major GN&C-related; orbital tasks include achieving the proper position,
velocity and attitude necessary to accomplish the mission objectives.
To do this, the GN&C; computer maintains an accurate state vector,
targets and initiates maneuvers to specified attitudes and positions,
and points a specified orbiter body vector at a target. These activities
are planned with several constraints in mind, including fuel consumption,
vehicle thermal limits, payload requirements and rendezvous/proximity
The GN&C; software
for the majority of on-orbit operations is called OPS 2 (on orbit),
which is further divided into major mode 201 (orbit coast, in which
the majority of attitude and pointing operations occur) and major
mode 202 (maneuver execute, in which OMS translations are targeted
and executed). GN&C; software is also used for the flight control
system checkout before deorbiting (OPS 8). In this configuration,
the crew checks out the navigation aid systems, the dedicated displays,
the RCS jets, the aerosurfaces and the hand controllers.
software available in OPS 2 has several important features. As before,
it propagates the orbiter state vector. During coasting flight,
the software uses a model of atmospheric drag acceleration to propagate
the state vector. If translational thrusts are anticipated, the
flight crew can set a flag for navigation to use IMU-sensed acceleration,
when above a noise threshold value. When this flag has been set
via an item entry on an orbit display, the flight crew can monitor
the thrust that is sensed.
option that may be available on orbit is called rendezvous navigation.
When this option is enabled by a flight crew input on the relative
navigation display, the software maintains a target state vector
and the orbiter state vector. In this mode, it is possible for navigation
to use external sensor data from the star tracker, crewman optical
alignment sight or rendezvous radar (based on reasonableness tests)
to compute the orbiter target state vector. This assumes that the
target vector is accurate.
On orbit, the
accuracy of the orbiter state vector depends on the accuracy of
the IMUs and the accuracy of the modeled drag acceleration. Since
there is currently no method on board the orbiter to compute independently
corrections to the state vector, periodic updates are sent from
Mission Control to correct any errors that develop with the onboard
available in navigation in OPS 2 is the landing site update function,
which allows the crew to select different runways to be used in
the entry guidance computations. The crew interfaces with this capability
through the universal pointing display only in major mode 201.
One of on-orbit
software's several features, universal pointing is used to compute
attitude changes required to point a specified orbiter body axis
at a specified target, to maneuver to a predetermined attitude,
to rotate the orbiter about a specified body axis or to maintain
attitude hold. Although the complete capabilities of the universal
pointing software are available only in major mode 201, a subset
is available in major mode 202 and OPS 8 (on orbit).
feature is PEG 7, or external delta-velocity, targeting for OMS
or RCS burns. This targeting scheme is identical to that used in
OPS 1 (ascent) and OPS 3 (entry). In this mode, guidance sends the
commands to flight control to execute a burn specified by an ignition
time and delta velocities in the local vertical coordinate system
at ignition. Commands continue to be generated until the original
delta-velocity requirement is met. This option is available in major
mode 202 via the orbit maneuver execute display.
The third guidance
capability is an on-orbit targeting scheme that is used to compute
the parameters required to move the orbiter to a specified target
offset position in a given amount of time. This feature, which is
used to do onboard targeting of rendezvous maneuvers, is enabled
via the orbit targeting CRT display. The actual thrusting period
is still accomplished via the orbit maneuver execute display.
The orbit flight
control software includes an RCS DAP, an OMS thrust vector control
DAP, a module called an attitude processor to calculate vehicle
attitude and logic to govern which DAP is selected. The attitudes
calculated by the attitude processor are displayed on the ADI along
with another crew display, universal pointing, which is available
in major mode 201. The vehicle attitude is used by the DAP to determine
attitude and rate errors.
The RCS DAP,
used in OPS 2 at all times except when an OMS burn is in progress,
controls vehicle attitudes and rates using RCS jet fire commands.
Either the larger primary jets or the less powerful vernier jets
are used for rotational maneuvers, depending on whether norm or
vern is selected on the panel C3 orbital DAP panel. That selection
depends on fuel-consumption considerations and how quickly the vehicle
must be maneuvered to satisfy a mission objective.
rates and dead bands, translation rate and certain other options
for the DAP may be changed by the crew during the orbit phase using
the DAP configuration display. The crew can load the DAP with these
options two ways at a time. One set is accessed by depressing the
DAP A push button on the orbital DAP panel and the other by depressing
the DAP B push button. For convenience, each planned DAP configuration
is given a number and is referenced to that number and to the DAP
used to access it. Typically, the DAP A configurations have larger
dead bands and higher rates than the DAP B configurations. The wide
dead bands are used to minimize fuel usage, while the tight dead
bands allow more precision in executing maneuvers or in holding
The RCS DAP
has both an automatic and a manual rotation mode. The one that is
used depends on crew selection of the auto or man push buttons on
the orbital DAP panel. The manual mode is also accessed when the
RHC is moved out of its detent (neutral) position. In both the automatic
and manual modes, the rotation rate is controlled by the selection
of DAP A or DAP B and the information loaded in the DAP configuration
display. Moreover, in automatic, the DAP determines the required
attitude to be achieved from universal pointing. It then computes
the RCS jet fire commands necessary to achieve these requirements
within the current set of dead bands. In the manual rotation mode,
the RCS DAP converts flight crew inputs with any of the three RHCs
to RCS jet fire commands, depending on whether pulse , disc rate
or accel is selected on the orbital DAP panel. Simply stated, in
pulse, a single burst of jet fire is produced with each RHC deflection.
The resultant rotational rate is specified on the DAP configuration
display. In discrete rate, jet firings continue to be made as long
as the RHC is out of detent to maintain the rotational rate specified
on the DAP configuration display. In acceleration, continuous jet
firings are made as long as the RHC is out of detent.
RCS DAP mode-local vertical/local horizontal-is used to maintain
the current attitude with respect to the rotating LVLH reference
frame. It is selected through the LVLH push button on the orbital
The RCS DAP
has only a manual translation capability, which is executed through
the forward or aft THC. Only the primary RCS jets are used. Deflections
of the THC result in the firing of the RCS jets, depending on which
transition DAP mode push button is selected on the orbital DAP panel.
In pulse, a single burst of jet fire results; in normal, there are
continuous jet firings with a specified subset of the available
jets; in high, all upfiring jets fire continuously in a Z translation;
and in low, a special technique is used to perform a Z translation
with the forward- and aft-firing RCS jets in order not to fire directly
toward a target (this avoids plume impingement and contamination
of a target payload).
The OMS thrust
vector control DAP is available when an OMS burn is executed in
major mode 202 via the orbit maneuver execute display. The TVC DAP
uses the guidance-generated delta-velocity requirements and converts
these into the appropriate OMS gimbal commands to achieve this target,
assuming auto is selected on the orbital DAP panel. It generates
the OMS fire commands; the OMS shutdown commands; and, if necessary
because of OMS engine failure, RCS commands required to maintain
attitude control. If manual is selected, the TVC DAP uses inputs
from the RHC to control attitude during the burn.
As with the
transition DAP, there are many subtleties in the operation of the