Missile tracking system with a thermal track link

A closed-loop missile tracking system (10) employs a missile (12) with a thermal beacon (22) and an optical beacon (24). A target designator (40) defines a boresight from a missile firing location, such as an aircraft, to a target. The closed-loop missile tracking system (10) employs a first tracker (48) and a second tracker (64) with a forward looking infrared (FLIR) sensor (52) to track the displacement of the optical beacon (22) and thermal beacon (24) from the boresight. The first tracker (48) generates a first set of azimuth and elevation error signals. The second tracker (64) further includes a video demultiplexing interface (70) which transforms serial multiplexed video signals, which are output by the FLIR sensor (52) and contain a field with M rows and L columns of pixels, into a demultiplexed parallel video signal. A video thermal tracker (VTT) (58) selects the N adjacent horizontal rows of pixels and generates a second set of azimuth and elevation error signals therefrom. The VTT (58) selects at least one of the first set of error signals, the second set or a combination thereof to guide the missile (12).

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to missile tracking systems and, more particularly, 
to a missile tracking system with two track links having distinct 
frequencies. 
2. Discussion 
Some missiles, such as tube-launched, optically-tracked, wire-guided (TOW) 
missiles, do not include on-board tracking electronics and therefore 
require the input of target tracking signals from remotely located 
tracking electronics. Such missile systems typically include a target 
designator which defines a boresight or line of sight (LOS) from a 
launching site to a target. When the missile is fired, the tracking 
electronics guide the missile down the boresight to the target using a 
closed-loop control strategy. In other words, as the missile moves away 
from the boresight defined by the target designator, the error signal 
generated by the tracking electronics increases proportionately. As the 
missile moves towards the boresight defined by the target designator, the 
error signal decreases proportionately. 
For tracking purposes, some missiles generate an optical beacon at 
near-infrared wavelengths which is received by tracking electronics 
associated with the aircraft. Still other missiles employ radar tracking. 
The tracking electronics generate azimuth and elevation error signals by 
identifying the displacement of the missile from the boresight. The 
tracking electronics transform the error signals from the launching site 
coordinate system, such as an aircraft coordinate system, to the missile 
coordinate system. The tracking electronics amplify the error signals and 
transmit the error signals to the missile. This closed-loop control 
continues to guide the missile down the boresight until the missile hits 
the target. 
Some targets, however, are protected by electro-optical jammers which 
transmit high intensity signals at near-infrared wavelengths. If the 
jamming signal has an amplitude higher than the amplitude of the beacon 
generated by the missile, the tracking electronics can be confused by the 
electro-optical jamming signal. If the jamming signal is successful, the 
tracking electronics will incorrectly identify the displacement of the 
missile relative to the boresight. As a result, the error signals 
generated by the tracking electronics are incorrect and the missile will 
be guided away from both the boresight and, more importantly, the target. 
Common battlefield conditions such as smoke also degrade the optical 
beacon generated by the missile and cause incorrect error signals to be 
generated by the tracking electronics. 
Therefore, a missile system which reduces the effects of electro-optical 
jamming and/or battlefield conditions such as smoke is desirable. 
As cuts in the military budget continue, competitive pressure increases to 
provide missile tracking systems with higher reliability and increased 
accuracy at lower cost. Therefore, a missile system which reduces the 
effects of electro-optical jamming and/or battlefield conditions such as 
smoke without substantially increasing the cost of the missile tracking 
system is also desirable. 
SUMMARY OF THE INVENTION 
A missile tracking system, according to the invention, for guiding a 
missile from a launching site to a target includes a missile with a 
controller connected to first and second beacon generators and a 
trajectory control means for controlling the trajectory of said missile. A 
designating means identifies a target and defines a boresight from said 
launching site to said target. A first tracking means generates a first 
error signal based on the position of a first beacon relative to said 
boresight. A second tracking means generates a second error signal based 
on the position of a second beacon relative to said boresight. An error 
signal selecting means, coupled to said first and second tracking means 
and said designating means, selects at least one of said first error 
signal, said second error signal, or a combination thereof to guide said 
missile. 
