2.4 Satellite Navigation, ADS-B, and Traffic Surveillance

Satellite Navigation, ADS-B, and Traffic Surveillance

Modern airspace operations rely on satellite-based positioning, automatic digital surveillance broadcasts, airborne collision avoidance systems, and digital search-and-rescue beacons. This section examines the technical principles of Global Navigation Satellite Systems (GNSS/GPS), Wide Area Augmentation Systems (WAAS), Automatic Dependent Surveillance-Broadcast (ADS-B), Traffic Alert and Collision Avoidance Systems (TCAS), and 406 MHz Emergency Locator Transmitters (ELTs).

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GNSS and GPS Architecture

The United States Global Positioning System (GPS / Navstar) comprises a baseline constellation of at least 24 operational satellites deployed across six orbital planes inclined at $55^\circ$ relative to the equator. Orbiting at a nominal altitude of approximately 20,200 km (10,898 NM) with a 12-hour sidereal period, each satellite repeats its ground track once every sidereal day.

Carrier Frequencies and Modulated Codes

GPS satellites continuously transmit right-hand circularly polarized (RHCP) RF signals driven by onboard atomic rubidium and cesium clocks:

• L1 Frequency (1575.42 MHz): Modulated with the Coarse/Acquisition (C/A) code (a 1.023 MHz pseudo-random noise Gold code repeating every millisecond), the military precision P(Y) code, and the 50 bps Navigation Message.
• L2 Frequency (1227.60 MHz): Carries the P(Y) code and modernized civil L2C code for dual-frequency ionospheric error compensation.
• L5 Frequency (1176.45 MHz): Allocated specifically within the Aeronautical Radionavigation Services (ARNS) band for high-integrity civil aviation safety-of-life operations.

The Pseudo-Range Measurement and Clock Bias Solution

The airborne GPS receiver measures the apparent transit time of the satellite signal by comparing the phase of the received incoming PRN Gold code against an internally generated replica code. Multiplying transit time by the speed of light yields the pseudo-range ($P_i$):

$P_i = \sqrt{(x_i - x)^2 + (y_i - y)^2 + (z_i - z)^2} + c \times \Delta t_{\text{clock}}$

Where $(x_i, y_i, z_i)$ are the known orbital coordinates of satellite $i$, $(x, y, z)$ are the unknown aircraft antenna coordinates, $c$ is the speed of light, and $\Delta t_{\text{clock}}$ is the time offset (clock bias) of the receiver's internal quartz crystal oscillator.

               (x1, y1, z1) Sat 1          (x2, y2, z2) Sat 2
                      \                         /
                       \  Pseudo-Range P1      /  Pseudo-Range P2
                        \                     /
                         v                   v
                      +-------------------------+
                      |    Aircraft Receiver    |
                      | Resolves 4 Unknowns:    |
                      |    (x, y, z, Δt_clock)  |
                      +-------------------------+
                         ^                   ^
                        /  Pseudo-Range P3    \  Pseudo-Range P4
                       /                       \
               (x3, y3, z3) Sat 3          (x4, y4, z4) Sat 4

Because an inexpensive quartz oscillator cannot match the sub-nanosecond accuracy of atomic clocks, every microsecond of clock error would introduce nearly 1,000 feet of ranging error. Therefore, four unknown variables must be solved simultaneously: $x$, $y$, $z$, and $\Delta t_{\text{clock}}$. A minimum of four satellites is mathematically required to yield a valid three-dimensional navigational position fix and eliminate receiver clock bias.

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RAIM and Differential Augmentation (WAAS)

Receiver Autonomous Integrity Monitoring (RAIM)

Under Instrument Flight Rules (IFR), the avionics system must continuously guarantee signal integrity without relying on external ground advisories. RAIM is an internal software algorithm that performs consistency checks among redundant pseudo-range measurements:

• Fault Detection (FD): Requires a minimum of 5 satellites with favorable Dilution of Precision (DOP) geometry (or 4 satellites plus barometric altimeter aiding). If one satellite transmits corrupted ephemeris or timing data, the redundant pseudo-range spheres will not intersect at a common coordinate, triggering an onboard RAIM alert that warns the pilot to revert to alternate navigation.
• Fault Detection and Exclusion (FDE): Requires a minimum of 6 satellites (or 5 satellites plus barometric altimeter aiding). FDE mathematically isolates the single faulty satellite, removes its pseudo-range from the navigation matrix, and recalculates a safe position solution without interrupting IFR flight.

