Method and device for guiding an aerodyne on a runway, particularly during the taxiing phase preceding take off

A method and device are provided for guiding an aerodyne on the runway, picularly during the taxiing phase preceding take off, the device including an inertial unit adapted for delivering signals representative of the North speed, of the East speed, of the azimuth and of the ground speed of the aerodyne. From these signals, a computer elaborates a parameter defining the axis of the runway, during an apprenticeship step during which the aerodyne, guided on site by the pilot, taxies along the axis of the runway, and stores this parameter at the end of the step. The computer then delivers to the means piloting the aerodyne a synthetic runway aberration signal between the axis of the runway thus stored and the position of the aerodyne delivered by the inertial unit.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and device for guiding an 
aerodyne on the runway, in bad visibility, particularly during the taxiing 
phase preceding take off. 
Generally, the systems employed for effecting take off in bad visibility 
use a radioguidance system comprising, a transmitter disposed on the 
ground at the end of the runway which transmits along the axis thereof a 
radiobeam defining a radioalignment currently called "localiser" and, a 
reception system located on board the aerodyne and coupled to a detector 
adapted for determining the angular aberration between the aircraft and 
the axis of the runway. This angular aberration may then be used for 
displaying a guide order, for example by means of a head up type display 
system, in the form of a conventional trend bar or any other known display 
system, such as a "PVD" (Para Visual Display) type system or even a system 
for displaying informations by projecting light symbols on the windscreen 
of the aerodyne. 
Now, at the present time numerous take off runways are not provided with 
radioguidance systems and cannot be used for poor visibility take offs. 
Furthermore, the runways having radioguidance systems become unfit for poor 
weather take offs, should the transmission of the radioalignment be 
stopped, for example because of a breakdown or of any other reason. 
The object of the invention is then to overcome these problems by means of 
a method using a completely airborne guidance device for determining a 
synthetic runway aberration (position of the aerodyne/axis of the runway) 
without the help of any equipment on the ground. 
SUMMARY OF THE INVENTION 
It is based on the discovery that the navigation systems used at the 
present time include more and more often inertial units whose information 
is used by on board computers, particularly for determining the position 
of the aerodyne. 
However, up to now, the use of these navigational systems has never been 
contemplated for on the runway guidance and, in particular, for 
determining the angular or metric aberration between the position of the 
aerodyne and the axis of the runway, in particular in the absence of 
information transmitted by ground installations, no parameter is available 
on board the aerodyne relative to the axis of the runway and its 
orientation. 
In order to solve this problem the invention proposes however a solution 
which advantageously allows the inertial navigational system equipping the 
aerodyne to be used for guiding it on the runway, in poor visibility, 
during the taxiing phase preceding take off. 
According to the invention, this method comprises more particularly the two 
following successive steps at least: 
a first step, called apprenticeship step, during which the aerodyne 
initiates the taxiing phase at low speed and is guided at view by the 
pilot along the axis of the runway, whereas, at the same time, a computer 
makes an estimation of at least one parameter defining this axis from the 
information supplied by the inertial unit and stores this parameter at the 
end of the step, and 
a second stage comprising the elaboration by the computer of a synthetic 
runway aberration signal between the position of the aerodyne, whose 
parameters are determined by the computer from information delivered by 
said unit, and the axis of the runway whose parameters have been 
previously stored, and guidance of the aerodyne using said synthetic 
runway aberration. 
Of course, the synthetic runway aberration signal thus obtained may serve 
for elaborating a guide order usable by a display system, for example of 
the head up type or by a servo control directing the aerodyne on the 
ground. 
It should be noted in this connection that during the first step the speed 
of the aerodyne, initially at rest, will progressively increase while 
remaining however sufficiently low so that even with poor visibility the 
pilot can guide the aerodyne on site strictly along the axis of the 
runway. 
During the second step, the speed of the aerodyne exceeds the threshold 
from which the pilot can no longer guide the aerodyne on site. Guidance is 
then provided through the synthetic runway aberration elaborated by the 
computer. 
The problem which then needs to be solved for implementing the invention is 
that of determining the synthetic runway aberration. In fact, the inertial 
unit equipping aerodynes of the present time do not have sufficient 
accuracy for on the runway guidance, mainly because of the two following 
causes of errors: 
a first cause of error due to alignment defects of the inertial unit in 
azimuth guidance (aberration between the true North and the North 
determined by the unit); 
a second cause of error affecting the North and East speeds determined by 
the unit, errors of this type being limited but evolutive in time 
(Schuler's sinusoid). 
