U.S. patent application number 12/467464 was filed with the patent office on 2009-09-17 for method and apparatus for generating and securely distributing long-term satellite tracking information.
Invention is credited to Charles Abraham, Sergei Podshivalov, Matthew Riben, Frank van Diggelen.
Application Number | 20090234571 12/467464 |
Document ID | / |
Family ID | 46205909 |
Filed Date | 2009-09-17 |
United States Patent
Application |
20090234571 |
Kind Code |
A1 |
Riben; Matthew ; et
al. |
September 17, 2009 |
Method and Apparatus for Generating and Securely Distributing
Long-Term Satellite Tracking Information
Abstract
A method and apparatus for generating and distributing satellite
tracking data to a remote receiver is disclosed. The method for
includes extracting from memory at least a portion of
long-term-satellite-tracking data, generating formatted data from
the at least a portion of long-term-satellite-tracking data, the
formatting data being in a format supported by the remote receiver,
applying security to the formatted data to prevent unauthorized
access to and/or tampering with the at least a portion of
long-term-satellite-tracking data; and transmitting the formatted
data to the remote receiver.
Inventors: |
Riben; Matthew; (Cupertino,
CA) ; Podshivalov; Sergei; (San Jose, CA) ;
van Diggelen; Frank; (San Jose, CA) ; Abraham;
Charles; (Los Gatos, CA) |
Correspondence
Address: |
MCANDREWS HELD & MALLOY, LTD
500 WEST MADISON STREET, SUITE 3400
CHICAGO
IL
60661
US
|
Family ID: |
46205909 |
Appl. No.: |
12/467464 |
Filed: |
May 18, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11277943 |
Mar 29, 2006 |
7548816 |
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12467464 |
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11333787 |
Jan 17, 2006 |
7443340 |
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11277943 |
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09993335 |
Nov 6, 2001 |
7053824 |
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11333787 |
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09884874 |
Jun 19, 2001 |
6560534 |
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09993335 |
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09875809 |
Jun 6, 2001 |
6542820 |
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09884874 |
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Current U.S.
Class: |
701/532 |
Current CPC
Class: |
G01C 21/00 20130101;
B64G 1/1014 20130101; G01S 19/05 20130101; G01S 19/258 20130101;
G01S 19/27 20130101 |
Class at
Publication: |
701/200 |
International
Class: |
G01C 21/00 20060101
G01C021/00 |
Claims
1. A method for distributing long term satellite tracking data to a
remote receiver comprising: extracting from memory at least a
portion of long-term-satellite-tracking data; generating formatted
data from the at least a portion of long-term-satellite-tracking
data, the formatting data being in a format supported by the remote
receiver; applying security to the formatted data to prevent
unauthorized access to the at least a portion of
long-term-satellite-tracking data; and transmitting the formatted
data to the remote receiver.
2. The method of claim 1, wherein applying security to the
formatted data comprises encrypting the formatted data to prevent
unauthorized access to the at least a portion of
long-term-satellite-tracking data.
3. The method of claim 1, wherein applying security to the
formatted data operates to ensure to the remote receiver that the
at least a portion of long-term-satellite-tracking data is
unadulterated.
4. The method of claim 1, wherein the long-term-satellite-tracking
data comprises at least one of: a plurality of satellite positions
with respect to time for a period of time into the future, and a
plurality of satellite clock offsets with respect to time for a
period of time into the future.
5. The method of claim 1, wherein the long-term-satellite-tracking
data comprises at least one of: data representative of satellite
positions, velocities or acceleration; data representative of
satellite clock offsets, drift or drift rates.
6. The method of claim 1, wherein the long-term-satellite-tracking
data is valid for a first period of time, wherein the at least a
portion of the long term satellite tracking data is valid for a
second period of time, and wherein the first period is longer than
the second period.
7. The method of claim 1, wherein applying security to the
formatted data comprises applying any of cryptosecurity,
transmission security, emission security, and traffic-flow security
to prevent unauthorized access to the at least a portion of
long-term-satellite-tracking data.
8. The method of claim 1, wherein applying security to the
formatted data comprises applying a security protocol and a
communication protocol to the formatted data, wherein the security
protocol is combined with a communication protocol.
9. The method of claim 1, further comprising: receiving from the
remote receiver a request for security credentials for removing the
security from the formatted data; and sending to the remote
receiver the security credentials.
10. The method of claim 9, further comprising: ensuring that, prior
to sending the security credentials, payment for the remote
receiver to obtain the at least a portion of
long-term-satellite-tracking data is accounted for.
11. The method of claim 10, wherein the payment for the remote
receiver to obtain the at least a portion of
long-term-satellite-tracking data is a fee arrangement.
12. The method of claim 9, further comprising: sending to the
remote receiver a request for information to substantiate that the
remote receiver authorized to access the at least a portion of
long-term-satellite-tracking data; and receiving from the remote
receiver the information to substantiate that the remote receiver
authorized to access the at least a portion of
long-term-satellite-tracking data.
13. The method of claim 12, further comprising: ensuring that,
prior to sending the security credentials, payment for the remote
receiver to obtain the at least a portion of
long-term-satellite-tracking data is accounted for, wherein the
information to substantiate that the remote receiver authorized to
access the at least a portion of long-term-satellite-tracking data
comprises information to validate the payment for the remoter
receiver to obtain the at least a portion of
long-term-satellite-tracking data.
14. The method of claim 1, wherein the transmitting the formatted
data comprises: transmitting the formatted data over a wireless
communications link.
