20-12-2012, 03:31 PM
GPS Landing System Reference Antenna
[attachment=44601]
Abstract
The GPS landing system (GLS) is a reality today, and will undoubtedly become the workhorse system in the future. GPS
aircraft navigation is currently utilized for aircraft en-route, terminal, and initial-approach navigation. It is expected that in
2009, a Category I GPS landing system will start its initial phase of a worldwide deployment. The ARL-1900 antenna was
designed specifically to satisfy the stringent requirements for the Category I, II, and III GPS landing system reference-receiver
stations.
A difficult problem for a Category I, II, and III GPS landing systems is the mitigation of ground-reflected multipath effects. The
antenna must provide coverage of the upper hemisphere while suppressing ground-reflected multipath. In addition, the
antenna must operate at the L1, L2, and L5 GPS frequencies, have right-hand circular polarization, and ideally have constant
carrier and code (group) delay throughout the coverage region. Over a period of 15 years, BAE Systems has developed the
ARL-1900 antenna, a unique antenna with near-ideal performance that satisfies the stringent requirements for a Category I,
II, and III GPS landing systems. This paper reviewsthe requirements for a GPS landing system reference antenna, presents
the design principles for the ARL-1900 antenna, describes its implementation, and presents performance data.
Introduction
The instrument landing system (ILS) was introduced in 1941. It
was selected by the ICAO (International Civil Aviation
Organization) in 1946 as the international all-weather landing aid
[1]. It is currently the primary worldwide aircraft landing system. It
uses a localizer antenna to provide horizontal guidance with
respect to the runway's centerline, and a glide-slope antenna to
provide vertical guidance with respect to the runway's glide path.
Distance-to-runway-threshold guidance is provided by marker beacon
antennas and/or a DME (distance measuring equipment)
antenna.
The microwave landing system (MLS) was originally
intended to replace the instrument landing system. The widespread
deployment initially envisioned by its designers in the 1970s and
1980s never became a reality. GPS-based landing systems (GLS),
notably WAAS (wide-area augmentation system), provide the same
level of positional accuracy and substantially larger coverage, with
no equipment needed at the airport. The dramatically lower cost
and performance advantages of a wide-area-augmentation-system
GPS landing system have led to the turning off of most existing
microwave landing systems in North America. The microwavelanding-
system mode of operation is basically the same as that of
the instrument landing system.
Angular Coverage
The antenna has to receive signals from satellites in the upper
hemisphere for all azimuth angles and elevation angles ranging
from 5° to 90°.
ARL-1900 Design Principles
The sharp cutoff [7, 8] requirement for the elevation-angle
radiation pattern, and the omnidirectional azimuth-angle radiation
pattern required for the coverage of the upper hemisphere, led to a
vertical collinear array antenna as the sole viable solution for this
set of antenna requirements. The radiation pattern of an array
antenna can be expressed as the product of two factors: the array
factor and the element factor. The product is strictly correct for an
infinite array, where all elements have the same environment. The
ARL-1900 antenna has 19 elements.
Carrier Delay and Code (Group) Delay
For the measurement of distance between a satellite and the
reference-receiver antenna, the antenna's spatial reference point
(phase center) should ideally have a constant-carrier-delay (phase)
pattern over the coverage region. For this ideal case, the code-delay
pattern over the coverage region is also a constant value when
measured with respect to the same point.
The array-factor excitation that is used for the ARL-1900 (see
[8], Table 1, for a representative excitation) produces an arrayfactor
pattern that has constant carrier delay and constant code
delay in the upper hemisphere. It has the ideal carrier-delay and
code-delay patterns. The phase center is at the center of element
10, the center element. In practice, because of mutual coupling
among the array elements, the signal in the zenith direction is
delayed about 4.5 em (see Figure 8a). This lowers the antenna
phase center so that it is located 4.5 em below the center element.
The resulting computed and measured carrier-delay patterns are
shown in Figure 8. The carrier delay variation was ±l.Ocm.
