Antenna, Waveguide, Display Subsystems and Interfaces to Navigation Equipment
Antenna
A slotted waveguide array, horizontally polarized, rotated at roughly 20–24 rpm (faster on high-speed craft). Beamwidth θ ≈ 70λ/D: a 1.8 m X-band antenna gives about 1.2°, a 3.7 m one about 0.6°. Vertical beamwidth is much wider (commonly 20–25°), which is why antenna height and sea state strongly affect clutter. Rotation rate sets picture update: 24 rpm is 2.5 s per sweep, during which a 20 kn ship moves about 26 m - hence afterglow and trails.
Waveguide
Rectangular guide carrying the TE10 mode, sized near 0.7λ × 0.35λ (X-band about 23 × 10 mm). Practical rules: keep flange faces clean and unmarked; use E-plane and H-plane bends and flexible sections instead of distorting the guide; pressurize with dry air or nitrogen from a dehydrator to keep moisture out and prevent arcing. Arcing shows as ozone or burnt smell, fluctuating magnetron current and lost range; moisture or a mismatched joint raises VSWR and can damage the magnetron.
Display and controls
Modern sets are raster PPIs with scan conversion and video processing. External controls include gain, sea (STC), rain (FTC), tune, range, VRM, EBL, heading marker, off-centering, target expansion, trails, interference rejection, parallel index, and setup menus for antenna height, heading alignment, range calibration and sector blanking.
Interfaces
- Heading: gyrocompass by synchro/stepper or NMEA HDT/HDG; required for true motion, ARPA and ECDIS overlay. Losing heading freezes the heading marker.
- Position and speed: GPS sentences (GGA, RMC, VTG) or speed log (VBW) for true motion and ARPA/AIS.
- AIS targets: NMEA VDM/VDO; ECDIS and ARPA links over IEC 61162 (NMEA 0183, 4800 baud) or NMEA 2000.
- NMEA 0183 allows one talker and several listeners; check baud rate, enabled sentences and a common ground.
Worked example
An ECDIS radar overlay is rotated about 7° from the chart, while ranges and echo shapes are correct. Correct range points to the position input being fine; a constant 7° offset points to heading - gyro repeater misalignment, a missing or wrong HDT/HDG sentence, or an incorrect heading calibration value in the radar. Random scatter would suggest interference or poor data, not a fixed bias.
Traps
- Assuming all scanners rotate at the same rate.
- Cleaning waveguide flanges with abrasives.
- Confusing VRM range accuracy with EBL bearing accuracy.
- Expecting a correct AIS overlay without a heading input.