U.S. patent application number 11/604525 was filed with the patent office on 2007-03-29 for satellite communication system.
This patent application is currently assigned to Northrop Grumman Space & Missions Systems Corp.. Invention is credited to Keith R. Jenkin, Roy K. Tsugawa.
Application Number | 20070072546 11/604525 |
Document ID | / |
Family ID | 37526624 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070072546 |
Kind Code |
A1 |
Jenkin; Keith R. ; et
al. |
March 29, 2007 |
Satellite communication system
Abstract
A satellite communication system and related method for
collecting data in space and transmitting it to a processing center
on the earth. The communication system includes at least one
satellite orbiting the earth that has a device for collecting the
data. The satellite transmits the data to a plurality of receive
only terminals on the earth. The terminals process the signals and
transmit the data on the communications link to the processing
center.
Inventors: |
Jenkin; Keith R.; (Seal
Beach, CA) ; Tsugawa; Roy K.; (Santa Barbara,
CA) |
Correspondence
Address: |
WARN HOFFMANN MILLER & LALONE PC;NORTHROP GRUMMAN CORPORATION
P.O. BOX 70098
ROCHESTER HILLS
MI
48307
US
|
Assignee: |
Northrop Grumman Space &
Missions Systems Corp.
Los Angeles
CA
|
Family ID: |
37526624 |
Appl. No.: |
11/604525 |
Filed: |
November 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09641654 |
Aug 18, 2000 |
7151929 |
|
|
11604525 |
Nov 27, 2006 |
|
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Current U.S.
Class: |
455/13.1 |
Current CPC
Class: |
H04B 7/18582
20130101 |
Class at
Publication: |
455/013.1 |
International
Class: |
H04B 7/185 20060101
H04B007/185 |
Claims
1. A satellite communication system comprising: at least one
satellite orbiting the earth, said at least one satellite including
a device for collecting data, said satellite transmitting signals
including the data to the earth; a plurality of terminals on the
earth receiving the transmitted signals from the at least one
satellite, said plurality of terminals processing the signals and
transmitting the data on a communications link on the earth; and a
processing center on the earth, said processing center receiving
the data on the communications link from the plurality of
terminals.
2. The system according to claim 1 wherein the plurality of
terminals are receive only terminals that do not transmit signals
to satellites.
3. The system according to claim 1 wherein the plurality of
terminals are unmanned terminals.
4. The system according to claim 1 wherein the communications link
includes fiber optic cables.
5. The system according to claim 1 wherein the at least one
satellite is a plurality of satellites that transmit signals
including data to the plurality of terminals.
6. The system according to claim 1 further comprising a satellite
operations center on the earth, said satellite operations center
receiving signals from the processing center concerning system
operations, and said satellite operations center sending signals to
the at least one satellite and the plurality of terminals for
changing system operations in response thereto.
7. The system according to claim 1 wherein the processing center
includes a first processor for processing mission data received
from a first satellite and a second processor for processing
mission data received from a second satellite.
8. The system according to claim 1 wherein each of the plurality of
terminals includes a GPS receiver for receiving GPS signals.
9. The system according to claim 1 wherein the device is a sensor
that collects weather data.
10. The system according to claim 1 wherein the device collects
data for one or more of the group consisting of television,
internet, telephone, video conferencing and financial data
communications.
11. A satellite communication system comprising: a plurality of
satellites orbiting the earth, each satellite including a device
for collecting data, and each satellite transmitting signals
including the data to the earth; a plurality of terminals on the
earth receiving the transmitted signals from the plurality of
satellites, said plurality of terminals being receive only
terminals that are unmanned, said plurality of terminals processing
the signals and transmitting the data on a communications link
including fiber optic cables on the earth; and a processing center
on the earth, said processing center receiving the data on the
communications link from the plurality of terminals.
12. The system according to claim 11 further comprising a satellite
operations center on the earth, said satellite operations center
receiving signals from the processing center concerning system
operations, and said satellite operations center sending signals to
the at plurality of satellites and the plurality of terminals for
changing system operations in response thereto.
13. The system according to claim 11 wherein the processing center
includes a first processor for processing mission data received
from a first satellite and a second processor for processing
mission data received from a second satellite.
14. The system according to claim 11 wherein each of the plurality
of terminals includes a GPS receiver for receiving GPS signals.
15. The system according to claim 11 wherein the device is a sensor
that collects weather data.
16. A method for transmitting data collected in space to the earth,
said method comprising: collecting data by at least one satellite
orbiting the earth; transmitting signals including the data from
the at least one satellite to the earth; providing a plurality of
terminals on the earth that receive the transmitted signals from
the at least one satellite; processing the signals at the plurality
of terminals; transmitting the processed signals from the plurality
terminals on a communications link on the earth; and receiving the
transmitted signals on the communications link at a processing
center on the earth.
17. The method according to claim 16 wherein providing a plurality
of terminals includes providing a plurality of receive only
terminals.
18. The method according to claim 16 wherein providing a plurality
of terminals includes providing a plurality of unmanned
terminals
19. The method according to claim 16 wherein transmitting the
processed signal on the communications link includes transmitting
the signals on fiber optic cables.
