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When tracking outbound, however,
you want to end up with the needle
on the same side as the wind, so,
although you are still looking for the
plus 20, minus 20 equation, the
needle would be pointing at 160°
RB. When you make your initial
turn, the needle looks like it's going
the wrong way, but it's something
you get used to.
In short, if the pointy end of the
needle moves to the right of a line
between 0° and 180°, fly right, as
drift is to the left, and vice versa.
DME
Distance Measuring Equipment is also
UHF-based, between 962 and 1213
MHz. It is actually secondary radar,
which measures the time difference
between paired pulses being sent
from the aircraft, and being received
back on different frequencies, 63
MHz away (there are 126 DME
channels). In other words, the
aircraft is the first to transmit on
UHF, then the DME transmitter
returns the signal, plus 63 MHz.
Two frequencies are used because,
otherwise, the first pulse received
would be the ground return from
below the aircraft. Similarly, the
ground station could self-trigger
from other sources, such as those
being bounced off a building. Jittering
is used on the PRF so the DME's
own pulse can be identified.
Instruments in the cockpit will not
only show your distance to a station,
but will calculate the rate of
movement and display the
groundspeed (just multiply the
distance flown in 6 minutes by 10 if
yours doesn't). It is normally based
with a VOR or TACAN and has a
range of about 200 nm, ± 6, with an
accuracy better than ½ nm or 3% of
the distance, whichever is the
greater. The reason it's not
completely accurate is because the
distance measured is the slant range
132 JAR Private Pilot Studies
from the station, and not from your
equivalent position on the ground,
although at long distances and lower
altitudes, this will be minimised.
GPS
The Global Positioning System was
originally set up by the US military,
using 24 satellites orbiting every 12
hours to give extreme accuracy at a
very much reduced cost compared
to, say, INS. Although they have
guaranteed to keep the system
running for the foreseeable future, in
National Emergencies it may be
unavailable, which is why it is still
not acceptable as a sole means of
navigation without traditional
systems as backup (that is, it can be
used as a primary aid in some areas).
It will give altitude information as
well as location, but 3D readouts
require 4 satellites. Selective
Availability, where there was a
deliberate fudging of the signal (by
dithering the clock signals) to make
it less accurate for non-military
receivers, is now inoperative.
However, for exam purposes, C/A
(Coarse Acquisition) Code is made up
from civilian signals on the L1
frequency, which is 1575.42 MHz. P
(Precise) Code is for the military and
broadcast on L1 and L2. Without
C/A, accuracy is to 30m, otherwise
it is 100m for about 95% of the
time. Differential GPS uses a ground
station within 70 nm to increase
accuracy to within 1-3 metres.
Satellites are kept in line by an
atomic clock at a ground master
station, and aircraft use crystal
oscillators. Code matching with a time
delay removes clock drift, which is the
system's major error.
At least 5 satellites should be visible
at any point over the Earth at any
time, though you could get a
problem flying through the odd
ravine way up North. The readings
are referenced to a mathematical
model (that is, an imaginary grid
system), the WGS-84 ellipsoid; they
can be converted to other models
inside most receivers.
The mask angle is the lowest angle
above the horizon from where a
satellite can be used. Signals from it
contain the time (from four atomic
clocks) and its position, plus error
correction. From this, the pseudo-range
of the satellite can be computed,
which is called that because it was
not directly measured, but calculated,
as a result of which it is subject to
error, particularly delays as signals
pass through the ionosphere.
For each satellite involved, the
pseudo range is added to the
ephemeris (or exact position in space),
and triangulation used to figure out
the receiver's position. As mentioned
above, you need 4 satellites for a 3D
position, which includes height
above Mean Sea Level, in other
words altitude. Such height readouts
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