Integrated GPS/Loran Navigation Sensor for Aviation Applications

by James H. Doty, David A. Anderson and Patrick Y. Hwang, Ph.D., Rockwell Collins, Inc., Linn Roth, Ph.D., Locus, Inc.

Portions of this work are being performed under subcontract SK-00-18 between Locus, Inc. and Advanced Management Technology, Inc. (AMTI) and under subcontract SK-02-02-001-00 between Rockwell Collins, Inc. and AMTI under a Federal Aviation Administration (FAA) Broad Information Technology Services (BITS) contract.

OutlineInterest in LoranGPS/Loran Integration ProgramBrassboard System Architecture and FeaturesFlight Test Results

DOTs Volpe Center study on GPS vulnerabilities spurred interest in independent, backup systems for both navigation and timing, i.e. critical infrastructure areasUSCG interested in utilizing Loran for backup harbor entrance and approach navigationFAA interest for improving NPA (Non-Precision Approach) integrity, continuity and availabilityCongress has provided $94M from 1997 to 2003 (including $25M in FY03) to further Loran developmentLoran infrastructure upgrade well underwayResurging Interest in Loran

GPS/Loran Integration ProgramThe FAA has funded efforts to develop resources to evaluate and demonstrate the use of Loran to enhance the integrity and continuity of airborne navigation systems Locus led the effort to develop a prototype combined GPS & Loran H-field antenna and embedded Loran receiver cardRockwell Collins led the integration of a Locus Loran receiver into a Collins Multi-Mode Receiver (MMR)Flight testing was performed by Ohio Universitys Avionics Engineering Center with support from Rockwell Collins and Locus

Purpose of GPS/Loran DemonstratorThe Brassboard demonstrator and integrated GPS/Loran antenna provide a test bed to investigate issues of accuracy, availability, integrity and continuity of an integrated GPS/Loran systemThey also provide a flight test resource for collecting GPS, Loran, and integrated data in real time

GPS/Loran Integration ProgramLocusRockwell CollinsGPS/Loran AntennaBrassboard GPS/LoranBreadboard GPS/Loran Test PalletLoran CardStandard SatMate 1020 Loran ReceiverStandard GNLU-930 MMRBreadboard GLIPLoran Card ICDGLIP CardGPS/Loran Integration EffortGPS/Loran Antenna DevelopmentGPS Antenna RequirementsH-Field Loran AntennaLoran Receiver

Brassboard MMR ArchitectureRockwell Collins integration processor card combines GPS and Loran dataLocus receiver card provides the Loran function and is installed on the MMR door in place of the MLS

Brassboard Navigation SolutionsBrassboard provides multiple position solutions as well as an integrity boundFederated and Integrated solutions share a common RAIM-FDE integrity functionManual controls allow for various test scenarios

Federated Position SolutionPosition solution is a weighted least squares utilizing both GPS and uncorrected Loran dataLoran data is kept independent having no GPS-derived ASF error correctionsWhen GPS is unavailable, reverts to Loran-only positionCauses discontinuity in position solutionMay use precomputed ASFs or corrections from ground stations to improve accuracy when GPS is unavailableNo Federated ASF corrections implemented in Brassboard

Integrated Position SolutionPosition solution is a weighted least squares utilizing GPS and ASF corrected Loran dataASF values computed using GPS-corrected positionWhen GPS is lost, position accuracy is maintainedASF corrections prevent discontinuity Integrity of corrected Loran may be maintained with proper ASF calibration procedureASF values may be calculated when GPS RAIM integrity is high and the values frozen when GPS integrity is lowThe procedure is similar to baro/pressure calibration described in TSO-C129a or the newer approach in Appendix G, RTCA/DO-229CBrassboard includes manual inputs to control ASF updates

Performance with Simulated GPS FaultGPS data was reduced to four SVs and ramping bias error injected into one SVASF estimator enabled for testIntegrated solution detected fault and excluded the faulty SV

Integrated GPS/Loran AntennaLocus, Inc. developed an integrated GPS/Loran antenna for use with the Brassboard System Utilizes an H-field Loran antenna to eliminate problems with precipitation static (p-static) Rockwell Collins worked with Locus to ensure GPS antenna compatibility with MMR

