12.3 Transponders, Altitude Encoders & Emergency Locator Transmitters

12.3 Transponders, Altitude Encoders & Emergency Locator Transmitters

FAA Airframe Subject Matter Focus: Technicians must master the installation, testing, and regulatory maintenance requirements for Air Traffic Control (ATC) Radar Beacon System (ATCRBS) transponders, automatic altitude reporting digitizers, Mode S data links, ADS-B broadcast equipment, and Emergency Locator Transmitters (ELTs). Key FAA regulations include 14 CFR §91.413 (24-calendar-month transponder inspection), Part 43 Appendices E and F, and 14 CFR §91.207 (ELT operational rules, G-switch tests, and battery replacement intervals).

1. Air Traffic Control Transponder Systems (ATCRBS)

The ATC Transponder is an airborne secondary surveillance radar (SSR) transceiver that receives interrogations from ground ATC radar stations (or TCAS-equipped aircraft) and transmits an amplified, coded reply sequence.

                  ATC TRANSPONDER RECEPTION & REPLY FREQUENCIES

      [ Ground Secondary Radar ] ── 1030 MHz (Interrogation) ──> [ Aircraft ]
      [ & TCAS Interrogators   ] <── 1090 MHz (Coded Reply)  ─── [ Transponder ]

Transponder Operating Modes

                MODE A / MODE C REPLY PULSE TRAIN STRUCTURE

     F1                                                         F2     SPI
    ┌──┐   ┌──┐   ┌──┐   ┌──┐   ┌──┐   ┌──┐   ┌──┐   ┌──┐   ┌──┐   ┌──┐   ┌──┐
    │  │   │C1│   │A1│   │C2│   │A2│   │C4│   │A4│   │X │   │B1│   │  │   │  │
    └──┘   └──┘   └──┘   └──┘   └──┘   └──┘   └──┘   └──┘   └──┘   └──┘   └──┘
     │<───────────────────── 20.3 µs Total Framing ───────────────>│      │
     0.0 µs                                                       20.3 µs 24.65 µs

Pulse Structure & Squawk Codes

• Framing Pulses: Every transponder reply is bounded by two framing pulses, F1 and F2, spaced exactly $20.3\ \mu\text{s}$ apart.
• Information Pulses: Twelve information pulses ($A_1, A_2, A_4, B_1, B_2, B_4, C_1, C_2, C_4, D_1, D_2, D_4$) are positioned at $1.45\ \mu\text{s}$ intervals between F1 and F2. Mode A codes use octal digits (0 to 7) because each digit is represented by three binary bits ($1 + 2 + 4 = 7$; digits 8 and 9 are impossible in octal).
• SPI (Special Position Identification): Activated when the pilot presses the cockpit "IDENT" button. Adds a pulse $4.35\ \mu\text{s}$ after F2, causing the aircraft target to bloom/highlight on the controller's radar scope for approximately 18 seconds.
• Critical Emergency Squawk Codes:
  • 7500: Hijacking / Unlawful Interference
  • 7600: Lost Radio Communications
  • 7700: In-Flight Emergency / Distress
  • 1200: Standard VFR flight in US airspace

2. Altitude Encoders & 14 CFR §91.413 Inspection Requirements

Altitude Encoding Digitizers (Blind Encoders)

Mode C and Mode S transponders require continuous altitude inputs. This data is generated by an encoding altimeter or a solid-state blind altitude encoder plumbed directly into the aircraft static pressure system.

• Standard Datum Reference: Encoders convert absolute static air pressure into standard pressure altitude referenced to 29.92 inches of mercury (1013.25 hPa) regardless of the local altimeter barometric setting. Ground ATC radar computers apply the local altimeter setting to correct the readout automatically.
• Gillham (Gray) Code Bus: An 11-wire parallel data bus where only one binary bit changes state between adjacent 100-foot altitude steps. This single-bit transition prevents ambiguous binary readouts during pressure altitude changes.
• Altitude Correlation Tolerance: Under 14 CFR Part 43 Appendix E, the pressure altitude reported by the transponder must correspond with the primary cockpit altimeter within $\pm 125\text{ feet}$ at all tested altitudes.

                    PITOT-STATIC & TRANSPONDER INTERFACE

     [ Static Ports ] ─── Static Line ───┬───> [ Primary Cockpit Altimeter ]
                                         │
                                         └───> [ Blind Altitude Encoder ]
                                                      │ (Gillham / RS-232)
                                                      ▼
                                               [ ATC Transponder ]
                                                      │ 1090 MHz
                                                      ▼
                                               [ Bottom Blade Antenna ]

Transponder Certification & Testing (14 CFR §91.413)

Under 14 CFR §91.413, no person may use an ATC transponder in controlled airspace unless, within the preceding 24 calendar months, the transponder has been tested, inspected, and found to comply with 14 CFR Part 43 Appendix F.

