Method and system for distinguishing between displacing targets and a surface of water

This invention relates to a method and system for distinguishing between displacing targets and a surface of water by transmitting pulses from a transmitter that is submerged in the water, the pulses being reflected and received by a receiver, also submerged in the water. The variations in the distance to the reflecting surface for consecutive pulses are measured and compared with a first value, determined inter alia from the state of the sea, and with another value, determined from the size of the target.

BACKGROUND OF INVENTION 
1. Field of Invention 
This invention relates to a method and system for distinguishing between 
displacing targets and a surface of water by transmitting pulses from a 
transmitter towards the surface of water, the pulses being reflected and 
received by a receiver. The invention is intended to be used in a 
proximity fuse arrangement, for torpedoes or mines, for example. 
2. Description of Prior Art 
The previously known devices use the distance to the surface of the water 
as a reference, and form in different ways an average surface of the water 
in order to avoid the problem of false output functions when the sea is 
high. This leads to several disadvantages since it is very difficult with 
this method to fulfill the requirement for a low probability of error when 
the sea is heavy, and at the same time the requirement for a high 
probability of a correct function when passing the target. It is further 
required, in the prior art, that the ship's sides be almost vertical in 
order to get a correct and accurate function. 
SUMMARY OF INVENTION 
The invention is based on the fact that the inclination of the surface of 
the water is normally limited to a certain maximum angle, the size of 
which can be determined from measured wave spectra. If a larger 
inclination is measured, this is due either to an error in measurement or 
to the presence of an object in the water. 
The invention is carried out in the following manner. From a transmitter, 
pulses (e.g., hydroacoustic pulses or laser pulses) are transmitted at 
constant intervals toward the surface of the water, where they are 
reflected and received by a receiver near the transmitter. The time from 
transmission to reception of the pulse is a measure of the distance to the 
reflecting object (the surface of the water or the ship). 
The largest variation A in the distance to the surface of the water that 
can be expected in the absence of a target is predetermined by 
calculations and/or measurements. Distinction between the target and the 
surface of the water is, according to the invention, accomplished by 
giving the method the special characteristics disclosed below, and as is 
evident from the attached claims.

