U.S. patent application number 12/434026 was filed with the patent office on 2011-07-07 for practical method for upgrading existing gnss user equipment with tightly integrated nav-com capability.
This patent application is currently assigned to Dalaware Corporation. Invention is credited to William J. Bencze, Clark E. Cohen, Bryan T. Galusha, Todd E. Humphreys, Brent M. Ledvina, Mark L. Psiaki.
Application Number | 20110163913 12/434026 |
Document ID | / |
Family ID | 43544821 |
Filed Date | 2011-07-07 |
United States Patent
Application |
20110163913 |
Kind Code |
A1 |
Cohen; Clark E. ; et
al. |
July 7, 2011 |
Practical Method for Upgrading Existing GNSS User Equipment with
Tightly Integrated Nav-Com Capability
Abstract
A practical method for adding significant new high-performance,
tightly integrated Nav-Com capability to any Global Navigation
Satellite System (GNSS) user equipment, such as GPS receivers,
requires no hardware modifications to the existing user equipment.
In one example, the iGPS concept is applied to a Defense Advanced
GPS Receiver (DAGR) and combines Low Earth Orbiting (LEO)
satellites, such as Iridium, with GPS or other GNSS systems to
significantly improve the accuracy, integrity, and availability of
Position, Navigation, and Timing (PNT)--in some cases by three
orders of magnitude, to enable high precision GNSS carrier phase
observable to be more readily exploited to improve PNT
availability--even under interference conditions or occluded
environments, and to enable new communication enhancements made
available by the synthesis of precisely coupled navigation and
communication modes. To achieve time synchronization stability to
the required sub-20 ps level between the existing DAGR and a
plug-in iGPS enhancement module, a special-purpose wideband
reference signal is generated by the iGPS module and coupled to the
DAGR via the existing antenna port, so that no hardware
modification of the DAGR is required.
Inventors: |
Cohen; Clark E.;
(Washington, DC) ; Humphreys; Todd E.; (Half Moon
Bay, CA) ; Ledvina; Brent M.; (San Francisco, CA)
; Bencze; William J.; (Half Moon Bay, CA) ;
Psiaki; Mark L.; (Brooktondale, NY) ; Galusha; Bryan
T.; (Oakland, CA) |
Assignee: |
Dalaware Corporation
San Mateo
CA
|
Family ID: |
43544821 |
Appl. No.: |
12/434026 |
Filed: |
May 1, 2009 |
Current U.S.
Class: |
342/357.29 ;
342/357.71; 342/357.77 |
Current CPC
Class: |
G01S 19/09 20130101;
G01S 19/43 20130101; G01S 19/36 20130101; G01S 19/46 20130101 |
Class at
Publication: |
342/357.29 ;
342/357.71; 342/357.77 |
International
Class: |
G01S 19/46 20100101
G01S019/46; G01S 19/31 20100101 G01S019/31; G01S 19/37 20100101
G01S019/37 |
Goverment Interests
[0001] This invention is made with Government support under Navy
Contract N00173-08-C-2074 awarded by the Naval Research Laboratory.
The Government has certain rights in the invention.
Claims
1. A method of upgrading existing Global Navigation Satellite
System (GNSS) user equipment, comprising the steps of: providing
first GNSS user hardware including a first oscillator driving a
precise coded reference signal generator arranged to generate a
reference signal and supply the reference signal to an existing
second GNSS user device, and a Navigation/Communication processor
arranged to generate position, navigation, and/or timing signals
upon receipt from the second GNSS user device of coherent
correlations of incoming GNSS signals and the reference signal;
providing an existing second GNSS user device separate from the
first GNSS user hardware, said GNSS user device including at least
one existing port and a second oscillator arranged to produce said
coherent correlations of incoming GNSS signals and the reference
signal generated by the add-on first GNSS hardware, and to send the
coherent correlations to the first GNSS user hardware, coupling the
first GNSS user hardware to the second GNSS user device via the
existing port without modifying hardware of the second GNSS user
device.
2. The method of claim 1, wherein the first GNSS user hardware
combines the coherent correlations from the second GNSS device with
its own raw precise correlations to derive a precise solution for
position, velocity, and/or time.
3. The method of claim 1, wherein the at least one existing port
includes an antenna port and a data port, the first GNSS user
hardware is arranged to send the reference signal to the second
GNSS user device through the antenna port, and the first GNSS user
hardware is arranged to send steering commands to the second GNSS
user device and receive back the coherent correlations through the
data port.
4. The method of claim 3, wherein the first GNSS user hardware
includes Iridium receive capability and the second GNSS user device
is a Global Positioning System (GPS) receiver.
5. The method of claim 4, wherein Iridium and GPS signals are input
through a common antenna.
6. The method of claim 4, wherein the first oscillator provides a
common precision oscillator reference for both the GPS and Iridium
r.f. timing and ranging measurements.
7. The method of claim 6, wherein the first oscillator provides a
carrier phase reference common to both the GPS and Iridium
platforms
8. The method of claim 4, wherein the second GNSS user device is a
Defense Advance GPS Receiver (DAGR).
9. The method of claim 4, wherein the first GNSS user hardware
further includes Iridium transmit capability.
10. The method of claim 9, wherein the first GNSS user hardware
also includes Y-code, M-code, and/or C/A code tracking
capability.
11. The method of claim 9, wherein the second GNSS receiver also
includes M-code capability.
12. The method of claim 9, wherein the second GNSS receiver also
includes Galileo GLONASS and/or COMPASS capability, or is a
commercial C/A code receiver.
13. The method of claim 1, further comprising the step of employing
a Kalman Filter in the first GNSS user device to model and estimate
the user equipment position, velocity, and time using the carrier
phase of the incoming signals.
14. The method of claim 13, further comprising the step of using a
reference station to calibrate the code and carrier phase of
satellites within view and telemeter the calibration data to a
communications satellite whose broadcast output is coupled into the
first GNSS user hardware and whose calibration data is employed to
improve the position, velocity, and time solution.
15. The method of claim 14, wherein the Kalman Filter includes
observable data from an inertial measurement unit (IMU).
16. The method of claim 14, wherein the first GNSS user hardware
includes a transmitter component that uses its improved position,
velocity, and time estimate to improve a time and frequency
synchronization of its transmissions.
17. The method of claim 16, wherein the transmissions are coupled
to a regional communications network.
18. The method of claim 15 where the transmissions are coupled to a
satellite network.
19. The method of claim 18, where the transmissions are coupled to
a global network of low earth orbiting satellites.
20. The method of claim 1 wherein the first GNSS user hardware is
an upgrade module that includes Iridium receive capability and said
Navigation/Communication processor is a central
Navigation/Communication processor for directing internal operation
of the upgrade module, said Navigation/Communication processor
including control loop drivers for Iridium correlators implemented
by a signal processor bank, a GPS P code generator, and a
configuration control for transmit and receiver functions as well
as an interface to the second GNSS user device.
21. The method of claim 20, wherein the Navigation/Communication
processor further includes an interface to a
micro-electromechanical-system (MEMS) inertial measurement unit
(IMU).
22. The method of claim 20, wherein the Navigation/Communication
processor further includes respective interfaces for receiving
signals from a barometer and/or magnetometer.
23. The method of claim 20, wherein the signal processor bank
synthesizes a replica code that matches what each Iridium satellite
is known to broadcast, the replica code consisting of a
pre-identified series of message bits known in advance by the user
to create direct sequence chips of a spread spectrum signal,
thereby enabling the time and carrier phase of an incoming Iridium
signal to be determined.
24. The method of claim 20, wherein the Navigation/Communication
processor actively controls a variable attenuator to ensure that a
P code reference signal is strong enough to be detected under
interference but not so strong as to be a source of unwanted
interference.
25. The method of claim 20, wherein the Navigation/Communication
processor monitors an Automatic Gain Control (AGC) and routes an
incoming composite GPS and navigation signal into the second GNSS
user equipment in order to track overall power emerging from
antenna terminals and regulate the power to a constant value.
