Patent Description:
In its basic infantry form, a Tube-launched, Optically tracked, Wire-Guided ("TOW") missile system includes a missile in a sealed tube, which is clipped to a launch tube (or rail) prior to use. When required, the missile tube is attached to the rear of the launch tube, the target sighted and the missile fired. The launch motor (also called the "kick" motor or booster) ejects the missile from the launch tube, at which point four wings indexed at <NUM> degrees just forward of the booster nozzles spring open forwards, four tail control surfaces or "fins" flip open rearwards, and sustained propulsion is subsequently provided by the flight motor (sustainer) which fires through lateral nozzles amidships and propels the missile to the target. An optical sensor on the sight continuously monitors the position of a light source (e.g. the thermal signature of the hot motor) on the missile relative to the line-of-sight in a projectile coordinate system, and then corrects the trajectory of the missile by generating electrical signals that are passed down two wires, or more recently an RF link, to command the control surfaces to move the missile up/down and left/right. After launch, the operator simply has to keep the cross-hairs of his sight pointing at the target, and the guidance system will automatically transmit corrective commands to the missile through the wire. The TOW missile in its current variations is not a fire-and-forget weapon, and like most second-generation wire-guided missiles has Semi-Automatic Command Line of Sight guidance. This means that the guidance system is directly linked to the platform, and requires that the target be kept in the shooter's line of sight until the missile impacts.

The guidance system includes a signal processor that is coupled to the optical sensor to generate a measured missile position (e.g., Azimuth/Elevation (Az/El)) in the sensor's FOV and a track valid flag indicating the missile is within the FOV. The difference between the measured missile position and a desired missile position in the FOV (e.g., the cross-hairs or a known offset from the cross-hairs) forms an error signal. As long as the track valid flag is true, a controller generates control surface commands in the projectile coordinate system to maneuver the missile to reduce the error. The controller typically generates one command (e.g., Az) to a first pair of control surfaces to move the missile left and right and another command (e.g., El) to a second pair of control surfaces to move the missile up and down. The control surface actuator on the missile may be an analog or digital controller responsive to an angle command or a "bang-bang" controller responsive to a duty cycle modulation of a binary command. This closed-loop process repeats until the missile engages the target or track of the missile is lost and an abort command is issued.

The optical sensor has a narrow FOV, typically a couple degrees. The TOW missile system is used to engage targets at large stand off distances, over a few thousand meters. The FOV must be narrow to support these ranges. As such, it is not uncommon for the missile to fly out of the FOV and lose track. This might, for example, occur if the gunner jerks the cross-hairs or a gust of wind hits the missile in flight.

If track is lost, the signal processor switches the track valid flag to false. The controller then holds the last valid control surface command (e.g., Az/El) until either track is re-established and the track valid flag is true or an abort command is issued. Holding the last valid control surface command maneuvers the missile along a straight-line path towards the desired missile position (e.g., the cross-hairs) based on the last measured missile position before track was broken. The straight-line path is the best estimate to intersect the tracker's FOV based on the last measured missile position.

A carrier tracking system is disclosed in <CIT>, which discloses a microprocessor of an electronics package programmed so that upon receipt of a start up signal, tracker conditioning is effected by starting a clock, timing sequence and determining pre-fire conditions. A time decision is then made. If the time is less than a preselected time, a decision is made whether the tracker is in handoff. Handoff results when the tracker loses the missile. If the answer is no a computer commands the missile to fly a standard track link, and the computer returns to start. If the tracker is in handoff, a decision is made whether the missile is in the field of view of the forward looking infrared (FLIR) sight. If not, a command <NUM> is given for the missile to fly a preprogrammed flight profile.

Improvements relating to missile guidance systems are disclosed in <CIT>, which discloses a system comprising an optical sight for aiming at a target, an optical tracker having an optical axis approximately aligned with that of the sight for tracking the missile in flight in the field of view of the tracker, the tracker being constructed and arranged to produce a continuous electrical output representing the radially -offset position of the missile in flight with respect to the projected tracker axis, whereby the tracker output continuously defines a guidance requirement for controlling the missile flight path onto the target, and means for deriving automatic electrical control signals dependent on the tracker output and for transmitting the control signals to a receiver in the missile as command signals to guide the missile automatically in accordance with the said guidance requirement, which means includes an electrical complex providing a variable output which is an electrical analogue of the said guidance requirement defined by the tracker output, means for automatically adjusting the complex continuously in response to variations in the tracker output during the flight of the missile under tracker control in such a way as to cause the output of the complex to conform continuously to the missile guidance requirement defined by the tracker output, and means responsive to a loss of the tracker output during flight for automatically utilising the output of the complex, in the condition to which it was adjusted immediately prior to the loss of tracker output to provide continued automatic missile guidance during the absence of tracker output.

