Typical Instrument Panel
Figure 10-33 shows the captain’s
instrument panel in a Boeing 747. At the upper left is the indicated airs
peed/mach meter. Below it is a DME indicator, and below that is an RMI. The
attitude director indicator has a decision height light in the upper right
corner of the instrument and a test button in the lower left corner. The two
blank windows at the top of the horizontal situation indicator shows No. 1 and
No. 2 INS distance to go.
Below the HSI is the captain's
flight director computer selector. At the top right is a panel which contains
flight director and autopilot mode annunciation in the center. The warning
light to the left is used by the auto throttle system, and the warning light to
the right is used by the autopilot system. The circles above the warning lights
are photo-electric cells used to regulate the illumination intensity on this
panel.
At the middle left in the radio
altitude indicator is the index bug (white triangle), set by the decision
height selector knob at the bottom. The barometric altimeter is to the right of
the radio altitude indicator and to the right of that are the three marker
beacon lights. To the right of the vertical speed indicator is a 24-hour clock.
Figure 10-33
 |
RMI Pointer Signal Development
Figure 10-34 is concerned only with the development of the signal which
causes the RMI needle to point at the VOR station.
The VOR/RMI needle position is relative to the case of the
instrument. Regardless of what the compass card may show (compass section of
the RMI is operative or not), as long as the signal to the needle is correct,
it points at the station in the same way that an ADF needle points at its
received station.
Remember that in order for this needle to point at the
station, two kinds of information must be combined (Figure 10-5). One is the
receiver position relative to the VOR station (on which radial it is located),
the other is the airplane heading. The combining of these two information bits
is accomplished in the differential synchro in the VOR receiver.
Heading, from one of the repeated heading transmitters in
the compass system, is given to the stator of the differential synchro. The
position of the rotor of the differential synchro is a direct function of the
particular radial on which the receiver is located. So the output from the
differential synchro is the combination of heading and position information.
This output goes to the stator of the “free
swinging rotor synchro” in the RMI indicator to which the RMI needle is attached.
The rotor lines itself up with the magnetic field appearing in the stator. If
we keep the airplane in the same position while changing its heading, as we do
on a compass rose, the field in the stator of the differential synchro moves in
accordance with heading changes. This causes the RMI needle to move by the same
amount. If the RMI compass card is functioning. The card and the needle move
together.
If we maintain a constant airplane heading while changing
radials, as in flying by a VOR station off to one side of the airplane, the
heading signal does not change, but the differential synchro rotor moves,
causing the RMI needle to move around, pointing to the station all the time.
Compare this diagram with Figure 10-17. The differential
synchro rotor is positioned according to the VOR radial position of the
receiver. An aircraft heading signal from the compass system is present in the
stator of that synchro; the signal handed over to the RMI needle synchro is
therefore the algebraic sum of position and heading.
Only two things can change the position of the RMI needle
– change in airplane position, or a change in airplane heading.
Figure 10-34
RMI Indication When Passing VOR
Figure
10-35 illustrates some RMI indications. As the airplane changes its position
with respect to the VOR station, it simultaneously changes its heading. The
result of this combination of signals always results in the needle pointing at
the station.
|
RMI Indication Around VOR
Figure 10-36 shows a variety of airplane
situations and the corresponding RMI indications. In order to see the indicator
as the pilot sees it in his panel, rotate the indicator so that the lubber the
appears at the top.
|
HSI VOR Presentation Illustrations
The primary purpose of the following series of
illustrations is to demonstrate the Horizontal situation indicator “map-like”
presentation of airplane position relative to a selected VOR course. Here 0˚
has been selected as shown by the course select cursor at 0˚ (a localizer
presentation would be similar). In the HSI, airplane position is represented by
the airplane symbol fixed to the glass, while radio course is represented by
they deviation needle.
The hypothetical
“tear drop” flight path does not cross the “to-from” radials (90˚ and 270˚), so
the “to” arrow is always in view. Only when the airplane crosses the selected
course (0˚ or 180˚) does the deviation needle move to the other side of center
(reference Figures 10-20 to 10-22, and 10-28 to 10-32).
Validity of the HSI course indication relative to the airplane symbol
results from rotation of the inner mask in following the compass card movements
as the airplane turns. For example, in this series, when the airplane is not on
the radial it is always to its left of the radial, yet the deviation needle
crosses over carter when the airplane crosses the 180˚ radial.
The flight path section of these illustrations
is rotated each time as necessary to show the airplane headed toward the top of
the page. This technique facilitates the viewer’s correlation of airplane
flight position with instrument information.