26. The method of claim 20, wherein the first GNSS user hardware
executes a reference input initialization and control routing
including the steps of: a. minimizing reference signal power; b.
measuring interference at an automatic gain control (AGC); c.
recording a benchmark based on the measured interference; d. step
incrementing reference signal power and measuring interference at
the AGC until a threshold is reached and recording a scale factor;
e. initializing reference signal lock; f. again measuring
interference at the AGC; and g. applying backoff.
27. Add-on first Global Navigation Satellite System (GNSS) user
hardware for upgrading an existing second GNSS user device by
adding tightly integrated Nav-Com capability without having to
modify the existing second GNSS user device, comprising: a first
oscillator driving a precise coded reference signal generator
arranged to generate a reference signal and supply the reference
signal to an existing second GNSS user device; a
Navigation/Communication processor arranged to generate position,
navigation, and/or timing signals upon receipt from the second GNSS
user device of coherent correlations of incoming GNSS signals and
the reference signal; and at least one port for coupling the add-on
GNSS user hardware to the second GNSS user device through an
existing port of the second GNSS user device without modifying
hardware of the second GNSS user device.
28. Add-on GNSS user hardware as claimed in claim 27, wherein the
add-on GNSS user hardware combines the coherent correlations from
the second GNSS device with its own raw precise correlations to
derive a precise solution for position, velocity, and/or time
29. Add-on GNSS user hardware as claimed in claim 27, wherein the
at least one existing port includes an antenna port and a data
port, the add-on GNSS user hardware is arranged to send the
reference signal to the second GNSS user device through the antenna
port, and the add-on GNSS user hardware is arranged to send
steering commands to the second GNSS user device and receive back
the coherent correlations through the data port.
30. Add-on GNSS user hardware as claimed in claim 27, wherein the
add-on GNSS user hardware includes Iridium receive capability and
the second GNSS user device is a Global Positioning System (GPS)
receiver.
31. Add-on GNSS user hardware as claimed in claim 30, wherein the
second GNSS user device is a Defense Advance GPS Receiver
(DAGR).
32. Add-on GNSS user hardware as claimed in claim 30, wherein the
add-on GNSS user hardware further includes Iridium transmit
capability.
33. Add-on GNSS user hardware as claimed in claim 32, wherein the
first GNSS user hardware also includes Y-code, M-code, and/or C/A
code tracking capability.
34. Add-on GNSS user hardware as claimed in claim 32, wherein the
second GNSS receiver also includes M-code capability.
35. Add-on GNSS user hardware as claimed in claim 32, wherein the
second GNSS receiver also includes Galileo GLONASS and/or COMPASS
capability or is a commercial C/A code receiver.
36. Add-on GNSS user hardware as claimed in claim 27, wherein the
Navigation/Communication processor includes a Kalman Filter in the
second GNSS user device to model and estimate the user equipment
position, velocity, and time using the carrier phase of the
incoming signals.
37. Add-on GNSS user hardware as claimed in claim 36, wherein a
reference station is used to calibrate the code and carrier phase
of satellites within view and telemeter the calibration data to a
communications satellite whose broadcast output is coupled into the
add-on GNSS user hardware and whose calibration data is employed to
improve the position, velocity, and time solution.
38. Add-on GNSS user hardware as claimed in claim 37, wherein the
Kalman Filter includes observable data from an inertial measurement
unit (IMU).
39. Add-on GNSS user hardware as claimed in claim 37, wherein the
add-on GNSS user hardware includes a transmitter component that
uses its improved position, velocity, and time estimate to improve
a time and frequency synchronization of its transmissions.
40. Add-on GNSS user hardware as claimed in claim 39, where the
transmissions are coupled to a regional communications network.
41. Add-on GNSS user hardware as claimed in claim 39, where the
transmissions are coupled to a satellite network.
42. Add-on GNSS user hardware as claimed in claim 41, wherein the
transmissions are coupled to a global network of low earth orbiting
satellites.
43. Add-on GNSS user hardware as claimed in claim 27, wherein the
first and second GNSS user equipment share a common antenna.
44. Add-on GNSS user hardware as claimed in claim 27, wherein the
add-on GNSS user hardware is an upgrade module that includes
Iridium receive capability, and the Navigation/Communication module
is a central Navigation/Communication processor contained in the
upgrade module for directing internal operation of the upgrade
module, said Navigation/Communication processor including control
loop drivers for Iridium correlators implemented by a signal
processor bank, a GPS P code generator, and a configuration control
for transmit and receiver functions as well as an interface to the
second GNSS user device.
45. Add-on GNSS user hardware as claimed in claim 44, wherein the
Navigation/Communication processor further includes an interface to
a micro-electromechanical-system (MEMS) inertial measurement unit
(IMU).
46. Add-on GNSS user hardware as claimed in claim 44, further
comprising respective interfaces to a barometer and/or
magnetometer.
47. Add-on GNSS user hardware as claimed in claim 44, wherein the
signal processor bank synthesizes a replica code that matches what
each Iridium satellite is known to broadcast, the replica code
consisting of a pre-identified series of message bits known in
advance by the user to create direct sequence chips of a spread
spectrum signal, thereby enabling the time and carrier phase of an
incoming Iridium signal to be determined.
48. Add-on GNSS user hardware as claimed in claim 44, wherein the
Navigation/Communication processor actively controls a variable
attenuator to ensure that a P code reference signal is strong
enough to be detected under interference but not so strong as to be
a source of unwanted interference.
49. Add-on GNSS user hardware as claimed in claim 44, wherein the
Navigation/Communication processor monitors an Automatic Gain
Control (AGC) and routes an incoming composite GPS and navigation
signal into the second GNSS user equipment in order to track
overall power emerging from antenna terminals and regulate the
power to a constant value.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method of upgrading existing
Global Navigation Satellite System (GNSS) user equipment, such as a
GPS receiver, in order to add high-performance, tightly integrated
navigation and communication (Nav-Com) capability without the need
to modify the existing equipment. The invention also relates to an
apparatus, which may take the form of a plug-in enhancement module,
for adding iGPS to existing GNSS user equipment.
[0004] In a preferred embodiment of the invention, the upgrade is
to a particular tightly integrated Nav-Com system known as iGPS,
which utilizes the carrier phase of signals received from Low Earth
Orbiting (LEO) satellites, such as Iridium to provide a
special-purpose wideband reference signal. In this embodiment,
upgrade to iGPS is achieved by: [0005] using an existing antenna
port in the GNSS user equipment to supply the GNSS user equipment
with a special-purpose wideband reference signal phase locked to
the reference oscillator of the apparatus; [0006] causing the
existing GNSS user equipment to produce coherent correlations of
incoming GNSS (or GPS) signals and the reference signal relative to
the GNSS user equipment reference oscillator; and [0007] sending
the coherent correlations back through an existing data port to a
Nav-Com processor for combination with additional correlations of
Iridium taken relative to the reference oscillator of the apparatus
to derive more precise solutions for position, velocity, and/or
time.
[0008] The GNSS user equipment may be a Defense Advanced GPS
Receiver (DAGR), with the Nav-Com processor and special purpose
reference signal generating components being provided in a single
unitary module that plugs into existing ports of the DAGR, without
need to modify the DAGR. This arrangement can significantly improve
the accuracy, integrity, and availability of Position, Navigation,
and Timing (PNT) in the DAGR, in some cases by three orders of
magnitude, using carrier phase with the potential to converge onto
sub-decimeter level position fixes with time frames on the order of
a minute anywhere on the globe. The invention enables the high
precision GNSS carrier phase observable to be more readily
exploited to improve PNT availability--even under interference
conditions or occluded environments. Furthermore, the invention
enables new communication enhancements made available by the
synthesis of precisely coupled navigation and communication modes.
With the proper design and integration, the easily attached upgrade
is capable of significantly lowering fielding and life cycle costs
to realize the advanced capability.
