Patent GB-1430550-A (GB)
Title: Systems for the transmission of positional information
IPC: G01S1/70
Filed: 1974-07-01
Abstract: 1430550 Optical position finding SAABSCANIA AB 1 July 1974 [2 July 1973] 29058/74 Heading H4D In a system for providing positional information to objects located within a solid angle defined by two orthogonally disposed fan beams, which are alternately emitted by an optical transmitter, and which are respectively caused to scan in elevation and in azimuth by a scanning device, a coding element is coupled to said scanning device. A binary word corresponding to each instantaneous angular position of a beam is produced, and is converted by the coding element into a corresponding time pericd between two successive light pulses from the transmitter. At a receiver on an object within said solid angle the leading edges of at least two successive light pulses of a beam are detected, the time period between a pair is determined, and a decoder provides the corresponding binary word defining an angular co-ordinate of the object with respect to the transmitting station. As described, a laser transmitter 1, Fig. 1, is fed from an exciter 2 and transmits alternately and periodically via a deflecting device 3 two fan-shaped beams (4), (5), Fig. 3 (not shown), which extend at right-angles to each other. A position pick-up 10 is coupled to the device 3, and produces binary words corresponding to its positions which are fed to one input of a code selector 11. An element 12 also provides an input to the code selector 11 representative of "ether information", e.g., command signals &c. Output from the code selector 11 is fed via a circuit block 13 to a counter 14. Pulses from a constant frequency oscillator 15 count down the counter 14 and when zero is reached it is detected by a detector 16, which activates the exciter 2, so that the laser 1 emits a light pulse. At the same time the next binary number is entered in the counter 14, and the process is repeated. With nine binary bits, 512 different binary words can be formed so that the period of time between successive light pulses can be varied from Tmin (a time period determined by the characteristics of the laser) to Tmax, where (Tmax-Tmin) is 512 times the pulse repetition period of the oscillator 15 (Figs, 5, 6, not shown). A receiver comprises a detector 17, Fig. 2, which is sensitive to laser radiation and which is coupled via a clipper 18 (having a clipping level control circuit 19) and a delay circuit 20 to an input of a counter 21. Also connected to an input of the counter 21 is a storage 23, which contains a binary number corresponding to Tmin. When a laser pulse falls on the detector 17, a pulse signal is generated which, via the clipper 18 and the delay circuit 20, reaches the counter 21 to release the pre-setting from the storage 23 and to open the counter to count pulses from an oscillator 22. When the next laser pulse is detected, a pulse signal is generated anew which, via the clipper 18, reaches on the one hand the delay circuit 20, and on the other hand a selector 28, which activates one of four storages 24, 25, 26, 27 concerned respectively with positional information in azimuth and elevation and "other information" in the azimuthal and elevational directions. Which storage is to be activated is determined by an address portion of the binary word in the counter 21. When the pulse signal reaches the counter, the process is repeated. The information in the said storages is utilized by computer means as required. The oscillator 22 has a pulse frequency which is a wholenumber multiple of the frequency of the oscillator 15, Fig. 1. Owing to the higher frequency, a word having more binary bits than the one transmitted is obtained and, by cancelling in the word received the least significant bits, the effect of disturbing pulses is reduced.