Patent GB-913869-A (GB)
Title: Improvements in or relating to computers for use with radio navigation systems
Inventor: SEWELL ARTHUR DAVID
IPC: G06F7/66
Filed: 1958-07-29
Abstract: 913,869. Radio navigation. GENERAL ELECTRIC CO. Ltd. Oct. 29, 1958 [July 29, 1958], No. 24471/58. Class 40 (7). [Also in Group XIX] Relates to a computer for computing on a craft the cartesian co-ordinates x, y or the longitude and latitude X, Y of the craft from the range differences q 1 =r 1 -r 0 , q 2 =r 2  - r 0  . . . of the craft with respect to a plurality of pairs of spaced radio beacons B1, B0; B2, B0 . . ., Fig. 4, comprising a hyperbolic navigation system. As described, the computer is a digital differential analyser comprising a plurality of interconnected digital integrators and the radio navigation system is a pulse or CW system comprising a master station B0 and three spaced slave stations B1,B2,B3, the known cartesian co-ordinates of the stations being (x 0 , y 0 ) . . . (x 3 , y 3 ). The ranges of the craft from the four stations B0 . . . B3 are denoted by r 0  . . . r 3 . The difference, e.g. q 1 , between a pair of ranges, e.g. r 1  and r 0 , is determined by known means, e.g. by phase comparison of signals derived from the received beacon signals, and a pulse is produced when the magnitude of q changes by a predetermined amount, the resultant pulse train comprising the measured incremental range difference signal dq. The incremental changes dx and dy in the x and y co-ordinates of the craft are given by where K, L, P, M, N and Q are appropriate coefficients (see below), E 1  is the error difference between (1) the range difference q 1  obtained by accumulating the measured incremental range difference signals d q1  and (2) the range difference (r 1  - r 0 ) computed from the computed values of dx, dy and the known values of x 0 , y 0  and x 1 , y 1  and E 2  and E 3  are corresponding error differences for the pairs of ranges r 2 , r 0  and r 3 , r 0 . If the curvature of the earth is neglected the quantities r 0  . . . r 3  are computed from equations of the form but for a spherical earth the equations are more complicated (see Fig. 8 below). The service area of the navigation system may be divided into four zones Z0 . . . Z3, Fig. 4, each containing one beacon and sufficiently accurate values of dx and dy as given by equations (1) and (2) are obtained if the coefficients K ... Q are taken to be constant trigonometric functions of the angles between the pairs of beacon base lines B0, B1; B0, B2; B0, B3; B1, B2; B2, B3; B3, B1 and between the base lines and the x axis, Fig. 5 (not shown), the values of the constant coefficients being different for each zone, e.g. K 0 , K 1 , K 2 , K 3 , L 0  . . . . When the craft is in any one of the zones Z1, Z2 or Z3 the computation may be effected by considering only the signals dq relating to beacons outside the zone and so two of the coefficients may be zero, e.g. in zone Z1 the constants K 1  and M 1  are zero. Also for zone Z0 the coefficients may be chosen so that so that in effect the computation is dependent only on the ranges of the craft from the beacons B1, B2, and B3. Alternatively, the computation accuracy may be improved by choosing values of the zone constants such that the computation is dependent at all times on the signals received from all four beacons. According to the present invention means are provided for automatically selecting the set of constants K . . . Q corresponding to the zone occupied by the craft. If the zones are defined by the hyperbolic curves q 1 =c 1 , q 2 =c 2 , q 3 =c 3 , Fig. 4, the control may be effected in response to the measured input signals dq (see Fig. 7 below). Alternatively, the zones may be defined by constant values of weighted sums of the range difference signals q 1 , q 2 , q 3  or by lines e,f parallel to the x and y axes or by a stepped line g having segments parallel to said axes; in the latter case the control may be effected in response to the computed signals dx, dy. Integrator units, Figs. 2a and 2b. The integrators employed are of known form and the Specification includes references to descriptions thereof in the technical literature. As shown in Fig. 2a each integrator comprises a digital integrand store IS having an integrand input II, a digital remainder store RS having an overflow output OO and a transfer device T interconnecting the two stores and having an " action " input AI. Each pulse applied to the integrand input II represents, according to its sign, a positive or negative increment dv of predetermined magnitude in a variable v and the pulses are accumulated in the integrand store IS so that the content of that store represents v+c, where c is a constant dependent on the initial content of the store IS. Each pulse applied to the action input AI represents, according to its sign, a positive or negative increment du of predetermined magnitude in another variable u and causes the content v+c of the integrand store IS to be added to or subtracted from the remainder in