The
principles of air navigation are the same for all aircraft, big or small. Air navigation involves successfully piloting an
aircraft from place to place without getting lost, breaking the laws applying
to aircraft, or endangering the safety of those on board or on the ground.
Air
navigation differs from the navigation of surface craft in several ways:
Aircraft travel at relatively high speeds, leaving less time to calculate their
position en route. Aircraft normally cannot stop in mid-air to ascertain their
position at leisure. Aircraft are safety-limited by the amount of fuel they can
carry; a surface vehicle can usually get lost, run out of fuel, then simply
await rescue. There is no in-flight rescue for most aircraft. And collisions
with obstructions are usually fatal. Therefore, constant awareness of position
is critical for aircraft pilots.
Navigation Techniques:
Good pilots use all means
available to help navigate. Many GA aircraft are fitted with a variety of radio navigation aids, such as Automatic direction finder (ADF) or non-directional
beacons (NDBs),
VHF Omni directional range (VOR) and satellites.
Automatic Direction Finder (ADF)
or non-directional Beacons (NDBs):
ADF
uses NDBs on the ground to drive a display which shows the
direction of the beacon from the aircraft. The pilot may use this bearing to
draw a line on the map to show the bearing from the beacon. By using a second
beacon, two lines may be drawn to locate the aircraft at the intersection of
the lines. This is called a cross-cut. Alternatively, if the track takes the flight
directly overhead a beacn, the pilot can use the ADF instrument to maintain
heading relative to the beacon, though "following the needle" is bad
practice, especially in the presence of a strong cross wind - the pilot's
actual track will spiral in towards the beacon, not what was intended. NDBs
also can give erroneous readings because they use very long wavelengths, which are easily bent and reflected by ground features and the
atmosphere. NDBs continue to be used as a common form of navigation in some
countries with relatively few navigational aids. In Pakistan NDB operates at
190 ± 525 KHz.
VHF Omni Directional Range (VOR):
VOR is a more sophisticated system, and is still the
primary air navigation system established for aircraft flying under IFR in
those countries with many navigational aids. In this system, a beacon emits a
specially modulated signal which consists of two sine waves which
are out of phase. The phase difference corresponds to the actual
bearing relative to true north that the receiver is from the station. The
upshot is that the receiver can determine with certainty the exact bearing from
the station. Again, a cross-cut is used to pinpoint the location. Many VOR
stations also have additional equipment called DME ( distance
measuring equipment) which will allow
a suitable receiver to determine the exact distance from the station. Together
with the bearing, this allows an exact position to be determined from a single
beacon alone. For convenience, some VOR stations also transmit local weather
information which the pilot can listen in to, perhaps generated by an Automated
Surface Observing System.
There
are two main types of VORs in operation (i.e. two types of actual ground
installation) but the aircraft receiver is the same for both. The receiver is
unaware which type of ground station is in use - it experiences the same
effects from both. It's the method of creation that differs.
The C VOR employs a rotating directional antenna.
Consider for a moment a directional antenna which has a transmission pattern of
one broad peak and one broad null in the horizontal plane. If we were to feed
this antenna with a VHF carrier and also rotate the antenna at 30 revs/second
(1800RPM) - think of how an AM receiver would view this rotation. An AM
receiver would see carrier amplitude modulated by a sine wave of 30Hz - the
phase of which would be determined by the receiver's position around the
station. A practical C VOR doesn't actually spin the aerial at 1800 RPM (although
the earliest ones did) - it uses electronic switching of an aerial array to
achieve the effect. Additionally, the reference signal is transmitted by FM
modulating it onto a 9960 Hz sub carrier (deviation +/-480Hz). The reference
signal provides the receiver its comparison to station north. The receiver
compares the AM 30Hz variable phase signal recovered with the decoded 30Hz
reference signal from the FM sub carrier and determines the radial position from
North on which the receiver lies.
The D VOR is a later and improved design
of VOR which suffers less from sitting errors. The C VOR requires a clear area
of at least 1500 ft in radius. The D VOR is more practical in crowded areas or
where there are tall buildings. However, it's a big structure around 100ft in
diameter. The D VOR reverses the usages of the two 30Hz signals. However, by
also reversing the direction of its rotating variable signal it produces
exactly the same result in the receiver. The receiver has no
"knowledge" that it's a D VOR as opposed to C VOR it's receiving and
operates as normal. In the D VOR the main VHF carrier is AM modulated at 30 Hz
providing the Reference signal. This is transmitted from a central
omnidirectional antenna and has the same phase all around the VOR for any
receiver.
