Characteristics Of A RADAR
1. Vertical Beam Width (VBW)
VBW is the vertical angle at the scanner contained between the upper and lower edges of the RADAR beam.
The upper and lower edges of the beam are taken to be the lines joining the half-power points above and below the centre of the beam.
If the VBW was too small, targets would be missed due to rolling and pitching.
If the VBW was too large, the RADAR energy sent out through the scanner would be spread out over a large vertical angle.
As per Performance Standards for Navigational RADAR (IMO), the RADAR should function without deterioration in performance when the vessel is rolling or pitching up to +/- 10º.
In commercial Marine RADAR sets, VBW is anything between 15º & 30º. The value of VBW depends on the constructional details of the scanner.
2. Horizontal Beam Width (HBW)-
HBW is the horizontal angle at the scanner contained between the leading and trailing edges of the RADAR beam.
The leading and trailing edges of the beam are taken to be the lines joining the half-power points ahead and behind the centre of the beam.
HBW causes all targets to appear larger in azimuth, by an amount equal to half the HBW on either side.
This is because echoing from a target commences when the leading edge of the beam touches the target and continues until the trailing edge of the beam has left the target. During this time, the scanner and the trace would have rotated thru an arc equal to the angular size of the target plus HBW. A point target thus appears as an arc subtending an angle equal to HBW, at the centre of the PPI. This is called Beam Width Distortion.
The value of the HBW of a RADAR set depends on the scanner, the larger the horizontal size, the smaller the HBW and vice versa.
In commercial Marine RADAR sets, HBW is between 0.6º to 2.0º.
3. Pulse length –
PL is the time taken for a pulse to leave the scanner i.e. the interval between the instant the leading edge of the pulse leaves the scanner and the instant the trailing edge does so.
PL is, therefore, usually expressed in micro-seconds but, the speed of radio waves being taken to be 300 m/microseconds, PL may also be expressed in metres. PL is controlled by the transmitter.
When the leading edge of the echo enters the receiver, the tracing spot on the screen becomes fat and bright and remains so until the entire echo comes in.
During this interval, the tracing spot would have covered a distance equal to half the PL in metres on a radial path.
The paint on the PPI would hence appear to have a radial depth equal to half the PL in metres.
To ensure range accuracy, the tracing spot is synchronised with the leading edge of the pulse. Hence the correct range of a target is the range of the nearest edge of its paint on the screen. PL, therefore, does not affect range accuracy. PL does affect range discrimination.
Short pulses are suitable for the shorter range scales as they give better Range Discrimination.
To cover longer ranges, however, long pulses would have to be sent out to allow for ATTENUATION (loss of energy) in the atmosphere.
Long pulses would be unsuitable for short ranges because all targets painted on the screen would appear too large in the radial direction.
Marine RADARs use PL between 0.05µ (15m/micro sec) to 1.00µ (300m/micro sec).
During normal operation, short pulses should be used.
When searching for specific targets such as buoys, light vessels etc., or when making landfall, long pulses may be used.
4. Pulse Repetition Frequency (PRF)-
PRF is the number of pulses sent out through the scanner in one second.
Commercial Marine RADAR sets usually have two or three values of PRF between 500 and 4000. A high value of PRF is preferable for a clear and detailed picture.
On longer-range scales, this is not possible because a greater interval between pulses is required for each pulse to go a long distance and come back necessitating a low PRF
Longer range scales, therefore, have a low PRF while the short-range scales have a high PRF.
The picture resolution of a RADAR set is governed by its RPM-PRF relationship.
The wavelength of a commercial Marine RADAR set may be about 3cm (9200 to 9500 mega Hertz called the X-band) or about 10 cm (2900 to 3100 MHz called the S-band).
RADAR pulses of different wavelengths are influenced differently by external factors.
6. Attenuation In The Atmosphere-
It is the loss of energy caused by scattering, absorption, diffraction, etc. It is greater when using waves of shorter length.
The picture obtained by S-band RADAR is less affected by sea clutter and rain clutter.
When a ray of energy passes very close to any object, it is bent slightly towards the object.
10cm waves are diffracted more than 3 cm waves.
The surface detection range of far off-targets using longer waves is thus greater than when using shorter waves.