Abstract:
A method is provided for determining position integrity in a system having a Global Navigation Satellite System (GNSS) component, such as, for example, a Global Positioning System (GPS) device. For successive alarm limits, with each alarm limit corresponding to a position integrity level, it is determined whether valid position integrity information is available. At the alarm limit at which valid position integrity information is first available, a corresponding position integrity level is determined. If no valid position integrity information is available for any of the alarm limits, a default position integrity level is then designated. An associated apparatus, system, and computer software program product are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/217,229, filed Jul. 10, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to navigational systems and, more particularly, to a method, apparatus, system, and computer software program product for determining position integrity in a navigational system having a Global Navigation Satellite System (GNSS) component. 
     BACKGROUND OF THE INVENTION 
     Global Navigation Satellite Systems (GNSS) such as, for example, GPS devices, are well known in the art and are commonly used to determine the geodetic latitude and longitude coordinates of mobile vehicles employing such devices. For simplicity, a GPS device will be discussed herein as an example of a GNSS, wherein the terms “GNSS” and “GPS” may be used interchangeably. However, it will be understood by one skilled in the art that the present invention is not restricted to a GPS device and may be applicable to other GNSS-type devices according to the spirit and scope of the present invention. 
     With a GPS device, information signals transmitted from a plurality of satellites to a GPS receiver are analyzed using known trilateration techniques in order to determine the geodetic coordinates of the receiver, wherein the geodetic coordinates are typically provided in latitude and longitude. The geodetic coordinates (latitude and longitude), however, may vary in accuracy due to, for example, atmospheric conditions, selective satellite availability, and the relevant positions of the satellites with respect to the line-of sight view of the satellites. Often associated with this variance in GPS accuracy is an integrity determination, which produces a warning if it is determined that the GPS accuracy is insufficient to be relied upon for navigational purposes. Accordingly, where a GPS integrity system is provided, a maximum horizontal position error, otherwise referred to as a “horizontal protection level” (HPL) may be determined and compared to an allowable radial error, otherwise referred to as a “horizontal alarm limit” (HAL). If the HPL is found to exceed the HAL, then a warning is issued that the geodetic coordinates should not be relied upon for accuracy. 
     One method of determining the integrity of a GPS system is the Receiver Autonomous Integrity Monitoring (RAIM) concept which is typically implemented in software in the GPS receiver and which employs an instantaneous self-consistency check during the determination of the geodetic coordinates. In order for RAIM to function as intended, a minimum plurality of satellite signals are required. Where such a minimum plurality of satellite and/or satellite signals are not available, the RAIM internal consistency check may not be available (“RAIM unavailable”), where, in turn, no horizontal position integrity information is available. In addition, the RAIM may also generate error values based upon the consistency check, which are then compared to predetermined error limits. Accordingly, should an error value exceed the corresponding error limit, a RAIM alarm may be generated to indicate the failure of the consistency check (“RAIM alarm”), where, in other words, horizontal position data may be available, but without integrity. In such instances, where RAIM is not available or a RAIM alarm is generated, the integrity of the geodetic coordinates may be questionable. Thus, there exists a need for a GNSS device capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated. 
     In some instances, the GPS device may be interfaced with other navigational equipment, wherein the GPS device may also be relied upon to provide location coordinates as well as position integrity information. For example, the GPS device may be interfaced with a Mode S transponder, via a processing unit, with the transponder configured to receive position integrity information from the GPS device as is known in the art. The position integrity information is converted into a corresponding code in a data stream, which is then transmitted by the transponder. The data thus transmitted by the transponder indicates the position of the vehicle carrying the GPS device as well as the level of integrity and/or accuracy of that position information. 
