Time reference scanning beam microwave landing system

The disclosure teaches a method, and an apparatus for carrying out the method, for determining the time between TO and FRO received waveforms from the time reference scanning beam (TRSB) of a microwave landing system (MLS). In accordance with the invention, the time interval between the TO and FRO received waveforms is determined by correlating the received waveforms with apriori waveforms representative of the TO and FRO waveforms respectively. The apriori waveforms are generated by having a process construct matched filters for these waveforms. A FRO received waveform is used to provide an apriori representation of a TO received waveform and vice-versa. In one embodiment of the invention, the TO and FRO waveforms which are used to construct the matched filters are time reversed.

BACKGROUND OF INVENTION 
(a) Field of Invention 
The invention relates to method steps for determining the time between TO 
and FRO received waveforms from the time reference scanning beam (TRSB) of 
a microwave landing system (MLS), and to apparatus for carrying out the 
method steps. More specifically, the invention relates to such method 
steps wherein the time interval between the TO and FRO received waveforms 
is determined by correlating the received waveforms, with apriori 
waveforms representative of the TO and FRO waveforms, respectively, and to 
apparatus for carrying out the method steps. 
(b) Description of Prior Art 
Known in the art are landing guidance systems wherein angular position 
information is derived from the time difference in received amplitude only 
signals. These received signals are created by a ground transmitting 
station which transmits a narrow beam of radio-frequency energy in a swept 
(or step) fashion and at a fixed angular rate. An example of such a 
landing system is the TRSB-MLS system. The back and forth sweeps are 
referred to as TO and FRO sweeps. 
These are five known processes or processors, which are discussed in the 
current literature on MLS processing. These are known as a matched filter 
processor, a dwell gate processor, a split gate processor, a single edge 
processor, and a dual edge processor. Each of these processes/processors 
attempts to overcome noise, multipath distortion, and possible transmit 
beam asymmetry. Multipath distortion causes the received waveform to be 
distorted in shape (amplitude versus time graph) when compared to the 
ideal. Under certain circumstances, the distorted TO received waveform can 
be a mirror image of the FRO received waveform in time when their 
respective amplitude versus time graphs are compared. Such disadvantages 
form part of the system which is taught in "Time Reference Microwave 
Landing System Multipath Control Techniques", by R. J. Kelly, Journal of 
the Institute of Navigation, Vol. 23, No. 1. This invention overcomes this 
multipath effect problem as well as optimizing the signal in a noisy 
environment. 
The first type, a matched filter processor, requires knowledge of an 
apriori received TO waveform and an apriori received FRO waveform. In case 
of a symmetric antenna transmit beam (and symmetric antenna received 
pattern) knowledge of only one is sufficient. In the event of multipath 
distortion of the received waveform, or if assumed apriori data is 
incorrect, this filter exhibits poor performance. This is due to the fact 
that basic apriori assumptions are no longer valid and thus increasing 
error results. This is documented in the paper given by R. J. Kelly and E. 
F. C. La Berge, "Comparison Study of MLS Airborne Signal Processing 
Techniques", 1978 I.E.E.E. document 78CH1336-7 NAECON. 
The second type, a dwell gate processor, sets a threshold at a 
predetermined level below the peak value of the received TO and FRO 
waveforms. This process determines the time at which the received 
amplitude rises through the threshold, and the time at which the received 
amplitude fails through the the threshold. The midpoint between the rise 
threshold time and the fall threshold time is the reference time for the 
received waveform. This is done for the TO and FRO received waveforms. The 
measurement sought is the time between these time reference points. This 
process does not exhibit optimum performance in either a white noise or 
multipath noise environment. 
The third type, a split gate process, finds the centroid of the received 
waveform by taking the difference of sums of a number of amplitudes on 
each side of a sampling point. The computation is performed about a 
sampling point which is shifted in the direction toward making the 
difference zero. Typically there are two sampling points. One is the peak 
amplitude, the other is variable. Linear interpolation is used to find the 
point at which the difference is zero. This process is superior to the 
dwell gate but inferior to the matched filter under white noise 
conditions, and inferior to the single edge processor under multipath 
interference conditions. 
The fourth type, a single edge processor, essentially compares the change 
in slope of the received waveform with the change in slope of an apriori 
waveform. The assumption in this process is that only one edge of the 
received waveform is distorted and so the process is performed on the edge 
assumed to be undistorted. Also this process is intended to function in 
conjunction with the dwell gate processor. In general, this processor has 
superior multipath performance but poor noise performance when compared 
with the other methods, and exhibits poor performance if beam amplitude is 
too low or if both edges are distorted. 