[attachment=44601]
Abstract
The GPS landing system (GLS) is a reality today, and will undoubtedly become the workhorse system in the future. GPS
aircraft navigation is currently utilized for aircraft en-route, terminal, and initial-approach navigation. It is expected that in
2009, a Category I GPS landing system will start its initial phase of a worldwide deployment. The ARL-1900 antenna was
designed specifically to satisfy the stringent requirements for the Category I, II, and III GPS landing system reference-receiver
stations.
A difficult problem for a Category I, II, and III GPS landing systems is the mitigation of ground-reflected multipath effects. The
antenna must provide coverage of the upper hemisphere while suppressing ground-reflected multipath. In addition, the
antenna must operate at the L1, L2, and L5 GPS frequencies, have right-hand circular polarization, and ideally have constant
carrier and code (group) delay throughout the coverage region. Over a period of 15 years, BAE Systems has developed the
ARL-1900 antenna, a unique antenna with near-ideal performance that satisfies the stringent requirements for a Category I,
II, and III GPS landing systems. This paper reviewsthe requirements for a GPS landing system reference antenna, presents
the design principles for the ARL-1900 antenna, describes its implementation, and presents performance data.
Introduction
The instrument landing system (ILS) was introduced in 1941. It
was selected by the ICAO (International Civil Aviation
Organization) in 1946 as the international all-weather landing aid
[1]. It is currently the primary worldwide aircraft landing system. It
uses a localizer antenna to provide horizontal guidance with
respect to the runway's centerline, and a glide-slope antenna to
provide vertical guidance with respect to the runway's glide path.
Distance-to-runway-threshold guidance is provided by marker beacon
antennas and/or a DME (distance measuring equipment)
antenna.
The microwave landing system (MLS) was originally
intended to replace the instrument landing system. The widespread
deployment initially envisioned by its designers in the 1970s and
1980s never became a reality. GPS-based landing systems (GLS),
notably WAAS (wide-area augmentation system), provide the same
level of positional accuracy and substantially larger coverage, with
no equipment needed at the airport. The dramatically lower cost
and performance advantages of a wide-area-augmentation-system
GPS landing system have led to the turning off of most existing
microwave landing systems in North America. The microwavelanding-
system mode of operation is basically the same as that of
the instrument landing system.
Angular Coverage
The antenna has to receive signals from satellites in the upper
hemisphere for all azimuth angles and elevation angles ranging
from 5° to 90°.
ARL-1900 Design Principles
The sharp cutoff [7, 8] requirement for the elevation-angle
radiation pattern, and the omnidirectional azimuth-angle radiation
pattern required for the coverage of the upper hemisphere, led to a
vertical collinear array antenna as the sole viable solution for this
set of antenna requirements. The radiation pattern of an array
antenna can be expressed as the product of two factors: the array
factor and the element factor. The product is strictly correct for an
infinite array, where all elements have the same environment. The
ARL-1900 antenna has 19 elements.
Carrier Delay and Code (Group) Delay
For the measurement of distance between a satellite and the
reference-receiver antenna, the antenna's spatial reference point
(phase center) should ideally have a constant-carrier-delay (phase)
pattern over the coverage region. For this ideal case, the code-delay
pattern over the coverage region is also a constant value when
measured with respect to the same point.
The array-factor excitation that is used for the ARL-1900 (see
[8], Table 1, for a representative excitation) produces an arrayfactor
pattern that has constant carrier delay and constant code
delay in the upper hemisphere. It has the ideal carrier-delay and
code-delay patterns. The phase center is at the center of element
10, the center element. In practice, because of mutual coupling
among the array elements, the signal in the zenith direction is
delayed about 4.5 em (see Figure 8a). This lowers the antenna
phase center so that it is located 4.5 em below the center element.
The resulting computed and measured carrier-delay patterns are
shown in Figure 8. The carrier delay variation was ±l.Ocm.