It’s radar-centric, nothing to do with the observer. The observer sees all the thousands of traces combined into the cursor that sweeps around the scope face. Traces that include a target reflection are brighter at the range of the target. Each trace corresponds to one revolution of of a flasher-type echo sounder.
Here’s my favorite small-boat sounder which I unfortunately murdered. British, and built with British germanium transistors that I couldn’t source in 1990 or so. Sixty foot or sixty fathom per revolution by changing the motor speed (so you can instantly tell by the ticking which range it’s on).
^^If your targets don’t fade as the cursor rotates around and are refreshed as it passes over them again, you’re looking at a video display rather than the kind I’m talking about. It moves the electron beam just like a television, scanning left to right and top to bottom on the screen, and refreshing the screen at a constant rate. The word “trace” in this type of display refers to the left-to-right movement of the beam, and retrace to its return to the left side in preparation to drop down slightly and move left to right again for the next scan line on the display. Hence “second-trace” becomes physically meaningless.
I don’t personally think that “second trip” is an improvement as it is also IMO physically meaningless – the trip it refers to is the motion of the trace from center to edge. But I’m having trouble proposing a better one that isn’t a sentence long. “Over-range false target” or some such.
To me is just seems that while the term might seem relatively concrete to a observer who has the equipment to see both it would be a bit more abstract to the mariner.
I do like over-range, maybe over-range return, but if you want to find it in Bowditch it’s second-trace.
In conclusion there is also a phenomenon called Sub-refraction. The effect of sub-refraction is to bend the radar rays upward and thus decrease the maximum ranges at which targets may be detected. The distance to the radar horizon is reduced. This condition is not as common as super-refraction. Sub-refraction can occur in polar regions where Arctic winds blow over water where a warm current is prevalent. If a layer of cold, moist air overrides a shallow layer of warm, dry air sub-refraction may occur.
Sub-refraction also affects minimum ranges and may result in failure to detect low lying targets at short range. It is important to note that sub-refraction may involve an element of danger to shipping where small vessels and ice may go undetected. The officer in charge of the watch should be especially mindful of this condition and extra precautions be administered such as a reduction in speed and the posting of extra lookouts.
The unambiguous range of a radar is the maximum range at which a target can be located so as to guarantee that the reflected signal/pulse from that target corresponds to the most recent transmitted pulse. The radar range is measured by the time delay between pulse transmission and reception. It is usually assumed that the received pulse is associated with the most recent transmitted pulse. Targets at ranges beyond the unambiguous range therefore appear closer because the received pulse will correspond to the previous transmitted pulse.
The maximum unambiguous range (Ru) is the longest range to which a transmitted pulse can travel out to and back again between consecutive transmitted pulses. In other words, Ru is the maximum distance radar energy can travel round trip between pulses and still produce reliable information.
Pw in the formula is in the order of 0.2µs and is negligible as compared to the Pulse Repetition Time. PRT = 1/prf so the formula kan be simplified to c/2prf. The unambiguous range can also be written as Ru = c/2fp. For a prf of 2000 Hz RU = 40.5 NM.
Time-keeping on this clock uses arithmetic modulo 12
A familiar use of modular arithmetic is in the 12-hour clock, in which the day is divided into two 12-hour periods. If the time is 7:00 now, then 8 hours later it will be 3:00. Usual addition would suggest that the later time should be 7 + 8 = 15, but this is not the answer because clock time “wraps around” every 12 hours.
We all have seen this at certain moments. Mutual Radar Interference is caused by other radars in the vicinity operating on a similar frequency to our ship’s radar. The interference shows up as bright spots scattered over the screen, or as a distinctive pattern of dotted lines curving outwards from the centre of the screen. It is more common on longer range scales as on shorter range scales only a few of the interfering pulses will be displayed.
If only one other radar is involved this is not too serious, but in busy traffic areas the clutter can be dense enough to cause confusion. An interference rejection circuit can minimise this problem. It works by rejecting any echo which does not return from any two successive pulses. While large targets will not be effected by IR, some small echoes may be lost. There is no IMO symbol for IR.
Normal procedure is to have the IR switched off as the sensitivity is lowered when switched on. As already mentioned there is the danger of missing smaller targets.
