Date: 8/15/00
104 : Preprint
Time: 9:58:50 AM
STScI Internal Use Only.
Paper delivered at:
SPIE International Symposioum on
Astronomical Telescopes and
Instrumentation 2000.
27-31 March, 2000 Munich, Germany
by William M. Workman III
header for SPIE use
Impact of Observing Constraints and Unplanned Events on Observatory
Efficiency and Long Range Plan Stability
William M. Workman IIIa, Wayne Kinzela, Tricia Royleb, Ian Jordana
a
Computer Sciences Corporation (CSC), Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218
b
Association of Universities for Research in Astronomy (AURA), Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218
ABSTRACT
In an era of increasing pressure to do more with less and make the most out of every budget dollar, HST science operations
have steadily been able to give its customers more by increasing observatory efficiency. While original mission goals for
observatory efficiency were targeted at less than 35%, HST now consistently achieves weekly schedules which are greater
than 50% efficient. Furthermore, special concentration on continuous viewing opportunities and science campaigns (i.e. – the
Hubble Deep Field) has yielded efficiencies exceeding 60%. More than fourteen years of applied operational experience and
system analysis by HST ground, flight, instrument, and user support systems personnel have resulted in the success.
However, these efficiency levels could be even higher were it not for the variety of constraints and unplanned events which
affect how and when the observatory can be used. Certain known spacecraft and instrument constraints impact efficiency
with little effect on long range plan stability since they can be accounted for in advance. For this class of concerns, planning
scenarios can be developed and analyzed to see what efficiencies might be achieved without these constraints. Unpredictable
events such as spacecraft safings and anomalies, targets of opportunity and quick turnaround director's discretionary science
reduce the overall stability of an observatory’s planned use as well as its efficiency. In this paper we will describe various
constraints and unplanned events, show their effects on HST observatory efficiency and stability, and discuss specific efforts
of the HST Long Range Planning group to minimize their impact.
Keywords: HST, observatory efficiency, long range plan stability, observing constraints, unplanned events
1. INTRODUCTION
Best efforts and intentions cannot overcome the inevitable limitations of use and occurrence of unplanned events that are
inherent in any astronomical observatory. The best that can be done is to identify all possible observing constraints in
advance and as accurately as possible estimate the amount of time which will be lost to events outside the control of long
range planning. Accurate knowledge of the former allows development of an efficient plan for observatory use given the set
of observations being requested, while unplanned events modify these results in terms of observatory efficiency and Long
Range Plan (LRP) stability. While high goals are set for both of these metrics, the LRP cannot remain a static entity without
significantly decreasing efficiency. Failure to fully understand an observatory’s constraints results in less than optimal
efficiency. Failure to be flexible in responding to unplanned events results in a very stable LRP at the expense of telescope
efficiency. Science planners must find a balance between assuring individual users of an observatory that they will get their
data when expected and maximizing the use of available observing time. At first glance it might appear that all that can be
done is to build a plan given the circumstances at the time and simply respond to changes in constraints and events as they
occur. However, our experience in planning Hubble Space Telescope (HST) science activities has shown that being proactive
maximizes its use and minimizes lost opportunities to the greatest degree possible. Had these issues not been a focus over the
past ten years, the HST cost per unit of science data returned would be almost twice what it is today.
This discussion presents an overview of the process used in planning HST science activities, constraints on HST which limit
observatory efficiency, unplanned events which reduce both efficiency and LRP stability, and specific actions taken to
minimize their impact. In the closing section we present suggestions, based on our experiences, for consideration by planners
in observatories just beginning to develop their planning systems as well as those who have been in operation for some time.
We hope that you will find something useful for your applications and encourage you to contact us for more information or
provide suggestions to us based on your experiences.
2. THE CURRENT PLANNING AND SCHEDULING SYSTEM
2.1.
The Normal Process Flow
The HST system, without perturbation, beats on a yearly rhythm. The Call for Proposals, proposal selection, and addition to
the LRP occurs roughly once per year, and this periodic series of events is referred to as a Cycle. Each successful General
Observer (GO) proposal is allocated time in HST orbits. Observers decide how to distribute their approved orbits among
visits which define a sequence of observations on an individual or set of related targets. When added to the LRP, each visit is
assigned a Plan Window (PW) which represents the time frame it is available for scheduling and execution. Plan windows
may be modified following the initial assignment because of changes to the visit, spacecraft constraints, or conditions in the
LRP as a whole.