The Distance Measuring
Equipment distance from the VOR station is shown in the upper left center of
the HIS. RMI needles always point at the station.
On
Selected Radial — "To" Arrow Points At Station
|
Leaving
Selected Radial At 35° Angle
– 3/4 Dot Deviation
Selected Radial Behind and to the Right – Deviation Needle pegged
Figure 10-39
 |
Parallel
To Selected Radial On The Right
Figure 10-40
|
Selected
Radial Ahead And The Right --
"To"
Arrow Does Not Point At Station
Figure 10-41
Two
Dots (10°)
Away From Selected Radial
Figure 10-42
On
Selected Radial – Deviation Needle Centered-
"To"
Arrow Points At Station
Figure 10-43
Two Dots Away From Selected Redial --
Deviation
Needle On Other Side Of Center
Figure 10-44
Selected
Radial Behind And To The Right--
"To"
Arrow Does Not Point At Station
Figure 10-45
Parallel To Selected Radial On The Right
(Reference
10-40)
Figure 10-46
Approaching
Radial At 28°
Angle – 1/2 Dot Deviation
Figure 10-47
On Selected Radial – "To" Arrow Points At Station
Figure 10-48
|
ATC
Transponder Principle
Figure
12-1 illustrates the ATC transponder principle.
The ATC is distinguished from the primary
surveillance radar. The primary surveillance radar, used by the air traffic control ground station, provides the
ground station operator with a symbol on his surveillance radar scope of every
aircraft in his area. The primary surveillance radar is a reflection type radar
system not requiring any response from the aircraft.
The primary and secondary surveillance radar antennas
are mounted on the same rotating mounting, and therefore both always look in
the same direction at the same time.
The secondary surveillance radar system uses what is
called an “ATC transponder” in the aircraft. The ATC transponder is a
transmitter/receiver which transmits in response to an interrogation from the
ground station secondary surveillance radar system. When no ATC reply is made
from the aircraft under surveillance, the indication on the ground radar scope
is a single short line, like the one at about 7 o’clock. If an ordinary ATC
reply is made, the aircraft indication on the radar scope is like the two close
to the sweep line.
The aircraft transponder reply can include a special
code which identifies that particular airplane on the scope. If the pilot
receives instructions from the ground station to do so, he presses his “ident”
button on his control panel. This causes the display on the radar scope to
change, so that the ground station operator can be positive of his particular
location on the radar scope. The transponder can also transmit aircraft altitude
information, which can then be displayed to the ground station operator.
The ATC transponder system is an outgrowth of the
war-time identification “friend or foe”, and all commercial aircraft presently
use it.
The Air traffic Control (ATC) system allows controllers
to track airplane movement on ground radar displays.
The ground station monitors the airplane’s altitude and
identification, and computes its range, bearing and airspeed.
The altitude and identification of the airplane is
transmitted to the ground station by an on-board transponder. This information
is sent in response to interrogation signals from the ground station.
`.
Figure 12-1
|
ATC Transponder System
Figure 12-5 shows the pilot’s control panel for the ATC system. The
switch in the lower left corner of the panel marked “altitude source” is a
two-position switch by which the pilot can select No. 1 or No. 2 central air
data computer to give altitude information to the ATC transponder. Two sets of
knobs of two each are below the “code” digital readout. Each knob controls one
digit. A total of 4,096 codes are available.
At the request of
the ground station, the pilot selects a particular code. The selector knob in
the upper left corner of the control panel selects the ATC system desired for
use. With the knob in “stand by” position, an otherwise long warm up can be
eliminated prior to actual use without putting the ATC system into full-time
operation.
The button marked
“ident” below the digital readout is the one which the pilot uses at the
request of the ground station for identification on the ground radar scope.
Each display is unique from other aircraft radar returns.
The upper right
switch is a mode select switch. Mode A is ordinary used for domestic operation
and mode B is ordinarily in Europe. Mode C puts
the transponder in condition for transmitting altitude information alternately
with code information (Figure 12-4). Since mode C is no longer used the
altitude information is transmitted by the aircraft transponder, if the altitude
report switch is on and either mode A or mode B is selected.
The ground station
transmits on 1,030 MHz. If the pilot has selected mode A (Figure 12-2), the
transponder will reply only to an interrogation which consists of two pulses
eight microseconds apart. If the pilot has selected mode B, the transponder
will reply only if the interrogation consists of two pulses separated by 17
microseconds.
|
Figure 12-2
ATC Transponder System (cont'd.)
