The 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 operations considerations.

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.

The navigation 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.

Another navigation 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 state vector.

Another feature 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).

Another guidance 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.

The rotation 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 attitude.

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.

Another manual 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 DAP panel.

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 orbital DAP.