15. A method for obtaining at a remote receiver
long-term-satellite-tracking data distributed over a communications
link, the method comprising: receiving at the remote receiver
formatted data, wherein the formatted data comprises at least a
portion of long-term-satellite-tracking data that is extracted from
memory of a device remote from the remote receiver, formatted in a
format supported by the remote receiver, and applied with security
to prevent unauthorized access to the at least a portion of
long-term-satellite-tracking data; and removing the security from
the formatted data.
16. The method of claim 15, wherein the security comprises
encryption, and wherein removing the security comprises decrypting
the encryption to access the formatted data.
17. The method of claim 15, wherein removing the security from the
formatted data comprises ensuring that the at least a portion of
long-term-satellite-tracking data is unadulterated.
18. The method of claim 15, wherein removing the security from the
formatted data comprises removing any of cryptosecurity,
transmission security, emission security, and traffic-flow security
used to prevent unauthorized access to the at least a portion of
long-term-satellite-tracking data.
19. The method of claim 15, further comprising: sending from the
remote receiver a request for security credentials for removing the
security from the formatted data; and receiving at the remote
receiver the security credentials, wherein removing the security
from the formatted data comprises substantiating the security
credentials to remove the security from the formatted data.
20. The method of claim 19, further comprising: supplying, in
response to a request for information to substantiate that the
remote receiver authorized to access the at least a portion of
long-term-satellite-tracking data, payment for the remote receiver
to obtain the at least a portion of long-term-satellite-tracking
data is accounted for.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part application of
co-pending U.S. patent application Ser. No. 11/333,787 filed Jan.
17, 2006, which is a continuation-in-part application of co-pending
U.S. patent application Ser. No. 09/993,335, filed Nov. 6, 2001,
which is a continuation-in-part of U.S. patent application Ser. No.
09/884,874, filed Jun. 19, 2001, now U.S. Pat. No. 6,560,534, which
is a continuation-in-part of U.S. patent application Ser. No.
09/875,809, filed Jun. 6, 2001, now U.S. Pat. No. 6,542,820. This
application contains subject matter that is related to co-pending
U.S. patent application Ser. No. 09/715,860, filed Nov. 17, 2000,
now U.S. Pat. No. 6,417,801. Each of the aforementioned related
patents and/or patent applications is herein incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to generating
satellite tracking information for earth orbiting satellites. More
specifically, the invention relates to a method and apparatus for
generating and distributing satellite tracking information through
a network or communications link.
[0004] 2. Description of the Related Art
[0005] A positioning receiver for the Global Positioning System
(GPS) uses measurements from several satellites to compute a
position. The process of acquiring the GPS radio signal is enhanced
in speed and sensitivity if the GPS receiver has prior access to a
model of the satellite orbit and clock. This model is broadcast by
the GPS satellites and is known as an ephemeris or ephemeris
information. Each satellite broadcasts its own ephemeris once every
30 seconds. Once the GPS radio signal has been acquired, the
process of computing position requires the use of the ephemeris
information.
[0006] The broadcast ephemeris information is encoded in a 900 bit
message within the GPS satellite signal. It is transmitted at a
rate of 50 bits per second, taking 18 seconds in all for a complete
ephemeris transmission. The broadcast ephemeris information is
typically valid for 2 to 4 hours into the future (from the time of
broadcast). Before the end of the period of validity the GPS
receiver must obtain a fresh broadcast ephemeris to continue
operating correctly and produce an accurate position. It is always
slow (no faster than 18 seconds), frequently difficult, and
sometimes impossible (in environments with very low signal
strengths), for a GPS receiver to download an ephemeris from a
satellite. For these reasons it has long been known that it is
advantageous to send the ephemeris to a GPS receiver by some other
means in lieu of awaiting the transmission from the satellite. U.S.
Pat. No. 4,445,118, issued Apr. 24, 1984, describes a technique
that collects ephemeris information at a GPS reference station, and
transmits the ephemeris to the remote GPS receiver via a wireless
transmission. This technique of providing the ephemeris, or
equivalent data, to a GPS receiver has become known as
"Assisted-GPS." Since the source of ephemeris in Assisted-GPS is
the satellite signal, the ephemeris information remains valid for
only a few hours. As such, the remote GPS receiver must
periodically connect to a source of ephemeris information whether
that information is received directly from the satellite or from a
wireless transmission. Without such a periodic update, the remote
GPS receiver will not accurately determine position.
[0007] The deficiency of the current art is that there is no source
of satellite trajectory and clock information that is valid for
longer than a few hours into the future, and it can be expensive to
send the ephemeris information repeatedly to the many remote
devices that may need it. Moreover, mobile devices may be out of
contact from the source of the Assisted-GPS information when their
current ephemeris becomes invalid.
[0008] Therefore, there is a need in the art for a method and
apparatus for providing satellite trajectory and clock information
that is valid for an extended period into the future, e.g., many
days into the future.
SUMMARY OF THE INVENTION
[0009] The present invention is a method and apparatus for
generating satellite tracking data (STD) that is valid for extend
periods of time into the future, i.e., long term STD or LT-STD. The
STD may contain future satellite trajectory information and/or
satellite clock information. The STD is derived by receiving at one
or more satellite tracking stations the signals from at least one
satellite and determining satellite tracking information (STI) from
the received signals. STI contains present satellite orbit
trajectory data and satellite clock information.
[0010] The STD may be provided to a remote satellite signal
receiver via a network or communications system. The satellite
system may include the global positioning system (GPS), GLONASS,
GALILEO, or other satellite systems that may use STD to enhance the
performance of the receiver. By using the LT-STD, a remote receiver
may accurately operate for days without receiving an update of the
broadcast ephemeris information as normally provided from the
satellites.