This is true but would be much more likely to come into play (i.e. third-trace, fourth-trace etc) at higher prf where the ranges involved are shorter and target strength higher.
Harking back to the echo sounder, I ended up with a fancier one where the motor speed was constant and very fast; and range selection was done electronically.** That one would suppress “second-revolution” echoes but display third- and fourth-revolution ones.
**Somewhat analogous to a raster-scan radar display, where the actual display is no longer a fundamental part of range determination but only a convenient way the device “chooses” to display information to the operator.
I like the clock - but say we label it using distance instead of time.
Using the example in the book at 500 pulse / sec and the radar is set on the 30 mile scale.
At time = 0 a pulse is sent out and the clock starts.
Min range aside sufficiency strong return signals get displayed as long as they make the clock at 30 mile cut off.
Then from 30 miles to 164 miles wait time with nothing displayed and it’s back at time = 0 again and we fire off another pulse.
I’ve seen good returns off high coastal mountains in Alaska at 120 miles displayed at the proper range. So with good ducting it wouldn’t be out of the question to get a good return at 164+ miles. No?
Staggered PRF is a transmission process where the time between interrogations from radar changes slightly, in a patterned and readily-discernible repeating manner. The change of repetition frequency allows the radar, on a pulse-to-pulse basis, to differentiate between returns from its own transmissions and returns from other radar systems with the same PRF and a similar radio frequency.
With staggered PRF the radar’s own targets appear stable in range in relation to the transmit pulse, whilst the ‘jamming’ echoes may move around in apparent range, inhibiting integration and reducing or even suppressing its impact on true target detection.
I had one of those! It had something like a “second trace” in shallow water if the sound hit the bottom, reflected off the boat, hit the bottom, and came back again. Say in 10 feet of water you would see a strong return at 10 and a weaker one at 20.
That Seafarer MkII on the 60 foot range could show triple or even quadruple on hard bottom if you turned the gain up. I don’t recall seeing doubling on the 60 fathom range but the vast bulk of my cruising was in sixty fathom or lesser depth (Gulf of Maine).
And it is exactly “second-trace” – just wrapped around the dial instead of straight out, because the echo sounder gives no bearing information and it’s mechanically much simpler to make it a spinning arm that triggers the transmitter (Seafarer used a reed switch with a little magnet in the arm) once a revolution. Each revolution corresponds to one center-to-edge trace on the radar CRT.
As can be seen there is a very large and wide back lobe!
Another source of concern are the reflections from side lobes that can sometimes cause false targets and/or “noise” on the display.
The longer the antenna relative to the radio wavelength is the narrower the main lobe will be but the price to pay is that more side lobes will be created. In receiving antennas, side lobes may pick up interfering signals, and increase the noise level in the receiver.
Side lobes can be responsible for creating false steady targets in the radar screen. In navigation best practice is, if possible, to always evaluate each target.
Another effect is when a large ship is close to ours the side lobes can cause a typical interference as shown.
In military radar systems these side lobes turn out to be a very significant weak point in any radar system that can be used to create confusion and misrepresent the target. They are considered as the Achilles heel of such systems,
Multiple false targets with multiple headings can be generated with a single side lobe transmitter. The radar antenna is pointing in the wrong direction when the largest return signal is received.
Combining the false signal with the real signal creates a false signal that appears real to the radar. The radar antenna is actually pointing in a significantly different direction from you. The error can be as much as 30 degrees. So, if this acquisition radar directs a weapon at you, it’s really pointing in the wrong direction.
Except strictly speaking it doesn’t meet the definition of “second-trace”.
From Bowditch:
Second-trace echoes (multiple-trace echoes) are echoes received from a contact at an actual range greater than the radar range setting.
It would be a case of multiple echos:
Multiple echoes may occur when a strong echo is
received from another ship at close range. A second or third
or more echoes may be observed on the radarscope at
double, triple, or other multiples of the actual range of the
radar contact
In the case of the echo finder with multiple echoes it does share the feature that the apparent range is a modulo function of the true range.
Oh, quite right! @yacht_sailor was talking about multiple bounces between sea floor and water surface which would show up as multiple pips at 2x, 3x, 4x the actual depth.
Whereas I was talking about a single deepwater echo that appears on the second, third or fourth revolution of the the spinning arm giving an apparent depth that is much less than the true depth.
