Integrated enroute and approach guidance system for aircraft

Data from long range aids such as the global positioning system (GPS) and an inertial navigation system (INS) and short range aids such as a microwave landing system (MLS) are used to smoothly and automatically transition an aircraft from the long range aids to the short range aids. During cruise a Kalman filter combines data from the global positioning system and the inertial navigation system to provide accurate enroute information. When the aircraft arrives in the vicinity of the airport and begins to acquire valid data from the microwave landing system, the Kalman filter is calibrated with the MLS data to permit Precision landing with GPS/INS data alone in case the MLS system subsequently fails. In addition, navigation information begins to be derived from a weighted sum of the GPS/INS and MLS data, the weighting being determined by distance from the airport. In a first region farthest from the airport, the GPS/INS data is given a 1.0 weighting factor; and in a second region nearest the airport, the MLS data is given a 1.0 weighting factor. In a third region intermediate the first and second regions, the GPS/INS data and MLS data are proportionately and complementarily weighted as a function of the distance from the airport. If the MLS system fails, the weighting system is disabled and navigation data is again derived from the GPS/INS combination. In addition, the data from both systems are monitored, and a cockpit alarm is sounded if the data diverges beyond a specified amount.

BACKGROUND OF THE INVENTION 
The invention relates to new and useful improvements in aircraft navigation 
and airspace control systems, and more particularly to such systems using 
the global positioning system and the microwave landing system. 
In recent years aircraft navigation and airspace control have been vastly 
improved by introduction of the global positioning system (GPS) and the 
microwave landing system (MLS). These systems supplement the existing 
inertial navigation systems (INS) or radionavigation systems currently 
used for enroute navigation, and instrument landing system (ILS) for 
terminal guidance. GPS uses special radio receivers in an aircraft to 
receive radio signals transmitted from an array of earth satellites. Using 
the information from the satellites, an aircraft receives and calculates 
its position within 50-150 feet in all three dimensions. The MLS replaces 
the VHF ILS with an approach and landing system using microwave signals. 
This provides a much more accurate and flexible landing system which 
permits the number of takeoff and landing operations at an airport to be 
sharply increased. 
In the present practice, the GPS, INS and MLS systems are used separately 
at the discretion of the pilot, or the GPS and INS data are combined to 
give integrated GPS/INS enroute navigation information. The transition 
from GPS/IN navigation used enroute to the MLS navigation needed for 
landing is manual and is determined by the pilot. This transition from one 
navigation system to another normally occurs at a time of high cockpit 
workload and navigational uncertainty and thus can decrease aircraft 
safety. In addition, the transition is abrupt and the flight path can be 
adversely affected by the differences due to errors in each device. Due to 
such discrete transitions, extra reliance must be placed upon ground 
controller vectoring, especially when combined with a specified time of 
arrival for spacing and sequencing. Since ground controllers are otherwise 
fully occupied, the extra work related to ground vectoring increases the 
chances of mistakes by controllers which can compromise safety. Further, 
each of these devices is subject to errors, and independent operation does 
not use one device to check the accuracy of the others. 
It is therefore an object of the present invention to provide an aircraft 
navigation system which integrates the GPS/INS and MLS navigation systems. 
It is another object of the present invention to provide an aircraft 
navigation system which gradually shifts between the GPS and MLS systems. 
It is a further object of the present invention to provide an aircraft 
navigation system which minimizes errors associated with switching from 
the GPS/INS navigation system to the MLS navigation system. 
It is an additional object of the present invention to provide a highly 
automated enroute, transition and terminal navigation system. 
It is yet another object of the present invention to provide an aircraft 
navigation system which uses the optimimum type of navigation information 
for each phase of an aircraft mission or flight. 
It is yet a further object of the present invention to provide 
crosschecking of several navigation systems to detect anomalous errors and 
to automatically compensate therefor. 
It is yet an additional object of the present invention to provide a 
multi-source aircraft navigation system which decreases cockpit workload 
in critical phases of a flight and thus decreases the chances of mishap 
resulting from crew distraction or navigational uncertainty. 
It is still another object of the present invention to provide an aircraft 
navigation system which minimizes the need for ground controller 
vectoring. 
It is still an additional object of the present invention to provide an 
integrated navigation system which relies soley upon MLS guidance for 
final terminal guidance in accordance with certification criteria for 
landing approaches. 