20. The method according to claim 16 further comprising sending
signals from a satellite operations center on the earth to the at
least one satellite and the plurality of terminals for changing
system operations.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
patent application Ser. No. 09/641,654, filed Aug. 18, 2000, titled
"Satellite Payload Data Communications and Processing
Techniques.
BACKGROUND OF THE INVENTION
[0002] This invention relates to satellite communication systems,
and more specifically relates to such systems in which satellite
data is processed by an earth processing center.
[0003] Satellite communications are taking on increased importance
as evidenced by the following patents issued in the name of one of
the inventors of the present invention: U.S. Pat. No. 5,867,530,
entitled "Method and Apparatus for Accommodating Signal Blockage in
Satellite Mobile Radio Systems," issued in the name of Keith R.
Jenkin, on Feb. 2, 1999 and U.S. Pat. No. 5,940,444, entitled "DARS
PSF With No Data Rate Increase," issued in the name of Keith R.
Jenkin and Stephen J. Toner on Aug. 17, 1999.
[0004] Prior satellite communication systems requiring earth
processing centers including, for example, weather satellite
systems. In such a system, one or more traditional ground stations
are used. The weather satellite collects data continuously and
saves it onboard, and then "dumps" that data as it over flies a
traditional ground station. Polar locations are chosen as sites for
traditional polar orbiting missions since the poles are overflown
on every orbit, thus minimizing the number of traditional ground
stations needed. (If the stations were located elsewhere, say near
the equator, a prohibitively large number of these expensive
facilities and sustaining staff ringing the globe would be needed
to avoid blind orbits.)
Significant Data Timeliness Compromise
[0005] Mission data is continuously collected and stored onboard
until a traditional ground station is encountered. This results in
data already being delayed by up to as much as approximately 100
minutes before it even reaches the ground, which for weather data
is highly undesirable.
Traditional Ground Station Complexity and Cost
[0006] Since there are very few downlink opportunities, and each of
them is usually critical to prevent blind orbits, the stations must
have extremely reliable communications with the satellite to avoid
unacceptable performance. Usually a bi-directional system is used
(both downlink and uplink) to first establish a valid link, then
command the satellite to begin the downlink process. Data integrity
can be checked in near real time on the ground, and handshaking
schemes can instigate the retransmission of data packets in more
sophisticated systems. A full time (24-7) crew is essential at
traditional ground stations for rapid repairs if needed, and also
man-in-the-loop scheduling conditions automated systems can't
handle (i.e. preemption situations). In remote regions the
continuous staffing required over many years becomes a major
consideration in program life cycle cost. In a case like McMurdo
(Antarctica) the environment is incredibly adverse, and logistics
become a major concern. While adding the example McMurdo is
attractive since the nominal maximum onboard storage time is
reduced to half an orbit instead of one orbit, the programmatic
impact is substantial.
Minimum Pass Limitation Of Prior Systems
[0007] Since a downlink to a traditional ground station is a
complex operation, a practical limit on the geometrically available
contact time is usually imposed. The ground station antenna (which
might service several other satellites too) needs to be slewed,
signal acquisition accomplished, and reliable communications need
to be established. Therefore, otherwise viable contacts at a
traditional ground station are discarded if the contact
opportunities are somewhat short, such as five minutes (of a
nominal 12 minute pass time). The preferred embodiment of the
present invention does not require a minimum pass limitation, since
it is a dedicated, mission-captive capability. Furthermore, even
"scraps" of mission data (small periods) are useful in the
preferred embodiment architecture, since it will be shown that all
received valid data, no matter how small or redundant, become
amalgamated or used as checks upon arrival at the processing
center.
Ground Communication Drawbacks of Prior Systems
[0008] Since the traditional ground stations are generally located
in remote, sparsely populated areas, taking advantage of
commercially financed, installed, and maintained fiber optic
networks is unlikely since there is no financial motivation for
servicing such geographic (polar) areas. This means communication
from traditional ground stations to the processing center (probably
in the U.S.) is expensive for the data rates (bandwidth) needed by
future weather satellites. Either dedicated, sole-user fiber is
needed, or perhaps a complex, risky, and expensive "hop" from the
station to a communications satellite and back to the U.S. is
needed. Or, a slow existing link might be used, but because of
limited bandwidth, data will again be delayed awaiting its turn in
a rate buffer queue for ground communication.
Frailty of Prior Systems
[0009] Since there are, practically speaking, several single point
failure opportunities in a traditional ground station system, each
point must have incredibly high (i.e. expensive) reliability and
sufficient availability. For instance, if a key station is down for
a prolonged period, say due to earthquake damage, or immediately
irreparable equipment failure, or staffing problems and so on,
critical data will be lost or arrive so late it's essentially
useless.
Spacecraft Complexity/Risk of Prior Systems
[0010] Since passage over a traditional ground station is on the
order of 10 minutes, and the stored data is from a nominal 100
minute orbit, high downlink data rates (a minimum of 10.times.
payload data rate) need a spacecraft pointable, high gain antenna
to keep spacecraft electrical power and transmitter needs
reasonable. This means either moving mechanical parts (an
articulated gimbal system), or possibly a phased array antenna
(complex). Since the spacecraft antenna is highly directional
(continuously dynamically pointed at the ground station) and since
transmit power is limited, only one ground station at a time can be
downlinked. Furthermore, if subsequent contact opportunities arise,
a gimbaled spacecraft antenna needs precious time to slew and
repoint.
