Patent GB-615480-A (GB)
Title: Improvements in or relating to navigational aid apparatus
IPC: G06G7/22
Filed: 1946-05-27
Abstract: 615,480. Calculating-apparatus. FRANKLIN, E. May 27, 1946, No. 16093. [Class 106 (i)] [Also in Group XL (c)] The invention relates to a computor for use on a mobile craft in conjunction with a hyperbolic navigational aid system which, being supplied with the path difference information and the parameters of the system, obtains the distance and bearing of one of the navigational aid stations and thence the distance and bearing of the craft's destination, the bearing and distance of the destination from the said station being known. In a hyperbolic navigation system having stations A, B and C, Fig. 1, located as shown, the position of a craft at P is determined by the time difference between the arrival of pairs of signals simultaneously sent out by stations A, B and A, C. The measurement of these time differences establishes the position of P with respect to two families of hyperbolµ, each of which is a line of equal time difference for the pair of stations to which it is referred and which form its foci. Members of one family, of which R, Q is an example, intersect with those of the other represented by T, U, and if the craft carries a map marked with these hyperbolic lines or co-ordinates its position can be determined from the measured time differences. This position information is not, however, useful in navigating the craft to a given place for which range and bearing, i.e. polar coordinate, rather than hyperbolic co-ordinate, information is required. With alpha, #, d1 and d2 being known constants of the system as shown ; r, # being the polar co-ordinates of the common station A with respect to the craft; and g1 and g2 being the path differences from the craft to A and B and A and C, respectively (derived from the measured time differences and the velocity of signal propagation) which are determined at the craft, then : are equations from which rand # may be determined. If it is desired to navigate to any point, say D, Fig. 2, of known distance and bearing S and gamma from A, the range and bearing required are L and # and may be found from: where # is the angle EPD as shown. Further, if it is desired to navigate a given course of bearing #<1>, Fig. 3, to D, the deviation PW=z from the course is given by: The Computor. An electro-mechanical computor, Figs. 4 and 5, which may be supplied with values of gl and g2 manually by the navigator, automatically by the navigation system receiver, or in correspondence with the manual adjustment of strobes on the receiver indicator, solves the above equations and gives final indications corresponding to L, # and z. Shafts 1 and 25 controlling linear potentiometers 2 and 24 control the supply of voltages proportional to -g1 and +g2 over lines 5 and 26. Shafts 8 and 29 set double potentiometers 6, 7 and 27, 28 to values corresponding to d1 and d2 and the voltage outputs are fed in push-pull to cosine potentiometers 15 and 36 set according to alpha - # and # - 0; values proportional to a and # being pre-set by shafts 73 and 75 and # being fed, as may be seen later, by shafts 69 and 70 and the subtraction being done by gearboxes 72 and 74. The output of these cosine potentiometers being d1 cos alpha - # and - d2 cos # - # is added to the voltages on lines 5 and 26 respectively and applied to terminals 53 and 44. Shafts 46 and 49 are mechanically coupled to shafts 1 and 8 to set the potentiometers they control at values corresponding to (d1+g1) and (dl - g1) respectively so that the voltage fed to point 60 is proportional to: (d1+g1)(d1 - g1) (g2 - d2 cos # - #). Similar control of potentiometers 54 and 56 provides a voltage proportional to: - (d2+g2)(d2 - g2)(g1 - d1 cos alpha - #). at point 61. If these two values are added (Eqn. 1) as in line 65, the resultant should be zero for the correct value of # and servo mechanism 66 adjusts shafts 67 and thus 69 and 70 until line 65 is at earth potential and it thus solves the equation for #. From potentiometers 76 and 79 a voltage proportional to (d1+g1)(d1-g1) is added to a voltage proportional to - (g1 - d1 cos alpha - 0) 2r from line 88, providing shaft 84 sets potentiometer 81 to a value corresponding to r, in line 93 and servo mechanism 94 controlling shaft 84 adjusts itself until this sum is zero so solving equation (2) for r. Voltages proportional to +r and - r are fed by lines 89 and 90 to combined sine and cosine potentiometer 95 adjusted according to #-gamma, gamma being preset by control 97 and the subtraction done in gear-box 98, voltages proportional to r sin #-gamma and r cos #- gamma are fed via terminals 101 and 102 to the second portion of the computor (Fig. 5). The value of S is preset on potentiometer 104 whose output is added to the input from terminal 102, and the resultant fed to push-pull amplifier 110 with cosine potentiometer 117 and sine potentiometer 118 across its output. A similar amplifier 113 with output fed across sine and cosine potentiometers 116 and 119 is connected to terminal 101. All the potentiometers are controlled by shaft 120 which takes up a position corresponding to # under the influence of servo-mechanism 128 fed with the sum of the outputs from potentiometers 116 and 117, i.e. r sin #-gamma sin # and (r cos #-gamma+S) cos 8, which sum it adjusts to be zero and so solves equation 5. The added output from potentiometers 118 and 119 (see equation 4) is fed to range indicator 135. # is added to gamma (shaft 136 being coupled to shaft 97) in gear-box 137 and shaft 140 carrying pointer 142 indicates the required bearing # (see equation 3). Range voltages +L and - L are fed across centreearthed potentiometer 150 controlled by shaft 145 having a rotation of (# - #<1>), #<1> being preset by shaft 143, the output is the distance off the desired course (equation 6) and is indicated by meter 152. It is pointed out that in some navigation systems the radiation of the signals is not simultaneous, but the signals from the B and C stations are delayed known amounts behind those of the A station and that in such cases corrections must be applied to the information fed to the computor. As the hyperbolµ branches have two points of intersection, there is a possibility of ambiguity; this may be overcome if values of rand # are known for one calibration point where shafts 84 and 67 may be set manually.