Still other objects will become apparent in the following summary and 
description of a preferred embodiment of the invention. 
SUMMARY OF THE INVENTION 
Data from multiple navigation systems are used to smoothly and 
automatically transition from a first system to a second system. Data from 
the first system is continuously received and used for navigation purposes 
until valid data begins to be received from the second navigation system. 
The data from the first and second navigation systems are then combined to 
provide a sum of the two sets of data which are complementarily weighted 
as a function of the distance from a predetermined point associated with 
the second navigation system. 
In another aspect of the invention, data from the second navigation system 
is used to calibrate the data from the first navigation system when valid 
data from the second navigation system is received. This permits the first 
navigation system to substitute for the second navigation system if the 
second system subsequently fails.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring first to FIG. 1, a typical integrated aircraft avionic system in 
which the present invention may be used is shown. The central element of 
the integrated avionic system is flight management system 10 which 
coordinates and controls the other components of the system, including a 
flight display system 12, a flight control system 14, a navigation sensor 
system 16, a communication system 18, mission avionics 20 and an air data 
system 22. 
Data from the various systems and results generated by flight management 
system 10 are displayed on one or more displays comprising flight display 
system 12. The aircraft may be controlled by an autopilot, autothrottle 
and other components comprising flight control system 14. Information on 
aircraft position may be provided by navigation sensor system 16, which 
can include a wide variety of functions including GPS, inertial 
navigation, TACAN, VOR/ILS, MLS, automatic direction finder, and radar 
altimeter. Communication with airport, enroute control facilities and 
other aircraft is provided by a communication system 18. In the case of 
military aircraft, special capabilities, including targeting radar and 
weapons may be provided by mission avionics 20. Finally, air data system 
22 provides air mass referenced performance data used in controlling the 
aircraft and estimating wind and other factors which affect aircraft 
guidance. 
Referring to FIG. 2, an operational scenario for the present invention is 
illustrated. An enroute aircraft 30 progresses along a flight path 31 
(dotted line) comprising several segments which are related to the present 
invention, toward a landing on an airport runway 32 at touchdown 
(landings), or air release point (for cargo or personnel air drops), 34. 
Associated with runway, or drop zone, 32 is an MLS transmitter 36. For 
arrivals a cruising aircraft 30 may travel along an enroute leg 38 to a 
waypoint 40 using GPS, or other RNAV system, and inertial navigation 
system information only for navigation. At waypoint 40 aircraft 30 may 
travel along a transition and initial approach segment 42 to a waypoint 44 
using a blend of GPS, INS, and MLS information. From waypoint 44 aircraft 
30 travels along final approach segment 46 with increasing and finally 
total dependence on MLS information. 
On departures aircraft 30 may follow a flight path 48 comprising several 
segments also associated with the present invention. Initially, departing 
aircraft 30 may travel along a takeoff segment 50 to a waypoint 52 using 
MLS for navigation. At a prescribed distance from the touchdown or airdrop 
point 52 aircraft 30 begins to use a blend of MLS, INS and GPS information 
to waypoint 56. At waypoint 56 aircraft 30 follows departure enroute 
segment 58 using GPS and INS information for navigation with decreasing 
weighting of MLS information. 
The waypoints for both aircraft arrival and departure are arbitrarily 
chosen, and the use of various navigation aids may not necessarily be 
bounded by the waypoints. Referring now to FIG. 3, a functional block 
diagram of the present invention is shown. To provide context, the various 
elements of FIG. 3 are grouped together to relate them to flight 
management system 10 and flight control system 14 as shown in FIG. 1. In 
addition, navigation sensor system 16 is also shown in FIG. 3 to indicate 
the source of input data. It should be understood that certain necessary 
but conventional functions well known in the art, such as coordinate 
reference transformations, reasonableness checking and initialization, are 
omitted from FIG. 1 to clearly show the unique features of the invention. 
The present invention uses two typical radio navigation sources, GPS and 
MLS, the former of which is normally used for enroute navigation and the 
latter for terminal area navigation. It should be understood, however, 
that the specific navigation systems are merely illustrative, and any 
other equivalent system could be used. In addition, the present invention 
uses INS information from the aircraft's onboard inertial navigation 
system, primarily for short term flight control and complementing the GPS 
data. Navigation sensor system 16 thus provides GPS, INS, and MLS data 
signals to flight management system 10 via the similarly labeled lines. 
