entry phase of flight begins approximately five minutes before entry
interface, which occurs at an altitude of 400,000 feet. At EI minus
five minutes, the orbiter is at an altitude of about 557,000 feet,
traveling at 25,400 feet per second, and is approximately 4,400
nautical miles (5,063 statute miles) from the landing site. The
goal of guidance, navigation and flight control software is to guide
and control the orbiter from this state (in which aerodynamic forces
are not yet felt) through the atmosphere to a precise landing on
the designated runway. All of this must be accomplished without
exceeding the thermal or structural limits of the orbiter.
The entry phase
is divided into three separate phases because of the unique software
requirements. Entry extends from EI minus five minutes to terminal
area energy management interface at an altitude of approximately
83,000 feet, at a velocity of 2,500 feet per second, 52 nautical
miles (59 statute miles) from the runway and within a few degrees
of tangency with the nearest heading alignment cylinder in major
to the approach and landing capture zone, defined as the point when
the orbiter is on glide slope, on airspeed, and on runway centerline,
which occurs below 10,000 feet and is the first part of major mode
305. The orbiter attains subsonic velocity at an altitude of approximately
49,000 feet about 22 nautical miles (25 statute miles) from the
landing begins at the approach and landing capture zone, an altitude
of 10,000 feet and Mach 0.9 and extends through the receipt of the
weight-on-nose-gear signal after touchdown, which completes major
RCS jets are inhibited at entry interface.
feet, a pre-entry phase begins in which the orbiter is maneuvered
to zero degrees roll and yaw (wings level) and a predetermined angle
of attack for entry. The flight control system issues the commands
to the roll, yaw and pitch RCS jets for rate damping in attitude
hold for entry into the Earth's atmosphere until 0.176 g is sensed,
which corresponds to a dynamic pressure of 10 pounds per square
foot, approximately the point at which the aerosurfaces become active.
When the orbiter
is in atmospheric flight, it is flown by varying the forces it generates
while moving through the atmosphere, like any other aerodynamic
vehicle. The forces are determined primarily by the speed and direction
of the relative wind (the airstream as seen from the vehicle). The
direction of the airstream is described by the difference between
the direction that the vehicle is pointing (attitude) and the direction
that it is moving (velocity). It may be broken into two components:
angle of attack (vertical component) and sideslip angle (horizontal
To rotate the
orbiter in the atmosphere, aerodynamic control surfaces are deflected
into the airstream. The orbiter has seven aerodynamic control surfaces.
Four of these are on the trailing edge of the wing (two per wing).
They are called elevons because they combine the effects of elevators
and ailerons on ordinary airplanes. Deflecting the elevons up or
down causes the vehicle to pitch up or down. If the right elevons
are deflected up and the left elevons are deflected down, the orbiter
will roll to the right-that is, the right wing falls and the left
wing rises. The fifth control surface is the body flap, located
on the rear lower portion of the aft fuselage. It provides thermal
protection for the three main engines during entry, and during atmospheric
flight it provides pitch trim to reduce elevon deflections. The
sixth and seventh control surfaces are the rudder/speed brake panels,
located on the aft portion of the vertical stabilizer. When both
panels are deflected right or left, the spacecraft will yaw, moving
the spacecraft's nose right or left, thus acting as a rudder. If
the panels are opened at the trailing edge, aerodynamic drag force
will increase, and the spacecraft will slow down. Thus, the open
panels are called a speed brake.
On the flight
deck display and control panel (panel F7 between the commander and
pilot) are the surface position indicators, which display the position
of each aerodynamic control surface.
The aft RCS
jets maneuver the spacecraft until a dynamic pressure of 10 pounds
per square foot is sensed; at this point, the orbiter's ailerons
become effective, and the aft RCS roll jets are deactivated. At
a dynamic pressure of 20 pounds per square foot, the orbiter's elevators
become effective, and the aft RCS pitch jets are deactivated. The
orbiter's speed brake is used below Mach 10 to induce a more positive
downward elevator trim deflection. At Mach 3.5, the rudder become
activated, and the aft RCS yaw jets are deactivated (approximately
control is maintained with the aerojet DAP, which generates effector
and RCS jet commands to control and stabilize the vehicle during
its descent from orbit. The aerojet DAP is a three-axis rate command
feedback control system that uses commands from guidance in automatic
or from the flight crew's RHC in control stick steering. Depending
on the type of command and the flight phase, these result in fire
commands to the RCS or deflection commands to the aerosurfaces.
