U.S. patent application number 10/473333 was filed with the patent office on 2004-11-25 for super-resolution processor/receiver to discriminate superimposed secondary surveillance radar (ssr) replies and squitter.
Invention is credited to Galati, Gaspare, Leonardi, Mauro.
Application Number | 20040233095 10/473333 |
Document ID | / |
Family ID | 11455409 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040233095 |
Kind Code |
A1 |
Galati, Gaspare ; et
al. |
November 25, 2004 |
SUPER-RESOLUTION PROCESSOR/RECEIVER TO DISCRIMINATE SUPERIMPOSED
SECONDARY SURVEILLANCE RADAR (SSR) REPLIES AND SQUITTER
Abstract
A super resolution processor/receiver to discriminate
superimposed secondary surveillance radar (Mode S, Mode A/C)
replies and squitter that uses frequency-domain analysis to
suppress interference between replies. In particular, the
processor/receiver uses a spectral super-resolution method to
estimate carriers of reply signals (every reply has a carrier that
could be slightly different from the others). These frequency
analyses, applied with particular timing referring to the signal
that effectively carried the information, allow to estimate the
information on SSR Mode S/Mode A/C replies.
Inventors: |
Galati, Gaspare; (Gondi,
IT) ; Leonardi, Mauro; (Roma, IT) |
Correspondence
Address: |
Young & Thompson
745 South 23rd Street
Arlington
VA
22202
US
|
Family ID: |
11455409 |
Appl. No.: |
10/473333 |
Filed: |
March 15, 2004 |
PCT Filed: |
April 3, 2002 |
PCT NO: |
PCT/IT02/00206 |
Current U.S.
Class: |
342/37 ; 342/175;
342/195; 342/29; 342/30; 342/32; 342/36; 342/40 |
Current CPC
Class: |
G01S 13/782
20130101 |
Class at
Publication: |
342/037 ;
342/029; 342/030; 342/032; 342/036; 342/040; 342/175; 342/195 |
International
Class: |
G01S 013/74; G01S
013/78 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2001 |
IT |
RM01A000176 |
Claims
1. Method for discriminating superimposed Secondary Surveillance
Radar (SSR) replies in a digitalized received signal, a reply
comprising a preamble and a data-block, the method being
characterised in that it comprises the following step:--detecting a
start time of one or more replies in said digitalized received
signal, for each detected start time, determining an end time of
the corresponding reply, generating time windows in at least a
portion of the digitalized received signal, performing a spectrum
super resolution method to the digitalized received signal in the
generated time windows, in order to estimate one or more carrier
frequencies in each generated time window, estimating the number of
replies and the relative carrier frequencies in the digitalized
received signal, and reconstructing said one or more replies.
2. Method according to claim 1, characterised in that said
digitalized received signal comprises a Log (Sigma) component
signal of a digitalized antenna signal.
3. Method according to claim 1, characterised in that the steps of
detecting a start time and determining an end time of one or more
replies comprise an envelope detection of the digitalized received
signal; in that the time windows are generated in the whole signal
and are partially superimposed; in that the step of performing a
spectrum super resolution method further estimate the number of
carrier frequencies in each generated time window; in that the step
of estimating the number of replies and the relative carrier
frequencies in the digitalized received signal is carried out
through using the estimations obtained in the step of performing a
spectrum super resolution method, wherein one or more power
thresholds are created through which power in each generated time
window is able to be associated with a pertaining reply; and in
that the step of reconstructing said one or more replies estimates,
for each reply in each generated time window, the presence and
position of reply pulses through using said created one or more
power thresholds, wherein the pertaining reply pulses are the ones
having the carrier frequencies closest to estimated value, a signal
being created for each reply, said signal being re-timed in such a
way that the preamble is first re-timed and the data-block is
re-timed afterward.
4. Method according to claim 1, characterised in that the steps of
detecting a start time of one or more replies and estimating the
number of replies and the relative carrier frequencies in the
digitalized received signal comprise the following sub-step
estimating the arrival time of a leading reply, by using a power
monitoring, detecting four preamble pulses, calculating the timing
of the reply, through analysing the detected power levels and the
timing of the detected pulses of the preamble, estimating the
carrier frequency through performing a spectrum super resolution
method onto said four preamble pulses, in order to obtaining a
frequency evaluation for each of said four pulses, and averaging
the four evaluations; in that the time windows are generated
according with the timing of the leading reply, corresponding to
the chip time on which the pulses of the reply are detectable; and
in that the step of reconstructing said one or more replies
estimates comprises a first frequency testing, in order to verify
in which of two temporal windows, related to a same bit of the
reply, the frequency found for the leading reply is, through
deciding for the time window wherein the same frequency of or the
frequency value closest to the estimated frequency of the preamble
is found.
