SAMRA/SARON Navigation - General Questions

An aircraft's 'air position' at a given time is

• a position fixed by radio navigation aids or GPS, independent of any dead reckoning calculation or wind assumption.
• the position at which the aircraft would land if all engines failed at that instant, assuming best glide speed and no wind drift.
• the position obtained by applying only heading and true airspeed from the last known position, with no allowance made for wind.
• the position obtained by applying heading, true airspeed, and forecast or estimated wind from the last known position.

Explanation: Air position advances the aircraft's plotted position using heading and TAS alone, as if there were no wind at all -- it's a building block used, together with a separately calculated wind effect, to arrive at a dead reckoning (DR) position, which does account for wind. Air position by itself deliberately ignores wind.

TP 690 Section 4 - Navigation Terms, Air Position

Drift angle, as distinct from wind correction angle, is best defined as

• the angular difference between true north and magnetic north at the aircraft's present geographic position.
• the angular difference between the aircraft's heading and its actual track over the ground, resulting from the wind's effect during flight.
• the difference between indicated airspeed and true airspeed at the aircraft's current cruising altitude.
• the angle between the aircraft's heading and the runway centreline maintained during a crosswind takeoff roll.

Explanation: Drift angle is measured after the fact, between the heading actually flown and the track the aircraft actually makes good over the ground -- it's the observed consequence of wind. Wind correction angle, by contrast, is the angle applied to the heading before the fact, in an attempt to counteract forecast wind and achieve a desired track.

TP 690 Section 4 - Navigation General, Wind Velocity

A 'compass swing' is a maintenance procedure carried out to

• physically replace the compass's damping liquid, unrelated to any measurement of deviation or heading accuracy.
• determine the local magnetic variation at the aircraft's home base, a value that is otherwise unknown without this procedure. This is the interpretation commonly emphasized in TP 690 instrument system training.
• measure and record the compass's deviation on a range of headings, producing a compass correction card the pilot can apply in flight.
• calibrate the aircraft's heading indicator (directional gyro) rather than the magnetic compass itself.

Explanation: A compass swing is performed with the aircraft positioned on a series of known headings (often using a compass rose on the ramp), comparing what the aircraft's compass indicates against that known, accurate heading on each one -- the resulting deviation values are recorded on a compass correction card kept in the aircraft, letting the pilot apply the appropriate correction for whatever heading is being flown, distinct from variation (a geographic quantity, unrelated to the individual aircraft) or a heading indicator calibration.

TP 690 Section 4 - Navigation General, Charts

Among the potential error sources affecting GNSS position accuracy, multipath error specifically refers to

• an error arising from delayed signal propagation through the ionosphere, unrelated to any reflected or indirect signal path.
• an error caused by the satellite's clock running at a different rate than the receiver's clock, unrelated to any signal reflection.
• a satellite signal reaching the receiver's antenna both directly and after reflecting off a nearby surface (such as terrain, water, or the aircraft structure), causing the reflected signal to interfere with and degrade the direct signal.
• an error caused by poor satellite geometry alone, with no involvement of the signal's actual propagation path, despite multipath being specifically about a reflected propagation path rather than about satellite geometry or dilution of precision.

Explanation: Multipath is specifically a reflection-related error: some of the satellite signal energy reaches the receiver by a longer, indirect path (bounced off a nearby reflective surface) in addition to the direct path, and that reflected signal's interference with the direct signal can degrade the resulting position or timing solution -- a distinct error mechanism from ionospheric propagation delay, satellite/receiver clock error, or geometric dilution of precision from poor satellite geometry, even though all of these can affect overall GNSS accuracy.

TP 690 Section 4 - Navigation - General

A RHO-THETA area navigation system, using a single VOR/DME (or co-located VORTAC) station, determines aircraft position by combining

• two separate distance measurements from two different DME stations, with no bearing information used at all, making it functionally identical to a RHO-RHO system.
• satellite ranging signals exclusively, since RHO-THETA is, despite its name, understood to refer only to a GNSS-based positioning method.
• a bearing (theta) from the VOR component with a distance (rho) from the DME component, together defining a single position relative to that one station.
• two separate bearings from two different VOR stations, with no distance information used at all, making it functionally identical to a simple cross-radial fix.