Station Scheduling Complexity of Prior Systems
[0011] Since several satellites may need servicing by the same
station, scheduling is complex amongst disparate systems to avoid
usage conflicts. As more satellites are launched using the same
stations, competition for use of the stations increases and
scheduling becomes increasingly complex and conflicting. Adding
additional antennas, electronics, and personnel at a traditional
ground station facility can help mitigate this situation, but is
expensive.
[0012] The present invention addresses the foregoing deficiencies
of prior systems and provides a solution.
SUMMARY OF THE INVENTION
[0013] In accordance with the teachings of the present invention, a
satellite communication system and related method are disclosed for
collecting data in space and transmitting it to a processing center
on the earth. The communication system includes at least one
satellite orbiting the earth that has a device for collecting the
data. The satellite transmits the data to a plurality of receive
only terminals on the earth. The terminals process the signals and
transmit the data on a communications link to the processing
center.
[0014] Additional features of the present invention will become
apparent from the following description and appended claims, taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic representation of a communication
system employing communication satellites and embodying the
invention.
[0016] FIG. 2 is an enlarged view of one of the satellites shown in
FIG. 1.
[0017] FIG. 3 is a schematic block diagram of a receptor terminal
made in accordance with the invention.
[0018] FIG. 4 is a schematic block diagram illustrating a form of
data recovery in the event of an anomaly in the system shown in
FIG. 1.
[0019] FIG. 5 is a schematic block diagram of a portion of a form
of processing center employing a plurality of computers.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Glossary of Terms Used in this Specification
[0020] Autonomous Mode: An alternative embodiment where the system
is completely autonomous, yet with coverage immunity to failures.
Improvements and repairs still can be made on the ground only.
[0021] Traditional Ground Stations: In the context of this
specification, this refers to large, complex, expensive, facilities
used for many years in the past to support communications with
various satellite systems.
[0022] Receptor: One embodiment may use, for example, a distributed
network of small extremely simple, and relatively inexpensive,
unmanned antenna/receivers that are receive-only. (Downlink signal
reception, but no uplink signals sent to the satellite(s)). These
are technically earth stations, but of a significantly reduced
complexity/cost class than the Traditional Ground Stations noted
above.
[0023] Checksum: This is one of several mathematical means of
verifying the integrity of a block of digital data, of varying
potency. For simplicity in describing one embodiment, checksum will
be referred to, but any of the several other, possibly more complex
and robust methods may be used for a specific application. A
checksum is simply the sum of all values in a known-size dataset.
If a subsequent checksum is done on the same dataset at a later
time (say, after communication transmission) and its checksum value
isn't precisely the same as the original checksum for the same
dataset, errors in the dataset are present. Checksum does not allow
correction of errors, but merely is a test (only) for data
integrity. The important point is that the amount of bits needed
for a checksum value (or other integrity-test-only method) is
extremely small compared to the amount of bits in the dataset
itself.
[0024] Virtual Spherical Coverage: A feature whereby the whole
earth can be mapped in a timely and confident (high data integrity)
manner, despite having intermittent communication contact
(satellite-to-ground downlinking) of less than four PI steradians
(spherical coverage).
[0025] Mission Data: The actual useful information from a
satellite, such as imagery produced from mission instruments, plus
any overhead needed such as headers and encryption. (Other data
from a satellite typically includes things such as satellite
housekeeping information, which is typically very low data rate in
comparison to the actual end-use mission data.)
[0026] Data Timeliness: The time from when data was collected, to
when it becomes useful to the end-user. In systems such as the
example mission cited below when configured as a legacy system
using traditional ground stations, the dominant timeliness
constituent is the delay between downlink contacts from the
physical constraints of orbit and ground station geometries. Also
referred to as data aging or data latency.
[0027] Preemption: For one of several possible reasons, a
geometrically possible communication opportunity (satellite is
within nominal communication range and adequate other conditions)
is not utilized for downlinking mission data. Examples of
preemption include: 1) ground station is preoccupied servicing
another satellite having higher communication priority, 2) a ground
station is down (inoperable) due to scheduled maintenance or
unscheduled failure of its own equipment or its communication to
the final data delivery point (e.g. ground communication to the
U.S.), 3) ground station staff insufficient, 4) severe weather at a
ground station (e.g. extreme wind requiring antennas to be caged),
5) RF interference avoidance with another component.
[0028] Blind Orbit: An orbit where a satellite has had no
opportunity to pass (downlink) Mission Data because of either a
geometrically impossible situation (no ground stations within
communication range on that orbit), or all communication
opportunities were preempted. This is a very undesirable situation
for missions where data timeliness is important, since onboard
stored mission data will include an additional whole orbit's worth
of delay by the time it is finally downloaded.
[0029] LEO, MEO and GEO: Grouping general classes of Earth orbiting
satellites by their gross altitude.
[0030] LEO: Low Earth Orbit (in the hundreds of kilometers altitude
range).
[0031] MEO: Medium Earth Orbit (in the thousands of kilometers
altitude range).
