DE102013107994B3 - Method for determining ionosphere gradient for landing airplane, involves transmitting satellite data messages to receiving stations of satellite navigation system through different transmission paths of satellites - Google Patents

Abstract

The method involves transmitting satellite data messages (11-14) to receiving stations (6) of a satellite navigation system through different transmission paths of satellites (1,2) of the satellite navigation system. The transmitted messages are evaluated. The determinant of the ionosphere gradient (10) is obtained during evaluation between satellite data messages. Independent claims are included for the following: (1) a computer program for determination of ionosphere gradient; and (2) an ionosphere gradient monitor.

Description

The invention relates to a method for determining an ionospheric gradient according to claim 1. The invention further relates to a computer program, with which such a method is carried out, according to claim 7 and an ionosphere gradient monitor with such, executed on a processor computer program according to claim 8.

In general, the invention relates to the field of monitoring the influences of the ionosphere on radio signals. In particular, the signals from global satellite navigation systems, such as the GPS or Galileo, are influenced by changes in the ionosphere, which can be triggered, for example, by solar flares. This, in turn, has an impact on ground-based augmentation systems for such satellite navigation systems, such as the GBAS used in aviation. Such augmentation systems, which are based on single-frequency transit time measurements, are currently certified for Category 1 instrument approaches up to 200-foot decision height and further developed for automatic landings without external vision (Category 3c). Because of the high demands on accuracy, availability and integrity, runtime errors caused by the ionosphere are a considerable source of error. Since such augmentation systems can be designed as differential systems, the difference between the propagation time errors on the ground and on the aircraft in particular plays a role important role. Such runtime errors can be caused by an ionosphere gradient. To certify GBAS for category 3c landings, ionospheric gradient monitoring is required to detect run-time errors caused by an ionospheric gradient with regard to their effect on the resulting position error greater than 1.5 m with a defined error detection probability (see frozen guest assessment). D Standards and Recommended Practices SaRPs incorporating the new version of ICAO Annex 10, Volume 1, Chapter 3.6.7.3.4.).

From the publication by Khanafseh S .; Pullen, S. & Warburton J. "Carrier Phase Ionospheric Gradient Ground Monitor for GBAS with Experimental Validation," Journal of Navigation, 2012, 59, 51-60 is a proposal known for ionospheric gradient monitoring. However, the proposal is relatively sensitive to the placement of the antennas of the ground receiving station. Even shifts in the millimeter range lead to evaluation problems. For example, such a millimeter vibration can be triggered by wind.

A generic method for determining an ionospheric gradient is also known from the publication by JING, Jing, et al. Detecting ionospheric gradients for GBAS using a zero space monitor. In: Position Location and Navigation Symposium (PLANS), April 2012 IEEE / ION. IEEE, 2012. p. 1125-1133.

The invention is therefore based on the object of specifying a method which can be better implemented in practice for the determination of an ionospheric gradient, which has a high accuracy of determination and, in particular, fulfills the requirements for category 3c for instrument approaches. Furthermore, a suitable computer program and an ionosphere gradient monitor should be specified.

This object is achieved according to claim 1 by a method for determining an ionospheric gradient, at least two satellite data messages are transmitted via at least two different transmission paths from one or more satellites of a satellite navigation system to one or more receiving stations of the satellite navigation system and then evaluated, wherein the evaluation by subtraction between the data contained in the satellite data messages or data of the same type determined therefrom, an ionospheric gradient determination quantity is obtained. The invention has the advantage that a highly accurate determination of an ionosphere gradient is made possible with simple and inexpensive means. The proposed method is characterized by insensitivity to low position variations of the antennas, as z. B. can occur due to wind, and is thus very practical implementation. In particular, the proposed method can certify GBAS systems and category 3c landings. Another advantage over the prior art mentioned above is that in the present invention the non-detectable zone present there, which is caused by the carrier phase ambiguity of the satellite navigation receiver, does not exist. Rather, in the present invention, a carrier phase measurement is not required, so that thereby caused further disadvantages are avoided. In some satellite navigation receivers, the carrier phase measurement is not very robust, because it can easily lead to phase jumps (so-called cycle clips).

The present invention thus eliminates sources of error such as wind, ambiguities in carrier phase measurement, and phase center variations. The method is very robust against bottom-side influences.