5. Method according to claim 4, characterised in that the sub-step
of estimating the carrier frequency performs a spectrum super
resolution method wherein the signal in the pulses is represented
by a number of complex sinusoids with unknown amplitudes and
frequencies and white additive noise; a linear prediction filter is
defined and used to calculate the unknown parameters of the signal;
eigenvalues and eigenvectors are used to calculate the transfer
function of the error prediction filter defined; on the basis of
the eigenvalues, a signal subspace and a noise subspace are defined
with the related eigenvectors; and the zeros on the unitary circle
of the filter transfer function, that correspond to the frequencies
of the complex sinusoids, are calculated.
6. Secondary Surveillance Radar (SSR) receiving and processing
apparatus characterised in that it is adapted to carry out the
method for discriminating superimposed SSR replies in a digitalized
received signal according to claim 1, the apparatus being further
characterised in that it comprises: means (3,15) for detecting a
start time of one or more replies in said digitalized received
signal, said means (3,15) receiving at an input said digitalized
received signal; means (11, 15) for determining, for each detected
start time, an end time of the corresponding reply; means (11, 16)
for generating time windows in at least a portion of the
digitalized received signal; means (12,17) for performing a
spectrum super resolution method onto the digitalized received
signal in the generated time windows, in order to estimate one or
more carrier frequencies in each generated time window, said means
(12,17) receiving at a first input said digitalized received signal
and being connected to said means (11,16) for generating time
windows; means (3,18) for estimating the number of replies and the
relative carrier frequencies in the digitalized received signal,
and means (4,19) for reconstructing said one or more replies, such
means (4,19) being connected to said means (3,18) for estimating
the number of replies and the relative carrier frequencies.
7. Apparatus according to claim 6, characterised in that means
(3,15) for detecting a start time of one or more replies in said
digitalized received signal also receives an expected reply
time.
8. Apparatus according to claim 6, characterised in that it is
adapted to carry out the method for discriminating superimposed SSR
replies in a digitalized received signal, the apparatus being
further characterised in that said means for detecting a start time
of one or more replies in said digitalized received signal
comprises envelope detecting means (15) adapted to detect the start
time, when signal power is higher than a threshold, and the end
time of said one or more replies; said means for estimating the
number of replies and the relative carrier frequencies comprise
carrier frequency estimating means (18) for estimating, on the
basis of the evaluation for each window, the number of replies and
the relative carrier frequencies in the digitalized received signal
and for creating one or more power thresholds; and said means for
reconstructing said one or more replies comprise re-timing and
reply reconstruction means (19) for estimating, for each reply in
each generated time window, the presence and position of reply
pulses through using said created one or more thresholds, wherein
the pertaining reply pulses are the ones having the carrier
frequencies closest to estimated value, the re-timing and reply
reconstruction means (19) creating a signal for each reply and
re-timing the same in such a way that the preamble is first
re-timed and the data-block is re-timed afterward; said re-timing
and reply reconstruction means (19) being connected to means (12,
17) for performing a spectrum super resolution method onto the
digitalized received signal.
9. Apparatus according to claim 6, characterised in that it is
adapted to carry out the method for discriminating superimposed SSR
replies in a digitalized received signal, the apparatus being
further characterised in that said means for detecting a start time
of one or more replies and said means for estimating the number of
replies and the relative carrier frequencies in said digitalized
received signal comprise preamble analysing means (3) including
preamble detecting means (8) for estimating the arrival time of a
leading reply, by using a power monitoring, and detecting four
preamble pulses, start and end detecting means (9) for calculating
the timing of the reply, through analysing the detected power
levels and the timing of the detected pulses of the preamble, said
start and end detecting means (9) being connected to said preamble
detecting means (8), and carrier frequency estimating means (10)
for estimating the carrier frequency through performing a spectrum
super resolution method onto said four preamble pulses, in order to
obtaining a frequency evaluation for each of said four pulses, and
averaging the four evaluations, said carrier frequency estimating
means (10) being connected to said preamble detecting means (8) and
to said start and end detecting means (9); said means (4) for
reconstructing said one or more replies includes frequency testing
means (13) for verifying in which of two temporal windows, related
to a same bit of the reply, the frequency found for the leading
reply is, through deciding for the time window wherein the same
frequency of or the frequency value closest to the estimated
frequency of the preamble is found, said frequency testing means
(13) being connected to said carrier frequency estimating means
(10), to said means (11, 16) for generating time windows and to
said means (12,17) for performing a spectrum super resolution
method, and reply reconstruction means (14) for reconstructing said
one or more replies, said reply reconstruction means (14) being
connected to said frequency testing means (13).