Figure 12-4
shows a digitized altitude or code reply by the aircraft transponder. This
figure shows all 14 possible pulse positions
and the identification pulse. The first and the last are framing pulses and will always be present (there is never
a pulse in the middle open space).
The other pulses will or will not be
present in their allotted spaces, depending upon the altitude code
(submitted by the central air data computer), or by the aircraft code (submitted by the control panel). By the relative
timing of the aircraft transponder transmission pulse groups and
identification pulses, the ground station
distinguishes between altitude and code pulses.
Figure 12-3
shows a simplified block of the aircraft transponder system. A single antenna
is used for both the receiver and the transmitter.
A duplex
arrangement switches the antenna back and forth between the receiver and
transmitter as required. The information from the receiver goes to the decoder
then to the encoder, which determines whether the transmitter will transmit The
air data computer supplies coded altitude information to the encoder, and the
control pane supplies selected code information to the encoder.
The two DME
systems and the two transponder systems are interconnected by a suppressor bus
which prevents transmission from more than one system at a time.
Figure 12-2
BOEING 737
MAINTENANCE
TRAINING MANUAL
OPERATION
ATCRBS
INTERROGATION SIGNAL
Operation sequence
The ground
secondary surveillance radar (SSR) and the airborne TCAS system interrogate the
airborne transponder in one of two modes. The type of interrogation is
determined by the spacing between two pulses (P1 and P3) transmitted on a
carrier frequency of 1030 ±2Mhz. Each
interrogation contains a third pulse at the same frequency which is not
transmitted by the SSR but by an omnidirectional antenna located with the
SSR. This pulse, P2, is transmitted 2
uses after the P1 pulse. By comparing
the relative amplitude of the P1 and P2 pulses, the airborne transponder can
determine whether or not the airplane is in the main lobe of the SSR. In the figure, airplane A in the main lobe
will receive a P1 pulse of a higher amplitude than P2 (at least 6 dB higher
required for response) , and recognize the interrogation as coming from the
main beam. The situation is reversed
for airplane B, and the P2 pulse will be of the same or greater amplitude than
the P1 pulse. The transponder in
airplane B will not reply.
BOEING 737
MAINTENANCE
TRAINING WLNUAL
OPERATION
ATCRBS REPLY SIGNALS
Operation sequence
The
transponder replies to mode A or mode C interrogations with coded pulses on a
carrier frequency of 1090 ±3 Mhz,
For a mode A
reply, the coded pulses represent a four digit octal code entered on the ATC
control panel by the pilot, The four digits give the transponder the capability
to reply with one of 4096 possible mode A codes. For a mode C reply, the coded
pulses represent altitude data from the Air Data Computer. The pulses within a mode C reply correspond
to a specific altitude value. Altitude
values range from -1000 to +126,750 feet in 100 foot increments.
 |
BOEING 737
MAINTENANCE TRAINING
MANUAL
OPERATION
Mode S interrogation
In a mode S
environment, both ATCRBS and mode S equipped airplanes can operate at the same
time. Mode S, TCAS and ATCRBS
interrogators may also be operating simultaneously in the same airspace. Mode S interrogators use "all
call" interrogations and discrete interrogations. There are two types of all call
interrogations. The mode S/ATCRBS all
call is used to interrogate ATCRBS transponders and to elicit the unique node S
address from mode S transponders; the mode S only all call elicits the mode S
address - an ATCRBS airplane will not reply.
After gaining the mode S address from a transponder, the interrogator
will "lock out" the transponder from replying to later all call
interrogations and interrogate the transponder using discrete interrogations.
An additional signal, the ATCRBS all call, is used to interrogate ATCRBS
transponders only; mode S transponders will not reply. Discrete interrogations are used to
interrogate mode S transponders only; ATCRBS transponders will not reply.
Mode S/ATCRBS
All Calls, ATCRBS Only All Calls and Mode S Only All Calls
Figure A shows
a mode S/ATCRBS all call or ATCRBS only all call interrogation. The P1, P2 and P3 pulses are the same as in
the ATCRBS interrogation. The
additional P4 pulse is ignored by an ATCRBS transponder and an ATCRBS
transponder will respond based on normal criteria. The mode S transponder evaluates the P4
pulse, if the P4 pulse is 1.6 usec,
the mode S transponder identifies the interrogation as a mode S/ATCRBS all
call/ and the transponder will respond (if not previously locked out) with its
unique 24 bit address. If the P4 pulse
is 0.8 usec, then the mode S transponder identifies the interrogation as an
ATCRBS only interrogation and will not respond. If the P4 pulse is absent, then the mode S
transponder will reply with the appropriated ATCRBS response. The mode S only all call is sent in the same
format as the mode S discrete interrogations except that the address being
interrogated is all logic ones. All mode
S transponders in the main Lobe of this interrogation and not locked out to
that interrogator will respond with their unique address.