According to the invention, a subtraction between data contained in the satellite data messages or data of the same type determined therefrom is carried out during the evaluation, and from this a determination variable of the ionospheric gradient is obtained. All that is required for this is to evaluate two satellite data messages on the ground side, and it makes sense to have them near a runway supported by a GBAS system. It is important that the two satellite data messages are evaluated over two different transmission paths. The two different transmission paths may be formed, for example, by the satellite data messages originating from the same satellite but being received at the bottom at different positions. The said diversity of the satellite data messages can also be generated by receiving data from two satellites located at different positions at the same position on the ground. This also forms two different transmission paths. Combinations of the aforementioned possibilities are also advantageously feasible, such as the use of two satellites and two reception positions on the ground. The two different transmission paths hereby ensure a certain spatial diversity of the satellite data messages, so that the different ways in which the satellite data messages travel through the atmosphere make it possible to detect a gradient in the ionosphere.

To realize the ground-side different receiving positions, for example, multiple receiving stations can be used with their respective antennas at different positions, or a receiving station can be used with differently positioned antennas. Again, a combination of these options is feasible.

The receiving stations or their antennas need not necessarily be permanently in the same position. It is only necessary that during a detection period in which the two satellite data messages are transmitted and received via different transmission paths, one or more receiving stations of the satellite navigation system and / or their antennas are fixedly positioned in the region of the earth's atmosphere below the ionosphere. The evaluation of the received satellite data messages may then take place during the acquisition period or after.

By determining the difference of certain data contained in the satellite data messages or data calculated therefrom, a determination parameter for the ionospheric gradient can be obtained directly. The determinant may directly indicate the ionospheric gradient numerically and / or include a detection signal, in the sense of a binary signal, indicating whether the ionospheric gradient exceeds a predetermined threshold.

Ultimately, it is important for ground-based augmentation systems that the pilots of approaching aircraft can be given the information as to whether or not the augmentation system is operating with sufficient accuracy for the landing task desired. Then the pilot can take over manual control at any time.

Another advantage of the invention is that it is relatively inexpensive in the form of software, that is, by a computer program, feasible. In particular, apparative supplements of a ground-based augmentation system are not required. This allows a cost-effective implementation of the invention.

According to an advantageous development of the invention, at least two of the receiving stations involved in the method or at least two antennas of the same receiving station or of different receiving stations are arranged parallel to the runway of an airport. According to a further advantageous development, at least two of the receiving stations involved in the method or at least two antennas of the same receiving station or of different receiving stations are arranged on both sides of the landing threshold of the airport runway. A combination of both developments is advantageous. In this way, the ionosphere gradient monitoring can be referred in particular to the particularly relevant area of the airport, in which a landing aircraft relies on a particularly accurate GBAS support. Especially in the area of the landing threshold, where the target landing point of landing aircraft is, so that a high level of security can be achieved.

• – Troposphäreneinfluss (T j / i),
• – Multipfad-Einfluss (MP j / i),
• – Empfängeruhrzeitdivergenz (cΔt_i),
• – Satellitenuhrzeitdivergenz (cΔt_j)_i
• – Konstantanteil des Ionosphäreneinflusses (I₀).

According to an advantageous development of the invention, at least one, several or all of the following influencing variables are eliminated in the method:

• - tropospheric influence (Tj / i),
• - Multipath influence (MP j / i),
• Receiver time divergence (cΔt _i ),
• - satellite clock time divergence (cΔt _j ) _i
• - Constant part of the ionospheric influence (I ₀ ).

This has the advantage that possible error influences in the determination of the ionosphere gradient, i. H. in the calculation of the determinant of the ionospheric gradient, be largely eliminated in the process according to the invention.

According to an advantageous embodiment of the invention, the determination quantity is transmitted to a landing aircraft. The transmission of the determinant may, for example, be effected by a data message from an airport ground station to landing aircraft. The systems present in the aircraft can then use the transmitted determinant for an automatic evaluation, for example to generate an automatic warning message for the pilot, or to certain automatic systems that are not or not fully deployed due to an excessive ionospheric gradient, automatically lock against activation.

The object mentioned at the outset is also achieved according to claim 7 by a computer program with which a method of the previously described kind is executed when the computer program is executed on a processor. The processor may be implemented, for example, as a microprocessor, microcontroller, FPGA or other computer.

The object mentioned at the outset is also achieved according to claim 8 by an ionospheric gradient monitor having at least one processor, a computer program of the type specified above, and receiving means for receiving the at least two satellite data messages. The ionospheric gradient monitor can be designed as a separate device. According to an advantageous embodiment of the invention, the ionospheric gradient monitor is part of a ground-based augmentation system of a satellite navigation system, in particular GBAS system, or is coupled to such a system.

