Pulse Systems Weather Radar Study Guide

Introduction to Radar Systems and Weather Forecasting

• Definition and History of Radar

  • Radar is an acronym for Radio Detection and Ranging, a term coined in 1942 by the US Navy.

  • It is a means of gathering information about distant objects (targets) by transmitting electromagnetic waves and analyzing the echoes.

  • It was developed independently and simultaneously just before World War 2 in Great Britain, the United States, Germany, and France.

  • Initial use: All-weather method for detecting approaching aircraft.

• The Need for Weather Radar

  • Weather forecasting was historically considered unreliable until the introduction of satellites.

  • Modern techniques still face challenges with rapidly changing conditions and lack of detailed information on the exact location and severity of bad weather.

  • Without airborne radar, many flights might face diversions or cancellations based solely on unreliable forecasts.

  • An airborne system is required to detect hazards such as turbulence, hail, and lightning.

Detection of Turbulence and Meteorological Hazards

• Turbulence Detection Challenges

  • Severe turbulence causes mechanical stress on aircraft, which may lead to damage or crashes.

  • Passenger comfort is a major commercial factor; frequent discomfort or sickness would reduce customers.

  • Turbulence (randomly moving air currents) is not currently amenable to direct detection by reliable techniques, though pulsed Doppler systems show potential.

• Indirect Detection via Precipitation and Electrical Activity

  • Current systems detect water droplets or electrical activity, both associated with convective turbulence in Cumulonimbus clouds.

  • Clear Air Turbulence (CAT): Has no detectable associated phenomena and provides no clues to its presence on conventional radar.

• The Ryan Stormscope

  • A weather avoidance system that uses different methods than radar.

  • It detects electrical activity (lightning discharges) using a directional antenna system to measure direction and intensity for establishing range.

  • It utilizes the electromagnetic radiation bursts produced by discharges, which are normally considered "noise" in other radio applications.

• Turbulence Correlation and Theory

  • Convective turbulence occurs due to large shear forces implying:

    • An up-draught supporting large raindrops formed from warm moist air.

    • A nearby down-draught that cannot support large raindrops.

    • Frictional forces causing charge separation.

    • Electrical discharge (lightning) due to charge separation in a saturated medium.

  • The ultimate justification for the association between turbulence, steep rainfall gradients, and electrical activity is recorded correlation from numerous flights.

Physics of Raindrop Illumination and Reflectivity

• Interaction with the Transmitted Pulse

  • The radar system illuminates the target with transmitted pulses.

  • When energy strikes a raindrop, three things occur:

    1. Refraction of some energy.

    2. Absorption of a small portion of the energy by the raindrop.

    3. Reflection of the remainder back to the antenna.

• The Reflectivity Factor ($Z$)

  • Reflectivity ($Z$) relates rainfall rate to the return signal strength.

  • Signal strength is proportional to the sixth power of the droplet diameter ($d^6$).

  • Strong signals indicate large droplets supported by rapid up-draughts.

  • A Steep Rainfall Gradient occurs when strong signals from one part of a cloud are adjacent to weak signals, indicating vertical wind shear (turbulence).

• Reflectivity Characteristics of Different Media

  • $Z$ is not an exact measurement as it varies by storm type and duration, but it indicates severity.

  • Water vs. Ice: Water reflects approximately 5 times more radar energy than solid ice particles of the same mass.

  • Hail: Specifically difficult to determine as it exists as dry, water-coated, or melted particles of irregular shapes.

  • Snow: Difficult to detect unless moisture content is sufficient for reflection.

Basic Principles and the Radar Mile

• Foundational Facts

  • Precipitation scatters RF energy.

  • The speed of propagation of an RF wave is known.

  • RF energy can be focused into a highly directional beam.

• Range Calculation

  • Range ($R$) is proportional to the elapsed time between transmission and reception ($t$):

  • $R = \frac{ct}{2}$

  • $c = 162,000\text{ nautical miles per second}$

  • The divisor 2 accounts for the two-way travel of the signal.

• The Radar Mile

  • The speed of light is approximately $300,000,000\text{ meters per second}$.

  • 1 Nautical Mile ($nm$) = $1852\text{ meters}$.

  • Time to travel 1 nautical mile = $6.2\,\mu s$.

