No
7
Subjects:
ADF: Identifying Tone
Antennas:
GPS Specifications
Databuses: ARINC-429 and Flight
Management
Autopilots: 2- and 3-Axis
ADF
Identifying Tone
Q. Are there any NDB (Non-Directional Beacon)
stations that still use 400 Hz tones for the Morse
Code identifier? ICAO (International Civil
Aviation Associaton) states that in addition
to 1020 Hz tones, 400 Hz may also be
used.
A.
The ICAO world standard does specify that an
NDB audio ident may be either 1020 or 400
Hz. But in many years of flying and tuning
hundreds of NDB's we've never heard a 400 Hz
tone. About the only navaid with 400 Hz
today is for an ILS Outer Marker. It
could be difficult to hear 400 Hz on
an ADF because low-frequency receivers are
prone to atmospheric noise in bad weather (when
they're needed most). Also, the audio of an
inexpensive ADF receiver is often corrupted by a
"chopping" noise, which is a byproduct
of the way an ADF receiver processes signals.
The reason for the odd audio
frequency---1020 Hz---dates back to when
ground stations generated tones with reference to
an electric motor. A common motor speed is
1800 RPM, which is also 30 revolutions per second,
or 30 Hz. Multiply that by 34 and the result
is 1020. Other navaids which use 30 Hz
multiples are the Localizer ident (1020 Hz)
and DME (1350 Hz, or 30 x 45). VOR ground
stations use these multiples three
times; 1020 for the audio ident and for two
other signals which contain bearing information (a
9960 Hz subcarrier, which is 332 x 30) and for 30
Hz modulation.
The future of audio idents for
navaids is not bright. A GPS receiver
doesn't even have an audio output! Imagine trying
to ID the Morse Code from six
satellites every ten minutes. The GPS manufacturer
thoughtfully put all that function on the chip.
Antennas
GPS Specifications
Q.
How do you select a GPS antenna? On what
parameters are the specifications based? I've
heard there are active and passive GPS
antennas. What is the difference? What
is a circularly polarized GPS antenna? (From a
reader in India.)
A. First, how
do you select a GPS antenna? Most often you
don't because it is supplied by the GPS receiver
manufacturer. Unless you have a large
aircraft, with a very long run for the antenna
cable, most installations can be done with the
coaxial cable and antenna supplied.
What are the
specifications for a GPS antenna? Basically,
they are L-band antennas, meaning they tune
to GPS satellite frequencies on 1575.42 MHz
(also known as L1) and 1227.6 MHz (L2).
The terms
"active" and "passive"
indicate whether the GPS antenna has a
self-contained amplifier inside the antenna
housing.. More properly called a
"preamplifier," it is used when wiring
runs are long and signals might be lost in the
transmission line. Most light aircraft,
however, have short wiring runs (usually from the
top of the cabin, down a door post and behind the
instrument panel) and don't require the extra
boost of the preamplifier. If a preamplifier
is used, it usually doesn't need extra wires to
deliver DC power to the antenna. Voltage is
sent up the same coaxial cable which brings
the signal down to the GPS receiver.
All GPS antennas have
"circular polarization, which is one of three
basic types; the other two are
"vertical" and
"horizontal". Polarization is
selected according to how the antenna will
function. Antennas for communications,
DME and transponder, for example, are vertically
polarized; ADF, marker beacon and VOR
antennas are horizontally polarized. The choice
depends on how the radio wave is launched from the
transmitting antenna (in the vertical or
horizontal plane) and physical
considerations of mounting different antenna types
on the aircraft.
Circular
polarization---a combination of horizontal and
vertical---is essential for GPS because the
transmitting stations are not fixed in
place. Because they are aboard satellites
circling the earth every two hours, travelling at
11,000 miles per hour, the airplane must pick
up signals from three or more satellites that can
be almost anywhere in the sky----directly overhead
to low on the horizon. Circular polarization
assures maximum signal strength under these
dynamic conditions.
