WO2007086899A2

WO2007086899A2 - Transponder landing system augmentation of the global positioning system

Info

Publication number: WO2007086899A2
Application number: PCT/US2006/012633
Authority: WO
Inventors: Karl Winner; Yong Ho Son
Original assignee: Advanced Navigation And Positioning Corporation
Priority date: 2005-03-29
Filing date: 2006-03-29
Publication date: 2007-08-02
Also published as: WO2007086899A3

Abstract

A GPS navigation system augmentation using a transponder landing system. Initially, the position of a reference ground station is determined. When an aircraft using an on-board GPS navigation system approaches the ground station, the ground station accurately determines the position of the aircraft using the aircraft's transponder replies. In addition, the ground station receives a current position for the aircraft measured by an on-board GPS navigation system. By comparing the position of the aircraft determined from the transponder replies with the GPS- calculated position provided by the aircraft, the ground station determine error information for the on-board GPS navigation system. The ground station then transmits the error information back to the aircraft. Using this error information, the on-board GPS navigation system can improve the accuracy of future position calculations for the aircraft.

Description

TRANSPONDER LANDING SYSTEM AUGMENTATION OF THE GLOBAL POSITIONING SYSTEM

RELATED APPLICATIONS

[01] This application claims priority under 35 U.S. C. §119(e) to U.S. Provisional Patent Application No. 60/666,510 entitled "Transponder Landing System Augmentation Of The Global Positioning System," filed on March 29, 2005, and naming Karl Winner et al. as inventors, which provisional patent application is incorporated entirely herein by reference.

FIELD OF THE INVENTION

[02] The present application relates to navigation systems. The invention finds particular application in improving the accuracy of global positioning system navigation systems used by aircraft.

BACKGROUND OF THE INVENTION

[03] Various navigation systems have been employed to assist a pilot in navigating an aircraft. One such system, the Instrument Landing System (ILS), is a commonly used navigation system that provides an aircraft with guidance for maintaining a desired glide path during a runway approach for landing. Typically, an ILS navigation system will include two independent sub-systems, one providing lateral guidance, the other vertical guidance to aircraft approaching a runway. An ILS navigation system offers a high level of precision. For example, an ILS navigation system configured to the "Category I" standard allows for landings in weather with 2400 ft (732m) visibility or 1800 ft (549m) in case of touchdown and centerline lighting, and a 200 ft ceiling (cloud base or vertical visibility). More advanced ILS navigation systems configured to the Category II or Category III standards offer even greater navigation precision, allowing landings in near-zero visibility. ILS navigation systems are prone to interference from nearby FM broadcasts, however, and require extensive terrain grading and property acquisition at some airport sites. Further, ILS navigation systems are vulnerable to guidance beam distortion from construction near an airport.

[04] Another navigation system is the Microwave Landing System (MLS). MLS navigation systems are much less commonly employed than ILS navigation systems, however, and are being phased out in response to economic concerns. Yet another navigation system uses Precision Approach Radar (PAR). PAR navigation systems are commonly used in military environments, but require a ground operator to verbally convey glide path guidance corrections to the pilot via a communications link.

[05] Still another navigation system is the Global Positioning System (GPS). The Global Positioning System (GPS) is a space-based radio-navigation system using a plurality of satellites consisting of a constellation of satellites and a network of ground stations used for monitoring and control. With a GPS navigation system, a GPS receiver concurrently receives encoded signals from three or more GPS satellites. My measuring the time delay between the received signals, the GPS receiver can determine its distance from each of the satellites. Using trilateration, the GPS receiver can then determine its position relative to the satellites.

[06] While GPS navigation systems offer a number of advantages over previous navigation systems, they also have a number of disadvantages. For example, a basic assumption in determining the delay between the encoded signals received from the satellites is that all of the signals are propagating at the speed of light. The actual propagation speed of an encoded signal will change, however, as the signals travel through the charged particles of the ionosphere and then through water vapor in the troposphere. GPS navigation systems thus have various shortcomings related to accuracy, integrity, availability, and jam resistance. As a result, conventional GPS navigation systems cannot reliably provide desired navigation precision useful for, e.g., guiding an aircraft during a landing approach.