[0009] 2. Description of Related Art
[0010] Tightly integrated Navigation and Communication opens up a
vast realm of new complementary capability for U.S. military,
civil, and commercial applications, especially if such
infrastructure is global in nature. Communications infrastructure
can improve navigation by providing real-time data and timing
aiding, while navigation infrastructure improves communication by
providing time and position aiding. Employing a global
infrastructure enables stakeholders to better enjoy economies of
scope and scale. The more tightly integrated the architecture of
the Navigation and Communication components, the greater the mutual
synergies can be achieved.
[0011] One especially notable example of an integrated global
Nav-Com system is iGPS, created by the fusion of the Iridium and
GPS global satellite constellations. The Navy has awarded a
contract to a Boeing-led team to use Iridium to provide
supplemental data, timing, and ranging information to authorized
GPS users. This additional information provides among other
benefits the means for significantly improved interference
rejection performance and faster acquisition of GPS with a dynamic
user platform [Glen Gibbons, "Boeing Wins NRL Contract to Continue
Iridium/GPS Development", Inside GNSS, September/October 2008].
[0012] In general iGPS, as well as the broader global Nav-Com
solution set, has the potential to significantly improve the
accuracy, integrity, and availability of Position, Navigation, and
Timing (PNT). The rapid angle motion of LEO satellites in the sky
dramatically increases spatial diversity over the traditionally
slow moving GPS satellites in high orbit. If the LEO and GPS
satellite carrier phase is employed, there is potential to lock
onto sub-decimeter level position fixes with time frames on the
order of a minute anywhere on the globe. See, for example, U.S.
Pat. Nos. 6,373,432, 5,812,961, and 5,944,770.
[0013] The use of carrier phase provides significant dividends for
users. The GPS L1 wavelength is approximately 19 cm. The intrinsic
precision for GPS is a small fraction of this wavelength. The
typical timing and ranging error budget for GPS L1 works out to be
on the order of 0.5 cm RMS or 20 ps in terms of time. This
precision is the key to achieving the overall position accuracy
just mentioned as well as integrity and interference rejection. The
iGPS infrastructure can be used to provide both data aiding (for
Iridium ephemeris and GPS data stripping) as well as time stability
transfer (calibrating the Iridium clock with a reference station
and broadcasting precise Iridium carrier phase corrections to the
user in real time), as described in U.S. Pat. No. 7,372,400.
[0014] The converse is also true. Once the user position and time
are well known, new capabilities related to improved communication
are possible. For example, a carrier based upon an ultra-stable
virtual clock can be established between the user and a satellite
because the user has full knowledge of the position and timing of
each. This enables robust coherent communication links to be
established to support, for example, interference resistant and low
probability of intercept communications.
[0015] Traditionally, carrier phase has not been exploited by the
military for navigation purposes. Instead, signal squaring
techniques are employed, which have the unfortunate effect of
squaring both the signal and the noise. The result has been wasted
GPS signal power at a time when the military is considering
development and launch of higher-power satellites to make up the
shortfall. iGPS infrastructure enables more efficient use of
existing GPS power.
[0016] Additional global integrated Nav-Com benefits result from
further synergies. With GPS user equipment and other devices there
is often a need to securely disseminate encryption keys. Without a
suitable infrastructure, the process can become cumbersome. For
example, with only a one-way data link, users and devices may not
be able to authenticate with the key management authority. A
robust, global, two-way communication system solves this problem by
enabling the user and device to authenticate each request to re-key
no matter where they are in the world. This ease of use enables key
dissemination to be both secure and effortless.
[0017] In the case of iGPS, the U.S. has an opportunity to rapidly
implement an existence proof of a LEO-based enhancement to GPS. The
Iridium satellites are already on orbit with a lifetime projected
to extend beyond 2014 (see "Iridium Satellite LLC Estimates
Constellation Life Span To Extend Well Beyond Original
Predictions," Iridium Satellite LLC Press Release, Feb. 26, 2003).
Under the above-listed Navy contract, the Boeing team will develop
global ground infrastructure and develop new flight software for
the existing Iridium satellite constellation by the beginning of
2011. This timetable will provide several years of a suitable
signal in space for the U.S. Military and other authorized users to
make effective use of the new capability before the Iridium
constellation degrades beyond its useful life.
[0018] However, a significant obstacle to implementing iGPS is the
cost and effort to outfit user equipment such as the Defense
Advanced GPS Receiver (DAGR), which remains user equipment of
choice with the military. The U.S. Military has currently fielded
several hundred thousand GPS units ("Rockwell Collins delivers
200,000th DAGR and 40,000th GPS engine to the U.S. Army", Rockwell
Collins Press Release, Apr. 18, 2008) and many more are already in
the process of procurement. The U.S. Government has purchased these
units for nearly $2,000 per unit.
[0019] If the Military or other users are to adopt iGPS, there
needs to be a straightforward way to take advantage of the
installed base of user equipment. Prior art has so far presented
two unpalatable approaches: (i) modify the existing user equipment
hardware to accept a new precision iGPS interface capable of tight
integration and (ii) completely replace the existing user equipment
with new tightly integrated iGPS user equipment designed from
scratch.
[0020] The first approach has caused significant concern because of
the economic and technical risk associated with introducing a
precision iGPS interface with tight integration. In particular,
since the DAGR does not provide for an external oscillator input,
one would have to be added. It is not clear how much this changeout
would cost and to what extent it would require replacement of DAGR
components. There is also a related logistical and configuration
control issue that having multiple versions of DAGR hardware would
become cumbersome to manage for the users and leadership because,
when hardware modifications are made, many of the overall
specifications will need to change and be managed. In addition,
there is also technical risk associated with the hardware
modifications. The carrier phase precision of iGPS for tight
integration requires 20 ps stability between the GPS and Iridium
signal processing components over the full range of environmental
conditions. The hardware components that are especially subject to
phase variation include the GPS RF front end, the Iridium RF front
end, and the GPS oscillator. While the navigation processing
algorithms can tolerate a slow drift of carrier phase bias between
the two components, if thermal or mechanical disturbances are
excessive, the system will be incapable of providing useful
performance. The DAGR layout compounds the technical risk because
the Iridium and GPS components are by necessity in different boxes
which will be subject to different thermal and mechanical
stress.
[0021] The second approach also encounters resistance. Given that
the U.S. has already made a significant investment in GPS
equipment, it is difficult to justify displacing existing
inventory.
[0022] What is needed is a means for demonstrating the far-reaching
benefits of a LEO-enhanced GPS Nav-Com system to its potential U.S.
military, civil, and commercial stakeholders. To this end what is
needed is an existence proof in the form of iGPS formed by
integrating Iridium and GPS wherein there is a practical and
attractive method to upgrade user equipment for existing users of
GPS. In other words, what is therefore needed is a practical method
for creating a tightly integrated global Nav-Com upgrade to an
existing DAGR that provides the full necessary precision and
performance without need for any hardware modifications.
SUMMARY OF THE INVENTION
[0023] It is accordingly an objective of the invention to provide a
method of upgrading GNSS user equipment with tightly integrated
Nav-Com capability.
[0024] It is a second objective of the invention to provide
practical method for upgrading existing GNSS user equipment with
tightly integrated Nav-Com capability that is retroactively
applicable to existing user equipment.
[0025] It is a third objective of the invention to provide user
hardware for GNSS user equipment that is retroactively applicable
to existing user equipment.
[0026] The invention thus provides a method of upgrading and
tightly integrating GPS user equipment with other GNSS systems to
enhance navigation performance without the need for hardware
modifications to the original GPS equipment, and user hardware for
implementing the method. The method provides a practical,
cost-effective means for bounding adverse differential phase bias
drift among critical r.f. components of tightly integrated Nav-Com
user equipment. In the preferred embodiment, the performance
upgrades enable tightly integrated global Nav-Com capability, and
are compatible with iGPS and any other GNSS system.
