Limitations of Marine RADAR
1. Range Discrimination.
2. Bearing Discrimination.
3. Factors Affecting Minimum Range.
4. Factors Affecting Maximum Range.
5. Range Accuracy.
6. Bearing Accuracy.
1. Range Discrimination-
It is the ability of the RADAR to clearly distinguish two targets on the same bearing with a slight difference in ranges as two separate targets.
The factor which governs range discrimination is PL.
Pulse length makes all targets appear larger in radial depth by an amount equal to half PL in metres.
If the gap between the two targets is less than ½ PL in metres, then these targets would appear as one target on the PPI.
As per IMO Performance standards for RADAR, two small targets on the same bearing and separated by a distance of 40 metres on 1.5 miles range scale (placed at 50 to 100% of PPI), shall appear as two separate targets on the PPI.
2. Bearing Discrimination
A RADAR's ability to distinguish two targets on the same range is slightly different in bearings as two separate targets.
The factor that governs bearing discrimination is HBW, which causes all targets to appear larger in Azimuth by equal to half HBW on either side, i.e. one full HBW.
So if the angle at the Electronic Centre made by the inner corners of two targets is equal to or less than HBW their paints would merge on the PPI and they would appear as one big target.
Bearing discrimination is expressed in degrees and as per Performance Standards for Navigational RADAR (IMO), it should not exceed 2.5º.
3. Factors Affecting Minimum Range
The minimum detection range of the RADAR set depends on:
a. The Pulse Length
Since the same waveguide and scanner are used for transmission and reception, a TR cell unit is fitted. This unit blocks the receiver branch of the waveguide during transmission to prevent the transmitted energy from short-circuiting into the receiver and damaging it.
The TR cell ensures that reception starts only after the transmission is over.
Hence targets closer than ½ PL in metres cannot be shown on the PPI because their echoes would come back before the reception starts.
The theoretical minimum range of detection is therefore represented by ½ PL in metres.
b. The De-ionisation Delay
A small delay occurs in the TR cell between the completion of transmission and the commencement of reception. This delay increases the minimum detection range.
c. The VBW And The Height Of The Scanner
The VBW and the height of the scanner above sea level affect the minimum range as these two factors govern the distance off at which the lower edge of the RADAR beam would strike the sea surface.
The VBW of a given scanner is fixed whereas its height above the sea level depends on the ship's draft at that time.
The higher the scanner above sea level, the greater the minimum range of detection and vice versa.
d. The Wavelength
The minimum detection range of small targets is better when using the X-band than when using the S-band. As per Performance Standards for Navigational RADAR (IMO), the minimum detection range, with a scanner 15 m High shall not exceed 40 m.
4. Factors Affecting Maximum Range
a. Height Of Scanner
The greater the height of the scanner above the sea level, the greater the detection range. However, two major disadvantages may be experienced with a very high scanner – increased minimum detection range and also more clutter.
b. Power Of The Set
The greater the power of transmission, the greater the expected maximum range. The peak power of transmission of commercial Marine RADAR sets is around 25 to 60 kilowatts.
c. Wavelength
S-band waves have greater max. range than the X-band due to less attenuation in the atmosphere and more diffraction.
d. Pulse Repetition Frequency
Each value of PRF has a maximum range to which it can measure.
Each PRF is allotted a range scale so that echoes from one pulse do not get mixed up with echoes from the next pulse.
e. Pulse Length
Long pulses having more energy and the ability to suffer more attenuation in the atmosphere, ensure better maximum ranges than short pulses.
f. VBW & HBW
The narrower the beamwidths, the greater the directional concentration of the transmitted energy and hence the greater the maximum range.
g. Receiver Sensitivity
The sensitivity of a receiver is its ability to amplify a very weak signal sufficiently without losing the signal in receiver noise. Receiver sensitivity is one of the very important factors that determine the maximum detection range of a target.
h. Nature Of Target
The height, horizontal size, nature of surface, shape, and material of a target and also its aspect greatly affect the maximum detection range.
i. Weather Effect
Rain, snow, hail, fog etc. cause attenuation, resulting in a decrease in detection range.
j. Anomalous Propagation
Super-refraction causes an increase in maximum detection range whereas sub-refraction causes a decrease.
k. Sea and swell
Rough sea and heavy swell cause rolling and pitching whereby the scanner goes well out of the vertical, reducing the maximum detection ranges of targets.
5. Range Accuracy
As per Performance Standards for Navigational RADAR (IMO), the error in the range of an object obtained using range rings or the VRM, should not exceed 1% of the maximum range of the scale in use, or 30 m whichever is greater.
Range accuracy of RADAR depends on:
a. Correct synchronisation between the transmission of the pulse and the commencement of the trace.
b. Uniformity And Rectilinearity of the time base.
Uniformity of the time base means that the speed of the tracing spot must be very steady. Rectilinearity means that each trace created should be a perfectly straight line. The speed of the spot should be half that of the radio waves.
c. The Scale Of Size Of The Tracing Spot
The scale size of the spot increases as the range scale is increased.
d. Height Of Scanner
When observing small targets very close by, the RADAR measures the range from the scanner to the target whereas the correct range should be the distance along the surface of the earth.
6. Bearing Accuracy
As per Performance Standards for Navigational RADAR (IMO), the RADAR bearing of an object whose echo appears on the edge of the display should be capable of being measured with an accuracy equal to, or better than +/- 1º.
Factors that govern bearing accuracy are:
a. Correct alignment between the heading marker and the scanner.
b. Correct alignment bet. The heading marker and the bearing scale.
c. Gyro error, if any when the display is Gyro-stabilised.
d. Rectilinearity of the trace.
e. Beamwidth distortion.
f. Scale size of the spot.