same target.
Limit: accuracy depends on geometry, timing quality and station distribution.
Limit: it is network-dependent, not purely local in the same way ADS-B is.

How It Appears in Tracking Pages

In many tracking systems, MLAT-derived aircraft are marked differently from direct ADS-B targets because the position is calculated by the network, not transmitted by the aircraft itself.

Aircraft Mode S reply → multiple network stations hear it → timing server computes position → map displays MLAT track

For users, the big idea is simple: MLAT helps put a dot on the map for aircraft that would otherwise remain positionless.