Integrated GPS/Loran Navigation Sensor
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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.

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Integrated GPS/Loran Navigation Sensor for Aviation Applicationsby 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.



  • Interest in Loran

  • GPS/Loran Integration Program

  • Brassboard System Architecture and Features

  • Flight Test Results


Resurging Interest in Loran

  • DOT’s Volpe Center study on GPS vulnerabilities spurred interest in independent, backup systems for both navigation and timing, i.e. critical infrastructure areas

  • USCG interested in utilizing Loran for backup harbor entrance and approach navigation

  • FAA interest for improving NPA (Non-Precision Approach) integrity, continuity and availability

  • Congress has provided $94M from 1997 to 2003 (including $25M in FY03) to further Loran development

  • Loran infrastructure upgrade well underway


GPS/Loran Integration Program

  • The 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 card

    • Rockwell Collins led the integration of a Locus Loran receiver into a Collins Multi-Mode Receiver (MMR)

    • Flight testing was performed by Ohio University’s Avionics Engineering Center with support from Rockwell Collins and Locus

Purpose of gps loran demonstrator

Purpose of GPS/Loran Demonstrator

  • The 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 system

  • They also provide a flight test resource for collecting GPS, Loran, and integrated data in real time

Gps loran integration program

GPS/Loran Integration Program


Loran Receiver

Standard SatMate 1020Loran Receiver

H-Field Loran Antenna

Loran Card

GPS/Loran Antenna Development

GPS/Loran Antenna

GPS/Loran Integration Effort

GPS Antenna Requirements

Brassboard GPS/Loran

Loran CardICD




Breadboard GPS/Loran Test Pallet

Rockwell Collins


Brassboard MMR Architecture

  • Rockwell Collins integration processor card combines GPS and Loran data

  • Locus receiver card provides the Loran function and is installed on the MMR door in place of the MLS

Brassboard navigation solutions

Brassboard Navigation Solutions

  • Brassboard provides multiple position solutions as well as an integrity bound

  • Federated and Integrated solutions share a common RAIM-FDE integrity function

  • Manual controls allow for various test scenarios

Federated position solution

Federated Position Solution

  • Position solution is a weighted least squares utilizing both GPS and uncorrected Loran data

  • Loran data is kept independent having no GPS-derived ASF error corrections

  • When GPS is unavailable, reverts to Loran-only position

    • Causes discontinuity in position solution

  • May use precomputed ASFs or corrections from ground stations to improve accuracy when GPS is unavailable

    • No Federated ASF corrections implemented in Brassboard

Integrated position solution

Integrated Position Solution

  • Position solution is a weighted least squares utilizing GPS and ASF corrected Loran data

    • ASF values computed using GPS-corrected position

  • When GPS is lost, position accuracy is maintained

    • ASF corrections prevent discontinuity

  • Integrity of corrected Loran may be maintained with proper ASF calibration procedure

    • ASF values may be calculated when GPS RAIM integrity is high and the values frozen when GPS integrity is low

    • The procedure is similar to baro/pressure calibration described in TSO-C129a or the newer approach in Appendix G, RTCA/DO-229C

    • Brassboard includes manual inputs to control ASF updates

Performance with simulated gps fault

Performance with Simulated GPS Fault

  • GPS data was reduced to four SV’s and ramping bias error injected into one SV

  • ASF estimator enabled for test

  • Integrated solution detected fault and excluded the faulty SV

Integrated gps loran antenna

Integrated GPS/Loran Antenna

  • Locus, 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 testing

Van Testing

  • The GPS/Loran system was tested in the Collins Mobile Navigation Lab van

  • The antenna mount was raised to reduce interference from other van systems


Flight Test Antenna Locations

GPS/Loran Antenna

Loran Antenna

  • Flight testing was performed on Ohio University’s Avionics Engineering Center (AEC) King Air, C-90SE twin turboprop

  • A combined GPS/Loran antenna was used with the Brassboard System

  • An additional Loran antenna was mounted on the underside of the tail and used with a standalone receiver for performance comparison

May 2003 flight testing

May 2003 Flight Testing

  • Five flights were performed near Cedar Rapids Iowa and Madison Wisconsin

  • The system worked accurately and without fault through all flights

Comparison of position solutions

Comparison of Position Solutions

  • GPS availability during the flight test was very good

    • Both 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 from cedar rapids ia to madison wi

ASF Values for Flight fromCedar Rapids, IA to Madison, WI

ASF values for strong stations show good continuity and stability

Orbit tests over altitude

Orbit Tests over Altitude

  • Little change in ASF values from 2,000 to 12,000 feet altitude

  • Some velocity/direction sensitivity observed

    • May be due to antenna inter-channel delays, filtering, data latency issues or bank-angle issues

Coasting performance

Coasting Performance

  • After low pass of the Cedar Rapids runway the GPS antenna was disconnected to simulate a GPS failure

  • “Coasting” Integrated solution overlays first runway pass with GPS

Coasting performance1

Coasting Performance

  • After low pass of the Cedar Rapids runway the GPS antenna was disconnected to simulate a GPS failure

  • “Coasting” Integrated solution overlays first runway pass with GPS

North/South offset reduced from 150 m to <10 m

Continuing work

Continuing Work

  • Rockwell Collins is performing additional flight testing on their Sabre 50 test aircraft

    • Investigating issues of data latency and antenna-induced Loran errors

    • Evaluating performance enhancements possible with inertial aiding of Loran using a low-cost MEMS AHRS

  • Locus is working to improve the performance and capabilities of their receiver and antenna

    • Improving H-field antenna channel balancing significantly reducing inter-channel delay

    • Adding automatic station selection and ASF map memory to receiver

    • Adding additional filtering to improve noise performance

Gps imu loran integration

GPS-IMU-Loran Integration















ASF Calibration

ASF Calibration













Calibrate Loran when GPS is good

Use Loran when GPS is bad

Gps imu loran flight test

GPS-IMU-Loran Flight Test

Ten hours of flight testing were performed on the Rockwell Collins Sabre 50 using:

GPS-Loran Antenna inside radome

  • Locus GPS-Loran H-field antenna

  • Rockwell Collins MMR with Locus embedded Loran card

  • Rockwell Collins AHC-3000A AHRS modified to add IMU outputs


GPS-Loran GNLU-930

Multi-Mode Receiver (MMR)

Taxi data evaluation

Taxi Data Evaluation

  • Raw data collected for a short taxi test

  • Post-processing was utilized to generate alternative solutions for performance comparison


Comparison of alternate solutions

Comparison of Alternate Solutions

Coasting IMU has diverging solution

IMU-Loran has accuracy and low noise

Loran-only has large bias

ASF-corrected Loran is accurate but noisy



  • Rockwell Collins and Locus participated in two coordinated FAA sponsored programs to develop a demonstration prototype GPS/Loran system

  • Performance demonstrated in both flight and van testing

    • Good coasting performance demonstrated after loss of GPS

  • The integrated GPS/Loran system provides enhanced integrity, availability, continuity and accuracy

    • ASF calibration procedure enhances Loran accuracy without compromising integrity

  • Integration with low-cost IMU shows promise of reducing position noise and latency



  • Mitchell 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 Collins

  • David W. Diggle of Ohio University’s Avionics Engineering Center led the flight testing of the Brassboard System and provided the raw data from the tests

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