The INS and GPS data signals are fed to a Kalman filter 70 whose function 
is to estimate inertial system errors. Kalman filters are discussed in 
Brown, Robert Grover, Random Signal Analysis and Kalman Filtering (John 
Wiley and Sons, 1983), which is incorporated herein by reference. 
The MLS data signal is fed through a single pole, single throw switch 72, 
or equivalent, to an independent monitor 74 and a first weighting device 
76. The MLS position and velocity signals are also fed to Kalman filter 70 
via line 73. These signals are used to improve the accuracy of Kalman 
filter 70 and to calibrate the GPS speed and positional errors relative to 
the airport terminal area. Switch 72 is closed only if a valid MLS signal 
is received. As the graph associated with device 76 indicates, weighting 
device 76 multiplies the signal from Kalman filter 70 by a factor between 
0 and 1.0 depending upon the distance from a reference point associated 
with the target runway. The specific distances used in the following 
description, and in the other weighting device to be described 
hereinafter, are exemplary; and other distances may be used as required. 
When the distance from the target runway reference point is less than 5 
nautical miles (graph region 76a), for example, the multiplier is 1.0; and 
when the distance is greater than 10 nautical miles (graph region 76c), 
the multiplier is 0. Between 5 and 10 nautical miles (graph region 76b) 
the multiplier varies linearly between 1.0 and 0 as a function of 
distance. 
The output of Kalman filter 70 is fed both to independent monitor 74 and to 
second weighting device 78. Similarly to device 76, as indicated by the 
graph, weighting device 78 multiplies the signal from Kalman filter 70 by 
a factor between 0 and 1.0 depending upon the distance from a reference 
point associated with the target runway. As with weighting device 76, the 
specific distances used in the following description are exemplary, and 
other distances may be used as required. When the distance from the target 
runway reference point is less than 5 nautical miles, for example (graph 
region 78a), the multiplier is 0; and when the distance is greater than 10 
nautical miles (graph region 78c), the multiplier is 1.0. Between 5 and 10 
nautical miles (graph region 78b) the multiplier varies linearly between 0 
and 1.0 as a function of distance. 
Both weighting devices 76 and 78 comprise specialized multipliers which 
multiply, or weight, an incoming signal by a factor determined by a 
control parameter. In this case the control parameter is the distance from 
the runway reference point. 
The output signals from weighting devices 76 and 78 are summed together in 
summer 80 to provide a NORMAL composite navigation signal on line 82. As 
may be observed the multipliers for weighting devices 76 and 78 are 
complementary such that at any distance from the target runway, the sum of 
the multipliers is 1.0. This results in a NORMAL composite navigation 
signal which proportionally blends the two input signals. While the 
multipliers in devices 76 and 78 are shown as having linear transfer 
characteristics between 5 and 10 miles, they may have any other shape so 
long as they are complementary and their sum is 1.0. 
The composite navigation signal on line 82 is coupled via a double pole, 
single throw switch, or equivalent, 84 and line 86 to guidance steering 
device 86 in flight control system 14. Guidance steering device 86 
compares the composite navigation signal on line 83 to a signal 
representative of the desired flight path provided by reference device 88 
to provide an error estimate used to generate control signals for the 
aircraft flight controls. Reference device 88 describes the desired 
aircraft horizontal and vertical flight path in mathematical terms, e.g., 
a straight line, a series of line segments or a curved path. The geometric 
descriptions of the flight path comprise conventional analytical 
expressions using latitude/longitude, ECEF (earth-centered, earth-fixed), 
or other suitable coordinate reference frame. 
Switch 84 couples the composite navigation signal on line 82 to guidance 
steering device 87 only if the MLS signal is valid. Otherwise, switch 84 
couples guidance steering device 87 via line 86 to REVERSIONARY signal on 
line 90, which is derived from the output of Kalman filter 70. In other 
words, the navigation signal used by guidance steering device 87 is 
derived from the combination of the INS/GPS and MLS signals when the MLS 
signal is valid and from the INS/GPS signal only, if the MLS signal is not 
valid. 
