2.3 Pulse Navigation and Surveillance: DME, Transponders, and Weather Radar

Pulse Navigation and Surveillance: DME, Transponders, and Weather Radar

Pulse avionics systems operate in the Ultra High Frequency (UHF) and microwave frequency bands. Unlike continuous-wave radios, pulse systems transmit short, high-power bursts of RF energy and measure precise time intervals or decode serialized pulse trains. This section examines Distance Measuring Equipment (DME), Secondary Surveillance Radar (SSR) transponders, and airborne weather radar systems.

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Distance Measuring Equipment (DME)

DME provides continuous line-of-sight distance measurement between an aircraft and a ground transponder. DME operates within the UHF band from 960 MHz to 1,215 MHz, providing 252 channels (126 X and 126 Y), each pairing an interrogation frequency with a reply frequency.

Interrogation and Reply Pulse Pairs

To prevent random atmospheric static or continuous-wave carriers from triggering replies, DME interrogators transmit pulse pairs rather than single pulses. Each Gaussian-shaped pulse has a duration of $3.5\ \mu\text{s}$:

• X-Channel Spacing: Interrogation pulse spacing is $12.0\ \mu\text{s}$; reply pulse spacing is $12.0\ \mu\text{s}$.
• Y-Channel Spacing: Interrogation pulse spacing is $36.0\ \mu\text{s}$; reply pulse spacing is $30.0\ \mu\text{s}$.

Interrogation frequencies and reply frequencies are separated by exactly $63\ \text{MHz}$.

Aircraft Interrogator                       Ground Transponder
       |                                             |
       |---- Pulse Pair Interrogation (12 µs) ------>|
       |                                             | [50.0 µs Fixed Delay]
       |<--- Pulse Pair Reply (63 MHz offset) -------|
       |
[Total Elapsed Time Δt]

Slant Range Calculation and the 50 µs Ground Delay

When the ground transponder receives a valid pulse pair, it introduces an intentional, ultra-precise fixed delay of exactly $50.0\ \mu\text{s}$ before transmitting the reply pulse pair. This fixed delay gives the ground station time to process the signal and allows airborne receivers to measure very close distances without zero-range blind spots.

Because electromagnetic waves propagate at the speed of light ($c = 300{,}000\ \text{km/s} = 161{,}875\ \text{NM/s}$), an RF pulse requires approximately $6.18\ \mu\text{s}$ to travel 1 nautical mile. For round-trip propagation, the elapsed time is $12.36\ \mu\text{s}$ per nautical mile:

$\text{Slant Range (NM)} = \frac{\Delta t - 50.0\ \mu\text{s}}{12.36\ \mu\text{s/NM}}$

Where $\Delta t$ is the total elapsed time between interrogation transmission and reply reception.

Slant Range vs. Horizontal Distance

DME measures straight-line slant range, not horizontal ground distance. By the Pythagorean theorem:

$D_{\text{slant}} = \sqrt{D_{\text{ground}}^2 + h^2}$

Where $h$ is aircraft altitude above the station. If an aircraft flies directly over a DME station at an altitude of 6,076 feet (1.0 NM above the ground), the DME indicator reads 1.0 NM, even though the horizontal distance over the ground is zero. Slant-range error becomes negligible only when the aircraft is at a horizontal distance greater than 1 NM for every 1,000 feet of altitude.

Search Mode vs. Track Mode

Because hundreds of aircraft can interrogate a single ground station simultaneously, the ground transponder replies to all valid pulse pairs. To recognize its own replies, the airborne interrogator introduces pulse jitter (intentionally randomizing the time between transmitted pulse pairs):

• Search Mode: Interrogates at a high rate (up to 150 pulse pairs per second). The internal range gate sweeps across time, searching for reply pairs that synchronize with the pseudo-random interrogation jitter pattern.
• Track Mode: Once locked onto synchronized replies (typically within 1 second), the interrogator drops its interrogation rate to 25 to 30 pulse pairs per second, tracking the time delay and conserving transponder duty cycle.

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ATC Transponders: Mode A, Mode C, and Mode S

Air Traffic Control transponders operate as the airborne component of the Secondary Surveillance Radar (SSR) system. Ground radars transmit interrogations on 1030 MHz, and airborne transponders transmit replies on 1090 MHz.

             Transponder Reply Pulse Train (Mode A/C)

  F1   C1   A1   C2   A2   C4   A4   X   B1   D1   B2   D2   B4   D4   F2       SPI
  |    |    |    |    |    |    |    |   |    |    |    |    |    |    |        |
  |----+----+----+----+----+----+----+---+----+----+----+----+----+----+---|........|
  0   1.45 2.90 4.35 5.80 7.25 8.70 10.15 11.60 13.05 14.50 15.95 17.40 18.85 20.30  24.65 µs

Interrogation Formats and Side Lobe Suppression (SLS)

Ground radar interrogates transponders using pulse pairs $P1$ and $P3$:

• Mode A (Identity): $P1$ and $P3$ pulses spaced $8.0\ \mu\text{s}$ apart.
• Mode C (Altitude): $P1$ and $P3$ pulses spaced $21.0\ \mu\text{s}$ apart.

To prevent transponders from answering interrogations originating from ground antenna side lobes (which would cause false multiple targets on ATC screens), the ground station incorporates Side Lobe Suppression (SLS):

• The directional antenna transmits main-beam pulses $P1$ and $P3$.
• An omnidirectional antenna transmits pulse $P2$, exactly $2.0\ \mu\text{s}$ after $P1$.
• If the aircraft is in the main radar beam, $P1$ is significantly stronger than $P2$ ($P1 \ge P2 + 9\ \text{dB}$); the transponder processes the interrogation and replies.
• If the aircraft is outside the main beam, the omnidirectional $P2$ pulse is equal to or greater than $P1$ ($P2 \ge P1$); the transponder's internal SLS logic suppresses replies for $35\ \pm 10\ \mu\text{s}$.