The invention will be explained in more detail using an exemplary embodiment using drawings.

Show it:

1 A schematic representation of an application of the invention in a GBAS system and

2 A diagram representing the minimum detectable error plotted over the elevation angle, and

3 A diagram showing the relationship between the distance between the reception positions for the two different satellite data messages over the elevation angle, plotted for various minimally detectable ionospheric gradients.

The 1 shows an example of two satellites 1 . 2 which are part of a satellite navigation system, for example GPS or Galileo. The satellites are in space. The signals of the satellites 1 . 2 can be received on the ground, for example from a receiving station 6 that with antennas 3 . 4 . 5 connected is. The antennas 3 . 4 . 5 are arranged at different positions. There may also be several receiving stations distributed at different positions. The receiving stations 6 or their antennas 3 . 4 . 5 are near a runway 7 an airport. It is advantageous, the receiving stations 6 or antennas 3 . 4 . 5 near a landing threshold 8th the runway 7 to arrange. The landing threshold 8th A runway is the point intended for landing aircraft to land.

By way of example, it is shown that from the satellite 1 Satellite data messages 11 . 12 . 13 from the antennas 3 . 4 . 5 be received. From an airplane 9 becomes a satellite data message 14 receive.

That in the 1 illustrated aircraft 9 should be on the runway 7 land. The landing can be supported by a GBAS system, so that the landing takes place fully automatically until touchdown. However, this is only possible with sufficient certainty if the aircraft 9 received satellite data messages 14 have substantially the same run-time errors caused by the ionosphere and other influences as those of the antennas 3 . 4 . 5 received satellite data messages 11 . 12 . 13 , If the runtime difference becomes too large, the positioning accuracy of the GBAS system becomes too poor for an automatic landing. According to the ICAO specification, the position error may not exceed 1.5 m at the landing threshold 8th be. This critical limit is already reached at an ionospheric gradient of 300 mm per kilometer of spatial extent.

The 1 also exemplifies an ionosphere front 10 that passes the airport area. By way of example, it is shown how the ionosphere density changes over a distance b by a value d. This results in a gradient g of the ionosphere front 10 , The ionosphere front 10 For example, it moves at a speed v in the direction of the arrow shown.

The ionosphere gradient is in 1 thereby represented with a measure for the transit time shift. The satellite data messages 11 . 12 . 13 go through high-delay areas, the satellite data message 14 an area with less propagation delay. This results in errors in the GBAS system. The receiving station 6 evaluates the satellite data messages 11 . 12 . 13 in the manner described below and determines therefrom a determinant for the ionospheric gradient 10 , Exceeds the ionospheric gradient 10 a critical limit of 1.5 m positioning error of the GBAS system, a corresponding detection signal via a data communication 15 from the receiving station 6 to the plane 9 sent. In the plane 9 the pilot is informed of the fact that there is an excessively high ionospheric gradient for an automatic landing approach using GBAS. The pilot can then take over the manual control.

The ionosphere layer is a dispersive medium in the upper part of the atmosphere, which is composed of ionized gas. Solar radiation generates free electrons and ions which lead to propagation delays of electromagnetic waves and consequently also influence satellite data messages. The ionosphere layer is not constant, but varies both temporally and spatially. As the determinant of the ionospheric gradient, a value expressed in mm / km can be determined, indicating the difference in ionospheric delays between two points one kilometer apart. For the determination of the ionospheric gradient, the following is proposed for an implementation of the present invention. It is initially assumed that the following model for the so-called pseudorange. ρ j / i = rj / i + I j / i + T j / i + c (Δt ^j + Δt _i ) + MP j / i + ε j / i (1)

i

laufender Index für die Empfangsstation bzw. die Empfangsantenne,

j

laufender Index für den Satelliten,

r j / i

der tatsächliche Abstand (range) zwischen Satellit j und Empfänger bzw. Empfangsantenne i,

I j / i

der Ionosphäreneinfluss,

T j / i

der Troposphäreneinfluss,

cΔt^j

die Satellitenuhrzeitdivergenz,

cΔt_i

die Empfängeruhrzeitdivergenz,

MP j / i

der Multipfad-Einfluss,

ε j / i

das Empfängerrauschen, das Gauß-verteilt mit N (μ = 0, σ_epsilon) sei.

