U.S. SPACE SHUTTLE RENDEZVOUS TECHNIQUE
This page was created to provide the general public with an explanation of how the US Space Shuttle performs a rendezvous. It is adapted from a paper given at the SPACE DYNAMICS conference in Toulouse, France in 1989.
SHUTTLE RENDEZVOUS AND PROXIMITY OPERATIONS
DON J. PEARSON
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION JOHNSON SPACE CENTER HOUSTON, TX
Originally presented at: COLLOQUE: MECANIQUE SPATIALE (SPACE DYNAMICS) TOULOUSE, FRANCE NOVEMBER 1989
The Shuttle program has brought a new dimension to NASA rendezvous and proximity operations capabilities. For this program, NASA needed to demonstrate that a large manned space vehicle could rendezvous and grapple totally passive spacecraft such as malfunctioned and reservicable satellites. But, to make the most efficient use of the National Space Transportation System (NSTS), NASA needed to load the payload bay with co-manifested cargo for delivery to orbit on the same mission it would accomplish satellite retrieval. The Shuttle could then go to orbit with a full cargo and perform a retrieval or servicing mission with a different cargo. This paper details the challenges encountered in providing this type of rendezvous capability and the resulting design decisions in the NSTS program.
Shuttle rendezvous missions can be divided into two basic groups: 1) a "ground-up rendezvous" which is a rendezvous with a target vehicle already onorbit at the time of Shuttle launch, and 2) a "deploy/retrieve rendezvous" which is a rendezvous with a payload previously deployed from the Shuttle on the same flight (figure 1). The remainder of this paper addresses the ground-up rendezvous mission scenario which demonstrates all the major rendezvous design requirements and resulting trajectory control plans.
Mission planners design a trajectory control scheme which is capable of supporting mission operations with co-manifested cargo and utilizing a fixed crew timeline. For ground-up rendezvous missions, co-manifested payload deployments and Shuttle separation maneuvers are embedded in a rendezvous trajectory plan which is controlled from the Johnson Space Center (JSC) Mission Control Center in Houston.
Ground-up rendezvous missions have four separate flight segments characterized by a unique mission design emphasis, guidance/navigation/and control (GN&C) support system, and crew activities. These four phases are: the launch phase, the orbit adjust phase, the relative navigation phase, and the proximity operations phase.
For rendezvous missions, the Shuttle is launched from the Kennedy Space Center near the time of day that the earth rotation brings the targets orbital plane over the launch site as depicted in figure 2.
Typically a launch window around the optimal launch time is less than one hour. The Shuttle is steered to the target vehicle's orbital plane with the main engines using a technique called yaw steering. Targets with an inclination of 28.5 degrees have rendezvous planar launch windows up to one hour long since the inclination matches the launch site latitude. Targets in higher inclinations (e.g. : 57 degrees) can have planar launch windows less than 5 minutes in duration.
At the time of launch, the target may be anywhere in its orbital plane. The angle between the Shuttle and the target is called the phase angle. The Shuttle orbital trajectory is controlled to drive the phase angle which exists at launch time to zero at the completion of the rendezvous. Due to performance limitations and constrained crew activity timeliness only certain phase angles at launch are permissible for rendezvous. This "phase window" can be anywhere from 10 minutes long (40 degrees of onorbit phase angle makeup capability) to 90 minutes long (360 degrees of phase angle makeup) depending on the target retrieval altitude, crew timeline, and crew day that the actual rendezvus occurs. The relationship of the phase window to the planar window changes each day during the launch period and depends on the target's orbital period and inclination.
ORBIT ADJUST FLIGHT SEGMENT
The Shuttle enters this flight trajectory regime after the ascent phase. The Shuttle Orbital Maneuvering System (OMS) is used at apogee to raise perigee so that the Shuttle can remain onorbit and begin orbital operations. Using the Shuttle "Standard Insertion" ascent procedure, the OMS engines are fired approximately two minutes after Shuttle Main Engine Cutoff (MECO) which occurs 8 minutes after launch to raise the apogee altitude above 185 kilometers (100 nautical miles). This maneuver is called the OMS-1 burn. Approximately 30 minutes later at apogee, the engines are again fired (OMS-2) to raise perigee to a safe altitude. For rendezvous missions, the OMS-2 burn is targeted as a posigrade phasing burn and is the first onorbit maneuver to begin the process of rendezvous. Another available ascent technique is called "Direct Insertion" in which the Shuttle main engine performance is used to establish a sufficiently high apogee at the time of MECO so that the OMS-1 burn is not necessary. The first maneuver onorbit is still called OMS-2 and is targeted in the same manner as for standard insertions.
