1,193,932. Radio navigation. J. P. CHISHOLM. 5 April, 1968, No. 16535/68. Heading H4D. Specification describes a navigation system wherein each of a plurality of units A, B and C, Fig. 1, can determine its range to any of the other units, wherein a marker pulse signal is transmitted from a first unit and its time of receiption S at a second unit is determined according to the time clock of the second unit, a marker pulse signal is subsequently transmitted from the second unit and its time of reception L at the first unit is determined according to the first unit's time clock, the elapsed time (L-M) between transmission of its own signal and the reception of the second unit's signal is determined according to the first unit's time clock, the delay (T-S) between reception of the first unit's signal and the transmission of the second unit's own pulse signal, is determined according to the second unit's time clock and said delay is subtracted from said elapsed time to give a time [L-M-(T-S)] proportional to the signal propagating line between the units. As shown in Fig. 2 a frame of 1 sec. duration is provided, divided into 100 time slots, each 10,000 (Ás long and each having a particular unit assigned to it, e.g. slot #1 is assigned to unit A, slot #2 to the unit B and slot # 3 to unit C. Each unit comprises an atomic clock which though stable may not be synchronized with the clocks of the other units. Thus the clock of unit B is 1000 Ás. later than that of unit B and that of unit C is 500 Ás. earlier than that of unit A. Each unit transmits a marker pulse 3000 Ás., M (as measured on its own clock) after the beginning of its time slot. Transmitted marker pulses in Fig. 2 are shown with arrow head up and received pulses with arrow head down. The transmitted pulse from A is received at unit B 240 Ás. later (see Fig. 1 for propagation times) and of unit C, 192 Ás. later. Due however to the other unit's clocks being out of synchronism with that of unit A, their clocks time the reception S of the marker pulse as 2240 Ás. and 3692 Ás. respectively. Unit B transmits its marker pulse at time 13,000 Ás. T on its own clock, which is received at unit A at a time 14,240 Ás. on its own clock. Unit B then transmits, 2000 Ás. after its pulse, a telemetry message, comprising, its identity B, its altitude 31,000 Ás., the time 13,000 Ás. on its own clock that it transmitted its marker pulse, the identity of the unit transmitting the marker pulse last received A, and the time 2,240 Ás. of reception of said pulse. This message is received and decoded at unit A, whence said delay (T-S) = 10,760 Ás. is determined and subtracted from the determined elapsed time (L-M) = 11,240 Ás. to give 480 Ás, or 2 x 240 Ás. All the signals are transmitted and received omnidirectionally so that A's marker pulse is received at unit C which transmits its own pulse and its telemetry message in time slot # 3 to enable the propagation time between units A and C to be similarly determined. The telemetry messages may be received at a central ground station whereby a matrix of the units positions may be produced. Fig. 4 illustrates unit B which comprises an atomic 1 mc/s. clock 18 feeding a flip/flop chain 20, which in turn feeds a binary encoder 30 which generates the coded times L and M and a time slot logic circuit 22 which produced enabling pulses at outputs 21, 23, 25 . . . 29 defining the time slots #1, #2 &c. Time slot #2 is assigned to unit B, so that the logic circuit output on line 23 is fed via a 3000 Ás. delay 26 to the transmitter assembly 28/14 whereby its marker pulse is transmitted by the unit at the required time. The local time of transmission 13,000 Ás. is produced by encoder 30 on line 32 and is fed as time M to computer 100 and is also entered in enabled store 42 which forms one of a line of stores recording the local clock reading when the any marker pulse (including the units own marker pulse) is received. The marker pulse control output of transmitter control 28 is delayed by 2000 ÁS. in means 34 and enables telemetry assembly 36. This comprises a digital altimeter, an identity code, a local clock reading section and a telemetry encoder section. The last received marker pulse was from unit A and this fact is stored in store 80. The local clock reading when said marker pulse was received was 2240 Ás. and this is stored in store 40. Stores 80, 40 and 42 are read out and together with the altimeter information give the required information for the telemetry signal. The marker pulse and telemetry signal is received at unit C which in turn transmits its marker pulse and telemetry signal which are received by unit B. According to its local clock unit C receives B's marker pulse at 14,644 Ás. and transmits its own marker pulse at 23,000 ÁS. This information, together with the information that the marker pulse from A is received at a local time of 3692 Ás. is telemetered back to unit B. The marker pulse from C is received at unit B at a local time 21,644 Ás. and this time is stored in unit 44 and is also fed as time L to computer 100. The telemetry signal from unit C is decoded at 19, and the telemetry contents C-265-23000-B-14644 stored in units 84, 94, 74 and 64 respectively. The contents of stores 74 and 64 are fed as times T and S respectively to computer 100. Computer 100 then computes the propagating time between units B and C as 144 Ás. which is then stored in unit 54. The propagation time to unit A is then determined at the beginning of the next frame. If additional storage units are provided unit B can " overhear " the transmissions between other units and store the propagation times between these units. Rather than telemeter both the transmit and receive times, e.g. 14,644 Ás. and 23,000 Ás. for unit C, the unit can telemeter the transmit time and the delay between the reception time and the transmit time.