10. Super resolution Processor/Receiver to discriminate
superimposed Secondary Surveillance Radar (SSR) replies and
squitter, according to claim 6, with the ability to discriminate
and decode superimposed replies using both time and frequency
analysis.
11. Processor/Receiver according to claim 10 characterised in that
it utilise super resolution, that is exceeds the classic resolution
limit (Raylegh limit) using non linear systems and model
analysis.
12. Processor/Receiver according to claim 11 characterised in that
estimates the information about the replies carriers and estimate
the replies timing using the time and frequency analysis with super
resolution techniques of the whole (or part) of the signals coming
from one or more replies.
13. Processor/Receiver according to claim 12 characterised in that
it estimates the information about the carrier frequencies and
timing using an analysis of the preamble of the leading reply with
time and spectral super resolution techniques.
14. Processor/Receiver according to claim 10 characterised in that
it applies the super resolutions method to time windows where is
"useful" signal".
15. Processor/Receiver according to claim 10 characterised in that
it uses a super resolution method to study the information bit
pulse's position.
16. Processor/Receiver according to claim 10 characterised in that
it applies a super resolution method on the first and on the second
part of interval related to each information's bit to estimate the
position of the pulse.
17. Processor/Receiver according to claim 10 characterised in that
it uses a super resolution method applied in superimposed time sub
windows on the interval in which power is detected.
18. Processor/Receiver according to claim 10 characterised in that
it has a reply reconstructor.
19. Processor/Receiver according to used for scopes different from
Mode S reception in which the pulse coding is used and is necessary
to discriminate superimposed signals, both for measurement (E. S.
M. Electronic support measurements) or communications scopes.
Description
FIELD OF THE INVENTION
[0001] This invention belongs to the field of Air Traffic Control
and of Surveillance Systems. In particular it is applied in
Cooperating Surveillance SSR (Secondary Surveillance Radar) Mode S
and/or Mode A/C Systems and utilises "Mode S Squitter".
BACKGROUND OF THE INVENTION
[0002] Mode S squitter require that an aircraft emits Mode S
replies spontaneous or on calling (replies to interrogations).
These replies are generated from on board equipment called
transponder. In this manner ground station (attive or passive) or
others aircraft acquire positions and others information about the
aircraft.
[0003] Present-day Mode S-Receivers decode replies if they do not
suffer from interference with other replies. The Mode S replies can
be decoded only if they are free from interference or at most,
interfered with one Mode A/C reply. No Mode S reply with Mode S
interference from other Mode S replies can be recognised and
corrected, before the present invention.
SCOPE OF THE INVENTION
[0004] This invention goes beyond the present limitations in the
capability to recognise and decode Mode S/Mode S interference by
suitably using and analysing the replies and the spontaneous
replies ("squitter") in the frequency domain besides the
traditional processing in the time domain, the latter being the
unique analysis used before this invention. The frequency analysis
can be used because each transponder produce a central frequency
(carrier frequency of the reply) that can be quite different from
the others transponders due to the calibrations of the generator,
inside a tolerance window defined by international regulations
(ICAO standards). A sharp analysis (high resolution) of the
frequencies allows the discrimination of superimposed replies that
one cannot obtain with time analysis only.
[0005] Model based or "super resolution" methods allow a
high-resolution frequency analysis. The present invention
preferably uses an algorithm based on the paper of Tufts-Kumaresan,
"Estimation of Frequencies of Multiple Sinusoids: Making Linear
Prediction Perform Like Maximum Likelihood" Proceeding of IEEE, vol
70, no. 9, September 1982, pp. 975-981.
