The Rendezvous FDO

Rendezvous FDO responsibilities

The FDO is prime for computing ALL rendezvous maneuvers until onboard sensor data becomes available.

Common rendezvous maneuvers:

NC - controls phasing between chaser and target in the future

NH - sets up chaser to be at a defined delta-height from the target at a future time

NSR - makes the chaser coelliptic with the target

NPC - controls and nulls the planar difference between the chaser and the target

Day-of-rendezvous set: NCC, Ti, MC1, MC2, MC3, MC4

Two types of Rendezvous profiles

Deploy-Retrieve

Deploy-Retrieve missions involve deploying a payload, performing a separation maneuver(s), phasing away from the payload, stationkeeping for a pre-defined time, and retrieving it during the same mission.

Typical Deploy-Retrieve payloads: SPAS, SPARTAN, WSF

Station-keeping constraints

Payload communications range

Orbiter-to-payload minimum range

Minimize perturbations and phasing maneuvers

Ground-Up

Ground-up missions are much more complex

The Orbiter must be launched into a "phantom plane" that will match the target's orbit or a planned "control box" some number of days later, taking into account orbital perturbations and all intervening maneuvers.

Ground-up missions require Launch Window/Launch Targeting computations and a plane change maneuver somewhere in the rendezvous profile.

OMS-2 is used as a phasing maneuver, as are separation burns associated with deployables.

Ground-up rendezvous missions usually have very tight propellant budgets and are much more sensitive to launch slips than Deploy-Retrieve missions.

Rendezvous Launch Window

Precise launch times computed from planar and phasing constraints

Planar window

Defined by amount of ascent performance and/or thermal constraints

Phase window

Defined by minimum altitudes, propellant available, etc.

Actual launch time and rendezvous day determined by the overlap of the two windows.

Day-of-Rendezvous timeframe

When Rendezvous RADAR and/or Star Tracker data is acquired on the target, the onboard relative navigation state is more accurate than ground solutions, and onboard maneuver computations become prime, with the ground solution serving in a backup role.

The FDO is responsible for evaluating the various ground solutions based on all available vector sources (ground, PROP, and FILT), discussing the maneuver with the Rendezvous GPO, and recommending further action to the Flight Director.

In the event that the onboard computers are unable to target the burns, the FDO can provide accurate solutions based on the onboard relative state.

In the event that the onboard state vector is corrupted by bad sensor data, the FDO will be able to “reset” the relative state.

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