[07] Because of the significant advantages offered by GPS navigation systems, a variety of augmentation systems have been developed to overcome the disadvantages in GPS navigation. For example, a modified form of GPS navigation, referred to as "Differential" GPS, is used to reduce errors in conventional GPS navigation systems. As described above, a GPS receiver typically uses timing signals from at least four GPS satellites to establish a position. As also discussed above, each of those timing signals may have some error or delay. Two receivers that are fairly close together, however, will tend to have the same errors.

[08] Accordingly, differential GPS uses a stationary reference GPS receiver with a known position. The reference GPS receiver calculates timing for received GPS signals, rather than using timing signals to calculate a position. More particularly, the reference GPS receiver determines what the travel time of each GPS signal should be, and compares it with the actual travel time for each received GPS signal. The difference is an "error correction" factor. The reference GPS receiver then transmits this error information, calculated for each GPS satellite, to local roving GPS receivers, so that they can use this error information to correct their own GPS position measurements.

[09] One augmentation system used with aircraft landing aid systems is the Wide Area Augmentation System (WAAS). With this system, the signals from the GPS satellites are received across a wide region by Wide Area Reference Stations (WRS). The position of each reference station is precisely surveyed, so that any errors in the received GPS signals can be detected. The GPS signal information collected by each the reference station is forwarded to a WAAS Master Station (WMS), which in turn uses the information to generate augmentation messages. More particularly, these messages contain information that allows GPS receivers to correct for errors in the GPS signals. The augmentation messages are sent to the GPS satellites, which then relay the augmentation information in the messages back down to GPS receivers. A GPS receiver configured to use WAAS can employ the augmentation message to improve the accuracy of its position calculation.

[10] Another aircraft landing aid system is the Local Area Augmentation System (LAAS). A LAAS navigation system typically uses at least four ground-based reference receivers, a LAAS ground facility (LGF), a VHF data broadcast transmitter (VDB), and complementary LAAS avionics installed on the aircraft. With this navigation system, signals from GPS satellites are received by the reference receivers, which calculate their position using the GPS signals. Like the differential GPS system described above, using the position information from the reference receivers, the LAAS ground facility measures errors in the GPS-calculated position, and produces a LAAS correction message based on the difference between the actual position of the reference receivers and their GPS-calculated position. This LAAS correction message is then sent via the VHF data broadcast transmitter to the LAAS avionics on an aircraft. The LAAS avionic uses the information to correct the GPS signals received by the aircraft's own GPS receiver.

[11] While WAAS and LAAS offer significant improvements in accuracy over conventional GPS navigation systems, both WAAS and LASS may be subject to jamming and spoofing, and may not be suitable as the sole means of precision guidance for an aircraft. Accordingly, it would be desirable to provide a navigation system that can augment conventional GPS navigation systems, particularly conventional GPS navigation systems employed on board aircraft during the approach of a runway for landing.

BRIEF SUMMARY OF THE INVENTION [12] The invention relates to the augmentation of GPS navigation systems, and in particular GPS navigation systems used on board aircraft. According to various examples of the invention, the position of a reference ground station is accurately determined. When an aircraft using an on-board GPS navigation system approaches the ground station, the ground station accurately determines the position of the aircraft using the aircraft's transponder replies. In addition, the aircraft transmits its current position calculated by its on-board GPS navigation system. By comparing the position of the aircraft determined from the transponder replies with the GPS-calculated position provided by the aircraft, the ground station can determine error information for the GPS position calculated by the on-board GPS navigation systems. The ground station then transmits the error information back to the aircraft. Using this error information, the on-board GPS navigation system can improve the accuracy of future position calculations for the aircraft.

[13] Advantageously, one difference between the LAAS system and various embodiments of the invention is the use of spatial errors on a measured location of the aircraft to derive differential time error corrections, rather than using multiple, spatially separated GPS receivers to provide localized differential time error correction to aircraft near those spatially discrete GPS receiver sites. This distinction allows various embodiments of the invention to provide positional error correction at the aircraft, versus timing error information that has been extrapolated for the GPS receiver system on the aircraft

BRIEF DESCRIPTION OF THE DRAWINGS

[14] Fig. 1 illustrates a computing environment that may be used to implement various examples of the invention.

