Video compression for missile terminal guidance

A method and apparatus for improving aim point accuracy in a programmable flight control missile includes a communication system for receiving at least one frame of imagery in a vision system mounted in a missile, predicting a subsequent frame content based on the content of the previous frame as a function of missile path height, missile velocity, and frame rate, and creating a residual image by determining the difference between a predicted subsequent frame content and actually received frame content. In the preferred embodiment, the residual image can be transferred at a high bit rate through a communication system into a controller at which location correction data can be generated and delivered to the communication system to the missile in flight. Preferably, the predicted subsequent video frame and the sensed subsequent frame content are centrally aligned before creating a residual image. Preferably, a small, high entropy block of pixels is considered to determine an x or y vector for the point of best match, and the residual image is selectively quantized and entropy coded for transmission.

TECHNICAL FIELD 
The present invention relates to communication systems for missile guidance 
including a method and apparatus for improving accuracy of the missiles 
movement toward a target by communication of visual information about the 
target area and correspondingly correcting the missile aim point by 
in-flight target updates. (IFTU). 
BACKGROUND ART 
The flight of missiles is currently controlled by loading initial target 
location data into the missile prior to launch and then guiding the 
missile to the target via a global positioning system. Although targeting 
is relatively precise considering the long range that the missile travels, 
inaccuracies of a few meters can be very important for particular types of 
targets. For example, the effect of a missile hitting the top of a dam is 
substantially different than the effect of an impact in the water behind 
the dam. Even when terminal guidance is employed, aimpoint improvement is 
possible through an IFTU, but only with low resolution images in a 
reasonable time frame or at a low frame rate. 
SUMMARY OF THE INVENTION 
The present invention overcomes the above-mentioned disadvantages by 
providing back link imagery which provides the basis for an in-flight 
target update. Data compression based on a priori knowledge reduces the 
amount of information that needs to be conveyed, and renders low bit rate 
communications to provide lower expense for complete data transmission. A 
number of relay communication techniques can be used, but preferably 
involve a geosynchronous satellite communication system. 
In the preferred embodiment, the transmission of video information from a 
camera mounted on the missile is symbolically compressed for transmission 
via the geosatellite to the missile operator. Although previously known 
video compression techniques such as MPEG 2 standard (Moving Picture 
Experts Group) developed under the auspices of the International Standards 
Organization could be utilized, the preferred embodiment employs symbolic 
compression based on a priori data about the content of the frame 
sequences. Rather than merely squeezing the redundancy out of the rasters 
spatially and temporally in the raster of pixels and the sequence of 
frames as the sequence of such rasters, symbolic compression employs the 
fact that the next succeeding frame is an affine dilation (a zoom or 
enlarged representation) of a subimage of the prior frame. As a result, 
the next frame can be predicted fairly accurately knowing the missile 
height, velocity, and the frame rate. As a result, the amount of the 
information that must be transmitted is substantially reduced because it 
can be re-created from the prior frame. 
In the preferred embodiment, super compression in the transmitter on the 
missile employs a technique for forming a residual image as the difference 
between a sensor generated image and an image that is predicted from the 
prior frame. The residual image is coded and transmitted to the receiver. 
Since the residual image will not be correct if frames are not aligned, 
the sensor generated image and the predicted image are aligned by shifting 
the images with respect to each other until a best fit is found. Moreover, 
since it is desirable to have a fixed bit rate, transmission of the entire 
first frame is preferably sent over a number of frame intervals. 
Accordingly, the frame is hierarchically coded. 
As a result, the present invention decreases the amount of image data 
required to be transmitted as back linked information. For example, in the 
preferred embodiment, the information sent back to the operator can be 
25-50 times less than the transfer of imaging information compressed from 
other known methods of iconic compression. As a result, the present 
invention reduces the required missile antenna aperture, the missile 
transmitter power required, or the invention permits increased image 
resolution or increased frame rate at which communication can be conveyed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, a missile control system 10 for terminal 
guidance is thereshown comprising a control source 12 from which the path 
14 of a missile 16 is initiated. The path 14 begins at a launch site 
remotely located from its termination at a target 18. The system 10 
includes a communication system 20, preferably a relay communication 
system in which a geosynchronous satellite is the relay station between 
source 12 and missile 16. Nevertheless, unmanned area vehicles (UAVs) can 
also be used to relay to provide high data rate, jam-resistant network 
that add two-way communication ability between the source 12 and the 
missile 16. 
As the missile 16 launched with basic mission plans guide them toward a 
projected target location. A video back link is transmitted from the 
missile 16 through beam 22 to the satellite 20 and through a beam 24 to 
the controller 12. Preferably, the communication signals are transmitted 
through a single beam throughout the path of a missile 16 to avoid 
complicated switching requirements and the power requirements for the 
signaling through multiple beams. 
In the present invention, compression is obtained by reliance upon a priori 
knowledge about the content of video images to provide symbolic 
compression that results in substantially greater compression ratios than 
can be achieved by iconic compression. Video image from a camera located 
on the missile 16 approaching a target provides a sequence of frames in 
which subsequent frames have a substantial relationship to the previous 
frames. In particular, the affine dilation is an enlarged or zoom image of 
the previous frame as demonstrated in the sequence of frames shown in FIG. 
3. Accordingly, the subsequent frame can be predicted at a predictor 30 
with a high degree of accuracy as a function of the missile height, the 
missile velocity and the frame rate of the camera. 
In the preferred embodiment, the residual image is created by the 
transmitter in the missile by generating two images for each frame. The 
first image is the actually sensed image generated by the sensor as well 
as the second image that is predicted from the prior frame. These two 
images or their representative signals are subtracted to form a residual 
image 36. Since the level of prediction is very high, the residual image 
36 has little energy and does not require a substantial bit rate. As a 
result, once the initial image is transmitted completely, the subsequent 
transmission of residual images provides accurate updating information 
regarding the target location at very low bit rates. 
In order to form a more accurate residual image 36, the first frame image 
and the second frame image are aligned at an aligner 32 by shifting the 
images with respect to each other until a "best fit" is found. In view of 
the large priori information, the center of the image can move only a 
small distance frame-to-frame, and the corresponding computation is 
minimal to align one image with the other. Preferably, a small, high 
entropy block of pixels can be matched to determine the x and y vectors 
for the point of best match. 
In addition, since a complete first frame includes a substantial amount of 
information, the bit rate required can be lowered by transmitting the 
information over several frame intervals. Accordingly, the frame is 
preferably coded hierarchically at filter 34 to reconstruct the image 
methodically after segments of information have been transmitted over 
several intervals. Preferably, the low frequency information is sent 
first, followed by data for increasing spatial frequencies. Quadrature 
mirror filters or wavelets could be used to form the hierarchical coding 
of the signal to be sent by the transmitter from the missile to the 
satellite to the controller. 
As a result, the present invention decreases the amount of data required to 
transmit back link information, for example, in the preferred embodiment 
by a factor of 25-50 over previous compression techniques, to obtain 
several advantages. In particular, the size of the antenna used on the 
missile can be substantially reduced over the size of antennas required to 
convey the amount of information previously required for conveying a 
series of frame images in previously known communication systems, and can 
substantially reduce the power required to transmit the information from 
the missile. Alternatively, the image resolution which can be conveyed is 
substantially improved over the previously known communication systems 
having limited bit rates, or the frame rate can be substantially 
increased. 
Having thus described the present invention, many modifications thereto 
will become apparent to those skilled in the art to which it pertains 
without departing from the scope and spirit of the present invention as 
defined in the appended claims.