Note: Descriptions are shown in the official language in which they were submitted.
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ACTIVE ANTENNA COMMUNICATION SYSTEM
The present application claims priority from a U.S. provisional patent
application, Serial No. 60/197,799, filed April 14, 2000, which is
incorporated
herein by reference for all purposes.
The present invention relates to wireless communication systems, and
more particularly, to a wireless communication system using an aircraft with
one or
more ground-based stations.
BACKGROUND OF THE INVENTION
The need for high-bandwidth, last-mile connectivity to voice and data-
stream end-users has been rapidly increasing for quite some time. This need
for
increased communication capacity exists both in urban locations that have a
substantial communications infrastructure, and in lesser-developed areas that
lack
such infrastructures. Communications signals can be delivered to end-users
through
a number of different types of communication systems. A wired, terrestrial
system
typically provides high speed communication for a large bandwidth signal.
However, the infrastructure for such a system is expensive and time consuming
to
build, maintain and upgrade, and it does not, by itself, support mobile
communications. A wireless system that uses transmission towers provides
reasonably high speed communication for a substantially more limited bandwidth
per the ground area served.
Geostationary Earth Orbit (GEO) satellites (at an altitude of about
36,000 kilometers) can also provide wireless communications to end-users, but
are
limited by bandwidth efficiency because of their extremely high altitude. Even
narrow-beam antennas mounted at such distances encompass large land areas.
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Therefore, GEO satellites are limited in their ability to serve high-bandwidth
communication needs in most areas, and particularly for densely populated
areas.
Furthermore, GEO satellites must be in equatorial orbits, which limits their
practical
use to equatorial land regions.
Medium and low Earth orbit (MEO and LEO) satellite systems (at
altitudes of 10,000 kilometers and 700-1500 kilometers, respectively) are
complex
in nature because end-user's are required to have equipment to track the
satellites'
relative movement across the sky. Non-geostationary satellites require
complex,
continuously adjusting, directional antennas that are able to gimbal through
large
angles. These antennas are needed both in the air and on the ground, typically
with
the ground antennas having secondary antenna systems adapted to switching
communications signals from one passing satellite to the next. Of course, none
of
. the above satellites are easily retrieved, e.g., for servicing.
Aircraft are used in a wide variety of applications, including travel,
transportation, fire fighting, surveillance and combat. Aircraft can be used
to relay
communication signals. Ground-stations for such a purpose would typically
require
either low-bandwidth, onlnidirectional antennas or large gimbal-angle
capabilities
(similar to ground-stations for MEO or LEO satellites) because such aircraft
would
travel substantial distances even if circling.
Unfortunately, normal gimbaled ground-stations axe expensive devices
that are susceptible to damage and wear-and-tear. Using wide-angle or omni-
directional antennas can avoid the use of gimbals. However, the larger
broadcast
angles require additional power and, more important, limit the reuse of
frequencies
by nearby ground-stations and/or nearby aircraft. Thus, the system-wide
bandwidth
is limited by the use of wide angle or omni-directional antennas.
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An exception to the above-stated requirement for the antennas to have
either wide broadcast angles or large-angle gimbals is where the aircraft is
capable
of station-keeping in a small station in the sky for a long period of time,
i.e., act as a
long-duration, suborbital high-altitude platform. Such an aircraft is
described in
U.S. Patent No. 5,810,284. This aircraft design is embodied by the well-known
Pathfinder, Centurion and Helios aircraft.
These aircraft are capable of maintaining position at stratospheric
altitudes for long periods of time, allowing ground-stations to use fixed,
narrow-
beam antennas (e.g., 2° or 3° bandwidth antennas having no
steering mechanisms
other than simple ones for initially acquiring the target). These narrow-beam
antennas allow for frequency reuse between multiple ground-stations and a
given
aircraft, as well as between one ground-station (or closely adjoining ground-
stations) and multiple aircraft. However, such aircraft can expend significant
resources (i.e., power) in maintaining the tight station necessary for using
such
narrow-beam antennas. The power is spent both in tight maneuvering, and in
quickly compensating for momentary variations in local flight conditions.
