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TitleNavigation Notes
Tags Longitude Orbit Latitude Equator
File Size426.0 KB
Total Pages50
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Page 2

Rhumb Line : A line which cuts all meridians at equal angles. It spirals to the poles at angles less than
90°. ATS routes are rhumb line tracks. Equator is a RL as well as GC. Parallels of Lat are RL tracks.
Meridians are GC track (Flying a constant direction).

Direction. It is measured with respect to the North, clockwise. All meridians point to North.

True Direction. A direction which is measured with respect to True North the symbol is and is
annotated as 005 T or 005 (T).The geographical north is not aligned with the magnetic earth since the
earth behaves as a bar magnet with its axis slightly aligned from the geographical axis. The exact
position of North an South pole with respect to the bar magnet is defined and known, however the
position of magnetic north depends on where the observation is taken from vis a vis the position of True
North.

Magnetic North. It is the direction which points to the magnetic north. The angular difference between
the True North and Magnetic North is known as Variation and is denoted as E or W depending on
whether the Magnetic North lies east or west of true North. Simply, if Magnetic North lies East of True
North the Variation is East and if Mag N lies W of True North Var is West. Thumb Rule - Var E Mag
Least, Var W Mag Best. It is applicable in any hemisphere.

The symbol for Magnetic north is and is denoted by 045 M or 045 (M).

Isogonal : Lines joining places of equal Variation. Agonal : Lines joining places of zero Variation.

Compass North. The direction measured wrt to Compass North is called Compass direction.

The symbol for Compass North is and is denoted as 005 (C) or 005 C.

Deviation. It is the angular difference between Magnetic North and Compass North and is termed as
Easterly if Compass North lies East of Magnetic North and Westerly if Compass North lies West of
Magnetic North. Deviation is obtained from Compass Card and it varies from aircraft to aircraft due to
inherent magnetic fields present in the ac and is different for different headings. Thumb Rule - Dev E
Com Least, Dev W Comp Best. It is applicable in any hemisphere.

Any line drawn on a map represents a True Track.

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Page 49

GPS

It utilises the range obtained from a constellation of satellites to fix position of an aircraft in the air or on
ground. It is a satellite based navigational aid.

Basic Data.

Total number of Satellites : 21+ 3 (stand by) =24.
Number of Orbital Paths : 06.
Inclination of Orbital Path : 55° to the Equator
Distance : 20, 200 kms above the surface of the earth.
Time taken for each orbit : 12 hours.
Number of satellites required for 2D fix : 3
Number of satellites required for 3D fix : 4 (also for initialisation of the system)
Number of satellites required for RAIM (Remote Autonomous Integrity Monitoring) : 5
(This is similar to Built in Test Equipment BITE)
Frequency : L1 -1575 MHz and L2 -1228 MHz in UHF Band.

Principle. GPS transmits a PRN (Pseudo Random Noise) of 1 milli sec duration in UHF band. Each
satellite has its own unique code. The information contained in PRN code is

(a) Position of satellite (azimuth and angle)
(b) Clock Time
(c) Clock Error
(d) Information on ionospheric condition
(e) Supplementary information

Two codes are transmitted, CA (Coarse Acquisition) and P (Precision) Code. Two services are provided
by GPS. These are

(a) Standard Position Service (SPS) using CA code which is available for civil use.
(b) Precise Position Services (PPS) using CA & P Code.

Pseudo Range. The receiver has an accurate crystal oscillator to provide time, however, the accuracy
does not compare with that of the satellite clock. The receiver clock is deliberately kept in error by a
small factor to ensure that correction process takes place in one way only. The initial calculated range is
called Pseudo Range. For example if the receiver clock is 1 milli second fast the receiver will
overestimate the range by 162 Nm. Therefore when the receiver sets about calculating the correct range
it knows that it must reduce Pseudo Range.

Errors of GPS

Ephemeris Error. This error is due to the disturbed position of the satellite. Any deviation from its
orbital path will induce this error. This is caused due to various reasons e.g. debris or gravitational effect
of Sun, Moon and other planets. To obviate this error satellite position is checked every 12 hours and
when necessary it is updated. The max permissible error is 2.5 m.

Satellite Clock Error. The clock is also checked atleast every 12 hours. The max permissible error is
1.5 m.

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