Wide Area Augmentation System (WAAS)

WAAS augments basic GPS to support precision approaches with vertical guidance. The architecture comprises:

1. Wide-Area Reference Stations (WRS): Exactly surveyed ground stations distributed across North America monitor GPS signals and detect clock drift, ephemeris errors, and ionospheric delays.
2. Wide-Area Master Stations (WMS): Calculate real-time differential correction vectors and ionospheric grid point delay models, along with strict integrity bounds.
3. Geostationary Satellites (GEO): WAAS master stations upload correction messages to geostationary communication satellites, which broadcast the differential data back to aircraft across North America on the standard GPS L1 frequency (1575.42 MHz).

In the FAA's measured performance, WAAS positions are typically accurate to better than about 1.5 meters (unaugmented GPS is accurate to several meters), and WAAS alerts users within seconds when a satellite or correction should not be used. This capability enables Localizer Performance with Vertical Guidance (LPV) approaches, allowing flight crews to descend to decision heights as low as 200 feet AGL (equivalent to Category I ILS minimums) at runways lacking terrestrial ILS transmitters.

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Automatic Dependent Surveillance-Broadcast (ADS-B)

ADS-B transforms air traffic management from active ground radar interrogation to automated broadcast surveillance:

• Automatic: Transmits continuously without interrogation from ground stations or flight crews.
• Dependent: Depends on onboard GPS/WAAS sensors for precise position and air data computers for pressure altitude.
• Surveillance: Provides identity, 3D position, velocity vectors, and intent.
• Broadcast: Radiates omnidirectionally to all ground stations and nearby aircraft equipped with ADS-B In.

       [GPS / WAAS Constellation]
              |
              v
      [Aircraft ADS-B Out]
         /           \
        / 1090ES      \ 1090ES or 978 UAT
       v               v
[Nearby Traffic]   [FAA Ground Broadcast Transceiver (GBT)]
 (ADS-B In)              |
                         +--> ATC En-Route / Terminal Automation
                         +--> Transmits TIS-B (Radar Traffic Uplink)
                         +--> Transmits FIS-B (Weather / NOTAMs Uplink)

14 CFR 91.225 Airspace Mandates

Under Federal Aviation Regulation 14 CFR 91.225, all aircraft operating in the following airspace must be equipped with certified ADS-B Out:

1. Class A Airspace: At and above 18,000 feet MSL up to FL600.
2. Class B and Class C Airspace: Within the lateral boundaries and from the surface up to the ceiling.
3. Mode C Veil: Within 30 nautical miles of designated Class B primary airports from the surface up to 10,000 feet MSL.
4. Class E Airspace: Above 10,000 feet MSL across the contiguous US (excluding airspace at and below 2,500 feet AGL).

Dual-Link Architecture: 1090ES vs. 978 MHz UAT

Ground Uplink Services: TIS-B and FIS-B

• Traffic Information Service-Broadcast (TIS-B): Ground stations track non-ADS-B aircraft via traditional secondary/primary radar and re-broadcast their position to ADS-B In equipped aircraft.
• Flight Information Service-Broadcast (FIS-B): Broadcast free of charge exclusively on the 978 MHz UAT link, providing real-time METARs, TAFs, AIRMETs, SIGMETs, NOTAMs, and high-resolution NEXRAD precipitation radar to the flight deck.

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Traffic Alert and Collision Avoidance System (TCAS)

TCAS operates independently of ground-based air traffic control, interrogating nearby Mode A, C, and S transponders to detect potential collision threats.