Mode A Reply Structure and Octal Coding

A Mode A/C reply consists of two framing pulses, F1 and F2, spaced $20.3\ \mu\text{s}$ apart. Between the framing pulses are 12 information pulse positions, grouped into four octal digits:

$\text{Octal Groups: } A\ (A1, A2, A4),\ B\ (B1, B2, B4),\ C\ (C1, C2, C4),\ D\ (D1, D2, D4)$

This provides $2^{12} = 4{,}096$ selectable squawk codes (0000 to 7777). Special squawk codes include:

• 7500: Unlawful Interference (Hijack)
• 7600: Lost Communications
• 7700: General In-Flight Emergency
• 1200: Standard VFR visual flight rules in US airspace

Pressing the cockpit IDENT switch adds a Special Position Identification (SPI) pulse $4.35\ \mu\text{s}$ after F2, which causes the aircraft target to bloom (highlight) on the controller's radar screen for 18 to 20 seconds.

Mode C Pressure Altitude Encoding

Mode C provides pressure altitude data referenced to standard atmospheric datum (29.92 in Hg / 1013.25 hPa) regardless of the pilot's barometric altimeter setting. The transponder receives this data from an encoding altimeter or Air Data Computer (ADC):

• Gillham Code (Parallel): Uses 11 parallel wires carrying an inverted, cyclic Gray code representing altitude in 100-foot increments from $-1{,}000$ to $+126{,}700$ feet. Because Gray code changes only one bit at a time between successive increments, it prevents transition glitches.
• ARINC 429 (Serial): Modern digital installations transmit altitude serially over a high-speed or low-speed twisted-pair databus using standard label 203.

Mode S (Selective Interrogation)

In dense traffic sectors, conventional Mode A/C transponders suffer from "synchronous garble" (overlapping pulse replies from multiple aircraft). Mode S resolves this by assigning every aircraft a permanent, globally unique 24-bit ICAO aircraft address (providing over 16.7 million distinct addresses).

• Ground radars selectively address specific aircraft using Differential Phase Shift Keying (DPSK) on 1030 MHz.
• Transponders reply on 1090 MHz using Pulse Position Modulation (PPM) at a data rate of 1 Mbps.
• Mode S transponders emit periodic, unsolicited broadcast bursts known as squitters (Acquisition Squitters for TCAS tracking, and Extended Squitters carrying ADS-B navigational position).

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Airborne Weather Radar

Airborne weather radar systems detect precipitation, severe turbulence, and terrain ahead of the aircraft. Operating in the microwave X-band (~9.3 GHz to 9.4 GHz / $\approx 3.2\ \text{cm}$ wavelength), the radar achieves an optimal balance between antenna reflector size (fitting inside the nose radome) and atmospheric backscatter efficiency.

Rayleigh Scattering and Precipitation Reflectivity

Weather radar does not detect clouds, fog, water vapor, or dry hail directly; it detects liquid water droplets. Backscattered RF power ($P_r$) follows Rayleigh scattering principles, where reflected energy is proportional to droplet diameter ($d$) to the sixth power and inversely proportional to the fourth power of wavelength ($\lambda$):

$P_r \propto \frac{d^6}{\lambda^4}$

Because reflectivity scales with $d^6$, a wet raindrop reflects exponentially more energy than tiny fog or cloud droplets. Furthermore, liquid water has a dielectric constant approximately five times higher than frozen ice. Wet hail coated with a liquid water sheen produces the most intense radar returns.

Precipitation intensity is commonly displayed in color levels:

• Green: Light precipitation (minimal turbulence).
• Yellow: Moderate precipitation.
• Red: Heavy precipitation (strong convective downdrafts).
• Magenta: Extreme precipitation on many radars. Doppler-capable radars also show detected turbulence, in magenta or white depending on the system.

Gyro Antenna Stabilization and Tilt Control

Aircraft Pitching Up (+5°) ----> [AHRS / Gyro]
                                      |
                               [Pitch Signal: -5° Correction]
                                      v
                         [Antenna Stabilization Drive]
                                      |
                                      v
     [Radar Antenna Maintains Selected Sweep Angle Relative to Horizon]

The radar antenna (a flat-plate slotted array) produces a narrow conical pencil beam ($3^\circ$ to $8^\circ$ beam width). To maintain the scanned volume relative to the Earth's horizon when the aircraft pitches and banks, the antenna pedestal receives real-time analog or digital pitch and roll stabilization signals from the Attitude Heading Reference System (AHRS).

The flight crew adjusts a manual tilt control knob to angle the antenna beam above or below the horizontal datum. At high cruising altitudes, tilting the antenna down too far paints ground clutter (hills, cities, terrain); tilting it up too high sweeps the beam over the tops of active thunderstorms (skimming the dry "anvil"), presenting a dangerous false indication of clear weather.

Ground Clutter and MAP Mode

Modern radar systems incorporate Ground Clutter Suppression (GCS) algorithms. In Weather Mode (WX), signal processing attenuates non-Doppler shifted stationary targets (terrain). In Ground Map Mode (MAP), the antenna receiver gain and STC (Sensitivity Time Control) circuits are adjusted to accentuate land-water boundaries, coastlines, and mountains for navigation.

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Microwave RF Radiation Safety Distances

Airborne weather radars transmit peak pulse powers ranging from tens of watts (solid-state units) to several kilowatts (magnetron units). FAA Advisory Circular AC 20-68B, Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar, sets out the ground-operation precautions: