System and method for monitoring integrity of a global navigation satellite system

A system and method for monitoring integrity of a Global Navigation Satellite System (GNSS) are provided. Integrity of a GNSS location is assessed based on a comparison of the GNSS location with one or more locations received from at least one other GNSS. Integrity of the GNSS location is also assessed based on a comparison of the GNSS location with one or more locations obtained from signals generated by one or more known located emitters. Integrity of the GNSS location is also assessed based on a comparison of the GNSS location with historical data, which may include contextual information of recent GNSS locations of a user equipment, measurements made by an inertial navigation system of the user equipment, and prior measurements made by the user equipment during similar paths. An integrity warning is outputted when one or more of the integrity assessments indicate a loss of integrity of GNSS.

FIELD

The subject technology generally relates to global navigation satellite systems, and, in particular, relates to systems and methods for monitoring a global navigation satellite system.

BACKGROUND

Satellite-based navigation may be used to navigate an aircraft, e.g., during landing of the aircraft. In order to ensure the safety of the aircraft, it is important to monitor the integrity of the navigation locations. Present integrity monitoring measures the consistency of locations obtained from different combinations of satellite signals within a constellation to detect anomalous signals from individual satellites. However, this approach does not handle the case of an anomaly in the control segment of a global navigation satellite system which could affect some or all satellites in the system, or the case of intentional spoofing of satellite signals in the region of the navigation user equipment.

SUMMARY

According to various aspects of the subject technology, a method for monitoring integrity of a Global Navigation Satellite System (GNSS) is provided. The method comprises assessing integrity of a GNSS location based on a comparison of the GNSS location with one or more locations received from at least one other GNSS. The method further comprises assessing integrity of the GNSS location based on a comparison of the GNSS location with one or more locations obtained from signals generated by one or more known local fixed emitters. The method further comprises assessing integrity of the GNSS location based on a comparison of the GNSS location with historical data, wherein the historical data comprise at least one of contextual information of recent GNSS locations of a user equipment, measurements made by an inertial navigation system of the user equipment, and prior measurements made by the user equipment during similar paths. The method further comprises outputting an integrity warning when one or more of the integrity assessments indicate a loss of integrity of the GNSS.

According to various aspects of the subject technology, a system for monitoring integrity of a GNSS system is provided. The system comprises a navigation user equipment, wherein the navigation user equipment is configured to assess integrity of a GNSS location obtained from the GNSS against one or more locations obtained from at least one other GNSS. The navigation user equipment is further configured to assess integrity of the GNSS location against locations obtained from signals generated by one or more known local fixed emitters. The navigation user equipment is further configured to assess integrity of the GNSS location based on historical data, wherein the historical data comprise at least one of contextual information of recent GNSS locations of a user equipment, measurements made by an inertial navigation system of the user equipment, and prior measurements made by the user equipment during similar paths. The navigation user equipment is further configured to output an integrity warning when one or more of the integrity assessments indicate a loss of integrity of the GNSS. The system further comprises a first communication interface configured to receive fixed emitter information and the historical data from a centralized database, and to load the received fixed emitter information and historical data into the user equipment prior to transiting a path. The system further comprises a second communication interface configured to receive navigational data of the user equipment, and to load the navigational data of the user equipment after transiting a path into the centralized database to update the historical data.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth to provide a full understanding of the subject technology. It will be apparent, however, to one ordinarily skilled in the art that the subject technology may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail so as not to obscure the subject technology.

A method and system for monitoring the integrity of locations from a Global Navigation Satellite System (GNSS) are disclosed. According to one aspect, the system can detect loss of integrity even should all satellite signals in a GNSS constellation simultaneously lose integrity while maintaining mutual consistency, such as from a control segment anomaly or intentional spoofing in the region of the navigation user equipment.

A GNSS location service such as the United States' Global Positioning System (GPS) may include an integrity function to monitor for reliable location information (for example, information related to a geographic location). The system may include one or more sensors for monitoring GNSS (e.g., GPS) signals and other information associated with the GNSS. The system assesses integrity of a GNSS location based on a comparison of the GNSS location with locations received from at least one other GNSS. Examples of GNSSs that the system may use include European Galileo, GLObal'naya NAvigatsionnaya Sputnikovaya Sistema (Global Navigation Satellite System or GLONASS), and China's Compass or Beidou. A GNSS may be augmented by a Satellite-Based Augmentation System (SBAS) (e.g., to improve accuracy, reliability, and availability). Examples of SBASs may include Wide Area Augmentation System (WAAS), and European Geostationary Navigation Overlay Service (EGNOS), etc.