Thus, according to another feature of the invention, for eliminating the 
first cause of error the method may include: 
(a) during the apprenticeship step: 
detecting by the unit the North and East speeds (VN and VE) as well as the 
azimuth .psi.o; 
estimating by the computer the transverse speed vT of the aerodyne (VT=VE 
cos .psi.o-VN sin .psi.o); 
calculating the aberration Yo seen by the unit between the aerodyne and the 
axis of the runway by intergration in time of the transverse speed vT 
during the apprenticeship phase; 
calculating the distance D travelled on the ground by the aerodyne by 
integration in time of the ground speed VG of the aerodyne, during the 
apprenticeship phase; 
determining, by the computer, at the end of the apprenticeship period the 
lane error (or false North error elaborated by the unit ) 
.DELTA..psi.o=Yo/D 
storing this lane error .DELTA..psi.o; 
(b) during the second step: 
estimating the transverse speet vT corrected by the lane error 
.DELTA..psi.o 
EQU (vT=VE cos (.psi.o-.DELTA..psi.o)-VN sin (.psi.o-.DELTA..psi.o)); 
elaborating, from this transverse speed, a synthetic runway aberration 
signal for guiding the aerodyne. 
Furthermore, to take into account the errors affecting the North and East 
speeds, the method of the invention may further include a first additional 
step preceding immediately the apprenticeship step, during which, with the 
aerodyne stopped and aligned as well as possible along the axis of the 
runway, the computer determines the mean value of the North speed and the 
East speed. The result of this calculation corresponds to the errors 
affecting these speeds, namely .DELTA.VNo and .DELTA.VEo. These values 
once elaborated are then stored then subtracted from the values VN and VE 
acquired during the apprenticeship phase, and serve for determining the 
false North error then the synthetic runway aberration. 
It is however clear that this solution does not allow the variation of the 
errors .DELTA.VNo and .DELTA.VEo to be taken into account during the 
apprenticeship and synthetic aberration calculation steps. 
It will be recalled in this connection that the errors affecting the North 
and East speeds are evolutive in time according to Schuler's sinusoid 
whose period is of the order of 84 minutes. To estimate these errors, the 
position should then be determined on this curve at the time when the 
measurement is made. Now, the computer has no information for making such 
an estimation. 
To overcome this problem, the invention proposes adding a second additional 
step for measuring, when stopped, the errors affecting the North and East 
speeds, namely .DELTA.VN.sub.1 and .DELTA.VE.sub.1, for example when the 
aerodyne is at the embarcation post, or even during the travel towards the 
take off runway. Thus, with the values .DELTA.VN.sub.1 .DELTA.VNo and 
.DELTA.VE.sub.1, .DELTA.VEo, the computer may estimate the slope of 
Schuler's sinusoid in the zone corresponding to these values and 
consequently, make the necessary error corrections during the 
apprenticeship step and the step determining the runway aberration of the 
aerodyne.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The diagram shown in FIG. 1 shows the consequences of one of the two main 
causes of errors which affect the determination of the synthetic runway 
aberration Y. 
This diagram shows particularly, by means of vectors, the true geographical 
North or "true North" and the North determined by the inertial unit ("IRS 
North"). Statistically, this IRS North may differ from the true North by 
an angular aberration .DELTA..psi.o of the order of 0.4.degree. because of 
an alignment defect of the unit. 
It will be recalled in this connection that, in accordance with the method 
of the invention, the angular aberration .DELTA..psi.o is elaborated by 
the computer in the following steps: 
1. calculating the estimated transverse speed vT*, by the relationship: 
EQU vT*=VE cos .psi.o-VN sin .psi.o 
with: 
.psi.o which is the azimuth of the aircraft stopped at the end of the 
runway and aligned as well as possible with the axis of the runway, this 
azimuth taking as reference the IRS North, 
VE=VE-.DELTA.VEo, that is to say the East speed (VE) detected, reduced by 
the aberration .DELTA.Veo measured when the aerodyne is stopped at the end 
of the runway and aligned in the axis thereof, 
VN=VN-.DELTA.VNo, that is to say the North speed VN detected, reduced by 
the aberration .DELTA.VN.sub.o measured when the aerodyne is stopped at 
the end of the runway and aligned along the axis thereof. 
2. calculating of the erroneous runway aberration Yo* which the aircraft 
forms with the axis of the runway (seen by this unit) during the 
apprenticeship step, during which the aerodyne taxies as well as possible 
on site along the true runway axis (shown by broken lines in FIG. 1). It 
should be recalled in this connection that because of the false North 
error, the inertial unit indicates an azimuth .psi.o (unit azimuth) 
different from that of the aerodyne and, consequently, an erroneous runway 
abberation Yo*. This erroneous runway aberration is obtained by means of 
the following relationship: 
##EQU1## 
in which: to is the time when the pilot actuated the throttle lever to 
begin taxiing on the ground before take off, 
tfin is the time marking the end of the apprenticeship step, for example 
the time when the aerodyne reaches the speed of 100 knots. 