[0011] As an example, a method for distributing LT-STD to a remote
receiver is disclosed herein. This method may include functions for
extracting from memory at least a portion of the LT-STD; generating
formatted data from such LT-STD so that the formatted data is in a
format supported by the remote receiver, applying security to the
formatted data to prevent unauthorized access to the LT-STD, and
transmitting the formatted data to the remote receiver. The
function of applying security to the formatted data may include
applying any of cryptosecurity, transmission security, emission
security, and traffic-flow security; any or all of which may use
encryption. The function of applying security to the formatted data
also operates to ensure to the remote receiver that the LT-STD is
unadulterated.
[0012] The method may also include receiving from the remote
receiver a request for security credentials for removing the
security from the formatted data, ensuring that payment for the
remote receiver to obtain the LT-STD is accounted for, and sending
to the remote receiver the security credentials. The payment for
the remote receiver to obtain the LT-STD may be a fee
arrangement.
[0013] The method may further include the functions of sending to
the remote receiver a request for information to substantiate that
the remote receiver is authorized to access the LT-STD, receiving
from the remote receiver the information to substantiate that the
remote receiver is authorized to access the LT-STD. This
information may include information to validate the payment for the
remote receiver to obtain the at least a portion of
long-term-satellite-tracking data.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] So that the manner in which the above recited features of
the present invention are attained and can be understood in detail,
a more particular description of the invention, briefly summarized
above, may be had by reference to the embodiments thereof which are
illustrated in the appended drawings.
[0015] It is to be noted, however, that the appended drawings
illustrate only typical embodiments of this invention and are
therefore not to be considered limiting of its scope, for the
invention may admit to other equally effective embodiments.
[0016] FIG. 1 depicts a system for creating and distributing
satellite tracking data (STD) to remote GPS receivers;
[0017] FIG. 2 depicts a method for forming the STD from the
satellite measurements made at satellite tracking stations;
[0018] FIG. 3 depicts a timeline of STD data that conforms to the
broadcast ephemeris format models as described in ICD-GPS-200C yet
spans many hours;
[0019] FIG. 4 depicts a flow diagram of a method that uses a least
squares estimation technique to update parameters in an orbit
trajectory model;
[0020] FIG. 5 depicts the error in the orbit model derived from the
STD, and compares the error to the error in the broadcast
ephemeris;
[0021] FIG. 6 depicts an example of a data table that could be used
in an STD database;
[0022] FIG. 7 is a flow diagram illustrating an example of a
distribution process for distributing long-term STD information;
and
[0023] FIG. 8 is a flow diagram illustrating another example of a
distribution process for distributing long-term STD
information.
DETAILED DESCRIPTION
[0024] FIG. 1 depicts a block diagram of a system 100 for creating
and distributing satellite tracking data (STD). The satellite
system may include the global positioning system (GPS), GLONASS,
GALILEO, or other satellite systems that may use STD to enhance the
performance of the receiver. The following disclosure uses GPS as
an illustrative system within which the invention operates. From
the following disclosure, those skilled in the art will be able to
practice the invention in conjunction with other satellite
systems.
[0025] A network of GPS tracking stations 102 is used to collect
measurement data from the GPS satellites 104. Such a network is
described in detail in U.S. patent application Ser. No. 09/615,105,
filed Jul. 13, 2000. The network could comprise several tracking
stations that collect satellite tracking information (STI) from all
the satellites in the constellation, or a few tracking stations, or
a single tracking station that only collects STI for a particular
region of the world.
[0026] An STD collection and computation server 106 collects and
processes the measurement data (this measurement data is referred
to herein as satellite tracking information (STI) that includes at
least one of: code phase measurements, carrier phase measurements,
Doppler measurements, or ephemeris data). In the preferred
embodiment, measurement data is obtained from both the L1 and L2
frequencies on which the GPS satellites transmit. Alternative
embodiments may use only one of these frequencies, and/or other
frequencies used by other satellite systems or by future versions
of the GPS system.
[0027] The server 106 via its logic, and in particular, its LT-STD
software 124 (described below) continuously or, alternatively,
periodically produces a set of long term satellite tracking data
(LT-STD) that includes: 1) accurate satellite tracking data (STD)
(e.g., a trajectory of each satellite and/or a clock offset
measurement) during the data collection period, 2) a prediction of
the future STD of each satellite, and 3) models that match the
future STD of each satellite.
[0028] In addition to any firmware and software, such as the LT-STD
software 124, the logic of the server 106 also includes a central
processing unit (CPU) 118, support circuits 122, and memory 120.
The CPU 118 may be any one of the many CPUs available on the market
to perform general computing. Alternatively, the CPU may be a
specific purpose processor such as an application specific
integrated circuit (ASIC) that is designed to process satellite
tracking information. The support circuits 122 are well known
circuits such as clock circuits, cache, power supplies and the
like. The memory 120 may be read only memory, random access memory,
disk drive storage, removable storage or any combination thereof.
The memory 120 stores executable software or instructions, e.g.,
LT-STD software 124, that, when executed by the CPU 118, causes the
system 100 to operate in accordance with the present invention.
[0029] The logic of the server 106 may also include an input/output
interface ("I/O") (not shown). The I/O interface provides an
interface to control the transmissions of digital information
between I/O devices (not shown) contained within, associated with
or otherwise attached to the server 106, between components of the
server 106 (shown and not shown) and/or between other components of
the system 100. The I/O devices (not shown) may be embodied as any
or any combination of (i) storage devices, including but not
limited to, a tape drive, a floppy drive, a hard disk drive or a
compact disk drive, (ii) a receiver, (ii) a transmitter, (iii) a
speaker, (iv) a display, (v) a speech synthesizer, (vi) an output
port, and (vii) a user input device, including a keyboard, a
keypad, a mouse and the like.
