The Orbit FDO
In some of the simplest terms, the Orbit Flight Dynamics Officer (FDO) determines where the shuttle is, where it is going, and where it has been.
With the trajectory defined, the FDO also generates, executes, and confirms all translational maneuvers (maneuvers that change the orbit size and/or shape) to meet specific payload requirements. Once these maneuvers are defined, the FDO is also responsible for determining landing opportunities and just as important, evaluate the weather conditions at landing sites around the world.
The FDO coordinates with the United Space Space Command at the Cheyenne Mountain Air Force Station in Colorado Springs to determine if orbital debris poses a threat to the shuttle. Other real time activities that the FDO performs include providing shuttle sighting opportunities (which you can view on the Shuttle web site) and state vector postings for a variety of satellites so other people can plot the shuttle ground track on home computers.
During the Space Shuttle era, there were three distinct certifications for the Orbit phase:
- Orbit FDO: Certified for all basic orbit trajectory operations. Supported Spacelab and RMS deploy shifts, planning shifts on any flight, and execute shifts on deploy or rendezvous flights other than the actual deploy or day-of-rendezvous shifts.
- Deploy FDO: An additional certification for the planning and deployment of satellites that required the use of upper-stage boosters (Inertial Upper Stage – IUS, Payload Assist Module – PAM, Transfer Orbit Stage – TOS, etc.). Supported all Orbit FDO and upper stage deploy (IUS, PAM, TOS, etc.) shifts
- Rendezvous FDO: An additional certification for the planning of rendezvous profiles and for the Day-of-Rendezvous execution shift for both ground-up and deploy-retrieve payloads. Supported day-of-rendezvous shifts. For more information about the Rendezvous FDO, please read this!
There were only a few of us who held all three Orbit phase certifications… 🙂
I have provided the contents of a presentation (below) that was given by the Orbit Flight Dynamics Officers whenever the question “What is a FDO?” is asked. There is quite a bit of detail in this pitch, and I have purposely left it in its original wording.
Don’t forget to check out the Orbit FDO displays!
State vector maintenance
- What is a state vector?
- Put simply, it’s where an orbiting vehicle is and where it is going at a given instant in time.
- Shuttle state vector formats
- Primarily, we use the Cartesian (X, Y, Z, Xdot, Ydot, Zdot) format in the Aries Mean-of-1950 (M50) coordinate system utilizing feet and feet per second units.
- We can support almost any other format for customers and other external commitments
- Different units
- Different coordinate reference frames (J2000, TDR, etc.)
- Different coordinate elements (Two-line Elements, Spherical)
- Ground-based Navigation Monitoring
- The FDO is responsible for maintaining the best knowledge of the Orbiter’s state vector in a ground-based ephemeris
- Raw state vector tracking data provided by TDRS S-band beacon tracking and ground station C-band “skin tracking”
- “Least Squares” fit of tracking data provides final state vector
- Ephemerides, based on best state vectors, are used for future planning purposes
- Upcoming maneuvers, deorbit targeting, deploy opportunities
- Acquisition events (TDRS, ground sites, sun/moon lighting, etc.)
- The FDO will maintain an ephemeris for each vehicle of interest
- On-board Navigation Monitoring
- The FDO is responsible for maintaining the quality of the on-board navigation state vector during non-day-of-rendezvous activities.
- The FDO will update the on-board state vector as required for targeted maneuvers and to ensure that adequate deorbit/entry capability is maintained.
- The predicted on-board state vector downtrack error must be less than 20 NM at next deorbit TIG (FR A4.1.3-1).
- The “rule-of-thumb” will have the FDO consider an update if the semi-major axis error is greater than 1000 feet or if a two-orbit prediction of downtrack error is greater than 20,000 feet.
- The FDO will provide a GO/NO-GO assessment of the current state vector for crew sleep periods.
- When possible, the FDO will uplink a new vector prior to crew sleep.
- If mission events do not allow pre-sleep vector uplink, and the current state vector is predicted to violate the criteria in FR A4.1.3-1, the FDO will make every effort to uplink the state vector as soon as possible after crew sleep begins.
- The FDO will uplink and maintain a target state vector for Rendezvous NAV target track activities.
- The FDO is also responsible for uplinking TDRS vector information to the SM machine.
- Targeting and modeling
- The FDO is responsible for targeting and modeling all translational maneuvers (except for some of the final “manual phase” rendezvous maneuvers).
- The FDO will provide a maneuver PAD and uplink maneuver targets to the GNC as appropriate and verify the resultant on-board solution.
- The FDO will monitor the crew procedures and the maneuver execution to determine the “actual” maneuver (from IMU sensed DVs).
- The FDO will provide a “No trim required” call if the maneuver was performed to the expected accuracy’s, defined as f(criticality).
- Maneuver Examples
- Orbit adjust
- Enhance EOM lighting or crossrange or increase number of deorbit opportunities
- Meet primary payload trajectory requirements (i.e., altitude requirements, groundtrack coverage, etc.)