In the automatic
mode, the orbiter is essentially a missile, and the flight crew
monitors the instruments to verify that the vehicle is following
the correct trajectory. The onboard computers execute the flight
control laws (equations). If the vehicle diverges from the trajectory,
the crew can take over at any time by switching to CSS. The orbiter
can fly to a landing in the automatic mode (only landing gear extension
and braking action on the runway are required by the flight crew).
The autoland mode capability of the orbiter is used by the crew
usually to a predetermined point in flying around the heading alignment
cylinder. In flights to date, the crew has switched to CSS when
the orbiter is subsonic. However, autoland provides information
to the crew displays during the landing sequence.
and pilot can select automatic or CSS flight control modes. The
crew can select separate modes for pitch and roll and yaw (roll
and yaw must be in the same mode). The body flap and speed brake
have automatic and manual modes.
provides automatic control in the pitch axis, and the automatic
roll and yaw provides automatic control in the roll and yaw axes.
During entry, the automatic mode uses the RCS jets until dynamic
pressure permits the aerosurfaces to become effective; the aft RCS
jets and spacecraft aerosurfaces are then used together until dynamic
pressure becomes sufficient for aerosurface control only.
the pitch axis is provided by the elevons, speed brake and body
flap. The elevons provide control to guidance normal acceleration
commands, control of pitch rate during slap-down (landing) for nose
wheel load protection, and static load relief after slap-down for
main landing gear wheel and tire load protection. The speed brake
provides control to guidance surface deflection (open/close, increase/decrease
velocity) command. The body flap provides control to null elevon
the roll and yaw axes is provided by the elevons and rudder. The
elevons provide control to guidance bank angle command during terminal
area energy management and autoland and control to guidance wings-level
command during flat turns, 5 feet above touchdown. The rudder provides
yaw stabilization during TAEM and autoland and control to guidance
yaw rate command during flat turn and subsequent phases.
When the orbiter
is in the automatic pitch and roll and yaw modes, the crew's manual
control stick steering commands are inhibited. In the CSS mode,
the crew flies the orbiter by deflecting the RHC and rudder pedals.
The flight control system interprets the RHC motions as rate commands
in pitch, roll or yaw and controls the RCS jets and aerosurfaces.
The larger the deflection, the larger the command. The flight control
system compares these commands with inputs from rate gyros and accelerometers
(what the vehicle is actually doing-motion sensors) and generates
control signals to produce the desired rates. If the crew releases
the RHC, it will return to center, and the orbiter will maintain
its present attitude (zero rates). The rudder pedals position the
rudder during atmospheric flight; however, in actual use, because
flight control software performs automatic turn coordination, the
rudder pedals are not used until the wings are leveled before touchdown.
The CSS mode
is similar to the automatic mode except that the crew can issue
three-axis commands, affecting the spacecraft's motion. These are
augmented by the feedback from the same spacecraft motion sensors,
except for the normal acceleration (velocity) accelerometer assemblies,
to enhance control response and stability.
or pilot's RHC commands are processed by the GPCs in the CSS mode
together with data from the motion sensors. The flight control module
processes the flight control laws and provides commands to the flight
control system, which positions the aerosurfaces in atmospheric
the roll and yaw axes is provided by the elevon and the rudder.
The elevons augment the RHC control. The rudder interface between
the roll and yaw channel automatically positions the rudder for
coordinated turns. A rudder pedal transducer assembly is provided
at the commander and pilot stations. The two rudder pedal assemblies
are connected to their respective RPTAs. Because of the roll and
yaw interface, rudder pedal use should not be required until just
before touchdown. There is an artificial feel in the rudder pedal
assemblies. The RPTA commands are processed by the GPCs, and the
flight control module commands the flight control system to position
In the CSS
mode, the commander's and pilot's RHC trim switches, in conjunction
with the trim enable/inhibit switch, activate or inhibit the RHC
trim switch. When the RHC trim switch is positioned forward or aft,
it adds a trim rate to the RHC pitch command; positioning it left
or right adds a roll trim.
(CSS mode) in the pitch axis is provided by the elevons, speed brake
and body flap. The elevons provide augmented control through the
RHC pitch command. The speed brake can be switched to its manual
mode at either the commander's or pilot's station by depressing
a takeover switch on the speed brake/thrust controller handle. Manual
speed brake control can be transferred from one station to the other
by activating the takeover switch. When the SBTC is at its forward
setting, the speed brake is closed. Rotating the handle aft, positions
the speed brake at the desired position (open) and holds it. To
regain automatic speed brake control, the push button must be depressed
again. In the manual mode, speed brake commands are processed by
the GPCs, and the flight control module commands the flight control
system to position the speed brake and hold it at the desired position.