[0030] The LT-STD produced by the LT-STD software 124 is stored in
an STD database 108. As described in more detail below, a
distribution server 110 is operable to perform a distribution
process for distributing the LT-STD information to GPS receivers
112 and/or authorized GPS receivers 113 over a communication
network, such as wireless communications system 114 and/or the
Internet 116.
[0031] The distribution server 110 includes logic in the form of
software, firmware and/or hardware (e.g., CPU, support circuits,
memory, I/O interface, I/O devices, etc.) substantially similar to
the server 106; excluding the logic for performing functions
particular to server 106 and including logic for executing
executable software or instructions that, when executed by the its
CPU (not shown), causes the distribution server 110 to operate in
accordance with the present invention. The details of the logic of
the distribution server 110, however, are not iterated here so as
to not obscure the present disclosure.
[0032] Referring now to FIG. 7, a flow diagram illustrating an
example of a distribution process 700 for distributing LT-STD
information is shown. Although this distribution process 700 may be
performed by any number of architectures, the distribution process
700 is described with reference to the system 100 of FIG. 1 for
convenience.
[0033] The distribution process starts at termination block 702 and
transitions to process block 704 at which the distribution server
110 accesses (e.g., queries and received from) the STD database 108
to gather a recent (e.g., the most) set of the LT-STD. After
process block 706, the distribution server 110 uses
trajectory-conversion software 111 to format, and if desired, to
secure via a security module 111a this recent set of LT-STD so as
to form a set of formatted data (collectively "formatted data"), as
shown in process block 706. The trajectory-conversion software 111
and security module 111a may format and secure the formatted data
according to a communication protocol and/or security protocol,
respectively.
[0034] The security protocol may include any standard, non-standard
and/or proprietary measure, control and/or rule (collectively
"security directives") for denying unauthorized entities (man or
machine) from accessing, modifying and/or adulterating the recent
set of data underlying the formatted data. Such security directives
are devised, however, so as to ensure to the authorized GPS devices
113 (i.e., GPS devices that are authorized to obtain the formatted
data) that the recent set of LT-STD underlying the formatted data
is authentic, unmodified and/or unadulterated.
[0035] To facilitate such functions, the security directives may
employ cryptosecurity, transmission security, emission security,
traffic-flow security and/or physical security; any and all of
which may use cryptography and/or cryptology (e.g., encryption and
decryption techniques). Details of the cryptosecurity, transmission
security, emission security, traffic-flow security, cryptanalysis,
and/or physical security may be found in Federal Standard 1037C
Glossary of Telecommunications Terms, which is incorporated herein
by reference, and available from the National Communications
System, ATTN: Ms J. Orndorff, 701 S. Courthouse Rd., Arlington, Va.
22204-2198. Telephone: (703) 607-6204.
[0036] Although the security protocol is discussed herein as being
separate from the relevant communication protocol, the security
protocol may be, alternatively, integrated into, integral to or
otherwise combined with the communication protocol. The
communication protocol, in turn, may be any standard, non-standard
and/or proprietary protocol for exchanging the formatted data over
the communication network. Typically, the communication protocol
follows or is a function of a format of the communication
network.
[0037] For example, the communication protocol for the
communication network when embodied as the wireless communications
system 114 is a wireless communication protocol, such as any of 1G,
2G, 2.5 and/or 3 G communication protocol (e.g. CDMA, UTMS, GSM,
etc.), wireless local area network protocol (e.g., IEEE 802.11),
wireless personal area network protocol, the Bluetooth standard,
and the like. If, on the other hand, the communication network is
embodied as the Internet 116 or other packet-data network, then the
communication protocol may be any packet-data protocol, such as a
protocol based-on, derived from or otherwise associated with the
Open Systems Interconnection (OSI) model, the Internet Protocol
model, etc.
[0038] At process block 708, the distribution server 110
distributes the formatted data over the communication network to
one or more of the GPS devices 112 that require the formatted data.
If the formatted data is also secured, then the distribution server
110, using one or more of the appropriate security directives,
distributes the formatted data over the communication network to
one or more of the authorized GPS devices 113. At process block
710, any of GPS devices 112 and/or any of the authorized GPS
devices 113 (or only the GPS devices 113, if secured) obtain the
formatted data in accordance with the communication protocol.
[0039] At optional process block 712, the authorized GPS devices
113 remove any of the security directives applied to formatted data
to recover the recent set of LT-STD. Before doing this, however,
the authorized GPS devices 113 may have to prove that they are
authorized to remove the security directives. This may be done by
way of substantiating appropriate security credentials, which may
include, for example, using a cipher or other key to strip the
security directives from the formatted data.
[0040] To substantiate the appropriate security credentials,
however, the authorized GPS devices 113 have to be authorized to do
so. This may be done by sending to the distribution server 110
(prior to or after distribution of the formatted data) respective
requests for the security credentials. If authorized, the
distribution server 110, in turn, sends to the authorized GPS
devices 113 their respective security credentials. Alternatively,
the authorized GPS devices 113 may obtain the security credentials
from a third party (not shown), such as a marketplace server, a
front office server, a service provider (man or machine), etc.
Alternatively, the authorized GPS devices 113 may be preconfigured
with the security credentials.
[0041] In any case, the authorized GPS devices 113 may require a
fee arrangement and payment-validating information to substantiate
payment of the fee arrangement to obtain the security credentials.
