AMC1 ACNS.D.ADSB.085 Geometric Altitude

ED Decision 2013/031/R

(a)     Geometric Altitude data source

The position source should output a vertical position accuracy metric to support the encoding of the Geometric Altitude GVA quality indicator.

GNSS position sources should provide the geometric altitude accuracy through the vertical figure of merit (VFOM). If that is the case, the vertical position source accuracy output by a GNSS receiver should have been qualified as per Appendix H Part 5 paragraph 5.

If the position source does not output a qualified vertical accuracy metric, the GVA parameter should be set to ‘zero’.

For general guidance on the GVA encoding, refer to Definition 20 in Appendix H of Subpart D.

(b)     Geometric Altitude Reference

A GNSS position source compliant with CS ACNS.D.ADSB.070 provides Geometric Altitude, in its native format, as geocentric height above the earth’s ellipsoid shape. Height Above Ellipsoid (HAE) is described by the WGS-84 format.

Another altitude reference is described by the earth’s geoid, a surface on which the gravitational potential is constant and which approximates the (local) mean levels of all the earth's seas. The difference between the mathematically idealised smooth ellipsoid and irregular geoid surfaces varies between +106m to -85m across the earth. The related Mean Sea Level (MSL) altitude is then established as the sum of the HAE altitude and those local differences (using look-up tables). MSL is sometimes also referred to as Height-Above-Geoid (HAG).

A position source that only provides HAG or MSL altitude (ARINC label 076) but not HAE (ARINC label 370) should not be interfaced to the ADS-B transmit unit unless the ADS-B transmit unit can properly convert HAG/MSL to HAE, using the same HAG/MSL model as the position source (typically NATO STANAG Appendix 6). This should be based on position source installation instructions that specify a deterministic method to perform conversion to HAE, and be demonstrated during ADS-B transmit unit design approval. It is expected that the respective compliance information is supplied by the position and velocity source, and ADS-B transmit unit manufacturers through a Declaration of Design and Performance (DDP) or an equivalent document.

Note: Horizontal position sources compliant with Class 3 equipment approved under ETSO-C145c/C146c are required to output HAE altitude. The requirement has been implemented from revision C of RTCA/DO-229 onwards.

(c)      Geometric Altitude Accuracy Quality Indicator Testing

If a qualified vertical accuracy metric is available, under nominal GNSS satellite constellation and visibility conditions, the transmitted GVA value should be a minimum of ‘one’.