[0027] In accordance with the principles of a preferred embodiment
of the invention, the objectives of the invention are achieved by a
method of upgrading existing Global Navigation Satellite System
(GNSS) user equipment that includes the steps of: providing first
GNSS user hardware including a first oscillator driving a precise
coded reference signal generator arranged to generate a reference
signal and supply the reference signal to an existing second GNSS
user device, and a Navigation/Communication processor arranged to
generate position, navigation, and/or timing signals upon receipt
from the second GNSS user device of coherent correlations of
incoming GNSS signals and the reference signal; providing an
existing second GNSS user device separate from the first GNSS user
hardware, said GNSS user device including at least one existing
port and a second oscillator arranged to produce said coherent
correlations of incoming GNSS signals and the reference signal
generated by the add-on first GNSS hardware, and to send the
coherent correlations to the first GNSS user hardware; and coupling
the first GNSS user hardware to the second GNSS user device via the
existing port without modifying hardware of the second GNSS user
device.
[0028] The objectives of the invention are also achieved by upgrade
hardware corresponding to the first GNSS user hardware of the
above-described method, the upgrade hardware being arranged to
generate a stable reference signal and supply it to the second GNSS
user device and to combine the coherent correlations from the
second GNSS device with its own raw precise correlations to derive
an improved solution for position, velocity, and/or time.
[0029] Preferably, in both the method and apparatus embodiments of
the invention, the at least one existing port used to couple the
first GNSS user hardware with the existing second GNSS user device
with the includes an antenna port and a data port, the first GNSS
user hardware is arranged to send the reference signal to the
second GNSS user device through the antenna port, and the first
GNSS user hardware is arranged to send steering commands to the
second GNSS user device and receive back the coherent correlations
through the data port.
[0030] According to still further aspects of the preferred
embodiments of the invention, the first GNSS user hardware includes
Iridium receive capability and the second GNSS user device is a
Global Positioning System (GPS) receiver, with the Iridium and GPS
signals preferably being input through a common antenna. In this
embodiment, the first oscillator provides a common precision
oscillator carrier phase reference for both the GPS and Iridium
r.f. timing and ranging measurements. In addition, the first GNSS
user hardware further includes Iridium transmit capability, while
either the first or second GNSS user receiver may include Y-code,
M-code, C/A code, Galileo, GLONASS, and/or COMPASS capability. A
reference station may be used to calibrate the code and carrier
phase of satellites within view and telemeter the calibration data
to a communications satellite whose broadcast output is coupled
into the first GNSS user hardware and whose calibration data is
employed to improve the position, velocity, and time solution.
[0031] The Navigation/Communication processor used in the preferred
embodiment of the invention may include a Kalman Filter to model
and estimate the user equipment position, velocity, and time using
the carrier phase of the incoming signals. Optionally, the Kalman
Filter may utilize observable data from an inertial measurement
unit (IMU).
[0032] In addition, the first GNSS user hardware may include a
transmitter component that uses its improved position, velocity,
and time estimate to improve a time and frequency synchronization
of its transmissions, with the transmissions being coupled to a
regional communications network, satellite network, or global
network of low earth orbiting satellites.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a schematic diagram of an upgraded GNSS system
including an existing GNSS user equipment and an enhancement module
constructed in accordance with the principles of a preferred
embodiment of the invention.
[0034] FIG. 2 is a block diagram of the system of FIG. 1.
[0035] FIG. 3 is a schematic diagram illustrating the timing,
ranging, and data signals used by the system of FIG. 1.
[0036] FIG. 4 is a flowchart of a reference input initialization
and control sub-routine according to a preferred method.
[0037] FIG. 5 is a schematic diagram illustrating a phase bias
drift sensitivity benchmark.
[0038] FIG. 6 is a schematic diagram illustrating the phase bias
drift sensitivity of the preferred method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] FIGS. 1 and 2 are top-level functional drawings showing an
upgraded GNSS system including existing GNSS user equipment,
illustrated as a Defense Advanced GPS Receiver (DAGR) 1, and
enhanced user hardware constructed in accordance with the
principles of a preferred embodiment of the invention, which may
optionally take the form of a separate add-on enhancement module 2
illustrated in FIG. 2. The enhanced user hardware is arranged to
plug into existing interfaces to the user equipment, which in the
exemplary DAGR include a two-way serial port 3 and an external
antenna input 4.
[0040] Those skilled in the art will appreciate that it is also
within the scope of the invention to adapt or modify the
illustrated user hardware and method to GNSS user equipment other
than a DAGR. In addition, those skilled in the art will appreciate
that while the invention eliminates the need to modify existing
GNSS hardware in order to implement tightly integrated Nav-Com
capability, such hardware may nevertheless be modified for other
reasons. Furthermore, even though the existing GNSS hardware is not
modified, it may be necessary to modify the software of the
existing GNSS user equipment to accommodate the enhanced PNT data
inputs (for example, as described below in connection with FIG. 6).
In the case of a DAGR, software upgrades can be made at the depot
level or even potentially in the field. In general, the software
provided in conventional GNSS user equipment may be readily changed
in cooperation with the user equipment manufacturer to accommodate
the higher precision inputs, enabling the existing equipment to
easily be retrofitted to accommodate the upgrade.
[0041] Although the specific packaging of the user equipment can
take many forms, FIG. 2 shows a preferred embodiment wherein the
Nav/Com enhancement electronics are contained in a separate add-on
module 2, which interfaces to the existing DAGR. As noted above, it
is one object of the invention to maintain the existing DAGR
hardware interfaces so as to make the upgrade as easy as possible.
Therefore, it will be practical to maintain configuration control
over the entire DAGR inventory with the mix of unmodified
DAGRs.
[0042] As illustrated in FIGS. 1 and 2, a common antenna 5 is used
for the GPS L1 and L2 receive signals and the Iridium L-band
transmit and receive signals. It will nevertheless be appreciated
that multiple antennas may be provided, particularly in case LEO
satellites other than Iridium are used or if a commercial off the
shelf (COTS) dual-band GPS active antenna is used. Each input is
bandpass filtered by respective blocks 6 and 7 for the appropriate
GPS or Iridium bands and then fed through a respective preamplifier
8,9.
[0043] Preferably, an oscillator 10 serves as the common timebase
for the transceiver. It is an object of this invention to keep the
component costs, including that for this oscillator 10, to a
minimum and therefore a low-cost crystal oscillator is illustrated
although, of course, one or more other oscillators with similar or
better performance may be substituted. The oscillator 10 drives a
synthesizer 11, which provides all the necessary A/D, D/A, and
local oscillator signals for the unit.
[0044] The Iridium input is digitized by an A/D converter 12 and
then fed through a Hilbert transformation processing function or
circuit 13, which provides a complex representation of the incoming
signal for the bank of signal processors 14. In an identical
manner, the complex digitized Iridium output of the bank of signal
processors 14 is combined and routed from the bank of signal
processors 14 through an inverse Hilbert transform processing
function or circuit 16 in preparation for D/A conversion by D/A
converter 17, and power scaling by a variable attenuator 18,
amplification by an amplifier 19, before being supplied to
modulator 20 for transmission through antenna 5.
[0045] The Iridium transmit and received functions may be switched
in accordance with a frequency and time division multiple access
scheme, such as the one described in the publication entitled
"Working Document on Service Link Characteristics of the Iridium
Satellite System," available from US WP 7D/14R2, Feb. 11, 2005. As
described therein, each 90 ms repeating frame is divided into a
20.32 ms downlink paging slot and four each 8.28 ms duplex uplink
and downlink telephony slots. The Iridium portion of the integrated
unit is capable of receiving both Iridium and GPS concurrently.
When the Iridium user equipment is transmitting, GPS tends to be
susceptible to out-of-band interference, so transmit blanking is
implemented for the GPS input. If only a single Iridium time slot
is used for the uplink, the 8.28 ms/90 ms duty cycle for GPS
downtime amounts to an acceptable .about.0.1 dB loss.