TCAS I vs. TCAS II Architecture

• TCAS I: Required on Part 121 passenger airplanes with 10 to 30 passenger seats and on Part 135 turbine-powered airplanes with 10 to 30 seats (14 CFR 121.356 and 135.180). It computes the range, bearing, and relative altitude of nearby traffic and issues a Traffic Advisory (TA) with an aural "TRAFFIC, TRAFFIC" and an amber symbol when an intruder's projected time to closest approach becomes too short. TCAS I does not provide vertical escape maneuvers.
• TCAS II: Required, with a Mode S transponder, on Part 121 turbine-powered airplanes over 33,000 lb maximum certificated takeoff weight (14 CFR 121.356). Its thresholds depend on altitude-based sensitivity levels: TAs come roughly 20 to 48 seconds and Resolution Advisories (RAs) roughly 15 to 35 seconds before closest approach. The system commands vertical escape maneuvers (e.g., "CLIMB, CLIMB" or "DESCEND, DESCEND") displayed on the Primary Flight Display (PFD) vertical speed tape.

Mode S Coordination and Directional Antennas

When two TCAS II-equipped aircraft converge, their transponders establish direct digital communication via 1030/1090 MHz Mode S datalinks. The transponders negotiate complementary avoidance maneuvers: if Aircraft A is instructed to climb, Aircraft B is commanded to descend, eliminating contradictory maneuvers.

TCAS II uses a four-element directional antenna on top of the fuselage, plus a bottom antenna that may be directional or omnidirectional. The receiver measures the relative phase and amplitude differences between the antenna elements (amplitude monopulse / phase interferometry) to determine the intruder's precise azimuth bearing.

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406 MHz Digital Emergency Locator Transmitters (ELT)

Under 14 CFR 91.207, U.S.-registered civil airplanes generally must carry an approved ELT, with exceptions listed in 91.207(f) (for example, scheduled air-carrier flights, training flights entirely within 50 NM of the departure airport, and aircraft equipped to carry only one person). While legacy 121.5 MHz analog ELTs have been decommissioned from satellite monitoring, modern installations utilize 406 MHz digital beacons monitored by the international Cospas-Sarsat satellite network.

[Aircraft Crash / G-Switch Tripped]
               |
               +---> 406.025 MHz (5W Digital Burst every 50s) ---> [Cospas-Sarsat Satellites]
               |        Includes: 15-Hex Beacon ID & GPS Position          |
               |                                                        v
               |                                              [Mission Control Center]
               |                                                        |
               |                                                        v
               |                                             [Search and Rescue Units]
               |
               +---> 121.500 MHz (100 mW Analog Swept Tone) -------> Local Direction-Finder DF

Signal Characteristics and Satellite Geo-location

• 406.025 to 406.028 MHz Digital Uplink: Transmits a 5-watt phase-modulated RF pulse burst lasting approximately 440 milliseconds every 50 seconds. The burst carries the beacon's unique 15-hexadecimal-character identification, which encodes a country code and an identifier such as the aircraft's 24-bit address, tail number, or a serial number, plus latitude and longitude in GPS-coupled units.
• Search Radius Reduction: Standard Doppler satellite localization on 406 MHz achieves a position fix within 1 to 2 nautical miles. When encoded with internal GPS navigation data, the search radius collapses to under 100 meters within minutes of activation (compared to a 12 to 15 NM search radius for legacy 121.5 MHz analog beacons).
• 121.5 MHz Analog Homing Tone: Modern 406 MHz ELTs continue to broadcast a low-power (approx. 100 mW) swept downward audio tone on 121.5 MHz to allow search-and-rescue teams to home in on the wreckage using handheld directional antennas.

Installation, G-Switch, and Battery Regulations

• Crash Sensor (G-Switch): An inertia switch activates an automatic ELT on a crash-level deceleration along its sensing axis. The ELT must be mounted with that axis oriented as the installation instructions show (typically arrow forward), and 91.207(b) requires fixed and deployable automatic ELTs to be attached as far aft as practicable.
• Battery Replacement (14 CFR 91.207(c)): ELT batteries must be replaced (or recharged, if rechargeable) when the transmitter has been in use for more than 1 cumulative hour, or when 50% of their useful life has expired, as established by the transmitter manufacturer under its approval. The new expiration date must be marked on the outside of the transmitter and entered in the aircraft maintenance record.
• 12-Month Inspection (14 CFR 91.207(d)): Each required ELT must be inspected within 12 calendar months after the last inspection for proper installation, battery corrosion, operation of the controls and crash sensor, and a sufficient signal radiated from its antenna.