[15] Fig. 2 illustrates a GPS navigation system augmentation tool that may be employed according to various examples of the invention. [16] Figs. 3A and 3B illustrate a flowchart describing a method of augmenting a GPS navigation system according to various examples of the invention.

[17] Fig. 4 illustrates the operation of a GPS navigation augmentation tool to determine a position of a ground station according to various examples of the invention.

[18] Fig. 5 illustrates the operation of a GPS navigation augmentation tool to determine a position of an approaching aircraft according to various examples of the invention.

[19] Fig. 6 illustrates the operation of a GPS navigation augmentation tool to provide GPS navigation correction information to an approaching aircraft according to various examples of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Operating Environment

[20] Various components of the system according to the invention may be implemented through .the execution of software instructions by a computing device, such as a programmable computer. An illustrative example of such a computing device 101 therefore is illustrated in Figure 1.

[21] As seen in this figure, the computing device 101 has a computing unit 103. The computing unit 103 typically includes a processing unit 105 and a system memory 107. The processing unit 105 may be any type of processing device for executing software instructions, but will conventionally be a microprocessor device. The system memory 107 may include both a read-only memory (ROM) 109 and a random access memory (RAM) 111. As will be appreciated by those of ordinary skill in the art, both the readonly memory (ROM) 109 and the random access memory (RAM) 111 may store software instructions for execution by the processing unit 105. [22] The processing unit 105 and the system memory 107 are connected, either directly or indirectly, through a bus 113 or alternate communication structure, to one or more peripheral devices. For example, the processing unit 105 or the system memory 107 may be directly or indirectly connected to one or more additional memory storage devices, such as a hard disk drive 115, a removable magnetic disk drive 117, an optical disk drive 119, or a flash memory card 121. The processing unit 105 and the system memory 107 also may be directly or indirectly connected to one or more input devices 123 and one or more output devices 125. The input devices 123 may include, for example, a keyboard, a pointing device (such as a mouse, touchpad, stylus, trackball, or joystick), a scanner, a camera, and a microphone. The output devices 125 may include, for example, a monitor display, a printer and speakers.

[23] With some implementations, the computing unit 103 may be directly or indirectly connected to one or more network interfaces 127 for communicating with a network. The network interface 127 translates data and control signals from the computing unit 103 into network messages according to one or more communication protocols, such as the transmission control protocol (TCP) and the Internet protocol (IP). These protocols are well known in the art, and thus will not be discussed here in more detail. An interface 127 may employ any suitable connection agent (or combination of agents) for connecting to a network, including, for example, a wireless transceiver, a modem, or an Ethernet connection.

[24] It should be appreciated that one or more of these peripheral devices may be housed with the computing unit 103 and bus 113. Alternately or additionally, one or more of these peripheral devices may be housed separately from the computing unit 103 and bus 113, and then connected (either directly or indirectly) to the bus 113. Also, it should be appreciated that both computers and computing appliances may include any of the components illustrated in Figure 1, may include only a subset of the components illustrated in Figure 1, or may include an alternate combination of components, including some components that are not shown in Figure 1.

GPS Navigation Augmentation Tool

[25] Figure 2 illustrates an example of a GPS navigation augmentation tool 201 that may be implemented according to various examples of the invention. As seen in this figure, the GPS navigation augmentation tool 201 includes ground station position module 203, a relative aircraft position module 205, an absolute aircraft position module 207, and an error determination module 209. As will be discussed in more detail below, the ground station position module 203 obtains position information from a GPS receiver 211 to identify the position of ground station. By using this position information in conjunction with location information 213 for the ground station, the ground station position module 203 accurately determines the position of the ground station.

[26] When an aircraft approaches the ground station, the relative aircraft position module 205 communicates with a transponder system 215 to determine the position of the approaching aircraft relative to the ground station. Based upon the position of the ground station previously determined by the ground station position module 203 and the relative position of the aircraft determined by the relative aircraft position module 205, the absolute aircraft position module 207 can accurately determine the actual position of the approaching aircraft. The absolute aircraft position module 207 then provides this accurate position information for the approaching aircraft to the error determination module 209.

