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Receiver Autonomous Integrity Monitoring(RAIM).

When GNSS equipment is not using integrity information from WAAS or LAAS, the GPS navigation receiver using RAIM provides GPS signal integrity monitoring. RAIM is necessary since delays of up to two hours can occur before an erroneous satellite transmission can be detected and corrected by the satellite control segment. The RAIM function is also referred to as fault detection. Another capability, fault exclusion, refers to the ability of the receiver to exclude a failed satellite from the position solution and is provided by some GPS receivers and by WAAS receivers.

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요약하자면 WAAS나 LAAS와 같은 보정시스템을 사용하지 않고 그대로 신호를 받을 경우 위성 자체에서도 잘못된 신호를 발신하는 경우가 있는데 이 경우에 자체적으로 위성에서 오신호를 수정하는데 2시간까지 걸릴 수 있단다. 이럴 경우 잘못된 Signal을 항공기에서 받게 되는데 이걸 걸러버리는게 RAIM. 그렇기 때문에 평소보다 많은 GNSS 신호가 필요하지 않을까? 그래서 최소 5개의 위성신호를 잡아야 한다고 되어 있으며, 4개의 위성신호만 잡힐 경우 Baro-aiding이 필요하다고 한다.

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아래 설명에 나와있다 시피 Fault Detection and Exclusion (FDE) 기능이 있는 GPS의 경우 미리 신호에서 제한다고 하니... 참고

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5 satellites, or 4 satellites and barometric altimeter input (baro-aiding), to detect an integrity anomaly. Baro-aiding is a method of augmenting the GPS integrity solution by using a non-satellite input source in lieu of the fifth satellite. Some GPS receivers also have a RAIM capability, called fault

detection and exclusion (FDE), that excludes a failed satellite from the position solution; GPS receivers capable of FDE require 6 satellites or 5 satellites with baro-aiding. This allows the GPS receiver to isolate the corrupt satellite signal, remove it from the position solution, and still provide an integrity-as-sured position. To ensure that baro-aiding is available, enter the current altimeter setting into the receiver as described in the operating manual. Do not use the GPS derived altitude due to the large GPS vertical errors that will make the integrity monitoring function invalid.

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https://www.icao.int/SAM/Documents/2009/SAMIG4/presentation%20ICAO_SAM_IG_RAIM_Volchansky_v4.pdf - Aerospace Engineer인 Lou Volchansky 가 2009년 10월 19일 ICAO SAM(South American) Implementation group 에서 발표한 PPT 자료.

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https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/briefings/media/GPS_WAAS_Oliver_02308_Knowledge.ppt - GPS/WAAS 등에 대해 FAA에서 정리하기 쉽게 만들어놓은 PPT인데 스윽 한번 읽어 보시길

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