The system also assesses integrity of the GNSS location based on comparisons of the GNSS location with locations obtained from signals generated by known local fixed emitters. Examples of known local fixed emitters include cellular base stations, radio stations, navigational beacons, etc. In one example, the system monitors signals generated by known local fixed emitters and determines location based on the known positions of the emitter and measurements of the signals using Frequency Difference of Arrival (FDOA), Time Difference of Arrival (TDOA), joint FDOA and TDOA or other technique.

The system further assesses integrity of the GNSS location based on comparing the GNSS location with historical data. Historical data may include recent GNSS locations of a user equipment, measurements made by inertial navigation system of the user equipment, and prior measurements made by the user equipment during similar flight paths. In one example, if an aircraft is flying at 600 mph, and a new (for example, current) GNSS (e.g., GPS) navigation fix is such that the aircraft would have had to fly 1800 mph from the last navigation fix to reach the new navigation fix, then the accuracy of the new navigation fix can be considered to be suspect. In another example, if the navigation history of a specific airline flight demonstrates that the airline flight always approaches to the East runway, and a new navigation fix indicates an approach to the South runway, then the new navigation fix may be considered suspect.

The system may fuse inputs from the GNSS being monitored, another GNSS, signals from known local fixed emitters, user equipment inertial navigation systems, and context and history from previous user equipment navigation paths, make one or more assessments of the integrity of the GNSS based on one or more of the inputs, and output an integrity warning when one or more of the integrity assessments indicate a loss of integrity of the GNSS. In one example, the integrity warning is outputted when one of the integrity assessments indicate a loss of integrity of the GNSS. In another example, the integrity warning is outputted when two of the integrity assessments indicate a loss of integrity of the GNSS.

FIG. 1illustrates an overall diagram of an exemplary integrity application to an aircraft. InFIG. 1, a GNSS (e.g., GPS)101provides navigation signals to a user equipment navigation receiver102, which is located in aircraft103. The user equipment receiver also receives signals from other GNSS107(e.g., the Galileo System, Global Navigation Satellite System, the Beidou System, etc.) and compares locations based upon received signals from other GNSS107with locations based upon signals received from GNSS101. Furthermore, the user equipment receiver also receives signals from known local fixed emitters108and compares locations based upon the received signals from known local emitters108with locations based upon signals received from GNSS101. InFIG. 1, data of known local emitters108, such as their geographical locations, are stored in emitter file109. Furthermore, the user equipment102has access to measurements made by inertial navigation system110. Furthermore, the user equipment receiver also assesses data stored as navigation history files111for historical data of past history of navigation paths. The emitter data file109and navigation history file111are supported from a centralized database112to provide storage and updates to this information.

The system compares locations based upon received signals of GNSS101with locations based upon received signals from other GNSS107to assess integrity of GNSS101. The system also compares locations based upon the received signals of GNSS101with locations based upon received signals from known local emitters108to assess the integrity of GNSS101. The system also compares locations based upon the received signals of GNSS101with measurements made by inertial navigation system110. The system also compares locations based upon the received signals of GNSS101with historical data stored in navigation history file111to assess the integrity of GNSS101. The system outputs an integrity warning, which may be provided to an integrity monitor104, when one or more of the integrity assessments indicate a loss of integrity of the GNSS.

In one example, GNSS101provides user equipment navigation receiver102with signals corresponding to inaccurate coordinates of aircraft103's location. This may be due to a control segment105anomaly in GNSS101or other cause. In this example, locations based upon signals received from GNSS101are compared with locations based upon signals received from other GNSS107, locations based upon signals received from known local fixed emitters108and historical data stored in navigation history file111. The system, upon determining that locations based upon signals received from GNSS101do not match locations based upon signals received from other GNSS107, locations based upon signals received from known local fixed emitters108and/or historical data stored in navigation history file111, outputs an integrity warning to integrity monitor104. In another example, an intentional spoofing106of user equipment navigation receiver102causes user equipment navigation receiver102to receive signals corresponding to inaccurate coordinates of aircraft103's location. The locations based upon received signals are also compared with locations based upon signals received from other GNSS107, locations based upon signals received from known local fixed emitters108and historical data stored in navigation history file111to determining integrity of the received signals, and an integrity alert is generated upon determining that integrity has been compromised.

