Patent GB-911972-A (GB)
Title: Improvements in or relating to navigation aids
Inventor: TOOLEY MICHAEL DENT
IPC: G01S1/02
Filed: 1957-11-29
Abstract: 911,972. Radio navigation. GENERAL PRECISION SYSTEMS Ltd. Dec. 1, 1958 [Nov. 29, 1957], No. 37304/57. Class 40 (7). A radio navigation computer and display unit is capable of taking the information from any suitable ones of a plurality of radio navigational aids including a hyperbolic co-ordinate device and combining this with local information about pitch, altitude, rate of climb and heading to give an indication of craft track utilizing rectilinear mutually perpendicular co-ordinates. The track is indicated as a permanently marked line on a chart and the presentation may include means for showing the instantaneous drift angle or the heading error. As described, the computer is fed with information from the following radio-navigation system receivers : Tacan, Decca Navigator, and Doppler; any one of which can be used singly, and bearing information equipments of direction-finder and omni-range beacon types, which need to be used in pairs. It is also possible to determine the distance and bearing of a single fixed station and feed this information to the computer. The local information may be derived from an artificial horizon device, altimeter and gyro-compass. The display unit comprises a chart 11 tensioned between rollers 12 and 13 and capable of being driven from one to the other by means of a motor 25. A potentiometer 26 on the drive gives an output proportional to the Y co-ordinate of the marking pen or stylus 19. The latter is supported on a carriage driven at right-angles to the direction of motion of the chart by motor 27 and endless band 17. A potentiometer 28 gives an output proportional to the X co-ordinate of the marking pen. As shown, the pen marks the underside of the chart (a transparent strip may be provided above the chart for it to press against) and the track is made visible by illuminating the chart from the underside. Besides the pen the carriage may also support a transparent rod 22 acting as the pointer of a mechanism 21 coupled to show the true heading of the craft. The pivot for the pointer is immediately under the pen and on the short end of the pointer is a transparent scale 23 with a centre zero. Against this scale can be read the recorded track to give the drift angle. If desired, once the drift angle is established the pointer may be offset by the amount of the drift to display the " track made good." The pointer may then be used as a direction indicator to steer to a given point on the chart so long as the drift remains the same. The y axis of the chart will not in general be the North-South line but will make an angle a therewith chosen so that the course it is desired to follow is substantially parallel with the y axis. It is arranged that a solenoid holds the pen against the chart and is energized in accordance with the signals received by the radio navigation receiver or receivers in use. Thus if signals are not being received or are weak and unreliable no trace is made. The current to the solenoid may be interrupted every minute intentionally to produce small gaps, serving as timing markers, in the track drawn. It is, of course, necessary to set into the display equipment initial information including the angle " a " of the chart in use. Use of bearings of two spaced stations. Where (x 1 , Y 1 ) and (x 2 , y 2 ) are the positions of the two stations on the chart in use and # and γ are the corresponding bearings of the aircraft from the stations then the position of the craft is given by y - y1 = - (x - x 1 ) cot (α - #) y - y 2  = - (x - x 2 ) cot (α - γ) (Fig. 6, not shown). The computer, which is described in detail with reference to Figs. 4A, 4B, 4C, 4D, and 4E (all not shown) solves these equations in the form: (y - <y 1 ) sin (α - #) + (x - x 1 ) cos (α - #) = 0 (y - y 2 ) sin (α - γ) + (x - x 2 ) cos (α - γ) = 0 The x and y information from potentiometers 28 and 26 is used to obtain values of (y - y 1 ) and (x - x 1 ) for application to a pair of sine-cosine resolvers positioned to represent (α - #) and the sum signal used to give an error voltage to drive the map feed motor 25 until the zero point is reached. At the same time a similar set of apparatus set according to (y - y 2 ) and (x - x 2 ) and with resolvers at (α - γ) provides an error signal to drive motor 27 so that both equations are satisfied and the pen is correctly positioned. The values of a, (x 1 , y 1 ), (x 2 , y 2 ) may be set in automatically from data punched or printed on the map margin. Provision can also be made to tune to the best pair of radio stations automatically. Use of Doppler derived information. Path speed (v) and drift angle (D) may be used in conjunction with locally obtained altitude variation (h), true heading (#), and pitch angle (Q), information to solve the equations: In solving these equations a potential proportional to the term in brackets in each equation is first obtained. This is then divided by the equivalent of cos 2Q and applied to the stator of a resolver with its rotor positioned in accordance with # + D. The angle a is subtracted by adjustment of the stator position so that the sine and cosine arm outputs of the resolver are respectively proportional to dx/dt and dy/dt. The outputs are applied to control the motors 27 and 25 to maintain the correct craft position once an initial position has been set manually. Use of bearing of, and slant range to, a ground station. Where d s  is the slant range, h the aircraft altitude and h g  the height of the ground station the ground distance d p  of the craft position from the station is first determined by use of the equations It will be noted that E is the elevation angle. For this purpose a signal proportional to d s  is fed to the stator of a resolver whose sine output is compared with (h - h g ) to give an error signal used to drive the resolver to an angular position representing E. The cosine output is then proportional to d p . The craft position can then be obtained from the equations where # is the true bearing of the ground station which has map co-ordinates (x 1 , y 1 ). Use of hyperbolic co-ordinates. The hyperbolic co-ordinates derived from the Decca Navigator receiver essentially provide path difference information. Utilizing two position fixing outputs I and m then it is known that:- l = distance of craft from master station - distance of craft from 1st slave station. m = distance of craft from master station - distance of craft from 2nd slave station. To avoid the complication of direct computation of x and y the value of l - (difference in the distance of the point represented by the pen from the master and the first slave station) is used to get an error signal supplied to correct the x indication and the corresponding value using m and the second slave station to give an error signal supplied to correct the y indication. When the null point is reached the craft position has been determined. The lane identification signals are utilized by the computer but in order to enable it to function properly the computer must be initially set with approximate values of the craft's co-ordinates. The various distances used in the computation are obtained in the computer by the use of addition, subtraction, and zero-error seeking resolver arrangements and switches are provided to enable any pair of hyperbolic co-ordinates out of the usual set of 3 to be used. It is explained with reference to Fig. 10 (not shown), how, if maps on Mercator's projection are used as the chart 11, the errors due to the form of projection can be corrected in the computer. Instead of pen and paper the display unit may use an electrically-heated stylus and heat-sensitive paper.