The fifth type, a dual edge processor, is a combination of two single edge 
processors. This was motivated by fact that both edges of the beam may be 
distorted and that an unbiased estimator such as a dwell gate or split 
gate processor would be required. The functioning is similar to the single 
edge processor in that both compare the change in slope of the received 
waveform with the change in slope of an apriori waveform. This is done for 
both edges and an average is taken. The multipath performance of this 
processor can be superior to that of the single edge processor. Again 
noise performance is poor compared with other methods. 
TRSB-MLS systems are also taught in U.S. Pat. Nos. 4,019,184, Dorey, Apr. 
19, 1977 and 4,017,862, Wild, Apr. 12, 1977. The Wild patent teaches the 
process of determining the time delay between the TO and FRO waveforms by 
cross-correlating two wavetrains produced by consecutive TO and FRO 
excitation. 
SUMMARY OF INVENTION 
It is an object of the invention to provide a method and apparatus which 
overcomes the disadvantages of the prior art. 
It is a further object of the invention to provide a method and apparatus 
which minimizes the error in the measurement of the time between the TO 
and FRO received waveforms of a TRSB-MLS under high noise and multipath 
interference conditions. 
In accordance with the invention there is provided a method for processing 
TO and FRO waveforms from the time reference scanning beam of a microwave 
landing system, which method includes the steps of detecting said TO and 
FRO waveforms, further steps for determining the time interval between the 
TO and FRO received waveforms, said further steps comprising: generating 
apriori waveforms representative, respectively, of said TO received 
waveform and said FRO received waveform; performing: a correlation process 
between said TO received waveform and the apriori waveform representative 
of said TO received waveform; and a correlation process between said FRO 
received waveform and the apriori waveform representative of said FRO 
received waveform; determining: the maximum correlation between the TO 
received waveform and the apriori waveform representative thereof; and the 
maximum correlation between the FRO received waveform and the apriori 
waveform representative thereof; to thereby determine, respectively, the 
times of the TO and FRO waveforms; and determining the absolute difference 
between said TO and FRO times. 
In accordance with the invention there is provided in a receiver means for 
receiving TO and FRO waveforms from the time reference scanning beam of a 
microwave landing system, which receiver means includes detector means for 
detecting said TO and FRO waveforms, processing means for determining the 
time interval between the TO and FRO received waveforms, said processing 
means comprising: means for generating apriori waveforms representative, 
respectively, of said TO received waveform and said FRO received waveform; 
correlation means for performing: a correlation process between said TO 
received waveform and the apriori waveform representative of said TO 
received waveform; and a correlation process between said FRO received 
waveform and the apriori waveform representative of said FRO received 
waveform; means for determining: the maximum correlation between the TO 
received waveform and the apriori waveform representative thereof; and the 
maximum correlation between the FRO received waveform and the apriori 
waveform representative thereof; to thereby determine, respectively, the 
times of the TO and FRO received waveforms; and means for determining the 
absolute difference between said TO and FRO times.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A received waveform in a TRSB/MLS receiver has the general appearance as 
illustrated in FIG. 1, that is, there is a TO "pulse" and FRO "pulse" 
separated by noise and other unwanted signals N. After some elementary 
processing, the waveforms, as appearing in FIG. 2, are detected and 
identified as well known in the art. Apriori waveforms are then correlated 
with a respective one of the TO or FRO received waveforms as shown in 
FIGS. 3A to 3C and 4A to 4C respectively. 
Concerning the apriori waveforms, it is contemplated to generate such 
apriori waveforms by having a process construct matched filters for these 
waveforms. The matched filters may be constructed using time reversed 
waveforms. With the time reversal process, a FRO received waveform is time 
reversed to produce the apriori representation of a TO received waveform, 
and a time reversed TO waveform is used as an apriori representation of 
the FRO received waveform. As can be seen in FIGS. 3 and 4, the apriori 
waveforms are time shifted until, in FIGS. 3C and 4C, the apriori 
waveforms coincide in time with the received waveforms. At such a time, 
the co-relationship between the apriori waveform and the received waveform 
will be at a maximum, so that to determine the time of the TO received 
waveform and the FRO received waveform, it is merely necessary to 
calculate the time shift of the apriori waveforms which produces the 
maximum corelation. 
This is accomplished mathematically as follows: 
Let the TO waveform be denoted as P.sub.1 (t) and the FRO waveform as 
P.sub.2 (t). Both P.sub.1 and P.sub.2 are functions of time. Let the 
digital representations of P.sub.1 and P.sub.2 be denoted as P.sub.1 (i) 
and P.sub.2 (i) respectively. Define the apriori waveforms as A.sub.1 
(t)=P.sub.1 (-t+X.sub.1) and A.sub.2 (t)=P.sub.2 (-t+X.sub.2). The 
parameters X.sub.1 and X.sub.2 are chosen so that A.sub.1 (t) and A.sub.2 
(t) are zero for t&gt;t.sub.ref. The parameter t.sub.ref in FIGS. 3A, B, C 
and 4A, B, C is the origin. Also, A.sub.1 (t.sub.ref) and A.sub.2 
(t.sub.ref) are non zero. Note that A.sub.1 (t) is a waveform whose shape 
is time reversed when compared with P.sub.1 (t), and similarly for A.sub.2 
(t) and P.sub.2 (t). 