SUMMARY OF THE INVENTION
[0006] It is a specific object of the present invention a method
for discriminating superimposed Secondary Surveillance Radar (SSR)
replies in a digitalized received signal, a reply comprising a
preamble and a data-block, the method being characterised in that
it comprises the following step:
[0007] detecting a start time of one or more replies in said
digitalized received signal,
[0008] for each detected start time, determining an end time of the
corresponding reply,
[0009] generating time windows in at least a portion of the
digitalized received signal,
[0010] performing a spectrum super resolution method to the
digitalized received signal in the generated time windows, in order
to estimate one or more carrier frequencies in each generated time
window,
[0011] estimating the number of replies and the relative carrier
frequencies in the digitalized received signal, and
[0012] reconstructing said one or more replies.
[0013] Preferably according to the invention, said digitalized
received signal comprises a Log(Sigma) component signal of a
digitalized antenna signal.
[0014] Still according to the invention, the method may be
characterised
[0015] in that the steps of detecting a start time and determining
an end time of one or more replies comprise an envelope detection
of the digitalized received signal;
[0016] in that the time windows are generated in the whole signal
and are partially superimposed;
[0017] in that the step of performing a spectrum super resolution
method further estimate the number of carrier frequencies in each
generated time window;
[0018] in that the step of estimating the number of replies and the
relative carrier frequencies in the digitalized received signal is
carried out through using the estimations obtained in the step of
performing a spectrum super resolution method, wherein one or more
power thresholds are created through which power in each generated
time window is able to be associated with a pertaining reply;
and
[0019] in that the step of reconstructing said one or more replies
estimates, for each reply in each generated time window, the
presence and position of reply pulses through using said created
one or more power thresholds, wherein the pertaining reply pulses
are the ones having the carrier frequencies closest to estimated
value, a signal being created for each reply, said signal being
re-timed in such a way that the preamble is first re-timed and the
data-block is re-timed afterward.
[0020] Always according to the invention, the method may be
characterised
[0021] in that the steps of detecting a start time of one or more
replies and estimating the number of replies and the relative
carrier frequencies in the digitalized received signal comprise the
following sub-step
[0022] estimating the arrival time of a leading reply, by using a
power monitoring, detecting four preamble pulses,
[0023] calculating the timing of the reply, through analysing the
detected power levels and the timing of the detected pulses of the
preamble,
[0024] estimating the carrier frequency through performing a
spectrum super resolution method onto said four preamble pulses, in
order to obtaining a frequency evaluation for each of said four
pulses, and averaging the four evaluations;
[0025] in that the time windows are generated according with the
timing of the leading reply, corresponding to the chip time on
which the pulses of the reply are detectable; and
[0026] in that the step of reconstructing said one or more replies
estimates comprises a first frequency testing, in order to verify
in which of two temporal windows, related to a same bit of the
reply, the frequency found for the leading reply is, through
deciding for the time window wherein the same frequency of or the
frequency value closest to the estimated frequency of the preamble
is found.
[0027] Furthermore according to the invention, the sub-step of
estimating the carrier frequency may perform a spectrum super
resolution method wherein
[0028] the signal in the pulses is represented by a number of
complex sinusoids with unknown amplitudes and frequencies and white
additive noise;
[0029] a linear prediction filter is defined and used to calculate
the unknown parameters of the signal;
[0030] eigenvalues and eigenvectors are used to calculate the
transfer function of the error prediction filter defined;
[0031] on the basis of the eigenvalues, a signal subspace and a
noise subspace are defined with the related eigenvectors; and
[0032] the zeros on the unitary circle of the filter transfer
function, that correspond to the frequencies of the complex
sinusoids, are calculated.
[0033] It is still an object of the present invention a Secondary
Surveillance Radar (SSR) receiving and processing apparatus
characterised in that it is adapted to carry out the aforementioned
method for discriminating superimposed SSR replies in a digitalized
received signal, the apparatus being further characterised in that
it comprises:
[0034] means for detecting a start time of one or more replies in
said digitalized received signal, said means receiving at an input
said digitalized received signal;
[0035] means for determining, for each detected start time, an end
time of the corresponding reply;
[0036] means for generating time windows in at least a portion of
the digitalized received signal;
[0037] means for performing a spectrum super resolution method onto
the digitalized received signal in the generated time windows, in
order to estimate one or more carrier frequencies in each generated
time window, said means receiving at a first input said digitalized
received signal and being connected to said means for generating
time windows;
[0038] means for estimating the number of replies and the relative
carrier frequencies in the digitalized received signal, and
[0039] means for reconstructing said one or more replies, such
means being connected to said means for estimating the number of
replies and the relative carrier frequencies.