The body flap can be switched to its manual mode at panel C3 by
moving a toggle switch from auto/off to up or down for the desired
body flap position. These are momentary switch positions; when released,
the switch returns to off .
In the entry
phase, the RCS commands roll, pitch and yaw. Lights on the commander's
panel F6 are used to indicate the presence of an RCS command from
the flight control system to the RCS jet selection logic; however,
this does not indicate an actual RCS jet thrusting command. The
minimum light-on duration is extended to allow the light to be seen
even for minimum-impulse RCS jet thrusting commands. After the roll
and pitch aft RCS jets are deactivated, the roll indicator lights
are used to show that three or more yaw RCS jets have been requested.
The pitch indicator lights are used to show elevon rate saturation.
entry subphase, the primary objective is to dissipate the tremendous
amount of energy that the orbiter possesses when it enters the atmosphere
so that it does not burn up (entry angle too steep) or skip out
of the atmosphere (entry angle too shallow), stays within structural
limits, and arrives at the TAEM interface with the altitude and
range to the runway necessary for a landing. This is accomplished
by adjusting the orbiter's drag acceleration on its surface using
bank commands relative to vehicle velocity. During TAEM, as the
name implies, the goal is to manage the orbiter's energy while the
orbiter travels along the heading alignment cylinder, which lines
up the vehicle on the runway centerline. A HAC is an imaginary cone
that, when projected on the Earth, lies tangent to the extended
different tasks during the entry, TAEM and approach and landing
subphases. During the entry subphase, guidance attempts to keep
the orbiter on a trajectory that provides protection against overheating,
overdynamic pressure and excessive normal acceleration limits. To
do this, it sends commands to flight control to guide the orbiter
through a tight corridor limited on one side by altitude and velocity
requirements for ranging (in order to make the runway) and orbiter
control and on the other side by thermal constraints. Ranging is
accomplished by adjusting drag acceleration to velocity so that
the orbiter stays in that corridor. Drag acceleration can be adjusted
primarily in two ways: by modifying the angle of attack, which changes
the orbiter's cross-sectional area with respect to the airstream,
or by adjusting the orbiter's bank angle, which affects lift and
thus the orbiter's sink rate into denser atmosphere, which in turn
affects drag. Using angle of attack as the primary means of controlling
drag results in faster energy dissipation with a steeper trajectory
but violates the thermal constraint on the orbiter's surfaces. For
this reason, the orbiter's bank angle (roll control) is used as
the primary method of controlling drag, and thus ranging, during
this phase. Increasing the roll angle decreases the vertical component
of lift, causing a higher sink rate. Increasing the roll rate raises
the surface temperature of the orbiter, but not nearly as drastically
as does an equal angle of attack command. The orbiter's angle of
attack is kept at a high value (40 degrees) during most of this
phase to protect the upper surfaces from extreme heat. It is modulated
at certain times to ''tweak'' the system and is ramped down to a
new value at the end of this phase for orbiter controllability.
Using bank angle to adjust drag acceleration causes the orbiter
to turn off course. Therefore, at times, the orbiter must be rolled
back toward the runway. This is called a roll reversal and is commanded
as a function of azimuth error from the runway. The ground track
during this phase, then, results in a series of S-turns.
If the orbiter
is low on energy (the current range-to-go is much greater than nominal
at current velocity), entry guidance will command lower-than-nominal
drag levels. If the orbiter has too much energy (the current range-to-go
is much less than nominal at current velocity), entry guidance will
command higher-than-nominal drag levels to dissipate the extra energy.
is used to control cross range. Azimuth error is the angle between
the plane containing the orbiter's position vector and the heading
alignment cylinder tangency point and the plane containing the orbiter's
position vector and velocity vector. When the azimuth error exceeds
an initialized-loaded number, the orbiter's roll angle is reversed.
rate and downranging are controlled by bank angles-the steeper the
bank angle, the greater the descent rate and the greater the drag.
Conversely, the minimum-drag altitude is wings level. Cross range
is controlled by bank reversals.
The entry thermal
control phase is designed to keep the thermal protection system's
bond line within design limits. A constant heating rate is maintained
until the velocity is below 19,000 feet per second.
