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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
Electricity & Radio 295
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 NAD83
model used for mapping in Canada
has the same properties, so GPS
positions can be related directly to
Canadian Aviation Chart positions).
Refer also to GEN 1.5.2 of the AIP.
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
should not be used for navigation by
themselves, as they are referenced to
the WGS-84 spheroid, so the current
altimeter setting should be put into
the receiver if you want to use them
(the techies at Garmin say the term
Mean Sea Level is used generically, as
the difference is close enough with
the accuracy available).
RAIM stands for Receiver Autonomous
Integrity Monitoring, for making sure
satellites are working properly, which
needs an extra satellite. For the bad
signal to be isolated, you need one
more than that (in fact, the 5th
satellite is for checking errors, while
the 6th is a stand-in).
Although it is tempting to use GPS
all the time, remember that it is
electrical, and therefore reserves the
right to go offline at any moment,
without warning. The antenna in a
GPS is live as well, and equally liable
to stop working.
Use it, by all means, but you should
always have an idea of your
approximate position, just in case it
fails, or you go out of satellite range,
so you don't get the embarrassment
of having to explain to your
customer just why you are landing to
find out where the map is. It's just
part of being a professional.
A GPS may have a database of
airspace and frequencies inside –
although not so important for VFR
use when you carry a CFS, it is still
the mark of a professional to keep it
up to date.
Errors
· Satellite clock drift
· Ephemeris (position)
· Propagation delay
· Receiver noise
· Multi-path reflection
· C/A Selective Availability
ILS
This is a precision approach system
using a VHF transmitter for
horizontal guidance (the localiser), and
a UHF one for vertical, which
usually produces a 3° glideslope with a
1.4° wide beam. The impression
given is that two narrowly focussed
296 Canadian Professional Pilot Studies
beams intersect to provide the
guidance but, in fact, the "beams"
are created electronically by the
equipment in the aircraft. This means
you can get on-course or on-glidepath
indications regardless of your position, as
was found by an Air New Zealand
767 in July 2000, which got down to
400', 6 miles short of the runway
(check your distance and altitudes,
and do not use equipment when it is
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