With the completion of OMS-2, the Shuttle is phasing towards the target and the crew performs post-insertion activities such as opening the payload bay doors and reconfiguring systems for onorbit activities. Depending on mission design requirements, the co-manifested cargo will be deployed later in the crew day or on Flight Day 2. Shuttle separation maneuvers are computed on the ground in the Mission Control Center (MCC) in Houston which are compatible with safe separation requirements and the rendezvous. In order to provide maximum flexibility, this highly mission unique phase of the rendezvous is controlled from the ground.
The Shuttle is tracked using the Telemetry and Data Relay Satellite (TDRS) S-band communication links and S-band and C-band ground tracking sites. The target vehicle is tracked with S-band ground tracking sites (if the target carries the appropriate transponder) and C-band tracking sites. C-band antennas are used to skin track the target and have generally been used on most Shuttle rendezvous missions to date because the targets have been passive disabled satellites (e.g. Solar Max, Syncom, Westar, Palapa). Determination of the semimajor axis (SMA) of each vehicle is the primary concern during rendezvous since energy errors cause secular downtrack dispersions. Generally, the Shuttle's SMA is known to within 500 meters (3sigma) and the target's SMA is known to better than 30 meters (3sigma) after two to three orbits of tracking. Shuttle SMA knowledge is degraded due to translation affects caused by attitude maneuvers, attitude deadbanding, Shuttle venting, and translational maneuvering trim limits.
Upon completion of navigation processing the Flight Dynamics Officer (FDO) in the MCC uses the Rendezvous Targeting Processor (RTP) to compute the remaining rendezvous maneuvers. This processor is a very generalized targeting program which allows flexible scheduling of maneuvers. Currently, up to 10 maneuvers of 24 different types can be computed as a single maneuver plan sequence targeted to flight planning end conditions. Maneuver types include: horizontal phasing and altitude adjustments, coelliptic maneuvers, plane change maneuvers, and two-impulse Lambert maneuvers. Scheduling options include geometric constraints (e.g. node crossing, apogee, elevation angle) and lighting constraints. Figure 3 contains an example of the targeting plan along with the resulting relative motion.
Upon completion of the rendezvous targeting the results are transferred into an ephemeris maintained in the MCC and the upcoming maneuver is uplinked to the crew. Onboard support is limited to crew execution of the ground targeted maneuvers using External Delta-V Guidance (EXDV). The uplinked maneuver is loaded by the crew into the Maneuver Execute software. Here an option exists to automatically maneuver to an inertial burn attitude for +X-RCS or OMS engine usage. Within 15 seconds of planned time of ignition (TIG) the crew enters an execute command to allow for automatic execution of the delta-V. EXDV guidance is then used during the burn: accelerometers sense the thrusting every second, decrement the commanded inertial delta-V to go (VGO), and compute the time to thrust termination. The engines shut down automatically and the crew then manually trims the maneuver residuals usually to better than 0.1 m/s. Options also exist for the crew to manually perform the full burn in their current attitude.
After each orbit maneuver, ground tracking is again used to update estimated states of both vehicles and the NAV-TARGET-BURN sequence is recycled. Premission, GN&C errors are analyzed in Monte Carlo simulations to evaluate trajectory and propellant dispersions and to determine the frequency of maneuvering. This phase of the rendezvous ends when the Shuttle closes to within 100 kilometers of the target and onboard sensors directly detect the target.