Visits are scheduled for execution onboard the spacecraft on a weekly basis throughout the Cycle. These week-long
segments, built approximately 3 weeks in advance of execution and referred to as flight calendars, represent a major
milestone for visits. Calendar generation involves the detailed queuing of spacecraft slews, Tracking and Data Relay Satellite
(TDRS) contact scheduling requests (negotiated with external agencies through a third party) for managing onboard data
storage, downlink and uplink opportunities, science instrument state transitions, solar array slews for power management,
scheduling of star-tracker update of the spacecraft inertial platform, scheduling of science and calibration parallel programs,
and other necessary spacecraft activities. The three week build cycle means that 3 calendars are in work at any given time and
that changes to visits become less likely once they are scheduled because of the process overheads involved.
2.2.
New Cycle Ingest
Before the Time Allocation Committee (TAC) meets, the current scheduling rate, failure rate, and remaining number of orbits
to be scheduled are determined. Combined with the planned nominal end date for the upcoming cycle, the available number
of GO orbits is computed from these values as well as the estimated number of Director’s Discretionary (DD), Guaranteed
Time Observer (GTO) and science calibration orbits that will be executed during the cycle.
After the awarding of telescope time, observers are given approximately 3 months to prepare their observations for execution
onboard HST. Successful proposers submit their Phase 2s, which are detailed exposure-by-exposure, visit-by-visit
instructions for executing their observations. The Cycle calibration requirements and their Phase 2s are defined after GO
Phase 2s have been submitted based in part upon their submissions. Proposals are then processed en-masse through the
institute’s proposal implementation and preparation software which validates the Phase 2s and transforms their instructions
into intermediate products for use by the planning and scheduling systems. The visits are then assigned plan windows enmasse by SPIKE (Science Planning Intelligent Knowledge Engine), which is a LISP-based software package capable of
producing PWs based upon visit constraints. Visits not receiving PWs typically fall into several categories:
•
•
•
Targets of Opportunity
Unschedulable Visits
SNAPs
SNAP proposals are not guaranteed to be executed and never receive plan windows since they are used only to fill gaps in
calendars otherwise not utilized by prime science. However the other categories provide a source of instability and
uncertainty for long range planning. Visits can be unschedulable for a wide variety of reasons: unsuitable or no valid guide
star availability, linked relative orient or timing constraint violations, or other incompatible observer requirements.
2.3.
Weekly Maintenance
When building the plan, the subscription level used is extremely important. During the Cycle, visits from many sources are
added to the plan nearly every week. For example, Target of Opportunity observations are activated, Directors Discretionary
time is awarded, failed observations are repeated, and originally unschedulable visits are made schedulable. These
perturbations tend to occur at a low level throughout the cycle. Past experience has shown that they can make up about 10%
of the observations that are scheduled. Subscription levels must be adjusted in the initial plan each Cycle to allow room for
these activities. In weeks when these events do not occur, unrestricted observations are pulled early in the LRP to fill out the
near term schedules.
3. HST OBSERVING CONSTRAINTS
3.1.
The Low Earth Orbit HST environment
The HST is in a low earth orbit (LEO) with an orbital period of approximately 96.5 minutes. This introduces a variety of
observing constraints which are described in the following sections.
3.1.1.
Orbit Plane and Target Viewing
The earth’s oblacity produces a retrograde precession of the HST orbit plane with an approximate 56 day sidereal period.
The declination of the target being viewed from this orbit determines the amplitude of the viewing time fluctuations, while
the right ascension governs its phasing. Depending on target declination, viewing times range from approximately 52 – 96.5
minutes per orbit. At a given declination, precession causes these times to fluctuate from roughly ± 1 minute at declinations
below 30 degrees to as much as 20 minutes or more at higher declinations. Reflected and scattered light also restrict the
science instruments, and to a lesser degree the pointing control system (PCS), from observing targets within 20 degrees of
the bright earth limb and 7.6 degrees of the dark earth limb. The target viewing is further modulated by these restrictions
which depend on the sidereal season. As the HST orbit degrades over time due to atmospheric drag, these viewing times
decrease as the apparent disk of the earth increases. As HST’s attitude is adjusted for each observation, the drag varies
causing a continuous change in the mean velocity of HST in its orbit. This results in weekly predictions of entry and exit
times for events like target viewing that are uncertain by a few seconds. This uncertainty can increase up to many minutes for
predictions beyond several weeks from execution since they are based on extrapolated linear fits to the changing HST orbit
model.
3.1.2.