3. calculating the distance D travelled on the ground between time to and 
time tfin 
##EQU2## 
in which VG is the ground speed of the aerodyne which may be calculated by 
the unit (VG.sup.2 =VN.sup.2 +VE.sup.2) or by any other means. 
4. Calculation of the aberration .DELTA..psi.o, from the relationship: 
EQU .DELTA..psi.o*=Yo*/D 
As mentioned above, this value .DELTA..psi.o* as well as the realigned 
azimuth of the axis of the runway (.psi.o*=.psi.o-.DELTA..psi.o*) are 
stored at the end of the apprenticeship period for determining the true 
runway aberration Y after the apprenticeship step. 
These different calculations may be made by means of the system whose 
organization is shown schematically in FIG. 2 which includes an inertial 
unit 1 (IRS) delivering signals representative of the North speed (VN), of 
the East speed (VEst), of the ground speed VG and of the realigned azimuth 
.psi.o* of the runway. 
These signals are transmitted to a computer 2 which determines the first 
derivative Y' of the runway aberration Y, that is to say the metric 
aberration between the aerodyne and the axis of the runway, this first 
derivative Y' being obtained from the formula: 
EQU (Y'=VT=VE cos (.psi.o*)-VN sin (.psi.o*)) 
This signal Y' is transmitted to the control block 3 of the head up display 
device 4 of the aerodyne. This block 3 is designed for providing, to the 
display device 4, a guidance order signal .delta.r, for example in the 
form of .delta.r=K(Y+.tau..sub.1 Y'+.tau..sub.2 Y"), according to a given 
control law similar to that used in the guidance systems using 
radioalignment ("localiser"). 
This block 3 further receives a YAW speed signal r1 delivered by a yaw gyro 
5 from which is determined, by means of the signal VG delivered by the 
unit 1, the second derivative Y" of the runway aberration Y. This 
calculation is worked out in block 6. It should be noted in this 
connection that the use of the yaw gyro is not obligatory, the yaw speed 
signal being able to be elaborated by the inertial unit 1 (connection r'1 
shown with broken lines). 
FIG. 3 shows schematically one embodiment of the computer 2 and of the 
control block 3. In this Figure, computer 2 has been divided into two 
units 8, 9, namely: a unit 8 for estimating the transverse speed Y' of the 
aerodyne with respect to the axis of the runway and unit 9 for computing 
the error .DELTA..psi.o*. 
The estimation unit 8 receives the signals VN, VE and .psi. delivered by 
the inertial unit 1. It elaborates the signal Y' and transmits it to unit 
9 through a switch SA.sub.1 and to the control block 3 through a switch 
SB.sub.1. 
This estimation unit 8 is further connected to the output of unit 9 by 
means of a circuit including a switch SB.sub.2. It further receives a 
control signal S.sub.1 whose function will be explained further on. 
The unit 9 for computing the aberration .DELTA..psi.o* further receives a 
signal representative of the ground speed VG through a switch SA.sub.2 and 
a signal S.sub.2 indicating the end of the apprenticeship step. 
It comprises more especially, as shown, two integrators I.sub.1 and I.sub.2 
whose inputs are connected respectively to the terminals A.sub.1, A.sub.2 
of switches SA.sub.1 and SA.sub.2 and whose outputs are connected at the 
time of transmission of the signal S.sub.2 to the two inputs of a divider 
shown by the block Q. These two integrators are initialized at o at the 
time of transmission of signal S.sub.1, that is to say at time to. 
The output of divider Q is connected to the input of a memory unit M whose 
access is controlled by the signal S.sub.2. Thus, from time to, these two 
integrators effect the integration in time of the transverse speed vT* and 
of the ground speed VG. 
At time tfin determined by the transmission of the signal S.sub.2, these 
integrators then deliver the values Yo* and D and, consequently, at the 
output of divider Q the value .DELTA..psi.o*=Yo*/D is obtained. 
Concurrently, the memory unit M enabled for writing by the signal S.sub.2 
stores this value which, because of the switching of SB.sub.2, is applied 
to the estimation unit 8. 
At the input of the control block 3 the signal Y' is applied (if switch 
SB.sub.1 is closed) to an integrator 11 (transfer function 1/s) which 
delivers a signal Y to a first input of the summator 12. This integrator 
11 is initialized at o at the beginning of the second step (at the time 
when the signal S.sub.2 is transmitted). This signal Y' is also 
transmitted to a network 13 with time constant .tau.1 connected by its 
output to a second input of the summator 12. 