     An interfaced GPS device may be classified as, for example, a “sole means of navigation” GPS receiver (“DO-229A GPS receiver”) as identified in a document entitled “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment”, document number RTCA/DO-229A, or a “supplemental navigation” device (“DO-208 GPS receiver”) as identified in a document entitled “Minimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS)”, document number RTCA/DO-208, wherein both documents are incorporated herein by reference in their entirety. Where a “sole means of navigation” GPS receiver is available to be interfaced with the transponder, a RAIM algorithm in the GPS receiver provides an HPL to the processing unit when RAIM is available and no RAIM alarm is present. However, if RAIM is not available or a RAIM alarm is present, an HPL is not provided to the processing unit and the transponder is thus not able to transmit any position integrity and/or accuracy information. Further, where a “supplemental navigation” device is provided, such a device is typically capable of determining geodetic coordinates, but may or may not be configured to execute a RAIM algorithm. Even if a RAIM algorithm is executed by the device, the device is typically not configured to return an HPL to the processing unit since “supplemental navigation” devices are not required to be capable of determining an HPL. Thus, there also exists a need for a method of determining position integrity information when a vehicle is equipped with a “supplemental navigation” device lacking the capability of determining an HPL. In addition, there exists a further need for a “sole means of navigation” GPS receiver capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated 
     Thus, there exists a need for a GNSS device capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated. There also exists a need for a method of determining position integrity information when a vehicle is equipped with a “supplemental navigation” device lacking the capability of determining an HPL. 
     SUMMARY OF THE INVENTION 
     The above and other needs are met by the present invention which, in one embodiment, provides a method for determining position integrity in a system having a Global Navigation Satellite System (GNSS) component. For an alarm limit in a plurality of successive alarm limits, with each alarm limit corresponding to a position integrity level, it is selectively determined whether valid position integrity information is available. At the alarm limit at which valid position integrity information is first available, a corresponding position integrity level is determined. If no valid position integrity information is available for any of the alarm limits, a default position integrity level is then designated. 
     Another advantageous aspect of the present invention comprises an apparatus for determining position integrity information, and transmitting at least one of position accuracy information and position integrity information, in a system having a GNSS component such as, for example, a Global Positioning System (GPS) device. The apparatus comprises a transponder configured to transmit a type code indicative of position accuracy and/or integrity, a navigational device capable of executing a Receiver Autonomous Integrity Monitoring (RAIM) algorithm to determine whether valid position integrity information is available, and a processing unit in communication with the transponder and the navigational device. The processing unit is configured to selectively provide successive alarm limits to the RAIM algorithm executed by the navigational device, wherein the processing unit is also configured to determine a valid position integrity level corresponding to the alarm limit at which valid position integrity information is first available. The processing unit is further configured to designate a default position integrity level if no valid position integrity information is available for any of the alarm limits. The processing unit is thereafter configured to direct to the transponder at least one of the valid position integrity level, the default position integrity level, and a position accuracy level corresponding to at least one of the valid position integrity level and the default position integrity level, from which the transponder thereafter determines the corresponding type code for transmission. 
     Still another advantageous aspect of the present invention comprises a system capable of determining position integrity information, and transmitting position accuracy information and/or position integrity information, in an apparatus having a GNSS component such as, for example, a Global Positioning System (GPS) device. The system comprises a computer device having a first processing portion for directing the execution of a RAIM algorithm to determine whether valid position integrity information is available. A second processing portion selectively provides successive alarm limits to the RAIM algorithm so as to determine a valid position integrity level corresponding to the alarm limit at which valid position integrity information is first available. A third processing portion designates a default position integrity level if no valid position integrity information is available for any of the alarm limits. A fourth processing portion then directs to a transponder at least one of the valid position integrity level, the default position integrity level, and a position accuracy level corresponding to at least one of the valid position integrity level and the default position integrity level, from which the transponder thereafter determines the corresponding type code for transmission. 
     Yet another advantageous aspect of the present invention comprises a computer software program product for determining position integrity information, and transmitting position accuracy information and/or position integrity information, in a system having a GNSS component such as, for example, a Global Positioning System (GPS) device. The computer software program product comprises a first executable portion configured to direct the execution of a RAIM algorithm to determine whether valid position integrity information is available. A second executable portion selectively provides successive alarm limits to the RAIM algorithm so as to determine a valid position integrity level corresponding to the alarm limit at which valid position integrity information is first available. A third executable portion is configured to designate a default position integrity level if no valid position integrity information is available for any of the alarm limits. A fourth executable portion then directs to a transponder at least one of the valid position integrity level, the default position integrity level, and a position accuracy level corresponding to at least one of the valid position integrity level and the default position integrity level, from which the transponder thereafter determines the corresponding type code for transmission. 
