20-04-2013, 03:27 PM
GPSfrom the Ground Up
[attachment=53220]
INTRODUCTION
With each passing year, GPS becomes a more integral
part of the general aviation (GA) cockpit. As displays get
larger, and as more functionality is added, GPS is starting
to bridge the gap between traditional avionics “boxes”
like VOR and ADF and the integrated flight management
systems found in airliners and corporate jets.
The result, paradoxically, is that GPS receivers themselves—
rather than becoming more prominent—are
fading into the background. Why? Because when all is
said and done, a GPS is a “black box” that generates
a latitude, longitude, altitude, direction, and speed.
What pilots are usually talking about when they use
the term “GPS” is not the receiver itself, but all the peripheral
hardware and software that’s wrapped around
it. And as the avionics industry has demonstrated,
there are few limits on the shape and sophistication of
that hardware and software.
An analogy: In 1975, computers were expensive and
exotic. Today they’re cheap, and no one thinks much
about them because they’re everywhere—hidden in
everything from car engines to refrigerators.
GPS is following a similar path, which is one reason we
won’t spend much time talking about how it actually
works. What we will talk about are ways to get the most
utility you can from it while ensuring that it remains a
tool that works for you—not the other way around.
Fundamentals
The technology of GPS is complex, but the basic
concept behind it is simple. A receiver pulls in signals
from multiple satellites, determines how long the signals
took to arrive, and uses that information to triangulate
its own position. If it can “see” three satellites, it
can figure out where it is over the ground. If it can see
at least four, it can determine its altitude as well.
Route Planning
For many pilots, one of the most attractive features of
GPS is direct, airport-to-airport navigation. That’s great
in theory, but in practice there’s more to it than punching
“Direct” and following the active course line. For
one thing, in the congested airspace over many parts of
the country, you’re not likely to be cleared “direct” on a
flight of any length. For another, special use airspace often
interferes with direct flights.
On the bright side, there’s no reason you can’t file a flight
plan that follows airways until clear of obstacles and goes
direct thereafter. Even so, it’s worth noting that in most
cases the time/distance savings of going direct rather than
via airways is smaller than you’d imagine.
Databases, Charts, and Equipment
There are a few other things to keep in mind as you prepare
for a flight with GPS. First, the navigational database:
For VFR flights (or if using the receiver for situational
awareness under IFR), there’s no requirement to have
a current database, but be wary of possible changes to
airports, airspace, and obstructions. In other words, bring
appropriate charts, and use them!
IFR is a little different. Databases are updated on a 28-
day cycle, and although the regulations don’t specifically
require a current database, FAR 91.103 requires
pilots to be familiar with “all available information”
pertinent to the flight. For that reason, it’s conceivable
that failing to update the database could lead to
trouble with the FAA. However, pilots are allowed to
use an expired database for IFR en route and terminal
(not approach) operations, so long as they verify that
the data is still correct.
Programming and Taxi
Time is money, so the old saying goes, and nowhere is
that more true than in aviation. None of us like to waste
time on the ground, but don’t get impatient: Get your
clearance and program the route into the GPS before
you taxi out (and preferably before starting the engine).
In a moving aircraft, all the button pushing and knob
twisting of flight plan entry is just too much of a distraction.
More than one runway incursion has been caused
by a “heads-down” pilot preoccupied with the GPS.
Arrival and Approach
As you near the destination airport, the GPS automatically
cycles back to terminal mode and full CDI deflection
decreases from either two or five miles to one mile.
Now the main consideration is getting ready for the approach
(if one is necessary). Approaches are like “mini
flight plans” stored in the databases of IFR-certified
receivers. Both RNAV and non-RNAV approaches are
included. If you’re flying a non-RNAV approach (an ILS,
for example), the receiver can perform backup navigation
and situational awareness duties, but remember
that the traditional navigation system should still be the
primary reference.
Once an approach is selected, the receiver will ask you
to choose either a specific transition (one of the initial
approach fixes), or “vectors to final.” Choosing vectors
to final basically puts the receiver in “hold” mode (i.e.,
turns autosequencing off) until the aircraft intercepts the
final approach course. Unless ATC has told you to expect
vectors, select the most likely transition for your direction
of arrival. This can be changed later if necessary—assuming
you know how to reprogram the receiver.