DETAILED DESCRIPTION 
Referring to FIG. 1, the following symbols are used: 
CL: output function 
RC: counting cell 
Z.sub.h : measured distance 
H.sub.Zi : integer part of Z.sub.h /s where s is the factor of 
digitalization 
H.sub.ZL : previous accepted value of H.sub.Zi 
k: number of consecutive pulses with .vertline..DELTA.i.vertline.&lt;A that is 
needed to reset H.sub.ZL to H.sub.Zi-1 
As explained above, pulses are transmitted at constant intervals toward the 
surface of the water where they are reflected and received by a receiver 
near the transmitter. The time from transmission to reception of the pulse 
is, accordingly, a measure of the distance to the reflecting object 
(typically, either the surface of the water or the surface of a target). 
The measured distances Z.sub.h are digitalized (block 3 of FIG. 1) to form 
digital value H.sub.Zi. Then, in a conventional analyzer provided in the 
receiver, the value 
##EQU1## 
is formed, where H.sub.i is the distance to the reflecting surface for 
pulse number i, and n-1 is the number of times .DELTA..sub.i has been 
rejected (block 3). As will be seen below, the value .DELTA..sub.i will be 
rejected until the presence of a target is indicated. 
Once the value .DELTA..sub.i is formed, the shifting operation (block 4) 
relative to the B-cells is performed. It is to be noted that the B-cells 
store pulses meeting the condition .DELTA..sub.i .gtoreq.B. 
.DELTA..sub.i is then compared with a first determined value A (block 5). 
If .DELTA..sub.i is less than A, then one of two conditions is indicated: 
either .DELTA..sub.i is equal to or less than -A (indicating an obvious 
error); or the magnitude of .DELTA..sub.i is less than A (indicating that 
the measured inclination is smaller than the largest expected inclination, 
and the pulse response must be from the surface). 
In any event, returning to block 5, if .DELTA..sub.i is less than A, the 
counting cell pointer RC is incremented (block 6). Then, if the counting 
cell pointer RC is less than 0, the n-count is incremented (block 12). On 
the other hand, if RC is equal to or greater than 0, RC is set to -1 
(block 8). Then, a further decision is made (in block 9) between the 
previously mentioned two conditions. That is to say, if .DELTA..sub.i is 
equal to or less than -A, indicating obvious error, the n-count is 
incremented (block 12); conversely, if .DELTA..sub.i is greater than -A, 
indicating pulse response from the surface, the previous accepted value 
H.sub.ZL for measured distance is replaced by the present measured 
distance H.sub.Zi (block 10), and the n-count is set to 1 (block 11). 
Returning to block 5, if .DELTA..sub.i is equal to or greater than A, a 
target is indicated, and the counting cell pointer RC is set to -k (k is 
defined above). Then, .DELTA..sub.i is compared to a further predetermined 
value B (block 14), the value B being a measure that corresponds to a 
certain minimum draft of water for targets of interest. Specifically, it 
is required that .DELTA..sub.i be equal to or greater than B. This 
condition must be satisfied in order not to get an output function as a 
result of detection of large fish, logs, or the like; in fact, in order to 
preclude such erroneous output functions, the latter condition must be 
satisfied a certain number of times in succession, depending on the size 
of the target, detection of which is desired. 
Thus, returning to block 14, if .DELTA..sub.i is equal to or greater than 
B, the B-cell counter b.sub.j is set to 1 (block 15). Then, referring to 
block 16, if the summation condition contained therein is not satisfied, 
the n-count is set to 1 (block 11); conversely, if the summation condition 
contained in block 16 is satisfied, an output function is provided (block 
17). 
In other words, the B-cells, which (as previously stated) store pulses 
satisfying the condition .DELTA..sub.i equal to or greater than B, are (as 
also previously stated) initialized in block 4. Then, each time the 
condition ".DELTA..sub.i equal to or greater than B" is met (block 14), a 
"1" is stored in the B-cell corresponding to that pulse (block 15). When a 
certain number of (e.g., two) consecutive pulses meets the condition of 
block 14, as determined by decision block 16, the output function is 
provided (block 17). If the condition of block 16 is not satisfied, as 
indicated by the "NO" exit therefrom, the procedure continues until the 
certain number of consecutive pulses meets the condition of block 16. 
In the event of an error in measurement (or if the condition has been 
satisfied by, e.g., a log), another condition can be introduced saying 
that if .DELTA..sub.i +m during a certain number of (e.g., two) 
consecutive pulses is .ltoreq.A, a target is considered not to be present, 
and so one returns to calculate .DELTA..sub.i. 
The above-described method has, in simulations, given very good results, 
and as well has provided good results against those targets that have 
sides that have large inclinations in relation to the vertical plane. The 
method of the invention can, of course, be realized in several ways, and 
is not limited only to hydroacoustic devices. It can be used with all 
devices wherein transmitted pulses are used to measure the distance to a 
reflecting surface. 
Further referring to FIG. 1, it is to be noted that the dotted-line portion 
of FIG. 1 (blocks 18 and 19a) is intended for use of the procedure in 
exercise conditions, such dotted-line portion being deleted during the 
performance of mere exercises. Thus, during exercise conditions, after 
completion of the function of block 17 (provision of the output function), 
the n-count is set to 1 (block 18), and block 19a is executed. During 
battle-type situations, completion of the functions of blocks 11 and 12, 
respectively, results in immediate execution of block 19b. 
Referring to FIGS. 2 and 3, the arrangement of the present invention 
comprises a pulse control circuit 20, a transmitter 21, a receiver 22, 
three comparison circuits 23, 25, 27, three condition circuits 24, 26, 28, 
and an activating circuit 29. The control circuit 20 starts transmitter 21 
and prevents receiver 22 from getting any pulses directly from the 
transmitter 21. Pulses are sent from the transmitter 21 with a frequency 
of 50 kHz, mentioned as an example. 
Echo pulses received by the receiver 22 are transmitted to the first 
comparison circuit 23. In this circuit, each successive distance to the 
water surface is calculated by virtue of the time difference between 
transmission of a pulse and reception of the echo pulse. Then, the 
difference in distance measured by pulses 31 and 32 in FIG. 3 is measured, 
curve S representing a water wave and curve T representing the side of a 
boat. 
Condition circuit 24 stores a value A representing the maximum change 
attributed to a wave. This value is, thus, calculated beforehand and 
represents the maximum angle and, therewith, the maximum difference that 
can be expected in the special case where the arrangement is to be used. 
The difference in distances of pulses 31 and 32 is expected to be less 
than A but the difference in distances of pulses 33 and 34 is expected to 
be greater than A because pulse 34 is reflected from the boat side T. 
After the pulse 34 is received, a signal is transmitted to the comparison 
circuit 25 which is then activated. Activation of circuit 25 indicates 
that a target has probably been found but it is not sure that the target 
is of the right size, that is, greater than a value decided beforehand. 
Distance values corresponding to the mentioned pulses 33 and 34 are also 
transmitted to the circuit 25, wherein pulse value 33 is stored, and the 
difference between the said two values is compared with a value B (see 
FIG. 3). Apparently, in FIG. 3, the difference is less than B. The next 
incoming pulse value 35, together with the pulse value 33, gives a new 
difference which is also less than B. The difference between pulse value 
38 and pulse value 33 is greater than B, however, so that a signal is 
transmitted from circuit 25 to circuit 27. 
Circuit 27 counts the number of differences which exceed the value B. When 
this number of differences is equal to a predetermined value P, a signal 
is transmitted to activiting circuit 29, and this causes detonation. 
Thus, three conditions must be fulfilled before a detonation is caused: (1) 
there must be something in the water that represents a greater angle than 
the calculated maximum wave angle of the sea per the value A; (2) 
immediately after this first condition is fulfilled, the difference 
between pulse value 33 and a following pulse value (not necessarily the 
next pulse value) must be greater than a predetermined value B (condition 
in depth); and (3) after this has happened, the examination of the second 
condition must be repeated a predetermined number of times P (condition in 
breadth). 
Although FIG. 2 illustrates the inventive arrangement in terms of discrete 
circuits for the sake of simplicity, in practice, the arrangement may be 
manufactured utilizing semiconductor (integrated circuit) technology so as 
to, for example, form the discrete circuits on a single chip. Normally, 
the constants A, B and P each have one value in dependence on where a 
torpedo associated with the arrangement is to be used (detonated), and 
depending on the type of targets for which the torpedo is intended. For 
example, a torpedo intended to be used in the Baltic sea has an 
arrangement with a value A calculated in advance from the wave structure 
of this sea so that it corresponds to the maximum wave angle to be 
expected for this sea. However, it is of course possible to make the value 
A adjustable depending on the particular sea for which the arrangement is 
intended. This adjustment may be made with a control voltage, the 
amplitude of which can be changed with a rheostat. Then, this voltage is 
provided by circuit 24 to the comparison circuit 23, as explained above. 
In the same way, the circuits 26 and 28 may be made adjustable as to 
parameters B and P depending on the particular kind of targets. 
While preferred forms and arrangements have been shown in illustrating the 
invention, it is to be clearly understood that various changes in detail 
and arrangement may be made without departing from the spirit and scope of 
this disclosure.