     Thus, embodiments of the present invention provide a method, apparatus, system, and computer software program product for determining position integrity in a system having a GNSS component. Embodiments of the present invention also provide a GNSS device capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated. Embodiments of the present invention are further capable of determining position integrity when a vehicle is equipped with a “supplemental navigation” device lacking the capability of determining an HPL. Thus, embodiments of the present invention provide distinct advantages over prior art navigational systems having a GNSS component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some of the advantages of the present invention having been stated, others will appear as the description proceeds, when considered in conjunction with the accompanying drawings, which are not necessarily drawn to scale, in which: 
     FIG. 1 is a schematic representation of a navigational system having a GNSS component according to one embodiment of the present invention. 
     FIG. 2 is a schematic representation of an airborne position message transmitted by a transponder component of a navigational system according to one embodiment of the present invention. 
     FIG. 3 is an exemplary table of position integrity levels, representative of a set of alarm limits, and corresponding position accuracy levels, transmission type codes, and Navigation Uncertainty Categories for Position (NUC P ) values according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     FIG. 1 schematically illustrates one embodiment of a navigational system having a GNSS component, such as, for example, a GPS device, wherein the navigational system is indicated generally by the numeral  100  and includes the features of the present invention. The navigational system  100  may comprise, for example, a “sole means of navigation” (DO-229A) GPS receiver  150 , a “supplemental navigation” (DO-208) GPS receiver  200 , a processing unit  250 , and a transponder  300  with a corresponding antenna  350 . The navigational system  100  is generally configured to form and transmit a position message  550  (and as shown in FIG. 2) to a receiving subsystem  400  with a corresponding antenna  450  in communication with a terminal application  500 . 
     In an airborne position message  550  as shown in FIG. 2, the type code field  600  indicates the message type as well as a type code  650  corresponding to a Navigation Uncertainty Category for Position (NUC P ). A correlation between the type code  650  and the NUC P  value  700  is shown in FIG.  3 . The NUC P  code  700  indicates a level of confidence in the latitude  750  and longitude  800  coordinates included in the airborne position message  550 . Generally, the NUC P    700  is determined from an integrity limit known as the “Horizontal Protection Limit” (HPL)  850 , wherein an HPL  850  is typically a distance range corresponding to a discrete NUC P  value  700 , as further shown in FIG.  3 . An HPL  850  is a measure of position integrity. More particularly, an HPL  850  represents the radius of a circle in the horizontal plane centered on the true position, which describes the region which is assured to contain the indicated horizontal position, meaning that the probability of the position fix being in error by more than the HPL  850 , without a RAIM alarm being detected, is less than 10 −7  per flight hour. 
     In instances where the navigational system  100  includes a “sole means of navigation” (DO-229A) GPS receiver  150 , an HPL  850  may be readily available from the RAIM algorithm executed within the DO-229A GPS receiver  150 . The HPL  850  received from the DO-229A GPS receiver  150  is then directed through the processing unit  250  to the transponder  300 . The transponder  300 , in turn, determines and sets a type code  650  corresponding to the HPL  850  received from the processing unit  250 , wherein the type code  650  is included in the type code field  600  as part of the position message  550  transmitted to the receiving subsystem  400 . The type code  650  received by the receiving subsystem  400  is then converted to the corresponding NUC P  code  700  prior to utilization by the terminal application  500 . Thus, the terminal application  500  is provided with the geodetic coordinates (latitude and longitude) of the navigational system  100  along with the accuracy and/or integrity of the transmitted coordinates. However, in some instances, an HPL  850  may not be available from the DO-229A GPS receiver  150  if RAIM is not available and/or if a RAIM alarm exists. 
     Where the transponder  300  is a Mode S transponder, the register for the position message  550  must be updated, for example, approximately every  200  milliseconds. Generally, the transponder  300  begins transmitting the position message  550  only after valid horizontal position integrity and/or accuracy information is received from the processing unit  250 . While position integrity and/or accuracy information is available, the transponder  300  transmits the position message  550 , for instance, twice per second at random intervals that are uniformly distributed over a range of 0.4 to 0.6 seconds relative to the previous position message transmission. If more than two seconds have elapsed without the transponder  300  receiving valid position integrity and/or accuracy information, the transponder  300  may clear the type code field  600  and the latitude  750  and longitude  800  position fields. However, the transponder  300  continues to update the altitude field  775  with current pressure altitude. Subsequently, the transponder  300  continues to transmit the position message  550  for the next 58 seconds or until valid position integrity and/or accuracy information becomes available. Typically, when 60 seconds have elapsed without valid position integrity and/or accuracy information, the transponder  300  stops transmitting the position message  550  until horizontal position integrity and/or accuracy information again becomes available. Thus, integrity and/or accuracy information in the form of a type code  650  or an NUC P  value  700  may not be available in instances where an HPL  850  is not available due to either, for example, unavailability of a RAIM or the presence of a RAIM alarm. 