According to another feature of the invention, the first tracking means is 
an optical track link operating at near-infrared wavelengths and the 
second tracking means is a thermal track link operating at far-infrared 
wavelengths. 
According to another feature of the invention, the second tracking means 
further includes sensing means for generating serial multiplexed video 
signals of a field of view including said boresight and said second 
beacon. The serial multiplexed video signal of said field of view includes 
M horizontal rows and L columns of pixels. The pixels in said field of 
view are sequentially ordered in said serial multiplexed video signal in a 
column-by-column manner. 
According to another feature of the invention, the second tracking means 
further includes interfacing means, coupled to said sensing means, for 
transforming said serial multiplexed video signal into a demultiplexed 
video signal. 
According to another feature of the invention, the second tracking means 
further includes row selecting means, coupled to said interfacing means, 
for selecting N adjacent horizontal rows of pixels from said M horizontal 
rows of pixels in said field of view, wherein N is less than M. 
According to another feature of the invention, the second tracking means 
further includes signal generating means, coupled to said row selecting 
means, for generating said second error signal from said N adjacent 
horizontal rows of pixels selected by said row selecting means. 
According to another feature of the invention, the missile tracking system 
further includes coordinate transforming means having an input coupled to 
said error signal selecting means for transforming a selected error signal 
from a coordinate system associated with said launching site to a 
coordinate system associated with said missile. 
In a further embodiment of the present invention, a missile tracking system 
for guiding a missile from a launch site to a target includes a missile 
with a controller connected to first and second beacon generating means 
for generating first and second beacons and a control means for 
controlling the trajectory of said missile. A designating means identifies 
a target and defines a boresight from said launching site to said target. 
A first tracking means generates a first error signal based on the 
position of said first beacon relative to said boresight. A second 
tracking means includes sensing means for generating serial multiplexed 
video signals of a field of view including said boresight and said second 
beacon. The serial multiplexed video signal includes M horizontal rows and 
L columns of pixels for said field which are sequentially ordered in a 
column-by-column manner. The second tracking means further includes an 
interfacing means, coupled to said sensing means, for transforming said 
serial multiplexed video signal into a demultiplexed video signal. The 
second tracking means generates a second error signal based on said 
demultiplexed video signal. An error signal selecting means, coupled to 
said first and second tracking means, selects at least one of said first 
error signal, said second error signal, or a combination thereof to guide 
said missile. A missile control means, coupled to said error signal 
selecting means, transmits guidance commands, related to said at least one 
of said first error signal, said second error signal, or said combination 
thereof, to direct said missile along said boresight. 
Still other objects, features and advantages will be readily apparent from 
the specification, the drawings and the claims which follow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides a second track link for tracking the missile 
if the primary track link is not operating properly due to electro-optical 
jamming electronics or battlefield conditions such as smoke. The secondary 
track link, such as a forward looking infrared (FLIR) sensor tracking a 
thermal beacon on the missile, is capable of tracking through battlefield 
conditions such as smoke and includes conventional algorithms to prevent 
jamming. A demultiplexing video interface transforms the serial 
multiplexed video signal output by the FLIR sensor into N selectable 
parallel channels suitable for input to a video thermal tracker. 
Referring to FIG. 1, a closed-loop missile tracking system 10 is 
illustrated and includes a missile 12 and tracking electronics 14. Missile 
12 includes a controller 20 coupled to an optical beacon generator 22 and 
a thermal beacon generator 24. Controller 20 is also coupled to a 
gyroscope (gyro) 32, a receiver 28 and yaw and pitch controls 36. 
Controller 20 may include an input/output interface (not shown). 