The fee may be any of a subscription-fee arrangement, one-time-fee
arrangement, one-time-activation-fee arrangement, prepaid-fee
arrangement, renewal-fee arrangement, etc. The fee arrangements may
be based on usage. For example, the fee arrangement may be based
upon (i) the particular recent set of LT-STD requested or to be
distributed, (ii) the number of times (i.e., a frequency) of
delivery of the recent set of LT-STD, (iii) a quantity of the
recent set of LT-STD requested or to be distributed, (iv) etc. Many
other fee arrangements are possible as well.
[0042] The payment-validating information may include, for example,
subscription information, credit information,
prepaid-fee-arrangement information, debit information, checking
account information, savings account information; and/or any other
information to substantiate that the authorized GPS devices 113
have paid or is operable to pay for the recent set of LT-STD.
[0043] By requiring the authorized GPS devices 113 to substantiate
the appropriate security credentials, the GPS devices 112 or other
entities (man and/or machine) may be denied from accessing the
recent set of LT-STD underlying the formatted data. This way, the
GPS devices 112 or other entities (man and/or machine) have to
undergo the foregoing to become one of the authorized GPS devices
113. This may require a fee arrangement and payment-validating
information to substantiate payment of the fee arrangement before
being able to obtain the security credentials.
[0044] The distribution process 700 terminates at termination block
714 after the GPS devices 112 and/or the authorized GPS devices 113
obtain the recent set of LT-STD from the formatted data. After the
GPS devices 112 and/or the authorized GPS devices 113 obtain the
recent set of LT-STD, which may include orbit data, the GPS devices
112 and/or the authorized GPS devices 113 may operate continually
for many days without needing to download fresh broadcast ephemeris
from the satellites or any other source.
[0045] FIG. 8 is a flow diagram illustrating another example of a
distribution process 800 for distributing LT-STD information. The
distribution process 800 is described with reference to the system
100 of FIG. 1 for convenience. For simplicity, the distribution
process 800 is also described below with reference to only one of
the authorized GPS devices 113 and the distribution server 110. The
distribution process 800, however, may be performed using any of
the GPS devices 112, and/or any of the authorized GPS devices
113.
[0046] The distribution process starts at termination block 802 and
transitions to process block 804. At process block 804, the
authorized GPS device 113 may establish respective a secure
communication session with the distribution server 110 via the
communication network so as to prevent unauthorized access to
and/or tampering with the LT-STD to be distributed to such
authorized GPS devices. This secure communication session may
employ any of the aforementioned security directives, including any
of the cryptosecurity, transmission security and traffic-flow
security directives, and may be, for example, embodied as a virtual
private network (secured, trusted, or otherwise). The secure
communication sessions may employ other secure communication
channels or tunneling as well.
[0047] As part of establishing the secure communication session,
the authorized GPS device 113 and/or the distribution server 110
may provide authentication consistent with the security directives
so as to ensure that the authorized GPS device 113 and the
distribution server are as they claim to be. The authentication may
be performed using any or any combination of a login name, a
password, a token, a card key, a fingerprint, retinal scan, or any
other of the security credentials noted above.
[0048] After the secure communication session is established, the
distribution server 110 accesses (e.g., queries and received from)
the STD database 108 to gather a recent (e.g., the most) set of the
LT-STD, as shown in process block 804. After process block 804, the
distribution server 110 uses trajectory-conversion software 111 to
format, and if desired, to secure via a security module 111a this
recent set of LT-STD so as to form the formatted data as shown in
process block 808. The trajectory-conversion software 111 and
security module 111a may format and secure the formatted data
according to the security directives of the appropriate
communication protocol and/or security protocol, respectively, for
the secure communication session.
[0049] These security directives are devised as above, so as to
ensure to the authorized GPS device 113 that the recent set of
LT-STD underlying the formatted data is authentic, unmodified
and/or unadulterated. If the secure communication session is
embodied as a virtual private network, then the secured
communication session may use, for example, encryption and
decryption techniques in accordance with the Advanced Encryption
Standard, RSA, Elliptic Curve Cryptosystems, etc.
[0050] At process block 810, the distribution server 110
distributes the formatted data via the secure communication session
to the authorized GPS device 113. At process block 812, the
authorized GPS device 113 obtains the formatted data in accordance
with the communication protocol. If also secured, then the
authorized GPS device 113 removes any of the security directives
applied to formatted data to recover the recent set of LT-STD, as
for example, described above with respect to optional process block
712 (FIG. 7).
[0051] At process block 814, the authorized GPS device 113 and the
distribution server 110 tear down the secured communication session
in accordance with the communication protocol used to establish the
secured communication session. The distribution process 800
terminates at termination block 814 after the authorized GPS device
113 obtains the recent set of LT-STD from the formatted data.
[0052] As above, after the authorized GPS device 113 obtains (or
any of the GPS devices 112-113 that use the distribution process
800 obtain) the recent set of LT-STD, which may include orbit data.
Accordingly the authorized GPS device 113 (or any of the GPS
devices 112-113 that use the distribution process 800) may operate
continually for many days without needing to download fresh
broadcast ephemeris from the satellites or any other source.
[0053] The orbit data distributed to the GPS devices 112 and/or the
authorized GPS devices 113 may be in the same format as broadcast
ephemeris or may be some other model format as defined by the GPS
devices 112 and/or the authorized GPS devices 113.
[0054] Herein the orbit data is generally referred to as a
satellite tracking model (STM). Loading or transferring the STM to
the GPS devices 112 and/or the authorized GPS devices 113 can be
accomplished in many ways, including, for example, the distribution
process 700 of FIG. 7. If any of the GPS devices 112 and/or
authorized GPS devices 113 are in the form of a personal digital
assistant (PDA) or other handheld device, for example, then the STM
may be loaded into or transferred to such GPS devices 112 and/the
authorized GPS devices 113 via (i) a cradle for the PDA, and (ii)
the communication network.
