Calibrated by gps if 6 satellites are usable at the

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calibrated by GPS, if 6 satellites are usable at the time of calibration, then calibration is done as described in paragraph (a)(3)(xxi) using all 6 satellites and not just 5 out of the 6. NOTE 2: This testing requirement does not preclude the use of alternative RAIM algorithms. If alternative RAIM algorithms are used, or if the same RAIM algorithm is used but tracks only 5 satellites, then the manufacturer shall establish a set of tests acceptable to and approved by the cognizant Aircraft Certification Office and demonstrate equivalent performance with those tests. (xxi) Pressure Altitude Input Requirements . If the equipment incorporates altitude aiding, the following requirements apply. An alternate acceptable algorithm for incorporating barometric altitude is specified in appendix G of RTCA/DO-229, "MOPS for GPS/Wide Area Augmentation System Airborne Equipment", January 16, 1996. 1. The equipment shall include a provision for automatic input of pressure altitude data. Pressure altitude shall be corrected/calibrated using GPS derived altitude data when and only when the maximum subset VDOP (VDOP max as defined below) is 5 and the test statistic S as defined in Equation 4 of Appendix F of RTCA/DO-208 is less than a threshold that corresponds to the 95th percentile point of the distribution of the test statistic in the presence of SA and given that no other errors are present. For example, in case of 5 satellites being used, the threshold is 3.841 x (33.3) 2 (from chi-square distribution table). These data shall be properly scaled to ensure the position accuracy is not degraded below that provided by GPS alone. An acceptable methodology of scaling is contained in this paragraph. Scaled altitude data shall be utilized in the navigation and RAIM detection functions when and only when these functions cannot be provided by GPS alone. This requirement does not preclude the use of altitude data for smoothing. NOTE: All equipment must be designed to automatically input at least pressure altitude data. As an option, the manufacturer may design a system which will accept an automatic input of pressure altitude corrected for the local barometric pressure setting applied by the aircraft's air data system in response to the pilot's action of setting the altimeter. Above a geometric altitude of 18,000 feet, equipment which uses pressure altitude corrected for the local barometric pressure 1Page 25
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TSO-C129a setting shall treat this input as pressure altitude. Pressure is to be calibrated with GPS; this calibrated value is defined as the current value of pressure altitude minus the difference between the pressure altitude and GPS estimate of vertical position as determined at the most recent time when the calibration was accomplished. At and below a geometric altitude of 18,000 feet, equipment which uses pressure altitude corrected for the local barometric pressure setting as an input shall correct this measurement with the difference between the WGS-84 ellipsoid altitude and mean sea level altitude at the current latitude and longitude.
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