Radar & ARPA – Types, Principle, Errors, Limitations, Setup

Principle: it Works on Echo Principle (Echo ranging by radar) which is based on a time interval between the transmission of an EM pulse into the environment and reception of its echo.

Type Of Radars:

X Band RadarS Band Radar
X Band 3cm, 9 GHZS Band 10 cm, 3 GHZ
Can detect SARTBetter Tar detecting in RAIN & heavy Weather
In fair weather target will show betterBetter for long range detection (radar horizon greater than 9 GHZ
For high discrimination, good sensitivity and tracking performance.Good sea clutter response, tgt less likely to be hidden.
Less affected by target swapBetter for and Rain clutter.
Can see small targets due to high Freq

Radar Errors:

  1. Indirect Echo (reflected): Range true but bearing of a vessel in direction of the obstruction. (mast, funnel or containers on a container ship)
  2. Multiple Echo: Bounce of energy back and forth, entering antenna in each bounce. . .  .
  3. Side Lobes: Leakage in energy
  4. Radar to radar interference: Transmission of other radar picked up by our radar.
  5. Second Trace Echoes: A sent no return, B sent–A return

Radar Limitations:

  1. Range Discrimination: Same Bearing different range, the ability of the RADAR set to clearly distinguish two small targets on the same bearing at slightly different ranges. The distance between the two targets is equal to or less than ½ PL.
  2. Bearing Discrimination: 2 targets on the same range but different bearing, separated by 2.5° d, the ability of the radar set to clearly extinguish two target of the same range and slightly different bearings. (Factor affecting bearing discrimination: High Beam Width)
  3. Minimum Range:
  4. Blind/Shadow sectors: Ship structure such as Mast or funnel might create a blind sector.

Ground Stabilization and Sea Stabilization:

Ground StabilizationSea Stabilization
Speed and Heading input from GPSInput from Gyro and Log
Gives course and speed over groundCourse and speed through water
Used for Navigation in pilotage, coastalUsed for Anti-collision
Set and Drift into accountDo not take set and drift into account
True aspect of vessel unknown True aspect of vessel known
Stop vsl, move along same rate of set & driftStop vessel appear to be stopped
Fixed target appear STATIONARYFix Tgt – Vector opp to set and same rate of drift


Related Questions:

  1. Radar Range and Bearing Accuracy:
    → Range Within 30m or 1% of the range scale in use (whichever is greater);
    → Bearing within 1 degree.
  2. How to check the performance of radar?
    → By “Performance Monitor” instruction in the user manual
  3. How often?
    → After start-up and once every watch
  4. Radar Availability:
    → Fully functional within 4 minutes after switching on.
    → Fully operational within 5 seconds from standby condition.
  5. What is Radar Stabilized? – When getting Heading input from the gyro.
  6. What happens if Gyro fails or Gyro input lost? – The radar will go into “HEAD-UP Orientation”
Radar Setup for Collision Avoidance:
  1. Relative Motion: Helps is maintaining situational awareness.
  2. CU / NU—I prefer CU, as the screen and outside of window view remain the same.
  3. True trail: To get the true aspect of a vessel
  4. Relative vector: To see the relative line of approach.
  5. Speed through LOG Sea stabilization mode.
Radar Startup & Setup:
  1. Check scanner for any obstruction (Halyard, working aloft, any notice near radar screen)
  2. Switch on radar (do setting till its warming up [What happens in warmup and how much time?])
  3. Select orientation, motion check heading and speed input (from were heading and speed input)
  4. Once you see Standby on-screen, Press “TRANSMIT
  5. Select Range, Pulse length.
  6. Check Heading marker is aligned with the fore-and-aft line of the vessel.
  7. Adjust (B-G-T-R-S)
    Brilliance: Such that clearly visible and do not hurt your eyes.
    Gain: Such as you can see fine speckles on screen. (What Gain does?)
    Tunning: Such that targets are best seen on screen (What Tunning does?)
    Rain: Only use this if it’s raining. (Always use before Sea clutter, but WHY?)
    Sea: If required in bad weather.
  8. Check Radar Performance by “Performance Monitor” (What is it?)

Thank you, do you have any questions related to Radar and Arpa? Let me know.