RAIM Availability prediction

RAIM Availability prediction

Main content 一 Background & research purposes 二 Related research in China and abroad 三 Theory and arithmetic 四 RAIM systems development 五 The vision of the future

1 Background & research purposes The basic for PBN procedure is the navigation system meet the requirement of accuracy, integrity, availability, continuity Integrity it that when the satellite positioning error exceeds the allowable limit, system has the ability to alarm timely. RNP operation based on satellite navigation system must ensure the integrity of the system. To deal with possible satellite navigation signal abnormalities.

Background & research purposes At present, there are 3 method to augment GPS system integrity : Aircraft Based Augmentation System ABAS Ground Based augmentation System GBAS Satellite Based augmentation System SBAS

Background & research purposes RAIM (Receiver Autonomous Integrity Monitoring ) is a algorithm setting in a GPS receiver, respond quickly to satellite failure and completely automatic, and without outside intervention. But the application of RAIM technology can not ensure 100% available : (1) RAIM requires the more than 5 better geometric satellite distribution, this has led some time RAIM is not available. (2) Short outage problem of GPS satellite will also make the RAIM coverage holes more serious RAIM Enroute Terminal NPA Fault detection 99.998% 99.990% 99.903% time 5min 10min 30min

Background & research purposes For civil aviation users, using RAIM availability prediction enhance the availability of GPS to avoid RAIM hole problem during a flight.

Background & research purposes The aim of RAIM Availability Forecast Algorithm is as far as possible to accurate simulate the airborne receiver RAIM calculation results. (1)The basic algorithm for constellation autonomous integrity is identical. (2)At the same time flight path, terrain data and performance of airborne receiver also needed.

2 Related system Some countries have established RAIM prediction system :FAA Australia German Eurocontrol AUGUR ATMB Above system does not take the terrain and receiver performance into considerations. Commercial system : Jeppesen IDS Naverus The relevant standards and documentation :AC20-138A AC90-96A TGL2Rev.1 TGL3Rev.1 TGL10 TSO-C129a TSO-C145b/146b ICAO PBN RTCA DO-208 RTCA DO-229D

3 The principle and algorithm 3.1 Satellite visibility calculation 3.2 RAIM algorithm 3.3 Impact Analysis of flight path 3.4 Impact Analysis of terrain masking 3.5 Impact Analysis of receiver performance

3.1 Constellation visibility calculation The broadcast ephemeris and almanac are used to compute the Satellite visibility in the future. broadcast ephemeris provide Satellite clock error, Kepler orbit parameters and Orbit perturbation correction. Broadcast ephemeris updated every 2hours. In a short period after updated, the satellite position can reach meter accuracy level. Using broadcast ephemeris to calculate satellite position, the accuracy decreases rapidly with the increase of relative reference time interval.

Almanac Almanac is the general form of broadcast ephemeris, include only the Kepler orbit and clock parameters correction parameters, provide low accuracy satellite position for the user. period of almanac validity is more than six mouth, Ephemeris those short-term effective perturbation correction is not much significance For longer Almanac Updated cycle, The use calculation of satellite position can only achieve kilometer level accuracy, but it could take a long time to maintain.

Almanac received or download Directly download from the GPS receiver to, data receiving cycle for 12minutes The latest almanac data is collected by USA Coast Guard navigation center, according to each GPS forecast cycle. By entering the www.navcen.uscg.gov web site, you can use transfer protocol (FTP) or hypertext transfer protocol (HTTP) for data acquisition GPS constellation state (NANU) data report GPS satellite momentary interruption of unavailable information, you can download from American Coast Guard navigation center too.

Almanac parameters

The calculation of satellite visibility Satellite elevation angle h can be calculated by using the receiver position and the satellite position All the satellites which elevation angle h h0 (generally 5 ) can be used by receiver at that time

3.2 RAIM Algorithm RAIM is a techniques checking the satellite measurements consistency by using overdetermined solutions. For fault detection(fd), at least 5 visible satellites are needed. If the satellite fault detected, the pilot in the cockpit can receive a warning, indicating that GPS is not available For fault detection and exclusion (FDE), at least 6 visible satellites are needed. This can be excluded from the fault satellite navigation solution, so that the operation can continue without interrupted.

RAIM algorithm flow and module RAIM algorithm flow can be expressed as the form. The key module is fault detection algorithm and Geometry Screening algorithm.

RAIM Fault detection Algorithm snapshot method: Calculation based only on the current redundant observation data. The current mainstream systems usually use snapshot method, snapshot method include: Pseudo range comparison least square residuals Parity space ARP HPL etc. The above method is validated basically equivalent.

HPL Algorithm P FA is False alarm rate,the probability of there is no fault but told the pilots have a fault;p MD is Missed Detection rate, the probability of there is a fault but not decteted.

HPL Algorithm HPL=SLOPE max pbias, pbias=σ uere *sqrt(λ) λnon central parameterχ 2 of non central density function, σ uere is the standard deviation of satellite pseudo range measurement error

UERE

RAIM prediction result RAIM prediction is a simulation algorithm, unable to calculate SSE as the test statistics. Only need to judge whether HPL will be greater than HAL Once somewhere calculated HPL more than HAL, then the RAIM is not available. In fact, the calculation of HPL is related to the amount of visible satellites, UERE, constellation geometry, PFA and PMD values.

3.3 Affect of flight path and attitude RAIM prediction should consider the impact of flight attitude and flight path

3.4 Affect of terrain masking RNP approach based on GNSS in complex terrain, need to research the impact of terrain on the satellite visibility, to improve the traditional RAIM algorithm. RAIM availability prediction algorithm combined with digital terrain model (DEM), the DTM data could be directly imported to the prediction.

Terrain visibility algorithm

Impact of antenna and the receiver performance GPS antenna prformance(rtca DO-228) GPS receiver performance(rtca DO-229D)

RAIM prediction system RAIM availability prediction system has been developed, different airport, elevation angle, RNP (HAL) could be predicted After a long time comparison, the caculation are identical with the Eurocontrol website. a variety of ways for publishment.

Airport Positon Caculation comparison

Caculation comparison Revelent airport comparison

RAIM prediction service At present providing the service for Air China, China Southern Airlines, Shenzhen Airlines and other companies.

4 Application of RAIM prediction At present, the ways for sending RAIM prediction are website, FTP and mail

5 The vision of future In view of the development and application of Beidou, research on Beidou and multi constellation. research RAIM algorithm based on multi sensor.

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