It is desirable to develop a communication system that provides for
high-bandwidth signals to a large number of low-priced, durable ground-
stations.
Various embodiments of the present invention can meet some or all of these
needs,
and provide further, related advantages.
SUMMARY OF THE INVENTION
In various embodiments, the present invention solves some or all of
the needs mentioned above by providing a communication system that provides
for
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the aircraft to have a larger flight station while still having the advantages
of using
narrow-beam ground-station antennas.
The communication relay system of the invention typically includes an
aircraft and a plurality of ground stations for which the aircraft relays
signals. The
aircraft is configured to stationkeep within a designated flight-station,
which is only
a portion of the aboveground field of vision that can include a plurality of
other
potential flight-stations. The aircraft includes a communication relay module,
which has one or more antennas for communicating with the ground-stations. The
ground-stations are located within a coverage area, and each ground-station
has at
least one antenna configured to communicate, via communication signals, with
at
least one of the antennas of the communication relay module.
A feature of the invention is that the beamwidth of the ground-station
antenna is narrow enough such that it is inadequate to illuminate (i.e.,
transmit to
and/or receive from) the entire flight-station at one time. Because the
aircraft can
move throughout the flight-station, each ground-station antenna is configured
to be
steerable under the control of an antenna controller, such that the ground-
station
antenna can maintain communication with the aircraft's communication relay
module as the aircraft moves throughout the flight-station. The antenna
controller is
configured to limit the steering of the antenna such that it avoids directing
the
antenna at any flight-station other than the designated flight-station.
Advantageously, most embodiments having this feature will have
lower power usage by the ground-station antenna, and will have less crosstalk
from
nearby communications using the same frequencies, as compared to having the
ground-station antenna have a beamwidth large enough to illuminate the entire
flight-station. Furthermore, the aircraft will have to complete fewer
maneuvers and
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expend less energy to maintain station as compared with a flight-station small
enough to be fully illuminated by the narrow-beam ground-station antenna.
An additional feature of the invention is that aircraft positional
information is transmitted from the aircraft to the ground-station and/or
received by
the ground station using a wide-beam or omnidirectional antenna, thereby
allowing
the ground station to receive the information without having its antenna
properly
aimed at the aircraft.
Other features and advantages of the invention will become apparent
from the following detailed description of the preferred embodiments, taken in
conjunction with the accompanying drawings, which illustrate, by way of
example,
the principles of the invention. The detailed description of particular
preferred
embodiments, as set out below to enable one to build and use an embodiment of
the
invention, are not intended to limit the enumerated claims, but rather, they
are
intended to serve as particular examples of the claimed invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustrative view of a preferred embodiment of a
communication system embodying the invention.
FIG. 2A is an elevational view of an aircraft used in the
communication system depicted in FIG. 1.
FIG. 2B is a plan view of the aircraft depicted in FIG. 2A.
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FIG. 3 is another illustrative view of the communication system
depicted in FIG. 1.
FIG. 4 is an elevational view of a flight-station, over a number of
ground-stations, as used in the communication system depicted in FIG. 1.
FIG. 5 is a plan view of an array of flight-stations, as used in the
communication system depicted in FIG. 1.
FIG. 6 is a plan view of an array of ground-level illuminations by
overlapping aircraft antenna beams that define cells within a coverage area,
as used
in the communication system depicted in FIG. 1.
FIG. 7 is an elevational view of directional ground antennas targeting
an aircraft within a flight-station, as used in the communication system
depicted in
FIG. 1.
FIG. 8A is a schematic view of a first embodiment of a steerable
antenna as used in a ground-station of the communication system depicted in
FIG. 1.
FIG. 8B is a schematic view of a second embodiment of a steerable
antenna as used in a ground-station of the communication system depicted in
FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention summarized above and defined by the enumerated
claims may be better understood by referring to the following detailed
description,
which should be read in conjunction with the accompanying drawings. This
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detailed description of particular preferred embodiments of a communication
system, set out below to enable one to build and use particular
implementations of
the invention, is not intended to limit the enumerated claims, but rather it
is intended
to provide particular examples thereof.
