Communications: directive radio wave systems and devices (e.g. – Directive – Including antenna orientation
Reexamination Certificate
2002-06-03
2004-11-30
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including antenna orientation
C342S354000, C342S355000, C342S358000
Reexamination Certificate
active
06825806
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to satellites and, more particularly, to antenna pointing and to wide field-of-view attitude acquisition of satellites.
2. Description of the Related Art
The diagram
20
of
FIG. 1A
illustrates a satellite
22
that orbits in an orbital plane
24
about the earth
26
. The satellite has a satellite body
28
which carries an antenna system
29
and solar panels
30
that generate power for the satellite. Although the satellite's orbital plane
24
may be coplanar with the earth's equatorial plane
32
, it is shown more generally as having an inclination
34
.
The satellite
22
may be in a synchronous orbit or alternatively, in a nonsynchronous orbit.
FIG. 1A
illustrates the synchronous alternative by showing the satellite in positions
22
A,
22
B and
22
C at exemplary times T
O
, T
O
+6 hours and T
O
+12 hours. The satellite
22
provides a service (e.g., communication service) to a service area
40
on the earth which is shown in corresponding positions
40
A,
40
B and
40
C.
FIG. 1A
also illustrates the nonsynchronous alternative by indicating that the satellites at positions
22
B and
22
C may be different satellites
36
and
38
. In the nonsynchronous alternative,
FIG. 1A
represents one instant in time (e.g., the time T
O
) and the satellites
22
,
36
and
38
serve respective service areas
40
A,
40
B and
40
C.
FIG. 1B
is an enlarged view of the satellite in position
22
A. This figure shows that the antenna system
29
of the satellite
22
generates a payload beam
42
which forms a payload footprint
44
on the earth
26
. The payload beam generally includes a large number of individual spot beams. In order to enhance the satellite's provided service and reduce the energy needed to provide that service, the payload footprint
44
is preferably coincident with its respective service area. Stated differently, it is important to reduce service error which is any difference between the payload footprint
44
and its respective service area.
The importance of reduced service error has created a need for satellite methods and structures that improve antenna pointing. Sources of error in antenna pointing include mechanical misalignment, thermal deformation, ephemeris error and orbit error. Most systems that improve antenna pointing depend upon sensed signals from attitude references (e.g., sun, stars and earth's horizon). A conventional attitude reference that improves antenna pointing is a beacon ground terminal that radiates a beacon signal. This provides the satellite's receiving antennas with a reference signal from a predetermined terminal location. Beacon systems, however, require additional satellite hardware and the cost of a dedicated beacon terminal.
Accordingly, various alternatives have been proposed. For example, U.S. Pat. No. 3,060,425 radiated a suppressed-carrier, double sideband signal from a plurality of antennas that were arranged transversely to a selected axis of a satellite. These signals were received at an earth-based terminal and demodulated to yield phases and amplitudes indicative of the satellite's attitude with respect to the selected axis. This method requires accurate interferometry equipment and is difficult to implement with conventional communication terminals.
In a method of U.S. Pat. No. 5,790,071, three different phase-shifted pulses, one sum pulse and two delta pulses, are generated on a satellite and transmitted to an earth-based terminal. The delta pulses and sum pulse are used to form two delta-to-sum ratios that indicate relative attitude between the satellite and the terminal. This method requires special phase shift patterns of the antenna which can not be used to generate regular service beams.
U.S. Pat. No. 4,599,619 and U.S. Pat. No. 4,630,058 apply two satellite-generated beacon beams, one regular beam from the satellite communication antenna and one broad beam from a separate antenna that covers a region including and greater than that covered by the regular beam. The beacon beams are received at ground terminals that are positioned near the periphery of the regular beam. Ratios of the regular beam to the broad beam are thereby produced and are used to determine pointing errors of the communication antenna. To practice this method, the satellite must carry the additional antenna and a large number of ground terminals must be appropriately positioned.
A method of U.S. Pat. No. 5,697,050 and U.S. Pat. No. 5,758,260 is directed to a satellite whose antenna generates a moving beam pattern on the earth's surface wherein the beam pattern comprises a plurality of sub-beams. A signal radiated from at least one ground-based transmitter terminal is received with the satellite's antenna and that received signal is retransmitted to the ground terminal. The gain of the received signal is determined at the ground terminal and compared to an expected gain to derive antenna pointing correction signals. This method is restricted to pointing of satellite receiving antennas that have moving beam patterns on the earth's surface.
U.S. Pat. No. 5,812,084 configures a satellite's phased-array antenna in a “straight-through” mode in which all radiating elements radiate with the same amplitude and phase. The antenna's attitude is then estimated based upon straight-through gains measured at two or more receiver sites. Most satellite service beams are, however, not generated in such a “straight-through” mode.
U.S. Pat. No. 4,910,524 oscillates the pointing direction of a satellite transmit beam to produce a periodic or repetitive displacement of a ground pattern, and measuring the resultant oscillatory variation in flux density at a ground station or ground stations to determine the antenna beam pointing errors. For most satellites, however, the addition of a deliberate oscillation of the payload would be an added burden on the satellite, and it is itself another source of antenna pointing error.
U.S. Pat. No. 6,150,977 measures the signal strength of a first spot beam at at least three unique locations on the ground to determine at least one attitude component of the antenna pointing error of a satellite antenna. The requirement that at least three unique ground measurement locations be provided for a single beam is unnecessarily restrictive.
The paper by Loh, “On Antenna Pointing for Communications Satellite” discusses many methods of determining satellite antenna beam pointing, including sun, earth, star and beacon sensors. There is also discussed a system of pointing based on a on-board multiple-beam-antenna (MBA) system. The MBA sensing system processes the magnitudes of signals received from a known uplink site by singlet beams of an on-board MBA system to provide the error for antenna pointing control. Providing good attitude information using this single uplink site taught by Loh requires that position of the uplink site in the singlet beam pattern provides good observability of attitude. Most combinations of uplink site and singlet beam pattern optimized for communication will not have good observability. The current invention addresses this by using multiple uplink sites.
Loh describes three techniques of closed loop control of antenna beam pointing classified under “A.2 On-Ground Sensors”. These are “Ratio of Signals at Various Sites”, “Downlink C/KT's measured by a Spectrum Analyzer” and “Location Determination Using Singlets of MBA”. However, the first technique simply describes and references the teachings of U.S. Pat. No. 4,630,058, discussed above. The second technique “assumes that the downlink C/KT's of each FDMA transponder is measured at a ground station by a spectrum analyzer. The ratios of measured C/KT's to the desired values are used as pointing error for footprint control.”
A system based on this is described in the section “Ground-based Closed-loop Satellite Antenna pointing Control System”, and depicted in
FIG. 10
(using four ground sit
Fowell Richard
Li Rongsheng
Liu Ketao
Wu Yeong-Wei A.
Koppel, Jacobs Patrick & Heybl
Mull Fred H.
The Boeing Company
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