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Advancement of GPS for AR&C

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Presentation on theme: "Advancement of GPS for AR&C"— Presentation transcript:

1 Advancement of GPS for AR&C
Janet W. Bell NASA / JSC May 23, 2002

2 Contributors NASA-JSC UT @ Austin / Center for Space Research:
Aeroscience & Flight Mechanics Divison Boeing Titan-LinCom (Dr. Kevin Key) GeoControls 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

3 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

4 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

5 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

6 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, 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

7 X-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)

8 GPS / INS Integration Pursue techniques with high probability to maximize performance: Tracking loop & filtering algorithms for rapid acquisition & measurement of GPS signals 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

9 X-GPSR Multipath Mitigation
Generic GPS Receiver Components RF Down Conversion: GHz to Intermediate Frequency ~1 MHz IF Tracking Loops: Maintain lock on incoming GPS Signals Navigation Algorithms: Generate Receiver’s PVT solution Multipath Mitigation Concepts 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

10 Adaptive Self-tuning GPS Filter (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

11 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

12 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.

13 RSW vs. Choke-Ring RH & LH CP Patterns
RH-CP Micropulse Choke-Ring L1 Antenna. RH-CP RSW L1 Antenna on a 14 in diameter circular ground plane. Choke-Ring Antenna - Broad pattern above horizon - Relatively insensitive to low angle multipath signals - Poor CP performance (large LH polarization) RSW Antenna Choke-ring Antenna RH & LH CP Patterns RSW Antenna - Broad pattern above horizon - Extremely insensitive to low angle multipath signals - Excellent CP performance (small LH polarization)

14 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.

15 Navigation Systems & Technology Lab Resources
NSTL 16A/1004 GPS Receivers / Nav Sensors / RSW Antennas GPS Pseudolites Motion Platform Roof-top 2-Axis Positioner ESD Certified Laboratory 1553, 422, 488 Ethernet RF VME’s, SUN’s, Power Hawk Rapid Development Lab 16A/1169 GPS Signal Generator 3-Axis Rate Table Real Time Simulation Platform Rapid Development Lab 16A/2115

16 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

17 Supplementary Data

18 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 Implementation/Metrics FY 02 03 04 05 06 Total Current State of the Art 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 . 2 Prototype Evaluation 3 Breadboard Evaluation TRL 4 Flight Version, Testing & Ground Demo 5 6 Ground FEU, Testing & Demo Space Demo 7

19 Possible Multipath Mitigation Schemes
RF Down Conversion: Take 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

20 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 Multipath Compensator Tracking Loops Navigation System Received Signal Navigation error Tracking error

21 Tracking 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

22 Navigation 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

23 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), with new adaptations (chaos analysis) that improve the scheme.

24 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.

25 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

26 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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