GPS World - January 2008 - (Page 65)

Avionics & Transportation | INNOVATION 95% VPL particular location indicates that the VPL is less than or equal to 50 meters during 95% of the day. When the VPL is less than 50 meters and the HPL is less than 40 meters, a user can perform an LPV approach with a 250-foot decision height (see the “Navigating Aircraft with GPS and WAAS” sidebar for an explanation of decision height). Since the VPL is almost always larger than the HPL, the VPL alone is a good indicator of system performance. The second graph shows the percent of the CONUS and Alaska region, which achieves various VPLs for 95% and 99% of the day. 95% LPV availability over 10% of Alaska 99% LPV availability over 1% of Alaska 95% VPL 95% LPV availability over 63% of Alaska 99% LPV availability over 32% of Alaska 20 m VPL 25 m VPL 30 m VPL 35 m VPL 40 m VPL 45 m VPL 50 m VPL 65 m VPL WAAS Shadows Whenever possible, each release of new capabilities is integrated into the system without interrupting WAAS service. Raytheon has developed a sophisticated set of five WAAS “Shadow Systems” which mimic the fielded system and allow a thorough end-to-end test of all software, hardware, and cut-over procedures. The shadow systems are capable of linking together refer- 95% VPL L PV200 L PV p FIGURE 5 Performance improvement in Alaska from additional reference stations ence stations, master stations, and GEO uplink stations. Network traffic from the operational WAAS is fed into Raytheon’s facility in Fullerton, California, where it is filtered and processed as if it went through a fielded master station. The actual output WAAS message is internally looped back into the system and inserted into the WRS data as if the messages were broadcast from a live GEO satellite. Each shadow system along with the fielded system is monitored with a set of automated tools that look at Navigating Aircraft with GPS and WAAS Before the advent of GPS, a common procedure for aircraft navigating from point A to point B was to fly over navigation beacons in a doglegged sequence of route segments. With GPS in place, direct “straight line” routes from A to B could be planned and flown. Such navigation procedures are known as area navigation or RNAV. Unaugmented GPS is capable of providing RNAV enroute and terminal navigation to position an aircraft in the vicinity of an airport. For landing, an aircraft’s electronics switches to approach navigation. Traditionally, approaches are classified as either precision or nonprecision, depending on the accuracy and capabilities of the navigational aids used. Precision approaches use both lateral (course) and vertical (glide slope) guidance to a decision height. If the required visual references, such as the approach lights or the runway environment, are not in view at this height, the pilot must fly a “missed approach,” which is a specified, controlled routing away from the runway. Nonprecision approaches provide lateral course guidance only, using a “minimum descent height.” This height is defined as the height below which an aircraft must not descend until visual reference has been established, typically between 250 and 500 feet (aviation operations in the U.S. and elsewhere normally use imperial units), depending on the particular airport. Unaugmented GPS is capable www.gpsworld.com of providing a non-precision approach, now referred to as lateral navigation (LNAV). On an LNAV approach, the pilot flies the final approach using lateral guidance, but when the aircraft reaches the final approach fix, the pilot descends to a minimum descent height using the barometric altimeter. WAAS provides the additional capability for aircraft to use GPS for vertical navigation (VNAV), hence the ability to fly LNAV/VNAV approaches. LNAV/VNAV is an approach in which a vertical glide slope guides the aircraft to a distance of about 3800 feet before the runway threshold at an average decision height of 350 feet. WAAS permits aircraft to fly to even lower minimum decision heights using Localizer Performance with Vertical Guidance (LPV). An LPV approach use lateral guidance from WAAS and vertical guidance provided by either the barometric altimeter or WAAS. An LPV approach enables descent to 200–250 feet above the runway, and can only be flown with a WAAS receiver. An LPV approach with a 200-foot decision height is sometimes called an LPV200 approach. LPV approaches are operationally equivalent to the legacy Instrument Landing System Category I approaches but do not require any navigation infrastructure to be installed at the runway. In November 2007, a total of 964 published LPV approach procedures and 1224 LNAV/ VNAV approach procedures were available for use in the United States. January 2008 | GPS World 65 http://www.gpsworld.com

Table of Contents Feed for the Digital Edition of GPS World - January 2008

GPS - January 2008 Contents Out in Front Expert Advice The Money-Go-Round u-Nav Latest Acquisition Apples to Apples Global SBAS 2008 GPS Receiver Survey Advertisers Index & Company Directory The Manufacturer's Road Year of the Who Working Indoor Up and Down Good, Better, Best Marketplace Classifieds Seen + Heard

GPS World - January 2008

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