The third input of this summator receives through a network 14 with time 
constant .tau.2, a signal representative of the second derivative Y" of 
the aberration signal Y, elaborated from the derivative of the signal 
.psi. delivered by the unit 1 (or determined from a yaw gyro) and from the 
ground speed VG of the aerodyne. 
At the output of the summator, a signal .delta.rc is then obtained of the 
form: 
EQU .delta.rc=Y+.tau.1Y'+.tau.2Y" 
This signal is then transmitted to the display system through a switch 
SB.sub.3 and a gain amplifier K. 
The operation of the above described device is then as follows: 
On coming onto the take off runway, the pilot disposes the aerodyne along 
the axis of the runway and stops. Concurrently, if that has not already 
been done, he starts up the guidance system, for example by pressing a 
button. 
During this stop, computer 2 estimates the errors .DELTA.VN and .DELTA.VE 
with which the North and East speeds are affected as well possibly as the 
position at that moment on Schuler's sinusoid for determining, from the 
slope of this sinusoid at this position, what will be the variation of the 
errors .DELTA.VE and .DELTA.VN during take off (duration of the order of 
30 seconds to two minutes). 
In fact, if we refer to FIG. 4 which shows Schuler's sinusoid whose period 
is 84 minutes, it is clear that at position A the variation of the errors 
.DELTA.VN and .DELTA.VE will not be very great, whereas at position B, 
this variation will be much greater. 
In practice, determination of the position on Schuler's sinusoid requires 
the knowledge of two positions on this curve. Now, the above mentioned 
stop at the end of the runway is not sufficiently long for determining two 
positions far enough removed for obtaining a significant result. This is 
why the invention proposes making the determination of the first point 
during a stop of the aerodyne preceding that made at the end of the 
runway. This stop may for example be the one normally made by the aerodyne 
in the passenger embarcation zone. It may also be carried out during the 
travel of the aircraft from the embarcation zone to the take off runway. 
Once the errors .DELTA.VE, .DELTA.VN have been determined, the 
apprenticeship step is begun. It begins when the throttle lever of the 
aerodyne is placed at maximum and, when, consequently, under the thrust of 
the engines the aerodyne begins to taxi along the axis of the runway. 
Action on the throttle lever initiates a control signal S.sub.1 which is 
taken into account by the computer 2 and closes the switches SA.sub.1 and 
SA.sub.2. This action defines then the time to which forms the lower limit 
of the integrations carried out for determining the aberration Yo* seen by 
the unit between the aerodyne and the non realigned axis of the runway and 
the distance D travelled on the ground. 
The apprenticeship step takes place during a period between time to when 
the aerodyne is still at speed O and time tfin when it reaches a 
predetermined speed, for example 100 knots. 
During the first part of this step, between the time to and the time t1 
corresponding to a predetermined speed, for example of 50 knots, the 
estimation unit 8 determines the transverse speed Y' of the aerodyne with 
respect to the axis of the runway as indicated above, whereas the 
computing unit 9 elaborates a false North error signal .DELTA..psi.o*. 
However, this signal is not reinjected into the elaboration unit 8 which 
will only receive a zero error signal (block 16) from switch SB.sub.2. 
Furthermore, because switches SB.sub.1 and SB.sub.3 are open, the control 
block 3 is disconnected from the display device, which supplies no 
information useful for piloting. The pilot then guides the aerodyne solely 
by his view of the runway. 
It should however be noted that, for informing the pilot that the device is 
operating, it is possible to provide an additional connection (shown with 
broken lines) connecting the output of the estimation unit 8 to the 
terminal 17 of switch SB.sub.3, this connection including a filtering cell 
18 with transfer function of the type 
##EQU3## 
This connection thus allows information relative to the transverse 
aberration Y to be displayed on the display device, this value being 
progressively deleted to disappear after a given time for example of the 
order of 2 to 5 seconds. 
In the second part of the estimation step (from 50 to 100 knots), the 
operation of the display device remains similar to the preceding one. 
