Advancement of GPS for AR&C
Janet W. Bell
NASA / JSC
281-483-5295
May 23, 2002
Contributors

NASA-JSC
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–
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–

Aeroscience & Flight Mechanics Divison
Boeing
Titan-LinCom (Dr. Kevin Key)
GeoControls
UT @ Austin / Center for Space Research:
– Dr. Glenn Lightsey

Texas A&M Commercial Space Center for Engineering:
– Dr. John Crassidis

University of Houston Applied Electromagnetics Laboratory :
–
–
–
–

Dr. Jeffery Williams
Dr. L. S. Shieh
Dr. G. Ron Chen
Steve Provence
CSDL
2
JSC GPS Navigation Experience
Includes:
 MAGR Flight Test Program
 GANE (GPS Attitude Navigation Experiment)
 STS-80 SPAS relative navigation RME
 First GPS/INS space-flights (for RLV Program)
– Litton LN-100G on STS-81
– Honeywell H764-G on STS-84
 SIGI Series of Flight Tests, starting with STS-86
 SOAR (SIGI Operational Attitude Readiness) STS-106
 X-38 SIGI flight tests (STS-100, -108, ’01)
 Operational ISS SIGI, STS-110, 04/02
3
Some Lessons

Complex / costly DDT&E & SE&I with use of proprietary
commercial GPS receivers targeted to military vs. space
– Must have Open Systems Architecture
– Perform precision (few-m/cm) navigation and attitude
determination investigations in ground / space
– Algorithms must be designed for space vs. retrofit
– Support integration with other sensors (INS, optics,etc.)
– Mitigate signal blockage, reflection multi-path, etc.
– GPS & INS are complementary technologies
– Low power / size / weight mandatory
– Miniaturization / MEMs a primary goal
4
Key GPS Technology Areas for AR&C
Open Architecture GPS Receiver
(X-GPSR: Experimental GPS Receiver)
 GPS Augmentation (INS, VisNAV etc.)
 Reduced Surface Wave Antenna
 Multipath Mitigation

5
X-GPSR Status To-Date

1997, developed open architecture Plessey chipset
GPS receiver for Houston Ship Channel Authority
heading determination (SCR)
 Modified SCR firmware to conduct GPS pseudolite
precision relative navigation investigations
 Cross-strapped 2 SCR’s to evaluate attitude capability
 Conducted periodic trades of GPS chipsets, chipsetbased receivers available worldwide
 Chipset-based GPS receiver benchmark underway,
5/02 – 9/02 (SCR, Zarlink Orion, GSFC PiVoT, JPL BlackJack,
SSTL SGR, Trimble Force 19, Novatel Millenium, etc.)

Initiated X-GPSR development
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X-GPSR Components



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Open architecture
L1 frequency
PVT / Attitude capable
Integrate with INS and other sensors
Kalman filter designed for space vs. retrofit
– Orbital dynamics model, including fast gravity
model (vs aircraft dynamics)
– Maneuver detection & measurement
Multipath Mitigation Methods
GPS antenna technology (RSW)
7
GPS / INS Integration
• Pursue techniques with high probability to
maximize performance:
• Tracking loop & filtering algorithms for rapid acquisition
& measurement of GPS signals (UT @ Austin)
• CSDL “Deep Integration”
When GPS signals are blocked, INS data actively controls GPS
correlators to account for frequency uncertainty and changing
pseudoranges. When GPS returns, the GPS correlators are
already positioned to detect lock. Reacquisition is rapid and
INS realigns.
• Select INS to optimize cost & requirements
• Several CSDL candidates, including MEMS
8
X-GPSR Multipath Mitigation
Generic GPS Receiver Components
A.
RF Down Conversion: 1.57542 GHz to
Intermediate Frequency ~1 MHz
B.
IF Tracking Loops: Maintain lock on
incoming GPS Signals
C.
Navigation Algorithms: Generate
Receiver’s PVT solution
Multipath Mitigation Concepts
D.
E.
F.
New Hardware: Feedback error estimates
of multipath to hardware that compensates
incoming signal for multipath
Tracking Loop Modifications: Use
multiple correlators for multipath
estimation or new state space approach to
tracking loops
Navigation Estimation Strategies:
Estimate multipath error as part of the
Kalman filter approach to navigation
Antenna
Pre-Amp
A. RF DownConversion
D. New Hardware
For Mitigating
Multipath
Local
Oscillator
B. IF
Tracking Loops
C. Navigation
Algorithms
E. Tracking Loop
Estimate of Multipath
F. Navigation Estimate
Of Multipath
Position
Velocity
Acceleration
Time
Typical Design
New Design Concepts
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Adaptive Self-tuning GPS Filter
(UH/Shieh/Chen)