Tracking electronics 14 include a targeting system 40 with a target sight 
and designator 44, a near-infrared tracker 48, a forward looking infrared 
(FLIR) sensor 52 and video display 54. A first or near-infrared tracker 48 
tracks optical beacon 90 and is coupled to a video thermal tracker (VTT) 
58 which is associated with a processor electronic box (PEB) 62. A second 
or optical tracker 64 tracks thermal beacon 94. FLIR sensor 52 and video 
display 54 are coupled to FLIR electronic box (FEB) 66. FEB 66, in turn, 
is coupled to PEB 62 and a video multiplexing interface or a video thermal 
tracker (VTT) interface 70. VTT interface 70 is coupled to VTT 58. An 
output of VTT 58 is coupled to a coordinate transformer 74 of a 
stabilization control amplifier (SCA) 78. Coordinate transformer 74 is 
coupled to a missile command amplifier (MCA) 82 which includes a 
transmitter 86. While transmitter 86 and receiver 28 are illustrated, it 
can be appreciated that if wires connect the tracking electronics 14 and 
missile 12, transmitter 86 and receiver 28 can be omitted or replaced with 
input/output interfaces. 
Tracking system 14 employs optical beacon generator 22 and thermal beacon 
generator 24 to track missile 12 and to generate error signals which are 
proportional to the displacement of the missile 12 from a boresight 
defined by target sight and designator 44 to the target. When the missile 
12 is fired, controller 20 initializes a missile coordinate system and 
gyro 32 (so that the missile is roll stabilized). Likewise, SCA 78 
initializes an aircraft coordinate system. Controller 20 activates optical 
generator 22 which begins transmitting an optical signal 90, preferably at 
near-infrared (0.9 micron) wavelengths. Likewise, controller 20 activates 
thermal beacon generator which transmits a thermal signal 94, preferably 
at far-infrared (10 micron) wavelengths. 
The first tracker or near-infrared tracker 48 receives optical beacon 90 
and generates azimuth and elevation error signals based upon the 
difference between the optical beacon and the boresight defined by the 
target sight and designator 44. The azimuth and elevation error signals 
are output via connection 100 to VTT 58. In prior missile control systems, 
the azimuth and elevation errors signals would then be output directly 
from near-infrared tracker 48 to coordinate transformer 74 of SCA 78. 
Video output from a FLIR sensor would not be used to generate the error 
signals. 
According to the present invention, the second tracker 64 includes FLIR 
sensor 52 which senses thermal beacon 94 and generates serial multiplexed 
video which is output to FLIR electronic box 66. FLIR electronic box 66 
generates two video signals. A first video signal is scan converted, 
preferably using an RS-170 format, for compatibility with video display 
54. Because the first video signal is delayed an equivalent of one frame, 
(or 1/30 seconds), it is unsuitable for use with a closed-loop tracking 
system. Such a delay would cause significant tracking problems. FLIR 
electronic box 66 also provides a second video signal which is serial 
multiplexed and is a nonscan converted video signal (or pseudo video). The 
pseudo video signal is typically used with conventional imaging 
electronics such as a video scene tracker. 
Preferably, the pseudo video signal is an analog serial multiplexed video 
signal having a peak voltage range from a -2.50 to +2.50 volts direct 
current (DC) and a pixel clock rate of 6.804 MegaHertz (MHz). The pseudo 
video signal is output via connection 102 to VTT interface 70. In a 
preferred embodiment, VTT interface 70 transforms the serial multiplexed 
pseudo video signal into a parallel video signal providing a minimum of 56 
parallel channels of which a group of eight adjacent channels are 
selectable by the VTT 58 at one time. Preferably, thermal beacon generator 
24 can be selectively switched on and off so that the thermal beacon can 
be accurately and distinctly identified from clutter. 
VTT 58 generates a second set of azimuth and elevation error signals from 
the parallel scanned FLIR sensor video. Thus while the function of the 
first tracker is performed by near-infrared tracker 48 alone, the function 
of the second tracker is performed by FLIR sensor 52, FEB 66, VTT 
interface 70, and VTT 58. 