[0046] A central Navigation/Communication processor 21 directs the
internal operation of the user equipment. It contains control loop
drivers for a plurality of Iridium correlators implemented by
signal processor bank 14, a GPS P code generator 22 (depicted for
illustrative purposes only as external to the processor 21), and a
configuration control 23 for the transmit and receiver functions as
well as the interface to the DAGR and an optional
micro-electromechanical-system (MEMS) Inertial Measurement Unit
(IMU) 24. Processor 21 also provides the master estimation
capability of the user equipment in the form of a Kalman Filter
which optimally combines the available information, including that
from Iridium, GPS, and the IMU or other sensors, such as a
barometer or magnetometer.
[0047] In order to secure the full benefits of integrated Nav-Com,
the Iridium signal processing block 14 provides means for
correlation and coherent detection of the incoming received signal.
Driver tracking loops command the numerically controlled
oscillators (NCOs) 25 in the signal processor 14. Iridium code
generators 26 synthesize a replica code that matches what each
Iridium satellite is known to broadcast. In the case of Iridium,
which was not originally designed to be a navigation system, this
replica code is selected to consist of a pre-identified series of
message bits known in advance by the user to create the direct
sequence chips of a spread spectrum signal. This architecture
enables the timing (code) and carrier phase of the incoming Iridium
signal to be determined precisely. The same architecture is used
for the transmit side to ensure that the outgoing timing (code) and
carrier phase can be precisely controlled.
[0048] The synthesizer also creates a sinusoidal local oscillator
at the GPS L1 band at 1575.42 MHz and a square code driver clock at
10.23 MHz. These clocks are fed to the P code generator 22, which
generates a reference receiver clock time P code signal at zero
Doppler. The Navigation/Communication processor 21 is able to
initialize the start time of the P code generator 22 to a commanded
value. Based on an estimate of the incoming interference power
supplemented by the Navigation/Communication processor's estimate
of the GPS signal power, the processor 21 actively controls a
variable attenuator 30 to ensure that the P code reference signal
is strong enough to be detected under interference but not so
strong so as to be an unwanted source of interference. An in-line
Automatic Gain Control (AGC) 31, monitored by the
Navigation/Communication processor 21, routes the incoming
composite GPS and reference signal into the DAGR 1. This allows the
processor 21 to track overall power emerging from the antenna
terminals as well as regulate the power to a constant value coupled
to the DAGR or existing GPS receiver hardware, which may or may not
have an AGC and/or SFAP or STAP type signal processing which may
also be controlled and integrated via the existing ports. The
active control of the reference signal power is described in more
detail below.
[0049] No encryption capability or authorization is required to
generate the GPS P code. Therefore, it will be readily appreciated
that the expansion hardware or module for the DAGR does not need to
be a controlled item under Selective Availability Anti-Spoofing
Module (SAASM) and, accordingly, can be made very practical and
easy to interface with an existing DAGR, which natively supports P
code tracking. If the target GPS receiver is a commercial C/A code
receiver, C/A code can be used as well. However, P-code is
preferred when possible because of its superior minimal cross
correlation properties.
[0050] The DAGR 1 illustrated in FIG. 1 contains 12 GPS tracking
channels, each of which can be operated within the military SAASM
encryption architecture. The SAASM architecture permits operation
of any given satellite to employ C/A code (the 1.023 MHz
coarse/acquisition signal), P code (the 10.23 MHz precision
signal), or Y code (the 10.23 MHz encrypted signal). The
Navigation/Communication processor 21 communicates with the DAGR 1
via the serial port 3. The serial port 3 enables the processor 21
to individually command the code and carrier Numerically Controlled
Oscillators (NCOs) for each SAASM channel and return the raw
in-phase and quadrature (I and Q) GPS correlations.
[0051] In the preferred embodiment, the existing 1 Pulse Per Second
(1 PPS) interface of existing GPS receivers remains available for
use without changing the cable in any existing integration in which
the GPS receiver is used. If GPS is not available, the Iridium or
other GNSS receive capability in the add-on module can continue to
accurately drive the 1 PPS via the existing interfaces.
[0052] FIG. 3 shows satellite and reference station configurations
in relation to the user equipment for the global system illustrated
in FIGS. 1 and 2. There can be one or more each of GPS satellites
40 and Iridium satellites 41 in view of the user equipment 1,2,5.
Reference stations 42 established worldwide serve to calibrate the
clock and ephemeris of each satellite in view. Each reference
station 42 incorporates an antenna 43 and signal processing (not
shown) that is functionally equivalent to signal processing of the
user equipment except that it is operated in a controlled
environment and may optionally be packaged in a rack mount.
[0053] In the illustrated embodiment, the GPS and Iridium
satellites 40,41 each share a common view of both the reference
station 42 and the user equipment 1,2,5 anywhere on the globe where
capability is needed. Each satellite broadcasts a PRN ranging code
modulated onto the carrier. Since Iridium was never designed to be
a navigation satellite and broadcast a PRN ranging code, an
equivalent facsimile is synthesized using one or more pre-defined
data bit streams. The reference stations 42 serve to calibrate the
satellite clocks in real time. Each reference station measures the
code and carrier phase of all satellites in view and telemeters
this information to the users via the Iridium data link. The user
equipment 1,2,5 tracks the code and carrier of each satellite 40,41
in view then makes its own measurements of code and carrier phase
and reads in the reference station data. Additionally, Iridium
provides a reverse data path, enabling the user equipment 1,2,5 to
uplink data to the satellite for relay, in this case, back to the
reference station 42.
[0054] Preferably, the downlink and uplink timing and ranging
functions of the preferred system are executed with
centimeter-level precision, i.e., consistent with variations that
are small relative to the carrier phase of the timing and ranging
signals. Such precision enables rapid angle motion of low earth
orbit (LEO) satellites to significantly improve accuracy and
integrity above stand-alone GPS. This precision is also key to
enable sustained coherent integration of a GPS signal subject to
interference. To accomplish this, a suitable flywheel, such as the
illustrated Inertial Measurement Unit (IMU) 24 of FIG. 1, is used
for feedforward to carry out GPS tracking in both time and space.
For example, if an IMU can provide a centimeter-level position
flywheel, and Iridium can provide a high-power, 20 ps level
(centimeter-level in terms of speed-of-light conversion) timing
source, then the GPS signal can be coherently integrated for a
sustained period of time. The overall integrated Nav-Com system
resembles a lock-in amplifier often employed in sensitive physics
experiments. Iridium, the real-time reference station calibration,
and a stable treatment of the various local oscillators used
between GPS and Iridium within the user equipment are important
design attributes. In the presence of interference, an ordinary GPS
signal is too faint to be registered.
[0055] By accurately modeling and estimating the satellite and user
position state including clock parameters, iGPS establishes a
replica carrier phase for the incoming GPS signal that to the
centimeter level almost exactly matches that of the incoming faint
signal--even though the user may be undergoing significant
dynamics. Over time the user equipment is able to integrate an
error signal, generating I and Q components that can be used as
observables to close the receiver tracking loops--even under
interference conditions. The intrinsic precision also enables
important uplink capability, and enables the provision of user
equipment capable of generating sustained phase stability in the
uplink channel--even under dynamics.
[0056] With a carrier stable to the centimeter-level of phase over
intervals of many seconds, it is possible to improve the uplink
interference rejection as well as engage in LPI communication. Such
uplink improvement is accomplished by enabling the user to
precisely spread the data modulation over frequency and time.
[0057] The Navigation/Communication processor controls the user
equipment processes and data flow and includes a Kalman Filter
comprised of a precise model based on the user state and a means to
optimally manage the user equipment process and measurement noise.
Table 1 provides a listing of the key Navigation/Communication
processor attributes.
