Digital radio altimeter validation system

Abstract

A digital radio altimeter validation system, equipped with an RF input / output interface and characterized by an incompressible latency, configured to receive a linear chirp (FMCW) continuous wave signal, linearly frequency modulated and quadratic phase, s(t), which can also be written in complex form in Cartesian coordinates, representing time, and configured to retransmit it with a configurable delay and deliver to the radio altimeter a signal exactly compensated in frequency for the latency by a linear extrapolation of its phase by calculating the difference between the current phase and the phase numerically delayed by the value to be compensated. Figure for the abbreviation: Figure 1

Description

Title of the invention: Digital radio altimeter validation system

[0001] The invention relates to a digital RF card for validating a radio altimeter or radar altimeter, with frequency-modulated continuous-wave radar signals or FMCW for an acronym of "Frequency-Modulated Continuous-Wave" in English.

[0002] A radio altimeter or radar altimeter is an instrument for measuring the distance of an aircraft above the ground, using the principle of radar.

[0003] Validating a radio altimeter requires delaying the emitted signal by a wide range of orders of magnitude, from a few tens of milliseconds to a few tens of milliseconds. Physical delay lines are bulky, expensive, and limited in the test scenarios they offer.

[0004] Some digital systems exist and offer more flexibility, but remain quite simplistic and do not allow the simulation of very low heights for servo-slope radio altimeters.

[0005] Prior art systems do not allow for the simulation of very low heights (close to 0 feet) and very high heights at the same time (close to 10,000 feet) in a flexible manner and in compatibility with slope-controlled radio altimeters.

[0006] Also, according to one aspect of the invention, a digital radio altimeter validation system is proposed, equipped with an RF input / output interface and characterized by an incompressible latency T, configured to receive a linear chirp (FMCW) continuous wave signal i(f) with frequency linearly modulated frequency = at+ fi and quadratic phase s(t) — +^+)0, which can also be written in complex form in Cartesian coordinates (fit) + 1 representing time, and configured for the retransmit after a configurable delay and deliver to the radio altimeter a signal IV) + exactly frequency compensated for latency T by an extra linear polarization of its phase by calculating the difference between the current phase and the phase numerically delayed by the value to be compensated T, so that the frequency of this signal has no apparent delay.

[0007] This compensation makes it possible to simulate zero heights, and it can be extended to high heights by adding a digital delay corresponding to the associated propagation delay.

[0008] According to one embodiment, the digital radio altimeter validation system comprises: a first converter configured to convert the signal emitted in complex form in Cartesian coordinates fit - t) + jQÿ - t) into coordinates polars in the form pe^'^ ; - a shifter configured to perform a left shift of one bit or binary multiplication by 2 of the phase of the output signal of the first converter - a delayer configured to apply a delay of said latency T to the phase of the output signal of the first converter; - a subtractor configured to subtract the output phase of the phase shifter from the output phase of the phase retarder; and - a second converter configured to convert the output signal of the digital system, having as its magnitude P that of the output of the first converter and as its phase the output phase of the subtractor 2tj)(t - t) - 4>(Z -2t) into a signal in complex form in Cartesian coordinates / '(0 + JÔV)-

[0009] A linear chirp is a linearly modulated signal with frequency f, and therefore with quadratic phase μ, which can respectively be expressed in the form f(t) = at + p and 2k + r = < function of the minimum frequencies fmi(1) and maximum frequencies fmax of the chirp and its duration T according to n - fmarfmin, P = fmin, and F the initial fraction of the phase ¢(0) / 2^ (Jl *“ «y

[0010] According to one embodiment, the first converter includes a CORDIC module configured to implement a numerical calculation by coordinate rotation for calculations of trigonometric and hyperbolic functions.

[0011] In one embodiment, the timer includes a FIFO queue.

[0012] According to one embodiment, the second converter comprises a module CORDIC configured to implement numerical calculation by coordinate rotation for calculations of trigonometric and hyperbolic functions.

[0013] Alternatively, the second converter includes a piecewise polynomial approximation module of the complex exponential.

[0014] According to another embodiment, the digital RF radio-altimeter validation card comprises: - a delay (Ret2) configured to apply a delay of said latency T to the signal received l(t) , Q(f ) from the radio altimeter by the RF card having already undergone an incompressible latency T , Z ( f - t ) , Q(t - T ), and deliver at output the signal received from the radio altimeter delayed by 2t , / ( f-2t ) , Q ( ?-2t ) ; - a complex multiplier (MultComplex) configured to apply complex multiplications to the received signal I(t) , Q(t) from the radio altimeter by the RF card having already undergone an incompressible latency , I ( t - T ) , QÇt-j) , and output the signals / 2 ( r - r ) - ( z -t) , 2J(rj) , +ϔ2( / -t) =p2( / -T); - a conjugate multiplier (MultConj) configured to apply complex multiplications to the output signals of the delay (Ret2) and the output signals_ 22(zt) et2 / (fT) Q(tx) of the complex multiplier (MultComp) to deliver the output signals Z3 = / / 2+ and e3 = / 2ei - Q / i; and - a divider (Div) configured to apply divisions to the output signals of the conjugate multiplier (MultConj) by the output r(t - t) 4- Q2(t - t) = p1(t - T) of the complex multiplier (MultComplex), and deliver the output signal / ( / ) + jQ( t) = pert2$(^^

[0015] The invention will be better understood upon examination of some embodiments described by way of non-limiting examples and illustrated by the accompanying drawings, in which: - Figure 1 schematically illustrates a digital RF card for validating a radio altimeter, according to one aspect of the invention; and - [Fig.2] schematically illustrates a digital RF card for validating a radio altimeter, according to another aspect of the invention.

