Patent GB-1430389-A (GB)
Title: Computing apparatus for tracking movinb objects
IPC: G01S13/937
Filed: 1972-06-21
Abstract: 1430389 Radar computer SOLARTRON ELECTRONIC GROUP Ltd 20 June 1973 [21 June 1972] 29007/72 Heading G4A [Also in Division H4] In a computer receiving time-spaced input positional signals each representative of a position of at least one moving object, Cartesian x, y pairs of digital signals are produced from the input positional signals, a predetermined number of such x, y pairs are stored, and digital means receiving the stored signals multiplies each of the stored x signals by a first set of predetermined coefficients and each of the stored y signals by a second set of predetermined coefficients, and summing the x and y products so formed so as to produce digital signals representing a specified set of time derivatives at at least one preselected common point in time on hypothetical functions x=f 1 (t) and y=f 2 (t) which substantially fit the x, y values, both of the hypothetical functions being either at least second order polynomials or spline functions. The invention is described as applied to an anticollision shipborne pulse radar (or analogous sonar) system but may be applied to an airborne pulse radar with three-dimensional data; both hypothetical functions are third order polynomials and a least squares fit is utilized for selecting the sets of predetermined coefficients. A store for the predetermined coefficients is provided and may be a read-only, or a randomaccess, memory. Detailed embodiment.-The particular description relates to the tracking of the movement of one other ship (B) by a pulse radar 12 on "own" ship (A) which produces I.F. echo pulses at 19, a synchronizing pulse output at 18 and an "index" pulse at 21 for each incremental azimuth of a revolution of aerial 16 with respect to true north. Computing.-The echo pulses are passed to AND gate 36 which also has inputs from 38, 40 which define an (r, #) window in which ship B only is expected. An accumulator 56 is caused to contain a digital number R B  representative of the sum of, e.g., four successively measured values of the range r of B from A and an accumulator 60 to contain a digital number # B  representative of the sum of four successive bearing values. By means of dividers 64, 66 digital numbers r<SP>1</SP> B , #<SP>1</SP> B , representing the range and bearing of the "radar centre" of ship B, are produced. Filters 68, 70 may be linear, i.e. it is assumed that over a short period, e.g. fifteen seconds, i.e. five aerial revolutions, the course of ship B is a straight line, and thereby produce digital positions r B , # B , corresponding to the middle of the fifteen second period, which are then converted to Cartesian signals x<SP>1</SP> B , y<SP>1</SP> B  (with ship A as origin). A device 76 produces digital positions x<SP>1</SP> A , y<SP>1</SP> A  representative of the position of ship A relative to an arbitrary fixed origin O and digital signals x A , y A  representative of the speed of ship A. Adders 74 produces digital sixteen-bit words x B , y B  (the Cartesian co-ordinates of ship B relative to the fixed origin) which are transferred to registers 78, 80 each capable of storing twenty sixteen-bit words. Random errors in x B , y B  are reduced by digital filters 82, 84 each comprising a multiplier 86, a coefficient store 88 and a summing unit 89; there is produced a digital representation of x r  of the third order polynomial which is a least squares fit with the signals x B  at a time near that represented by the most recent stored signal x B , and a digital representation of x M  (analogous to x r ) at a time in the middle of the interval between the most recent and the oldest stored signals x B . Under good radar conditions only the eight most recent values of x B  are utilized but fifteen may be employed under poor radar conditions. Analogous representations y r , y m  are formed. A circuit 90 produces a digital signal v B  = ##x M <SP>2</SP> + #y M <SP>2</SP> and a circuit 92 affords a digital signal α B  = tan<SP>-1</SP>(y r /x r ); such signals may be decoded and applied to afford an alpha-numeric display or a print-out. Further mathematical operations are described, to correct for inaccuracy in x r  and y r , the corrected magnitudes being combined with signals x A  and y A  to produce signals x<SP>1</SP> B , y<SP>1</SP> B  representative of the speed of B in x, y directions relative to A. The most recent x B , y B  signals from registers 78, 80 are added to x<SP>1</SP> A , y<SP>1</SP> A  signals from computer 76 to form signals x<SP>11</SP> B , y<SP>11</SP> B  which represent the position of B with respect to A as origin. Tracking.-Comparators 106, 110 position the r, 0 window to ensure that only ship B echoes pass through AND gate 36. Cathode-ray tube display.-Smoothed analogue values of x B , y B  drive the plates of a cathode-ray tube 122 which thereby displays the track 123 of ship B and analogue signals dependent on x r , y r  produce a dotted line 131 indicating the calculated course of B. Similar processing produces displays 123a, 131a corresponding to ship A. Trial inputs of trial heading and speed of A may be inserted into computer 76 to afford a display of a trial course. Distance, and time, of closest approach are calculated by a processor 134 and may drive the above-mentioned alpha-numeric display or printer. A visible and/or audible alarm 140 is triggered when the calculated closest-approach distance falls below a predetermined value. Programming may replace some of the discrete circuits shown.