Databuses
ARINC-429
& FLight Management
Q. Can
you describe the operation of the
ARINC-429 databus and how it is related to a
Flight Management System? (From an
avionics technician student in New York City.)
A. ARINC 429 was adopted
by the airlines about 25 years ago for their first
generation of digital airliners, the Boeing
757/767. The databus had several
purposes: to reduce the number of wires,
reduce weight, reduce space, and to act as an
interface between systems. Before ARINC-429,
avionics and instruments
produced signals of varying voltages,
currents, frequencies and waveforms. That
was no problem in earlier days because there was
little need for systems to "talk" to
each other. But the microprocessor
revolution changed all that. By gathering
signals from many systems and converting them to a
standardized digital form, the airplane could now
compute many new useful functions, for
example; blend all navigation sources (VOR, DME, GPS,
etc) for the most accurate solution, generate
electronic displays to depict flight instruments,
produce moving maps, head-up displays and others.
This
also made possible the Flight Management System,
which enables a pilot to enter a trip
(waypoints, altitude, destination, etc.) and fly
it most efficiently with regard to time and fuel.
To compute the best flight path, the FMS is
interconnected with the flight control computer
(autopilot), the thrust (engine) management
computer and cockpit displays and warnings.
Sharing all this data would be virtually
impossible under the old analog system, but is
greatly simplified by converting all signals to an
ARINC-429 format.
Information is transmitted on an ARINC- 429 bus in
32-bit long "words" which contain
identifying labels and data. Speed of the bus is
either 12-14.5 kilobits/second or 100
kilobits/second. The higher speed is
for data which is rapidly changing, such as in the
flight control system, or where large amounts of
data need to be transmitted.
ARINC-429 has worked very well, especially in how
it simplifies the addition of the many new
electronic systems being installed aboard
aircraft. Its technology, however,
is aging. ARINC-429 is a simplex bus,
meaning it cannot mix signals of
various systems into one wire. A separate
cable, therefore, is needed from each signal
source directly to the system(s) it talks
to. Also, the bus is slow by
today's standards.
These problems have been largely
solved in 429's replacement; ARINC-629, now
installed on the Boeing 777. The new databus
is much faster (2 megabits/second) and affords
much greater reduction in wiring because it is a multiplex
databus. Instead of signals traveling
on separate cables, they are combined into a
single wire. (They are kept apart by a timing
system which inserts data into unoccupied time
slots.)
For a
complete description of ARINC-429 and other
avionics databuses, see the book,
"Principles
of Avionics Databuses".
Autopilots:
2- and 3-axis
Q.
Can you clear up the confusion between a
2- and 3-axis autopilot?""
A.
The problem arises because most of us
learned that an airplane maneuvers around three
axes; pitch, roll and yaw. These are
controlled by elevator, aileron and rudder.
But autopilots often control other things and
call them "axis".
For example, some say that a
3-axis autopilot controls roll, heading and
altitude. But a closer look reveals that roll and
heading are both controlled in the roll axis. The
autopilot operates the aileron to both roll the
aircraft (to keep wings level, for example) or
turn the aircraft to, and hold, a desired
heading.
Altitude hold is accomplished by
controlling the second axis, pitch.
Control of the third axis, yaw, is
usually found in higher performance aircraft,
especially turbojets. Its main function is to
continuously operate the rudder to neutralize
small yaw changes which cause
"fishtailing", and discomfort to the
passengers.
In evaluating an autopilot, therefore, it's
helpful to clearly define the desired
functions. The following categories, for
example, are used by one manufacturer, S-Tec:
Single
Axis Autopilot - Roll
Single Axis Autopilot - Pitch
Two-Axis Autopilot - Roll and Pitch
Yaw Damper
Optional Equipment is then added, such as altitude
alerter, automatic electric trim, remote
annunciator, single-cue flight director.etc.
