Ramon Lopez/WASHINGTON DC
An independent risk assessment by the Johns Hopkins University Applied Physics Laboratory has concluded that, with some improvements and augmentations, the Global Positioning System can be "...the only navigation system installed in the aircraft and the only navigation service provided by the US Federal Aviation Administration."
The US aviation agency, which co-sponsored the study with the Aircraft Owners and Pilots Association and the US Air Transport Association, hopes the outcome will quell GPS critics, who believe that a back-up ground-based navigation system, such as Loran-C, is needed as an insurance against intentional and unintentional interference (Flight International 27ÊJanuary-2 February.
The report says the GPS system, working with the Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS) can satisfy all requirements for operating in oceanic areas, through to Category 3 approaches.
It says that risks to GPS signal reception can be managed, but steps must be taken to minimise the effects of intentional interference.
The recent decision to add a secondary military frequency, with an added signal modulation that could be used for civil applications, has helped the situation. The report says the second civil frequency "...will completely remove the requirement for ionospheric corrections...and will improve the corrections provided by WAAS."
There have been at least three instances where the GPS system has been accidentally jammed, but the study concludes that unintentional interference "...is not a major risk factor." It says the second civil frequency will further reduce the concerns, and actions required to counter intentional jamming will "...readily address this risk."
It deemed intentional interference the largest risk area, but it noted that planned avionics are designed to quickly recognise the onset of this threat. While aircraft can be vectored away from jammed regions, the threat poses no safety risk, although disruptions in traffic flow could follow. The study says methods to detect and stop GPS jammers must be put in place.
The study also recommends regulations to limit radio frequency emissions at satellite radio navigation frequencies, and development of air traffic control procedures to cover GPS signal outages. The FAA should consider additional means for aircraft-based interference suppression. These might include antenna nulling and signal-processing techniques and integration with inertial navigation instrumentation.
Source: Flight International