  • Radar Mile Factor = $12.4\,\mu s$ (The time required for a pulse to travel 1 nautical mile and return).

Radar Display Systems

• Quantities Displayed

  1. Range

  2. Bearing

  3. Intensity of echo

• Plan Position Indicator (PPI)

  • The standard display type; aircraft/antenna is at the center, with distance shown as concentric circles.

  • A radial trace sweeps in unison with the antenna rotation.

• Cathode Ray Tube (CRT) and Intensity Modulation

  • Uses a beam of electrons velocity-modulated by signal strength.

  • Strong signals = higher electron velocity = brighter spot on the phosphor screen.

  • In a rho-theta display, the origin is at bottom center.

  • Every microsecond of round-trip time ≈ $0.081\text{ nautical miles}$.

  • For a $20\text{ nm}$ range selection, the time-base duration is approximately $250\,\mu s$.

• Modern Electronic Flight Instrument Systems (EFIS)

  • Replaces electromechanical and CRT displays with multi-color Liquid-Crystal Displays (LCD) or Light Emitting Diode (LED) screens.

  • Primary Flight Display (PFD): Integrates EADI, Air Data Computer, and EHSI data.

  • Navigation Display (ND): Combines EHSI data with moving maps, TCAS, and Weather Radar.

Operational Characteristics: Frequency and Pulse Parameters

• Frequency Bands

  • X-band ($8,000\text{ to }12,500\text{ MHz}$ / $3.2\text{ cm}$ wavelength): Provides high resolution, used by the majority of radars. Prone to signal absorption/scattering by small raindrops (attenuation distortion).

  • C-band ($4,000\text{ to }8,000\text{ MHz}$ / $5.5\text{ cm}$ wavelength): Better penetration through heavy precipitation but lower resolution.

• Pulse Width

  • Determines Minimum Range: A $2\,\mu s$ pulse results in a minimum range of approximately $1/6\text{ nautical mile}$.

  • Determines Range Resolution: A $2\,\mu s$ pulse occupies $2000\text{ feet}$; targets within $1000\text{ feet}$ of each other will merge on the display.

  • Weather radars use longer pulses ($2\text{ to }5\,\mu s$).

  • Pulse Compression: A technique (not on existing airborne units) using frequency modulation to achieve both high resolution and long-range power.

• Pulse Repetition Frequency (PRF)

  • Duty Cycle = $\text{Pulse Width} \times \text{PRF}$. Constant average power prevents overheating.

  • Second Trace Echoes: If PRF is too high, echoes from a previous pulse may appear on current trace (PRF < \frac{c}{2R}).

  • Example: For a max range of $200\text{ nm}$, the round trip is $2500\,\mu s$. A PRF of $500\text{ Hz}$ (Period = $2000\,\mu s$) would cause a second trace echo at $40\text{ nm}$ for a real target at $200\text{ nm}$.

  • Typical PRFs: $100\text{ to }250\text{ Hz}$ (modern), $400\text{ Hz}$ (older synchronous units).

• Beam Width and Bearing Resolution

  • Gain ($G$) is inversely proportional to range: $G = \frac{1}{\theta^2}$.

  • Narrow beams are preferred for increased range and better bearing resolution.

  • With a $4^{\circ}$ beam width, two targets separated by $3.5\text{ nm}$ at a range of $\ge 50\text{ nm}$ will merge.

Control Features and Signal Processing

• Tilt and Stabilization

  • Scanning is typically $\pm 90^{\circ}$ azimuth.

  • Tilt control: Adjusts the beam constant elevation relative to the horizontal (typically $\pm 15^{\circ}$).

  • Line-of-Sight Stabilization: Combines pitch ($P$) and roll ($R$) signals with azimuth angle ($\theta$) via a resolver. Output = $P \cos(\theta) + R \sin(\theta)$.

• Sensitivity Time Control (STC)

  • Adjusts receiver gain with range to compensate for the fact that signal strength decreases with distance.

  • Target power decreases with the inverse square ($1/R^2$) for beam-filling targets and inverse fourth power ($1/R^4$) for others.

  • Operates usually out to $30\text{ or }70\text{ nm}$.

• Automatic Gain Control (AGC)

  • Noise-derived AGC: Used in older analog systems to keep receiver noise output constant. Gated to sample noise just before transmission.