Visit structure versus Target Viewing
Since a proposal’s visit structure is created well in advance of execution, assumptions must be made about how much time
will be available per orbit for a given target declination. The HST ground system currently uses a table of target viewing
times to structure observer’s exposures with overheads into sets of activities with similar telescope pointings. The duration of
the activities assigned to each set is constrained by this table and a schedulability parameter that describes the percentage of
the physical orbits per year the visit should be structured to fit. For example, if the minimum target viewing time per orbit at
55 degrees declination is 55 minutes, then a visit which is structured to require just 55 minutes per orbit should be
schedulable in 100% of the orbits available to that target. This visit structure provides maximum planning flexibility and is
very efficient in orbits where the target viewing time is equal to the minimum. But at this declination, the viewing time also
may reach a near maximum of 66 minutes where the 55 minute visit structure does not take full advantage of the available
time on target. On the other hand, increasing alignment duration to use the 66 minutes would reduce the planning flexibility
to only 10% of available orbits. The chosen solution is to select durations which maximize per-orbit-efficiency and planning
flexibility.
For the past five years a default value of 30% has been used in an effort to balance efficiency and flexibility for as many
visits as possible in all declination ranges. Since visits currently are not structured for 100% of available orbits, their
structure is in effect a constraint that affects plan window assignment.
3.1.3.
Continuous and near continuous viewing opportunities
The poles of the HST orbit lie 28.5 degrees from Earth'
s equatorial pole and combine with the HST earth limb avoidance to
define a variable circle of roughly 4 – 16 degrees in radius centered on the orbit pole near ± 61.5º declination. Orbital
precession causes these northern and southern continuous viewing zones (CVZ) to sweep completely around the equatorial
pole in right ascension. In principle, continuous viewing opportunities increase HST observing efficiency since the entire
orbit can be filled with science exposures and earth occultation does not occur. However, CVZ activities add implicit timing
restrictions that create potential conflicts. Targets in this declination range are only visible in the telescope’s CVZ once every
56 days allowing up to seven CVZ opportunities per year. Proposer specifications may reduce this to only one or two
opportunities by requiring that the uninterrupted time on target be longer than the available time in most of the opportunities.
An individual CVZ window duration may be as little as one orbit or up to 9 days in duration depending on the distance of the
target from the HST orbit pole and the position of the sun relative to the target during the CVZ time. In addition, the South
Atlantic Anomaly (SAA), described in more detail later, restricts the longest uninterrupted observation to about 5-6 orbits
regardless of the total available CVZ window size.
While observations near the orbit pole contribute to increased open-shutter-time efficiency overall, the benefits at the edges
of the CVZ declination range tend to be negated by the target imposed planning restrictions. For example, a visit which is
restricted to just one or two orbits per year may just as likely require an orbit in the middle of a sequence of SAA-free orbits
as in SAA-intersected orbits. This eliminates an opportunity for a 5 or 6 orbit visit that requires SAA-free time and forces the
planning system to find other visits of the right orbit duration to fit around the time critical CVZ activity. Depending on the
visit pool, there may not be a good selection of observations to fill in the gaps. When this occurs, the overall efficiency of the
telescope has been diminished for the sake of an individual observation.
An alternative is to preference towards, but not require, scheduling in CVZ windows for targets which can use them. By
splitting a non-interruptable CVZ observing sequence into multiple contiguous subsets, near-CVZ as well as pure
opportunities are made available. Near-CVZ windows are interrupted by earth occultation but provide from 60 minutes to
almost a full orbit of target viewing for a week or more on either side of the pure CVZ opportunity. Using these windows, the
visit is no longer time critical and is flexible enough to help maximize the overall telescope efficiency at minimal cost to the
per orbit efficiency. There are two operating overhead times involved in structuring a visit to take advantage of this
flexibility. First, from zero to a few minutes of overheads are required for each uninterruptable subset depending on the
instrument and detector mode being used. Secondly, each interruption by earth occultation requires ~5 minutes for guide star
reacquisition for the next observing subset. While the instrument overhead is fixed by the visit structure, the guide star
overhead applies only if the visit is actually scheduled outside of the CVZ window. Regardless, the total additional overhead
is usually small compared to the total observing time and the overall benefit. Currently, this strategy is only applied when
conflicts between visits are identified and cannot be resolved by other means.
3.2.
South Atlantic Anomaly
Earth’s offset magnetic dipole allows solar and extrasolar particle radiation to penetrate closer to its surface in the region
known as the South Atlantic Anomaly (SAA) and produces a many orders of magnitude increase in the ionizing radiation
density at HST’s altitude. Data contamination increases for all detectors and presents safety concerns for some instruments.