VTT 58 performs the additional functions of selecting between the first set 
of azimuth and elevation error signals generated using the optical beacon 
90 and near-infrared tracker 48 and the second set of azimuth and 
elevation error signals generated from the thermal beacon 94 and the 
second tracker 64. Preferably, VTT 58 can generate a hybrid set of azimuth 
and elevation error signals from a combination of the first and second 
sets of error signals. Coordinate transformer 74 translates the selected 
azimuth and elevation error signals output by VTT 58 from the aircraft 
coordinate system to the missile coordinate system and outputs yaw and 
pitch error signals via connection 106 to MCA 82. Transmitter 86 sends the 
yaw and pitch errors to receiver 28 of missile 12. Receiver 28, controller 
20 and yaw and pitch controls 36 of missile 12 correct the missile 
trajectory. 
VTT 58 selects between the first and second azimuth and elevation error 
signals or generates the hybrid set based on a quality factor associated 
with the first and second sets of azimuth and elevation error signals. The 
quality factor is determined by examination of the signal-to-noise ratio 
for each error signal. The signal-to-noise ratios are then related to a 
weighing factor that is assigned to the first and second azimuth and 
elevation error signals. 
VTT 58 utilizes the azimuth and elevation error signals generated by 
near-infrared tracker 48 and optical beacon generator 22 unless the 
quality factor thereof drops below a predetermined threshold. In such a 
case, VTT 58 switches to the azimuth and elevation error signals generated 
by the thermal beacon 94 and FLIR 52, and VTT 58. In degraded conditions 
where both the near-infrared and thermal tracking are degraded due to 
smoke, dust, and/or other atmospheric effects, the near-infrared and 
thermal tracking error signals are summed together based on a weighing 
function assigned to each. If a jammer is detected, a hybrid set of error 
signals is not generated and either the near-infrared or the thermal 
sensor error signals are used alone. 
When only the first and second sets of error signals are employed (without 
the hybrid set), the optical track link is considered the primary track 
link. It is monitored for its signal quality throughout the missile 
flight. If the quality of the optical track link is degraded due to 
electro-optical jamming measures or battlefield conditions such as smoke, 
missile tracking is transferred to the thermal track link. Since the 
missile is already flying down the boresight defined by the target 
designator 44, there is no step input to the closed-loop guidance system 
as the change is made between the first and second sets of error signals. 
Once the missile tracking is transferred to the thermal track link, the 
optical track link is no longer used for the remainder of the missile's 
flight. 
The pseudo video signal output by FLIR sensor 52 is a serial multiplexed 
video signal. For example, assuming left to right scanning of the object 
scenes, the first pixel of the first row is followed by the first pixel of 
the second row, . . . , and the first pixel of the M.sup.th row. In other 
words, the pseudo video signal outputs the left-most column of pixels 
first. Then the second pixel of the first row is output and is followed by 
the second pixel of the second row, . . . , and the second pixel of the 
M.sup.th row. In other words, the pseudo video signal then outputs the 
second column of pixels (from the left). This sequence continues until the 
right-most column of the field is output. Note that the pseudo video 
signal may start with the right-most column first and end with the 
left-most column when the FLIR sensor 52 is scanning the object scene 
right to left. 
Conventional VTT 58 require N adjacent channel video signal inputs where 
each channel video signal contains one horizontal row of pixels from the 
field (where N is less than M). In a preferred embodiment, M equals 120 
and N equals 8. VTT interface 58 demultiplexes the pseudo video signal and 
allows the VTT to select the N adjacent channel video signals. 
A first embodiment of a video demultiplexing interface or VTT interface 70' 
according to the present invention is illustrated in FIG. 2. FLIR sensor 
52 generates the pseudo video signal at output 128 which is amplified by a 
differential buffer amplifier 130. Buffer amplifier 130 is coupled to a 
low pass filter 134 which, in turn, is connected to N sample and hold 
circuits 136, 138, . . . , and 142. An output of each of the N sample and 
hold circuits is coupled to an input of an automatic gain control (AGC) 
amplifier 146, 148, . . . , and 152. An output of each of the N AGC 
amplifiers is coupled to an input of an offset correction amplifier 156, 
158, . . . , and 162. An output of each of the N offset correction 
amplifiers is coupled to an input of a low pass filter 166, 168, . . . , 
and 172. Outputs of each of the N low pass filters are coupled to N 
channels 176, 178, . . . , 182. As can be appreciated by skilled artisans, 
FIG. 2 illustrates N sample and hold circuits. For example, in a preferred 
embodiment, eight sample and hold circuits are employed. Therefore in this 
example N equals eight. It should be understood that the third through the 
seventh sample and hold circuits are represented by symbols " . . . " in 
FIG. 2. This same designation is employed FIG. 2 for the AGC, offset 
correction, and low pass filter circuits. 
