Communications: directive radio wave systems and devices (e.g. – Air traffic control – Secondary surveilance radar
Reexamination Certificate
2004-03-15
2004-11-16
Gregory, Bernarr Earl (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Air traffic control
Secondary surveilance radar
C342S029000, C342S030000, C342S032000, C342S036000, C342S040000, C342S175000, C342S195000
Reexamination Certificate
active
06819282
ABSTRACT:
FIELD OF THE INVENTION
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
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.
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
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.
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
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:
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.
Preferably according to the invention, said digitalized received signal comprises a Log(Sigma) component signal of a digitalized antenna signal.
Still according to the invention, the method may be 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.
Always according to the invention, the method may be 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.
Furthermore according to the invention, the sub-step of estimating the carrier frequency may perform 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.
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:
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;
means for determining, for each detected start time, an end time of the corresponding reply;
means for generating time windows in at least a portion of the digitalized received signal;
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;
means for estimating the number of replies and the relative carrier frequencies in the digitalized received signal, and
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.
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.
Always according to the invention, the appara
Galati Gaspare
Leonardi Mauro
Gregory Bernarr Earl
Universta' Degli Studi Di Roma “Tor Vergata”
Young & Thompson
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