GPS World - January 2008 - (Page 63)

Avionics & Transportation | INNOVATION L1 GPS signal L2 GPS signal L1 GEO signal L5 GEO signal GPS satellite ephemeris Global Positioning System satellite Geostationary Earth orbit satellite L1 signal w/WAAS message L5 signal L2 GPS L1 GPS signal signal GPS ephemeris Signal generator The System WAAS is a wide-area differential GPS system that processes measurements from reference stations throughout North America to compute ionospheric corrections and satellite orbit and clock corrections (see FIGURE 1). GPS measurements are processed by a network of Wide Area Reference Stations (WRSs) located throughout the coterminous United States (CONUS), Alaska, Puerto Rico, Hawaii, Mexico, and Canada. L1 and L2 measurements from the WRSs are used by the Wide Area Master Stations (WMSs) to form WAAS corrections and integrity information. Dual-frequency (L1/L2) measurements are used to calculate ionospheric delays. However, L2 measurements are not usable by aircraft equipment because the L2 signal does not fall in a protected radio frequency band, a requirement for such a safety-of-life service. Thus, WAAS provides ionospheric corrections for the single-frequency L1 users who cannot adequately correct for ionospheric delay on their own. These corrections are provided in the form of a latitude-longitude grid, hence the term ionospheric grid delays. Users’ receivers interpolate the grid delays to obtain corrections for a particular satellite at a particular location. The ionospheric grid points (IGPs) cover North America. Signal quality data measured by the reference receivers is used in the Safety Processor to detect GPS signal deformation which could result in a hazard similar to the “GPS SV19 failure.” That failure occurred in 1993 when the pseudorandom noise code modulation on the satellite’s signal became distorted, leading to tracking errors and larger than normal positioning errors. The assembled WAAS message is output to the GEO Uplink System (GUS). The GUS chooses data from one of the WMSs and uploads the data to a geostationary WAAS satellite, which broadcasts the data to users. The GEOs broadcast both an L1 and L5 signal. Currently, the L5 signal is only used by the GUS itself to calculate ionospheric delay. In the future, when L5 signals are available from GPS satellites, the GEO L5 signal will contain a WAAS message for dual-frequency L1/L5 users. www.gpsworld.com GEO Uplink System L1 GEO signal WAAS message Comparator WAAS Msg Processor 1 GUS Receiver GUS Processor WAAS Msg Processor 2 User GUS (1 pair per GEO) Operations and Maintenance 1-2 Wide Area Reference Station L1/L2 GPS measurement L1 GEO range measurement GPS ephemeris WAAS message Signal quality data Wide Area Master Station TCN WAAS message WRE A WRE B WRE C TCN Terrestrial Communication Network Correction Processor 2 Safety Processor 2 WRS 1-38 WMS 1-3 p FIGURE 1 WAAS architecture Redundancy. Redundant system components are used to increase system reliability and mitigate hazardous information from a failed piece of hardware. Each of the 38 reference stations has three WAAS Reference Equipment (WRE) units, each with its own co-located antenna, receiver, and data processor. The system actively uses data from two of the WREs at a reference station. Data from the third WRE at each reference station is used during system maintenance or whenever the system detects a WRE failure. Stations are connected into the system through the Terrestrial Communications Network (TCN), which is divided into two separate and diverse networks designated Ring 1 and Ring 2. The backbone of each ring consists of high reliability dual T1 circuits with enough capacity to ensure that every message is delivered on each ring even if one leg of the network goes down. Each WMS consists of two correction processors and two safety processors. A hardware comparator performs a bitby-bit comparison of the output WAAS messages from the two safety (validation) processors. If one or more bits mismatch, the correction and validation component (C&V) faults itself and another C&V automatically takes over. Each GUS receives a WAAS message from each WMS. In the event that a WMS fails to send a message, the GUS switches to a different WMS. A pair of GUS sites is assigned to each GEO satellite. In the event that one GUS site fails or during system maintenance, the other GUS site automatically takes over. The two GEO satellites are positioned such that most users in North America will have dual GEO coverage. In the event of a satellite or uplink failure, users’ receivers automatically switch to the other available satellite. Enhancements Under the “WAAS Full LPV” contract awarded by the FAA, WAAS is undergoing a series of enhancements designed to improve overall system reliability and extend the LPV service region. These enhancements result in a significant availability improvement in CONUS and Alaska as well as expanding LPV service into Canada and Mexico. A comparison of performance requirements for the WAAS Initial Operating Capability (IOC) — achieved in 2003 — and the Full LPV Contract is given in TABLES 1 and 2. The tables provide availability and accuracy figures for LNAV and LPV approaches. The primary difference between LNAV and LPV is the size of the Horizontal Alarm Limit (HAL) and Vertical Alarm Limit (VAL), which refer to the January 2008 | GPS World Comparator Correction Processor 1 Safety Processor 1 63 http://www.gpsworld.com

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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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