Radar Flashcards
Horizontal beam width
HBW can vary between less than 1 degree up to 5 degrees for less expensive radars. Typically 0.8 - 1.2 for expensive radars. The narrower the HBW the more accurate bearings will be and will distinguish targets close together at similar range.
Vertical beam width
The vertical angle of the beam is typically 20 - 25 degrees. With half above the horizontal plane of the scanner and the other half below.
Explain procedure for starting up your radar.
(Brian Goes To Church)
On/Off/Standby
Brilliance - Set brightness of display, set range
Gain - Controls the degree of amplification of incoming signal, turn up until speckling appears then back a small amount.
Tuning - Adjusts the frequency generated in the receiver which will be mixed with returning signals. Must produce correct frequency or no picture will be displayed. On some sets this function is automatic.
Clutter - Reduce the Sea clutter (echoes of waves)
Rain clutter, rain can be visible on the screen and mask targets. This control reduces all echoes so that only their leading edges show, which can help detect targets in rain clutter.
Shadow and blind sector.
Many vessels have structures, such as funnel of the mast, that cause an obstruction to the radar energy being transmitted. This can cause sections where no energy is transmitted or received known as a blind sector.
As energy bends slightly around such obstructions these areas where reduced energy is transmitted and received is known as shadow sectors and will exist on either side.
3 Ways to check your blind sector
Use a target passing the sector and note when it disappears and reappears.
Turn the gain up until the sector becomes visible
You can physically measure the obstruction and the angle from your scanner
Different types of False Echoes
Radar to Radar interference
Multiple echoes
Side lobe echoes
Indirect echoes
Second trace echoes
Sea stabilised
Inputs needed
Used for collision avoidance to confirm aspect of target vessel
Course steered (Gyro compass)
Speed trough the water
Ground stabilised Radar
List 3 inputs
Used for pilotage
Inputs needed
- Course over ground
- Speed over ground (doppler log)
- GPS position
Checking your heading marker
Accuracy of the heading marker can be checked by switching to head up display and placing an object directly ahead of the vessel and checking that the object lines up with the heading marker.
Describe with aid of a sketch the effect of pulse length on range discrimination
Long pulse gives poor range discrimination
Short pulse gives good range discrimination
False echoes
Radar to Radar Interference
Radar to Radar Interference. Large number of bright dots scattered over the screen usually in a radial spiral pattern. Caused by receiving radar energy transmitted by other vessels at or near the frequency of the observer’s radar.
False echoes
Multiple echoes
Multiple Echoes. These echoes may appear when the observer’s ship and another ship are passing on parallel courses at close range. Some of the returning radar energy may reflect between the two hulls one or more times before returning to the scanner. This will result in one or more false echoes on the same bearing as the real echo, but at multiples of its distance. These echoes are always weaker than the true target and are rarely troublesome. They will usually disappear as soon as the vessels have passed the beam-on position.
False Echoes
Sidelobe Echoes
Sidelobe Echoes. These echoes are also associated with good targets at close range. A very small portion of the transmitted radar energy is radiated in side lobes at various angles either side of the main beam. This is too small to be reflected under normal conditions but may reflect from strong targets at close range. The echoes will display on either side of the true target at the same range. Removable can be effected by careful adjustment of the Sea Clutter control.
False echoes
Indirect Echoes
Indirect Echoes. Indirect reflection from an obstruction on the observing ship can cause the radar energy to be reflected, strike a target on some other bearing, and return by the same route.
This will result in a faise echo appearing on the screen on the bearing of the obstruction, but at the same range as a target. An obstruction that causes an indirect echo will most likely also cause a blind or shadow sector as well. For this reason indirect echoes are usually seen in these sectors, often right ahead and right astern. The danger of these echoes is that a real target, especially right ahead, may be mistaken for an indirect echo. A small alteration of course will cause an indirect echo to either disappear, or move with the ship’s head, whereas a real target will stay in the same position. If obstructions are large the effect can be minimised by fitting radar scattering material to the obstruction to scatter the energy.
False Echoes
Second-trace Echoes
Second-Trace Echoes.
Echoes that return from a range greater than the scale in use will not show. During conditions of super refraction or ducting echoes may be received from greater distances, even after the following pulse has been transmitted. If these then return during the time the receiver is open they will appear on the screen during the second trace after they were transmitted. The resultant echo will be at an incorrect (closer) range and will also be distorted.
Changing the range scale may cause such an echo to disappear or to jump in range.
Spurious return
Often referred to as radar to radar interference. This is caused when another vessel in close proximity to your own vessel is using a radar on the same wavelength as your own radar.
State 6 factors which influence the detection range of radar
Pulse length
Height of scanner
Radar power
Propagations
Seastate
Precipitation
Which radar controls can be manipulated to search for a target within area of precipitation
Rain Clutter (Fast Time Constant)
Rain can be visible on the screen and mask other targets. This control reduces all echoes so that only their leading edges show, which can help detect targets lost in the rain clutter.
3 cm wavelength radar is more affected than 10 cm.
Parallel Indexing (PI)
Parallel Indexing (PI). This is a useful method of checking on cross-track drift by continuously monitoring progress relative to the movement of the echo of a radar-conspicuous fixed object. A lighthouse or headland will apparently track past own ship, depicted as being at the centre of the screen, on a line parallel and opposite to the ship’s ground track. Any cross-track tendency will become apparent by the target moving off the parallel line.
Outline possible danger using ground stabilised information for collision avoidance
It is essential that the course and speed of own vessel, used in radar plotting, is the course and speed through the water. Using course and speed over the ground (COG and SOG) can give a false impression of the aspect of an approaching vessel.
Prior to starting up radar state 2 safety precautions
Ensure no permit to work is open
Visually check for obstructions around scanner
State the effect of a mis aligned heading marker on true and relative bearings
Both true and relative bearing will have the same error as the heading marker.