[27] With various examples of the invention, the GPS navigation augmentation tool 201 will be employed with aircraft that have their own, on-board GPS navigation system. More particularly, these embodiments of the invention will be employed for aircraft that are capable of transmitting position information calculated by the on-board GPS navigation system to the ground station. This aircraft GPS position information is received by a GPS interface unit 217, and relayed to the error determination module 209. By comparing the GPS position information received from the approaching aircraft with the aircraft position determined by the absolute aircraft position module 207, the error determination module 209 can determine the apparent timing errors for the GPS signals being used by the GPS navigation system on board the aircraft.

[28] Once it has determine the timing errors for the aircraft GPS navigation system, the error determination module 209 transmits this timing error information back to the aircraft GPS navigation system through the GPS interface unit 217. The aircraft GPS navigation system can then use this error information to more accurately calculate the position of the aircraft.

[29] With various examples of the invention, one or more of the ground station position module 203, relative aircraft position module 205, absolute aircraft position module 207, and the error determination module 209 may be implemented using a programmable computing device, such as the computing device 101 illustrated in Fig. 1. Correspondingly, one or more of the ground station position module 203, relative aircraft position module 205, absolute aircraft position module 207, and the error determination module 209 may be embodied as software instructions for implementing the functions of these modules stored on a computer-readable medium.

[30] Of course, with other examples of the invention, one or more components of the GPS navigation augmentation tool 201 may be implemented using other devices, such as purpose-specific electronic circuits, analog computing circuits, optical processors or any other suitable devices. Further, while each of GPS receiver 211, transponder system 215 and GPS interface unit 217 are illustrated in Fig. 2 as being separate from the GPS navigation augmentation tool 201, it should be appreciated that one or more alternate embodiments of the invention can include some or all of one or more of these components. For examples, some embodiments of the GPS navigation augmentation tool 201 may incorporate some portion of the GPS interface unit 217.

[31] The operation of the GPS navigation augmentation tool 201 will be described in more detail below, with reference to the flowchart illustrated in Fig. 3 and the diagrams illustrated in Figs. 4-6.

Determination OfA GPS Position

[32] Initially, in step 301, the GPS navigation augmentation tool 201 determines highly accurate position information for a ground station 401. For example, as shown in Fig. 4, the ground station 401 employs a GPS receiver 211. The GPS receiver 211 receives GPS signals S₁, S₂,... S_n from a plurality of three or more GPS satellites 403₁, 403₂,...403_n. With various examples of the invention, the GPS receiver 211 will measure the position of the ground station 401 over a period of days, in order to improve the accuracy of the position information for the ground station 401. As previously noted, the GPS receiver 211 provides the collected GPS position information for the ground station 401 to the ground station position module 203.

[33] Still further, with some examples of the invention, the GPS navigation augmentation tool 201 may optionally employ additional location information 213 to improve the accuracy of the position information for the ground station 401. This location information 213 may include, for example, ephemeris information for one or more of the GPS satellites 403_1? 403₂,...403_n. The location information 213 may alternately or additionally include survey information for the topography around the ground station 401. With these examples of the invention, the ground station position module 203 will employ the location information 213 to compute pseudorange corrections for the GPS position information measured by the GPS receiver 211. Typically, the pseudorange corrections will be valid over a local area in the vicinity of the ground station 401. [34] Various techniques for determining highly accurate GPS position information over time are well known in the art, and thus the determination of GPS position information for the ground station 401 will not be discussed here in further detail. It should be appreciated, however, that some examples of the invention may employ any other suitable positioning method to determine accurate position information for the ground station 401. It also should be appreciated that, with various examples of the invention, the GPS navigation augmentation tool 201 may omit the ground station position module 203 altogether. With these examples of the invention, the position of the ground station 401 can be separately determined and provided directly to the absolute aircraft position module 207.

[35] As also illustrated in Fig. 4, an approaching aircraft 405 will have its own, on-board GPS navigation system 407. Like the GPS receiver 211, the on-board GPS navigation system 407 will have a GPS receiver that receives GPS signals R₁, R₂,... R_n from a plurality of three or more GPS satellites 403i, 403₂,...403_n, and then analyzes the signals R₁, R₂,... R_n to determine the position of the aircraft. Of course, while Fig. 4 illustrates the on-board GPS navigation system 407 receiving signals R₁, R₂,... R_n from the same GPS satellites 403i, 403₂,...403_n as the GPS receiver 211 for convenience, it will be understand that the particular combination of satellites 403i, 403₂,...403_n relied upon by the on-board GPS navigation system 407 will depend upon the actual location of the aircraft and the time at which the position measurement is made.