Integrity Monitoring via Other Global Navigation Satellite Systems (GNSS)

FIG. 2illustrates example Global Navigation Satellite Systems. Block201represents Global Positioning System (GPS) GNSS. Block202represents Galileo GNSS. Block203represents Global Navigation Satellite System (GLONASS) GNSS. Block204represents Beidou GNSS. Each of the GNSS systems may represent GNSS101. Each of the GNSS systems may represent other GNSS107. In one example, GNSS101is GPS GNSS201, and other GNSS107include Galileo GNSS202, GLONASS GNSS203and Beidou GNSS204.

FIG. 3illustrates an example process for assessing integrity of GNSS based on comparing the GNSS location with one or more locations received from other GNSS. In block301, system receives location of aircraft103as determined from GNSS101. In block302, system receives location of aircraft103as determined from other GNSS107. Integrity of the GNSS location is assessed in block303by comparing the location of aircraft103as determined from signals from GNSS101to the location of the aircraft as determined from other GNSS107. InFIG. 3, the comparison is made by computing the distance between the two determined locations. In block305, if the difference between the two determined locations is greater than a first threshold value, the process proceeds to block306, where an integrity warning is outputted and provided to an integrity monitor104. If the difference between the two determined locations is not greater than the first threshold, the integrity warning is not outputted in block307.

Integrity Monitoring via Known Local Fixed Emitters

FIG. 4illustrates example types of known local fixed emitters. Block401represents Cellular base stations, which are ubiquitous worldwide. Block402represents radio stations. Block403represents navigational beacons, which are especially pertinent to the scenario of aircraft landings at an airport or air base. Additional types of known local fixed emitters may generate signals that are used to assess integrity of the GNSS location.

FIG. 5illustrates four example methods of navigating based upon signals from fixed emitters which may be known and found local to the area of a user navigation receiver. Measuring the Doppler shift of the frequency of the signals501provides a means of navigation based upon the known locations of the emitters, including the method disclosed in U.S. Pat. No. 8,040,275, “Method and Apparatus for Geographic Positioning,” issued Oct. 18, 2011, and incorporated by reference herein in its entirety. Measuring the time difference of arrival of the signals502provides a means of navigation based upon the range from known locations of the emitters. Measuring the angle of arrival of the signals503provides a means of navigation based upon the known locations of the emitters. Measuring the received signal strength of the signals504provides a means of navigation based upon the range from known locations of the emitters.

FIG. 6illustrates a flow diagram of the top level logic for using known local fixed emitters as an input to integrity monitoring. In block601, system receives location of aircraft103as determined from signals from GNSS101. In block602, system receives location as determined based on measurements of signals from known local fixed emitters108. In block603, a difference is computed between the location as determined from signals from GNSS101and the location as determined from the known local fixed emitter108. In block605, the difference is compared to a second threshold. If the difference is over the second threshold, then an integrity monitoring warning can be provided in block606. Alternatively, if the difference between the two determined locations is not greater than the second threshold, the integrity warning is not outputted in block607. This approach detects both the case of a fault in the GNSS control segment propagating to all the satellites in that system, as well as intentional spoofing of all the signals in that GNSS, through use of the independent information from the known local fixed emitters602.

In one example, the second threshold has a value that is different than the value for first threshold. In one example, the values of the first threshold and the second threshold may be based on accuracy of determining the location of the aircraft based on signals from other GNSS107and from known local fixed emitters108. In a case where the location of the aircraft can be more accurately determined from other GNSS107, the first threshold value may be smaller than the second threshold value. Alternatively, if the location of the aircraft can be more accurately determined from signals from known local fixed emitters108, the second threshold value may be smaller than the first threshold value.

