An aircraft display system, comprising: a display device; and a processor in operable communication with the display device and adapted to receive at least data representative of a desired course of the aircraft, data representative of required navigation performance for the aircraft, and data representative of estimated position uncertainty for the aircraft, the processor configured, in response to at least these data, to determine a lateral deviation distance of the aircraft from the desired course, and render a horizontal situation indicator on the display device that includes: (i) an aircraft symbol representative of a top-down view of the aircraft, the aircraft symbol rendered at a position that is representative of actual aircraft position relative to the desired course, (ii) an RNP boundary graphic representative of at least one RNP boundary for the aircraft, the RNP boundary graphic rendered at a position relative to the rendered aircraft symbol, (iii) an EPU graphic representative of the EPU for the aircraft, the EPU graphic comprising a pair of line segments (215-1, 215-2) symmetrically disposed on either side of rendered aircraft symbol and separated from each other by a separation distance that is proportional to the EPU for the aircraft, one line segment rendered left of the rendered aircraft symbol, and another line segment rendered right of the rendered aircraft symbol, (iv) a lateral deviation indicator representative of the determined lateral deviation distance, the lateral deviation indicator separated from the aircraft symbol by a distance representative of the determined lateral deviation distance, and characterised by (v) a corrective graphic if the processor determines that the lateral deviation distance is greater than a first predetermined lateral deviation distance, the corrective graphic comprising a graphic representative of a corrective direction and a graphic numerically indicating the lateral deviation distance. a display device; and a processor in operable communication with the display device and adapted to receive at least data representative of a desired course of the aircraft, data representative of required navigation performance for the aircraft, and data representative of estimated position uncertainty for the aircraft, the processor configured, in response to at least these data, to determine a lateral deviation distance of the aircraft from the desired course, and render a horizontal situation indicator on the display device that includes: (i) an aircraft symbol representative of a top-down view of the aircraft, the aircraft symbol rendered at a position that is representative of actual aircraft position relative to the desired course, (ii) an RNP boundary graphic representative of at least one RNP boundary for the aircraft, the RNP boundary graphic rendered at a position relative to the rendered aircraft symbol, (iii) an EPU graphic representative of the EPU for the aircraft, the EPU graphic comprising a pair of line segments (215-1, 215-2) symmetrically disposed on either side of rendered aircraft symbol and separated from each other by a separation distance that is proportional to the EPU for the aircraft, one line segment rendered left of the rendered aircraft symbol, and another line segment rendered right of the rendered aircraft symbol, (iv) a lateral deviation indicator representative of the determined lateral deviation distance, the lateral deviation indicator separated from the aircraft symbol by a distance representative of the determined lateral deviation distance, and characterised by (v) a corrective graphic if the processor determines that the lateral deviation distance is greater than a first predetermined lateral deviation distance, the corrective graphic comprising a graphic representative of a corrective direction and a graphic numerically indicating the lateral deviation distance. (i) an aircraft symbol representative of a top-down view of the aircraft, the aircraft symbol rendered at a position that is representative of actual aircraft position relative to the desired course, (ii) an RNP boundary graphic representative of at least one RNP boundary for the aircraft, the RNP boundary graphic rendered at a position relative to the rendered aircraft symbol, (iii) an EPU graphic representative of the EPU for the aircraft, the EPU graphic comprising a pair of line segments (215-1, 215-2) symmetrically disposed on either side of rendered aircraft symbol and separated from each other by a separation distance that is proportional to the EPU for the aircraft, one line segment rendered left of the rendered aircraft symbol, and another line segment rendered right of the rendered aircraft symbol, (iv) a lateral deviation indicator representative of the determined lateral deviation distance, the lateral deviation indicator separated from the aircraft symbol by a distance representative of the determined lateral deviation distance, and