The following variables are used:

i

running index for the receiving station or the receiving antenna,

j

running index for the satellite,

r j / i

the actual distance (range) between satellite j and receiver or receiving antenna i,

I j / i

the ionospheric influence,

T j / i

the troposphere influence,

cΔt ^j

the satellite clock time divergence,

cΔt _i

the receiver time divergence,

MP j / i

the multipath influence,

ε j / i

the receiver noise, which is Gaussian distributed with N (μ = 0, σ _epsilon ).

The pseudo-orange correction results for each receiving station i and each satellite j as follows: PRC j / i = ρj / i - rj / i - cΔt ⁱ (2)

Here is rj / i the actual distance and cΔt ⁱ the satellite clock correction. Accordingly contains PRC j / i the mistakes for every pseudo orange ρ j / i. PRC j / i = Ij / i + Tj / i + cΔt _i + MPj / i + εj / i (3)

The ionospheric influence contained herein can be given as follows: I j / i = I ₀ + f _I (λ, φ) (4)

Here, I _{0 is} a constant and f _I (λ, φ) is a function of the latitude λ and longitude φ, based on the puncture point through the ionosphere. The same applies to the tropospheric influence, which can be stated as a function of the elevation angle θ and the air humidity h as follows: T j / i = T ₀ + f _T (θ, h) (5)

Accordingly, the multipath influence can be given as a function of the azimuth angle Φ and the elevation angle θ of a satellite as follows: MP j / i = MP ₀ + f _MP (Φ, θ) (6)

T ₀ and MP ₀ are here again respective constants.

For each receiver, a receiver time correction is now performed as follows:

This is used to average over all PRC values (according to the expression given above the curly brace) of a receiving station i. The averaging can be designed, for example, as a moving averaging. By this step, a receiver clock time divergence cΔt ⁱ included in all measurements of the same receiving station is eliminated. Furthermore, the constant components of the ionospheric influence, the tropospheric influence and the multipath influence are eliminated by this step. Therefore: PRC j / sca, i = f _I (λ, φ) + f _T (θ, h) + f _MP (Φ, θ) + ε ^ j / i (8)

als Ergebnis der Fehlerfortpflanzung. Der bodenstations-spezifische zufällige Multipfad-Einfluss wird dabei während der Installation abgeschätzt und in einer Größe σ_pr,gnd, berücksichtigt. Die Größe σ_pr,gnd charakterisiert das Rauschen und den Multipfad-Einfluss nach Durchführung der Empfängeruhrzeitkorrektur gemäß Gleichung 7 als eine Zufallsvariable, die normal verteilt ist. Es ist erforderlich, die Größe σ_pr,gnd erstmalig für eine bestimmte Empfangsstation zu ermitteln und später kontinuierlich während des Betriebs einer Bodenstation eines bodengestützten Augmentierungssystems zu aktualisieren. Dementsprechend kann der Einfluss der Funktion f_MP in der Größe σ_ε = σ_pr,gnd(θ j / i) berücksichtigt werden. Fortan kann die Größe σ_pr,gnd zur Charakterisierung des Rauschens der Bodenstation und des Multipfad-Einflusses als eine um den Mittelwert Null normalverteilte Größe genutzt werden, die abhängig vom Elevationswinkel ist. Aus Sicherheitsgründen wird bei GBAS-Systemen ein gewisser Überschuss berücksichtigt. Hierzu kann entweder das Maximum der Größe σ_pr,gnd für einen gegebenen Elevationswinkel verwendet werden, oder es wird der nächst größere „airborne accuracy designator” für alle Stationen eines GBAS-Systems verwendet.Here, the indices i and j have been omitted in the angular arguments of the functions. ε ^ j / i is also Gauss-distributed with

as a result of error propagation. The ground station-specific random multipath influence is estimated during the installation and taken into account in a quantity σ _{pr, gnd} . The quantity σ _{pr, gnd} characterizes the noise and the multipath influence after performing the receiver time correction according to Equation 7 as a random variable which is normally distributed. It is necessary to first determine the quantity σ _{pr, gnd} for a particular receiving station and later to update it continuously during the operation of a ground station of a ground based augmentation system. Accordingly, the influence of the function f _MP in size σ _ε = σ _{pr, gnd} (θ j / i) be taken into account. From _now on, the quantity σ _{pr, gnd} for characterizing the noise of the ground station and the multipath influence can be used as a variable normally distributed around the mean zero, which is dependent on the elevation angle. For security reasons, GBAS systems take into account a certain surplus. For this either the maximum of the size σ _{pr, gnd} for a given elevation angle can be used, or the next larger "airborne accuracy designator" is used for all stations of a GBAS system.