TABLE-US-00001 TABLE 1 Navigation/Communication Processor
Architecture Definitions Processor Attribute Parameter List Aiding
Information GPS Data Bits GPS Clock Iridium Clock Iridium Ephemeris
Observables IMU 3 axis Rates and Accelerations GPS Early, Punctual,
Late Correlations Iridium Early, Punctual, Late Correlations Kalman
Filter States UE Clock Bias, Rate DAGR Clock Bias, Rate 3 axis
Position 3 axis Velocity 3 axis Attitude 3 axis IMU Accelerometer
Bias 3 axis IMU Rate Gyro Bias Power for each GPS Signal GPS
Carrier Phase Biases Iridium Carrier Phase Bias, Rate, and
Acceleration Control Outputs GPS Correlator NCO Commands Iridium
Correlator NCO Commands Iridium Transmit NCO Commands
[0058] The Aiding Information listed in Table 1 is that provided
via data link from the Iridium satellite. The observables are the
raw measurements collected from GPS, Iridium, and the IMU. In
addition, optional sensors that also include a low-cost
magnetometer and barometric altimeter may be provided. The Kalman
Filter States listed in Table 1 are those nominally estimated in
the user equipment in a practical scenario. Finally, the NCO
control outputs are listed which drive the signal processing
hardware. These commands are calculated as functions of the Kalman
Filter state parameters. Proper power control of the reference
input signal to the DAGR is crucial. If the power is too high, the
reference will jam the DAGR. If it is too low, the signal will be
undetectable and the system will lose its utility. The strategy
adopted to control the signal is chosen to track the incoming
interference level but be subject to an additional backoff. This
ensures that the overall performance is driven by the incoming
interference. As an additional safeguard during operations, the
incoming GPS signal power is monitored to ensure that it is
independent of the reference signal power.
[0059] FIG. 4 is a flowchart of an initialization and operational
control method that embodies principles of a preferred embodiment
of the invention. The first step 100 upon device power up is for
the add-on user hardware to turn off the reference signal, followed
by the step 110 of measuring the incoming interference noise and
the step 120 of recording the measured interference noise in order
to obtain a benchmark of incoming noise power against which to
compare with the reference signal. The reference signal power is
incremented in dB steps and interference is measured (step 130)
until the AGC function detects that the reference power dominates
the incoming interference power (steps 140 and 150). Based on this
calibration, the add-on GNSS user equipment carries out the step
160 of calculating and recording a scale factor between the
commanded reference signal power and the detected input power to
the AGC. Once the calibration is complete, the reference signal is
expected to be readily accessible to the DAGR signal processor with
high carrier-to-noise ratio.
[0060] The next step 170 is to initialize a reference signal lock.
In order to do so, an approximate time can be obtained by sending
round trip messages across the serial port of the existing GNSS
user equipment. An approximate time and frequency search window is
derived based on the serial port messages and apriori information
about the likely oscillator frequency offsets. Then, one of the
DAGR correlator channels is allocated to the reference signal, and
the Navigation/Communication processor 21 commands the channel to
sweep through the defined frequency and time search window to
acquire and track the reference signal.
[0061] Upon reference signal track, steady state is achieved and
initialization step 170 is complete. In step 180, the interference
is then measured at the AGC, so that in step 190, nominal operating
mode establishes a backoff for the reference signal of
approximately 30 dB from the incoming interference level.
[0062] As a further sanity check, the Navigation/Communication
processor 21 can introduce a dither power level onto the reference
signal with a time constant on par with the GPS integration time
constant and potentially greater than 10 seconds. The Kalman filter
in the Navigation/Communication processor 21 of the embodiment
illustrated in FIG. 1, which estimates the power of the incoming
GPS signals, nominally tracks the power of each. If the tracked
signal power becomes appreciably correlated with the reference
power, the reference power may be too high and should be adjusted
downward.
[0063] It is an advantage of the preferred embodiment of the
invention that it provides a common precision oscillator reference
for both the GPS and Iridium r.f. timing and ranging measurements,
especially for the carrier phase components. A stable carrier phase
reference common to both the GPS and Iridium platforms unlocks the
previously untapped efficiency of coherent processing techniques
for a GPS incoming signal power level in the military and other
applications described above. In particular, the stability of the
GPS and Iridium platforms must be good to within a small fraction
of the 19 cm L band wavelength of each other. The requirement
translates into about 0.5 cm in terms of distance or 20 ps in terms
of time. This is a demanding level of precision, especially when
biases in electronics that change with the environment, such as
temperature or mechanical stress, induce fluctuations. Although the
Kalman Filter can be designed to accommodate a limited degree of
bias fluctuation, the practical benefits of integrated Nav-Com are
quickly lost in practical operational scenarios if such
fluctuations are excessive. The result is user equipment that
cannot operate.
[0064] Without loss of generality FIG. 5 shows a traditional
approach to integrating existing GPS user equipment with new
Iridium transceiver capability so as to maintain tight
inter-channel phase coherence. In addition to the data port that is
required for such integration, a common oscillator is provided by
physically altering the receiver 78 and introducing a common
hardware oscillator 79 that is shared between GPS and Iridium. As
illustrated, the common oscillator 79 is connected to the GPS code
synthesizer 80 and in addition is connected through an added port
81 to an Iridium code synthesizer 82. The GPS code synthesizer 80
is connected to respective signal inputs including common antenna
83, GPS filter 84, pre-amplifier 85, and digitizer 86, while the
Iridium code synthesizer is connected to common antenna 83, Iridium
filter 87, pre-amplifier 88, and digitizer 89.
[0065] The following analysis traces the bias sensitivity of this
conventional approach. In particular, the stability of node B
versus that of node D will be evaluated against a requirement to
stay stable to the level of <<20 ps for intervals of 1 minute
or longer. Many details are neglected for the purposes of this
analysis, including the fact that many GPS receivers have r.f.
front ends downconverting to an intermediate frequency. However,
without loss of generality, a direct downconversion to baseband is
assumed for this analysis. Further analysis shows that similar
conclusions will be reached supporting the improved stability of
the preferred embodiment shown in FIG. 6.
[0066] The analysis assumes a BPSK signal which is the format of
the GPS waveform. Since Iridium employs QPSK, the BPSK analysis
remains applicable by the principle of superposition. The incoming
signal at node 1, s.sub.1(t), is given by
s.sub.1(t)=d(t)c(t)cos [.omega..sub.0t+.phi.(t)]
where d(t) is the data modulation, c(t) is the PRN chipping
sequence, .omega..sub.0 is the nominal center angular frequency of
the band, and .phi.(t) is the incoming carrier phase of the
received signal. The signal propagates through signal paths A-B and
A-D, which adds biases b.sub.21 and b.sub.41 respectively.
s.sub.2(t)=d(t)c(t)cos [.omega..sub.0t+.phi.(t)+b.sub.21]
s.sub.4(t)=d(t)c(t)cos [.omega..sub.0t+.phi.(t)+b.sub.41]
The oscillator has a nominal phase output assumed to be receiver
clock time, t.sub.3(t), as measured at node C.
t.sub.3(t)=t+.tau.(t)
where .tau. is the instantaneous receiver clock bias as measured at
node C. The common oscillator drives a synthesizer with two taps,
one for the GPS processing path and one for the Iridium processing
path. Each synthesizer and associated interconnect electronics
routed to nodes B and D induce path biases b.sub.32 and b.sub.43.
The resulting receiver clock time as measured at each node is given
by
t.sub.2(t)=t+.tau.(t)+b.sub.23 and
t.sub.4(t)=t+.tau.(t)+b.sub.43
Each signal is then sampled at nodes B and D by a separate A/D
converter. Each A/D converter samples the signal when its input of
receiver clock time reaches a multiple of the sampling period T as
follows for the case of node B:
t.sub.2(t)=t+.tau.(t)+b.sub.23=kT
[0067] Solving for the time t.sub.A/D[k,2] at which sample k is
made at node B,
t.sub.A/D[k,2]=kT-.tau.(t.sub.A/D[k,2])-b.sub.23
Inserting the sampling time for node B into the received signal
expression for node B,
s.sub.2(t.sub.A/D[k,2])=d.sub.k,2c.sub.k,2 cos
[.omega..sub.0kT-.omega..sub.0.tau.(t.sub.A/D[k,2])-.omega..sub.0b.sub.23-
+.phi.(t.sub.A/D[k,2])+b.sub.21]
The equivalent expression for node D is
s.sub.4(t.sub.A/D]k,4])=d.sub.k,4c.sub.k,4 cos .left
brkt-bot..omega..sub.0kT-.omega..sub.0.tau.(t.sub.A/D[k,4])-.omega..sub.0-
b.sub.43+.phi.(t.sub.A/D[k,4])+b.sub.41.right brkt-bot.