Integrity Monitoring via Past Navigation History

FIG. 7illustrates a flow diagram of the top level logic for using past navigation history to train and operate a Hidden Markov Model (HMM). The HMM method of utilizing contextual knowledge to enhance understanding of navigation sensor inputs, includes the method disclosed in U.S. application Ser. No. 12/858,034, “Contextually Aware Monitoring of Assets,” filed Aug. 17, 2010 , and incorporated by reference herein in its entirety. Past navigation history for a specific path701is accumulated over multiple days 1, 2, . . ., N. Past navigation history for additional N paths701is also accumulated over multiple days 1, 2, . . ., N. This navigation history data is used to train the HMM702for a user planning to transit a navigation path that has been transited many times before in the navigation history701. The HMM is operated703in the user navigation receiver during transit of the navigation path. The HMM703takes as an input the GNSS locations704from the navigation receiver being monitored for integrity. The HMM703also takes in measurements from a user inertial navigation system705. Based upon the training702and the external inputs704and705, the HMM703predicts the navigation path706. This predicted path706may be compared with past navigation history to determine whether it is inconsistent707. Should the predictions be inconsistent with the past history then an integrity monitoring warning708can be provided. Should the predictions be consistent with the past history then the HMM703can be updated from the external inputs704and705to refine the accuracy of the HMM for making future path predictions706.

FIG. 8illustrates a flow diagram of the top level logic for using past navigation history as an input to integrity monitoring. In block801, system receives a location from the GNSS101being monitored for integrity. In block802, the system receives measurements from a user inertial navigation system. In block803, the system receives a location from previous navigation history. In block804, the locations and measurements from blocks801-803are compared to determine one or more differences between the location from GNSS101and one or more of the locations from blocks802and803. This comparison may be for example between a military supply aircraft that has been landing at an airbase daily for months with the same landing approach, to the navigation receiver location solution which is being monitored for integrity. Any variance in the GNSS locations as compared to the same landing paths previously taken could indicate an anomaly or integrity condition with that GNSS location.

In block805, the difference is compared to a third threshold. If the difference is over the third threshold then an integrity monitoring warning can be provided in block806. Alternatively, if the difference is not greater than the third threshold, then no integrity warning is outputted in block807. Note that this approach detects both the case of a fault in the GNSS control segment propagating to all the satellites in that system, as well as intentional spoofing of all the signals in that GNSS, through use of the independent information from the inertial navigation system802and the past navigation history803.

Fusion of Multiple Inputs for Integrity Monitoring

FIG. 9illustrates a flow diagram of the top level logic for fusing multiple inputs for integrity monitoring. The multiple inputs shown include integrity assessment from other GNSS in block901, integrity assessment from known local fixed emitters in block902, and integrity assessment from historical navigation data in block903. Each integrity assessment may indicate a loss of integrity, for example, when the respective difference exceeds the respective threshold. In one example, when one or more of these inputs indicate a loss of integrity in block904, an integrity monitoring warning is provided in block905. Alternatively, when none of these inputs indicate a loss of integrity in block904, an integrity warning is not provided in block907. In another example, when two or more of these inputs collectively indicate a loss of integrity, an integrity monitoring warning is provided in block905. Alternatively, if two or more of these inputs do not collectively indicate a loss of integrity, an integrity warning is not provided in block907.

Consider the example discussed forFIG. 8in which a military supply aircraft has been landing at an airbase on a daily basis for months with the same landing approach. An adversary could arrange to spoof signals not only from one GNSS but from all GNSS, and/or could jam signals from local fixed emitters either as part of an attack upon the military aircraft landing or as another part of a military campaign. Fusion of the integrity monitoring check based upon previous navigation history could show that the spoofed GNSS signals are inducing false locations causing the aircraft to descend at much lower altitudes than previously recorded in past landings, so that the integrity warning provided could warn off a crash condition. It is through fusion of all of these methods of monitoring integrity that the error cases of multiple or all anomalous satellite signals in a GNSS constellation or constellations may be monitored and detected. Though this example was provided of a military scenario, this method for integrity monitoring also applies to other scenarios such as terrorism versus domestic airports.

System with Centralized Database to Provide Advanced Integrity Monitoring

FIG. 10illustrates a system level view of the concept of the disclosure. One or many sets of user equipment navigation receivers102are supported by the system, to provide emitter file data109and navigation history file data111to the navigation receiver102prior to transiting a navigation path. The data to populate each of these files109and111is stored and maintained in a centralized system database112. A first communications interface1001is used to load emitter data and navigation history data1002into the user navigation receiver102prior to transiting the navigation path. After the path has been transited, a second communications interface1003is used to return the recent navigation data1004for update to the centralized system database112.

FIG. 11illustrates examples of the centralized system database. The database of fixed emitters1101may contain fields of emitter location, emitter identification and emitter frequency. The database of user navigation history1102may contain waypoints with fields of location, time, velocity, acceleration and heading.

All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims.