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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
Electricity & Radio 133
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 Pooley's, 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
Radar
The use of radar improves aircraft
spacing and improves safety - the
word stands for Radio Direction and
Ranging. It works on the basis that
radio pulses will reflect off objects
and a proportion of the energy will
return to the transmitter. You can
calculate the distance between them
because the speed of the radio wave
is known, and the direction the aerial
was pointing in at the time will
supply the bearing. The “blips”
representing the objects are
displayed on a Cathode Ray Tube (TV
screen) and an air traffic controller
can see the relative positions of
aircraft reflecting pulses:
The word pulses, mentioned above, is
the operative word – short bursts of
electromagnetic energy are mixed
with relatively long periods of silence
(in electronic terms, this means
somewhat less then a thousandth of
a second).
This is known as primary radar, and it
has a few limitations. First of all,
radio signals weaken over distance
and, since they have to make two
journeys, the range of a target is
necessarily limited. Secondly, the blip
is quite large, and aircraft very close
together cannot be distinguished.
Finally, radio waves can be bent by
the atmosphere or screened by
objects, such as mountains or
buildings, and different aircraft
return signals differently, in terms of
shape or surface.
134 JAR Private Pilot Studies
Secondary Surveillance Radar
This is a development of a system
introduced during the Second World
War called Identification Friend or Foe
(IFF), which was supposed to
distinguish between friendly and
enemy aircraft (friendly aircraft had a
small transmitter that gave a
distinctive periodic elongation to the
blip on the screen).
SSR improves on the primary radar
mentioned above by using
secondary equipment to provide
more information, hence the name.
Participating aircraft carry a
transponder (which stands for
transmitter/responder) that receives the
interrogation pulse from the transmitter,
superimposes information on it and
sends it right back on another paired
frequency. This means, first of all,
that the range of operation can be
doubled immediately, and, secondly,
that the blip on the screen can be
made much smaller, together with
information that makes it more
easily identifiable to ATC, because
the pulses can be coded. Computer
trickery can provide predicted tracks
and collision warnings, amongst
other things.
A controller for example, will give
you a number to squawk, which you
dial up on the transponder in your
aircraft, and which will appear next
to your blip with a height readout,
depending on the transponder.
There are standard numbers to
squawk, when not otherwise
　
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