[0016] The present invention relates to a digital radio altimeter validation system, equipped with an RF input / output interface and characterized by an incompressible latency T, configured to receive a linear chirp FMCW continuous wave signal f(t) = at + p and with quadratic phase 5(t) = + / W, which can also be written in complex form in co Cartesian ordinates I(t) + j&t\ 1 representing time. The digital radio altimeter validation system is also configured to retransmit the signal after a configurable delay and deliver to the radio altimeter a signal I '(0 + jQ'(Ù) exactly frequency compensated for the latency T by a linear extrapolation of its phase by calculating the difference between the current phase and the phase numerically delayed by the value to be compensated T.

[0017] Fig. 1 schematically represents a digital RF card for validating a radio altimeter, according to one embodiment of the invention.

[0018] The digital system is also configured to deliver to the radio altimeter a corrected signal with a delay of 2t in complex form in Cartesian coordinates ZV) + jQ\t) corresponding to the signal reflected at a predetermined height, in which the received signal is compensated by a linear extrapolation of its phase, from the current phase and the phase numerically delayed by the value to be compensated T.

[0019] The digital radio altimeter validation system comprises: - a first converter Convl configured to convert the signal emitted in complex form in Cartesian coordinates l(t - r) + jQ^t - t) into polar coordinates in the form; - a Dec shifter configured to perform a left shift of one bit or binary multiplication by 2 of the phase of the output signal of the first converter Convl; - a delay Ret configured to apply a delay of said latency T to the phase of the output signal of the first converter Convl; - a subtractor Sub configured to subtract the output phase of the shifter Dec from the output phase of the retarder Ret; and - a second converter Conv2 configured to convert the output signal of the digital system, having for modulus P that of the output of the first converter Convl and for phase the output phase of the subtractor Sous 2<|)( t- t) -¢)( c2t) , into a signal in complex form in Cartesian coordinates IV)+jQ '(t).

[0020] It is proposed to use a digital system to simulate the propagation channel, in particular by introducing a delay using a FIFO queue to simulate a height. This makes it possible to perform all the desired tests.

[0021] Unfortunately, the RF input / output interface of the digital system has an incompressible latency T due to propagation times, which prevents the simulation of heights less than hmin = ct / 2; c corresponding to the speed of propagation of the wave in its medium (generally assimilated to the speed of light in a vacuum).

[0022] However, the fact that it is digital allows this delay to be compensated by applying a signal with a negative phase slope (non-causal). In other words, it is possible to use complex operators to compensate for the phase by the amount by which it has varied over the duration T.

[0023] For example, the received continuous frequency-modulated background can be of the form e2jn\at2l2 +fit+y), and the chirps principle can be linear with variable slope of frequency f ( t) = at+p ■

[0024] Phase correction is compatible with zero propagation time for testing an FMCW radio altimeter.

[0025] It is possible to write the frequency of a linear chirp as f(t) — OCt + [5, and therefore the associated chirp can be written s(t) = — pe2jn^.l0au+pdu+yj — pe2X«t2 / 2 +Pi+y)'

[0026] The received signal r then essentially corresponds to the emitted signal s, with a delay T and an attenuation A. By mixing the two, a pure beat frequency is obtained: BF(t) = s(t) xr (t) = s(t) x A s*(t- t); BfM = Ap* xxe = Ap? x

[0027] The frequency of this signal is Îbf ~ aT, and we can deduce the height of the carrier as u _ cxt _ CXW. “ 2 2a

[0028] It is possible to fix the value of a at the transmitter and attempt to determine the value of fsF at the receiver, i.e., to vary the value of a in order to obtain a known frequency Σbf. This latter principle, less complex to implement, is that of servo-slope radio altimeters.

[0029] By studying the phase shifts of the emitted signal, it appears that 4>( t - t) -(|)(t-2T) =4>(t) - <j)(t-t) -2rrat2, soit (|) ( t ) = 2(|) ( t - T ) - ([) ( 1-2T ) + 2rraT2- For a linear chirp, a is constant and therefore 2naT2 also: by mixing the received signal delayed by T with the signal delayed by 2t, we obtain a frequency compensated signal.

[0030] The first Convl converter may include a CORDIC module configured to implement a numerical calculation by coordinate rotation for calculations of trigonometric and hyperbolic functions.

[0031] The Ret retarder may include a FIFO first in first out queue.

[0032] The second converter Conv2 may include a CORDIC module configured to implement a numerical calculation by coordinate rotation for calculations of trigonometric and hyperbolic functions.

[0033] Alternatively, the second converter Conv2 may include a piecewise polynomial approximation module of the complex exponential.

