GB2111787A

GB2111787A - Distance measuring system

Info

Publication number: GB2111787A
Application number: GB08137225A
Authority: GB
Inventors: Rolf Johannessen; Francis Giles Overbury
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-12-10
Filing date: 1981-12-10
Publication date: 1983-07-06
Also published as: GB2111787B

Abstract

A distance measuring system for marine use in conjunction with a bearing information transmitter or "radio lighthouse" e.g. of the type in Patent No. 2062394. A continuous 3KHz tone is broadcast as a sideband of the radio lighthouse transmission. Ship-borne equipment includes a standard VHF communications receiver and means for transmitting an interrogation signal to the radio lighthouse comprising a short burst of 3KHz phase locked to the tone received at the ship. The radio lighthouse equipment measures (25) the phase difference between the continuous tone and the received interrogation signal and calculates (26) from this the distance to the ship. This distance is then transmitted (27) to the ship as a digitally encoded signal. <IMAGE>

Description

SPECIFICATION Distance measuring system This invention relates to a distance measuring system for marine use in conjunction with other navigational aids.

British Patent 1,429,743 (R. Johannessen-J.C.

Greenwood 10-1) discloses that apparatus for transmitting bearing information including two fixed omnidirectional aerials spaced a predetermined distance apart, means for transmitting from one aerial a composite signal comprising a combination of a VHF carrier frequency signal periodically modulated by an audio frequency signal and the unmodulated carrier frequency signal such that the modulated and unmodulated carrier frequency signals are in antiphase, means for transmitting from the second aerial the periodically modulated carrier frequency signal only, means for cyclically varying the phase of one of the transmitted signals relative to the other transmitted signal, and means for distinctively altering the audio frequency modulation of the carrier frequency signal at the commencement of each cyclic variation in phase of the one transmitted signal.

Such an apparatus will transmit signals which can be picked up by any VHF communications receiver, however humble. The transmission creates in effect a radio vector rotating slowly about a fixed point, i.e.

the site where the two aerials are located. This vector is defined, to the VHF radio listener, by a point in time when the received signal has a minimum audio frequency modulation. This listener will hear first a distinctive signal which serves to identify the particular shore station being received and the start of a period in time. He then hears a series of equally spaced clearly distinguished audio signals. Beginning at the end of the distinctive time signal the listener counts the number of regularly spaced audio signals until the audio frequency modulation reaches a null. The count so obtained can be directly translated into a bearing from or to the shore station, e.g. by reference to a table.Thus, with the aid of no more than a VHF radio receiver, a bearing table (which could be memorised) and a chart (which could also be memorised) a sailor can determine his bearing to a fixed point on land and navigate using that bearing.

British Patent application 7936772 (specification serial No. 2062394 A) C. Earp-R. Johannessen-F.G.

Overbury 159-33-28) discloses an improvement over the earlier patent in which a single sideband is used to carry the audio tone.

However, such apparatus as disclosed in the above referenced prior art gives only bearing information. A navigator can calculate a position "fix" if he can obtain two intersecting bearings from two shore stations both of which are in range. With only one bearing available he cannot compute his position unless he can determine his distance from the single shore station which is giving him a bearing.

According to the present invention there is provided a distance measuring system comprising means for radiating as a single sideband of a VHF transmission of an apparatus for transmitting bearing information, in which another sideband is a signal periodically modulated with a first audio frequency signal, a VHF radio signal modulated with a continuous fixed frequency different audio frequency signal, means for receiving from a remote equipment a VHF radio signal modulated with a burst of audio frequency signal of the same frequency and phase as the continuous audio frequency signal received at the remote equipment, means for measuring the phase difference between the radiated continuous audio signal and the audio frequency modulation on the received burst of audio signal, means for computing the distance between the radiating equipment and the remote equipment as a function of said phase difference and means for transmitting an encoded signal conveying said distance information.

Such a system is ideally suitable for incorporation with an apparatus for transmitting bearing information as disclosed in application No. 7936772 where the bearing and distance modulations can be superimposed on the same carrier.

As for the remote equipment, e.g. a VHF receiver by means of which bearing information can be obtained, only minimal modification is required. All that is needed is an audio frequency local oscillator output when a distance measurement is required.

An embodiment of the invention will now be described with reference to the accompanying drawings in which Figure 1 illustrates schematically the basic elements of both the shore station and the ship-borne equipment of a marine distance measuring system, and Figure 2 illustrates the relationship between the sidebands of a conventional marine VHF channel used by the equipment of Figure 1.