[0055] The transmission may be alternatively accomplished by
broadcasting the LT-STD (or a model representing all or a portion
of the LT-STD) without knowledge of the specific location of the
GPS devices 112 and/or the authorized GPS devices 113. As such, the
distribution server 110 generally does not require the GPS devices
112 and/or the authorized GPS devices 113 to send any information
to it through the communication network. When the LT-STD is
broadcast using the security protocol, the authorized GPS devices
113 may need to be preconfigured with the security credentials or
obtain the security credentials from the third party as noted
above.
[0056] Referring back to FIG. 1, since GPS is a ranging system in
and of itself, the data transmitted by the GPS satellites can be
used to determine the range, range-rate and clock offsets to the
GPS satellites from a set of tracking stations. This set of
observations generated by the tracking stations 102 is used in the
orbit determination process, and in the estimation of the satellite
clock characteristics. The set of monitoring stations 102 could be
a single station, a public network such as the Continuously
Operating Reference System (CORS), or a privately owned and/or
operated network.
[0057] FIG. 2 illustrates the preferred embodiment of a process for
computing LT-STD. The process begins at step 202 with the
collection of satellite measurements from the network of tracking
stations. Measurements such as code phase, (CP), carrier phase
(CPH), and Doppler may be used for GPS satellite tracking
information. At step 204, the measurements are used to compute the
satellite trajectories and clock offsets over the periods during
which the data was collected. This step is performed using standard
GPS processing techniques and software packages well known in the
art. Examples of this type of software are GIPSY from the Jet
Propulsion Laboratory (JPL), GEODYN from NASA Goddard Space Flight
Center (GSFC), and the commercial product, MicroCosm, from Van
Martin Systems.
[0058] At step 206, the satellite trajectories and clock offsets
from step 204 are propagated into the future with the same software
package, using standard orbit models, such as gravity, drag, solar
radiation pressure, tides, third body effects, precession,
nutation, and other conservative and non-conservative forces
effecting the satellite trajectory. These are normally the same
force models that are used in the estimation of the satellite
orbits during the data fit interval. A subset of these models, such
as those for drag and solar radiation pressure, are adjusted during
the orbit estimation process described in step 204 to best fit the
trajectory. This combination of known and estimated force models
parameters is used in the propagation 206 to provide the propagated
orbit for time outside the data fit interval. The clock offsets for
GPS satellites are typically very small, and change linearly over
time. These clock offsets are propagated into the future using
standard models, such as a second order model containing clock
offset, drift, and drift rate.
[0059] At step 208, the propagated satellite trajectories and/or
clock offsets are stored as STD in a database. At step 210, the
trajectory conversion software converts the LT-STD data into a
model and format expected by the GPS device to which the model is
to be provided. At step 212, the prescribed model or information is
output. The prescribed model may be output in accordance with the
distribution process 700 of FIG. 7. For use with existing GPS
receivers, such as the GPS devices 112 and/or the authorized GPS
devices 113 (collectively hereinafter "GPS devices 112-113"), the
preferred embodiment of the model is the GPS ephemeris model as
described in ICD-GPS-200 and an ephemeris model is generated from
the LT-STD for each 4 hour period as illustrated in the timeline
300 of FIG. 3, i.e., a different model 301, 302 and so on is
generated for each six hour period. As such, the plurality of
models 301, 302 and so on cumulatively span the length of the
available LT-STD.
[0060] In an alternate embodiment, at step 204 (FIG. 2), the
satellite trajectories and clock offsets may be estimated using the
data broadcast by the satellites and the standard equations given
in ICD-GPS-200c.
[0061] The orbit model is a mathematical representation of the
satellite trajectory that describes the trajectory as a function of
a small number of variables and eliminates the need to provide
satellite position vectors explicitly as a table of time vs.
satellite positions. An example of an ephemeris model is the
classic six element Keplerian orbital model. Although this model
lacks long term accuracy, it is a functional ephemeris model for
providing satellite trajectory information as a function of a small
number of variables. In the preferred embodiment, the model used to
describe the trajectory is GPS standard ephemeris, specified in
ICD-GPS-200c, following the same conventions and units. This is the
preferred method to provide maximum compatibility with existing GPS
receivers, such as the GPS devices 112-113. However, other orbit
models could also be used to represent the satellite trajectory.
Orbit models can be selected to provide increased accuracy, longer
duration fits, more compact representation of the trajectory, or
other optimizations required in an application.
[0062] This invention is different from the current art in that the
orbit model provided to the GPS devices 112-113 is not the
ephemeris data broadcast by the GPS satellites. Current art
downloads the ephemeris broadcast from the GPS satellites and
retransmits that data to GPS devices. In this invention, the
broadcast ephemeris data is not required at any stage and is not
used in the preferred implementation.
[0063] The broadcast ephemeris data provided by the GPS satellites
cover a specific time period (typically 4 hours) and the end of
that time the information becomes unusable. For example, if a
device receives a broadcast ephemeris that will expire in 5
minutes, the device would need the new broadcast ephemeris before
operating outside that 5 minute interval. With this invention, the
STD may be formatted for the time period required by the device.
This time period may be for the current time forward or may be for
some time interval in the future. For example, a device, such as
any of the GPS devices 112-113, may request orbit information in
the standard GPS ephemeris format for the current time. In this
case, the ephemeris provided to any of GPS devices 112-113 would be
valid for the next 6 hours. Any of the GPS devices 112-113 could
request orbit information for the next 12 hours in the standard GPS
format, which, for example, could be supplied as two six hour
ephemeris orbit models. In addition, different orbit models and
formats that support different accuracies and standards can be
generated from the LT-STD.