     In some instances, the navigational system  100  may not be equipped with a DO-229A GPS receiver  150 , but instead may be equipped with only a “supplemental navigation” (DO-208) internal GPS receiver  200 . Further, a DO-208 GPS receiver  200  may sometimes be configured to execute a RAIM algorithm as an internal consistency check. However, a DO-208 GPS receiver  200  typically does not output dynamically calculated HPL  850  values. Thus, where a navigational system  100  has only a DO-208 GPS receiver  200 , a type code  650  will not be transmitted by the navigational system  100  since an HPL  850  is not available. Generally, the same occurs if an HPL  850  is not available in a navigational system  100  having both a DO-229A GPS receiver  150  and a DO-208 GPS receiver  200 . 
     It has been discovered that, where a navigational system  100  includes only a DO-208 GPS receiver  200 , that the HPL value  850  that is used in designating a corresponding NUC P    700  may be determined by introducing and using an appropriate Horizontal Alarm Limit (HAL)  825  in the RAIM algorithm of a DO-208 GPS receiver  200 . As shown in FIG. 3, an HPL value  850  corresponding to a particular NUC P  value  700  falls within a defined distance range. For example, an NUC P    700  of seven corresponds to an HPL value  850  that is greater than or equal to 25 meters, but less than 0.1 nautical miles. In this instance, an HPL value  850  equal to 0.1 nautical miles would be outside the allowable HPL values  850  corresponding to an NUC P    700  of seven. Thus, since any determined horizontal position accuracy of 0.1 nautical miles or greater would be outside the distance range of a NUC P    700  of seven, an appropriate and corresponding HAL  825  would be equal to 0.1 nautical miles. In other words, establishing an HAL  825  equal to 0.1 nautical miles for an NUC P    700  of seven considers all HPL values  850  less than 0.1 nautical miles. 
     In instances where the navigational system  100  is configured to implement a HAL  825 , the selected HAL value  825  is directed from the processing unit  250  to the DO-208 GPS receiver  200  for use in the RAIM algorithm therein. The RAIM algorithm in the DO-208 GPS receiver  200  is then executed using the HAL value  825  from the processing unit  250 . If RAIM is available, the RAIM algorithm determines the consistency of the position calculated by the DO-208 GPS receiver  200  and compares that consistency to the HAL value  825 . If the consistency of the measurements returned by the RAIM algorithm are less than the HAL value  825 , then no RAIM alarm is produced and the processing unit  250  is notified accordingly. The processing unit  250  then sets an HPL value  850  of slightly less than the HAL value  825  used in the RAIM algorithm, at about the upper range limit of the corresponding HPL distance range. In some instances, the processing unit  250  may produce a Horizontal Figure of Merit (HFOM) value  900  in lieu of and corresponding to the HPL value  850 . An HFOM is defined as a 95% containment value on the accuracy of the position fix. As shown in FIG. 3, HFOM  900  ranges are accuracy values, wherein the range bounds are roughly half the range bounds of the corresponding HPL  850  ranges. Either the HPL  850  or the HFOM  900  may be used by the transponder  300  to determine the appropriate type code  650  to be transmitted by the navigational system  100 . 
