I assume that there is more sophisticated equipment out there now but we used a pinger on the same frequency as the echo sounder which gave a second trace so we could see how far the equipment was off the bottom.
Clever. Echo sounder version of a RACON, or the transponders used to keep track of aircraft.
Does it randomly ping or answer the ping from the fishfinder?
It would have to answer, as it has to be synchronized to the outgoing pulse from the sounder. In effect it turns a little target into a big one.
So if the depth finder is set on the 10 meter scale and the depth is 11 meters the return from the bottom will show up on the display as if the depth was 1 meter.
If the depth finder had a feature similar to the radar than on the 10 meter range one full rotation of the indicator would be 54.7 meters ( (164/30) * 10) but the receiver would cut off at 10 meters.
In other words on the 10 meters scale instead of sending a pulse the equivalent of every 10 meters the pules would be at intervals of 54.7 meters.
Additional wrinkles because of speed of the pulse (sound vs light) and the range scales involved.
This wonât happen on a flasher-type echo sounder because the prf is always equal to the time taken by one revolution of the light â itâs actually triggered by the light passing the zero point on the scale.
The sounder is the equivalent of an A-scope radar presentation except that itâs bent in a circle so thereâs no retrace interval. Like the A-scope it gives range and signal strength (to some extent anyway, by the brightness of the flash), but no bearing information. The A-scope radar display shows strength directly by the height of the return pulse and bearing by a separate indicator thatâs mechanically linked to the direction of the scanner. Indeed, the operator might be rotating the scanner manually.**
But as soon as your pulse generation isnât directly tied to the rotation of an arm carrying a light you can now choose prf arbitrarily, and quite intentionally space the pulses farther apart so as to create a dead zone outside the chosen range. Now instead of the light triggering a pulse as it crosses zero, rather the sweep of the electron beam is triggered by the outgoing pulse, and followed by a very rapid return of the beam to the initial point where it waits for the next pulse. Video-type echo sounders can and I presume do do the same thing for the same reason.***
**and the PPI or Plan Position Indicator type of radar display thatâs the customary form today combines the distance and bearing information into a single display by two modifications of the A-scope display: using brightness rather than a sideways deflection to indicate signal strength, and physically drawing the trace in a direction that corresponds to where the scanner is pointing.
***And it wouldnât be impossible in a flasher type â you could pulse say every third revolution of the light, and turn off the receiver after the first revolution.
Yes, agreed.
This is what Iâm saying but youâre saying it in a less confusing way, without the ratios.
So the standard depth finder has no unambiguous range. However at the 100 meter range scale if the the pulse was sent at t = 0 only on every third revolution and the receiver was shut off for the next two revolutions than the unambiguous range would be 300 meters.
Agree all.
Confusion all around with false shallow water echoes. And then of course Multiple Echoes: In shallow waters echoes may be received from the first, second & third reflections from the sea bed. Correct echo is the first one, second & third to be ignored. If the gain is turned up too high it might even show a straight line.
The false echoes are also mentioned in the 1977 edition volume 1, pages 152/3 of the American Practical Navigator.
That thread on the cruiser forum is interesting.
The first post describes seeing what has to be a second-trace.
Both exhibited similar behaviour as follows:
Depth reading are fine until the actual depth gets to about the limit of the transducerâs ping (for the NASA this was ~100m and for the new p319 itâs ~140m), at which point they will read out ridiculously shallow depths (anywhere between 2-10m).
Then run with this for a while:
That sounds to me as though there is a source of reflections of sound somewhere on your vessel which gives a very weak signal, but is close to the transducer. It could be a propeller, skeg, rudder tab, self-steering autovane, even a sacrificial anode close to the beam. As soon as the main signal is sufficiently weak that this extraneous reflection becomes comparable or is larger than the reflected signal from the sea bed, the sounder switches to measure the distance to the source of reflection, bearing in mind the angle of acceptance of the transducer.
In the end the OP agrees that second-trace is the most likely explanation.
The OP does misstate the problem here:
reading are fine until the actual depth gets to about the limit of the transducerâs ping
The limit of the transducerâs range would vary, bottom characteristics etc. The salient point is that the depth was at the limit of the range scale.
There is also another discussion on the same forum where the Moderator Adelie obviously is on to the problem of the second-trace and gives a good description of what is actually happening.
It seems that the problem is solved when the sophisticated CHIRP technology is used. CHIRP has been used by the military, geologists and oceanographers since the 1950âs. CHIRP sounders use a precise sweep pattern of many frequencies (i.e., 28-60 kHz or 130-210 kHz). Traditional sounders use two frequencies, 50 kHz and 200 kHz. The long transmit CHIRP pulse transmits more energy in the water column, up to 10-1000 times more energy on target! The result is that it the sonar can sound much deeper, also there is a better resolution which is a benefit for fishfinding.
Back in the day, as a 20 year old radarman on a FRAM DD off the coast in SVNAM in MRIV, I recall changing the range on the SPS-10 radar, and seeing all the ships on the gun line, and the CVAâs on Yankee Station, and the cruisers on NPIRAZ. Not only that, but the coastlineâs of VNAM, China and Hinan Is. It was fascinating - right up to the point when the Chief Radarman said something to the effect of: âYeah, and âtheyâ [referring to NVNAM] can see us tooâ!
I was no longer fascinated. .
After the WWII we sailed with mostly ex wartime Liberty, T-2, Victory etc ships where the norm for echo sounders was the Sub Signal Companyâs Fathometer 896A flasher with a depth range of 200 feet or fathoms. The transmitter and receiving frequency was 20 kHz and it used a magneto-striction transducer.
Fathometer on the SS John W. Brown Liberty ship. - Flickr: Ferd Brundick
After that almost every new build ship was equipped with Kelvin Hughes echosounders with recording paper, either wet paper as the MS11 and MS21 or dry as the MS26. The depth range was 720 feet or fathoms with 9 ranges of 80 feet or fathoms but it could work also beyond that by adding 720 feet or fathoms to the reading. I have seen readings of 1200 - 1300 fathoms. That large range is partly due to the low frequency of 14.8 kHz and the power of about 1000 Watt packed in a 2 mS pulse.
Another important reason was that it used a magneto-striction transducer in combination with a double walled, with air in between, parabolic reflector which produced a relatively narrow beam that increased the pulse energy considerably.
An example of multiple echoes. Note that the much weaker second reflection gives a much better definition as now the mud layer above the rocks is visible contrary to the much stronger first reflection were the mud and rock layers is blotted out by the strong echo signal.
That must be an RDF head on the right?
Yes, it is a Mackay RDF with rotatable loop antenna.
Itâs still a thousand watts (joules per second) no matter how long or short the pulse. Total pulse energy of the above pulse would be 0.2 joules.
I recalculated as follows U= 1/2 CV^2 where C = 4 /uF and V = 1000 then U = 2 joules. In joules per second or Watts 2/2 mS = 1000. I see now that I made a mistake by earlier writing 0.2 mS instead of 2 mSâŚ
It is a Mackay 106B. There is nothing that I could find on the internet about that RDF. I know that the filaments of the tubes were connected to a trickle charged 6V battery and the high tension was provided by a 90V dry battery.