Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1999-09-29
2003-10-14
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication over free space
Repeater
C455S427000
Reexamination Certificate
active
06633551
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a high-reliability spacecraft arrangement in which overlapping antenna beams are sequentially generated, as for control of the pointing of an antenna.
BACKGROUND OF THE INVENTION
This invention relates to spacecraft for cellular communications systems, and more particularly to such systems which provide coverage between terrestrial terminals in a region by way of a spacecraft, where some of the terrestrial terminals may be mobile terminals, and some may be gateways which link the cellular system with a terrestrial network such as a public switched telephone network (PSTN).
A salient feature of a spacecraft communication satellite is that all of the electromagnetic transmissions to the user terminals originate from one, or possibly a few, spacecraft. Consequently, the spacecraft communication antenna must form a plurality of beams, each of which is directed toward a different portion of the underlying target region, so as to divide the target area into cells. The cells defined by the beams will generally overlap, so that a user communication terminal may be located in one of the beams, or in the overlap region between two beams, in which case communication between the user communication terminal and the spacecraft is accomplished over one of the beams, generally that one of the beams which provides the greatest gain or signal power to the user terminal. Operation of spacecraft communication systems may be accomplished in many ways, among which is Time-Division Multiple Access, (TDMA), among which are those systems described, for example, in conjunction with U.S. Pat. Nos. 4,641,304, issued Feb. 3, 1987, and 4,688,213, issued Aug. 18, 1987, both in the name of Raychaudhuri. Spacecraft time-division multiple access (TDMA) communication systems are controlled by a controller which synchronizes the transmissions to account for propagation delay between the terrestrial terminals and the spacecraft, as is well known to those skilled in the art of time division multiple access systems. The TDMA control information, whether generated on the ground or at the spacecraft, is ultimately transmitted from the spacecraft to each of the user terminals. Consequently, some types of control signals must be transmitted continuously over each of the beams in order to reach all of the potential users of the system.
More specifically, since a terrestrial terminal may begin operation at any random moment, the control signals must be present at all times in order to allow the terrestrial terminal to begin its transmissions or reception (come into time and control synchronism with the communication system) with the least delay.
When the spacecraft is providing cellular service over a large land mass, many cellular beams may be required. In one embodiment, the number of separate spot beams is one hundred and forty. As mentioned above, each beam carries control signals. These signals include frequency and time information, broadcast messages, paging messages, and the like. Some of these control signals, such as synchronization signals, are a prerequisite for any other reception, and so may be considered to be most important. When the user communication terminal is synchronized, it is capable of receiving other signals, such as paging signals.
Communication spacecraft are ordinarily powered by electricity derived from solar panels. Because the spacecraft may occasionally go into eclipse, the spacecraft commonly includes rechargeable batteries and control arrangements for recharging the batteries when the power available from the solar panels exceeds the power consumed by the spacecraft payload. When a large number of cellular beams are produced by the antenna, a correspondingly large number of control signals must be transmitted from the spacecraft. When one hundred and forty beams are transmitted, one hundred and forty control signals must be transmitted. When the power available from the solar panels is divided between the information and data transmission channels of the spacecraft, the power available to the synchronization and paging signals may be at a level such that a user communication terminal in an open-air location may respond, but a similar terminal located in a building may not respond, due to attenuation of electromagnetic signals by the building.
FIG. 1
is a simplified block diagram of a spacecraft or satellite cellular communications system
10
, as described in U.S. patent application Ser. No. 08/986,611, filed Dec. 8, 1997 in the name of Kent et al. In system
10
, a spacecraft
12
includes a transmitter (TX) arrangement
12
t
, a receiver (RX) arrangement
12
r
, and a frequency-dependent channelizer
12
c
, which routes bands of frequencies from the receiver
12
r
to the transmitter
12
t
. Spacecraft
12
also includes an array of frequency converters
12
cv
, which convert each uplink frequency to an appropriate downlink frequency. Spacecraft
12
includes a power source which includes a solar panel (SP) illustrated as
12
s
, and a power converter (PC) or conditioner
12
p
for converting the solar array power into power suitable for powering the transmitter, receiver, and converters, and other devices on the spacecraft, such as, for example, attitude control systems. A transmitting antenna
12
at
mounted to the spacecraft body by a two-axis gimbal
12
gt
generates a plurality
20
of spot beams, one or more spot beams for each frequency band. Some of the spot beams
20
a
,
20
b
, and
20
c
of set
20
are illustrated by their outlines. Each antenna beam
20
x
(where x represents any subscript) defines a footprint on the surface
1
of the Earth below. The footprint associated with spot beam
20
a
is at the nadir
3
directly under the spacecraft, and is designated
20
af
. The footprint associated with spot beam
20
c
is designated
20
cf
, and is directed toward the horizon
5
, while the footprint
20
bf
associated with spot beam
20
b
is on a location on surface
1
which lies between nadir
3
and horizon
5
. It will be understood that those antenna beams which are illustrated in “lightning bolt” form also produce footprints. As is known to those skilled in the art, the footprints of antenna beams from a spacecraft may overlap (overlap not illustrated in FIG.
1
), to provide continuous coverage of the terrestrial region served by the antennas. Spacecraft body
12
b
also carries, by way of a two-axis gimbal
12
gr
, a receiving antenna
12
ar
, which produces spot beams which are intended to be identical to those of transmitting antenna
12
at.
Spacecraft
12
also includes a further transmit-receive antenna
72
a
, which produces a single, or possibly two or three, broad transmit beam(s) and corresponding receive beam(s), such as those designated as
20
d
and
20
e
, which are illustrated by “lightning bolt” symbols in order to simplify the drawing.
For completeness, it should be noted that each separate antenna beam forms an infinite number of more-or-less concentric “footprints” centered about the maximum-beam-intensity point on the ground, with each being a fraction of a decibel (dB) greater than the next inner footprint. When “a” footprint is discussed, a selected energy distribution across the “footprint” is assumed. Thus, a common assumption is that the beam intensity will not vary more than 3 dB across the footprint, which defines the extent of the footprint by the 3 dB contour of the antenna beam. Similarly, overlap of the beams is taken to mean overlap at the designated beam intensity. It should further be noted that a receiving antenna also preferentially receives signals within a receiving “beam,” and for a given antenna, the receiving “beam” is “dimensionally” identical to the transmitting beam, in that it has the same beamwidth and gain.
As illustrated in
FIG. 1
, a group
16
of mobile terrestrial user terminals or stations includes three user terminals, denominated
16
a
,
16
b
, and
16
c
, each of which is illustrated as having an upstanding whip antenna
17
a
,
17
b
, and
17
c
, respectivel
Kent Edward Jay
Tanzini John
Do Nhat
Duane Morris LLP
Lockheed Martin Corporation
Nguyen Chau
LandOfFree
High-rel beacon signal sequencer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High-rel beacon signal sequencer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-rel beacon signal sequencer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3172664