  • Preset Gain: Found in modern digital systems; gain is constant regardless of output changes.

• Iso-Echo Contour

  • Inverts signals exceeding a predetermined level to show heavy precipitation as dark holes within bright areas.

  • Narrow "paint" around the hole indicates a steep rainfall gradient and likely turbulence.

Microwave Components and Waveguides

• Waveguide Fundamentals

  • Metallic pipes used to transfer high-frequency energy via electromagnetic fields.

  • Advantages: neglible radiation loss, low dielectric loss (uses air), and the least copper ($I^2R$) loss compared to coaxial cables.

  • Dimensions: The "a" dimension (wide) determines the operating frequency range. The "b" dimension (narrow) determines the power handling capability.

• Types of Waveguides and Joints

  • Rectangular, Circular, and Ridged: Ridged guides have lower cut-off wavelengths and greater bandwidth but higher attenuation.

  • Flexible Waveguide: Used for short sections requiring movement/vibration; higher power loss due to interior ribbing.

  • Choke Joint: A semi-permanent joint providing electrical continuity with low power loss.

  • Rotating Joint: Necessary for the interface between stationary guides and the rotating antenna; typically circular in cross-section.

• Pressurization and Termination

  • Waveguides are pressurized with dry air or nitrogen to prevent dust/moisture ingress and condensation.

  • Dummy Load: Resistive load (graphite/sand or high-resistance rod) at the end of a guide to absorb energy as heat without causing standing waves.

• Transmitting Devices

  • Magnetron: Self-contained power oscillator used in radar.

  • Traveling Wave Tube (TWT): High-gain ($40\text{ dB}$ or more), low-noise wider bandwidth amplifier.

  • Gunn Diode: Semiconductor oscillator growing from GaAs on InP. X-band Gunn diodes ($8-12.4\text{ GHz}$) typically produce $300\text{ mW}$ output at $3.5\%$ efficiency.

Antenna Systems and Scanning

• Parabolic Reflector

  • Functions like a headlight; uses a dipole feed at the focus.

  • Suffers from "spill-over," leading to the height ring (ground returns from directly below).

• Flat Plate Planar Array

  • Consists of waveguide strips with staggered vertical slots.

  • Twice as efficient as parabolic versions; higher gain, narrower beam, and least side lobe power.

  • Beam width for a $28\text{-inch}$ diameter plate is approximately $3.5^{\circ}$ wide by $3.6^{\circ}$ high. Gain is around $34.5\text{ dB}$.

System Configuration and Operation (WXR Example)

• Receiver/Transmitter (R/T)

  • Peak power: $125\text{ W}$; average power: $500\text{ mW}$.

  • Operating frequency: Approximately $9.3\text{ GHz}$.

  • Weight: $32\text{ lb } (15\text{ kg})$.

• Modes of Operation

  • TEST: Initiates self-test and shows test patterns.

  • WX: Normal precipitation processing.

  • WX+T: Normal precipitation plus turbulence detection (active for ranges of $50\text{ nm}$ or less).

  • MAP: Ground mapping for features like coastlines/cities.

• Installation and Maintenance Safety

  • MPEL (Maximum Permissible Exposure Level): $10\text{ mW/cm}^2$. Most vulnerable tissues are eyes and testes.

  • Ground Operation Rules:

    • Never transmit from a stationary scanner.

    • Do not operate during refueling/defueling.

    • Do not operate near explosives or large reflecting objects.

    • Never look down an open waveguide.

• Maintenance Inspection

  • Check waveguide for cracks, dents, and corrosion.

  • Drain holes must face downward to remove condensation.

  • Antenna backlash check: $1/4\text{ inch}$ movement at the edge of a $30\text{-inch}$ dish equals nearly $1^{\circ}$ backlash.

Testing and Troubleshooting

• Self-Test Criteria

  • The antenna moves up/down and left/right, then centers.

  • EHSI displays "WXR" (Line 1) and "TEST" (Line 2).

  • Failure Codes on Line 3:

    • R/T: Failed receiver transmitter.

    • ANT: Failed antenna.

    • CTL: Failed control panel or waveguide switch.

    • ATT: Failed IRU attitude input.

    • WEAK: Calibration fault.

    • DSP: Invalid range word.

    • STAB: Stabilization circuit off.