In accordance with the invention, there is provided a method and apparatus as defined by claims <NUM> and <NUM>.

The existing approach of holding the last valid control surface command (e.g., Az/El) when track is lost to maneuver the vehicle along a straight-line path towards the desired vehicle position (e.g., the cross-hairs) based on the last measured vehicle position before track was broken is the best estimate to intersect the tracker's FOV assuming the vehicle did not move (e.g., rotate or shift) after track was broken.

However, if the vehicle does move after track is broken, the chance that the straight-line path based on the last measured vehicle position may fail to intersect the tracker's FOV increases significantly. A small rotation of or shift in position of the vehicle may put the vehicle on a straight-line path that does not intersect the tracker's FOV.

The present invention provides an increased likelihood of recapture of a remotely-sensed command guided vehicle given vehicle motion after track is broken. Upon loss of a valid track of the vehicle, the guidance system generates a delta actuator command including an orthogonal component orthogonal to the straight-line path as a next sample of a time-based alternating signal. The guidance system adds the time-based delta actuator command to the nominal actuator command, which is "held" upon loss of valid track, to maneuver the vehicle in first and second orthogonal directions back and forth across the straight-line path to increase an area of vehicle motion relative to the tracker's FOV. The penalty is a reduction in energy efficiency. In certain embodiments, this is accomplished without modification to guidance system hardware or the existing tracking valid or invalid guidance algorithms.

In different embodiments, the vehicle is a land, air, sea or space based vehicle. The actuators may comprise aerodynamic control surfaces such as fins, wings or canards, continuous or <NUM>-shot thrusters or propellers, rudders, or rockets. The commands may be transmitted over a wire, WIFI, radio, laser, optical or infrared link to the vehicle. The transmitted commands transmitted may be configured for analog, digital or bang-bang control. Remote tracking may use optical, infrared, radar or sonar sensing to determine vehicle position in the FOV.

Without loss of generality, the present invention will be described in the context of a Tube-Launched, Optically Tracked, Wire-Guided (TOW) missile in which Azimuth (AZ) and Elevation (EL) actuator commands are generated at the missile launcher and transmitted over a pair of wires to actuate Az and El pairs of fins to guide the missile.

Referring now to <FIG>, <FIG> and <FIG>, an embodiment of a TOW missile system <NUM> includes a missile <NUM> and a missile launcher <NUM> to launch and command guide the missile <NUM> to a target Missile <NUM> includes a motor <NUM>, a plurality of fins <NUM> and a fin controller <NUM> configured to adjust the fins <NUM> to maneuver the missile in first and second orthogonal directions in a projectile coordinate system. Missile <NUM> is typically provided in a sealed tube <NUM> that is mounted on the missile launcher <NUM>.

Missile launcher <NUM> includes a platform <NUM> for mounting and launching the missile <NUM>, a sight <NUM> for placement of cross-hairs on a target, an optical sensor <NUM> having a field of view (FOV) <NUM> that includes the cross-hairs, a communication link <NUM> such as a pair of wires between the missile launcher <NUM> and the missile <NUM>, a guidance computer <NUM> for generating the Az and El actuator commands to maneuver the missile <NUM> towards the cross-hairs when track is valid and to maneuver the missile <NUM> for recapture when track is lost, and a transmitter <NUM> for transmitting the actuator commands over the communication link <NUM>. Together components may be referred to as the Tracker.

