Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
1999-08-05
2001-08-28
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S357490, C342S417000
Reexamination Certificate
active
06281841
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system for determining phase differences between signals and which is preferably utilised in a direction finding technique using satellite positioning systems (such as NAVSTAR or GLONASS) and in particular, but not solely the use of the technique to direct weapons in an artillery (or similar) system.
DESCRIPTION OF THE PRIOR ART
Infantry weapons simulation systems using lasers fitted to infantry weapons and laser beam sensitive apparel are well known. However, laser technology cannot be used in artillery simulation systems because artillery is usually employed out of sight of the targets. A useful simulation system must be able to indicate to targets if they have been “hit” when ground contours or ground cover impedes direct line of sight between weapons and targets. It would be desirable if the pointing direction of the barrel of an artillery piece could be ascertained accurately and quickly enough to allow a computer calculation of the point of impact of simulated shell fire which point could be radioed to soldiers in a battle exercise. If those soldiers were provided with satellite positioning receivers, such as GPS (Global Positioning System) receivers, and computing resources a comparison could be made of their position and the computed point of impact of the shells to provide a kill status.
Known devices for ascertaining direction of high accuracy include precision gyroscopes or inertial guidance systems. Such devices are very expensive.
GPS is used to determine direction of travel of an object by simply ascertaining the direction between two successive computed positions. Attempts have also been made to use the GPS system to determine the direction of pointing of a stationary object. U.S. Pat. No. 5,146,231 discloses a GPS direction finder which, in addition to a conventional omni-directional antenna, has a second antenna which is highly directional. From knowing the positions of the GPS receiver and a GPS satellite a bearing to the satellite is obtained and the directional antenna is used to determine the orientation of the direction finder axis with respect to the satellite.
The system disclosed in U.S. Pat. No. 5,146,231 has a number of disadvantages. In an artillery effects simulation the aim is to make the simulation equipment invisible to the trainees so that they can learn to operate the weapons in a realistic environment. The directional antenna is necessarily large to provide precision and must be rotated to measure direction, compromising the aims of the simulation. In addition, two antenna would be required to provide azimuth and altitude values for the direction finder. Finally, the direction finder is based on locating the null in the received signal as the antenna is moved, however the GPS signal is filled with noise and is difficult to locate precisely.
U.S. Pat. No. 5,461,387 discloses a GPS direction finder which uses a single multimode antenna to generate mode one and mode two signals and by determining the ratio of the amplitude of these signals the phase difference between the signals is ascertained which in turn produces a bearing to a GPS satellite. The system disclosed is optimised for low cost and small size and does not have sufficient precision to allow a simulation of the effects of artillery.
A method of direction measurement using a number of independent GPS receivers has been described by Cannon et al, ION GPS-92, Albuquerque, N.Mex. This method relies on accurately measuring the phase of the signal received from three or four satellites. Such accurate phase measurement requires that the receiver is locked on to the information structure of the GPS signal. The requirement of a clear view of three or preferably four satellites can be a problem in some applications. For example, in an artillery exercise situation a gunner will seek cover for his weapon and this will reduce the number of GPS satellites that can be “viewed” by the weapon direction finder. Further, there are practical disadvantages in constructing GPS receivers capable of accurate phase measurement. Still further, for the artillery simulation application, there are practical difficulties in obtaining sufficiently accurate measurements of the phase difference between two antennae when using separate receiver channels (a ‘dual channel’ system) to process the signals from each antenna. In dual channel systems, direct phase measurements are made between the output of separate correlators. As relative timing errors inherently occur between channels in a dual channel system, this type of system is inaccurate.
There is available a software package produced by the Department of Geometics of the University of Calgary and sold under the trade mark HEAD which receives input of partial (fractional number of cycles) phase difference measurements between signals from two GPS antennae and then determines the absolute phase difference (both integer and fractional parts) unambiguously between the signals for each satellite (phase difference may be used to determine direction as explained later in this specification). This is achieved by looking at all the available information from the satellite signals and then determining what phase difference value will fit in for all of the information obtained (effectively similar to solving a series of simultaneous equations by elimination). However, this system requires at least five satellites to be visible in order to work and therefore is unable to continually provide phase difference information when less than five satellites are visible.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a directional finder which can function with a clear view of only two satellites and which goes some way towards overcoming the other disadvantages mentioned above.
In one aspect the invention consists in a method of determining the direction an object is pointing, using a position locating system and at least two omnidirectional microwave antennas spaced apart by a predetermined distance on a chosen axis of said object comprising:
(a) receiving at said antennas microwave signals from satellites at known positions,
(b) phase shifting the signal from a first of said antennas through a range of phase angles,
(c) summing the phase shifted signal with the signal from a second of said antennas,
(d) extracting from the summed signal the signal from a first satellite using correlation techniques,
(e) determining the magnitude of said correlated signal as a function of the phase shift of the phase shifted signal,
(f) determining the phase shift at peak magnitude,
(g) calculating the angle between said first satellite and said object axis using the angle of phase shift and said predetermined spacing of the said two antennas,
(h) resolving the ambiguity associated with the unknown integer number of complete cycles in the phase difference of the signals received by the two antennas;
repeating steps (a) to (h) for signals from a second satellite to determine the angle between said second satellite and said object axis,
deriving from said position locating system the ground position of the second antenna and the positions of said two satellites,
calculating from the solid angles between each of said two satellites and the object axis, the positions of said satellites and the ground position of the second antenna, the two possible azimuth and elevational directions of the object axis, and resolving the ambiguity in direction by using directional information from another source.
In a further aspect the invention consists in a method of determining the direction in which a gun barrel is pointing, using a position locating system and at least two omnidirectional microwave antennas mounted on said barrel and spaced apart by a predetermined distance, which method comprises:
receiving at said antennas microwave signals from satellites at known positions,
phase shifting the signal from a first of said antennas through a range of phase angles,
summing the phase shifted signal with the signal from a second of said antenn
Jacobson Price Holman & Stern PLLC
Phan Dao
Techno International Limited
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