Since Fiscal Year 2000, Congressional appropriations for the LORAN (LOng RAnge Navigation) System have exceeded $120 million. Congress has designated that these monies be administered by the Federal Aviation Administration and used for system modernization and research. Toward that end, the bulk of the funding has been turned over to the U.S. Coast Guard, the system operator, for improvements in transmitters, timing and frequency equipment, and upgraded system control. The remaining money has been applied to research in the areas of non-precision approach for landing of aircraft, and harbor approach and harbor entrance for maritime shipping. In addition, since the release of a report on the vulnerability of the Global Positioning System (GPS) on September 10, 2001 authored by the DoTVolpeTransportationSystemsCenter, LORAN has been in contention for use as a backup navigation and timing system for GPS. The figure below shows primary (solid line) and fringe (dashed line) coverage of the continental United States and Canada. In addition, LORAN chains are operational in Korea, Japan, Western Europe, Saudi Arabia, and Russia (a system called Chayka).
LORAN Coverage in the Continental United States
The Avionics Engineering Center (AEC) at Ohio University has been active in the field of LORAN research for many years and is currently doing work in specific areas which support issues dealing with non-precision approach for landing. These include: 1) the use of H-field aircraft antennas to mitigate the problem of precipitation static (P-static) build-up on aircraft which results in loss of navigation, 2) the characterization of atmospheric noise in the presence of P-static and thunderstorms and its effect on LORAN signal-to-noise ratio (SNR), and 3) the determination (measured and calculated) of additional secondary factors (ASFs) which affect the propagation of the LORAN ground wave.
LORAN has been in use since World War II as a position, navigation, and timing system. However, LORAN has typically had fairly large (100-500 meter) errors in its position solution performance. In addition, its use as an airborne navigation system has been hampered by problems of integrity, availability, continuity, and accuracy caused by climatic (changes in propagation path), aircraft-induced (precipitation static), and atmospheric (lightning) effects.
The introduction of new navigation systems has gradually reduced the use of LORAN as a primary means of point-to-point navigation, especially for aviation. Most notably, the GPS has provided the capability for worldwide navigation using a single system with accuracy, integrity, availability, and continuity performance far exceeding that typical of LORAN.
Recent events have resulted in a change in thinking regarding the need for a positioning and navigation system which can provide a backup to GPS. GPS, of course, is considered to be a primary means for aircraft positioning and navigation in the NAS. In addition, the timing and frequency communities began to realize their need for a backup system to reduce their dependence on GPS. In order to facilitate the evaluation of LORAN as a backup, two panels were formed to bring together people competent to: 1) determine the current capabilities of the system, and 2) suggest changes that would be required to allow LORAN to serve as a suitable backup. One of these panels was the LORAN Integrity and Performance Panel (LORIPP) formed by the Federal Aviation Administration (FAA) LORAN Program Office. It is through this panel, and under the direction of the FAA LORAN Program Office, that AEC conducts its research work.
ACTIVE RESEARCH AREAS / PROGRAM SUPPORT
P-static and Atmospheric Noise Characterization
AEC is conducting research to evaluate the effects of P-static and atmospheric noise on LORAN performance. These effects can have a significant impact on the SNR of LORAN signals. AEC has been conducting flight tests for the past several years to collect data in the presence of these atmospheric noise conditions. This data will allow the effects of the atmospheric noise on the LORAN SNR to be characterized. Once determined, knowledge of these effects will play a major role in the accuracy, integrity, availability, and continuity analysis of the LORAN system.
This task is being accomplished by collecting data under varying weather conditions, determining the effects those weather conditions have on the LORAN signal-to-noise ratio, and mitigating those effects when and where required. Collecting data that would allow these weather-related effects to be observed required that a data collection system capable of capturing radio frequency (RF) signals in the LORAN frequency band be fielded.
Flight tests have been conducted at several locations under varying weather conditions. LORAN data was collected using a two-channel data collection device to simultaneously collect radio frequency (RF) data from two independent antennas. Both E-field and H-field antennas are used to allow for comparison of the data so analysis of the performance of each antenna in varying environments can be accomplished. An identical data collection system is used to simultaneously collect ground data to be used for comparison purposes.