In the equilibrium
glide phase, the orbiter effects a transition from the rapidly increasing
drag levels of the temperature control phase to the constant drag
level of the constant drag phase. Equilibrium glide is defined as
flight in which the flight path angle, the angle between the local
horizontal and the local velocity vector, remains constant. This
flight regime provides the maximum downrange capability. It lasts
until drag acceleration reaches 33 feet per second squared.
drag phase begins at 33 feet per second squared. Angle of attack
is initially 40 degrees, but it begins to ramp down until it reaches
approximately 36 degrees by the end of this phase.
phase is entered as the angle of attack continues to ramp down,
reaching about 14 degrees at TAEM interface, with the vehicle at
an altitude of some 83,000 feet, traveling 2,500 feet per second
(Mach 2.5), and 52 nautical miles (59 statute miles) from the runway.
At this point, control is transferred to TAEM guidance.
entry phases, the orbiter's roll commands keep the orbiter on the
drag profile and control cross range.
steers the orbiter to the nearest of two heading alignment cylinders,
whose radii are approximately 18,000 feet and whose locations are
tangent to and on either side of the runway centerline on the approach
end. Normally, the software is set to fly the orbiter around the
HAC on the opposite side of the extended runway centerline. This
is called the overhead approach. If the orbiter is low on energy,
it can be flagged to acquire the HAC on the same side of the runway.
This is called the straight-in approach. In TAEM guidance, excess
energy is dissipated by an S-turn, and the speed brake can be used
to modify drag, lift-to-drag ratio and the flight path angle under
high-energy conditions. This increases the ground track range as
the orbiter turns away from the nearest HAC until sufficient energy
is dissipated to allow a normal approach and landing guidance phase
capture, which begins at 10,000 feet at the nominal entry point.
The orbiter can also be flown near the velocity for maximum lift
over drag or wings level for the range stretch case, which moves
the approach and landing guidance phase to the minimum entry point.
At TAEM acquisition,
the orbiter is turned until it is aimed at a point tangent to the
nearest HAC and continues until it reaches way point 1. At way point
1, the TAEM heading alignment phase begins, in which the HAC is
followed until landing runway alignment, plus or minus 20 degrees,
is achieved. As the orbiter comes around the HAC, it should be lined
up on the runway and at the proper flight path angle and airspeed
to begin the steep glide slope to the runway.
In the TAEM
prefinal phase, the orbiter leaves the HAC, pitches down to acquire
the steep glide slope, increases airspeed and banks to acquire the
runway centerline, continuing until it is on the runway centerline,
on the outer glide slope and on airspeed.
and landing guidance phase begins with the completion of the TAEM
prefinal phase and ends when the orbiter comes to a complete stop
on the runway. The approach and landing interface airspeed requirement
at an altitude of 10,000 feet is approximately 290 knots, plus or
minus 12 knots, equivalent airspeed, 6.9 nautical miles (7.9 statute
miles) from touchdown.
is initiated at this point to guide the orbiter to the minus 19-
to 17-degree glide slope (which is more than seven times that of
a commercial airliner's approach) aimed at a target approximately
0.86 nautical mile (1 statute mile) in front of the runway. The
descent rate in the latter portion of TAEM and approach and landing
is greater than 10,000 feet per minute (approximately 20 times higher
than a commercial airliner's standard 3-degree instrument approach
angle). The steep glide slope is tracked in azimuth and elevation,
and the speed brake is positioned as required.
1,750 feet above the ground, guidance sends commands to keep the
orbiter tracking the runway centerline, and a preflare maneuver
is started to position the orbiter on a shallow 1.5-degree glide
slope in preparation for landing, with the speed brake positioned
as required. At this point, the crew deploys the landing gear.
is begun at approximately 80 feet to reduce the sink rate of the
vehicle to less than 9 feet per second. After the spacecraft crosses
the runway threshold-way point 2 in the autoland mode-navigation
uses the radar altimeter vertical component of position in the state
vector for guidance and navigation computations from an altitude
of 100 feet to touchdown. Touchdown occurs approximately 2,500 feet
past the runway threshold at a speed of 184 to 196 knots (211 to
225 mph). As the airspeed drops below 165 knots (189 mph), the orbiter
begins derotation in preparation for nose gear slap-down.
system used from entry to landing consists of the IMUs and navigation
aids (TACAN, air data system, microwave scan beam landing system
and radar altimeter). The three IMUs maintain an inertial reference
and provide delta velocities until MSBLS is acquired.