Determination OfAn Aircraft Position

[36] Next, in step 303, the GPS navigation augmentation tool 201 determines the position of an approaching aircraft 405. With various embodiments of the invention, this aircraft position information can be obtained by using a conventional transponder system 215. As known in the art, transponders are typically deployed on aircraft to facilitate the function of monitoring and controlling the aircraft. For example, many aircraft employ the Air Traffic Control Radar Beacon System (ATCRBS). The operation of this system is illustrated in Fig. 5. Initially, a ground based transmitter 501, commonly referred to as a Secondary Surveillance Radar (SSR), will transmit interrogation signals Ii to the aircraft 405. The interrogation signals Ii may be transmitted at, e.g., 450-120 interrogations/second. When a transponder 503 on the aircraft 405 receives an interrogation signal Ii, it transmits a transponder reply Tj. Typically, the reply Ti is sent after a 3.0μs delay, and will provide the information requested in the interrogation signal Ii , such as identify information for the aircraft 405.

[37] As known in the art, multilateration can be used to accurately determine the position of the aircraft 405 from its transponder replies Ti. More particularly, using multilateration, the transponder system 215 includes a plurality of separated receiving stations at or near the ground station 401. Each of these stations records the time at which it receives a transponder reply Ti from the aircraft 405. By comparing the differences in the receipt times, the relative aircraft position module 205 can determine the distance of the aircraft 405 relative to the ground station 401. With various embodiments of the invention, the relative aircraft position module 205 also will determine the angle-of- arrival of the aircraft using interferometry techniques to extract I/Q components of the transponder reply signal. Both the use of multilateration and these interferometry techniques are well known in the art, and thus will not be described here in detail.

[38] Thus, using the ground measurements of the location of the aircraft 405 obtained from the transponder replies Ti, the relative aircraft position module 205 accurately calculates the spatial location of the aircraft 405 relative to the position of the ground station 401. Further, as the aircraft 405 gets closer to the transponder system 215, the angular accuracy relative to the size and position of the aircraft becomes better. By using the position of the aircraft relative to the ground station 401, and the position of the ground station 401 determined by the ground station position module 203 (as discussed above), the absolute aircraft position module 207 determines the position of the aircraft 405 in step 305. Moreover, because the determined position of the ground station 401 is very accurate, and the relative position of the aircraft 405 to the ground station 401 is very accurate, the actual position of the aircraft 405 obtained by the absolute aircraft position module 207 also will be very accurate. The absolute aircraft position module 207 then provides this aircraft position information to the error determination module 209 in step 307.

Correction Of Global Positioning Satellite Information

[39] As previously noted, various embodiments of the invention will be employed with aircraft that have both on-board GPS navigation systems (including GPS receivers) and ability to send the position information measured by those GPS receivers to the ground station 401. For example, many aircraft use avionics that implement the Automatic Dependent Surveillance-Broadcast (ADS-B) system. Aircraft using this system continuously broadcast their current position and altitude, their category of aircraft, airspeed, identification, and whether it is turning, climbing or descending. This information is transmitted over a dedicated radio datalink. Typically, an ADS-B system for an aircraft will obtain the aircraft's position information from an on-board GPS navigation system. A similar system, the Automatic Dependent Surveillance-Address (ADS-A) system, will transmit data to a specific address.

[40] Accordingly, in step 309, the aircraft 405 transmits its GPS position information (i.e., its position as determined by the on-board GPS navigation system 407) to the error determination module 209. More particularly, the aircraft 405 transmits its GPS position information to the GPS interface unit 217 at the ground station 401, which then relays that information to the error determination module 209. In response, the error determination module 209 compares the GPS position measured by the on-board GPS navigation system 407 of the aircraft 405 with the position of the aircraft 405 determined at the ground station (i.e., by the cooperation of the ground station position module 203, the relative aircraft position module 205, and the absolute aircraft position module 207).