Van TestingThe GPS/Loran system was tested in the Collins Mobile Navigation Lab vanThe antenna mount was raised to reduce interference from other van systems

Flight Test Antenna LocationsFlight testing was performed on Ohio Universitys Avionics Engineering Center (AEC) King Air, C-90SE twin turbopropA combined GPS/Loran antenna was used with the Brassboard SystemAn additional Loran antenna was mounted on the underside of the tail and used with a standalone receiver for performance comparison

May 2003 Flight TestingFive flights were performed near Cedar Rapids Iowa and Madison WisconsinThe system worked accurately and without fault through all flights

Comparison of Position SolutionsGPS availability during the flight test was very goodBoth the Integrated and Federated to solutions were dominated by the low-noise GPS measurements Integrated, Federated and GPS-Only agreed to within 2.5 meters RMS radial position difference

Un-aided Loran solutions matched GPS to better than 250 m RMS (0.13 nautical miles)

ASF Values for Flight fromCedar Rapids, IA to Madison, WIASF values for strong stations show good continuity and stability

Orbit Tests over AltitudeLittle change in ASF values from 2,000 to 12,000 feet altitudeSome velocity/direction sensitivity observed May be due to antenna inter-channel delays, filtering, data latency issues or bank-angle issues

Coasting PerformanceAfter low pass of the Cedar Rapids runway the GPS antenna was disconnected to simulate a GPS failureCoasting Integrated solution overlays first runway pass with GPS

Coasting PerformanceAfter low pass of the Cedar Rapids runway the GPS antenna was disconnected to simulate a GPS failureCoasting Integrated solution overlays first runway pass with GPS

North/South offset reduced from 150 m to

Continuing WorkRockwell Collins is performing additional flight testing on their Sabre 50 test aircraft Investigating issues of data latency and antenna-induced Loran errorsEvaluating performance enhancements possible with inertial aiding of Loran using a low-cost MEMS AHRSLocus is working to improve the performance and capabilities of their receiver and antennaImproving H-field antenna channel balancing significantly reducing inter-channel delayAdding automatic station selection and ASF map memory to receiverAdding additional filtering to improve noise performance

GPS-IMU-Loran IntegrationIMUGPSKalmanFilterLoranGeometryGPS-AHRSSolutionASF FilterASF CalibrationASFMeasurementsRAIM-FDELoranCalibrate Loran when GPS is goodIMUGPSKalmanFilterGPS-AHRSSolutionASF FilterASF CalibrationASFCorrectionRAIM-FDELoranUse Loran when GPS is bad

GPS-IMU-Loran Flight TestGPS-Loran GNLU-930Multi-Mode Receiver (MMR)AHC-3000AGPS-Loran Antenna inside radomeTen hours of flight testing were performed on the Rockwell Collins Sabre 50 using:Locus GPS-Loran H-field antenna Rockwell Collins MMR with Locus embedded Loran cardRockwell Collins AHC-3000A AHRS modified to add IMU outputs

Taxi Data EvaluationRaw data collected for a short taxi testPost-processing was utilized to generate alternative solutions for performance comparisonStart

Comparison of Alternate SolutionsLoran-only has large bias Coasting IMU has diverging solutionASF-corrected Loran is accurate but noisyIMU-Loran has accuracy and low noise

SummaryRockwell Collins and Locus participated in two coordinated FAA sponsored programs to develop a demonstration prototype GPS/Loran systemPerformance demonstrated in both flight and van testing Good coasting performance demonstrated after loss of GPSThe integrated GPS/Loran system provides enhanced integrity, availability, continuity and accuracyASF calibration procedure enhances Loran accuracy without compromising integrityIntegration with low-cost IMU shows promise of reducing position noise and latency

AcknowledgmentsMitchell J. Narins, of the Navigation and Landing Product Team, AND-702, of the Federal Aviation Administration directed the GPS/Loran program performed by Locus and Rockwell CollinsDavid W. Diggle of Ohio Universitys Avionics Engineering Center led the flight testing of the Brassboard System and provided the raw data from the tests