Rate aided tracking for launch transient survivability

#Maintaining target track during missile launch transient perturbations is a long standing problem for lock-on-before-launch (LOBL) target tracking seekers. The launch transient must be accomodated by the seeker when the target range is unusually large; consequently, target signal-to-noise (S/N) and apparent target size are at their minimum. Such conditions severely limit the ability of conventional target trackers to follow any large target motion resulting from a launch transient induced sensor rotation. A prior art solution of the launch transient problem has been to increase the performance capability of the sensor stabilization platform to achieve the degree of missile airframe to sensor isolation which limits to an acceptable level the amount of target motion that must be tracked. However, for high performance airframes the degree of stabilization performance required becomes a major cost driver for LOBL seekers.

#For rate aided tracking, pitch and yaw rate information and missile roll rate information are used by the tracker to predict target motion during the launch transient. This auxillary information is used to electrically reposition the target track gate and thereby compensate for any target motion that results from sensor rotation during the launch period. This reduces the work load of the tracker from that of tracking a perceived large target motion to that of tracking the difference between the large motion and an estimate of that motion. This tracking method increases the ability of the target tracker to maintain lock-on during target pertubations induced by undesirable platform motion before launch.

#Rate aided tracking is applicable to any LOBL seeker that uses a rate stabilized sensor platform and a tracker with a capability of being electronically re-aimed. The advantage of rate aided tracking is that seeker stabilization performance requirements can be greatly reduced because of the increased capability of the traget tracker. The decrease of the seeker stabilization performance also reduces cost.

#The single FIGURE is a block diagram of a preferred embodiment of the system for providing platform rate information to the tracker.

#Referring now to the drawing, the rate aiding circuitry 10 is shown in dashed lines and is coupled with a standard target tracker 26 and to rate stabilized platform and associated circuitry. A typical rate stabilized platform will involve two degrees of freedom, i.e., it will respond to changes with reference to two coordinate axes normally referred to as x for yaw and y for pitch. It does not respond to changes in the third axis normally referred to as z for roll. The single FIGURE shows a stable platform 4 and related circuitry for providing x and y coordinate axis processing. The y axis processing 6 is shown generally and is identical to the x axis processing discussed in detail.

#In a tracking system as shown generally in the single FIGURE, missile motion results in forces being applied to components and systems within the missile housing. Stable platforms such as platform 4 are sensitive to these motions. Thus, when missile motion forces are applied during the transition of a missile from a stationary position within a launcher to unemcumbered flight position free of the launcher, transient forces affect the missile and stable platform therein. These transient forces are manifest in the inertia of the platform, in the gimbal springs, and through friction, to cause a physical or mechanical misalignment of a target sensor 24 from the electro-optically established target LOS and of the rate sensor 34. A torque motor 36 is coupled to the stable platform to respond to forces on the platform to induce a stabilizing influence on the platform. The torque motor can also provide an offsetting or neutralizing response to undersirable motion. In the drawing, these correctional forces are depicted as a mechanical force in dashed lines 33 applied from the torque motor into the target sensor 24 and to rate sensor 34.

#In the system target sensor 24, normally composed of an optical telescope system and a detector, converts an impending target scene into an electrical signal 25. The x and y coordinate position of the target within this electrical signal is determined by the angular displacement .theta. of the optical axis of the sensor 24 with respect to the inertial line-of-sight (LOS) between the target position and the tracking system reference position or optical axis. The sensor output electrical signal 25 is operated upon by the tracker 26 to extract the target's x coordinate 28. The tracker output 28 is multiplied by a velocity constant in multiplier 30 to produce an output rate command 32 that is coupled back to the stable platform.