VTT interface 70' further includes a controller 188 having a channel select 
output and a sample clock output at 190 which is coupled to a second input 
of each of the N sample and hold circuits 136, 138, . . . , and 142. FLIR 
sensor 52 includes a plurality of control outputs which are coupled to an 
input of control logic circuit 188. The control outputs include an 
odd/even signal 194, a pixel clock signal 196, a column clock signal 198, 
and an active video signal 200. VTT 58 includes several control outputs 
including a DC compensation strobe signal 204 which is coupled to a second 
input of each of the N offset correction amplifiers 156, 158, . . . , and 
162. A gain select signal 206 of the VTT 58 is coupled to a second input 
of each of the N AGC amplifiers 146, 148, . . . , and 152. A band select 
signal 208 of VTT 58 is coupled to an input of controller 188. 
In use, the pseudo video signal 128 output by FLIR sensor 52 is input to 
and amplified by differential buffer amplifier 130. The output of buffer 
amplifier 130 is routed through low pass filter 134 to minimize noise in 
the video signal. Preferably, low pass filter 134 has a cutoff frequency 
of 9.3 MHz. A channel select signal and a sample clock signal 190 and the 
filtered pseudo video signal are coupled to first and second inputs of the 
N sample and hold circuits 136, 138, . . . , and 142. 
The serial multiplexed pseudo video signal 128 output by FLIR sensor 52 
contains successive fields. Each field is defined by a plurality of pixels 
in M horizontal rows and L columns. The serial multiplexed pseudo video 
signal output by FLIR sensor 52 includes pixels arranged serially in a 
column by column manner. The pseudo video signal must be demultiplexed 
into parallel rows of pixels so that VTT 58 can select N horizonal rows of 
the M horizontal rows in a field (where N is less than M). VTT 58 requires 
parallel input of the select N horizontal rows. 
To that end, the controller 188 triggers sample and hold circuit 136 to 
select a first designated pixel from a first column. The next sample and 
hold circuit 138 selects the second designated pixel from the same column 
and the next row. The Nth sample and hold circuit 142 selects the Nth 
designated pixel from the same column. Column clock 198 signals a new 
column and the process is repeated for each of the L columns of the field. 
Software associated with controller 188 and/or VTT 58 periodically monitors 
a field for a peak pixel signal and adjusts the gain for the field based 
on the peak. In a preferred embodiment, the peak pixel signal is measured 
for each field. VTT 58 outputs the gain via gain select signal 206. Thus 
the gain of each pixel of a field is adjusted uniformly. In other words, 
the eight sample and hold circuits 136, 138, . . . , 142 output N adjacent 
horizontal rows, one pixel at a time. AGC 146, 148, . . . , and 152 
optimize the amplitude of the pixels with respect to a predetermined 
threshold level based on a peak pixel amplitude. VTT 58 generates gain 
select signal 206 which controls the gain provided by AGC 146, 148, . . . 
, and 152. 
To minimize the effects of direct current (DC) offset during high gain 
operation, offset correction amplifiers 156, 158, . . . , and 162 are 
employed. Periodically, the input to buffer amplifier 130 is shorted with 
switch 164 and the DC offset in each of the N channels is sampled and 
stored. When switch 164 opens, the stored DC offset compensation values 
are summed with the associated channel's video signal. The DC compensation 
strobe signal 204 defines the timing for the DC offset compensation 
function. Preferably switch 164 is a field effect transistor (FET). 