[41] In step 311, the error determination module 209 uses the difference between the positions to determine the apparent timing error in the GPS receiver of the on-board GPS navigation system 407. With various examples of the invention, the error determination module 209 may use any desired technique to calculate the apparent timing error for the on-board GPS navigation system 407. For example, with some embodiments of the invention, the error determination module 209 may employ the same techniques used by differential GPS systems. As noted above, these systems use a GPS receiver with a known position to determine what the propagation time of GPS signals should be, compares this value with the propagation times for GPS signals actually received at the GPS receiver, and determines the difference as an "error correction" factor. Of course, any other suitable techniques for obtaining timing error information from two positions may be employed. These techniques are well known in the art, and thus will not be discussed here in further detail.

[42] Then, in step 313, the error determination module 209 transmits the determined timing error information to the aircraft 405. For example, as illustrated in Fig. 6, the error determination module 209 may send a data broadcast Cj from the ground station 401 the aircraft 405. This timing error information (or timing correction information) may be sent to the aircraft 405 via, e.g. an existing LAAS VHF signal. With the illustrated example of the invention, the GPS interface unit 217 includes the transmission facilities used to transmit the error information to the aircraft 405. Alternate embodiments of the invention, however, may employ transmission facilities separate from the GPS interface unit 217 to send the error information to the aircraft 405.

[43] Using this timing error information, the on-board GPS navigation system 407 can very accurately and reliably remove bias from the GPS signals it receives while communicating with the ground station 401. As a result, the GPS position information subsequently calculated by the on-board GPS navigation system 407 will be very accurate, and will provide safer and more reliable navigation guidance for the aircraft. Of course, the timing error information may be transmitted to the on-board GPS navigation system 407 using any desired communication protocol, and may employ any desired data format.

Implementation OfAn Augmentation System

[44] It should be appreciated by those of ordinary skill in the art that various implementations of the invention can provide a number of advantages and services to aircraft equipped with various navigation systems. For example, aircraft equipped with LAAS receivers will be able to receive, from various embodiments of the invention, precision approach guidance to land at an airport equipped with these embodiments. That is, these embodiments of the invention may provide a LAAS compliant uplink containing derived time corrections based on the spatial difference between the measurements on the aircraft (GPS) and ground (TLS). These LAAS compliant message may additionally include other information, such as, for example, Waypoints for precision landing.

[45] Still further, interoperability between various examples of the invention and aircraft that are not equipped with LAAS receivers can be achieved by up-linking ILS-like VHF/UHF Localizer and glide slope (Ci) signals generated using the timing error information. These ILS-like VHF/UHF Localizer and glide slope (Ci) signals can be generated using the techniques and methods disclosed in U.S. Patent No. 6,469,654 to Winner et al., entitled "Transponder Landing System" issued on October 22, 2002, which patent is incorporated entirely herein by reference. [46] In addition to the ILS-like signals for provide guidance to aircraft equipped with transponders that operate in only Mode A, embodiments of the invention can provide Mode S based aircraft specific guidance to aircraft equipped with transponders that operate in Mode S. As known in the art, transponders that operate in Mode S have provisions for transferring arbitrary data both to and from the transponder. Thus, various embodiments of the invention may use the uplink to the aircraft 405 to provide additional navigation, surveillance and landing guidance using the Mode-S ground to aircraft data format.

[47] As will be appreciated from the foregoing description, various examples of the invention provide a number of features and advantages. For example, embodiments of the invention provide interoperability between existing National Airspace System (NAS) components while using existing RF frequencies already allocated to serve the NAS. Further, the use of various examples of the invention can eliminate the requirement for more costly ground navigational aids currently being employed. Still further, the invention offers a ground based navigational solution for terrain challenged airports without the costs of installing other space-based augmentation systems (SBAS) or ground-based augmentation systems (GBAS). Various examples of the invention also will provide a safe, reliable, and extendable precision landing solution that is cost effective and supports all existing aircrafts. Still further, the improved positioning accuracy offered by various examples of the invention may allow for increased throughput at airports without sacrificing safety, and permit airlines to achieve higher air traffic density and more reliable landing and takeoff at airports.

