ระบบเรดาร์สภาพอากาศ

ภาพรวม

Airborne weather radar is a pulse-Doppler radar system operating in the X-band (9 GHz, 3 cm wavelength) that detects and maps precipitation — rain, hail, and snow — ahead of the aircraft. By measuring the intensity of radar returns from water droplets and their radial velocity (Doppler shift), modern weather radars can detect convective cells, estimate hail probability, identify areas of severe turbulence, and warn of windshear conditions near airports. The system displays color-coded precipitation levels on the navigation display, giving crews timely awareness to deviate around hazardous weather.

Weather radar became standard on commercial jet transports following a series of fatal weather-related accidents in the 1950s and 1960s. The 1974 crash of Eastern Air Lines Flight 66 into windshear at JFK accelerated development of radar-based windshear detection. Today, dual-channel predictive windshear systems are mandated by FAR 121.356 for US air carriers, and equivalent regulations apply worldwide. Modern systems such as the Honeywell RDR-4000 and Garmin GWX 80 offer fully automatic tilt management and sector scan, significantly reducing the pilot workload required to operate the radar effectively.

หลักการทำงาน

A flat-plate phased-array antenna behind the radome in the aircraft nose transmits pulses of microwave energy. These pulses scatter off precipitation particles, and a fraction returns to the antenna. The radar receiver measures the time delay (for range), amplitude (for precipitation intensity), and Doppler frequency shift (for radial velocity). Processing algorithms classify returns into precipitation levels (green, yellow, red, magenta for extreme) and compute derived products such as turbulence intensity (from velocity variance), hail index (from reflectivity profiles), and windshear alerts (from velocity divergence near the runway).

The antenna tilts automatically or manually in elevation to scan multiple beam positions, building a 3D picture of weather ahead. Ground clutter suppression filters remove returns from terrain. In predictive windshear mode, the radar scans a 60-degree sector in the forward arc out to about 5 nm, looking for the velocity divergence signature characteristic of a microburst — a downburst that creates a strong headwind-to-tailwind transition on approach.

ส่วนประกอบหลัก

• Flat-Plate Phased-Array Antenna: Typically 12–30 inches in diameter (larger on widebodies), mounted on a gimbal behind the radome. Phased array designs steer the beam electronically for faster sector scanning.
• Radar Receiver/Transmitter (R/T): The transmitter generates the microwave pulses; the receiver amplifies and digitizes the backscatter signal for processing.
• Weather Radar Computer: Processes raw radar returns into precipitation maps, turbulence overlays, windshear alerts, and hail probability using proprietary algorithms.
• Radome: The composite nose cone must be aerodynamically smooth and electromagnetically transparent to X-band radar while surviving bird strikes and lightning. Damage or moisture ingress significantly degrades radar performance.
• Control Panel: Mounted on the instrument panel or glareshield, controls gain, tilt, range, mode selection, and Doppler function. Modern systems use automatic gain and tilt.

การใช้งานบนเครื่องบิน

• Boeing 737-800: Honeywell RDR-4B or Rockwell Collins WXR-2100 MultiScan, displaying on the ND in weather overlay mode with automatic tilt management for hands-off operation.
• Airbus A320-200: Thales RDR-1600 or Honeywell RDR-4000, integrated with the ECAM for predictive windshear audio/visual alerts independent of the ND display.
• Boeing 777-300ER: Honeywell RDR-4000 with MultiScan technology, scanning up to 40 beam positions per second and displaying turbulence as a magenta overlay on the ND.
• Boeing 787-9: Honeywell RDR-4000 integrated with the Common Core System, sharing terrain database information with EGPWS to suppress ground clutter in mountainous terrain.

ข้อดีและข้อจำกัด

Weather radar provides the primary means for pilots to identify and avoid convective weather, reducing the risk of structural damage, upset, or loss of control in severe turbulence. Predictive windshear capability gives crews at least 15 seconds of warning before encountering a microburst, sufficient time to initiate a go-around. The Doppler turbulence function gives qualitative indication of clear-air turbulence intensity, which is otherwise undetectable by traditional reflectivity radar.

Key limitations include the inability to detect clear-air turbulence (CAT) directly — CAT lacks precipitation and produces no radar return. Dry turbulence in mountain waves or jet streams is similarly invisible. Radar shadows behind intense cells can conceal additional hazardous weather. Interpretation requires crew training and experience; poorly managed tilt settings can cause pilots to miss weather above or below the current scan plane. Satellite-based weather uplinks via datalink (XM Weather, SiriusXM, ACARS) supplement but do not replace airborne radar for real-time hazard detection.