The output of each of the N offset correction amplifiers 156, 158, . . . , 
and 162 is coupled an input of low pass filters 166, 168, . . . , and 172. 
Preferably, low pass filters 166, 168, . . . , and 172 have a cutoff 
frequency of 7.6 kHz. Low pass filters 166, 168, . . . , and 172 optimize 
the signal to noise ratio while maintaining an optimum spread function for 
a point source. A higher cut-off frequency would provide minimum 
distortion to the true signal, but would permit more noise to be present 
thus lowering the signal-to-noise ratio. A lower cut-off frequency would 
improve the signal-to-noise ratio, but also would result in an 
unacceptable loss in the peak energy of the true signal. The image of a 
point in object space can be equated to an energy mountain and effects on 
this image can be evaluated using mathematical expressions for a point 
spread function. 
Controller 188 controls the operation of VTT interface 70' and receives 
four control signals from FLIR sensor 52 and a band select signal from VTT 
58. The odd/even signal 194 is a logic signal that provides the column 
scan direction, left-to-right or right-to-left. The active video signal 
200 is a logic signal that is true whenever the video in each field is 
valid. The column clock signal 198 is a logic timing signal whose 
transition to the low state determines the timed location of each valid 
video column. The pixel clock signal 196 is a logic timing signal that 
indicates the timed location in each video column where the data for each 
video pixel is valid. 
After the entire field is input and is routed through the channels, the 
output of each of the N low pass filters 166, 168, . . . , and 172 
represents one channel of video that is required for input to VTT 58 for 
missile tracking. 
VTT 58 includes a multiplexer (not shown) coupled to an analog to digital 
(A/D) converter (not shown) which converts the N-channel analog video 
signal to an N-channel digital video signal. A direct memory accessing or 
addressing (DMA) processor (not shown) inputs the N-channel digital video 
signal directly in the VTT memory. 
As can be appreciated, video interface 70' demultiplexes the pseudo video 
output by FLIR sensor 52 and allows VTT 58 to select N of the M horizontal 
rows of pixels. As a result, VTT 58 can be used to generate a second set 
of azimuth and elevation error signals and to select between the first and 
second sets (or a hybrid thereof) of azimuth and elevation error signals. 
The second thermal tracking link prevents the loss of a missile when 
successful electro-optical jamming overrides the primary optical tracking 
link or when battlefield conditions such as smoke degrade the primary 
optical tracking link. The thermal tracking link is generally not affected 
by typical battlefield smoke. Conventional algorithms can successfully 
prevent jamming the thermal track link. By formatting the pseudo video 
signal output by FLIR sensor 52 to a conventional VTT format, existing 
FLIR sensor and VTT technology can be employed with modest modifications. 
A second video demultiplexing interface or VTT interface 70" is illustrated 
in FIG. 3. For purposes of clarity, reference numerals from FIG. 2 will be 
used in FIG. 3 where appropriate. VTT interface 70" includes a sample and 
hold circuit 220 having one input coupled to an output of low pass filter 
134 and second input coupled to a sample clock 222 of controller 224. A 
gain select output 206 of VTT 58 is coupled to a first input of an 
automatic gain control (AGC) amplifier 228 and a second input is coupled 
to an output of sample and hold circuit 220. An output of AGC amplifier 
228 is coupled to a first input of an offset correction amplifier 232. A 
second input of offset correction amplifier 232 is coupled to DC comp 
strobe 204 of VTT 58. 
An output of offset correction amplifier 232 is coupled to a first input of 
analog to digital (A/D) converter 236. A second input of A/D converter 236 
is coupled to a converter timing output 238 of controller 224. An output 
of A/D converter 236 is coupled to a first input of digital filter 240. A 
second input of digital filter 240 is coupled to a filter timing output 
244 of controller 224. An output of digital filter 240 is coupled to an 
input of direct memory accessing or addressing (DMA) output processor 250 
which transfers the digital filtered video data directly to VTT memory 
254. 