The code and data for each channel are wiped off downstream. Any
phase contributions of the modulation and its processing are
neglected herein. The carrier phase difference for any sample k
between node C and node D is then taken directly as the difference
between the cosine arguments for each signal as follows:
.DELTA..phi. 24 [ k ] = .angle. s 2 ( t A / D [ k , 2 ] ) - .angle.
s 4 ( t A / D [ k , 4 ] ) = .omega. 0 kT - .omega. 0 .tau. ( t A /
D [ k , 2 ] ) - .omega. 0 b 23 + .phi. ( t A / D [ k , 2 ] ) + b 21
- .omega. 0 kT + .omega. 0 .tau. ( t A / D [ k , 4 ] ) + .omega. 0
b 43 + .phi. ( t A / D [ k , 4 ] ) - b 41 ##EQU00001##
[0068] For an LEO satellite moving at <.about.8 km/sec, the time
tagging error at baseband is assumed to be sufficiently small such
that the incoming signal phase, .phi., and receiver clock bias,
.tau., is independent of choice of sample time reference. The term
.omega..sub.k becomes as a scale factor for unit conversion between
time and phase angle. Simplifying to retain the substantive terms
of the instantaneous r.f. bias behavior,
.DELTA..phi..sub.24.apprxeq.(b.sub.21-b.sub.41)-.omega..sub.0(b.sub.23-b-
.sub.43)
[0069] The implications of this result for a traditional benchmark
configuration in FIG. 5 do not readily support tightly integrated
Nav-Com. Two or more separate mechanical housings are required for
the overall integrated device if hardware changes are to be
minimized. For many practical integrations, it is either necessary
or desirable to spatially separate the housings and interconnect
each via cable. Each of the bias terms above will be subject to
separate temperature sensitivity. Each housing may operate at a
different temperature. Any given pair of bias terms has the
potential to be balanced for differential cancellation, but it can
be readily appreciated that such balancing is difficult, especially
if the existing housing and electronics of the DAGR is to remain
unaltered.
[0070] For example, the term b.sub.23 and its thermal sensitivity
profile are internal to the DAGR and is not under the control of
the user equipment integrator. A counterpart term b.sub.21 may or
may not span different hardware platforms, but nevertheless is
subject to a different thermal and mechanical environment that
makes it difficult for an integrator to differentially cancel
against term b.sub.23 in a robust and practical way. Similarly, the
term b.sub.41 may or may not share the same mechanical housing
external to the DAGR (for example if an external antenna is used)
and will not readily cancel against the term b.sub.12, which must
always span housings into the DAGR. Last, term b.sub.41 may or may
not share a mechanical housing outside of the DAGR but must
nevertheless balance term b.sub.43 which must always span different
housings and is therefore subject to thermal, mechanical, and other
environmental stress.
[0071] Taken in the aggregate, environmental stresses, including
thermal and mechanical, can introduce unacceptable excursions when
the requirement is sustained stability to the 20 ps level. It can
be readily appreciated that such environmental stresses cannot be
readily controlled or mitigated without physical alterations to the
existing DAGR hardware. Unfortunately, such physical alteration
defeats the core objective of offering a practical upgrade path for
tightly integrated Nav-Com.
[0072] FIG. 6, on the other hand, depicts the subject invention in
a simplified form that can be readily analyzed for environmental
phase stability in the same manner as FIG. 5. As before the purpose
of the next analysis is to assess the intrinsic stability between
node e of the DAGR and node d of the add-on module. The add-on
module receiver clock time is generated at node c of the add-on
module is given by
t.sub.3(t)=t+.tau.(t)
The receiver clock time at nodes d and e are also given as
t.sub.4(t)=t+.tau.(t)+b.sub.43
t.sub.5(t)=t+.tau.(t)+b.sub.23+b.sub.52
A new interface reference signal, r(t), is modulated with GPS P
code, p(t). The P code is generated by the add-on module and
conveyed to node e as follows:
r 5 ( t ) = p ( t 5 ( t ) ) cos [ .omega. 0 t 5 ( t ) ] = p ( t +
.tau. ( t ) + b 23 + b 52 ) cos [ .omega. 0 ( t + .tau. ( t ) + b
23 + b 52 ) ] ##EQU00002##
[0073] Even prior to tracking any incoming signals, the Kalman
filter begins propagating estimates of the iGPS add-on module clock
bias, .tau.(t), and the DAGR clock bias, .tau..sub.DAGR(t). The
reference signal pseudorange measurement has high signal to noise
ratio and is assumed to heavily weight the carrier phase precision
of the reference signal to effectively phase lock the oscillators
from the add-on module and the GPS receiver. The DAGR replica of
the same signal is given by
r ^ DAGR 5 ( t ) = p ( t DAGR 5 ( t ) ) cos [ .omega. 0 t DAGR 5 (
t ) ] = p ( t + .tau. DAGR ( t ) + .rho. ( t ) ) cos [ .omega. 0 (
t + .tau. DAGR ( t ) + .rho. ( t ) ) ] ##EQU00003##
The pseudorange observable, .rho.(t), as measured by the DAGR
correlator steered by the Navigation/Communication processor in the
add-on module in phase and delay lock is then
.rho.(t)=.tau.(t)-.tau..sub.DAGR(t)+b.sub.23+b.sub.52
Although upon power up only the relative module clock biases are
observed, the actual clock bias can be resolved once the receiver
is tracking satellites. Once the iGPS add-on receiver, the existing
GPS receiver, or both start tracking, enough information becomes
available for the Kalman filter to begin estimating position, time,
and, to the extent applicable, the other states listed in Table 1
above.
[0074] Employing an identical analytical approach for the new
method, the signal as measured at node e is given by
s.sub.5(t)=d(t)c(t)cos
[.omega..sub.0(t+b.sub.52)+.phi.(t)+b.sub.21]
The aggregate signal available at node e is then the sum of the
incoming s.sub.5(t) and r.sub.5(t). Because the inner details of
the DAGR or any other GPS or GNSS receiver are generally
proprietary or otherwise protected, a simplified model of the front
end is assumed. Again, without loss of generality, a direct down
conversion is assumed for the purpose of this analysis, which is
assumed to be readily extensible to other specific cases. The DAGR
is assumed to have a distinct sample interval, T.sub.DAGR, versus
the Nav-Com add-on module with sample rate, T. As above the A/D
sample times are computed in terms of receiver clock time and then
solved in terms of the actual time each measurement, m, is
made.
t.sub.5(t)=t+.tau..sub.DAGR(t)=mT.sub.DAGR
t.sub.A/D[m,5]=mT.sub.DAGR-.tau..sub.DAGR(t.sub.A/D[m,5])
The resulting A/D converter output at node e is
s.sub.5(t.sub.A/D[m,5])=d.sub.m,5c.sub.m,5 cos
[.omega..sub.0mT.sub.DAGR-.omega..sub.0.tau..sub.DAGR(t.sub.A/D[m,5])+.om-
ega..sub.0b.sub.52+.phi.(t.sub.A/D[m,5])+b.sub.21]
The signal and A/D clock at node d are treated in the same way as
before
s.sub.4(t.sub.A/D[k,4])=d.sub.k,4c.sub.k,4 cos .left
brkt-bot..omega..sub.0kT-.omega..sub.0.tau.(t.sub.A/D[k,4])-.omega..sub.0-
b.sub.43+.phi.(t.sub.A/D[k,4])+b.sub.41.right brkt-bot.
[0075] Before the phase of the two signals can be compared, it is
vital that the signal processing take into account the time tag of
the r.f. sampling events, especially for the r.f. phase terms,
.omega..sub.0mT.sub.DAGR and .omega..sub.0kT. In general, both r.f.
and baseband terms must have a sampling stability <<20 ps for
a sustained period of time, preferably for times on the order of 1
minute or more. For convenience of analysis, A/D samples between
the DAGR and Nav-Com add-on module are paired such that they occur
at approximately the same time. In general, because the two clocks
are running asynchronously and because two separate sampling rates
are assumed, there is no expectation that any given samples across
platforms will occur at exactly the same time.