air data-obtained after deployment of the two air data probes at
approximately Mach 3-is needed from entry through landing as inputs
to the guidance, flight control and crew display. TACAN provides
range and bearing measurements and is available at approximately
145,000 feet, nominally accepting the data into the state vector
before 130,000 feet. It is used until MSBLS acquisition, which provides
range, azimuth and elevation commencing at approximately 18,000
feet. Radar altimeter data are available at approximately 9,000
and operation are completely automatic, but the crew has the necessary
controls and displays to evaluate TACAN system performance and to
take over if required. When the distance to the landing site is
approximately 120 nautical miles (138 statute miles), TACAN begins
interrogating six navigation region stations. As the spacecraft
proceeds, the distances to the remaining stations and to the next-nearest
station are computed, and the next-nearest station is selected automatically
if the spacecraft is closer to it than it is to the previous locked-on
station. Only one station is interrogated if the distance to the
landing site is less than approximately 20 nautical miles (23 statute
miles). Again, TACAN automatically switches from the last locked-on
navigation region station to begin searching for the landing site
station. TACAN azimuth and range are provided on the CRT displaying
the horizontal situation. TACAN range and bearing cannot be used
to produce a good estimate of the altitude position component, so
navigation uses barometric altitude derived from the air data system
and operation are also completely automatic, and the flight crew
can evaluate system performance and take over if necessary. MSBLS
acquisition occurs at approximately 18,000 feet and about 8 nautical
miles (9.2 statute miles) from the runway. The range and azimuth
measurements are provided by a ground antenna located at the end
of the runway and to the left of the runway centerline. Elevation
measurements are given by a ground antenna to the left of the runway
centerline, about 2,624 feet from the runway threshold.
the commander's and pilot's altitude director indicators become
two-axis balls displaying body roll and pitch attitudes with respect
to local vertical/local horizontal. These are generated in the attitude
processor from IMU data. The roll and pitch error needles each display
the body roll and pitch attitude error with respect to entry guidance
commands by using the bank guidance error and the angle of attack
error generated from the accelerometer assemblies. In atmospheric
flight, the roll attitude error and the normal acceleration error
are displayed by the roll and pitch error needles, respectively.
The sideslip angle is displayed on the yaw error needle. The roll
and pitch rate needles display stability roll and body rates by
using stability roll rate, rate gyro rate and pitch rate. The yaw
rate needle displays stability yaw rate. After main landing gear
touchdown, the yaw error with respect to runway centerline and nose
gear slap-down pitch rate error are displayed on the roll and pitch
error needles. During rollout, the pitch error indicator indicates
pitch error rate.
the commander's and pilot's horizontal situation indicators display
a pictorial view of the spacecraft's location with respect to various
navigation points. The navigation attitude processor provides the
inputs to the HSI until the communications blackout is passed, at
approximately 145,000 feet. TACAN is then acquired and accepted
for HSI inputs at about 130,000 feet until MSBLS acquisition at
approximately 18,000 feet some 8 nautical miles (9.2 statute miles)
from the runway.
When the approach
mode and MSBLS source are selected for the commander's and pilot's
HSI, data from the MSBLS replaces TACAN data. MSBLS azimuth, elevation
and range are used from acquisition until the runway threshold is
reached, and azimuth and range are used to control rollout.
At an altitude
of 9,000 feet, radar altimeter 1 or 2 can be selected to measure
the nearest terrain within the beamwidth of the altimeters. This
indication is given to the altitude/vertical velocity indicator
radar, altitude and meter display from 5,000 feet to landing.
The left and
right air data system probes are deployed by the flight crew at
about Mach 3. This system senses air pressures related to orbiter
movement through the atmosphere for updating the navigation state
vector in altitude, guidance in steering and speed brake command
calculations, flight control for control law computations, and for
display on the alpha Mach indicators and altitude/vertical velocity
The AMIs display
essential flight parameters relative to the spacecraft's travel
in the air mass, such as angle of attack, acceleration, velocity
and knots of equivalent airspeed. The source of data for the AMIs
is determined by the position of the air data select switch. Before
the deployment of the air data system probe, the AMIs receive inputs
from the navigation attitude processor. When the air data probes
are deployed, the left or right air data system provides the inputs
to all AMIs except the acceleration indicator, which remains on
the navigation attitude processor, and the radar altitude. Neither
is operational until the orbiter descends to 5,000 feet.
The three rate
gyro assemblies of the flight control system measure and supply
output data proportional to the orbiter's attitude rates about its
three body axes, while the three accelerometer assemblies measure
and supply output data proportional to the orbiter's normal (vertical)
and lateral (right and left) accelerations. These assemblies are
incorporated into the flight control system for augmenting stability
because of the orbiter's marginal stability in its pitch and yaw
axes at subsonic speeds.