Part 43 Appendix F Test Parameters

1. Radio Frequency Tolerance: Reply carrier frequency must be $1090 \pm 3\text{ MHz}$.
2. RF Peak Output Power: Minimum peak pulse power must be at least $70\text{ Watts}$ (for Class 2A/2B equipment on light aircraft below 15,000 ft) or $125\text{ Watts}$ (for Class 1A/1B equipment on aircraft operating above 15,000 ft or $>200\text{ kts}$). Maximum power must not exceed $500\text{ Watts}$ ($+27.0\text{ dBW}$).
3. Receiver Sensitivity (MTL): Minimum Triggering Level (MTL) must be between $-73\text{ dBm}$ and $-77\text{ dBm}$ for ATCRBS transponders.
4. Side-Lobe Suppression (SLS): The transponder must reply to main-beam interrogation pulses (P1 and P3 equal amplitude) and must be suppressed (prevented from replying) when side-lobe pulses (P2 amplitude equal to or greater than P1) are received, preventing ghost targets.

3. Emergency Locator Transmitters (ELT)

An Emergency Locator Transmitter (ELT) is a self-contained, battery-powered emergency beacon designed to activate automatically during an aircraft crash deceleration to transmit homing distress signals to search and rescue (SAR) satellites and ground teams.

                  COSPAS-SARSAT 406 MHz DIGITAL ELT SYSTEM

     [ Crash Deceleration ] ──> [ Internal G-Switch Closes ]
                                            │
                                            ├─> 121.5 MHz Analog (Homing)
                                            │
                                            └─> 406.025 MHz Digital Burst (5W)
                                                      │ (24-Bit Hex ID + GPS)
                                                      ▼
                                            [ LEOSAR / GEOSAR Satellites ]
                                                      │
                                                      ▼
                                            [ Rescue Coordination Center ]

Analog (121.5 MHz) vs. Digital (406 MHz) ELTs

4. ELT Installation, Inspection & Battery Maintenance (14 CFR §91.207)

Structural Mounting & G-Switch Inertia Sensor

• Airframe Location: Under 14 CFR §91.207 and AC 43.13-2B, the ELT must be installed as far aft as practicable in the aircraft fuselage (typically in the empennage or baggage compartment forward of the vertical stabilizer), where structural crash damage is statistically least severe.
• Longitudinal Orientation: The ELT must be mounted with its primary inertia switch (G-switch) sensitive axis aligned parallel to the longitudinal axis of the aircraft (pointing in the direction of forward flight).
• Inertia Activation Threshold: Standard mechanical ball-and-spring or solid-state G-switches are calibrated to trigger automatically when subjected to a longitudinal deceleration force of $5.0\text{G} \pm 2\text{G}$ ($2.3\text{ to } 7.0\text{G}$) with a duration of at least 11 milliseconds.

                  14 CFR §91.207 REGULATORY MANDATES

     ┌─────────────────────────────────────────────────────────────────┐
     │ 1. INSPECTION INTERVAL (14 CFR §91.207(d)):                     │
     │    Inspected within 12 calendar months for:                     │
     │    - Proper installation and structural mounting security       │
     │    - Battery corrosion and terminal integrity                   │
     │    - Operation of controls and crash sensor (G-switch)          │
     │    - Radiated signal output using test receiver / dummy load    │
     ├─────────────────────────────────────────────────────────────────┤
     │ 2. BATTERY REPLACEMENT CRITERIA (14 CFR §91.207(c)):            │
     │    Battery must be replaced (or recharged) when:                │
     │    - Transmitter in cumulative use for MORE THAN 1 HOUR         │
     │    - 50% of useful battery life (or storage life) has EXPIRED   │
     │    - New expiration date legibly marked on outside of case      │
     ├─────────────────────────────────────────────────────────────────┤
     │ 3. OPERATIONAL TESTING RESTRICTIONS:                            │
     │    - 121.5 MHz testing ONLY during FIRST 5 MINUTES of any hour  │
     │    - Radiated RF test limited to NO MORE THAN 3 AUDIO SWEEPS    │
     │    - 406 MHz digital test: use internal self-test mode ONLY     │
     └─────────────────────────────────────────────────────────────────┘

Operational Testing Rules per FAA AIM & AC 91-44A

1. Authorized Time Window: Live operational testing of an installed 121.5 MHz ELT using the cockpit remote switch must ONLY be conducted within the first 5 minutes past any UTC / clock hour (e.g., 10:00 to 10:05).
2. Audio Sweep Limit: Operational radiation must be restricted to no more than 3 audio sweeps (lasting approximately 1 to 2 seconds). Tests conducted outside this window or exceeding 3 sweeps can trigger airborne search and rescue false alarms.
3. 406 MHz Self-Test: For 406 MHz ELTs, technicians must use the manufacturer's built-in self-test (BIST) sequence. The self-test transmits an inverted synchronization frame that is intentionally ignored by Cospas-Sarsat distress satellites.