Guidance and steering device 87 describes the aircraft situation with 
respect to the reference flight path provided by reference device 88 in 
terms of positional and velocity (path vector) errors. Device 88 computes 
the perpendicular, horizontal and vertical distance from the reference 
flight path to the sensed/estimated aircraft position by taking the vector 
difference and converting it into scalar error quantities. Similarly, the 
velocity error from the reference path direction is also calculated. 
The positional and velocity errors are summed and differenced with other 
aircraft feedback parameters, such as attitude and heading, to produce 
stable control signals which command the aircraft control surfaces. The 
control inputs cause the aircraft to modify its flight path so that the 
error from the reference flight path is continually corrected toward zero. 
There are many forms of feedback control systems well known in the art 
which are used to implement the guidance and steering functions. One 
example is described in Etkin, Bernard, Dynamics of Flight (John Wiley and 
Sons, 1959), which is incorporated herein by reference. 
Independent monitor 74 receives and compares both the MLS information and 
the information from Kalman filter 70. Independent monitor 74 evaluates 
the integrity of the MLS signal by comparing it with the integrated 
solution provided by Kalman filter 70 and provides an alert or automatic 
reversion when a difference greater than a predetermined threshhold is 
detected. The threshhold value may be constant, or it may be a function of 
flight phase. It may be implemented as a straightforward mathematical 
difference. 
In operation, Kalman filtered GPS and INS position and velocity data are 
used as the primary source for navigational guidance when aircraft 30 is 
enroute on segment 38 of the flight path (see FIG. 2). When aircraft 30 
comes into range of valid MLS signal coverage, the MLS position and 
velocity data are fed to Kalman filter 70 (FIG. 2) to reconcile 
differences between the GPS and MLS data for the airport terminal area. 
The Kalman filtered solution and the MLS solution are blended together in 
a complementary fashion and then gradually separated as a function of 
range, such as in the range 5 to 10 nautical miles in the present 
exemplary embodiment, such that only MLS data is used for the close-in 
terminal guidance. The reverse process occurs on the takeoff leg of a 
flight. This serves three purposes. First, it meets the certification 
criteria for precision terminal guidance using MLS only. Second, it 
enables the Kalman filtered solution to act as an independent performance 
monitor of MLS guidance, which is especially important at airports having 
only one MLS ground transmitter or receiver. Third, it enables switching 
to the Kalman filtered GPS/INS solution should the MLS become invalid. The 
GPS/INS solution can maintain the same accuracy as the MLS solution for a 
considerable time since the Kalman filtered solution is calibrated to the 
MLS system when the MLS signal is initially acquired. If the MLS failure 
occurs during the last portion of the approach, i.e., during flare and 
touchdown, the reversionary guidance would be sufficiently accurate and 
reliable to complete the maneuver. If the GPS/INS system were not 
calibrated, the GPS/INS solution alone would not permit a precision 
approach to be continued. 
Thus, it may be seen that the present invention provides a single, blended 
3-dimensional airspace navigation and guidance solution for an entire 
flight from takeoff to landing. This solution derived from GPS, MLS and 
other enroute and terminal navigation systems is optimized for the 
requirements of each mission phase, especially the accurcies needed for 
takeoff, landing and precise aerial delivery. In addition, real time 
calibration and cross checking of independent navigation sources is 
performed. This provides alerts when anomalous errors due either to 
equipment failure or human errors are detected. Further, if a failure of 
the MLS occurs, an automatic reversionary mode permits a flight maneuver 
to be continued with the same or higher approach minima as determined by 
certification and performance accuracy requirements. Continuation of an 
approach down to Category II minima (100 feet DH) without MLS signals, or 
perhaps Category III landings, may be attained. Further in an autoland 
system, the present invention provides calibrated GPS as a third 
independent source for voting and monitoring purposes, thus enabling a 
Category III approach to be completed even if an MLS receiver fails, 
wherein two MLS receivers are normally operational. 
The operational features of the present invention offer large benefits, 
including improved flight safety; fewer aborted missions, takeoffs and 
landings; more efficient air traffic management; higher airspace handling 
capacity; and increased crew confidence for terminal area operations. 
While particular embodiments of the present invention have been shown and 
described, it should be clear that changes and modifications may be made 
to such embodiments without departing from the true scope and spirit of 
the invention. It is intended that the appended claims cover all such 
changes and modifications.