[0032] GEO: Geosynchronous Earth Orbit (an altitude of around
36,000 kilometers, if circular, resulting in the satellite having
an orbital period the same as Earth's rotation (one day), causing
it to appear stationary overhead to an observer at a fixed location
on Earth).
[0033] Sun-Synchronous-Polar-Orbit: At certain circular orbit
altitudes and associated inclinations (e.g. around 800 Km and a few
degrees of inclination) a satellite's orbit plane follows the
Sun-Earth annual cyclic vector angular motion in inertial space.
Such an orbit is advantageous to missions, such as the weather
satellite mission. This orbit results in the entire Earth being
mapped in a relatively short time at desirable constant sun
illumination angles, owing to the combined dynamic geometry of the
Earth's daily rotation, plus orbital motion, plus the cross-track
swath of the satellite sensor's field-of-view. For global weather
observation from relatively low altitudes (e.g. LEO versus GEO),
this is an ideal scenario: full spherical coverage from consistent
observation angles updated fairly frequently.
[0034] Solid State Recorder (SSR): Recently satellites have been
implementing onboard data storage with solid state recorders, in
lieu of earlier data storage implementations such as mechanical
tape recorders. SSRs are essentially large amounts of RAM (as in a
computer) plus some controlling circuitry. Obviously the
elimination of moving parts (which eventually wear out and are
prone to failure) is a reliability advantage. More importantly for
system 10 is the random addressing capability (the "R" in RAM),
programmability, and asynchronous operation, as will be shown.
[0035] Code and Coding: Two disparate uses: 1) Code referring to
computer program instructions, and 2) Coding referring to data
overhead for error detection/correction algorithms.
[0036] Satellite Operations Center. Usually a single facility for
controlling satellites. Here commands are sent to the satellite to
specify its operation.
[0037] Processing Center Where mission data arrives to be converted
to useful information for the intended end use. For instance, in
connection with the preferred embodiment, weather maps are produced
by analyzing (via computer) multi-spectral imagery data collected
by the satellite with algorithms that can convert that raw data to
a useful end product format.
[0038] The invention is useful for many possible satellite/system
configurations of varying characteristics and missions. However,
the embodiments described below and for purposes of illustration,
but not of limitation, in connection with an LEO circular
Sun-Synchronous-Polar-Orbit weather mapping satellite system. For
this example, timely global mapping is important to mission
objectives. (Weather data is highly perishable, since weather
conditions are highly dynamic.) Such a weather satellite system may
have several sensors (data collection instruments), spanning a
large range of the electromagnetic spectrum in observation of
weather-related phenomenology below the satellite. Such sensors
typically produce a steady stream of mission data, typically of
high density (high data rates, such as that needed for
multi-spectral imaging).
[0039] Referring to FIG. 1, one embodiment includes a satellite
communication system 10 comprising a weather satellite 20 circling
the earth E in an LEO 21. Another identical weather satellite 22
circles the earth E in another LEO (not shown). System 10 also
includes a processing center PC and receptor terminals A, B and C
which are linked to center PC by a wide band optical fiber network
30, including links 32, 34 and 36.
[0040] The satellites 20 and 22 of this embodiment have an orbital
period of about 100 minutes (about an hour and a half per orbit),
and have a Mission Data rate of about 20 Mbps, a continuous stream
of the amalgam of all sensor data (megabits per second). The data
stream may consist of raw data, or compressed data of either the
lossy or non-lossy variety. The data stream also includes any
overhead information as noted, including data encryption details.
For simplicity, this example assumes that the Mission Data is
always at a fixed, constant rate coming from the mission sensors,
which is adequate for illustration. An actual system may have a
variable rate in practice, for efficiency (compression), or
scene-content variation (e.g. some sensors may not function at
nighttime) or other reasons.
Global Fiber Optics Ground Communication Network 30
[0041] A wide band network, such as network 30, is known to those
skilled in communications. There are many references in the open
literature describing the present and future status of wide band
networks, such as fiber optic cables. All of the significantly
populated continents are encircled in fiber optics cable, such as
cables 32, 34 and 36 that can pass a minimum of 40 Gigabits per
second. In the next couple of years, the entire globe (with the
exception of arctic Polar Regions having nil population and
commerce) will be a "spider web" of multi-terabit fiber optic
cable. This is due to fiber optics technology where many optical
spectral bands are used in a single fiber strand to produce nearly
a two order of magnitude increase in communication data rates. This
embodiment uses this extremely fast and inexpensive communication
from it's receptors, such as A, B and C, located most anywhere on
earth, back to the Processing Center, PC. The bandwidth proportion
needed by a weather satellite system, such as the one shown in FIG.
1, is on the order of one-one-hundred-thousandth of the capability
of a terabit optical fiber, which is essentially epsilon and
therefore very inexpensive. Some embodiments also may be used for
applications other than a weather satellite system, such as
television, HDTV, Internet, telephone (including video soon),
videoconferencing, and financial data communications. Thus, some
embodiments take advantage of this global inexpensive
communications capability financed by commercial/consumer markets,
instead of the traditional method of utilizing mission-dedicated
ground communication means.