The process of finding the measurement of time between the TO and FRO 
received waveforms is as follows: 
(1) Form the integral I. 
##EQU1## 
(2) Maximize I.sub.1 by varying .tau. until, for a specific value say 
.tau.=T.sub.1, 
.vertline.I.sub.1 .vertline. has maximum value. 
(3) Form the integral I.sub.2 
##EQU2## 
(4) Maximize I.sub.2 by varying .tau. until, for a specific value say 
.tau.=T.sub.2, 
.vertline.I.sub.2 .vertline. has maximum value. 
The ".vertline. .vertline." denote absolute value. The absolute value is 
not required if A.sub.1, A.sub.2, P.sub.1 and P.sub.2 are non negative 
valued functions. 
The measurement sought is: T=.vertline.T.sub.2 -T.sub.1 .vertline. 
For digital representations, the sums S.sub.1 and S.sub.2 are to be 
maximized: 
##EQU3## 
so that S.sub.1 is maximum at n=n.sub.1 and S.sub.2 is maximum at 
n=n.sub.2. If the time interval between the sampling insances is constant 
and is defined to be T.sub.s, then the measurement that is sought is 
T=T.sub.s .vertline.n.sub.2 -n.sub.1 .vertline.. 
The matched filters may also be constructed without using time reversed 
waveforms. The procedure is the same as with the time reversed waveforms 
except that the waveforms are not time reversed. However, a FRO received 
waveform is used to produce the apriori representation of a TO received 
waveform, and a TO waveform is used to produce an apriori representation 
of the FRO received waveform. 
The difference is mathematically represented with the following changes in 
the above formulae: 
EQU A.sub.1 (t)=P.sub.1 (.+-.t+X.sub.1) 
and 
EQU A.sub.2 (t)=P.sub.2 (.+-.t+X.sub.2). 
Turning now to FIG. 5 of the drawings, a receiver, indicated generally at 
1, includes an antenna 3 and a processor 5 for removing noise and other 
unwanted signals. Such a processor is well known in the art and requires 
no further description here. 
The means for carrying out the inventive steps include an integrator 7 and 
an apriori waveform generator 9. As above discussed, when the processor 
performs using analog techniques, an integration is performed of the 
product of the received waveform and its apriori representation. Thus, the 
received waveform is fed to one terminal of the integrator through 
conductive means 6 whereas the apriori waveform is fed to another terminal 
of the integrator through conductive means 8. When the apriori waveform 
constitutes the time reversed received waveform, the received waveform is 
provided to an input of the waveform generator 9 through the dotted line. 
The output of the integrator is fed to a maximum detector, and the time 
T.sub.1 is determined as above described. The time T.sub.2 is similarly 
determined, and the absolute difference of these two is calculated in 
subtractor 13. 
FIG. 6 illustrates a digital approach to carrying out the process. In FIG. 
6, the analog signal is converted to a digital signal in A/D converter 15. 
The output of this converter is fed to a summation means 70 at one 
terminal thereof, while the other terminal thereof is fed with the output 
of apriori waveform generator 9. Once again, the maximums are detected and 
the times T.sub.1 and T.sub.2 are determined as above described. The 
difference is then calculated in subtractor 13. 
It will be appreciated that the apparatus illustrated in FIGS. 5 and 6 are 
purely schematic representations of useable apparatus. A more practical 
approach is shown in FIG. 7. Referring to FIG. 7, the system will once 
again include receiving antenna 3, a processor 5, and an analog to digital 
converter 15. The output of the converter is fed to direct memory access 
(DMA) means 17 which has access to both a memory device 19 and a 
microprocessor 21. The microprocessor is appropriately programmed to carry 
out the process as above described. 
The inventive method and apparatus overcome the disadvantages suffered by 
the prior art for the following reasons: 
1. Mathematically, the correlation process is optimum. No other process is 
superior in additive white Gaussian noise. 
2. When time reversed TO and FRO received waveforms are used as the apriori 
waveform, any multipath distortion which affects a waveform will also 
affect its apriori waveform. Thus, the distortion in both the received and 
its apriori waveforms should be very similar so that these distortions 
will not upset calculations of received times. 
3. The method does not rely on any assumptions as do some of the prior art 
methods. In addition, the method does not rely on any particular beam 
geometry so should there by any change in beam geometries, the 
calculations will not be adversely affected. 
Although several embodiments have been discussed, this was for the purpose 
of illustrating, but not limiting, the invention. Various modifications 
which will come readily to the mind of one skilled in the art, are within 
the scope of the invention as defined in the appended claims.