- Payload separation
- Separation from non-propulsive payload (RMS deploy)
- Separation from upper stage prior to SRM firing
- Rendezvous or station-keeping
- Collision avoidance
U.S. Space Command interface
- The United States Space Command (USSPACECOM) provides several important services for the Shuttle Program:
- Satellite surveillance and tracking
- Prediction of close conjunctions between the Orbiter and other space objects
- “Threat and Warning” notifications
- The FDO is responsible for all verbal contact with USSPACECOM
- The FDO and Flight Director share responsibility for written traffic as per the ICD.
- The FDO sends and receives trajectory-related messages
- The Flight Director receives “Threat and Warning” messages
- The FDO and USSPACECOM coordinate for Computations of Miss Between Orbits (COMBOs).
- COMBO messages are sent to the FDO from the Space Defense Operations Center (SPADOC) according to a schedule agreed to before launch.
- The FDO sends an Orbiter vector to SPADOC via the JSC Message Center once per shift and after major trajectory events.
- SPADOC will perform COMBO analysis and report the results to the FDO within an hour of receiving the Orbiter vector.
- If an object is within 5x25x5 km of the Orbiter, SPADOC will inform the FDO and increase ground tracking station tasking on the object.
- The FDO will confirm the spacing seen by SPADOC (all final decisions will be based on this spacing).
- If the object is within 2x5x2 km of the Orbiter, the FDO and Flight Director will coordinate an avoidance maneuver and determine if the maneuver should be performed.
- Due to various inaccuracies, even if object is within the 2x5x2 km “box”, the probability of a collision is still less than 1-in-100,000.
- According to FR A4.1.3-6, the maneuver “should not compromise primary payload or mission objectives”.
- Daily Deorbit Opportunities
- The FDO is responsible for the generation of daily deorbit opportunities (DOPS) covering the entire span of the flight.
- The DOPS will take all planned maneuvers into account or will be annotated as to the trajectory differences.
- Daily Primary Landing Site (PLS) and End of Mission (EOM) selections will be annotated.
- A DOPS is generated pre-flight. In addition, during the flight, the need to generate a new DOPS is evaluated after every major trajectory event or significant state vector update.
- Weather/PLS Evaluation
- The FDO is responsible for coordinating with the JSC Spaceflight Meteorology Group (SMG) to evaluate the daily weather conditions at the three CONUS landing sites (EDW, KSC, and NOR).
- Forecast sheets will be distributed to FLIGHT and CAPCOM.
- Based on weather conditions and mission constraints, the FDO will recommend a no-comm Primary Landing Site (PLS) for the next day.
- The final PLS selection for the next two days will be uplinked on the Mission Summary message.
- EDW is preferred for the no-comm PLS due to more margin for energy downmode and systems failures as well as more stable year-round weather.
Other External Interfaces
- Once per day, the FDO sends RADAR Restriction messages, asking USSPACECOM to lower RADAR power levels at any given site if the ICD radiation limits will be exceeded.
- The FDO is responsible for maintaining contact with landing sites and keeping track of site status, NAVAids, landing aids, and convoy force status.
- The FDO is responsible for generating Shuttle Sighting Opportunities and vectors for PAO.
- On End of Mission (EOM) – 1 day, the FDO will provide Deorbit/Landing Data for each of the EOM opportunities being considered.
Like every position in the Mission Control Center, the FDO has a number of team members supporting the front room FDO. Flight Control is a team effort, and everyone works together in synchronicity, with trust and respect built from thousands of hours of simulation, documentation and procedure review, various technical discussions, and after-hours camaraderie!
The Orbit FDO has these primary support positions:
- TRAJ – Trajectory Officer
- Seated in the FCR next to FDO
- FDOs are always first certified as TRAJ prior to FDO certification
- Primary TRAJ responsibilities
- Orbiter mass properties
- LSO duties (contacts with landing sites, etc.)
- Support the FDO
- NAV – Ground Navigation
- Coordinate/evaluate all incoming ground/TDRS tracking data
- Compute state vector from tracking data
- Assist FDO in ground and onboard vector evaluation
- Console in the Trajectory MPSR
- Make all MOC inputs based on FDO instructions
- Console in the TRAJ MPSR
- Coordinate, w/NAV, tracking site data flow and data requests
- Console in TRAJ MPSR
- Profile Support
- During rendezvous flights, this position assists the FDO with rendezvous profile evaluation and determination
- Console in the TRAJ MPSR
- LSO – Landing Support Officer
- Provide DoD and State Department interfaces with various agencies, organizations, and personnel at the landing sites around the globe
- Conduct daily status briefings with external parties, with information provided by the FDO
- Provide daily NOTAMs (Notices to Airmen) for the regular crew messages
- SMG provides world-wide weather data to FDO and Flight