Controller 224 sets timing and otherwise controls the operation of VTT 
interface 70". Controller 224 receives four control signals from FLIR 
sensor 52 and band select signal 208 from VTT 58. Each of the control 
signals from FLIR sensor 52 and VTT 58 operate in a manner similar to the 
first embodiment illustrated in FIG. 2. 
In use, the pseudo video signal output by FLIR sensor 52 is input into and 
amplified by differential buffer amplifier 130. Low pass filter 134 
minimizes noise in the pseudo video signal. The filtered video and a 
sample clock output 222 are coupled to sample and hold circuit 220 which 
ensures that the serial video output thereof represents only valid pixel 
data. AGC amplifier 228 optimizes the serial video amplitude with respect 
to a fixed video threshold level in a manner similar to the first 
embodiment of FIG. 2. To that end, VTT 58 generates a gain select control 
signal 206 for AGC amplifier 228 as previously described. 
To minimize the effects of DC offset during high gain operation, an offset 
correction amplifier 232 is used. Periodically, the input to the buffer 
amplifier is shorted with switch 164 and the DC offset caused by high gain 
operation of buffer amplifier 130, low pass filter 134, sample and hold 
circuit 220, and AGC 228, is sampled and stored. When the switch 164 
opens, the stored DC offset compensation values are summed with the serial 
video. The timing signal for the DC offset compensation function is 
defined by the DC comp strobe 204 and is generated by VTT 58. 
The serial video output from the offset correction amplifier 232 along with 
a converter timing signal 238 are coupled to inputs of A/D converter 236. 
The output of the A/D converter 236 is preferably a multi-bit serial 
digital signal. The output of A/D converter 236 and a video band select 
signal are routed to digital filter 240. Digital filter 240 inputs the 
serial digital video into each of the N selected video channels and 
recursively filters the video data therein. Video outside the selected N 
channels is ignored. The band select signal 208 determines which N 
adjacent channels of the M video channels are to be processed. Digital 
filter 240 defines a 3 decibel (dB) cutoff frequency for each of the 
selected video channels. Preferably the cutoff frequency is 7.4 kHz. 
Digital filter 240 further provides a maximum signal to noise ratio while 
maintaining an optimum spread function for a point source. 
An output timing signal 256 and the output of digital filter 240 are input 
to DMA output processor 250. DMA output processor 250 provides the control 
necessary to take the processor of VTT 58 off line and to transfer the 
digital filtered video data directly to VTT memory 254. After video data 
in each of the selected N channels is recursively filtered, it is output 
directly to the VTT processor memory 254. The video data from each of the 
N selected channels is transferred sequentially to VTT processor memory 
254. The video from the remaining M-N channels is ignored. Preferably, M 
equals 120 and N equals 8. 
In a highly preferred embodiment, tracking system 10 consists of a standard 
M65 system with a FLIR sensor and a laser target designator added to an 
M65 telescopic sight unit. The standard M65 system is manufactured by 
Hughes Aircraft and the night targeting system upgrades to the M65 
telescopic sight unit are manufactured by TAMAM, a division of Israel 
Aircraft Industries, or Kollsman, a division of Sequa Corporation. 
Preferably the missiles employed are tube-launched, optically-tracked, 
wire-guided (TOW) missiles having both thermal and optical beacons. 
As can be appreciated from the forgoing, the missile tracking system 
according to the present invention provides two track links for tracking a 
missile. If the primary track link is not operating properly due to 
battlefield conditions such as smoke or electro-optical target jamming 
electronics, a secondary link can be employed to properly guide the 
missile to the target. A secondary track link, such as the FLIR sensor 
tracking the thermal beacon, can track through battlefield conditions such 
as smoke and may be used with conventional algorithms to prevent jamming. 
VTT interface, according to the invention, transforms analog serial 
multiplexed video signals into N parallel channels which can be selected 
by and input to a VTT. 
Various other advantages of the present invention will become apparent to 
those skilled in the art after having the benefit of studying the 
foregoing text and drawings, taken in conjunction with the following 
claims.