[0034] According to another embodiment, as illustrated in [Fig.2], the digital radio altimeter validation system comprises: - a delay (Ret2) configured to apply a delay of said latency T to the received signal I(t) , Q( / ) from the radio altimeter by the RF card having already undergone an incompressible latency T , l(t - T ) , Q ( t- T ), and deliver at output the received signal from the radio altimeter delayed by 2t , / ( ) , Q ( t -2t ) ; - a complex multiplier (MultComplex) configured to apply complex multiplications to the received signal / (f), Q (?) from the radio altimeter by the RF card having already undergone an incompressible latency T , I( t - T ) , ? - T ) , and output the signals / 2(?-t)-g2(tT) ,2 / (?-t) , +ϔ2(?-t) = p2' - a conjugate multiplier (MultConj) configured to apply complex multiplications to the output signals of the delay (Ret2) and the output signals / 2(zt)-Q2( / -t) and 2Z(fT) Q(tT) of the complex multiplier (MultComp) to deliver the output signals Z3 = Z / 2+ 0.¾ and = Z^ - ô / p and - a divider (Div) configured to apply divisions to the output signals of the conjugate multiplier (MultConj) by the output Z2(t - t) + Q2(t - t) = p2 of the complex multiplier (MultComplex), and deliver the output signal + jQ(t) =

[0035] Having a digital architecture makes it possible to create complex dynamic scenarios with a wide variety of possible heights.

[0036] The phase compensation performed allows the frequency value to be restored.

[0037] The present invention is simple to implement and relatively insensitive to phase noise.

Claims

Claims

1. Digital radio altimeter validation system, provided with an RF input / output interface and characterized by an incompressible latency T, configured to receive a linear chirp (FMCW) continuous wave signal s(t) linearly modulated in frequency f(t) =af + ^and with quadratic phase s(t) — which can also be written in complex form in Cartesian coordinates Z(0 + jQ[t\1 representing the time, and configured to retransmit it according to a configurable delay and deliver to the radio altimeter a signal ZV) + jQ'(t) exactly compensated in frequency for the latency T by a linear extrapolation of its phase by calculating the difference between the current phase and the phase digitally delayed by the value to be compensated T.

2. Digital radio altimeter validation system, according to claim 1, comprising: - a first converter (Convl) configured to convert the signal transmitted in complex form in Cartesian coordinates I(t) + j&t} into polar coordinates in the form; - a shifter (Dec) configured to carry out a left shift of one bit or binary multiplication by 2 of the phase of the output signal of the first converter (Convl); - a delayer (Ret) configured to apply a delay of said latency T to the phase of the output signal of the first converter (Convl); - a subtracter (Sous) configured to subtract the output phase of the shifter (Dec) from the delayer output phase (Ret);and - a second converter (Conv2) configured to convert the output signal of the digital system, having as module P that at the output of the first converter (Convl) and as phase the output phase of the subtractor (Sous) 20( t - T ) - <|) ( r-2x ) , into a signal in complex form in Cartesian coordinates IV) + jQ'(Ù-;

3. Digital radio altimeter validation system, according to claim 2, in which the first converter (Convl) comprises a CORDIC module configured to implement a digital calculation by coordinate rotation for calculations of trigonometric and hyperbolic functions.

4. A digital radio altimeter validation system according to claim 2 or 3, wherein the delayer (Ret) comprises a FIFO queue.

5. Digital radio altimeter validation system, according to one of claims 2 to 4, in which the second converter (Conv2) comprises a CORDIC module configured to implement a digital calculation by rotation of coordinates for calculations of trigonometric and hyperbolic functions.

6. Digital radio altimeter validation system, according to one of claims 2 to 4, in which the second converter (Conv2) comprises a piecewise polynomial approximation module of the exponential.

7. Digital radio altimeter validation system, according to claim 1 or 2, comprising: - a delayer (Ret2) configured to apply a delay of said latency T to the signal received I(t), Q(t) from the radio altimeter by the RF card having already undergone an incompressible latency T, I (tT), Q(t - T), and output the signal received from the radio altimeter delayed by 2t, l( / -2t), Q( / -2t); - a complex multiplier (MultComplexe) configured to apply complex multiplications to the signal received, Q( / ) from the radio altimeter by the RF card having already undergone an incompressible latency T , / ( t - t ) , Q(tl) , and output the signals / 2 ( t - t ) - Q1 ( t - t ) , 2Z(rT) Ô(fT) , and / 2(ZT) +g2(zT) =p2(r_T);- a conjugate multiplier (MultConj) configured to apply complex multiplications to the output signals of the delay (Ret2) and the output signals - Q2(tt) and 21 ( £ - T ) Q(t - T) of the complex multiplier (MultComp) to output the signals / 3 = V2 + Q,Q2 and Q. = I2Q. - QM- and - a divider (Div) configured to apply divisions to the output signals of the conjugate multiplier (MultConj) by the output Z2(tt) + ^2(zt) = p2(f- t ) of the complex multiplier (MultComplexe), and output the signal; / (0 + jQ(t) = pe. / (2^(fT)^(f-2T)).

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