With reference to FIG. 1, a communication system embodying the
invention includes one or more ground-stations 102, one or more aircraft 104
and
preferably one or more satellites 106. The ground-stations are located in
cells 108
that are targeted by directional antennas of the aircraft. Each airplane is
stationkeeping within a limited flight-station at stratospheric altitudes,
e.g., between
the altitudes of 50,000 feet and 70,000 feet. Preferably each flight-station
is set at
the same altitude as the other flight-stations. The aircraft uses one-way or
two-way
communication signals to relay ground-station communications to other ground-
stations and/or satellite networks.
Airplane
The invention preferably includes the use of an airplane as a
substantially geostationary platform having moderately tight station-keeping
requirements. In accordance with the present invention, the preferred airplane
is of
a design similar to that of the Pathfnder, Centurion and/or Hellos aircraft.
While
the preferred airplane's design is described below, further details are
provided in
U.S. Patent No. 5,810,284, which is incorporated herein by reference.
Nevertheless,
it is to be understood that other aircraft, such as helicopters, balloons,
blimps, kites
or other types of airplanes are within the scope of the invention.
With reference to FIGS. 1, 2A and 2B, the preferred aircraft 104
embodiment is a flying wing airplane, i.e., it has no fuselage or empennage.
Instead, it consists of an upswept wing 112, having a substantially consistent
airfoil
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shape and size along the wingspan. Preferably, six, eight or fourteen electric
motors
114 are situated at various locations along the wingspan, each motor driving a
single
propeller 116 to create thrust. Preferably, two, four or five vertical fins
118a - 118d,
or pods, extend down from the wing, with landing gear at their lower ends.
The preferred airplane 104 is solar-powered, and includes fuel cells to
store energy for continuous day and night flight. It is therefore ideally
suited to fly
continuous, unmanned missions of over a week to ten days, (e.g., 200 hours)
and
more preferably, of 3000 hours, or longer. Alternatively, it can be designed
to
derive some or all of its power from hydrogen fuel (such as liquified hydrogen
to be
used in either a fuel cell or a conventional motor), fossil fuels or other
stored fuels,
or combinations of fuel sources such as solar power by day and stored non-
renewable or partially renewable fuels by night.
The aircraft 104 is longitudinally divided into preferably five or six,
modular segments sequentially located along the wingspan. These segments range
from 39 to 43 feet in length, and have a chord length of approximately eight
feet.
Thus, the aircraft has length of approximately eight feet, and preferably has
a
wingspan of approximately 100, 120, 200 or 250 feet. The airplane's wing
segments
each support their own weight in flight so as to minimize inter-segment loads,
and
thereby minimize required load-bearing structure.
The fins 118a - 118d extend downward from the wing 112 at the
connection points between segments, each fin mounting landing gear front and
rear
wheels. The fins are configured as pods to contain elements of the aircraft,
such as
electronics, and/or various payloads. One of the pods, a "control pod" is used
to
carry control electronics, including an autopilot principally embodied as
software, to
control the motors and elevators. In addition, the pods carry sensors,
including
global positioning system equipment, as well as communications equipment.
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The airplane also includes a communication relay module that
includes the aircraft's antennas for transmitting to and/or receiving from the
ground-
stations. The aircraft's antennas have moderate beamwidths, preferably on the
order
of 10° - 20°.
As a result of the above design, the preferred embodiment of the
aircraft is light (less than 1 pound per square foot of wing area), travels at
relatively
slow air speeds (from 13 knots at low altitudes to 100 knots at high
altitudes), and
needs relatively little electrical power from the arrays of solar cells in
order to stay
airborne. The relatively slow flight capabilities of the airplane aid the
airplane's
capability for long-duration flight and tight maneuvering during
stationkeeping.