SSR Programmability and Flexibility
[0042] Referring to FIG. 2, satellite 20 includes an on-board
processor 23, which stores mission data in an SSR 24 that may take
the form of a recirculating memory. In the past, satellite data
storage methods such as mechanical tape recorders would operate in
a mode of continuous recording while out of ground station contact,
then high rate playback during a downlink pass, in a fairly rigid
sequential modality. SSR 24, on the other hand, can be treated like
RAM in computer 23: access to all memory data at any time and in
any order is possible, both for recording (writing), and
non-destructive and highly selective playback (reads). The
preferred embodiment uses these SSR features of random access and
non-destructive reading in several ways.
[0043] This embodiment uses globally distributed simple and
inexpensive "receptors" A, B and C. These are small, unmanned
antenna installations located at easy access points to the global
fiber optic network. Referring to FIG. 3, exemplary receptor
terminal A includes a small dish antenna 40 approximately two to
four meters in diameter, an antenna pointing gimbal control unit 44
(e.g. servos and encoders), an appropriate antenna feed 48, a
receiver/demodulator 50 which downconverts the carrier signal
received from one of satellites 20 or 22, and interfacing
electronics 60 to the commercial fiber optics network link 34. A
phased array antenna could also be used.
[0044] Receptor terminal A receives the open-loop broadcast signal
from an over flying satellite, collects the RF signal the satellite
is transmitting, demodulates the signal to a digital format, adds
simple periodic "wrap" tagging headers of time and location and
synchronization bit patterns around an arbitrarily sized "macro
packet," and forwards the raw bit stream via commercial optical
fiber link 34 to the processing center PC. There are no complex
data operations at the receptor, it merely acts as a bridge from
the satellite broadcasts to the processing center. No data analysis
or assessment or decisions are made, nor are any processing
operations done by the receptors. They simply collect data and pass
it along. Since the receptors are very simple and small and located
on existing fiber communications channels, their deployment and
operation is very cost effective.
[0045] It will be shown that a modest number of receptors is needed
to implement this embodiment because the embodiment employs virtual
spherical coverage which mitigates the fact that several gaps exist
in true geometric coverage opportunities. In this embodiment, 12
receptors are used to achieve reasonable coverage. Depending on the
needs of the system, at least twice that many receptors may be
deployed.
[0046] Still referring to FIG. 3, receptor terminal A also includes
a GPS antenna 70 and a GPS receiver 72 which can, for example,
receive the time of day. A computer with ephemeris tracking
software 80 controls gimbal control unit 44 in a well known manner.
A mass data storage unit 90 stores mission data in case of
problems.
[0047] Referring to FIG. 4, system 10 includes a satellite or
mission operations center 100 comprising an adaptive logic
controller 102 which automatically and transparently adapts system
10 to any dynamic operational scenario without compromising data
quality or incurring data loss. Controller 102 provides receptor
failure mitigation, and automatic system acclimation/adoption of
new receptors added to the system. In a sense, the system 10
"learns" as it operates, and continuously, and independently adapts
to system configuration changes. In other words, system 10 provides
a practical virtual spherical coverage system with data integrity
and minimal, if any, data loss. System 10 also provides near real
time data collection to data delivery time, a robust and forgiving
mode of operation, a life cycle cost advantage, easy growth and
expandability. System 10 also provides nearly continuous downlink
coverage for LEO and MEO satellites.
[0048] Referring again to FIGS. 1 and 2, satellites 20 and 22
comprise conventional LEO satellites consisting of the usual,
normal subsystems for providing power, attitude control, orbit
maintenance, a benign thermal environment, and command/status
communication, etc. The satellites also include sensors 27 and 28
for gathering weather data.
[0049] The satellite mission data collection subsystem for
satellite 20 comprises medium data rate (say 4.times. sensor data
rate) data downlink equipment 25 including programmable SSR 24
(random access), conventional data multiplexing, coding, and
encryption functions, and a fixed, wide field of coverage broadcast
antenna.
[0050] The function of this medium data rate satellite downlink
communicating subsystem is to transmit mission data continuously,
and to store it for short periods during communication gaps and
transmit it along with real time mission data at the next receptor
encounter. This broadcasting on cue is metered by periodically
updated, preloaded commands and a frequently updated onboard stored
digital contact zone reference map.
[0051] Referring still to FIG. 1, system 10 includes simple low
cost, modest reliability, unmanned receive-only terminals A-C
located at generally equally globally distributed convenient sites.
The receptors are in reasonable proximity to the worldwide network
of fiber optic communication channels 32, 34 and 36 for fast and
cheap passage of received mission data to a common mission
Processing Center PC. The receptors can, for example, be mounted on
the rooftops of existing facilities, including government embassies
(U.S. and or friendly countries), commercial global communications
provider's facilities (e.g. MCI or AT&T), military
installations, suitable traditional ground station sites, or any of
several global satellite command and control facilities. Anywhere
cheap fiber optics communication is available. Note that if a
location critically needs a receptor for ultimate mission data
timeliness in that locale but fiber communication isn't within
practical reach, a "bounce" to a convenient comsat could be set up
with existing available equipment, or perhaps a microwave or
coaxial cable, or a conventional communications link (e.g. T1) to
the closest fiber optic network interface. A receptor could be
installed on military or commercial ships having adequate
communication links, with five-space motion backed out of pointing
commands.
