Radar Second -Trace / Unambiguous Range

Here is the page from The Use of Radar at Sea

Here it’s called Second-trace echoes:

A quick check looks like 500 pulse/sec is typically ppr (or ppf pulse-repetition frequency).

According to this if the rr is on 30 mile range second-trace echoes have to be beyond 162 miles. It does say it is unusual but it is in fact possible and several such occurrences have been have been reported.

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Interesting Different name same thing.

Yeah., Bowditch uses the same term:

Second-trace echoes (multiple-trace echoes) are echoes received from a contact at an actual range greater than the radar range setting. If an echo from a distant target is received after the following pulse has been transmitted, the echo will appear on the radarscope at the correct bearing but not at the true range. Second-trace echoes are unusual, except under abnormal atmospheric conditions, or conditions under which super-refraction is present. Second trace echoes may be recognized through changes in their positions on the radarscope in changing the pulse repetition rate (PRR); their hazy, streaky, or distorted shape; and the erratic movements on plotting.

Makes sense in the context of radar sets where the electron beam actually traces out radii on the CRT and intensifies for targets. Not so much with raster-scan sets.

I don’t understand why it would be different. If a strong echo comes back wouldn’t it just get painted on the screen?

Physically it’s no difference at all. It’s just that modern radars don’t really have a trace as such – what comes out of the receiver is video.

The original PPI scopes fired an electron beam at the center of the CRT and started sweeping it out to the edge at the same time as the transmitted pulse, retracing (returning to center) before the next pulse. The direction of the sweep was synchronized to where the scanner was pointing. These scopes depended on slow-decaying phosphors to stay lit long enough to show shapes.


That makes sense. If the radar could be put on scale of more than 162 miles you’d see a second trace (or return) there? That term doesn’t seem very appropriate to a marine radar as they typicality don’t have a scale with a range that far. Maybe a big land based weather radar or some such?

No, in that case you’d see the target on the first sweep, at its proper distance.

The second-trace stuff comes when the initial sweep finishes, retraces, the next pulse is sent and now the target return shows up as a nearby target but displaced in rotation by one increment.**

**Which of course would be far to small a difference to see, much less than the width of the electron beam itself.

I thought that the intensifying effect of the phosphor, when the ‘writing’ of a target occurred, was called blooming. No blooming on a digital screen.

Ok, I see. So on radar with a range over 162 miles two traces at different ranges might be seen? The one at the correct range is the “first trace”?

That terminology doesn’t seem too helpful to the mariner as he will never see the “first trace”, only the second one.

Blooming is an undesirable spreading effect caused by high beam intensity. It would cause a target to paint as wider and deeper than it really is. In television or other raster-scan displays it’s also a spreading or contracting of the entire image or a section of it when image brightness changes (I think probably because of less than optimal “stiffness” of the power supply. LCD screens are of course not subject to this.

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It’s rather that on a radar with 500 pps rate, a strong target at range over 162 miles which would normally be far out of display range will potentially display as a target within the selected range. Bearing accuracy will be correct but you must add 162 miles to the displayed range to get the actual range.

Same effect that you get with a rotating-type echo sounder where the return echo shows up after the lamp has gone around once (or potentially twice or thrice).

The larger models of JRC JMA-91 3 cm and 10 cm radars have this range of pulse widths/repetition frequencies. Pulse width is narrow at short ranges for better range discrimination, and wide at long ranges for better signal strength:


I meant the range scale. I’m saying that if the mariner switches his radar to the say, 180 miles scale (which he doesn’t have) he would see both traces, first and second. Correct?

No, because on that scale (which would necessitate a prf less than 500 pps) the target would display once, at the correct range. However a target considerably outside the 180 mile range might display at a closer range.

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^^At this rate we’ll be worrying about returns off the moon before too long. :wink:

Ok, so it sound like only one trace will ever be seen at one time. If the return is being shown at the correct scale and the operator redueces scale and sees the same object but displayed at the wrong range that is called the second-trace.

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'Zackly. Side note – what I’m calling the trace is the movement of the electron beam from the center of the scope to the periphery, timed to coincide with the outgoing pulse and of duration equal to twice the selected range divided by ~300 million meters per second. When you change display ranges you’re changing the duration and hence speed of the trace (as well as whatever adjustments in transmitter or receiver). The thousands of individual traces combine visually to form the cursor which sweeps around the face of the CRT. The instantaneous brightness of the trace is proportional to received signal strength.

Retrace is the much faster return to the center (maybe with beam suppressed) in preparation for the next pulse.

The point I was trying to make is that the term “second-trace” seems like it’s more applicable to an observer who can actually observe both the first and second traces.

From the point of view of the bridge watch-stander the term “second trace” makes less sense because the first trace can not be seen. In other words the range scale cannot be switched to see the other trace because the marine radar will not have that range. No need for it because of the radar horizon.

. So just accept this is the second-trace and take my word on the first one.