[0064] Fitting the LT-STD to the desired orbit model can be
accomplished in a number of mathematical methods. The preferred
embodiment is a least-squares fit of the orbit model parameters to
the trajectory data. Other methods, such as Kalman filters or other
estimators can also be used to obtain the orbit model parameters
that best fit the trajectory data. These techniques of fitting data
to orbit models are well known to people skilled in the art of
orbit determination and orbit modeling.
[0065] The least squares technique provides an optimal fit of the
trajectory data to the orbit model parameters. FIG. 4 depicts a
flow diagram of a method of generating an orbit model using a least
squares estimation technique. One embodiment of LT-STD is a table
representation of time, position, and clock offset for each
satellite, as shown in FIG. 6. The time, position, and clock offset
can be in any time/coordinate system. For the purpose of simplicity
and illustration, the time/coordinate system is GPS time and
Earth-Centered-Earth-Fixed (ECEF) position in the World Geodetic
Survey 1984 (WGS-84) reference frame.
[0066] At step 402, the STD for the desired time interval is
extracted from the STD database. The orbit model parameters are
initialized to the orbit model values obtained by a similar process
for the previous interval. This guarantees that the initial orbit
model parameters are a good fit at least for the beginning of the
desired time interval. The rest of the process 400 will ensure that
the parameters are adjusted so that they become a good fit for the
entire time interval.
[0067] In the preferred embodiment there are 15 orbital parameters
to be adjusted: [0068] Square root of semi-major axis (meters 1/2)
[0069] Eccentricity (dimensionless) [0070] Amplitude of sine
harmonic correction term to the orbit radius (meters) [0071]
Amplitude of cosine harmonic correction term to the orbit radius
(meters) [0072] Mean motion difference from computed value
(radians/sec) [0073] Mean anomaly at reference time (radians)
[0074] Amplitude of cosine harmonic correction term to the argument
of latitude (radians) [0075] Amplitude of sine harmonic correction
term to the argument of latitude (radians) [0076] Amplitude of
cosine harmonic correction term to the angle of inclination
(radians) [0077] Amplitude of sine harmonic correction term to the
angle of inclination (radians) [0078] Longitude of ascending node
of orbit plane at weekly epoch (radians) [0079] Inclination angle
at reference time (radians) [0080] Rate of inclination angle
(radians/sec) [0081] Argument of perigee (radians) [0082] Rate of
right ascension (radians/sec) Although it will be readily apparent
that more terms may be used, for better fits, or, fewer terms may
be used for a more compact model.
[0083] At step 404, the orbit model is used to predict what the
trajectory would be, the predicted data is denoted the "Model
Trajectory Data" (MTD). If the model were perfect, the MTD would
coincide exactly with the STD. At step 406, the MTD and OTD are
compared to see how closely the orbit model fits the orbit data. In
the preferred embodiment, the comparison step 406 is performed by
summing the squares of the differences between each trajectory
point in the OTD and the corresponding point in the MTD, and
comparing the resulting sum to a threshold. If the fit is "good",
the model parameters are deemed "good" and the process stops at
step 410. If the fit is not good then the model parameters are
adjusted at step 408. There are many techniques well known in the
art for adjusting model parameters to fit data. For example, in
FIG. 5, the six-hour ephemeris model was adjusted to fit six hours
of OTD using a subspace trust region method based on the
interior-reflective Newton method described in Coleman, T. F., and
Y. Li, "On the convergence of reflective Newton methods for large
scale nonlinear minimization subject to bounds", Mathematical
Programming, Vol. 67, Number 2, pp. 189-224, 1994, and Coleman, T.
F., and Y. Li, "An interior, trust region approach for nonlinear
minimization subject to bounds", SIAM Journal on Optimization, Vol.
6, pp. 418-445, 1996. There are standard computer packages, e.g.,
MATLAB Optimization Toolbox, which may be used to implement these
methods.
[0084] Steps 404, 406 and 408 are repeated until the model
parameters are found that fit the OTD well.
[0085] When fitting an orbit model to trajectory data, there are
many choices of which orbit model to choose. The preferred
embodiment is to use orbit models with parameters that have been
defined in well-known standards. In one embodiment, the ephemeris
parameters defined in the GPS interface control document,
ICD-GPS-200c, are used. The ICD-GPS-200c definition includes a bit
that specifies a 4-hour fit or a 6-hour fit. Typically, the
satellite data is broadcast in 4-hour fits and, by the time this
data is obtained by the observer of the satellite, the data is
often near the end of its fit interval. In one embodiment of the
current invention, sequential 6 hour windows of STD are used to
create 6-hour ephemeris models, using the technique described in
FIG. 4 and the accompanying text. This produces a set of ephemeris
models as illustrated in FIG. 3. Although these particular 6-hour
models are not available without this invention, the models
nonetheless are defined using standard parameters (i.e.
ICD-GPS-200c) and will be understood by any device that was
designed to be compatible with said standard, such as any of the
GPS devices 112-113.
[0086] Alternatively, the transmission time for the model may be
dynamically determined in response to various transmission network
characteristics, e.g., cellular telephone rate structures, data
transmission bandwidths, low network utilization periods, low
network congestion periods and the like. Thus, the invention
determines present value of the specific characteristics and
compares the present value to a threshold. In response to the
comparison, the invention will transmit or not transmit the model.