Flight-Stations
With reference to FIGS. 1 and 3-6, each airplane 104 stationkeeps,
i.e., maintains a substantially geostationary position relative to the ground-
stations
102. This substantially geostationary position is a flight-station 132 having
a center
point 134, and an allowed lateral and altitudinal wandering distance. Thus,
the
High-station is typically a cylindrical shaped section of airspace, where the
cylinder
shape extends longitudinally in a vertical direction. Preferably, the flight-
station is
at an altitude of around 60,000-70,000 feet, above normal air traffic and
atmospheric disturbances (e.g., storms). At this altitude, the maximum
strength
winds have lower speed than the winds at lower jet-stream regions.
Preferably, each aircraft 104 is maintained in a separate flight-station
132 that is separated from the other flight-stations by a separation distance
136. At
any given time, each aircraft could be at any location within its flight-
station (as
depicted in FIG. 5). The separation distance both assures that one airplane
does not
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fly within the beamwidths of another's associated ground antennas, and serves
to
protect the airplanes from striking each other.
Ground-Station
With reference to FIGS. 1 and 3, the ground-stations 102 within each
5 cell 108 are terrestrial communication nodes that preferably broadcast
signals to,
and/or receive signals from, one or more of the aircraft 104. The ground-
stations
are typically far more numerous than the number of cells (i.e., there are
numerous
ground-stations in most cells). Ground-based communications equipment is
connected to the ground-stations, and typically includes one or more end-user
10 terminals (i.e., communications equipment for one or more end-users). Each
ground-station includes one or more narrow-beam antennas that can each
broadcast
signals to, and/or receive communication signals from, antennas of the
communications module on one of the aircraft.
The ground-station antennas preferably have a narrow beamwidth,
e.g., around of 2°, 2.5°, 3° or 4°, providing for
a high potential bandwidth at
reasonable power levels. These antennas have a steering mechanism that
provides
for the aim of the antenna to be tweaked on the order of 3° or
6° from a nominal
position, which is on the order of one to three times the beamwidth of the
ground-
station antenna. The communication system includes one or more controllers to
instruct and thereby control the ground-station antennas' steering. A separate
controller can be in each ground-station, or a single controller can be
located in
either the aircraft or a controlling ground-station. The controlling ground-
station
can be in contact with the aircraft, which relays control information to the
other
ground-stations, or directly in communication with the regular ground-
stations.
Furthermore, a controller can be co-located in a number of system components,
such
as partially in the airplane and partially in each ground-station.
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A single ground-station can include multiple ground-station antennas
that can be aimed at, and access signals from, different aircraft, thus
increasing the
available bandwidth. Separate controllers can control the different antennas,
or a
single system controller can control all the ground-station antennas.
The ground-station also includes an initial-aim adjustment
mechanism. This mechanism will typically be a manually adjusted and locked
system that includes some type of signal strength indicator to aid in setting
the
nominal aim of the antenna to the center point 134 of a flight-station 132.
Ground-station Cells
The antennas on each airplane 104 are configured and targeted to
illuminate an area 142 of the ground that is substantially filled by one cell
I08.
These preferably hexagonal cells can be of varied sizes, which are preferably
commensurate with the beamwidth of the airborne antennas at a distance equal
to
the cruising altitude of the airplane. The airplane's antennas can be targeted
to
illuminate overlapping ground areas so as to achieve complete cellular
coverage
over a coverage area 144. The coverage area might typically have a radius on
the
order of 10 to 30 miles.
The airplane antennas are carried in one or more payload modules on
the airplane 104. Using gimbals, the antennas maintain their attitude, and are
decoupled from the roll-pitch-yaw and translational motion of the aircraft.
Preferably all of the aircraft antennas are mounted on a single, gimbaled
platform to
limit the number of active gimbals. Thus, each aircraft antenna's aim is
maintained
on its respective cell I08.
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Antenna Beam Manipulation
With reference to FIG. 7, under the present invention the size of the
flight-station 132 is larger than the narrow-beam beams 152 of the ground-
stations
102 could cover without moving. By using a slight tweaking (i.e., minimal
steering)
of the direction of the ground-station antennas, the communication system can
enjoy
the benefits of having narrow-beam ground antennas (which would otherwise
require flight-stations on the order of X0.5 mi laterally and X0.1 mi
vertically from a
central reference point), while the aircraft can enjoy the benefits of having
a larger
flight area, such as about ~ 1.5 mi laterally and ~ 1.0 mi vertically from a
central
reference point.