In the shore station an r.f. oscillator 10 has its output fed to a VHF power amplifier 11 where the r.f.

carrier is continuously modulated by a stable 3 KHz audio tone from generator 12. This continuously modulated signal is fed, via circulator 13,to aerial 14 from whence it is radiated. Conveniently this 3 KHz tone may be radiated as one side band only of the carrier which is used in a bearing information transmitter of the type disclosed in application 7936772, from which the periodically modulated audio tone, e.g. of 1 KHz, is radiated as another sideband. Many of the channels in the International Marine VHF band are duplex channels. Channel 88, for instance, has one slot at 162.025 MHz used for shore-to-ship communication and one slot at 157.425 M Hz used for ship-to-shore. Each slot can have, for example, upper and lower sidebands which are used for separate functions.Thus the shore-toship slot at 162.025 MHz can use a 1KHz upper sideband to transmit a bearing information signal in the manner disclosed in application 7936772 and a 3KHz lower sideband to radiate the continuous audio tone for distance measurement purposes.

In the ship-borne equipment the two sidebands of the 162.025KHz carrier are received at aerial 15 and fed to a conventional VHF receiver 16. The 1KHz bearing tone and the 3KHz range tone are demodulated with the aid of a local r.f. oscillator 17. After passing through l.F. amplifier 18 the 1KHz tone is filtered out by audio filter 28 and used to determine the ship's bearing on the shore station. The 3KHz continuous tone is derived via audio filter 29 and, after amplification, is fed to a phase measuring circuit 19.

When the mariner wishes to interrogate the shore station a short burst of 3KHz tone in phase with the 3KHz tone received at the ship is used to modulate the output of an r.f. carrier oscillator 20 in power amplifier 21. This interrogation signal is transmitted on both sidebands of the 157.425MHz slot.

At the shore station the interrogating burst is received in receiver 22 and demodulated using local oscillator 24 and IF amplifier 23. The demodulated 3 KHz signal is fed to a phase comparator 25 together with a 3 KHz output from generator 12. The differ- ence in phase between the two signals is translated into a distance measurement in computer 26 and a coded reply signal is derived in encoder 27 which can then be transmitted to the ship.

The interrogation signals and coded reply signals are accompanied by ship identification signals. High accuracy can be achieved from integration due to the continuous transmission from the shore station giving plenty of time to phase lock the on board oscillator. Spectrum saviny is achieved by superimW position of the bearing and range information on the same carrier. Also, additional spectrum saving can be effected by the use of a spare duplex channel for the shipishore transmission. No significant internal modification of the normal ship-borne equipment is required, the additional equipment being in the form of add-on equipment between receiver-transmitter and aerial. The shore station can improve the accuracy of the system by incorporating correction circuits to counteract the known delays in the receiver circuits, particularly those arising in narrow band filters.

Claims

1. A distance measuring system comprising means for radiating as a single sideband of a VHF transmission of an apparatus for transmitting bearing information, in which another sideband is a signal periodically modulated with a first audio frequency signal, a VHF radio signal modulated with a continuous fixed frequency different audio frequency signal, means for receiving from a remote equipment a VHF radio signal modulated with a burst of audio frequency signal of the same frequency and phase as the continuous audio frequency signal received at the remote equipment, means for measuring the phase difference between the radiated continuous audio signal and the audio frequency modulation on the received burst of audio signal, means for computing the distance between the radiating equipment and the remote equipment as a function of said phase difference and means for transmitting an encoded signal conveying said distance information.

2. A system according to claim 1 wherein said first audio frequency is situated approximately in the centre of the pass-band of a VHF communications receiver and said second audio frequency is situated substantially at the upper limit of said pass-band.

3. A system according to claim 1 or 2, wherein said remote equipment comprises means for receiving the VHF radio signal modulated with said continuous fixed frequency audio frequency signal and means for transmitting a burst of audio frequency signal in audio phase with the received signal when a distance measurement is required.

4. A system according to claim 3 wherein said continuous fixed frequency audio frequency signal is transmitted in one slot of a duplex channel in the International Marine VHF band and the remote equipment transmits said burst in the other slot of said channel.

5. A distance measuring system substantially as described with reference to the accompanying drewings.

6. A method of determining the distance between a fixed station and a remote mobile station wherein the fixed station transmits, as a single sideband of a VHF transmission of an apparatus for transmitting bearing information on another sideband, a continuous fixed audio frequency signal, the remote mobile station receives the continuous fixed audio frequency signal and transmits an interrogation signal being a burst of audio frequency signal having the same frequency and audio phase as the received continuous signal, the fixed station receives the interrogation signal and compares the audio frequency phase with that of the continuously transmitted signal to calculate the distance between the stations, the fixed station then transmitting an encoded signal conveying the distance information to the remote station.