Guidance computer <NUM> includes a signal processor <NUM> coupled to sensor <NUM>. The signal processor <NUM> is configured to determine a missile position in the FOV <NUM> and set a track valid flag equals true if the missile <NUM> is detected in the FOV <NUM> and to generate a measured missile position in the FOV <NUM> and set the track valid flag equals false if the missile <NUM> is not detected. A first summing node <NUM> forms a difference of the measured missile position and a desired missile position (e.g., missile position command) in the FOV (e.g., the cross-hairs) as an error signal. A controller <NUM> is configured to generate "guidance commands" (step <NUM>) as a new nominal actuator command (e.g., including Az and El components) to use a latest command based on the error signal (step <NUM>) if the track valid flag is true (step <NUM>) and to hold a last valid nominal actuator command (step <NUM>) to place the missile <NUM> on a straight-line path <NUM> from a last known position of the missile <NUM> towards the desired missile position in the FOV <NUM> if the track valid flag is false (step <NUM>). A recapture module <NUM> is configured to generate "positions" (step <NUM>) as a delta actuator command based on last positions including an orthogonal component orthogonal <NUM> to the straight-line path <NUM> as a next sample of a time-based alternating signal (step <NUM>) if the track valid flag is false (step <NUM>) and a delta actuator command as a sequence of zeroes (step <NUM>) if the track valid flag is true (step <NUM>). A second summing node <NUM> sums the nominal actuator command and the delta actuator command to form a total actuator command. Transmitter <NUM> transmits the total actuator command from the missile launcher <NUM> over the communication link <NUM> to the missile <NUM> in flight to the fin controller <NUM> to control the plurality of fins <NUM> to command guide the missile <NUM> to maneuver in the first and second orthogonal directions to flying along an alternating path <NUM> back and forth across the straight-line path <NUM> to increase an area of missile motion relative to the tracker's FOV <NUM> until the missile <NUM> re-enters the tracker's FOV <NUM> and valid track is re-established or an abort command is issued. Increasing the area of missile motion relative to the tracker's FOV <NUM> as compared to the area for a straight-line path increases the likelihood of intersecting the tracker's FOV <NUM>.

Referring now to <FIG>, in an embodiment the recapture module <NUM> generates at least an orthogonal component <NUM> as a time-based alternating signal (e.g., sine wave, triangle etc.) and possibly an inline component <NUM> in a tracker coordinate system to define a generic search area <NUM>. Using an angle theta <NUM> between a straight-line path <NUM> in the tracker coordinate system defined by the last valid actuator command and an Azimuth axis <NUM>, the recapture module <NUM> transforms the orthogonal and inline components <NUM>, <NUM> into Az and El components <NUM>, <NUM>, respectively, in a projectile coordinate system. As shown, the orthogonal and inline components <NUM>, <NUM> and generic search area <NUM> are independent of the straight-line path <NUM>. The Az and El components <NUM>, <NUM> are a function of both the orthogonal and inline components <NUM>, <NUM> and the straight-line path <NUM>.

In this example, angle theta <NUM> is defined as the angle between inline component <NUM> (straight-line path <NUM>) and the Azimuth axis <NUM>. The matrix transformation is given by: <MAT>.

Referring now to <FIG>, in an embodiment inline and orthogonal components <NUM>, <NUM>, respectively, form a delta actuator command <NUM> that defines a generic search area <NUM> in a tracker coordinate system. As shown, orthogonal component <NUM> is a time-based sine wave of increasing amplitude that defines the basic form of search area <NUM>. A bias term may be added to the time-based sine wave to shape the area of vehicle motion. The bias term may include a ramp or exponential function of time. Inline component <NUM> represents an additional component to the last valid (and held) command that defines the straight-line path and is optional. If included, inline component <NUM> may include a constant term <NUM> or <NUM>, a ramp <NUM> or an exponential function of time to control the rate of motion of the missile <NUM> along the straight-line path. As shown, delta actuator command <NUM>, hence the search area <NUM> changes as a function of the inline component <NUM>. The inline and orthogonal components <NUM>, <NUM> may be selected as a function of at least one of an elapsed time of travel, a range to a target, a vehicle velocity, the straight-line path and an elapsed time since the last valid track. Selection may be affected by such factors as the energy efficiency of the resulting path or the ability of the missile <NUM> to reach the target.

Referring now to <FIG> through <FIG>, differences in the existing and proposed recapture algorithms are illustrated for different loss of track conditions and different motion of the missile following loss of track.

In a first example shown in <FIG> and <FIG>, a missile <NUM> leaves the tracker's FOV <NUM> following a path <NUM> horizontally to the left and undergoes no motion following the loss of track prior to execution of the recapture maneuver. As shown in <FIG>, following the existing recapture algorithm, the last actuator command is held to place the missile on a straight-line path <NUM> back towards the expected location of the desired position in the tracker's FOV <NUM>. As shown in <FIG>, following the proposed recapture algorithm, a delta actuator command that follows a time-based alternating signal of increasing amplitude is added to the last actuator command (held) to place the missile on an alternating path <NUM> about straight-line path <NUM> back towards the expected location of the desired position in the tracker's FOV <NUM>.

As shown in <FIG>, the last actuator command includes an Az component <NUM> that moves the missile <NUM> to the right and an El component <NUM> to counteract the effects of gravity. Together these components would place the missile <NUM> on a straight-line path horizontally to the right back towards the tracker's FOV <NUM>. These components are the same for both the existing and proposed recapture algorithms. As previously shown in <FIG>, the delta actuator command includes a ramp inline component <NUM> and an increasing amplitude sine wave orthogonal component <NUM>. Because theta is zero in this example, the Az component <NUM> equals the inline component <NUM> and the El component <NUM> equals the orthogonal component <NUM> per the matrix transformation in equation <NUM>. A total Az component <NUM> is the sum of Az components <NUM> and <NUM> and a total El component <NUM> is the sum of El components <NUM> and <NUM>.

