Air Navigation is an art of taking an aircraft from one place to another by shortest and safest route. Air Navigation compared to road navigation is different because there are various factors which affect an aircraft operating in three dimensions. An air navigator is confronted with three questions in the air:
- Where am I ?
- Why am I here ? and
- What to do next ?
We study the subject of Air Navigation to find answers to the above three questions.
Shape of Earth:
The earth is not a true sphere. It is flattened at two diametrically opposite ends.
The shape of the earth is called:
- a) Ellipsoid – A solid structure from an ellipse
- b) Oblate spheroid – A structure obtained by rotating an ellipse about its minor axis
- c) Geoid – Earth like
Compression: Flattening of earth at two ends is called Compression.
Equatorial Diameter – Polar Diameter ⁄ Equatorial Diameter = 1/297 or say = 1/300
P.S: Equatorial Diameter = 6884 nm, 7926 sm, 12784.6 km
Polar Diameter = 6861 nm, 7900 sm, 12705.6 km
For all practical purposes the earth is considered as a Perfect Sphere.
Axis: The earth is rotating about an imaginary axis which passes through the centre of the earth and two diametrically opposite compressed ends. Earth’s axis defines North and South Direction.
Poles: The points where the imaginary axis cut the earth are called poles.
North Pole: The pole overlooking which the earth appears to rotate anti-clockwise is called North Pole.
South Pole: The pole overlooking which earth appears to rotate clockwise is called South Pole.
EAST: The direction towards which the earth is rotating
WEST: The direction from which the earth is rotating
A circle on the surface of the earth whose centre and radius are same as that of the earth.
Properties of Great Circle:
1) Any number of great circles can be drawn through two points which are diametrically opposite.
2) Only one great circle can be drawn through two points which are not diametrically opposite.
3) Only one great circle can be drawn through two diametrically opposite points and a third point.
4) The smaller arc of the great circle provides the shortest distance between two places.
5) The plane of a great circle divides earth into two equal halves.
6) Radio waves travel along the great circle path on the surface of the earth.
7) Equator is a great circle.
8) Meridians are semi-great circles.
9) The meridians cut the equator at a constant angle of 90º.
Elsewhere a G/C cuts the meridians at changing angles. To fly the shortest distance between two places we must fly the G/C track, but the changing angles make such a flight quite difficult unless flying due N/S along a meridian.
10) A great circle provides the greatest circumference.
Equator: A great circle whose plane is at right angles to the earth’s axis.
- – It is equidistant from the poles
– It divides earth into Northern and Southern hemi-sphere
– Plane of equator lies in East-West direction
Meridian: A semi great circle touching North and South pole.
– It indicates North and South Direction
– Every point on the earth has its own meridian passing through it
Greenwich Meridian: A meridian passing through Greenwich in UK.
– It is also called zero meridian or prime meridian
– Greenwich anti-meridian, is the complementary meridian of the Greenwich Meridian
– The angle between Greenwich Meridian and its anti-meridian is divided into 180º.
Standard Meridian: A meridian nominated by a state as its standard meridian. It is generally separated by 15º or its multiples from Greenwich Meridian.
Longitude: The smaller arc of the equator intercepted between Greenwich meridian and meridian of the place, expressed in degrees and is said to be East or West depending upon whether the place is to the East or West of Greenwich.
– It is expressed in maximum 180º East or West.
– It may also be defined as angle subtended at the centre of the earth by the smaller arc of equator intercepted between the Greenwich meridian and meridian of the place and is said to be East or West depending upon whether the place is East or West of Greenwich.
Change of Longitude (Ch long or D long):It is the smaller arc of equator intercepted between meridians of two places and expressed in degrees.
- It is said to be East or West Ch long depending upon whether the change is towards East or West of the reference meridian.
- It may also be defined as angle subtended at the centre of the earth by the smaller arc of equator intercepted between
Small Circle: Any circle on the surface of the earth whose centre is not same as that of the earth.
– Any circle which is not a great circle.
Latitude: Arc of a meridian intercepted between equator and a place, expressed in degrees North or South depending upon whether the place is North or South of the equator.
– It may also be defined as angle subtended at the centre of the earth by the arc of meridian intercepted between equator and a place and said to be North or South depending upon whether the place is to the North or South of equator.
– It is measured 90º North or South of equator because the equator has the largest circumference.
Parallel of Latitude: A small circle whose plane is parallel to the plane of equator is called parallel of latitude
– All the places on the same parallel of latitude have the same latitude
– The parallels of latitude run in East-West direction
– Parallel of latitudes cut the meridians at right angle
– Parallels of latitudes are Rhumb lines
Change of Latitude (Ch lat or d lat): It is the arc of meridian intercepted between parallels of latitude of two places.
– It is the said to be North or South depending upon whether the change is towards North or South with respect to place of reference.
Rhumb Line: A regularly curved line, on the surface of the earth, which cuts all meridians at the same angle.
– All meridians are Rhumb lines.
– Equator and parallels of latitude are Rhumb lines
– Only one rhumb line can be drawn through any two points
– A rhumb line does not represent the shortest distance between two places, but it is convenient to follow.
– Rhumb line is used for single heading flying.
Position Report: The position of the place is reported by:
- a) Latitude and longitude of the place; example Bangkok 1359N 10039E.
- b) Bearing and distance from a known reference point, for example 040 Lucknow 25 ie. 040º from Lucknow at a range of 25 nm.
- Over a prominent place: example ‘ Overflying Lucknow’.
Graticule: The combination of parallels of latitude and longitude is called graticule.