A minimum sector altitude (MSA) represents the lowest altitude which may be used that provides a minimum clearance of 300 m (1000 ft) above all objects located in an area contained within a sector of a circle of 46 km (25 NM) radius centred on a significant point, the aerodrome reference point (ARP) or the heliport reference point (HRP). [Definition - ICAO Doc. 8168, Vol. II - PANS-OPS]
The minimum sector altitudes are encoded using the class SafeAltitudeArea. Its safeAreaType attribute indicates if the features encodes a minimum sector / safe altitude ('MSA') or an emergency safe / sector altitude ('ESA' - an area which extends to 100 NM and has one single emergency altitude value, as defined by the TERPS standard).
The centre of the MSA is modelled by the isBasedOn association with a SignificantPoint. Usually the centre is either the Navaid the procedure is based on or the ARP of the associated AirportHeliport.
Depending on the design of the MSA, the SafeAltitudeArea may have one or more sectors modelled through the isDefinedBy association with SafeAltitudeAreaSector. Each SafeAltitudeAreaSector isDefinedBy a CircleSector and has a buffer associated with it (typically 4 or 5 NM). The value of the buffer is encoded in the bufferWidth attribute.
Figure - Subsector delimited by a DME arc (source: ICAO Doc 8168, Vol. II, Figure I-4-8-2)
The CircleSector class has attributes that describe a circular sector of the MSA, including the value of the minimum sector altitude that applies in that sector:
- a circular sector is defined between the fromAngle and the toAngle. To indicate if the direction of the arc is clockwise or counterclockwise, the attribute arcDirection is used with the values 'CWA' (clockwise) and 'CCA' (counterclockwise);
- the angleDirectionReference attribute indicates if the angles are measured towards the significant point ('TO') the MSA isBasedOn or from this point ('FROM').
- the CircleSector may also be a subsector. For example, an MSA may be based on a VOR/DME where DME distances are used to create subsectors with radius values between 10 and 15 NM). The innerDistance and outerDistance attributes are used in this case.
the lowerLimit attribute provides the value of the MSA for the respective sector. Because the MSA is defined as an altitude, the lowerLimitReference should have the value 'MSL'.
The CircleSector class also has upperLimit and upperLimitReference attributes, which are relevant for other usages of this class, but which do not have a particular relevance in the context of the MSA and are expected to be left empty.
The resulting geometry of each SafeAltitudeAreaSector may be provided as Surface object through the isPortrayedBy association. It offers the sector geometry data in format that can be directly used by a GML viewer. However, the authoritative data is defined by the CircleSector class.