In particular, the airplane can be operated on average with less power
than would be needed to maintain a smaller station, and the airplane can
stationkeep
in more difficult whether conditions, such as strong winds, high altitude-
penetrating
thunderstorms, turbulence and vertical air motions. Furthermore, from the
reliability standpoint, it will not have to maneuver as often or as violently,
and its
antenna platform will be more easily stabilized with more limited deflections.
In conducting ground-station antenna steering under the invention, the
ground-station antenna controller is preferably configured to steer the
antenna
beams) to move throughout an entire flight-station. They are further
preferably
configured to limit the ground-station antenna beam steering such that the
beams
avoid crossing into any flight-station other than a particular, designated
flight-
station. This configuration might occur in control system software or
hardware,
because the amount of beam-steering that is necessary will depend on the
relative .
positions of the ground-station and flight-station, and on the size and shape
of the
flight-station. In particular, wide flight-stations will require higher
movement
capability from ground-stations directly underneath the flight-station than
from ones
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substantially distanced from the aircraft's location. Similarly, tall flight-
stations
will require higher movement capability from ground-stations substantially
distanced from the aircraft's, location than from ones directly underneath the
flight-
station. These geometric requirements can be readily calculated by the
controller.
Turning now to FIGS. 8A and 8B, ground-station antennas will
typically include a feed horn 202 and a main dish 204. The antennas might also
include a secondary reflector 206.
The preferred actuators for tweaking the antenna steering are low
powered and long lived. Because they do not need to deflect over large angles,
they
can be simple mechanisms that are lower cost and can have more reliability
than
large-angle gimbal systems. Among the types of mechanisms that can be used are
servomotors, stepper motors, piezoelectric actuators and bimetallic strips.
Gimbals
can also be used in some embodiments of the invention.
The steering of the antennas can be reoriented mechanically in a
number of different ways. For example, an entire antenna assembly could be
repositioned. More preferably, however, only a portion of the antenna
assembly,
such as the main dish (see FIG. 8A), the secondary mirror (see FIG. 8B), or
the feed
horn could be repositioned to a deflected position 208. Repositioning the feed
horn
or the secondary mirror is preferred, as they axe typically smaller devices.
If
repositioning either the feed horn or the secondary mirror is used, in might
be
necessary to use a larger main dish than would be needed for a fixed antenna.
Other types of antennas are also within the scope of the invention. For
example, a phased array could be used, i.e., a group of antennas in which the
relative phases of the respective signals feeding the antennas are varied in
such a
way that the effective radiation pattern of the array is reinforced in a
desired
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direction and suppressed in undesired directions. In such a case, the antenna
could
be steered electronically. Likewise, an array of narrow-beam antennas targeted
in a
pattern that covers the entire flight-station could be selectively used by a
control
system as a single, steerable antenna. Thus, not all embodiments require a
physical
motion to steer the antenna.
Beam-Steering Control System
In order for the ground-station antennas to steer such that their beams
follow the airplane as it moves throughout the flight-station, the ground-
station must
gain antenna-steering information (i.e., information about the required
vertical and
horizontal ground-station antenna orientation manipulations). This information
can
be developed in a number of different ways, in number of different control
system
embodiments. Typically, this information will be generated from aircraft-
location
information, as well as from information on the relative positions of the
ground-
stations with respect to the flight-station.
In a first embodiment of a ground-station antenna-steering control
system for the invention, the airplane's location is established by the
airplane, such
as by using a global positioning system (GPS) reading. The information is then
transmitted to each ground-station, either encoded within the carrier signal
normally
transmitted to each cell, or via a separate, narrow channel broadcast using
broad
beam or omnidirectional antennas to transmit and/or receive the information.
The information can be provided in a number of formats. For
example, the information can be sent as an absolute geographic position, a
relative
position of the aircraft with respect to the cell, or a relative position of
the airplane
with respect to the flight-station. Alternatively, the information can be
transformed
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into antenna steering information for each given cell and/or each group of one
or
more ground-stations and then transmitted.