To summarize the Orbit Adjust flight segment, ground support for independent navigation of each vehicle independently is essential since the target is too far away to be tracked directly by the Shuttle. The generalized targeting routine requires significant computation capability (precision propagation over multiple crew days nested in loops several levels deep) which is prohibitive for onboard support. The MCC FDO is also highly trained to utilize the flexibility of this software for mission support and contingency operations. The maneuvers are spaced far enough apart (usually several revs) to permit realtime mission planning and ground tracking. Finally, the crew can concentrate on mission unique payload operations rather than trajectory control.
RELATIVE NAVIGATION FLIGHT SEGMENT
Ground controllers design the orbit adjust flight segment to terminate with the Shuttle less than 100 kilometers directly behind the target at orbital noon on the morning of the crew day on which the retrieval will be completed. At this point onboard sensors can optically detect the target and dedicated crew rendezvous operations begin. Maneuvers are scheduled closer together, usually less than one rev apart. The objective of this phase is to approach to within two kilometers of the target using only passive skin tracking of the target from the Shuttle even with potential Shuttle hardware failures. Star trackers onboard the Shuttle (figure 4) can track objects brighter than third magnitude. This is the first time in the mission that target physical characteristics affect the rendezvous operations. The target attitude profile and reflectance must be such that the Shuttle can track the target from orbital noon through orbital sunset as the approach continues. Star tracker angle measurements are made 30 times per second, averaged, and output to the onboard GN&C general purpose computers (GPCs).
Onboard relative navigation software (reference 1) as shown in figure 5 selects data once every 8 seconds for processing
and a Kalman filter is used to estimate the relative state. This 13 element filter estimates relative position and velocity (6 elements), unmodeled accelerations (3 elements), and sensor biases(4 elements). Measurement residuals are computed and data automatically rejected if it fails to meet incorporation criteria. The relative state information is combined with the onboard propagated target inertial state to compute the estimated Shuttle inertial state. The output of the rendezvous navigation software is current target and Shuttle estimated inertial states. Typically the relative state converges after the first star tracker pass to within 2300 m (3 sigma) and 0.8 m/s (3 sigma).
The crew then uses the onboard navigation results to compute two-impulse Lambert targeted maneuvers using the onboard Orbit Targeting Specialist Function (OTSF) as shown in figure 6. Premission offset points and transfer times are stored in the flight software data initialization load (I-loads) and are used in the targeting. The maneuver time relative to a reference time, the transfer time, and the "aim point" are the components of a "target set". The aim point is a point relative to the target in a local vertical / local horizontal (LVLH) coordinate frame where the Shuttle is to be at the end of the transfer. Up to 40 different target sets are defined premission in the I-loads. Additionally, the crew can manually overwrite any of the target sets should the need arise. An option exists to schedule a maneuver at an elevation angle.
Two different two-impulse targeting algorithms exist in the OTSF. One algorithm uses a Lambert targeting technique: current Shuttle and target inertial states are predicted forward to the maneuver time and the relative state is computed and displayed to the crew. The target inertial vector is then predicted forward through the transfer time, is combined with the "aim point" and a desired Shuttle inertial position vector is then constructed. A conic Lambert targeting routine then computes the conic transfer orbit. Current onboard software is limited to less than one rev transfer times. The resulting transfer orbit is then predicted forward through the transfer using a higher fidelity prediction scheme (4x2 gravity model and Jacchia drag model). This results in the predicted Shuttle state at the end of the transfer missing the aimpoint slightly due to the non-conic effects. The aimpoint is then automatically adjusted slightly (figure 7) and the cycle repeated until convergence is reached. Current convergence criteria for this Lambert targeting technique is 80 meters.
The resulting maneuver is displayed to the crew and automatically transferred to the Maneuver Execute logic. With Lambert targeting, a Lambert guidance capability is available. The EXDV guidance scheme discussed before is augmented every four seconds with a recomputation (retargeting) of the remaining maneuver. So now the Shuttle can steer during the burn rather than remaining inertially pointing.
The other targeting scheme available is based on Hill's closed form relative motion technique. These equations have been expanded to include constant differential acceleration terms and they support multi-revolution transfer times (reference 2). Current onboard inertial states are predicted forward to the ignition time and the relative state is computed. This initial relative state and the desired relative state at the end of the transfer are used directly to compute the LVLH velocity required at TIG. Again, a precision prediction scheme is wrapped around the targeting scheme and the aimpoint miss is evaluated. If it exceeds the tolerance (one percent of the relative range at the end of the transfer), the aimpoint is modified and the process repeated. The resulting maneuver is again automatically transferred to the Maneuver Execute software and is executed by the crew using EXDV guidance.