[0052] Still referring to FIG. 1, system 10 includes a traditional
satellite operation center 100 for sending operating commands to
satellites 20 and 22. In system 10, center 100 is identical to a
traditional command center and operation, except that SSR and
medium data rate transmission commands and onboard maps are
included in the command stream, which are prepared from system
health information gleaned from mission data reception at the
processing center PC.
[0053] Referring to FIG. 1, the PC provides a facility where all
mission data from the satellite(s) arrives to be analyzed and
formatted for end use by specialized science-based algorithms, a
common function of weather systems. At the PC, data integrity is
continuously tested ("checksums" and/or one-on-one redundant data
duplicates compared from circuitous routes) and any necessary
system reconfiguration commands forwarded to the satellite 20 and
22. Note that these are very low data rate and occasional commands
that can be sent through a variety of existing channels, such as
SOC 100, a data transmission satellite (e.g. TDRS), a traditional
ground station, or any of many distributed satellite command
facilities. Note that the processing center PC and operations
center 100 may reside in one and the same physical (shared)
facility, in a convenient location such as a city in the
continental U.S.
[0054] Referring to FIG. 1, Mission Data collection is achieved in
a conventional manner by satellites 20 and 22. Sensors 27 and 28
continuously observe the Earth and its atmosphere producing a
continuous stream of data. Onboard electronics then condition the
data, multiplexing various data sources, compressing, encrypting,
and other common data formatting operations. As noted above, the
data rate for convenient illustration here is assumed at a constant
rate, but could also be variable.)
[0055] In satellite 20, data is routed to two destinations: the
first to the SSR 24 (FIG. 2) for continuous onboard storage, and
the second to a multiplexer at the medium data rate broadcast
downlink transmitter. At the multiplexer it is always continually
broadcast in real time as data is collected. At the SSR destination
it is also always continuously stored incrementally in time
(sequence).
[0056] SSR 24 (FIG. 2) can be configured as a "ring buffer." This
is a logical memory arrangement where data is continuously written
at the "head" pointer as it continuously advances around the
(virtual) "ring." The size of the ring is such that several orbits
of mission data are stored at any time (e.g. five orbits). As the
head pointer advances, it will eventually catch the "tail" pointer
and begin overwriting the data that is (in this example) five
orbits old. At any instant, all data from a time exactly five
orbits worth prior is stored, and instantly randomly
accessible.
[0057] In the satellite 20 ring buffer implementation, data is
never released, but only gets finally overwritten as the head
chases the tail of five orbits prior. Thus, data is always
available for delayed transmission (when a contact gap exists) or
when data that was assumed successfully received is not, as
reported/requested in later (asynchronous) uplink
commands/reception status from the operations center, and
retransmission of poor quality (noisy) or missing data is
needed.
[0058] So, system 10 asserts that its anticipated transmissions to
expected active contacts will be successfully received by
receptors, such as A-C, and forwards the transmissions to
processing center PC, open loop, according to its onboard contact
availability map. When a gap in coverage is indicated by the map,
that data (being continuously stored in the ring buffer 24) is
marked for later transmittal, is retrieved from the SSR 24 and is
multiplexed along with the continuously transmitted real time data.
The system 10 is anticipatory: it downlinks saved gap data when the
satellite is confident it will successfully be received. (If that
fails, the data will still be retrieved at the next opportunity
once the missing data is reported.)
[0059] Referring to FIG. 4, if for some reason a segment of that
open loop data never makes it successfully to the processing center
PC when anticipated, it can be requested for retransmission
(automatically) by the SOC 100 logic. Such a request would occur,
for example, if a receptor was physically damaged in a storm, or
lost its source of electrical power. The coverage map is then
updated for future use with that receptor removed and uplinked at
the next opportunity to the satellite to correct its future contact
profile assertions. (The center 100 has the same coverage map and
knows when data should arrive.) Note that even though the system 10
is nominally a very simple open loop arrangement, it can retrieve
missing or noisy data segments well within a reasonable delay
time.
[0060] A traditional system, always has forced substantial delay
since contact opportunities are widely spaced. If a traditional
expected ground station contact is missed, serious excess latency
results, rendering the data essentially useless for dynamic weather
use. Thus, a traditional system critically needs and relies on
every one of its infrequent downlink opportunities, whereas the
system 10 recovers quickly and is immune to data tardiness if a
receptor pass is missed.
[0061] Another embodiment ("Autonomous") is possible. For some
applications, the alternative may be a potential additional but
possibly acceptable burden on downlink channel use. It is akin to
the patents referenced in the background section. The satellite
would continuously broadcast time-shifted data copies from earlier
periods in its orbit, in a post facto fashion. For example, if,
say, four layers of time shifted data were continually broadcast
assertively (open loop) to receptors having typically a ten minute
pass, about a quarter of an orbit's prior data could be received
automatically by a single receptor in that vicinity. As is
described in the reference patents, a similar technique for
increased downlink and ground communications would be to reduce the
quality (lossy compression) or to partially select critical data
sections, and thus reduce the data rate of the redundantly
transmitted layers. Data would still get to the destination in a
timely and complete manner, but with a slightly reduced fidelity.