[0040] Still according to the invention, the means for detecting a
start time of one or more replies in said digitalized received
signal may also receive an expected reply time.
[0041] Always according to the invention, the apparatus may be
further characterised in that
[0042] said means for detecting a start time of one or more replies
in said digitalized received signal comprises envelope detecting
means adapted to detect the start time, when signal power is higher
than a threshold, and the end time of said one or more replies;
[0043] said means for estimating the number of replies and the
relative carrier frequencies comprise carrier frequency estimating
means for estimating, on the basis of the evaluation for each
window, the number of replies and the relative carrier frequencies
in the digitalized received signal and for creating one or more
power thresholds; and
[0044] said means for reconstructing said one or more replies
comprise re-timing and reply reconstruction means for estimating,
for each reply in each generated time window, the presence and
position of reply pulses through using said created one or more
thresholds, wherein the pertaining reply pulses are the ones having
the carrier frequencies closest to estimated value, the re-timing
and reply reconstruction means creating a signal for each reply and
re-timing the same in such a way that the preamble is first
re-timed and the data-block is re-timed afterward.
[0045] said re-timing and reply reconstruction means being
connected to means for performing a spectrum super resolution
method onto the digitalized received signal.
[0046] Still according to the invention, the apparatus may be
further characterised in that
[0047] said means for detecting a start time of one or more replies
and said means for estimating the number of replies and the
relative carrier frequencies in said digitalized received signal
comprise preamble analysing means including
[0048] preamble detecting means for estimating the arrival time of
a leading reply, by using a power monitoring, and detecting four
preamble pulses,
[0049] start and end detecting means for calculating the timing of
the reply, through analysing the detected power levels and the
timing of the detected pulses of the preamble, said start and end
detecting means being connected to said preamble detecting means,
and
[0050] carrier frequency estimating means for estimating the
carrier frequency through performing a spectrum super resolution
method onto said four preamble pulses, in order to obtaining a
frequency evaluation for each of said four pulses, and averaging
the four evaluations, said carrier frequency estimating means being
connected to said preamble detecting means and to said start and
end detecting means;
[0051] said means for reconstructing said one or more replies
includes
[0052] frequency testing means for verifying in which of two
temporal windows, related to a same bit of the reply, the frequency
found for the leading reply is, through deciding for the time
window wherein the same frequency of or the frequency value closest
to the estimated frequency of the preamble is found, said frequency
testing means being connected to said carrier frequency estimating
means, to said means for generating time windows and to said means
for performing a spectrum super resolution method, and
[0053] reply reconstruction means for reconstructing said one or
more replies, said reply reconstruction means being connected to
said frequency testing means.
[0054] According to the invention, it is provided a super
resolution Processor/Receiver to discriminate superimposed
Secondary Surveillance Radar (SSR) replies and squitter with the
ability to discriminate and decode superimposed replies using both
time and frequency analysis.
[0055] The Processor/Receiver may be characterised in that it
utilises super resolution, that is exceeds the classic resolution
limit (Raylegh limit) using non linear systems and model
analysis.
[0056] The Processor/Receiver may still be characterised in that
estimates the information about the replies carriers and estimate
the replies timing using the time and frequency analysis with super
resolution techniques of the whole (or part) of the signals coming
from one or more replies.
[0057] The Processor/Receiver according to the invention may also
be characterised in that it estimates the information about the
carrier frequencies and timing using an analysis of the preamble of
the leading reply with time and spectral super resolution
techniques.
[0058] The Processor/Receiver according to the invention may
furthermore be characterised in that it applies the super
resolutions method to time windows where is "useful" signal".
[0059] The Processor/Receiver according to the invention may still
be characterised in that it uses a super resolution method to study
the information bit pulse's position.
[0060] The Processor/Receiver according to the invention may also
be characterised in that it applies a super resolution method on
the first and on the second part of interval related to each
information's bit to estimate the position of the pulse.
[0061] The Processor/Receiver according to the invention may
furthermore be characterised in that it uses a super resolution
method applied in superimposed time sub windows on the interval in
which power is detected.
[0062] The Processor/Receiver according to the invention may still
be characterised in that it has a reply reconstructor.
