advancement of gps for ar&c janet w. bell nasa / jsc 281-483-5295 may 23, 2002 janet w. bell...
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Advancement of GPS for AR&CAdvancement of GPS for AR&C
Janet W. Bell
NASA / JSC
281-483-5295May 23, 2002
Janet W. Bell
NASA / JSC
281-483-5295May 23, 2002
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ContributorsContributors NASA-JSC
– 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
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JSC GPS Navigation ExperienceJSC 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
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Some LessonsSome 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
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Key GPS Technology Areas for AR&CKey GPS Technology Areas for AR&C
Open Architecture GPS Receiver (X-GPSR: Experimental GPS Receiver)
GPS Augmentation (INS, VisNAV etc.)Reduced Surface Wave AntennaMultipath Mitigation
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X-GPSR Status To-DateX-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, chipset-
based 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 ComponentsX-GPSR Components
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)
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GPS / INS IntegrationGPS / 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
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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. New Hardware: Feedback error estimates of multipath to hardware that compensates incoming signal for multipath
E. Tracking Loop Modifications: Use multiple correlators for multipath estimation or new state space approach to tracking loops
F. Navigation Estimation Strategies: Estimate multipath error as part of the Kalman filter approach to navigation
A. RF Down- Conversion
C. Navigation Algorithms
B. IF Tracking Loops
Local Oscillator
Antenna Pre-Amp
Position Velocity
Acceleration Time
Typical Design
F. Navigation Estimate Of Multipath
E. Tracking Loop Estimate of Multipath
D. New Hardware For Mitigating Multipath
New Design Concepts
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Adaptive Self-tuning GPS FilterAdaptive Self-tuning GPS Filter (UH/Shieh/Chen)(UH/Shieh/Chen)
Focus– Minimize the effects of
noise, particularly multipath, on the pseudorange measurements
– Provide accurate and rapid pseudorange solutions in poor environments, using:
• Adaptive control
• Uncertain noise estimation
• Nonlinear system model
Chaotic System Model
Adaptive System Block
Nonwhite bounded noise
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Adaptive Self-tuning GPS filterAdaptive 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)AntennasReduced Surface Wave (RSW)Antennas
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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Space Waves
Lateral Waves
Surface Waves
Due to surface and lateral waves, conventional patch designs are sensitive to their support structure and low angle multipath signals.
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RH-CP Micropulse Choke-Ring L1 Antenna.
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RHCPLHCP
RH-CP RSW L1 Antenna on a 14 in diameter circular ground plane.
RSW vs. Choke-Ring RSW vs. Choke-Ring
RH & LH CP PatternsChoke-ring Antenna
RSW Antenna
Choke-Ring Antenna
- Broad pattern above horizon - Relatively insensitive to low angle multipath signals - Poor CP performance (large LH polarization)
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 WorkAreas 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
VME’s, SUN’s, Power Hawk
GPS Pseudolites
1553, 422, 488
Ethernet
RF
Rapid Development Lab 16A/1169
GPS Signal Generator
3-Axis Rate Table
Real Time Simulation Platform
Rapid Development Lab16A/2115
ESD Certified Laboratory
GPS Receivers / Nav Sensors / RSW Antennas
Roof-top2-AxisPositioner
MotionPlatform
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JSC Navigation Systems & Technology JSC Navigation Systems & Technology LabLab
- 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
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Supplementary DataSupplementary Data
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Implementation/Metrics
Products/Benefits• ProductsA 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
• CustomersMSFC RITAT Testbed, SLI Contractors , Nav Sys Designers, NASA Centers, U.S. Labs, Universities, multiple ground/space applications
• X-cutting/Unique to ProjectOvercomes proprietary limitations; GPS & pseudolite modes
• Current State of the ArtComplex and costly development and integration due to proprietary receivers; vendor receivers targeted to military vs. space.
• Performance MetricsMeets 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.
• RisksContinuation of funding and availability of key personnel.
• Participants JSC, U of Texas Austin, Texas A&M, U of Houston
Experimental GPS Receiver (X-GPSR) For Advancement of SLI Navigation Systems
7
5
3
4
6
2T
RL
FY 02 03 04 05 06 Total
Prototype Evaluation
Ground FEU, Testing & Demo
Breadboard Evaluation
Space Demo
Flight Version, Testing & Ground Demo
.
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Possible Multipath Mitigation SchemesPossible Multipath Mitigation Schemes
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
RF Down-Conversion
Navigation Algorithms
IF Tracking Loops
Local Oscillator
Antenna Pre-Amp
Navigation Estimate Of Multipath
Tracking Loop Estimate of Multipath
New Hardware For Mitigating
Multipath
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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
New Hardware for Multipath MitigationNew Hardware for Multipath Mitigation
MultipathCompensator
TrackingLoops
TrackingLoops
Nav
igat
ion
Syst
em
Received Signal
Navigation error
Tracking error
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Tracking Loop ModificationsTracking Loop Modifications
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 MultipathNavigation Estimate of Multipath
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 onin 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 achievetheir goal of chaotic system tracking.
The system has a strong history of working (1981 version still in use today in military), with new adaptations (chaos analysis) that improve the scheme.
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CSDL CSDL
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/GPSCSDL 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 & AlternatesMEMS & 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