This totally open loop, virtual spherical coverage mode could be a
backup, fail safe modality. If the satellite is for any of several
reasons not receiving a normal uplink command profile, the
satellite could automatically default and reconfigure to this
totally autonomous mode. This would still provide full Earth
coverage, even if receptors randomly fail. In other words, the
system is completely automated, and any repairs needed for
continuous coverage are made on the ground (e.g., repairing
inoperable receptors or supplementing them with nearby additional
receptors).
[0062] In the case of noisy data receipt, for perhaps a temporary
reason such as a severe local storm at a receptor (e.g. "rain
fade"), then the coverage map will still get updated as a
precaution for further use, and that noisy data gets subsequently
recovered, since the system 10 also gracefully and automatically
recovers from both a temporary compromise in a receptor's
performance, as well as from prolonged permanent outages. Note that
noisy data (beyond correction capabilities) can be short term
("bursty") in system 10, and the whole pass over a receptor is
mostly valid and saved, and only the noisy parts recouped as
needed.
[0063] System 10 automatically recognizes and adjusts the coverage
map when a new receptor is brought on line. There is no need to
tell the system it has been added and to look for it. Also note
that receptor deployment and installation is remarkably easy: a
contracted local technician simply unpacks the unit (in maybe two
or three convenient pieces), secures the receptor stanchion in
approximately the appropriate North/South orientation (via a
magnetic compass or handheld GPS unit), and squares it vertically
with a bubble level. Once power is provided and network
communication (fiber optics connection) is established by
conventional commercial methods, installation is complete. This
crude initial orientation is adequate, since the receptor is
initially commanded in a simple search pattern remotely from the
operations center, pointing the receptor antenna generally in the
direction at the right time when a known system 10 satellite will
over fly. Since the system 10 satellites are continuously
transmitting real time data, no coordination or cooperation is
necessary. The processing center PC will suddenly start receiving
valid data from the new receptor and adjust the common coverage map
accordingly. Any receptor misalignment is dialed in as a bias to
its future pointing commands from the operations center. Since the
satellite (instant) location is very accurately known from its
orbital elements, and the location of a receptor is also accurately
known and fixed (via a onetime GPS handheld measurement), the
precise pointing locii for all satellite/receptor dynamic
combinations is easily and accurately calculated. There is no need
for constant "hunting" and handshaking for acquisition. The
receptor is simply told where and when to point, and passes along
whatever it receives.
[0064] At least three means of insuring data integrity (successful
and complete data receipt) are available and implemented in system
10:
[0065] Traditional error detection/correction overhead bits
embedded in the data stream right on the satellite. (The usual
digital communications procedure).
[0066] By comparing a delayed (time-shifted) "checksum" (etc.)
stream of each original data packet (embedded in the downlink
stream, before receptor receipt and passage to the processing
center) with a checksum calculated for packets as they arrive at
the processing center. According to the preferred embodiment, every
so often, the checksums of all previous packets collected in an
orbit are sent as a burst along with the real time mission data.
(This is a trivial impact on overall transmitted data rate, thus
costing nothing in bandwidth.) So, when the processing center PC
receives mission data, it also gets the checksums of all data sent
for 100 minutes prior to that time. These delayed
spacecraft-calculated checksums can be compared to checksums
calculated again by the PC on the same data received after
transmission. If there is a difference, then the suspect data can
be requested for retransmission via the next command opportunity,
since it is still resident and intact in the satellite SSR 24. This
alleviates a remote but possible situation, of particular concern
for military uses, of weather data tampering. Imagine someone
clever enough to intercept and alter data en route from a receptor
to the processing center. (Practically speaking an essentially
impossible, yet remotely conceivable task, since the data is
probably compressed, encrypted, and laced with several layers of
data quality tests and error correction means.) So, if the
seemingly impossible tampering were successfully accomplished, say,
to make it look like rain in Spain when in fact it is a sunny day,
the act would be detected within minutes. (The next receptor
downlink checksums wouldn't match). Future reception from a
tampered receptor's data would of course be suspect, and
disregarded until the problem has been removed.
[0067] Duplicate receipt of identical packets can occur if the
contact patterns of two or more receptors overlap, which is a
likely situation. Thus, the processing center PC can receive
duplicate data from circuitous routes and physical locations. These
can be compared one-bit-for-one-bit exactly after error correction
and normal goodness tests have been passed, and if there is a
difference, something is suspect. (The system 10 can always tell
multiple receptors overlapping in coverage to selectively not
transmit those zones to save fiber communications costs and overly
redundant data retrieval.)
[0068] System 10 also provides for satellite command intrusion
immunity. A valid concern is the potential of a renegade individual
or group sending unauthorized commands to a spacecraft for whatever
reason. As such, traditional ground stations are highly secured to
alleviate the possibility of intrusion. The system 10, however,
preferably is a downlink-only system. It is impossible to tamper
with a receptor and send erroneous commands to a satellite, since
there is no physical mechanism (transmitter and associated feed and
logic and modulation electronics). Thus, the concern of locating
receptors in questionable areas is not of concern for sending false
commands to the satellite(s).
