Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2001-03-30
2003-09-02
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S442000
Reexamination Certificate
active
06614396
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and device to correct the variation or drift of the phase difference &Dgr;&PHgr; between two signals generated by a frequency signal Fe=2&PHgr;&OHgr;, a drift in the value of Fe prompting the drift in the value of the phase difference.
The invention can be applied especially to the correction of the drift in the value of the QDM (the code commonly used in navigation to designate the angle &thgr; between a moving body and the magnetic north) in order to meet certain standards such as EUROCAE ED-22B, RTCA, ARINC MARK 2 AIRBORNE VOR RECEIVER 711-9. This drift results from a variation in the rotation frequency.
The invention can be applied for example but not exclusively, to air, sea or river navigation.
2. Description of the Prior Art
FIG. 1
shows the principle of operation of a system used, on board a moving body or aircraft, for example an aircraft, to determine the direction of this moving body at a given place, as seen from a radio beacon. A continuous wave is sent in a cardioidal radiation pattern rotating at 30 rpm. At the same time, an omnidirectional antenna sends a reference signal at 30 Hz with frequency modulation. The on-board receiver detects the amplitude modulation at 30 Hz caused by the rotation of the pattern and the reference signal. The azimuth of the receiver seen from the station or VOR beacon is equal to the phase shift between the two signals E
1
and E
2
. E
1
is the modulation due to the rotation of the pattern and E
2
is the reference transmitted.
A variation in the 30 Hz frequency prompts a linear drift in the phase difference and the beat introduced into the signal as can be seen in
FIGS. 2 and 3
.
FIG. 4
is an exemplary processing chain according to the prior art.
A standard digital VOR receiver technique consists in applying a separator filter to process two frequency bands, the AM modulated low-frequency band and the FM modulated high-frequency band, applying a frequency discriminator to recover the FM modulating frequency and comparing the phases of the two signals.
The composite signal VOR, after reception by a receiver
10
, is separated by means of a separator
11
into an AM modulated low-frequency signal (LF) and an FM modulated high-frequency signal (HF).
The low frequency signal (LF) is sent to a linear filter
12
and then sampled
13
in order to determine its phase &PHgr;
1
by means of a Fourier transform.
The high frequency signal (HF) is sent first of all to a non-linear filter
14
and then sampled
16
in order to determine its phase &PHgr;
2
in implementing a Fourier transform. The non-linear signal has a frequency discriminator
15
which, in particular, has the function of recovering the FM modulating frequency.
Using the two phases &PHgr;
1
and &PHgr;
2
, the value of the phase shift &Dgr;&PHgr; corresponding to the QDM (plus or minus the parameter of the phase-shift terms introduced by the filters) is determined for example by a computer programmed accordingly.
During the signal processing method, the operation of sampling the (LF) and (HF) signals by Fourier transform may generate spurious lines when the period of the analyzed signal is not a multiple of the temporal window of analysis.
To overcome this problem, there are known ways of using standard windows such as a generalized Hamming window to reduce the side lobes of the spectrum while widening the major lobe or the Kaiser lobe associating an infinite spectrum with a time signal with finite support. These two methods however have limited performance characteristics and disturb the signal-to-noise ratio for equal integration time.
It is also possible to use a DFT type filter bank method. In this case, the computation power needed is very great and leads to investment costs that are incompatible with certain applications.
FIG. 3
gives a view, in a referential system where the X-axis is the time axis and the Y-axis represents the drift in the QDM, of:
The curve (I) which corresponds to the value QDM(Fe) for the reference frequency Fe or the corresponding period &OHgr;.
The curve (II) which corresponds to the drift in the QDM and the curve (III) which corresponds to a beat prompted by the drift in the value of the reference frequency Fe.
Hereinafter in the description, the term QDM designates the angle taken with respect to the magnetic north, also known as the bearing.
The invention consists especially of the use of a relationship linking the drift in the QDM with a variation in the reference frequency and a frequency estimator carefully positioned in the processing chain to correct this drift.
It can also use a trapezoidal sampling window, for example to carry out the Fourier transform.
SUMMARY OF THE INVENTION
The invention relates to a method to correct the drift in the phase shift (&Dgr;&PHgr;, &thgr;) between two signals (S
1
) and (S
2
), said signals being generated by a signal having a reference frequency Fe or a corresponding period &OHgr;, the drift in the phase shift resulting from a drift in frequency Fe or in period depending on a given relationship R((F,&OHgr;), (&thgr;, &Dgr;&PHgr;)).
The invention comprises at least the following steps:
determining the value of the phase shift (&Dgr;&PHgr;, &thgr;) from the phase &PHgr;
1
of the signal S
1
and the phase &PHgr;
2
of the signal S
2
,
estimating F
est
the value of frequency Fe, or the corresponding period &OHgr;,
bringing this value (&Dgr;&PHgr;, &thgr;) into an interval of given values [QDMmin, QDMmax] containing the value QDM(Fe) obtained for the reference value Fe taking account of the values &Dgr;&PHgr; and F
est
and of the given relationship R((F,&OHgr;), (&thgr;,&Dgr;&PHgr;)).
According to one embodiment, the method comprises, for example, a step of processing by Fourier transform using a sampling window whose shape is substantially trapezoidal.
The invention also relates to a method to correct a drift in the phase shift (&Dgr;&PHgr;, &thgr;) between two signals, S
1
having a phase &PHgr;
1
and S
2
having a phase &PHgr;
2
, the two signals being generated by a signal with a frequency Fe or with the corresponding period, the drift in the phase shift resulting from the drift in Fe or the drift in the period according to a given relationship R((F,&OHgr;), (&thgr;, &Dgr;&PHgr;)). The method comprises at least one step to determine at least one of the phases (&PHgr;
1
, &PHgr;
2
) by Fourier transform using a substantially trapezoidal sampling window.
The invention also relates to a device for the correction of the drift in the phase shift (&Dgr;&PHgr;,&thgr;) between two signals (S
1
) and (S
2
), said signals being generated by a signal with a reference frequency Fe, the drift being linked to the drift in frequency Fe according to a given relationship R((F,&OHgr;), (&thgr;, &Dgr;&PHgr;)). The device comprises at least:
a separator of the signals S
1
and S
2
, means to determine the value of the phase &PHgr;
1
and &PHgr;
2
for each of the signals and means to determine the value of the phase shift &Dgr;&PHgr;,
an estimator of the frequency Fe or of the corresponding period positioned after the separator,
a device comprising, in memory, the relationship R((F,&OHgr;), (&thgr;, &Dgr;&PHgr;)) receiving the values &Dgr;&PHgr; and the estimated frequency or the estimated corresponding period, adapted to bringing the value of &Dgr;&PHgr; to a value within an interval of given values [QDMmin, QDMmax] containing the value QDM(Fe) obtained for the reference value Fe (or period).
According to one embodiment, the device comprises for example means to process the signals (S
1
) and/or (S
2
) by Fourier transform, the sampling window having a substantially trapezoidal shape.
The estimator to estimate the frequency value may be of the Prony's estimator type.
The invention also relates to a VOR receiver comprising a device for the correction of a drift in the phase shift (&Dgr;&PHgr;, &thgr;) between two signals (S
1
) and (S
2
), said signals being generated by a signal having a r
Blouet Christophe
Pina François
Issing Gregory C.
Thales
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