It is worth noting that the antenna-orientation information and/or the
aircraft location information represent a small amount of data requiring a
very low
5 data-rate to transmit, and little transmission power. That information needs
to reach
every ground-station that has an antenna targeting the aircraft. Each ground-
station
will have to conduct elevation and azimuth angle steering appropriate for its
geographical position relative to the aircraft. If one broad-beam antenna on
the
plane is used to send plane orientation information to reach all users, then
either the
10 information should be coded to each ground-station, specifying that
antenna's
steering requirements, or each ground-station needs to compute its own
steering
needs based on the airplane's position information.
In a second embodiment of a ground-station antenna-steering control
system for the invention, each airplane's location can be established by a
ground-
15 based central-control station, such as by using radar ranging and direction
finding.
The information can be telemetered to the. plane and relayed to the ground-
stations
in a manner similar to that discussed above for the first embodiment of a
ground-
station antenna-steering control system. Likewise, that information can be
transmitted to the ground-stations through other means such as available
ground
communication systems or separate wireless transmissions. Again, the
information
can be provided in a variety of forms, such as aircraft location information
or
antenna steering instruction information.
In a third embodiment of a ground-station antenna-steering control
system for the invention, the airplane's location is established by each
ground-
station, such as by an autonomous tracking system based on the aircraft's
transmission signal strength. In this type of system, the ground-station
antenna is
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periodically steered through small angles and the signal strength is compared
at each
position. Stronger signals indicate that the antenna is closer to being
centered on the
aircraft. Of course, once the ground-station is locked onto its respective
airplane, it
will stay locked onto it without transmissions of information from the
airplane.
It should be noted that if the third embodiment's antenna loses track
of the airplane, such as might occur when the system is powered down, it can
conduct a search pattern covering its range of motion, which should cover the
entire
flight-station. This ability might also be necessary for other embodiments if
the
antenna-steering information is sent to the ground-station embedded in the
normal
transmissions of the aircraft, which would be lost when the antenna lost track
of the
aircraft. The use of omnidirectional antennas by the control system generally
eliminates the need for significant or frequent scanning.
Other Considerations
The principles of small-angle antenna steering for the ground-station
antennas can be adapted to a wide range of flight-station sizes, such as X15
miles
laterally and ~5 miles vertically, or X20 miles laterally and ~3 miles
vertically.
However, as the stationkeeping loosens, secondary effects become more
relevant,
such as signal strength variations as distances vary significantly, or
interference with
other users of the same frequency that were otherwise shielded by strict
directionality. Furthermore, the spacing between one flight-station and nearby
flight-stations might need to be increased.
Small-angle steering could also be used on the airplane antennas as
well as the ground-station antennas, adding a fine control onto the large-
angle
gimbals that stabilize the antennas during flight.
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In the overall communication system, there may be ground-stations
that do not use only small angle adjustments. Such ground-stations would
include
mobile ground-stations, and ground-stations that are designed to switch
communication between different aircraft (e.g., for aircraft command and
control).
It should be understood that using limited directional adjustments in
ground-stations adds price and complexity over fixed ground-stations, but it
provides many benefits related to efficiency, energy supply, and maneuvering
ability
of the stratospheric airplane serving as a relay station. Thus, the total
communication system effectiveness, cost, and reliability can be improved
under
many embodiments of the invention.
The resulting system can be used for two-way communications
between ground stations and other locations, one-way broadcasts to the ground-
stations, or even one-way broadcasts by the ground-stations. Thus, it should
be
understood that the above descriptions of antennas illuminating cells or
flight
stations are a reference to the antennas' beam width taken at a distance, and
not
necessarily to an antenna configured to transmit communications rather than
only
receive communications.
While particular forms of the invention have been illustrated and
described, it will be apparent that various modifications can be made without
departing from the spirit and scope of the invention. Thus, although the
invention
has been described in detail with reference only to the preferred embodiments,
those
having ordinary skill in the art will appreciate that various modifications
can be
made without departing from the scope of the invention. Accordingly, the
invention
is not intended to be limited by the above discussion, and is defined with
reference
to the following claims.