The existing recapture algorithm follows a straight-line path back towards the tracker's FOV. The proposed recapture algorithm increases the search area with respect to the FOV and thus provides a higher likelihood of intersecting the tracker's FOV. In this case because there was no unknown motion of the missile after loss of track, both the existing and proposed recapture algorithms result in a successful recapture.

In a second example shown in <FIG> and <FIG>, a missile <NUM> leaves a tracker's FOV <NUM> following a path <NUM> at an angle theta of <NUM> degrees to the left and undergoes a clockwise rotation of approximately <NUM> degrees following the loss of track prior to execution of the recapture maneuver. As shown in <FIG>, following the existing recapture algorithm, the last valid actuator command is held to place the missile <NUM> on a straight-line path <NUM> back towards the expected location of the desired position in the tracker's FOV <NUM> based on the last valid position of the missile <NUM>, resulting in a failed recapture. As shown in <FIG>, following the proposed recapture algorithm, a delta actuator command that follows a time-based alternating signal of increasing amplitude is added to the last actuator command (held) to place the missile <NUM> on an alternating path <NUM> about straight-line path <NUM> back towards the expected location of the desired position in the tracker's FOV <NUM>, resulting in a successful recapture. Because theta is <NUM> degrees in this example, the Az and El components <NUM>, <NUM>, respectively, are a composite of any additional inline component and the orthogonal component. In this case, because of the rotation of the missile following loss of track, the straight-line path provided by the existing recapture algorithm fails to recapture the tracker's FOV eventually producing an abort command. However, the alternating path around the straight-line path provided by the proposed recapture algorithm intersects and successfully recaptures the tracker's FOV.

In a third example shown in <FIG>, a missile <NUM> leaves the tracker's FOV <NUM> following a path <NUM> at an angle theta of <NUM> degrees to the left and undergoes an abrupt vertical shift following the loss of track prior to execution of the recapture maneuver. As shown in <FIG>, following the existing recapture algorithm, the last actuator command is held to place the missile <NUM> on a straight-line path <NUM> back towards the expected location of the desired position in the tracker's FOV <NUM> based on the last valid position of the missile <NUM>, resulting in a failed recapture. As shown in <FIG>, following the proposed recapture algorithm, a delta actuator command that follows a time-based alternating signal of increasing amplitude is added to the last actuator command (held) to place the missile on an alternating path <NUM> about straight-line path <NUM> back towards the expected location of the desired position in the tracker's FOV <NUM>, resulting in a successful recapture. Because theta is <NUM> degrees in this example, the Az and El components are a composite of any additional inline component and the orthogonal component. In this case, because of the abrupt vertical shift of the missile following loss of track, the straight-line path provided by the existing recapture algorithm fails to recapture the tracker's FOV eventually producing an abort command. However, the alternating path around the straight-line path provided by the proposed recapture algorithm intersects and successfully recaptures the tracker's FOV.

The proposed recapture algorithm provides for a more robust recapture of the missile given the possibility of unknown missile motion (e.g., rotation or shift) after track is lost and prior to initiating recapture than the existing straight-line recapture algorithm. The penalty is reduced energy efficiency.

Claim 1:
A method of recapture of a remotely tracked command guided vehicle within a tracker's field-of-view , FOV, the vehicle including a plurality of actuators configured to perform maneuvers in first and second orthogonal directions, wherein the vehicle is an air, space, aerially tracked land, or aerially tracked sea based vehicle, the method comprising:
upon loss of a valid track of the vehicle,
a) holding (<NUM>) a last actuator command based on a last measured vehicle position before track was broken as a nominal actuator command that maneuvers the vehicle on a straight-line path from a last known position of the vehicle towards the tracker's FOV;
b) generating (<NUM>) a delta actuator command including an orthogonal component orthogonal to the straight-line path as a next sample of a time-based alternating signal;
c) summing the nominal actuator command and the delta actuator command to generate a total actuator command;
d) transmitting the total actuator command from the remote tracker to the vehicle to control the plurality of actuators to command guide the vehicle to maneuver in the first and second orthogonal directions to move the vehicle along an alternating path back and forth across the straight-line path to increase an area of vehicle motion relative to the tracker's FOV; and
repeating the steps a) through d) until the vehicle re-enters the tracker's FOV and valid track is re-established or an abort command is issued.