The three IMUs
constitute an all-attitude stabilized platform that also measures
and supplies output data proportional to the spacecraft's attitude
(rotation) and acceleration (velocity). They augment the rate gyro
assemblies and accelerometer assemblies.
The rate gyro
assembly pitch rate (rotation) and the accelerometer assembly normal
acceleration (velocity) are used to generate elevon (elevator) deflection
commands. The rate gyro assembly yaw rate (rotation) and the accelerometer
assembly lateral acceleration generate the rudder deflection required
for directional stability. The rate gyro assembly roll rate (rotation)
generates the elevon (aileron) deflection command required for lateral
(roll) stability. The speed brake and body flap positions generate
the elevon deflection required for trim near neutral to maximize
roll effectiveness of the elevons.
In the entry
phase, navigation software functions as it did during the deorbit
phase (three state vectors corresponding to each IMU) except that
additional external sensor data are sequentially incorporated. These
data provide the accuracy necessary to bring the orbiter to a pinpoint
landing and, to some extent, to maintain vehicle control. The TACAN
system, which becomes available at about 156,000 feet, provides
slant range and magnetic bearing to various fixed stations around
the landing site. It is used until the orbiter is approximately
1,500 feet above the ground, at which point it is rendered ineffective
by ground reflection. The air data system, which includes two transducer
assemblies attached to a probe on the left side of the vehicle and
two on the right side, provides pressures from which angle of attack,
Mach number, equivalent airspeed, true airspeed, dynamic pressure,
barometric altitude and altitude rate are computed. Only barometric
altitude is used by navigation. The other parameters are used by
guidance and flight control as well as for display to the flight
crew. The probes are normally deployed around Mach 3. The MSBLS
precisely determines slant range, azimuth and elevation relative
to the landing runway. For landing at runways with MSBLS ground
stations, MSBLS data become available at 20,000 feet for processing
One other tool
used by navigation is a drag altitude software sensor, which uses
a model of the atmosphere to correlate the drag acceleration measured
by the IMUs to altitude. This measurement, then, is only as good
as the atmospheric model on which it is based. The model is not
perfect. However, it has been determined through testing and analysis
that drag altitude data are important in keeping downrange and altitude
errors bounded during the blackout portion of entry (from approximately
265,000 to 162,000 feet). During this time, the ground is unable
to uplink state vector corrections to the orbiter, and TACAN data
are not available because of the heat-generated ionization of the
atmosphere around the vehicle.
also maintains a statistical estimate of the expected error in the
state vector. This is called a covariance matrix and is propagated
along with the state vector. When an external sensor, such as TACAN,
becomes available to the navigation software, a check is made to
see if the data lie within the current expected range of error.
Flight crew controls are provided on an onboard CRT horizontal situation
display to force the software to accept or inhibit the external
sensor data whether or not the data lie within the expected range.
Another control on the display may be selected to allow the software
to use the external sensor data to update its state vector so long
as the data lie within the expected range.
minutes before entry interface, the crew adjusts the software to
major mode 304. During this mode, which lasts until TAEM interface,
five CRTs become available sequentially and are used to monitor
auto guidance and the orbiter trajectory compared to the planned
entry profile. The five displays are identical except for the central
plot, which shows the orbiter's velocity versus range or energy/weight
versus range with a changing scale as the orbiter approaches the
landing site. This plot also includes static background lines that
allow the crew to monitor the orbiter's progression compared to
planned entry profiles.
Once TAEM interface
is reached, the software automatically makes a transition to major
mode 305. The CRT vertical situation 1 display then becomes available.
It includes a central plot of orbiter altitude with respect to range.
This plot has three background lines that represent the nominal
altitude versus range profile, a dynamic pressure limit in guidance
profile and a maximum lift-over-drag profile. At 30,000 feet, the
scale and title on the display change to vertical situation 2, and
the display is used through landing. When the approach and landing
interface conditions are met, a flashing A/L appears on the display.
CRT display used during entry is the horizontal situation. In addition
to providing insight into and control over navigation parameters,
this display gives the crew orbiter position and heading information
once the orbiter is below 200,000 feet.
The entry trajectory,
vertical situation and horizontal situation CRT displays, then,
are used by the flight crew to monitor the GN&C; software. They can
also be used by the crew to determine whether a manual takeover