[0076] The following development provides an explicit calculation
of the elapsed time between any given cross-platform samples.
First, the receiver clock time for each sample is recalled as
t.sub.5(t.sub.A/D[m,5])=t.sub.A/D[m,5]+.tau.(t.sub.A/D[m,5])+b.sub.23+b.-
sub.52
t.sub.4(t.sub.A/D[k,4])=t.sub.A/D[k,4]+.tau.(t.sub.A/D[k,4])+b.sub.43
Next, a matched pair of A/D samples, k.sub.m and m.sub.k, is
selected such that the sample times are near each other, i.e.,
within a sample or so. If a .about.50 MHz sample clock is assumed,
then a given pair of samples will generally occur within 0.02 .mu.s
of each other. For any given pair of samples, the difference in
time between sample epochs in receiver clock time is given by
.delta.t.sub.LO as follows.
.delta.t.sub.LO=t.sub.5(t.sub.A/D[m,5])-t.sub.4(t.sub.A/D[k.sub.m,4])
Substituting the above relationships for receiver clock time, the
time difference is shown to apply to elapsed time after accounting
for the component biases as follows
.delta.t.sub.LO.apprxeq.t.sub.A/D[m.sub.k,5]-t.sub.A/D[k.sub.m,5]+b.sub.-
23+b.sub.52-b.sub.43
It is assumed that the clock bias does not change appreciably
within the time span of .delta..sub.LO. The next step is to
substitute the following conversions between sample times and
receiver clock times
t.sub.A/D[k.sub.m,4]=k.sub.mT-.tau.(t.sub.A/D[k.sub.m,4])-b.sub.43
t.sub.A/D[m.sub.k,5]=m.sub.kT.sub.DAGR-.tau..sub.DAGR(t.sub.A/D[m.sub.k,-
5])
such that
.delta.t.sub.LO.apprxeq.m.sub.kT.sub.DAGR-.tau..sub.DAGR(t.sub.A/D[m.sub-
.k,5])-k.sub.mT+.tau.(t.sub.A/D[m.sub.k,5])+b.sub.23b.sub.52.
Substituting the above expression for inter-platform pseudorange
evaluated at each DAGR sample epoch,
.rho.(t.sub.A/D[m,5])=.tau.(t.sub.A/D[m,5])-.tau..sub.DAGR(t.sub.A/D[m,5-
])+b.sub.23+b.sub.52.
An explicit expression for .delta.t.sub.LO is then given by
.delta.t.sub.LO.apprxeq.m.sub.kT.sub.DAGR-k.sub.mT+.rho.(t.sub.A/D[m.sub-
.k,5])
The parameters on the right are all available to the receiver in
real time. The resulting phase rotation
.omega..sub.0.delta.t.sub.LO can then be applied to DAGR raw
correlations to the extent necessary to project them onto a common
time base with those from the Nav-Com add-on module.
[0077] It is well known that the set of GPS P codes and Y codes are
effectively orthogonal. For the purpose of this sensitivity
analysis, it assumed that (i) the phase of signal s.sub.5 can be
independently tracked from the phase of signal r.sub.5 and (ii)
that any phase bias contribution from code and data wipeoff can be
neglected. Calculating the incoming signal phase relative to the
reference phase at node e, the following expression is
obtained:
.DELTA..phi. 54 [ m k , k m ] = .angle. s 5 ( t A / D [ m k , 5 ] )
- .angle. s 4 ( t A / D [ k m , 4 ] ) - .omega. 0 .delta. t LO
.apprxeq. .omega. 0 m k T DAGR - .omega. 0 .tau. DAGR ( t A / D [ m
k , 5 ] ) + .omega. 0 b 52 + .phi. ( t A / D [ m k , 5 ] ) + b 21 -
.omega. 0 k m T + .omega. 0 .tau. ( t A / D [ k m , 4 ] ) + .omega.
0 b 43 - .phi. ( t A / D [ k m , 4 ] ) - b 41 - .omega. 0 m k T
DAGR + .omega. 0 k m T - .omega. 0 .rho. ( t A / D [ m , 5 ] )
##EQU00004##
The above expression for the known pseudorange of the reference
signal as measured via the DAGR correlators is substituted to
eliminate reference to the DAGR clock bias. Also substituted is the
above expression for .delta.t.sub.LO. Furthermore, as in the
analysis for FIG. 5, it is assumed that the baseband values of
relative clock bias, .tau., and satellite carrier phase, .phi.,
stay effectively constant across the chosen sampling epochs. The
tolerance for baseband time tags is relatively loose compared with
the overall 20 ps requirement for phase stability. Assuming that a
LEO satellite velocity relative to user does not exceed 8 km/sec
and that the frequency error of the user equipment oscillators is
also bounded by this velocity (equivalent to much less than 40 kHz
at L band), then a time tag accuracy of 0.1 .mu.s is sufficient to
bound phase errors to a small fraction of a wavelength.
Simplifying, the final result is given by
.DELTA..phi..sub.54.apprxeq.(b.sub.21-b.sub.41)-.omega..sub.0(b.sub.23-b-
.sub.43)
[0078] This is the identical equation from the previous analysis.
However, it is readily appreciated that by applying the method
described herein and from inspection of FIG. 6, all four of the
above r.f. biases are now in the complete control of the designer
of the add-on unit. The biases no longer depend in any way on the
bias characteristics of the DAGR. By this analysis, the design is
independent of the environment associated with the integration. The
designer is free to use practical techniques such as pairing
components along the two r.f. paths or pairing components along the
two clock paths to achieve differential cancellation. Furthermore,
since it is now possible to place all the bias sensitive components
on the same circuit card or even substrate of a single chip, the
designer also has practical means to ensure that thermal,
mechanical, or any other environmental stresses are minimized or
balanced.
[0079] Taken in the aggregate, the invention enables a stock DAGR
to be upgraded at the depot level without changes to the hardware.
Since only the software is changed in the DAGR, it remains useful
for its original purposes when it is in stand-alone form.
Therefore, such a DAGR with software modifications does not pose
practical impact to logistics, operations, or maintenance.
[0080] The combination of the tightly integrated upgrade unit and a
built-in global communications link also enables certain benefits
to logistics. The software could be placed in the flash memory of
the iGPS add-on component. DAGR reprogramming is already carried
out via the serial port to which the iGPS add-on component must
connect to anyway. A reprogramming switch could be used to change
iGPS add-on component into a reprogramming mode, and the standard
DAGR reprogramming could be used. Another approach would be to send
the new DAGR software load via Iridium.
[0081] Using standard interfaces described herein, the DAGR is
upgraded to become iGPS user equipment capable of tightly
integrated global Nav-Com. Because the invention enables practical
upgrade of existing DAGRs, there is no need to procure new military
GPS receivers if the Military is to find benefit with iGPS during
the remaining lifetime of the Iridium constellation. Therefore, the
invention facilitates the demonstration of iGPS itself by making it
practical to field and test. Furthermore, iGPS as a hybrid
constellation of LEO and MEO navigation satellites with integrated
Nav-Com capability, the invention helps provide an existence proof
for more advanced future global integrated Nav-Com to be
developed.
[0082] Having thus described a preferred embodiment of the
invention in sufficient detail to enable those skilled in the art
to make and use the invention, it will nevertheless be appreciated
that numerous variations and modifications of the illustrated
embodiment may be made without departing from the spirit of the
invention. For example, the method described herein, once
understood by one skilled in the art, can be readily broadened to
include a family of embodiments as well as user equipment that
includes iGPS and regional networks as well as other LEO and GNSS
global Navigation and Communication applications. It is therefore
intended that the invention not be limited by the above description
or accompanying drawings, but that it be defined solely in
accordance with the appended claims.
* * * * *