the store RS. The remainder store RS applies an output pulse of predetermined polarity to the output OO each time the remainder accumulated in the store RS reaches the numerical capacity of that store so that each output pulse from OO represents an increment dw of predetermined magnitude of a variable w such that Fig. 2b shows the schematic representation of the integrator which is used in Figs. 3 and 6 to 8, the polarity of the output pulses being indicated by the sign adjacent the output lead. If it is required to integrate with respect to time a train of pulses of constant recurrence frequency is applied to the action input AI; such an input is represented in Figs. 3 and 6-7 by dt and is referred to as a machine rate signal. Special forms of integrators.-A " servo " integrator (designated S in Figs. 3 and 6-8) is an integrator that is operated in a condition in which the integrand is at (or nearly at) maximum positive or maximum negative value so that an incremental change in the integrand causes a change in the sign of the integrand and thereby a change in the sign of the output integral. The servo integrator also has a feedback loop including one or more other integrators and the incremental output dw of the servo integrator is applied as an input to one of the feed-back integrators, the incremental output dz from a feed-back integrator being applied as an input to the servo integrator and the system operating to maintain the resultant accumulated integrand v+z of the servo integrator equal to zero. A " hard adder " (designated HA in Figs. 3 and 6-8) is a form of servo integrator in which a plurality of incremental variables dv 1 , dv 2  . . . is applied to the integrand input of the integrator together with the incremental output dw from the integrator, the system operating to maintain dw=dv 1 +dv 2 + . . . .  Fig. 3 embodiment. Before commencing operation the values (x<SP>1</SP>-x 0 ) . . . (x<SP>1</SP> - x 3 ) and (y<SP>1</SP> -y 0 ) . . . (y<SP>1</SP> - y 3 ) corresponding to the known or estimated position x<SP>1</SP>, y<SP>1</SP> of the craft are stored in the integrand stores of the integrators 12 to 19, respectively, the values of r 0 , - r 1 , - r 2  and - r 3  correspondirg to the position x<SP>1</SP>, y<SP>1</SP> are stored in the integrand stores of integrators 24 to 27 and the appropriate values of the zone constants K, L, P, M, N and Q are stored in the integrand stores of integrators 4 to 9. Assuming that the incremental output signals dx and dy are in fact produced by the integrators 10 and 11, the signal dx is applied to the action and integrand inputs of integrators 12-15 and the signal dy is applied to the action and integrand inputs of integrators 16-19. The signal dx is accumulated in the integrand store of integrator 12 so that the stored integrand becomes (x - x 0 ) and the integral output is an incremental signal - (x - x 0 ) dx. Similarly incremental signals +(x - x 1 ) dx ... + (x - x 3 ) dx and - (y - y 0 ) dy, +(y - y 1 ) dy . . . (y - y 3 ) dy are produced by integrators 13 to 19, respectively. The signals - (x - x 0 ) dx and - (y - y 0 ) dy from integrators 12 and 16 are combined in the integrand store of a servo integrator 20 with the output from an integrator 24 whose action and integrand inputs are connected to the output of integrator 20. If the output from integrator 20 is denoted by dr 0  then the output from integrator 24 is +r 0 dr 0  and the combined accumulated integrand of integrator 20 is This integrand is maintained equal to zero by the servo action of integrator 20 and is therefore in accordance with the range equation so that the output +dr 0  from integrator 20 represents the incremental change in the range r 0  of the craft from the station B0. Signals - dr 1 , - dr 2  and - dr 3  are produced by the servo integrators 21, 22 and 23 in a corresponding manner. The signals +dr 0  and -dr 1  from the integrators 20 and 21 are combined in the integrand store of a hard adder integrator 1 with the output dE 1  thereof and due to the servo action of the integrator 1 its accumulated combined integral q 1  - (r 1  - r 0 )+E 1  is maintained equal to zero so that E 1  represents the error difference between the computed range difference (r 1  - r 0 ) and the measured range difference q 1 . In a similar manner incremental signals dE 2  and dE 3  corresponding to range difference errors E 2  = (r 2  - r 0 ) - q 1  and E 3  = (r 3  - r 0 ) - q 3  are produced by integrators 2 and 3. The signals dE 1 , dE 2  and dE 3  are multiplied respectively by the zone constants K, L and P in integrators 4, 5 and 6 and accumulated in the integrand store of integrator 10 which has a machine rate action input dt so that the output from integrator 10 is given by when G is the gain of the integrator 10. A corresponding output is obtained from the integrator 11 by applying the signals dE 1 , dE 2  and dE 3  to integrators 7, 8 and 9. The output incremental signals dx and dy from the integrators 10 and 11 may be accumulated to provide a visual or other ind