Method for detecting and excluding multiple failures in a satellite
View Patent ↗The present invention relates to a method for detecting and excluding at least one pseudo-range measured between a satellite and a receiver for receiving signals transmitted by different satellites of a radio-positioning constellation when said pseudo-range is faulty, characterized in that said method includes the steps of: (a) determining an estimation of the position of the receiver from the pseudo-ranges measured by the receiver, (b) estimating, from the thus-estimated position, biases in the measured pseudo-ranges; (c) processing the thus-obtained biases in order to derive a value representative of the probability of a fault for each pseudo-range; (d) preselecting, on the basis of the resulting values, a given number of pseudo-ranges which are most likely to be faulty; (e) determining, for each combination of pseudo-ranges from among the thus-preselected pseudo-ranges, a value of a test variable representative of the likelihood the combination is faulty; (f) selecting, on the basis of the values of the thus-obtained test variables, at least one combination of pseudo-ranges which are most likely to be faulty; and (g) comparing the test variable associated with the selected combination with a predetermined threshold, and transmitting the signals to the receiver or outward on the basis of the result of said comparison. The invention also relates to an integrity-monitoring system and to a vehicle therefor.
1. A method for detection and exclusion of at least one pseudorange measured between a satellite and a receiver receiving signals emitted by different satellites of a radio-positioning constellation, when this pseudorange is faulty, characterised in that it comprises steps of:
(a) Determining estimation of the position of the receiver from the pseudoranges measured by the receiver;
(b) Estimating, from the accordingly estimated position, biases in the pseudoranges measured;
(c) Processing accordingly obtained biases to deduce therefrom a value representative of the probability of failure for each pseudorange;
(d) Pre-selecting, as a function of the accordingly obtained values, a given number of pseudoranges the likeliest to present failure;
(e) Determining, for each combination of pseudoranges among the accordingly pre-selected pseudoranges, a variable test value representative of the likelihood of the failure of the combination;
(f) Selecting, as a function of the accordingly obtained variable test values, at least one combination of pseudoranges failure of which is the likeliest;
(g) Comparing the test variable associated with the selected combination at a predefined threshold and transmission of a signal to the receiver or outwards as a function of the result of this comparison.
2. The method according to claim 1 , in which a test variable
T
j
,
j
∈
[
1
,
∑
P
=
1
K
C
M
P
]
is calculated at step (e) for all possible combinations of 1 to K pseudoranges among the M pseudoranges pre-selected at step (d), K being a fixed whole number less than or equal to M.
3. The method according to claim 2 , in which the calculation of each test variable T j is carried out by means of a statistical test based on estimation of the bias vector b t ′ corresponding to a bias vector b i , whereof the components not associated with the considered combination of measurements are set to zero.
4. The method according to claim 3 , in which the statistical test used follows a χ 2 distribution with P degrees of freedom, P being the number of pseudoranges of the considered combination.
5. The method according to any one of claims 2 to 4 , in which the predefined threshold used during step (g) is adapted in a threshold T d,j specific to each combination, the ratio
R
j
=
T
j
T
d
,
j
being constructed and used in place of the test variables T j .
6. The method according to claim 1 , in which K is the maximum number of faulty pseudoranges in question, and M is a number greater than the maximum number of faulty pseudoranges in question.
7. The method according to claim 1 , in which step (b) also provides the estimation {circumflex over (k)} of the instant of emergence of estimated biases.
8. The method according to claim 7 , in which step (b) comprises the sub-steps of:
(b1) Estimating a plurality of bias vectors b t , each under the assumption of the emergence of a failure at an instant kε[t−L+1,t] a sliding window of analysis of fixed size L;
(b2) Calculating, at each instant k in question, a ratio R t [k] of likelihood between the assumption of the emergence of a failure at the instant k and the assumption without failure, as a function of estimated biases under the assumption of the emergence of a failure at the instant k;
(b3) Estimating the instant of emergence of the likeliest failure {circumflex over (k)} from likelihood ratios;
(b4) Selecting the bias vector corresponding to the assumption of the emergence of a failure at the instant {circumflex over (k)}.
9. The method according to claim 8 , in which step (b) also comprises an alert sub-step (b5) if the likelihood ratio associated with the instant {circumflex over (k)} reaches a predefined threshold.
10. The method according to any one of claims 7 to 9 , in which a bias vector b t under the assumption of the emergence of a failure at an instant k is obtained from the innovation calculated by a Kalman filter at the instant k, said Kalman filter performing step (a).
11. The method according to claim 7 , in which the bias are expressed as a function of time t and of the index i linked to a pseudorange, in the form b i,t [k]=α i αβ i (t−k).
12. The method according to claim 11 , in which the representative values of the probability of failure for each pseudorange obtained at step (c) are failure criteria Γ i expressed in the form Γ i =[{circumflex over (α)} i ,{circumflex over (β)} i ](C i ) −1 [{circumflex over (α)} i ,{circumflex over (β)} i ] T .
13. The method according to claim 1 , in which the M pseudoranges the likeliest to present failure are the M pseudoranges associated with the M largest values of {Γ i } iε[1,N] , M being the given number of pseudoranges to be pre-selected.
14. The method according to claim 5 , in which the combination of the likeliest faulty measurements is the combination associated with the ratio R E P such as R E P =max{R j } jε[1,2 M ] .
15. An integrity-control system ( 20 ), comprising data-processing means ( 21 ), linked to a receiver ( 10 ) receiving signals transmitted by different satellites of a radio-positioning constellation and supplying the system ( 20 ) with pseudoranges measured between satellites of said constellation and the receiver ( 10 ) on which the means ( 21 ) execute a method according to claim 1 , on completion of which a signal is transmitted to the receiver ( 10 ).
16. A system according to claim 15 , characterised in that it is coupled to an inertial navigation device ( 30 ) according to an AAIM context.
17. A vehicle ( 1 ) equipped with a system according to claim 15 or 16 .