Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-04-15
2002-01-29
Hellner, Mark (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S033000, C701S215000
Reexamination Certificate
active
06342853
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to aviation navigation systems and more specifically to a system for using differential global positioning system (DGPS) receivers to replace dual instrument landing system (ILS) receivers for use with autopilot systems for Category III precision approaches.
BACKGROUND ART
The aviation industry relies upon numerous navigation aids in order to safely take off, navigate enroute, and land aircraft. Among these, the instrument landing system (ILS) is the internationally accepted and standardized navigation aid for landing aircraft at properly equipped airports. GPS, however, is increasingly being accepted as an alternative to traditional navigation aids, including even ILS.
Essentially, GPS is a space based radio positioning network for providing users equipped with suitable receivers highly accurate position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous orbits around the earth.
FIG. 1
shows the constellation
100
of GPS satellites
101
in orbit. The GPS satellites
101
are located in six orbital planes
102
with four of the GPS satellites
101
in each plane, plus a number of on orbit spare satellites (not shown) for redundancy. The orbital planes
102
of the GPS satellites
101
have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles) and typically complete an orbit in about 12 hours. This positions each of the GPS satellites
101
in such a manner that a minimum of five of the GPS satellites
101
are normally observable (above the horizon) by a user anywhere on earth at any given time.
The orbiting GPS satellites
101
each broadcast spread spectrum microwave signals encoded with positioning data. The signals are broadcast on two frequencies, L
1
at 1575.42 and L
2
at 1227.60, with the positioning data modulated using bi-phase shift keying techniques. A user receives the signals with a GPS receiver. The GPS receiver is adapted to demodulate the positioning data contained in the signals. Using the positioning data, the GPS receiver is able to determine the distance between the GPS receiver and a corresponding transmitting GPS satellite. By receiving signals from several of the GPS satellites
101
and determining their corresponding range, the GPS receiver is able to determine its position and velocity with a greater accuracy than conventional radio navaids.
Applications of CPS to aircraft navigation are currently partitioned into two main areas. The first area includes the en route terminal, and non-precision approach phases of flight. The second area is the precision approach phase of flight. This is a natural partition for both historical and practical reasons. The GPS signals commonly available to civilian users are referred to as the standard positioning service (SPS). The accuracy of SPS is specified by DOD to be within 100 meters horizontal positioning accuracy 95% of the time and 300 meters 99.99% of the time. The 100 meter accuracy specification currently is sufficient, i.e., at least as accurate or better than current approved navigation systems, for all phases of flight down to and including non-precision approaches. SPS, however, is not sufficiently accurate for vertical guidance in the precision approach phase of flight.
The lateral and vertical navigation sensor accuracies for precision approach traditionally have been based on three categories of approach:
Category I (CAT-I), Category II (CAT-II), or Category III (CAT-III). A precision approach is where an aircraft relies primarily upon instruments for landing, due to bad weather or other constraints. The operational definitions of these categories are based on visibility (runway visual range) and landing decision height. Category III has the most stringent requirements, including very stringent equipment redundancies, lateral guidance in roll out for Category IIIb and Category IIIc, and other requirements. Table 1 below summarizes the requirements specified by the traditional categories.
TABLE 1
Traditional categories of precision approach
Category
Runway visual range
Decision height
CAT-I
1800-2400
ft
200
ft
CAT-II
1200
ft
100
ft
CAT-IIIa
>700
ft
<100
ft
CAT-IIIb
150-700
ft
<50
ft
CAT-IIIc
<150
ft
0
ft
FIG. 2A
shows a schematic diagram a down view of an airport runway
20
relative to a flight path
21
of an aircraft
22
, and
FIG. 2B
shows a corresponding side view. The aircraft
22
has executed a CAT-II approach. The aircraft
22
follows a glide slope as it approaches the runway
20
, creating a flight path
21
. A decision height is that height above the runway at which aircraft
22
must declare a missed approach if the runway is not yet in view. In the present example, a CAT-II approach, required RVR
24
is 1200 ft and the required decision height
25
is 100 ft above runway
20
. Thus, CAT-IIIc allows flight right down to the runway surface (decision height of 0 ft) with potentially zero RVR. As can be expected, requirements for CAT-IIIc approaches are very stringent.
Before GPS can be used by the aviation community for precision approaches, GPS aviation electronics (avionics) need to be certified for these most demanding and flight critical phases of flight. Aircraft flying precision approaches utilize the well known ILS technology. ILS uses a ground based azimuth transmitter (localizer) and a ground based elevation transmitter (glide slope) which define a precision approach flight path to be followed. By employing ILS receivers, a properly equipped aircraft is able to fly down the ILS flight path to land on the runway.
Current CAT-III ILS systems used in newer aircraft employ dual ILS receivers on board the aircraft for added redundancy and fault detection. The aircraft's autopilot compares the outputs of these two receivers, and if they disagree by more than a certain predetermined amount, a fault is declared, and the aircraft must execute a missed approach. The CAT-III ILS systems are routinely accurate to within 2 ft in the vertical dimension at 100 feet above the runway surface. They are required to work under extreme weather conditions and at life-critical levels of performance (for example, the probability of a missed detection of failure not exceeding 5×10
−9
). This is the standard at which CAT-III GPS systems are expected to perform.
There are several prior art methods being considered for employing GPS for CAT-III auto landings. One method involves using differential GPS (DGPS) techniques to improve the accuracy and fault detection capability of SPS. DGPS involves placing a local area augmentation system DGPS transmitter near the airport. The transmitter broadcasts DGPS corrections and integrity data to nearby aircraft which use the data to determine their accurate DPGS positions. Although the DGPS positions tend to be sufficiently accurate in the horizontal dimension, they have much less margin in the vertical dimension. In addition, the code phase measurements used in typical prior art DGPS systems tend to be noisy in comparison to ILS determined measurements. Thus, even though DGPS is potentially more accurate overall than ILS, the noise characteristics are such, and the ILS comparison threshold is set so low, that two GPS receivers will disagree enough that a fault would get declared by the autopilot on approximately 50% of the approaches. This is because ILS receivers exhibit very low noise, even though the accuracy of the signal is usually inferior to DGPS. This relationship is illustrated below.
FIG. 2C
shows a comparison of the relative accuracy and error characteristics of ILS versus DGPS. Graph
30
shows the angular error of two ILS receivers (e.g., ILS #
1
and ILS #
2
). The vertical direction of graph
30
represents angular error and the horizontal direction represents time. In graph
30
, ILS #
1
and ILS #
2
both begin the approach at zero angular error (
Braisted Paul
Kalafus Rudolph M.
Hellner Mark
Trimble Navigation Limited
Wagner , Murabito & Hao LLP
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