However, in this case, switching of switches SB.sub.1, SB.sub.2, SB.sub.3 
which marks the premature end of the apprenticeship step (tfin), may be 
carried out in the two following cases: 
1. Following malfunctions such as: 
a reduction of the speed of one of the critical engines of the aerodyne due 
for example to a breakdown of this engine, causing a considerable yaw 
torque, 
a reduction of the longitudinal speed of the aerodyne, making take off 
impossible if not dangerous, 
prohibitive lateral acceleration for example following a tyre burst, 
a reduction of visibility causing the pilot to interrupt take off; 
2. When the aerodyne reaches a predetermined speed, for example 100 knots. 
In both cases, because of the closure of switch SB.sub.2, the false North 
error .DELTA..psi.o*, delivered by unit 9 at the end of the apprenticeship 
step, will be transmitted to the estimation unit 8 which will elaborate a 
transverse speed signal Y' corrected for this error in accordance with the 
procedure which will be described hereafter with reference to FIG. 5. 
Because of the closure of switch SB.sub.1, this signal Y' will be 
transmitted to the control block 3 which will transmit to the display 
system (switch SB.sub.3 being closed) the control signal .DELTA.r. 
In the example shown in FIG. 3, the second derivative of the metric 
aberration Y is elaborated from the ground speed VG and of the drift with 
respect to time .psi.' from the geographic azimuth .psi., in accordance 
with the relationship: 
##EQU4## 
However, in the case where the aerodyne has available yaw speed information 
r1 (yaw unit or gyro), this second derivative could also be obtained from 
the relationship: 
EQU Y"=VG.times.r1 
Such as shown in FIG. 5, the estimation unit 8 includes three storage units 
MEM.sub.1, MEM.sub.2, MEM.sub.3 whose inputs (writing) receive 
respectively the mean sliding values of the azimuth, of the North speed 
and of the East speed elaborated from the signals .psi., VN and VE 
delivered by the inertial unit. Calculation of these sliding values is 
made by circuits shown by blocks 21, 22 and 23 which deliver at any moment 
a mean value in a given period of time which has just elapsed. The storage 
units MEM.sub.1, MEM.sub.2, MEM.sub.3 are controlled by the signal S.sub.1 
so that when this signal S.sub.1 is transmitted they store respectively 
the mean values elaborated by blocks 21, 22, 23 and which in fact form the 
values .psi.o, .DELTA.VNo and .DELTA.VEo. The output of the storage unit 
MEM.sub.1 is connected to the inputs of the sine function generator 25 and 
a cosine function generator 24 through a subtractor 26 whose (-) input is 
connected to the terminal B.sub.2 of the switch SB.sub.2 which delivers 
the signal .DELTA..psi.o* when the control signal S.sub.2 is transmitted. 
The subtractor 26 will consequently deliver either the mean signal .psi. 
during the apprenticeship phase and the signal 
.psi.o*=.psi.o-.DELTA..psi.o* when the control signal S.sub.r is 
transmitted. 
The storage unit MEM.sub.2 is connected by its output to the negative input 
of a subtractor 27 which receives at its positive input the East speed 
signal VE delivered by the unit. The output of this subtractor 27 will 
therefore deliver the signal VN=VN-.DELTA.VNo. 
Similarly, the storage unit MEM.sub.3 is connected by its output to the 
negative input of a subtractor 28 whose positive input receives the North 
speed signal VN delivered by the unit. The output of this subtractor 28 
consequently delivers the signal VE=VE-.DELTA.VEo. 
The output of subtractor 27 is connected to an input of a multiplier 29 
whose other input receives the signal transmitted by the sine function 
generator 25. 
At the output of this multiplier 29 is obtained then a signal of type VN 
sin .psi.o*. Similarly, the output of subtractor 28 is connected to an 
input of the multiplier 30 which further receives the signal transmitted 
by the cosine function generator 24. This multiplier delivers then a 
signal of type VE cos .psi.o*. 
The outputs of the two multipliers 29, 30 are connected respectively to the 
two inputs of a subtractor 31 which accordingly delivers, depending on the 
case, the signal: 
EQU VT=VE cos .psi.o-VN sin .psi.o 
during the apprenticeship stage or 
EQU Y'=VT=VE cos .psi.o*-VN sin .psi.o* 
when the signal S.sub.2 is transmitted. 
Of course, the invention is not limited to the embodiment described above. 
It could in fact include two or more inertial units, connected to the same 
computer. This latter could furthermore receive information concerning for 
example the speed of the engine or engines of the aerodyne, the position 
of the throttle lever, so as to transmit control signal S.sub.1 of 
switches SA.sub.1 SA.sub.2 when the throttle lever is in its maximum 
position and for transmitting the control signal S.sub.2 when: 
the ground speed is greater than or equal to 100 knots, or 
the derivative of the rotational speed of a critical engine is less than a 
negative value, 
the derivative of the ground speed with respect to time is less than a 
predetermined threshold value, for example 2 knot/second, or 
the lateral acceleration of the aerodyne exceeds a predetermined threshold 
value.