[0063] The Processor/Receiver according to the invention may also
be used for scopes different from Mode S reception. Namely, it can
be used in all case in which the pulse coding is used and is
necessary to discriminate superimposed signals, both for
measurement (E.S.M. Electronic support measurements) or
communications scopes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] The foregoing aspects and other features of the invention
are explained in the following description, taken in connection
with the accompanying drawings, wherein:
[0065] FIG. 1 is a schematic diagram of a first embodiment of a
receiving and processing apparatus according to the invention;
[0066] FIG. 2 is a schematic diagram of the preamble analyser of
the apparatus of FIG. 1;
[0067] FIG. 3 is a schematic diagram of the super resolution
message processor 4 of the apparatus of FIG. 1; and
[0068] FIG. 4 is a schematic diagram of a second embodiment of a
receiving and processing apparatus according to the invention.
[0069] In the figures, similar elements are indicated by the same
reference numerals.
DETAILED DESCRIPTION
[0070] This invention describes an equipment that is composed by
various elements.
[0071] A first embodiment of the apparatus according to the
invention is shown in FIG. 1. The apparatus can work in the case of
interfering Mode S replies (also more than 2 replies). The only
requirement is that the preamble of the leading reply is received
without superimposing pulses; in this condition the receiver can
decode (with high probability) one of the superimposed replies (the
leading).
[0072] With reference to FIG. 1, unit 1 comprises an antenna and an
analog receiver adapted to operate in a SSR-Mode S equipment. The
output of the antenna and receiver unit 1 is connected to the input
of a sampler 2, which carries out a sampling of the video signal
through digitalizing the signals coming from unit 1.
[0073] The output of the sampler 2 is connected to a preamble
analyser 3 and to a super resolution message processor 4. The
preamble analyser 3 analyses the signal received in input and
detects the preamble, determining and the reply start-time t.sub.in
and the carrier frequency f.sub.c of the preamble (reply) and
outputting them towards the super resolution message processor 4.
The processor 4 analyses the signal received from the sampler 2,
discriminating pulses that belong to the reply-preamble using
super-resolution techniques.
[0074] The output of the processor 4 is connected to a message
decoding and error correcting unit 5 and to a monopulse estimator
6. Unit 5 outputs the reply information bits, while the estimator 6
outputs aircraft position data. In particular, the monopulse
estimator 6 receives a further signal from the sampler 2 in order
to estimate the aircraft position.
[0075] The outputs of the decoding and error correcting unit 5 and
of the monopulse estimator 6 are connected to an aircraft tracking
and communication channel management unit 7, which allows the
initialisation and the update of the tracks corresponding to the
aircrafts. In particular, unit 7 outputs an expected reply time
(only for roll call reply) towards the preamble analyser 3 and an
expected address (only for roll call reply) towards the decoding
and error correcting unit 5.
[0076] FIG. 2 shows a block diagram of the preamble analyser 3 of
the apparatus of FIG. 1. A preamble detector 8 receives in input
the signal coming from the sampler 2 and estimates the arrival time
of the leading reply by using a power monitoring and detects the
four preamble pulses. Furthermore, the preamble detector 8 verifies
if the required timing is respected and calculates the timing of
the reply. The preamble detector 8 preferably uses the method
described by J. L. Gertz and V. A. Orlando in "SSR Improvements and
collision avoidance systems panel SSR mode S system working
group-1. Improved squitter reception update", SICASP/WG-1 WP/1/585,
2 Jun. 1997.
[0077] The output of the preamble detector 8 is connected to both a
start and end detector 9 and a carrier frequency estimator 10.
[0078] The start and end detector 9 calculates the reply timing and
prevent false detection. Particularly, it detects the reply timing
(specifically the reply start time) analysing the detected power
levels and the timing of the detected pulses of the preamble. The
start and end detector 9 outputs the estimated reply timing also to
the carrier frequency estimator 10.
[0079] The carrier frequency estimator 10 applies a spectrum super
resolution method to the four preamble pulses, preferably 0.5
microseconds pulses, obtaining four frequencies valuations, one for
each pulse. It then averages the four evaluations to obtain the
estimate of the carrier frequency. The super resolution method is
preferably based on the Tufts and Kumaresan algorithm described in
the aforementioned paper. More preferably, the method uses a model
where the signal in the pulses is represented by a number of
complex sinusoids with unknown amplitudes and frequencies and white
additive noise; a linear prediction filter is defined and used to
calculate the unknown parameters of the signal. The method uses
eigenvalues and eigenvectors to calculate the transfer function of
the error prediction filter defined. Based on the eigenvalues two
subspaces are defined: signal subspace and noise subspace with the
related eigenvectors. Using only signal eigenvectors augments the
resolution and the efficiency of the method. Finally, the method
calculates the zeros on the unitary circle of the filter transfer
function that correspond to the frequencies of the complex
sinusoids.
