RADAR RAdio Detection and Ranging Background Measuring Distance

RADAR RAdio Detection and Ranging

Background Measuring Distance Measuring Direction Radio waves travel at a constant speed, so the time between the pulse been transmitted and its echo being received gives an indication of distance Radio waves travel at 162000 nautical miles per second – 300 metres per microsecond If a pulse is received 100 milliseconds after it was transmitted its travelled 30000 metres – 30 kilometres Distance to object and back so object is 15 kilometres away – 8 nautical miles Radar uses a continuous rotating aerial called a scanner to measure the direction from which returning echoes are received The same aerial is used to transmit the pulses Rotates at about 20 rpm

Terminology Pulse – the short burst of microwave energy transmitted by the radar Echo – the short burst of microwave energy returned to the radar from the target Target – the object that returns the echo Contact – the representation of a target on a radar screen – the blob Millisecond – one thousandth of a second Microsecond – one millionth of a second

Frequency A small craft radar transmits and receives at a frequency of about 9. 4 gigahertz – 9400 million waves per second – 60 times higher than a VHF radio – with a wavelength of just over 3 centimetres This is why small craft radar is referred to as 3 centimetres or X band to distinguish it from 10 centimetres or S band on larger ships.

Limitations of Small Craft Radar Power – small craft radar transmits at 1. 5 – 5 k. W (1500 -5000 Watts) this is 1000 times more than a marine VHF Commercial ships are 25 – 50 k. W The echoes that the radar depends on are very much weaker than the original transmission, so a low power radar is likely to miss objects detected by a powerful one, small targets and distant targets are effected most Antenna Size – the job of the antenna is to focus the outgoing transmission into a narrow beam and to measure the direction from which the echo returns – Antenna width 120 cms – 1. 8 degrees beam width 30 cms – 8 degrees width Display Size – a small display is less clear than a larger one – gaps between contacts and smaller contacts can be missed Display Resolution – most modern displays are similar to TVs and Computer screens – made up of dots (pixels) the more dots the better resolution

Switch on and Set Up Power on/off – turn on and let the set do its automated checks and then warm up, approx 2 minutes Gain – is the radars counterpart of the squelch on a VHF radio, it regulates the sensitivity of the receiver – to adjust turn it up until the screen is filled with speckles and then turn it down till the speckles just disappear Range – is used to adjust the area represented, if smaller the images will show more detail, changing the range also changes the duration of the pulses and the interval between them

The Radar Picture Head Up – in basic models with no compass input head up is the norm Advantage – target on the left side are to port and right to starboard North Up – need input from an electronic compass, advantage is that they are orientated as a chart is Disadvantage is that many small boat owners find it difficult to interpret a collision situation Course Up – shows the course that you are steering very much like head up Sea Clutter – radar pluses from the face of nearby waves Rain Clutter – just like sea clutter it’s a mass of contacts forming a large irregular blob, not just rain but can be caused by Cumulonimbus clouds that are full of water These can be adjusted out with Anti Clutter FTC

Understanding the Picture The width of the radar beam means even a small target may produce quite a large contact as it start producing echoes as soon as one edge of the beam touches it and produces echoes till the beam has swept over it On a radar with 6 degree beam a small target such as a buoy could appear 6 degrees across Gaps between objects – buoys or headlands – will appear smaller, harbour entrances will not show until it is close enough for the beam to pass straight through without touching the sides On radar with a 6 degree beam width a harbour entrance 200 metres wide is unlikely to show up as a gap until it is less that a mile away This is known as bearing discrimination

Vertical Beam Width A radar beam is usually higher than it is wide, for small craft radar the vertical beam width is about 25 – 30 degrees. This si to ensure that when the boat pitches and rolls that at least part of the beam will still be pointing towards the horizon The vertical beam width also explains why rain clouds produce strong echoes – at a range of 6 miles a typical small boat radar will see clouds up to an altitude of 3000 metres

Three Point Fixing by radar is possible but not recommended All practical marine radars are equipped with an Electronic Bearing Line – a straight line radiating out from the centre to the edge of the screen. The bearing represented by the EBL is displayed in a data box If the radar is not interfaced to a compass the bearing show will be relative to the heading clockwise Objects such as churches, lighthouses do not usually show up on radar The bearing on the edge of a piece of land needs to be adjusted for the beam width On head up radar you will have to adjust the bearing for course being steered Fix using radar range – Identify landmarks on the chart as well as on the radar Measure ranges as accurately as possible, use shortest range Measure as quickly as possible Choose landmarks that are well spread Measure ranges that are changing quickly last Most of this is like taking a 3 point fix with a hand bearing compass

Pilotage by Radar can be a very powerful pilotage tool particularly in poor visibility or darkness but using it effectively takes practice Pilotage by radar can be buoy hopping or just heading for open water up a channel The radar version of eyeball navigation suffer similar problems. Keeping track of where you are and where you should be going next Be aware of objects that will not show up on radar such as shallow water Be careful that you are not being swept of track by tide or wind

Collision Avoidance Relative motion – Your vessel is always at the centre of the radar screen, if you are moving towards a stationary object (a buoy) the radar picture will give the impression that you are stationary and the buoy is moving towards you If you are dealing with a moving object then its relative motion depends on its own movement as well as yours Assessing risk – steady bearing test can be carried out by the EBL which is the equivalent of a hand bearing compass If you are not sure if there is a risk of collision plot the range and bearing of the contact on a plotting paper sheet or the screen and after a few plots you should be able to see how close the contact will get to the centre of the screen – where you are Most small craft radar have a guard zone which allow the operator to mark areas on the screen to sound an alarm whenever a contact appears in a marked area