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

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

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

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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