[attachment=53220]
INTRODUCTION
With each passing year, GPS becomes a more integral
part of the general aviation (GA) cockpit. As displays get
larger, and as more functionality is added, GPS is starting
to bridge the gap between traditional avionics “boxes”
like VOR and ADF and the integrated flight management
systems found in airliners and corporate jets.
The result, paradoxically, is that GPS receivers themselves—
rather than becoming more prominent—are
fading into the background. Why? Because when all is
said and done, a GPS is a “black box” that generates
a latitude, longitude, altitude, direction, and speed.
What pilots are usually talking about when they use
the term “GPS” is not the receiver itself, but all the peripheral
hardware and software that’s wrapped around
it. And as the avionics industry has demonstrated,
there are few limits on the shape and sophistication of
that hardware and software.
An analogy: In 1975, computers were expensive and
exotic. Today they’re cheap, and no one thinks much
about them because they’re everywhere—hidden in
everything from car engines to refrigerators.
GPS is following a similar path, which is one reason we
won’t spend much time talking about how it actually
works. What we will talk about are ways to get the most
utility you can from it while ensuring that it remains a
tool that works for you—not the other way around.
Fundamentals
The technology of GPS is complex, but the basic
concept behind it is simple. A receiver pulls in signals
from multiple satellites, determines how long the signals
took to arrive, and uses that information to triangulate
its own position. If it can “see” three satellites, it
can figure out where it is over the ground. If it can see
at least four, it can determine its altitude as well.
Route Planning
For many pilots, one of the most attractive features of
GPS is direct, airport-to-airport navigation. That’s great
in theory, but in practice there’s more to it than punching
“Direct” and following the active course line. For
one thing, in the congested airspace over many parts of
the country, you’re not likely to be cleared “direct” on a
flight of any length. For another, special use airspace often
interferes with direct flights.
On the bright side, there’s no reason you can’t file a flight
plan that follows airways until clear of obstacles and goes
direct thereafter. Even so, it’s worth noting that in most
cases the time/distance savings of going direct rather than
via airways is smaller than you’d imagine.
Databases, Charts, and Equipment
There are a few other things to keep in mind as you prepare
for a flight with GPS. First, the navigational database:
For VFR flights (or if using the receiver for situational
awareness under IFR), there’s no requirement to have
a current database, but be wary of possible changes to
airports, airspace, and obstructions. In other words, bring
appropriate charts, and use them!
IFR is a little different. Databases are updated on a 28-
day cycle, and although the regulations don’t specifically
require a current database, FAR 91.103 requires
pilots to be familiar with “all available information”
pertinent to the flight. For that reason, it’s conceivable
that failing to update the database could lead to
trouble with the FAA. However, pilots are allowed to
use an expired database for IFR en route and terminal
(not approach) operations, so long as they verify that
the data is still correct.
Programming and Taxi
Time is money, so the old saying goes, and nowhere is
that more true than in aviation. None of us like to waste
time on the ground, but don’t get impatient: Get your
clearance and program the route into the GPS before
you taxi out (and preferably before starting the engine).
In a moving aircraft, all the button pushing and knob
twisting of flight plan entry is just too much of a distraction.
More than one runway incursion has been caused
by a “heads-down” pilot preoccupied with the GPS.
Arrival and Approach
As you near the destination airport, the GPS automatically
cycles back to terminal mode and full CDI deflection
decreases from either two or five miles to one mile.
Now the main consideration is getting ready for the approach
(if one is necessary). Approaches are like “mini
flight plans” stored in the databases of IFR-certified
receivers. Both RNAV and non-RNAV approaches are
included. If you’re flying a non-RNAV approach (an ILS,
for example), the receiver can perform backup navigation
and situational awareness duties, but remember
that the traditional navigation system should still be the
primary reference.
Once an approach is selected, the receiver will ask you
to choose either a specific transition (one of the initial
approach fixes), or “vectors to final.” Choosing vectors
to final basically puts the receiver in “hold” mode (i.e.,
turns autosequencing off) until the aircraft intercepts the
final approach course. Unless ATC has told you to expect
vectors, select the most likely transition for your direction
of arrival. This can be changed later if necessary—assuming
you know how to reprogram the receiver.