     In determining the appropriate HPL value  850  and/or the appropriate HFOM value  900  used by the transponder  300 , embodiments of the present invention may operate, for example, in accordance with the correlation chart shown in FIG.  3 . In such instances, the processing unit  250  is implemented to determine an HPL value  850  by using an appropriate HAL value  825  in the DO-208 GPS receiver&#39;s  200  internal RAIM algorithm. The resulting HPL value  850  is then sent by the processing unit  250 , either as the HPL value  850  or as the corresponding HFOM value  900 , to the transponder  300 , from which the transponder  300  determines the corresponding type code value  650 . Initially, the processing unit  250  sets the HAL  825  to slightly above the upper HPL  850  limit for the highest NUC P    700  value. Thus, for a highest NUC P    700  of seven, the processing unit  300  sets a HAL value  825  equal to 0.1 nautical miles, which is slightly above the upper range limit of the corresponding HPL  850 . If the DO208 GPS receiver  200  determines that RAIM is available and does not return a RAIM alarm for the provided HAL of 0.1 nautical miles, the processing unit  300  then selects a value of slightly less than 0.1 nautical miles for the HPL value  850 , corresponding to slightly less than 0.05 nautical miles for the HFOM value  900 , either of which are then sent to the transponder  300 . The transponder  300  then selects a corresponding type code  650  equal to eleven (further corresponding to a NUC P    700  value of seven) for transmission as a portion of the position message  550 . 
     However, if the DO208 GPS receiver  200  detects a RAIM alarm or indicates that RAIM is not available, the processing unit  250  then supplies a HAL value  825  corresponding to the next successively greater HPL range  850  which, as shown in FIG. 3, would be slightly greater than the highest HPL value  850  corresponding to the next lower NUC P  value  700  of six. The next HAL value  825  supplied to the RAIM algorithm would therefore be 0.2 nautical miles. If the test is passed, namely that RAIM is available and no RAIM alarm exists, the processing unit  250  then sets the HPL value  850  to slightly less than 0.2 nautical miles, corresponding to, for instance, to an HFOM value  900  to slightly less than 0.1 nautical miles. With these values, the transponder  300  then transmits a type code  650  of twelve in the position message  550  which corresponds to an NUC P  value  700  of six. Should this subsequent test fail, however, the processing unit  250  proceeds sequentially to the next successively greater HAL values  825  which, according to the chart shown in FIG. 3, would be successive values of 0.5 nautical miles and 1 nautical mile in the defined sequence of HPL ranges  850 . 
     If the processing unit  250  exhausts the possible HAL values  825  corresponding to the HPL ranges  850  used in the RAIM algorithm of and supported by the DO208 GPS receiver  200 , the processing unit  250  then reverts to a default accuracy/integrity value corresponding to an NUC P  value  700  of zero. Accordingly, as shown in FIG. 3, the processing unit  250  sets the HPL value  850  to slightly more than  20  nautical miles, corresponding to an HFOM value  900  of slightly more than  10  nautical miles. A corresponding type code  650  of  18  is then included in the position message  550  by the transponder  300 , corresponding to the NUC P  value  700  of zero, which means that the transmitted position has no or uncertain integrity. In other words, the transmitted geodetic coordinates of the navigational system  100  are reliable only to the extent of being within a relatively large distance range measured in nautical miles. These dynamic tests using HAL values  825  are repeated periodically so as to ensure that the processing unit  250  is continually updating the transponder  300  with an HFOM value  900  and/or an HPL value  850  such that the transponder  300  transmits the most current type code  650  for updating the position of the vehicle having the transmitting navigational system  100 . 
     Note that it will be appreciated by one skilled in the art that a navigational system  100  as described herein may be realized in many different manners consistent with the spirit and scope of the present invention. Therefore, it will be further appreciated that the described navigational system  100  as described herein supports a corresponding apparatus and methodology. In addition, the described navigational system  100  may be implemented in software, hardware, or a combination of software and hardware, as will be appreciated by one skilled in the art so as to support a corresponding system based upon a computer device and associated computer software. 
     Thus, embodiments of the present invention provide a method, apparatus, system, and computer software program product for determining position integrity in a system having a GNSS component, such as a GPS device. Such a navigational system  100  having a GNSS component is capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or RAIM alarm has been generated. Such a navigational system  100  may be configured with, for example, a “supplemental navigation” DO208 GPS receiver  200  and/or a “sole means of navigation” DO-229A GPS receiver  150  and provides a method of determining the integrity of the measured geodetic coordinates even if other methods are used by the system. Embodiments of the present invention are particularly advantageous for determining position integrity information when the vehicle is equipped with a “supplemental navigation” DO-228 GPS receiver  200  lacking the capability of determining an HPL value  850 . Thus, embodiments of the present invention provide distinct advantages over prior art navigational systems having a GNSS component. 
     Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.