By forming the difference between two adjusted pseudorange corrections of two satellite data messages i and k, which according to the invention were transmitted via different transmission paths, the following results: PRC j / sca, ik = PRC j / sca, i - PRC j / sca, k = f _I (λ j / i, φ j / i) - f _I (λ j / k, φ j / k) + f _T (λ j / i, φ j / i) - f _T (λ j / k, φ j / k) + ε (9)

The difference f _T (λ j / i, φ j / i) - f _T (λ j / k, φ j / k) behaves like the remaining tropospheric influence after applying the corrections in a differential satellite navigation system. This remaining tropospheric impact may be determined in accordance with document RTCA DO 253C, "Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment", Radio Technical Commission for Aeronautics, Tech. Rep. 253C, 2008, as follows:

Here Δh is the height difference between two receiving positions of a satellite navigation system, that is, for example, between two receiving stations or between two antennas of one or more receiving stations. θ is the elevation angle of the satellite. N _R is the refractive index, h ₀ is a tropospheric scale height of a ground station type 2 message broadcast. As can be seen, the correction factor at antennas or receiving stations at the same height is zero. At different heights, a residual error arises mainly due to modeling divergences of the refractive index.

This error σ _tropo must be taken into account when the antennas are at different heights.

Assuming the same height of the antennas: PRC j / sca, ik = PRC j / sca, i - PRC j / sca, k = f _I (λ j / i, φ j / i) - f _I (λ j / k, φ j / k) + ε (12)

The distribution of uncertainty in the obtained difference size PRC j / sca, ik, reproduced by ε , Gaussian is distributed with a mean of zero, as shown below:

For a GAST-D system, the manufacturer must comply with a GAD C soil accuracy expert. This is in the 2 for the required course to GAD C, plotted over the elevation angle, reproduced. The 2 also shows by way of example the accuracy _results achieved by the method _{according to} the invention, represented by the profile of the quantity σ _{pr, gnd} . As you can see, the requirements are met.

The 3 shows the minimum detectable error on the basis of a family of curves, in which the distance between the reception positions (antenna distance) was varied as a parameter to the following values: 300 m, 400 m, 500 m, 600 m and 1,000 m.

Claims (9)

Method for determining an ionospheric gradient ( 10 ), characterized in that at least two satellite data messages ( 11 . 12 . 13 . 14 ) via at least two different transmission paths from one or more satellites ( 1 . 2 ) of a satellite navigation system to one or more receiving stations ( 6 ) of the satellite navigation system and are evaluated thereafter, wherein the evaluation by difference formation between in the satellite data messages ( 11 . 12 . 13 . 14 ) or data of the same type determined therefrom a determinant (PRC j / sca, ik) of the ionosphere gradient ( 10 ) is won. Method according to Claim 1, characterized in that at least two of the receiving stations ( 6 ) or at least two antennas ( 3 . 4 . 5 ) of the same receiving station ( 6 ) or different receiving stations parallel to the runway ( 7 ) of an airport. Method according to one of the preceding claims, characterized in that at least two of the receiving stations ( 6 ) or at least two antennas ( 3 . 4 . 5 ) of the same receiving station ( 6 ) or different receiving stations on both sides of the landing threshold ( 8th ) of a runway ( 7 ) of an airport. Method according to one of the preceding claims, characterized in that at least one, several or all of the following influencing variables are eliminated in the method of determining the ionospheric gradient: - tropospheric influence (Tj / i), - Multipath influence (MP j / i), Receiver time divergence (cΔt _i ), satellite time divergence (cΔt _j ), constant ionospheric influence (I ₀ ) Method according to one of the preceding claims, characterized in that the determination variable (PRC j / sca, ik) the ionospheric gradient ( 10 ) and / or the determinant contains a detection signal indicating whether the ionospheric gradient exceeds a predetermined threshold. Method according to one of the preceding claims, characterized in that the determination variable (PRC j / sca, ik) to a landing plane ( 9 ) is transmitted. Computer program with which a method according to one of the preceding claims is executed when the computer program is executed on a processor. An ionospheric gradient monitor having at least one processor, a computer program executed by the processor according to claim 7 and receiving means for receiving the at least two satellite data messages ( 11 . 12 . 13 . 14 ). Ionosphere gradient monitor according to claim 8, characterized in that the ionosphere gradient monitor is part of a ground-based augmentation system of a satellite navigation system, in particular part of a GBAS system, or is coupled to such a system.

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