To date, flight and ground tests have been conducted in Michigan's Upper Peninsula out of the Hancock/Houghton County Memorial Airport, over northeast Ohio, and near the Kendall-Tamiami Executive Airport south of Miami, FL. Detailed information is available in the following references:
- Characterization of Atmospheric Noise in the LORAN-C Band, Manish Lad et. al., Proceedings of the International LORAN Association (ILA-32), Boulder, CO, November 2003.
- LORAN-C Band Data Collection Efforts at OhioUniversity, Curtis Cutright et. al., Proceedings of the International LORAN Association (ILA-32), Boulder, CO, November 2003.
- Characterization of Atmospheric Noise and Precipitation Static in the LongRange Navigation (LORAN-C) Band for Aircraft, Manish Lad, Masters Thesis, OhioUniversity, August 2004.
- Analysis of the Effects of Atmospheric Noise on LORAN-C, Curtis Cutright et. al., Proceedings of the International LORAN Association (ILA-33), Tokyo, Japan, October 2004.
Additional Secondary Factors Investigation
The FAA has been investigating the ability of LORAN-C to meet Required Navigation Performance (RNP) 0.3 requirements for accuracy, availability, integrity, and continuity. The use of locally measured and/or calculated LORAN-C Additional Secondary Factors (ASFs) is key to LORAN meeting those accuracy requirements for non-precision approach and landing guidance. AEC, using its Beechcraft C-90SE King Air has been collecting LORAN-C data for the past three years at six airports situated along the United States? East Coast and throughout the Midwest. Flights to these airports have been conducted semi-annually (late winter and late summer) in an effort to determine and characterize the behavior of ASFs as a function of seasonal variations and to determine if a single set of ASFs can cover the entire approach area for an airport.
At each airport, LORAN-C data are collected on the ground using all-in-view LORAN-C receivers with H-field and E-field antennas. WAAS-augmented GPS position data are collected simultaneously for use as a truth reference. Following the collection of data, a number of stabilized approaches (using ILS when possible) are performed at the airfield. These flight paths commonly extend beyond the approach perimeter for the airport due to Air Traffic Control considerations.
In the initial stages of this program, all data collected were post-processed to: 1) generate airfield-specific ASFs, 2) produce ASF-corrected LORAN-C tracks, and 3) determine lateral error information showing the difference between the ASF-corrected LORAN-C tracks and GPS truth?the latter provided by a WAAS-augmented GPS airborne receiver. More recently, the ground and air data-collection systems have been upgraded to allow ASFs to be calculated immediately after the ground data have been collected. These ASFs are then loaded into the all-in-view LORAN-C receiver aboard the aircraft, the LORAN-C position data are corrected in real-time, and both raw and real-time ASF corrected data are logged along with WAAS-augmented GPS truth data.
Additional information is available in the following reference:
- LORAN Additional Secondary Factor Correction Study for Aviation, G. Linn Roth, Ph.D., Proceedings of the Institute of Navigation, GNSS-2004, San Diego, CA, September 2004.
AEC provides flight test support for receiver testing. To date, AEC aircraft have flown LORAN‑C receivers developed by Rockwell Collins, Cedar Rapids, Iowa and FreeFlight Systems, Waco, Texas. Both companies? receivers have been designed to capitalize upon an integrated receiver concept using both GPS and LORAN-C. The following figure shows data from two low approaches to Runway 08 at the Cedar Rapids, Iowa Airport (CID) using the Rockwell Collins GPS-LORAN Integrated Processor (GLIP). The GPS antenna is removed from the GLIP during the second approach as indicated in the figure. The aircraft position track of the successive go‑ around, approach, and landing is generated using an ASF-calibrated LORAN position output from the GLIP. The GPS track is provided by a separate reference receiver aboard the aircraft.
Complete details of the testing are available in the following reference:
- "Integrated GPS/LORAN Navigation Sensor For Aviation Applications," J.H. Doty et. al., Proceedings of the Institute of Navigation, GPS/GNSS-2003, Portland, OR, September 2003.