– Minimize the effects of
noise, particularly
multipath, on the
pseudorange
measurements
– Provide accurate and rapid
pseudorange solutions in
poor environments, using:
Chaotic System Model
Adaptive System Block
Nonwhite bounded noise
Focus
• Adaptive control
• Uncertain noise
estimation
• Nonlinear system model
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Adaptive Self-tuning GPS filter

Objectives
– Pseudorange
measurement results
resistant to nonwhite
noise
– Fast and accurate
pseudorange solution
with a small number of
GPS satellites,
pseudolites or
combination
– Minimal computational
processor load

Approach
– Adaptive controller &
nonlinear model
– Multipath mitigation
with uncertain noise
analysis
implementation
– Real-time parameter
identification of
nonlinear system
model
– Digital Redesign
techniques to reduce
model complexity
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Reduced Surface Wave (RSW)Antennas
Due to surface and lateral waves,
conventional patch designs are sensitive
to their support structure and low angle
multipath signals.
Shorted Annular Ring (SAR) RSW antenna
– Outer radius designed to eliminate surface and
lateral waves
– Inner radius designed to resonant at the design
frequency.
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RSW vs. Choke-Ring
0
0
30
-30
-10
60
30
RHCP
LHCP
-30
-60
-20
60
-40
-40
-30
-30
-20
240
210
180
-40
-40
-10
90
-90
120
150
-60
-20
-30
90
RHCP
LHCP
-10
-30
-20
-10
-90
Choke-Ring Antenna
- Broad pattern above
horizon
- Relatively insensitive to low
angle multipath signals
- Poor CP performance
(large LH polarization)
240
120
150
210
RSW Antenna
Choke-ring Antenna
RH & LH CP Patterns
RH-CP RSW L1 Antenna on a 14
in diameter circular ground plane.
180
RH-CP Micropulse Choke-Ring
L1 Antenna.
RSW Antenna
- Broad pattern above horizon
- Extremely insensitive to low angle multipath signals
- Excellent CP performance (small LH polarization)
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Areas of Continuing Work

Multipath rejection
– Improved feed and fabrication techniques to enhance
pattern performance.

Stable phase center
– Study the general phase center characteristics of
microstrip patch antennas.
– Measurement of phase center for RSW antennas.
– Improved feed techniques.

Dual band (L1 & L2) operation
– Development of dual band RHCP RSW designs.
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Navigation Systems & Technology Lab Resources
NSTL
16A/1004
Motion
Platform
GPS Receivers / Nav
Sensors / RSW Antennas
GPS Pseudolites
Roof-top
2-Axis
Positioner
ESD Certified Laboratory
VME’s, SUN’s,
Power Hawk
Real Time
Simulation Platform
GPS Signal Generator
3-Axis Rate Table
Rapid Development Lab 16A/1169
Rapid Development
15 Lab
16A/2115
JSC Navigation Systems & Technology Lab
- To develop, test & evaluate advanced space navigation
systems and technologies
- Evaluate GPS stand-alone and by fusing with multiple
sensor technologies ( RF, INS, optics, Magnetometers, etc.)
- Current Technology Investigations:
• Pseudolite-Enhanced Relative Position & Attitude Det.
Investigates use of a localized GPS-like satellite constellation for GPS
applications where signal blockage is an issue
•
Experimental GPS Receiver (X-GPSR)
• Reduced Surface Wave (RSW) Antenna for GPS
• Mini-Aercam (ISS co-orbiting vehicle)
FIRE precision relative navigation filter
• VisNAV optical sensor for precision relative navigation
Supplementary Data
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Experimental GPS Receiver (X-GPSR)
For Advancement of SLI Navigation Systems
Products/Benefits
• Products
A non-proprietary, configurable, modifiable GPS receiver capable of
performing precision navigation and attitude determination investigations
in ground and space applications (X-GPSR)
• Benefits
- Overcomes proprietary issues prevalent throughout industry
- Growth path to integrate with different bus architectures
(PCI, VME, 1553, etc.)
- Benchmark for nav systems & GPS/INS filters in MSFC RITAT Testbed
- Growth path to SLI flight navigation system & MEMS scale
• Customers
MSFC RITAT Testbed, SLI Contractors , Nav Sys Designers, NASA Centers,
U.S. Labs, Universities, multiple ground/space applications
• X-cutting/Unique to Project
Overcomes proprietary limitations; GPS & pseudolite modes
FY
Implementation/Metrics
• Current State of the Art
03
04
05
06
.
2
Prototype Evaluation
3
Breadboard Evaluation
TRL
Complex and costly development and integration due
to proprietary receivers; vendor receivers targeted to
military vs. space.
• Performance Metrics
Meets SLI navigation requirements; supports GPS
and pseudolite modes; cost reduction in
development turn-around time by providing for open
evaluation of multiple nav systems; cost reduction by
providing path to SLI flight system.
• Risks
Continuation of funding and availability of key
personnel.
• Participants
JSC, U of Texas Austin, Texas A&M, U of Houston
02
4
Flight Version, Testing &
Ground Demo
5
6
Ground FEU, Testing & Demo
7
Space Demo
Total
Possible Multipath Mitigation Schemes
Antenna
Pre-Amp
New Hardware
For Mitigating
Multipath
RF
Down-Conversion
Navigation Estimate
Of Multipath
Tracking Loop
Estimate of Multipath
IF
Tracking Loops
Navigation
Algorithms
Local
Oscillator




