Global Positioning System for Civil Air Navigation

CENTER FOR ADVANCED AVIATION SYSTEM DEVELOPMENT

n December 1993, the Depart- Iment of Defense declared ini- tial operational capability for a new high-technology radionavi- gation system, the Global Posi- tioning System (GPS). GPS has the potential to improve the accuracy and coverage capabili- ties presently provided by other ra d i o n av i g ation systems. However , based on studies done by The MITRE Corporation’s Center for Advanced Aviation System Deve- lopment (CAASD) and others, the Federal Aviation Administra- Figure 1: WAAS Architecture (Generic) tion (FAA) and the aviation com- munity recognize that while GPS There are two primary methods expedite GPS use for precision pr ovides a beneficial operati o n a l for ensuring the integrity of GPS approaches, CAASD helped to capability to many users avi a tor s signals. In the receiver autono- define a concept to add differ- operating under instrument mous integrity monitor ing (RAIM) ential corrections and ranging flight rules will need GPS aug- me thod fir st published by MITRE, informa tion to the inform at i o n mentation to meet their opera- signals from redundant sate l l i t e s rel a yed by geostationary satel- tional requirements. are used to check the accuracy lites. As part of this effort, of the position determination. CAASD also developed and test- Through analysis and testing, Using an analytical model of ed algorithms to mitigate the CAASD helped the FAA identify RAIM, CAASD has led perform- effects of signal delays in the various aspects of GPS performance and availability assessments ionosphere and to ensure the ance that must be improved to of RAIM algorithms over the integrity of broadcast ionospher- su p p o r t high-availability GPS fli g h t past several years. In addition, ic delay corrections. These algo- operations, including precision CAASD conceived the second rithms are employed in the approach and airport surface method of ensuring the integrity FAA’s National Satellite Test Bed operations. These components of GPS signals—the GPS wide-area (NSTB) for GPS. At the FAA’s included signal integrity, which in t e g rity broa d c a s t. This techn i qu e request, CAASD helped to can be improved by providing uses ground monitor stati o n s to establish performance require- users with more timely notifica- check the GPS signals and trans- ments requirements for the tion of unusable signals; signal mit an integrity broadcast via Wide Area Augmentation System ac c u r a c y , which can be impr oved geostationary satellites that (WAAS) and is taking a lead role by transmitting differential cor- notifies users of the status of in the transfer of technology to rections to users; and system the signals. the prime contractor. WAAS will ava i l a b i l i t y , which can be impr oved consist of a network of North with satellite redundancy or reli- To enhance the GPS accuracy American ground stations, geo- ability enhancements. and availability, as well as to stati o n a r y satellite transponders, and GPS satellites to provide a global positioning system (LDGPS) User Group (SNUG) to define policy publicly accessible, en route- that provides corrections via a considerations for transition from navigation-through-precision Very High Frequency (VHF) data conventional navaids to sat e l l i t e approach capability (as illustrated bro a d c a s t to aircr a f t within airpo r t navigation. CAASD is also sup- in Figure 1). line-of-sight distances to provide po r ting the FAA on the Interagen c y all-weather approach capability at GPS Exec u t i v e Board (IGEB) to To ensure tha t the WAAS design any runway within the coverage determine the long-term ramifica- ach i e ves the necessary degree of area (see Figure 2). CAASD’s role tions to GPS users of improve- in t e g rity for precision approa ch , in specification development and ments to the basic satellite con- the FAA established a WAAS Integ- validation includes analyses of stellation performance and its rity Per for mance Panel (WIPP). The signal integrit y and availability, maintenance. CAASD’s support WIPP consists of CAASD, industr y, pseudolite siting, and pseudolite of the FAA’s GPS augmentation and academia and will wor k with the prime contractor thro u g h o u t the impl e m e n t ation of WAA S .

To help the FAA confirm the necessary schedule for the WAAS prime contractor to implement recommendations from the WIPP, CAASD initiated development of a Red Team, led by MITRE’s Economic and Decision Analysis Center (EDAC) and drawing on expertise from MITRE’s Software Engineering Center (SWEC).

To help the FAA determine the number of geostationary satel- Figure 2: LAAS Architecture (Generic) lites necessary to provide signal standards. In addition to LAAS ranging, differential corrections, programs, the SNUG, and the specification activities, CAASD is integrity broadcast services, and IGEB is helping shape the coun- also providing technical expertise the number and location of refer- try ’ s future civil navigation archi- to the FAA/industry partnerships ence stations to meet specified tecture by providing key benefits to develop LAAS equipage and requirements, CAASD developed to users at reasonable costs. a Satellite-Based Augmentation ensure the international standard- System (SBAS) Worldwide Avail- ization of LDGPS applications. For more inform a tion, contac t : ability Tool (SWAT). The SWAT Finally, CAASD is working with predicts the performance of an the FAA to ensure the integrity Kelly Markin integrated network of satellites, and safety of a future LAAS sys- 703.883.5819 ground stations, and communica- tem that will support landings to [email protected] tion links. It has also been used Category III standards. Center for Advanced Aviation by FAA to assess the operational To help the FAA plan the transi- System Development (CAASD) impacts of GPS/WAAS outages tion to future precision approach throughout the National Airspace and landing systems, CAASD is The MITRE Corporation System (NAS). evaluating candidate system 7515 Colshire Drive CAASD is also working with the architectures, using a hybrid of McLean, VA 22102-7508 FAA to construct a ground system existing systems, including GPS, USA specification for the Local Area WAAS and LAAS. CAASD is work i n g www.mitrecaasd.org Aug m e n t ation System (LAAS). The with the FAA and aviation users LAAS is a local-area differential through a Satellite Navigation