A standard trajectory profile has been developed for the Relative Navigation flight segment. Ground controllers have placed the Shuttle on a relative trajectory that will touch a point 75 kilometers (40 nautical miles) directly behind the target at orbital noon (figure 8). At that point, a ground computed phasing burn is performed to approach to 15 kilometers (S nautical miles) behind the target in either one or two orbital revolutions. A star tracker pass occurs immediately after the phasing burn and a second star tracker pass one rev later if the mission design uses a two revolution transfer. After the second star tracker pass, a Lambert maneuver is targeted onboard to correct for inplane and out-of-plane trajectory dispersions observed with the star tracker. GN&C Monte Carlo simulations indicate that the Shuttle will miss the 15 kilometer point by less than 1000 meters (3 sigma) as shown in figure 9.
After the midcourse correction burn, the Shuttle Ku-band system is used in the rendezvous radar mode to track the target. The system performs as either a rendezvous radar to detect and track onorbit spacecraft or as a wide-band two-way communications system for data exchange with the ground via TDRS. The functions of radar and high bandwidth communications are not available simultaneously since they share a common high-gain antenna mounted on a deployable assembly on the starboard side of the Shuttle payload bay. During launch and entry, the deployable assembly is stowed internal to the payload bay (figure 10). The radar operates in the 13.8 Ghz range and uses pulse-Doppler techniques to extract range and range rate information. Inertial pointing angles are also sent to the relative navigation software for processing. Radar specifications list a range of 18 kilometers (10 nautical miles) for a target with a one square meter reflectivity. Maximum range is limited to 50 kilometers (27 nautical miles). Historically, targets are detected at approximately 37 kilometers (20 nautical miles ). Radar data is incorporated every eight seconds and the relative state navigation uncertainty is reduced to 275 m and O.5 m/s (3 sigma) by the time the Shuttle reaches the 15 km (8 nmi) range. The maneuver at the 15 km range is called the transition initiation (Ti) maneuver. It is the maneuver where the Shuttle transitions to actually targeting directly for intercept with the target. Just prior to Ti, rendezvous GN&C systems are evaluated to determine if the crew is "GO FOR Ti". If there is some concern about continuing the rendezvous, a new maneuver can be executed at Ti to interrupt the sequence and allow for stationkeeping to resolve the situation.
Upon completion of Ti, the Shuttle again uses rendezvous radar information to update its knowledge of the relative state and five midcourses are scheduled to correct the trajectory during the next hour. If the radar has failed, the orbital lighting is such that optical tracking with the star tracker is used immediately following Ti.
MANUAL PHASE FLIGHT SEGMENT
By the completion of the last midcourse, orbital sunrise has occurred, the range has decreased to two kilometers, and the relative rates are under 2 m/s (6fps). Additionally, the relative trajectory has been designed such that the inertial line of sight (LOS) rates to the target are nearly zero at manual takeover and the Shuttle is below the target altitude so that the background view is dark. The target appears as a bright star frozen against a stellar background.
Because of single point failures possible with the rendezvous radar, this final rendezvous phase has been designed to allow for manual piloting since no relative navigation tracking support can be assured. Two minutes after the last midcourse, the crew changes the Orbiter's automated pointing mode from target-track to inertial hold. The trajectory control requirements now are to control range-rate as a function of the decreasing range while maneuvering perpendicular to the line of sight to keep the target centered in the crew optical alignment sight (COAS) mounted on the upper windows (figure 11).
A standard range-rate versus range braking schedule has been developed which minimizes plume impingement on the target and yet allows the range to decrease to 120 meters (400 ft) in 20 minutes as the Shuttle reaches the "V-bar" downtrack position (figure 12). As the Shuttle nears the V-bar, the relative rates have now decreased to under 0.2 m/s. The remaining trajectory profile design is highly dependent on the payload configuration. Payload attitude control and susceptibility to Shuttle plume impingement are major drivers in the design of the final approach.