BOEING 737
MAINTENANCE TRAINING MANUAL
OPERATION
Mode S discrete interrogation
Figure B shows
a mode S discrete interrogation. There
are 24 different discrete interrogations possible and these discrete
interrogations are called uplink formats.
Uplink formats can be 112 or 56 bits in Length. Examples of uplink formats are: short air to air surveillance;
surveillance, altitude request; and surveillance, identity request. In the uplink format, pulses P1 and P2 are
of the same amplitude; ATCRBS transponders will not reply. 1.5 usec after the beginning of the P2 pulse,
the mode S interrogator begins the P6 pulse.
The P5 pulse is a 4 MHz continuous wave. 2.75 usec after the. Beginning of the P2
pulse, the P6 signal reverses the phase of the 4 MHz wave,. This reversal is called the sync phase
reversal. If the mode S transponder
senses the sync phase reversal, the transponder will evaluate the address to
which the interrogation is addressed. If the interrogation is addressed to that
transponder, the transponder will reply.
Another pulse, P5 is in phase with the beginning P6 wave and is
transmitted from an omnidirectional antenna at the same time as the sync phase
reversal occurs. If the mode S
transponder is in the side lobe of the interrogator, the PS pulse will mask the
sync phase reversal and the transponder will not respond. All mode S uplink formats have the address
of the transponder being interrogated encoded in the interrogation. The message data bits follow the sync phase
reversal. The logic ones and zeros are
sent using differential phase shift keying (DPSK). The 4 Mhz P6 signal after the sync phase
reversal is divided into either 112 or 56 bit times. If during a bit time the phase of the
signal is reversed, that bit is a logic one; if the phase of the signal is not
reversed, the bit is a logic zero.
BOEING 737
MAINTENANCE TRAINING MANUAL
OPERATION
Mode S Lock Out
When a
transponder receives an all call interrogation, the transponder will respond
with its unique mode S address if the transponder is not locked out to that
interrogator. Mode S interrogators (ground stations and TCAS
airplanes) send a lock out signal to mode S transponders after obtaining the
transponder's address and placing the transponder on the interrogator's roll
call. Transponders on the roll call are interrogated based on the
interrogator's estimate of the position of that target. At the time of interrogation, the lock out
of that transponder is updated to keep the transponder locked out to that
particular interrogator's ATCRBS or mode S all call requests. Transponders can be Locked out up to 15
different interrogators and to one nonspecific interrogator at the same time. The transponder tracks the around of time which
it is been locked out to each interrogator.
If no lock out update is received in 18 seconds, the transponder will
automatically "unlock" itself to that interrogator's all calls. Also, transponders may be unlocked before the
18 seconds has expired by the interrogator.
Mode S Acquisition
Usually,
interrogators transmit mode S/ATCRBS all calls and mode S all calls to obtain
the mode S transponder’s address. However, in high traffic areas, transponders
can be directed by the interrogator not to respond to every all call but
respond to 1 all call in 4, 8, or 16. Because the transponders to which this
command has been given respond in a random fashion, the interrogator has a
better chance of receiving a response from only one airplane instead of getting
responses from two or more airplanes at the same time. This method of acquiring
transponders for the roll call is called stocastic acquisition. Also, ground
stations can coordinate handling targets by communicating on land lines. If a
target is leaving the area of control for one ground station and entering
another, the first ground station can pass that airplane’s address and other
data on phone lines to the next ground station. Thereby, the receiving ground
station does not have to go through the acquisition process for that
transponder.
BOEING 737
MAINTENANCE TRAINING MANUAL
OPERATION
Mode S Replies
Figure c shows
a mode S reply. The first four pulses
form a preamble. The data bits (56 or 112) are transmitted using pulse position
modulation (PPM). PPM sends a logic
one if the pulse is in the first portion of the bit time and a logic zero if
the pulse is in the last portion of the bit time. Bit time is 1 usec. Responses are called downlink formats and
the downlink format used by the transponder is based on the uplink format received
from the interrogator.