[0069] Satellite weather systems frequently calibrate their sensors
and data with ground truth. (In situ actual measurements using
ground based instruments, or weather stations.) This data is, of
course, available from many sources. However, it would be very easy
to mount measurement instruments on each receptor, perhaps on the
receptor stanchion directly. Local weather data could be passed to
the operations center, on low bandwidth communication means,
continuously. Such weather information, in addition to providing a
controlled source of ground truth for the system, could also be
helpful in remotely assessing a data error condition. For example,
if rain fade is the cause of noisy data (and not, say, RF
interference, etc.), then a local weather report directly from the
receptor would be useful. The weather instruments could be
rudimentary (temperature, wind conditions, humidity and others), or
more complex such as automatic cloud cover assessment devices. A
simple video camera and microphone at the receptor could also be
handy for remote receptor health assessment.
[0070] A significant cost savings and simplification of the
processing center ("PC") occurs when system 10 is used instead of a
traditional ground station architecture. In a traditional system,
data arrives at the processing center in bursts, as spacecraft in
the constellation rapidly downlink to a station data collected
during one orbit of the spacecraft around the earth. There are long
quiet periods at the PC when the spacecraft are away from station
coverage for prolonged stretches. Since these PC abrupt ingest
bursts can be from any one of the spacecraft in a constellation, an
obvious approach is to share the PC computing capacity among the
various spacecraft for computing resource efficiency. However,
there is substantial complexity and development cost associated
with managing one large computer resource to service several
different spacecraft simultaneously and asynchronously. In System
10, however, data arrives at the PC from all spacecraft more or
less continuously, with the exception of brief receptor coverage
gap zones. This makes practical a PC architecture where each
spacecraft and its payload have their own, dedicated processing
hardware and software, since there is no advantage to sharing
computer resources. This then allows 1) Simple system growth Oust
add another computer when a new spacecraft is launched, and
test/debug the new system without perturbing the existing system),
2) Easily accommodated differences in spacecraft sensor payload
suites (its own computer/software services its own unique payload),
3) Simple failure mitigation (a patch specific to a particular
sensor flaw is specific to its own unique and dedicated computer),
4) A new spacecraft and its new computer can utilize the latest in
computer hardware instead of forced use of dated equipment, 5)
Software improvements can be done and tested on a spare parallel
computer without disturbing the working system, and 6) Global
computer failures are isolated to a single spacecraft/payload.
[0071] Referring to FIG. 5, a processing center (PC) having the
foregoing advantages may comprise two separate processors 1 10 and
1 12 which use the same type of hardware. They have identical but
separate operating systems, and they execute different algorithms
required for the processing of signals from different satellites.
Processor 110 is dedicated to processing mission data received from
satellite 20, whereas processor 112 is dedicated to processing
mission data received from satellite 22. The data from the
satellites is switched to the proper processor by a controller 108.
By using this technique, processors may be lower capacity and lower
cost than providing a single high speed processor to process data
for both satellite 20 and satellite 22. In addition, each processor
may be programmed to handle any algorithms which are unique to the
type of data being processed from a particular satellite. When a
new satellite comes on line, a new processor is added and may be
programmed to handle the data from the new satellite. Since
satellite systems advance rapidly in capability, this technique
ensures that the processing center will not become obsolete when a
new satellite begins to feed data to the processing center. The
existing satellites and their respective computers may continue to
function. The new satellite can be brought on line by merely
programming a new computer to handle its needs while the existing
computers continue to function as in the past.
[0072] In summary, this invention offers at least the following
advantages over the known traditional systems:
[0073] 1. Near-zero final weather product timeliness: continuous
data from all satellites instead of bursts of large amounts of
widely separated (in time) data bunches;
[0074] 2. Robustness;
[0075] 3. Growth and redundancy;
[0076] 4. No orbital phasing control required; any initial phasing,
any orbit drift is accommodated
[0077] 5. Downlink bandwidth can be in the tens of Mbps;
[0078] 6. "Preemption" concerns disappear; planned, random,
pronounced, or permanent;
[0079] 7. Unmanned (no human errors, training, housing,
management);
[0080] 8. Security issues and concerns eliminated or more easily
controlled;
[0081] 9. Life-Cycle-Cost reduction; more economical overall
(inexpensive terminals, no staffing);
[0082] 10. Spacecraft simplified, more reliable (no gimbaled
antenna: fixed/shaped beam);
[0083] 11. Can be an independent adjunct to "Traditional" ground
stations (enhancement, backup);
[0084] 12. No need to artificially crop physical contact
opportunities (no minimum pass time imposed);
[0085] 13. No S/C-to-Ground Station coordination, cooperation,
scheduling (mostly gone);
[0086] 14. Mix and match orbits (e.g. different missions, different
altitudes/periods) (no competition for station time);
[0087] 15. Potential external funding since the system could be
utilized by other future satellite systems;
[0088] 16. No concern about simultaneous downloads to same terminal
(physically impossible);
[0089] 17. Simple deployment and installation;
[0090] 18. An excellent approach to system autonomy (Autonomous
Mode).
[0091] Those skilled in the art will recognize that only the
preferred embodiments of the invention have been described in this
specification. These embodiments may be modified and altered
without departing from the true spirit and scope of the invention
as defined in the accompanying claims.
* * * * *