[0080] FIG. 3 shows a block diagram of the super resolution message
processor 4 of the apparatus of FIG. 1. On the basis of the reply
start-time t.sub.in received from the preamble analyser 3, a time
window generator 11 generates temporal windows according with the
timing of the leading reply, corresponding to the chip time on
which the pulses of the reply can be detected. Pulse position
modulation is used in Mode S replies: two temporal windows are
defined for each information bit, and a pulse shall be only in one
of said temporal windows corresponding to the value of the bit.
[0081] The time window generator 11 is connected to a super
resolution processing unit 12 that applies a super resolution
method and estimates the carrier frequencies inside the temporal
windows generated by the time window generator 11.
[0082] Both the time window generator 11 and the super resolution
processing unit 12 are connected to a frequency testing unit 13,
which also receives the carrier frequency f.sub.c of the preamble
(reply) from the preamble analyser 3. The frequency testing unit 13
verifies in which of said temporal windows, related to the same
bit, is the frequency found for the leading reply. In particular,
the frequency testing unit 13 decides for the time interval wherein
the same frequency of or the frequency value closest to the
estimated frequency of the preamble is found.
[0083] The frequency testing unit 13 is in turn connected to a
reply reconstruction unit 14 that reconstructs the replies using
the information from the previous units. The used reconstruction
method is preferably one of the method described in the
aforementioned document by J. L. Gertz and V. A. Orlando.
[0084] In order to better understand the present invention, in the
following the operation modes of the embodiment shown in FIGS. 1-3
are described, similar operation modes being valid also for other
embodiments.
[0085] The antenna and receiver unit 1 carries out procedures
comprising receiving from the antenna signals through two squinted
mainlobes, and construction of three types of signals: log(Sigma),
log(Delta) and f(Delta/Sigma) carried on an intermediate frequency.
These signals are used to reach a fine target azimuth estimate
through monopulse techniques. Said three signals are digitalized by
the sampler 2.
[0086] The preamble analyser 3 uses the Log(Sigma) signal to detect
the reply, while the monopulse estimator 6 uses log(Delta) and
f(Delta/Sigma) signals to estimate the azimuth. The preamble
analyser 3 estimates the arrival time and the carrier of the reply,
through using super resolution techniques. The super resolution
message processor 4 uses these evaluations of the arrival time and
the carrier of the reply.
[0087] The super resolution message processor 4 generates the time
windows where the pulses of the message must be located (preferably
according to international standard requirements). Mode S replies
use a PPM modulations; consequently, on the basis of the start time
of the replies it is possible to define two time windows for each
bit of the message. The pulse related to a specific bit must be
located in one of said two windows, according to the information
bit. The carrier frequency estimator 10 carries out a frequency
super resolution estimation method in said two windows in order to
find the values of frequency in the two related windows for each
bit. The method estimates both the position of the pulse, for the
bit under consideration, and the window in which is found the
carrier frequency of the reply (evaluated from the preamble
analyser 3). In this way the full data block is estimated and the
reply is reconstructed.
[0088] Another operation mode of the apparatus according to the
invention comprises the detection of the pulses, preferably through
estimating the envelope in the middle of the time windows, and
performing the super resolution method only when the envelope is
higher then a threshold in both the bit time intervals.
[0089] The reconstructed reply by the super resolution message
processor 4 appears at the input of the message decoding and error
correcting unit 5 and at the input of the monopulse estimator 6.
Unit 5 decodes the reply, corrects errors and outputs the
information bits included inside the reply. Estimator 6 calculates
the monopulse estimate of azimuth and range and outputs such
position information. Then, all the information coming from the
reply, i.e. information bits and position information, is used by
unit 7 to track and to manage all the target in the coverage area
of the receiving and processing apparatus and to manage the whole
air-ground-air communications.
[0090] A second embodiment of the apparatus according to the
invention is shown in FIG. 4. Such second embodiment is more
complex than the apparatus shown in FIGS. 1-3, but it allows to
recognise both the interfering replies also in the case of
superimposed preambles, that is wherein a second reply arrives
before the end of the preamble of a leading reply. The apparatus of
FIG. 4 operates without requiring the timing information (coming
from a preamble analysis) and estimates the timing using the whole
signals. Another important feature of the apparatus of FIG. 4 is
that it estimates the number of interfering replies and the
corresponding carrier frequencies, using the whole signal. Since
the second embodiment still carries out a super resolution method,
it may be considered as a generalisation of the first embodiment of
FIGS. 1-3.