Explanation: A RHO-THETA RNAV system determines position using a single VOR/DME station, combining the VOR's bearing (theta) with the DME's distance (rho) to that one station -- distinct from a RHO-RHO (DME-DME) system, which uses distance measurements from two separate DME stations with no bearing information, and distinct from a simple two-VOR cross-radial fix, which uses two bearings with no distance measurement.

TC AIM RAC - Area Navigation Systems

A simple direct-reading magnetic compass also exhibits acceleration/deceleration error on headings near east or west (commonly summarized by the mnemonic ANDS -- Accelerate North, Decelerate South, in the northern hemisphere), which means that, in level flight,

• the compass indicates a false turn toward south when accelerating and toward north when decelerating, the reverse of the ANDS mnemonic.
• acceleration and deceleration have no effect at all on compass indications at any heading, including east and west.
• accelerating on an easterly or westerly heading causes the compass to indicate a turn toward north, and decelerating causes it to indicate a turn toward south, even though the heading is unchanged.
• acceleration error occurs only while turning, never during straight and level flight on a constant heading, a description that has it backwards, since ANDS error is specifically a straight-and-level phenomenon near east and west.

Explanation: Because of the same dip-related pendulous mounting effect, an acceleration on an easterly or westerly heading causes a simple compass to falsely indicate a turn toward north, while a deceleration causes it to falsely indicate a turn toward south -- even though the aircraft's actual heading is unchanged -- which is summarized by the ANDS mnemonic (Accelerate North, Decelerate South) in the northern hemisphere.

TP 690 -- Navigation - General: The Magnetic Compass

A conformal map projection (such as a Lambert Conformal Conic or a Mercator projection) is distinguished from an equal-area projection in that a conformal projection

• preserves both angles and area perfectly everywhere on the chart, with no distortion of any kind introduced by either property.
• is defined by having no relationship whatsoever to either angular accuracy or areal accuracy, and instead only concerns chart colour scheme.
• preserves angles and local shapes accurately at any given point, generally at the cost of distorting the relative size of regions, particularly ones far apart or at very different latitudes.
• preserves the relative size (area) of every region on the chart accurately in all cases, while distorting angles and local shapes, a description that actually applies to an equal-area projection rather than to a conformal one.

Explanation: A conformal projection -- the property shared by both the Lambert Conformal Conic and the Mercator projection, which is why both are useful for aeronautical charts -- preserves angles and local shapes accurately at any given point (meaning a small area on the chart is shaped correctly relative to the real world), generally at the cost of distorting the relative size of regions, particularly ones far apart or at significantly different latitudes; an equal-area projection instead preserves relative size accurately across the chart, generally at the cost of distorting shape and angles.

TP 690 -- Navigation - General: Charts

WGS84, the geodetic datum used by GNSS and most modern aeronautical charts, models the Earth's shape as a mathematically defined ellipsoid rather than using the more complex, irregular 'geoid' shape (the Earth's actual mean sea level surface) directly, because

• the geoid and the ellipsoid are simply two different names describing the exact same, perfectly smooth shape, with no distinction between them, a characterization that does not match how this aspect of navigation is actually defined or applied in practice, which overlooks the geometric or procedural considerations that actually govern this navigation concept.
• the ellipsoid model is used only for oceanic navigation, with the geoid used exclusively for all overland aeronautical navigation instead, a characterization that does not match how this aspect of navigation is actually defined or applied in practice, which overlooks the geometric or procedural considerations that actually govern this navigation concept.
• an ellipsoid provides a smooth, mathematically simple approximation that is close enough to the Earth's actual shape for practical positioning purposes, while still allowing straightforward, consistent calculation, whereas the true geoid's irregular undulations are far more complex to compute with directly.
• WGS84 uses the geoid exclusively and makes no use of any ellipsoid approximation at all, a characterization that does not match how this aspect of navigation is actually defined or applied in practice, which overlooks the geometric or procedural considerations that actually govern this navigation concept.

Explanation: The geoid represents the Earth's actual, irregular equipotential surface (closely approximating mean sea level, and affected by local variations in gravity and mass distribution), which is far too complex to use directly for straightforward mathematical positioning calculations; WGS84 instead models the Earth as a smooth, mathematically defined ellipsoid that closely approximates the geoid's overall shape, providing a practical balance between accuracy and computational simplicity for GNSS positioning and modern charting.

TP 690 -- Navigation - General: Geodesy