For example, the invention may monitor the network traffic and
determine the least congested time to transmit the model. Many
wireless networks have time varying rates. For example, cellular
telephone use is often less expensive on weekends compared to
mid-week rates. A useful embodiment of the current invention is to
create a satellite tracking model that is valid for the period
between inexpensive rates (example: valid from one Saturday to the
next), and transmit the model during the time that the rate is
inexpensive. As such, the model is transmitted for less cost than
if the models were transmitted during a peak rate period. Also, or
as an alternative, one may define and send the model to coincide
with periods of low data use on the network--whether the network is
wireless or not (e.g. the internet). Those skilled in the art will
realize that many other transmission time optimization
characteristics can be used to determine when it is best to
transmit the model to the receiver(s), such as the GPS devices
112-113.
[0087] FIG. 5 shows an example of Satellite Tracking Data (STD)
that was generated for a time interval of greater than six hours.
Then, using the technique described by FIG. 4 and accompanying
text, parameters of an ICD-GPS-200c ephemeris model were adjusted
to give a best fit to 6 hours of the STD. The orbit modeled by this
6-hour ephemeris was then compared to the true trajectory, and for
comparison, the true trajectory was also compared to the orbit
modeled by the broadcast ephemeris. The results are shown in FIG.
5, illustrating how the broadcast ephemeris loses validity while
the ephemeris created by this invention maintains its validity with
approximately one meter of error.
[0088] The clock offset of GPS satellites is easily modeled by
three parameters. In the preferred embodiment, the measured clock
offset is modeled by the three parameters defined in ICD-GPS-200c.
These parameters represent clock offset, drift, and drift rate. The
parameters are adjusted in a similar way to the method 400
described above to give a model that best fits the measured data
over the time interval.
[0089] Alternative embodiments may use longer fit intervals, such
as 8, 14, 26, 50, 74, 98, 122, or 146 hours for each ephemeris
model. These fit intervals are envisaged in ICD-GPS-200c, but are
seldom, if ever, available from the broadcast ephemeris. Under the
current invention, models with these fit intervals may be generated
even when the broadcast ephemeris is limited to a 4-hour fit
interval.
[0090] Alternative embodiments of the STD data may include observed
satellite velocity, acceleration, clock drift, or clock drift rate
and these terms may be used in the process of fitting a model in
ways which are well known in the art.
[0091] Another embodiment of an orbit model uses the spare data
bits in the current ephemeris format of a conventional GPS signal
to provide additional model parameters that would improve the data
fit over long time intervals. For example, subframe 1 has 87 spare
bits that are available for additional parameters. This technique
allows for more parameters to describe the orbital motion of the
satellites without compromising the standard data format. This new
ephemeris model is based on the current ephemeris model with
additional correction terms used to augment the model to support
the longer fit intervals with greater accuracy.
[0092] Yet another embodiment of an orbit model is to develop a new
set of orbital parameters that describe the satellite orbit which
are different, in part or in their entirety, from the GPS ephemeris
model parameters. With the goal of making the fit interval longer,
different parameters may provide a better description of the
satellite orbit. This new set of parameters could be defined such
that they would fit into the existing data structures, however,
their implementation and algorithms for use would be different.
[0093] Still a further embodiment of an orbit model would be to
develop a new set of orbital parameters that would not fit into the
existing GPS ephemeris model format. This new set of parameters
would be developed to better address the trade-off between the
numbers of parameters required, the fit interval, and the orbit
accuracy resulting from the model. An example of this type of
ephemeris parameter set is Brouwer's theory that could be used as
is or modified to account for GPS specific terms. Brouwer's theory
as described in Brouwer, D. "Solution of the Problem of Artificial
Satellite Theory without Drag", Astron J. 64: 378-397, November
1959 is limited to satellites in nearly circular orbits such as GPS
satellites.
[0094] Another embodiment is to use a subset of the standard
ephemeris parameters defined in ICD-GPS-200c. This approach is
particularly useful when bandwidth and/or packet size is limited in
the communication link that will be used to convey the orbit model
to a remote GPS receiver, such as any of the GPS devices 112-113.
In one such embodiment, the fifteen orbit parameters described
above, and in ICD-GPS-200c, may be reduced to a subset of 9
parameters, by setting all harmonic terms in the model to zero:
[0095] Square root of semi-major axis (meters 1/2) [0096]
Eccentricity (dimensionless) [0097] Mean motion difference from
computed value (radians/sec) [0098] Mean anomaly at reference time
(radians) [0099] Longitude of ascending node of orbit plane at
weekly epoch (radians) [0100] Inclination angle at reference time
(radians) [0101] Rate of inclination angle (radians/sec) [0102]
Argument of perigee (radians) [0103] Rate of right ascension
(radians/sec) Process 400 is then executed using this subset of
parameters. This reduces the amount of data that must be sent to
the remote GPS receiver. The remote GPS receiver can then
reconstruct a standard ephemeris model by setting the "missing"
harmonic terms to zero. There are a large number of alternative
embodiments to reduce the size of the data, while still providing a
model that fits the STD, including: [0104] removing parameters from
the model, and replacing them with a constant, such as zero--as
done above--or some other predetermined value that is either stored
in the remote GPS receiver or occasionally sent to the receiver;
[0105] the resolution of the parameters may be restricted in the
process 400--this too reduces the amount of data that must be sent
to the remote GPS receiver; and/or [0106] parameters, which are
similar among two or more satellites, may be represented as a
master value plus a delta, where the delta requires fewer bits to
encode; an example of this is the parameter Eccentricity, which
changes very little among different GPS satellites. Some of these
approaches reduce the ability of the model to fit the data over a
period of time (e.g., six hours). In this case, the fit interval
may be reduced (e.g. to four hours) to compensate.
[0107] While the foregoing is directed to the preferred embodiment
of the present invention, other and further embodiments of the
invention may be devised without departing from the basic scope
thereof, and the scope thereof is determined by the claims that
follow.
* * * * *