[0091] With reference to FIG. 4, an envelope detector 15 receives
from the sampler 2 the Log(Sigma) signal. The envelope detector 15
detects the start time (when signal power higher than a threshold)
and the end time of the reply or replies. Particularly, the
envelope detector 15 has not to take account of the fluctuation due
to the pulses of the replies. Specifically, in the case time
intervals in which there is no significant signal power are too
short, such time intervals are discarded. With respect to the
apparatus of FIG. 1, the envelope detector 15 replaces the preamble
detector 3.
[0092] The envelope detector 15 is connected to a sliding
sub-window generator 16, which generates time windows partially
superimposed within the time interval generated from the envelope
detector 15. Preferably, time window length is 0.5 microseconds,
and subsequent windows are superimposed each other for 0.25
microseconds. According to the invention, the smaller is the
interval the higher is the performance. With respect to the
apparatus of FIG. 1, the sliding sub-window generator 16 operates
similarly as the time window generator 11, but the former does not
use timing information and generates the windows in the whole
signal.
[0093] The sliding sub-window generator 16 is connected to a super
resolution processor 17, which receives also the Log(Sigma) signal
from the sampler 2. The super resolution processor 17 carries out a
super resolution method, preferably the aforementioned Tufts and
Kumaresan algorithm, in the time windows generated by the
sub-window generator 16 in order to estimate the carrier
frequencies in each window. For each time window, the processor 17
outputs the number of carriers and the relative frequencies. With
respect to the apparatus of FIG. 1, the super resolution processor
17 operates like the super resolution processing unit 12.
[0094] The super resolution processor 17 is connected to a carrier
frequency estimator 18 and to a re-timing and reply reconstruction
unit 19. On the basis of the evaluation for each sub-window, the
carrier frequency estimator 18 estimates the total number of
replies in the signal under test and the relative carrier
frequencies (this is necessary since there is no estimate coming
from the preamble). Considering the number of detected frequencies
in each sub window and the relative carrier frequencies, the
estimator 18 creates thresholds through which it is possible to
associate the power in each sub window with the pertaining
reply.
[0095] Using the threshold created by the carrier frequency
estimator 18, the re-timing and reply reconstruction unit 19 for
each reply estimates, in each sub-window, the presence and position
of the pulses (the pertaining pulses are the ones having the
carrier frequencies closest to estimated value). In this way the
system creates two or more signals: one for each reply under test.
These signals are then re-timed according to the international
specifications. The preamble is firstly re-timed, since its form is
known. Using the information coming from the re-timing of the
preamble, the data-block is then re-timed.
[0096] In the following, the operation modes of the embodiment
shown in FIG. 4 are described, similar operation modes being valid
also for other embodiments.
[0097] After receiving and digitalizing of the signal through the
antenna and receiver unit 1 and the sampler 2, respectively, the
apparatus monitors the channel through the envelope detector 15.
When a power level compatible with one or more SSR Mode S replies
is detected, the remaining units are activated for processing the
reply signal. The sliding sub-window generator 16 generates sub
windows within the time interval wherein reply power is detected,
in such a way that the super resolution method can be applied
therein. The super resolution processor 17 applies the super
resolution method on the signal coming from the sampler 2 in each
sub window, in order to estimate the number of sinusoids and the
relative frequencies. The carrier frequency estimator 18 uses all
the informations (i.e., number of frequencies and frequency values)
regarding the sinusoids belonging to each sub window to estimate
the number of replies under test and their carrier frequencies.
Using these evaluations the apparatus can handle each sub windows;
henceforth it determines in which window there are reply
pulses.
[0098] Having ascertained the presence of reply pulse(s) in a sub
windows, the re-timing and reply reconstruction unit 19 is able to
reconstruct the replies and then re-time the reply according to the
international requirements. After the reconstruction of the reply,
the apparatus is able to estimate range and azimuth, to decode the
message and to correct the errors through the message decoding and
error correcting unit 5 and the monopulse estimator 6. The aircraft
tracking and communication channel management unit 7 generates
tracks and manages communication between ground stations and
aircrafts.
* * * * *