By Trans Global Training

Ever reviewed a flight playback on Flightradar24 and seen an aircraft take off into thin air, track parallel to the runway through farm fields, jump in sharp 90-degree steps, or abruptly vanish on short final into a coastal airfield?

When flight crews or operations report these anomalies, the immediate question directed at maintenance is often:

“Is our transponder transmitting an erroneous GPS position?”

The answer is almost always no. In fact, on many classic transport-category aircraft flying commercially today, the aircraft isn’t transmitting any GPS coordinates at all.

To understand why tracking apps—and occasionally air traffic control—display these “ghost tracks,” we have to explore how transponder technology evolved, the regulatory grandfathering rules governing ADS-B Out, and the mathematical methods ground stations use to locate an airframe.

The Evolution: From IFF to Mode S

Civil Secondary Surveillance Radar (SSR) grew out of military IFF (Identification Friend or Foe) systems so air traffic controllers wouldn’t have to rely solely on primary radar skin-paints. Over several decades, transponder systems evolved in distinct steps:

1. Mode A: Identity Only (Squawk)

Mode A is an active interrogation-and-reply architecture. Ground radar sweeps the airspace on 1030 MHz. When the onboard transponder receives this interrogation, it replies on 1090 MHz with a 4-digit octal code (0000 to 7777) set on the cockpit control unit.

• Position payload:None. The ground radar determines where the plane is simply by knowing the mechanical azimuth angle of its rotating antenna (bearing) and timing the microsecond transit delay of the reply pulses (range).

2. Mode C: Pressure Altitude

Mode C uses the same 1030/1090 MHz mechanism, but the reply packet encodes barometric pressure altitude supplied directly by the aircraft’s Air Data Computer (ADC) or an encoding altimeter (referenced to 1013.25 hPa).

• Position payload:None. Radar still calculates bearing and range; the aircraft only provides the vertical coordinate.

3. Mode S ELS (Elementary Surveillance)

In dense airspace, classic Mode A/C caused severe RF congestion—too many transponders answered every radar sweep simultaneously (known as “garbling”). Mode S (“Select”) solved this by assigning every airframe a unique, permanent 24-bit ICAO address.

• Interrogators can address specific airframes individually.
• Alongside squawk and altitude, Mode S downlinks the Flight ID (the callsign entered by the crew).
• Crucially: Under baseline Mode S Elementary Surveillance (ELS), the transponder does not receive or transmit GPS coordinates.

Why Don’t All Commercial Aircraft Transmit GPS?

Modern airspace relies heavily on ADS-B Out (Automatic Dependent Surveillance–Broadcast). Under ADS-B Out, an upgraded Mode S transponder uses 1090 MHz Extended Squitter (1090ES) to autonomously broadcast 112-bit data bursts twice a second. These broadcasts contain high-integrity latitude, longitude, and velocity directly from an approved GNSS/SBAS sensor.

Under European regulations (such as Commission Implementing Regulation EU No 1207/2011 / SPI IR), ADS-B Out is mandated for aircraft over 5,700 kg MTOM or with cruise speeds exceeding 250 knots. However, the regulation contains a vital exemption: the 7 June 1995 cutoff date.

Because retrofitting classic regional turboprops and older airframes with digital data buses, SBAS GPS receivers, and DO-260B transponders was cost-prohibitive, aircraft whose original Certificate of Airworthiness was issued prior to 7 June 1995 are grandfathered. Provided no standard economic OEM modification exists, these aircraft remain fully legal for commercial IFR operations equipped solely with Mode S ELS.

On these classic airframes, the transponder wiring leaves the Extended Squitter discrete open or disabled, and no navigation data lines enter the transponder rack.

How Flight Trackers Locate Non-GPS Aircraft: The MLAT Math

If an aircraft doesn’t broadcast GPS coordinates, how does a tracking app put a live icon on the map?

The answer is Multilateration (MLAT).

When a Mode S transponder replies on 1090 MHz, that radio wave expands outward at the speed of light.

1. Multiple ground stations receive the exact same pulse.
2. Because the aircraft is at varying distances from each station, the signal reaches each antenna at slightly different nanoseconds.
3. The time difference of arrival (TDOA) between any two stations defines a curved hyperbolic line of position.
4. By calculating where the hyperbolas from at least four synchronized receiver stations intersect, a central server calculates the aircraft’s 3D position.

       [Receiver Station A] (t = 0.000000s)
               \
                \
  [A/C] --------> (1090 MHz Mode S Pulse)
                /
               /
       [Receiver Station B] (t = 0.000014s)
               |
       [Receiver Station C] (t = 0.000021s)
               |
       [Receiver Station D] (t = 0.000029s)

   Intersection of Hyperbolas = Calculated Lat/Long

Why Tracking Discrepancies Happen

Official air traffic service providers and public tracking services both rely on multilateration, but the hardware behind them is very different:

• Certified ATC Systems (WAM & A-SMGCS): Air navigation authorities use Wide Area Multilateration (WAM) networks and airport surface movement systems (A-SMGCS). These stations use atomic clocks, surveyed antenna positions, and redundant high-speed links.
• Crowdsourced Flight Trackers: Platforms like Flightradar24 rely on thousands of volunteer-hosted receivers (often simple SDR USB sticks on Raspberry Pis) connected over domestic broadband.

This difference explains why tracking errors occur:

1. Low-Altitude Departure Skew (The “Parallel Ghost Track”)

Right after takeoff, line-of-sight signals to ground receivers can bounce off terminal structures, metal hangars, and blast fences:

• Multipath propagation: A reflected signal travels a slightly longer physical path, arriving microseconds late.
• Sub-optimal receiver geometry: Close to ground level, only a few local receivers may have line-of-sight, often clustered on one side of the airfield.
• The result: The MLAT algorithm calculates a false intersection point, placing the aircraft a mile or two off to the side—frequently running parallel to the runway. Once the aircraft climbs through a few thousand feet into clean line-of-sight with five or more widely distributed receivers, the math resolves and the plot snaps back to the real track in sharp 90-degree steps.

2. The Coastal / Mountain Disappearance

Modern aircraft equipped with ADS-B Out only require a single receiver to pick up their broadcast GPS packet to appear on tracking platforms all the way to touchdown.

For a legacy Mode S aircraft, as soon as it descends below surrounding terrain into a coastal fjord or valley, inland receivers lose direct line of sight. The moment the receiver count drops below four, the MLAT equation cannot be solved. The aircraft vanishes or freezes on the map, even while flying a standard approach and communicating normally with local ATC radar.

Deepen Your Systems Knowledge

Whether troubleshooting legacy ARINC 572 / Gillham parallel encoding altitude lines on classic regional aircraft or complex integrated ARINC 429 digital avionics buses on modern airliners, understanding core system architecture is essential for rapid defect rectification:

• Working on classic regional turboprops? Explore our ATR 42/72 Series General Familiarisation Course for an in-depth look at traditional air data interfaces, legacy avionics shelves, and core system architectures.
• Maintaining modern glass-cockpit digital platforms? Check out our Boeing 737 MAX General Familiarisation Course to master modern DO-260B ADS-B Out architectures, integrated ADIRU interfaces, and digital flight management networks.

👉 Continue Reading:Curious how collision avoidance operates without satellite navigation? Read Part 2: TCAS Demystified – How Collision Avoidance Works Without GPS. available on 28 October 2026.