RF Down Conversion: Take 1.57542 GHz down to Intermediate
Frequency (IF)
IF Tracking Loops: Maintain lock on incoming GPS Signals
Local Oscillator: Used to generate a reference signals
Navigation Algorithms: Generate Receiver’s PVT solution for
users
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New Hardware for Multipath Mitigation



Takes input from tracking loop estimates of multipath
and navigation estimate of multipath
Use Xt-1 estimate of multipath to compensate signal at Xt
Use loop error & covariance to determine amplitude and
direction of compensation
Navigation error
Received
Signal
Multipath
Compensator
Tracking
Loops
Tracking
Loops
Navigation System
Tracking error
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Tracking Loop Modifications

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Digital Signal Processing of Uncertain Noise Parameters
– Multipath fits in the category of uncertain noise
– Use novel state space techniques to estimate multipath
Use of several correlators to estimate multipath effects
– A 4 RF receiver with 4 x 12 channels could be designed
to track 1 sv 4 times
– Track early and late with variable chip sizes for
correlation peak estimation (and therefore, multipath
estimation)
Use FIRs to estimate/compensate multipath
– Use knowledge of navigation message to determine
error between received signal and expected signal
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Navigation Estimate of Multipath

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Estimate the multipath as part of the Navigation solution
– Use phase along with the pseudorange in filter
• Phase has small multipath
• Phase has ambiguity
• Possibly use the “Code Minus Carrier” observable to
estimate multipath
– Include channel multipath in a Kalman filter implementation of
PVAT
Derive a multipath mapping algorithm
– The algorithm should be computational efficient
– The algorithm could be applied to any large space structure
Apply a multipath mapping algorithm to space based platforms
– Use “in situ” data to refine the mapping algorithm for a
particular space based vehicle
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Some notes on Digital Redesign & the Nonlinear Model
Digital Redisgn is a technique for converting a continuous time control system into a
digital system. Industry primarily uses the Bilinear transform method, but often with poor
results. Dr. Shieh developed the adaptive, self-tuning approach in 1981 (see below).
The method is very well received in the controls community.
Overall approach:
The foundation is from a system Dr. Shieh worked on
in 1981 for the Red Stone Arsenal in Huntsville Alabama.
At that time, it was the very first parameter identification
techniques of it’s kind. He revised it over the years. In 1999,
he and Dr. Chen began seeing if the system
could track and control a chaotic system. With slight
modifications / improvements they’ve been able to achieve
their goal of chaotic system tracking.
The system has a strong history of working (1981 version still in use today in military),
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with new adaptations (chaos analysis) that improve the scheme.
CSDL
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CSDL Deep Integration Details
– GPS tracking loop is built into the Nav filter. Filter
accepts I & Q signals from correlator and then drives
GPS oscillator.
– Kalman filter replaced by non-linear estimator with
adaptive gain as a function of measured S/N ratio.
– Longer coherent integration period obtained by using
knowledge of the bits associated with data message
Draper's Deep Integration technique
– Not dependent on proprietary GPS designs. Open
architectures will work.
– Not dependent on specific INS devices. MEMS, IFOGs
– GPS need only be a "component" chipset capable of I/O
outputs and control of correlators.
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CSDL Integrated INS/GPS

Deep Integration Provides:
– Code tracking: 15 to 20 db anti-jam performance
improvement against Gaussian jammers. Hangs on to
signal longer at onset of GPS blockage
– Re-acquisition: 2 to 3 times better error tracking range vs.
tightly-coupled systems. Increasing parallelism of
correlators further improves re-acquisition
– Overall, shortens the no-GPS period, shortens INS-only
flight
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MEMS & Alternates

MEMS
– Draper is a world leader in MEMS technology
– Draper MEMS devices are used in a 9 in3, 3 watt package
fired from a 5" Naval gun
– Where MEMS devices meet performance requirements,
MEMS provides an extremely robust, low cost/volume/power
INS solution
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