[48] Still further, various embodiments of the invention could be used to negate the need for dual frequency GPS satellites to eliminate ionospheric error contributions. They may also reduce the need for multiple GPS receivers in LAAS navigation systems, since the embodiments of the invention do not measure time delays directly, but rather deduces them based on spatial errors. Various examples of the invention also may be used to provide a secondary method of navigational aid in compliment to WAAS navigation systems, or in lieu of WAAS navigation systems prior to their development and installation. Still further, various examples of the invention may allow Multiple- In/Multiple-Out with segmented, curved, and other approaches to terrain challenged and non-terrain challenged airports.

[49] Advantageously, aircraft manufacturers may continue to use and install current LAAS solutions in flight management systems (FMS). Because various embodiments of the invention can conform to the interfaces used by deployed flight management systems, there is no need to change existing flight management systems, thereby avoiding any requirements for new flight management systems certification.

Conclusion

[50] While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. For example, while particular components and processes have been described as performing various functions, it should be appreciated that the functionality of one or more of these components and processes may be combined or otherwise reorganized. Thus, while some examples of the invention have been described with a separate relative aircraft position module 205 and absolute aircraft position module 207, other embodiments of the invention may combine the functions of these modules for implementation by a single component. Also, while the ground station has been described as being in single location, with various examples of the invention, one or more components described as being part of the ground station may be separately located over a local area. [51] Further, while a particular order of process steps was described above with reference to various examples of the invention, other examples of the invention may perform one or more of these process steps in an alternate order. For example, with some embodiments of the invention, the GPS navigation augmentation tool may receive an aircraft's GPS position before determining the aircraft's position relative to the ground station. Still further, while various examples of the invention have been described with reference to the GPS satellite positioning system, it should be appreciated that the invention may be employed to augment any other space-based navigation system, such as navigation systems using the Russian Global Navigation Satellite System (GLONASS) satellite positioning system.

Claims

What is claimed is:

1. A method of providing navigation information to an aircraft, comprising: obtaining a first position measurement of an aircraft from a ground station; receiving a second position measurement of the aircraft made at the aircraft using a space-based positioning system; comparing the first position measurement of the aircraft with the second position measurement of the aircraft to obtain a position difference; employing the position difference to calculate a timing error of a space-based positioning system receiver at the aircraft; and providing the calculated timing error to the aircraft.

2. The method recited in claim 1, wherein the first position measurement of the aircraft is obtained using transponder signals transmitted from the aircraft.

3. The method recited in claim 1, wherein the calculated timing error is provided to the aircraft via a VHF signal.

4. The method recited in claim 1, wherein the calculated timing error is provided to the aircraft in a message format compatible with the Local Area Augmentation System.

5. The method recited in claim 1, wherein the calculated timing error is provided to the aircraft through Instrument Landing System like VHFAJHF Localizer and glide slope signals generated based upon the calculated timing error.

6. The method recited in claim 1, further comprising obtaining the first position measurement of the aircraft from the ground station by determining a position of the ground station using a space-based positioning system; and determining a relative distance between the ground station and the aircraft by multilateration of transponder signals transmitted by the aircraft.

7. The method recited in claim 1, wherein the space-based positioning system is the Global Positioning System.

8. The method recited in claim 1, wherein the space-based positioning system is the Global Navigation Satellite System.

9. A navigation system, comprising: an aircraft position module that determines a first position measurement for an aircraft; and an error determination module that receives a second position measurement of the aircraft made at the aircraft using a space-based positioning system, compares the first position measurement of the aircraft with the second position measurement of the aircraft to obtain a position difference, employs the position difference to calculate a timing error of a space-based positioning system receiver at the aircraft, and provides the calculated timing error to the aircraft.

10. The navigation system recited in claim 9, further comprising a ground station position module that determines a position of the ground station.

11. The navigation system recited in claim 9, further comprising a relative aircraft position module that determines a distance between the aircraft and the ground station, and wherein the aircraft position module is an absolute aircraft position module that determines the first position measurement for the aircraft based upon the distance determined by the relative aircraft position module and a previously-determined position for the ground station.

12. The navigation system recited in claim 9, further comprising a spaced-based positioning interface unit for communicating with the space-based positioning system receiver at the aircraft.

13. The navigation system recited in claim 12, wherein the spaced-based positioning interface unit receives the second position measurement of the aircraft from the aircraft.

14. The navigation system recited in claim 12, wherein the spaced-based positioning interface unit transmits the calculated timing error the aircraft.