The preferred technique is the V-bar approach. As the Shuttle nears the V-bar at orbital noon, the automated attitude control mode is switched from inertial to an Shuttle tail-to-Earth pointing mode. This is used to sense the crossing of the V-bar. At that point, normal to the LOS rates are nulled to keep the Shuttle on the V-bar and the closing rate is adjusted per a new braking schedule. As the Shuttle continues the approach, orbital dynamics require the crew to periodically fire the aft engines to maintain a LOS approach along the V-bar. Since these engines are canted, incremental braking occurs each time these engines are fired. The range-rate can be adjusted by braking in either the "Norm-Z" control mode or the "Low-Z" control mode (figure 13). Although the Norm-Z mode is more fuel efficient, thruster plumes are directed at the target. Plume overpressures can cause vehicle attitude rates which result in the target tumbling and being unretrievable. Plume contamination can seriously degrade scientific apparatus. The Low-Z mode provides braking by firing opposing forward and aft jets. This is extremely fuel inefficient (400 kilograms for 1 m/s) but is necessary to approach sensitive payloads. Controllability is also markedly degraded due to RCS cross-coupling. The LOS approach to the target continues as orbital sunset occurs. It is necessary to be within 60 meters for optical tracking during the night pass and it is desirable to be within 20 meters at orbital sunset so that the payload bay floodlights can illuminate the payload for grappling operations. The rendezvous radar drops out at a range of 25 meters due to transmit/receive switchover limitations. Grappling is performed at a range of about 10 meters.
The V-bar approach is desirable because it is relatively fuel efficient, the constant earth horizon orientation is a good piloting reference, and closing rates can be easily and immediately nulled with subsequent efficient stationkeeping should some Shuttle or payload system anomaly occur.
Other techniques for final approach are possible and are usually based on either inertial or LVLH LOS approach techniques. The Long Duration Exposure Facility (LDEF) is to be retrieved in December 1989 by continuing the inertial approach through the V-bar crossing for 20 more minutes and then performing an LVLH approach radially down on top of LDEF(figure 14). The Shuttle will yaw as necessary during the LVLH approach to set up the proper geometry for grapple. Approaches along any LVLH LOS direction have recently been proposed (reference 1) to allow even more design flexibility.
Retrieval of targets without the standard Shuttle grapple fixture are also possible through the use of the Manned Maneuvering Unit (MMU) as demonstrated in the Westar and Palapa retrievals on STS-51A. Unique crew techniques for arresting target rotation rates and attaching grappling hardware have also been demonstrated with the SYNCOM Salvage mission on STS-51I.
With all of these approach techniques, propellant consumption from the forward reaction control system (RCS) for the manual phase is relatively large. Thrusting is frequent and maneuvers are performed in all three axes to control the relative trajectory yet maintain the COAS pointing towards the target. Because the forward RCS tank is constrained (there is no fuel sharing with the aft tanks), considerable analyses are performed to minimize forward RCS propellant usage in order to assure a reasonable success probability with a failed rendezvous radar. Mission designers were rewarded when Shuttle missions to retrieve the Solar Max Satellite on STS-41C and the disabled Westar and Palapa satellites on STS-51A proved that the techniques were refined sufficiently to allow for two Shuttle retrievals on any mission!
Shuttle rendezvous and proximity operations trajectory control techniques have been shaped by many factors: Shuttle system design constraints (limited forward RCS, single point radar failures), mission operations constraints (large launch windows, flexibility, contingency profile interrupts), and crew training requirements. The resulting trajectory control design primarily uses ground support for the launch and orbit adjust phases, standardized crew techniques utilizing onboard software for the relative navigation flight phase, and mission unique manual control for a flexible proximity operations phase.
1. E.L. Freddolino, "Space Shuttle Operational Level C Functional Subsystem Software Requirements Document: Guidance, Navigation, and Control" STS-83-0006B June 30, 1987
2. D.J. Pearson, "The Glideslope Approach" , AAS/GSFC International Symposium on Orbital Mechanics and Mission Design, April 24, 1989