Getting a Bearing on ASF Directional Corrections 32 nd Annual Technical Symposium International Loran Association 5 Nov 2003 Boulder, CO

11/5/03USCGA-JJMA Presentation for ILA 322 Authors  CAPT Richard Hartnett  US Coast Guard Academy  Gregory Johnson  John J. McMullen Assoc. Inc.  Dr. Peter Swaszek  University of Rhode Island  Ken Dykstra  John J. McMullen Assoc. Inc.

11/5/03USCGA-JJMA Presentation for ILA 323 Outline  Introduction  Problem identification  Error Sources and Solutions  Summary

11/5/03USCGA-JJMA Presentation for ILA 324 Introduction  LORAN-C as “the” backup for GPS  Accuracy, availability, integrity, continuity  Accuracy:  Needs:  Aviation: NPA – RNP 0.3 (309 meters)  Maritime: HEA (8-20 meters)  Limitations:  Spatial and temporal variations in TOA (ASF) observed by the receiver and presented to the position solution algorithm

11/5/03USCGA-JJMA Presentation for ILA 325 Loran  Multi-station, multi-chain, all-in-view receiver  TOA vs. TD measurements  Separate “errors” into spatial, temporal, and directional components  DSP-based receivers  H-field vs. E-field antennas  Grid approach vs. waypoints

11/5/03USCGA-JJMA Presentation for ILA 326 Current Research  USCGA/JJMA activities:  Explore ways to mitigate TOA variation  Current approach:  Model the ASF sources  Spatial effects  Temporal effects  Model the directional effects (H-field)  Estimate parameters/values for the models  “Correct” the TOA observations using the models Of interest today

11/5/03USCGA-JJMA Presentation for ILA 327 ASF Variation Model  Assume that the variation in the observed TOAs can be decomposed into 3 independent, additive terms:  Spatial term:  Due to differences in the Loran signal propagation path  Temporal term:  For short term TOA variability on a local region  Directional term:  Based on the relative bearing of the Loran tower to the vessel Key assumptions

11/5/03USCGA-JJMA Presentation for ILA 328 Block diagram of approach TOAs From Rcvr TOAs To Position Algorithm ASF Temporal Correction ASF Spatial Correction Antenna Direction Correction Live Monitor Update Recent Position Current Heading

11/5/03USCGA-JJMA Presentation for ILA 329 Problem identification  First noticed directional TOA in our data sets over a year ago, started investigating:  Have numerous examples of this:  Airplane rotation data on ground  Convair 580  Ohio University King Air  Locus antenna/Satmate rcvr  Megapulse antenna/DDC rcvr  Airplane rotation data in air  Convair in CA both rcvrs/antennas  Rockwell data – Locus antenna/satmate  Boat rotations  T-boats  Both Megapulse antenna/DDC and Locus antenna/satmate  Field data, various antennas  DDC, aircraft antenna, small antennas, split loop antennas

11/5/03USCGA-JJMA Presentation for ILA 3210 Graphs of TOA variation Field – DDC – Split Loops Field – SatMate - LocusTboat – DDC - Megapulse Convair – PC104 - Megapulse

11/5/03USCGA-JJMA Presentation for ILA 3211 Expected Position Error  Given that there are TOA variations as the antenna is rotated, what is the effect in the position domain?

11/5/03USCGA-JJMA Presentation for ILA 3212 Model of TOA Variations

11/5/03USCGA-JJMA Presentation for ILA 3213 Resulting Position Error

11/5/03USCGA-JJMA Presentation for ILA 3214 Position Error relative to center

11/5/03USCGA-JJMA Presentation for ILA 3215 Error Sources  Time delay  Coupling  Antenna phase issues  Reradiation\Local Effects

11/5/03USCGA-JJMA Presentation for ILA 3216 Time Delay  Unequal phase delay between the two channels leads to directional TOA variation  Sources  Amplifier/Filters  Cables  Antennas

11/5/03USCGA-JJMA Presentation for ILA 3217 Time Delay – Theoretical Formulation Ideal Actual

11/5/03USCGA-JJMA Presentation for ILA 3218 Theoretical Effect of Time Difference

11/5/03USCGA-JJMA Presentation for ILA 3219 Measured Time Delay Loran Simulator (ICS660 card) RF Front End (Freq Devices gain modules and filters) Ch 1 Ch 2 RF inRF out Loran Receiver (ICS 650 card) Ch 1 Ch 2

11/5/03USCGA-JJMA Presentation for ILA 3220

11/5/03USCGA-JJMA Presentation for ILA 3221 S/w compensation for time delay

11/5/03USCGA-JJMA Presentation for ILA 3222 Coupling  Some coupling seen with combined loops vs. with split loops  Maximize strong signal on one loop  Should be zero on other loop  This is not the case with some H-field antennas  Maximize strong signal on one loop  Signal is NOT zero on other loop

11/5/03USCGA-JJMA Presentation for ILA 3223

11/5/03USCGA-JJMA Presentation for ILA 3224 Single Loop Phase Issue

11/5/03USCGA-JJMA Presentation for ILA 3225 Strongest Beam Only

11/5/03USCGA-JJMA Presentation for ILA 3226 Strongest Beam and Delay Correction

11/5/03USCGA-JJMA Presentation for ILA 3227 Summary  Split loops (coupling)  Works – when strong signal is max. in one loop is zero on other loop  Time delay correction in software  Works - compensate for unequal delay in antenna and/or amplifier  Loop Phase Shift effect  Use antenna loop with largest magnitude only

11/5/03USCGA-JJMA Presentation for ILA 3228 Final Remarks  When faced with tremendous coupled challenges we find the best strategy is to do what most engineers on the team do – blame Mitch!  In the future, for harbor mapping we will use both E and H-field antennas – mappings may not be the same -  We are highlighting “solvable challenges” with H-field antennas  New Locus antennas  New Reelelektronica antennas  Presentation by Wouter tomorrow  Calibration is clearly an important issue – Must be done in lab and after installation!

11/5/03USCGA-JJMA Presentation for ILA 3229 Acknowledgements  Mitch Narins, FAA AND  JJMA Team  Christian Oates  George Sanders  Ruslan Shalaev  Tim Waldie

Questions?