Ranging system errors include timing error, uniform common datum error, pixel error, pulse width error, and active distance circle error. Among them, the pixel error and pulse width error do not need to be maintained and corrected, so the following only describes the distance error caused by the timing error, the uniform common datum error, and the movable distance circle error.

(i) Timing errors

Causes:Differences in the length of the signal transmission line between the antenna unit and the processor unit can lead to differences in scanning timing and thus timing errors. If more than one radar antenna is installed, the distance measurement error of the selected antennas has to be compensated.

Typical phenomenon:On the 0.25 n mile range, echoes from “flat” targets (e.g., breakwaters, bridge abutments) appear concave or convex on the screen, and the distance to the target echo is not accurately displayed.

(ii) Harmonization of public datum errors

Radar measurements of target range, bearing and other tracking data are referenced to the Common Common Reference Point (CCRP). The Marine Radar Performance Standard states that if multiple antennas are installed, the radar system must provide a method of compensating for the positional deviation of each antenna. The radar CCRP deviation compensation setup must be completed during the initialization phase of the installation. Inaccurate amount of deviation compensation will result in an error in the target range measurement on the radar display relative to the CCRP.

Before setting, you need to calibrate the radar timing error, and then set and adjust the CCRP position according to the steps specified in the technical manual. Take Furuno FAR-2××8 series radar as an example, the operation of changing the reference position of CCRP is as follows: Use the trackball to move the cursor to the “REF POINT” indication at the top of the screen, and then left click to select [ANT] or [CCRP] according to the requirement. Where [ANT] means to use the radar antenna position as the reference position, and [CCRP] means to use the position of the driver's console comprehensive information display (CONNING) as the reference position.

(iii) Activity distance marker circle error

The radar is equipped with at least 2 VRMs for accurate measurement of the target range, and the VRM errors will directly cause the target range measurement to deviate. For PPI radars, the VRM error correction can be checked at any time, or at least once per flight or per month, whichever is shorter. According to the radar performance standard, the system accuracy of the RR should be 1% or 30m, whichever is greater, of the maximum range used.

The azimuth system errors include beam width error, pixel error, first line error, compass repeater error, common datum error, antenna main flap deviation angle and beam asymmetry error, etc. Among them, the beam width error, pixel error, antenna main flap deviation angle and beam asymmetry error do not require maintenance correction. Among them, the beam width error, pixel error, antenna main flap deviation angle, and beam asymmetry error do not need to be corrected. In the following, only the first line error, the compass indication error and the bearing error caused by the unified common reference point are introduced for the attention of the electrician and technician. The Radar Performance Standard states that, irrespective of the type of stabilizing sensor and transmission system, the accuracy of the radar display bearing correction should be within 0.5° when the ship's slew rate is in general conformity with the appropriate class specification.

(i) First-line error

The first line is a scanning line with enhanced brightness displayed on the screen when the main antenna flap is pointing parallel to the ship's keel. It is the datum line for radar bearing measurement, and its error will lead to the relative bearing and true bearing error of target measurement. The checking of the first line error should be carried out every voyage or every month (whichever is smaller) to ensure the accuracy of the bearing measurement.

(ii) Indication error of the compass repeater

A compass repeater error will result in a fixed error in the radar headline and target true bearing readings. This is eliminated by comparing the radar compass repeater with the main compass bearing and adjusting the compass repeater to agree with the main compass. Checks for this error are normally carried out once a cruise or once a month, whichever is the lesser.

(iii) Harmonization of public datum errors

Inaccurate CCRP error compensation will result in an error in the target orientation measurement relative to the CCRP. To change the reference position of the Furuno FAR-2××8 series radar, move the cursor to the “REF POINT” indication and click to select [ANT] (antenna reference) or [CCRP] (cockpit CCRP reference).

Distance and first line calibration specific steps:

1. (1) Press and hold [5:HL OFF] key while pressing the [1:HL OFF] key to enter the initialization menu (INITIALIZE).
2. (2) Selection [2:ECHO ADJ] Enter the distance and first line calibration screen.
3. (3) Selection [3:HD ALIGN] Calibrating the bowline error limits the error to 0.2° or less。
4. (4) Selection [4:TIMING ADJ] Distance error calibration is achieved by adjusting the transmit delay.

Initialization setup parameters include: radar ranging and azimuth correction, antenna position, and comprehensive information display system (Conning) position. See Table 1-3-1 for details.