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How A radar works

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BASICALLY a radar set mea­sures relative range and bearing - though most modern ones can be interfaced with an electronic compass to give an azimuth sta­bilised display. Technology aside, probably the two common­est sources of error when mea­suring range and bearing are the fault of the operator: recording the wrong measurement and mistaken target identity - of which more later. For now, let's look at the radar's role with regard to accuracy. BEARING ACCURACY This is radar's least accurate feature. The effect of beam width on discrimi­nation has already been discussed and it should come as no surprise that it has an effect on bearing accuracy. When taking bearings of headlands an allowance should be made for half the beam width - which is all very well in theory but it's often difficult to relate the radar picture to what's actually shown on the chart, especially on low­lying coastlines. When taking radar bearings of an isolated contact the elec­tronic bearing line, EBL, should pass through the centre of the echo. If the radar isn't azimuth stabilised, you'll have to note the yacht's heading at the same instant - by no means an easy task, especially when the vessel is yawing. (Incidentally, if you ask a helmsman to call out his heading, miraculously they're always right on course! You may have to press him to be honest.) When side echoes are present you should appreciate that they may not be symmetrical. So, a tempo­rary adjustment of the gain control may be advisable to reduce this effect. Bearing accuracy is greatly influ­enced by yawing. Unless azimuth sta­bilised, it's virtually impossible to achieve any degree of accuracy in any­thing but quite calm conditions. Azimuth stabilisation rates highly on my list of optional extras, and is well worth the relatively small extra expense. A saving grace in dense fog is that often the wind is very light and a reasonable plot can be achieved. On a recent trip across the English Channel, fog set in unexpectedly whilst crossing the shipping lanes and I was able to plot quite successfully for a period of about seven hours in flat calm condi­tions. It was interesting to note that one of the larger contacts which passed us by less than a mile was doing about 25 knots and, as far as one could tell, was sounding no fog signal! A very common cause of bearing error is Heading Marker misalignment. An electronic switch in the scanner unit causes the Heading Marker to flash. This switch is activated by the scanner and should fIre the Heading Marker at the instant the main radar lobe is point­ing straight ahead. The radar beam isn't necessarily perpendicular to the scanner - the small angular difference being known as the 'angle of squint'. It's usually a simple job both to assess heading marker misalignment and correct it. The manufacturer's instruc­tions should be followed but the basic principle involves aligning the yacht's heading with a small contact - such as an isolated buoy - which can't be mis­taken for any other contact, and noting the angular discrepancy between the heading marker and the relative bearing of the contact. On some sets the error is removed by adjusting the electronic switch in the scanner unit by trial and error (and with the scanner switched off, of course!) On more modem sets the adjustment is normal­ly made through the software, by means of a function located in the menu. It's a very useful exercise to check radar bearings against visual bearings, as it will not only demonstrate the limitations of the method, but reveal any possible errors in the EBL. Ideally, the check should be made when the yacht's heading is absolutely steady to min­imise the effect of yawing. Under these conditions it should be possible to measure bearings with the radar to an accuracy of about 10 or so but not if she's swinging about. But, however good your set, general­ly it's better to use range measurement than bearings. RANGE ACCURACY This is the most impressive feature of radar. For a contact which actually lies on a range ring the accuracy of a small boat radar, such as the Raytheon Pathfmder, should be better than + /­1.5% of the range scale in use or 22m, whichever is the greater. When using the variable range marker, VRM, for contacts not on a range ring, some operator error may be expected but with care this should be minimal. The VRM edge nearest to the centre of the display should be aligned with the cor­responding edge of the contact. One of the most common sources of error lies with careless use of the controls. It used to be common practice to check the accuracy of the VRM against the fIxed range rings, and I still do this although modern set are unlikely to have a discrepancy. You might also care to check radar ranges against charted ranges, for example when you pass between two small navigational marks. By adding the two radar ranges togeth­er you should get the same as the charted dis­tance between the two marks. A very large scale chart would be needed to show up any difference. At very short range a contact that lies within the minimum range capability of the radar will paint on the display at the minimum range. In theory the minimum range is equal to half the pulse length but this figure is exceeded in practice due to other factors involved in the transmis­son of the pulse. You're unlikely to fmd any real problem because of this but it's worth considering when operating on minimum range scale, for example in a buoyed channel. Another small error may occur at very short range due to the height of the scanner in extreme cases. This is known as Pythagoras' effect (because the radar measures the hypoteneuse) and will show a low-lying contact as being further away than it really is. You may also notice on the minimum range scale that any straight line contact, such as a dock wall, tends to bend towards you producing what is known as an hour glass effect. Normally, if you check out the handbook you'll fmd one of the menu boxes will let you correct this.

 

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