Thus observations are typically not executed during passage through the SAA. HST passes through some part of the SAA in
about 60% of the physical orbits each day, spending from one second to 30 minutes in each SAA passage depending on
where the orbit track intersects the region. As the earth rotates and the orbital plane precesses, the SAA and its interaction
with the HST orbit follows a nearly day long cycle of 8 or 9 orbits that intersect the SAA followed by 7 or 6 orbits that do not
intersect the SAA. The SAA passages thus cause interruptions to HST observing that follow an orbital and nearly day-long
cycle. For Wide Field Planetary Camera (WFPC) and Space Telescope Imaging Spectrograph/ Charged Coupled Device
(STIS/CCD) observations, the SAA passages require the observations to be suspended during the SAA passage. However,
the STIS Multi-Anode Microchannel Array (MAMA) detectors, are not permitted to be in or transitioning to an operating
state while in the SAA. Additionally, to prevent the cycling of the high voltage power supplies each SAA intersected orbit, it
was decided to operate the STIS/MAMAs only during the SAA-free orbits of a day.
To achieve maximum scheduling efficiency, this implies that ~60% of the visit orbits scheduled must be scheduled using
SAA intersected orbits. For this to occur, the visits must have structures which fit well in the physical orbits where target
viewing time has been interrupted or reduced by an SAA passage (requiring more orbits to obtain the desired science
exposure time). The visits must also be scheduled at times during the 56 day sidereal precession cycle when the SAA passage
will occur while the selected target is being occulted by the earth. How well the SAA passages can be hidden in occultation
depends on the distribution of available targets throughout the Cycle, the types of scheduling restrictions placed on the visits
by observers, and the geometry of the HST orbit with respect to the Sun. (Note that about 40% of the HST observations use
STIS/MAMA. Thus the target pool for SAA hiding is substantially reduced since that instrument cannot be used during SAA
intersected orbits whether or not they are hidden in earth occultation.) Because the SAA lies primarily south of the equator,
HST mostly observes while physically in the northern half of SAA intersected orbits. Thus SAA hiding is maximized for
targets which are in the general direction of the right ascension of the northern-most point of the orbit plane.
3.2.1.
The Northpoint weeks
However, there is a period of time once every orbital synodic precession period (~48 days) when the right ascension of the
north point of the orbit will be aligned with the right ascension of the Sun. Unfortunately, at these times, most of the sky that
contain targets optimal for SAA hiding cannot be observed because of the 50 degree solar avoidance angle restriction. Thus
every 48 days conditions are maximized to produce a decrease in the number of orbits scheduled because statistically fewer
targets are available that can schedule in SAA intersected orbits. Theoretically, the scheduling impacts around these north
point (NP) weeks can be minimized by appropriate target selection. In practice however, target availability and user
specified restrictions combine to thwart even the best efforts to maximize efficiency in many of these intervals.
Scheduled Orbits per week
SAA Free
SAA Intersected
NP time
120
Scheduled Orbits
100
80
60
40
20
10-Jan-00
6-Dec-99
1-Nov-99
27-Sep-99
23-Aug-99
19-Jul-99
14-Jun-99
10-May-99
5-Apr-99
1-Mar-99
25-Jan-99
21-Dec-98
16-Nov-98
12-Oct-98
7-Sep-98
3-Aug-98
29-Jun-98
25-May-98
20-Apr-98
16-Mar-98
9-Feb-98
5-Jan-98
0
SMS Date
Figure 1. Distribution of scheduled TAC orbits (SAA-free vs SAA-intersected)
As an example, Figure 1. displays the number of TAC-awarded orbits that were scheduled per week between January 1998,
and January 2000. The data show whether or not observations were scheduled when the HST orbit intersected the SAA.
Several important features are apparent. First, the system is able to schedule observations in the SAA free orbits at a very
consistent level. Second, the majority of the variation in the total number of orbits scheduled per week is due to variation in
the number of orbits scheduled in the SAA intersected part of the day. The variations exhibit long term and cyclic patterns.
In particular, in late December, 1998, the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) operations ceased
as the NICMOS cryogen was exhausted. At that time, the average total scheduled TAC orbits declined, from about 88 orbits
per week to 80 orbits per week. NICMOS visit structure allowed scheduling around the SAA passages fairly well. With
NICMOS gone, the HST scheduling system was left with mostly STIS/CCD and WFPC observations that are difficult to
schedule in SAA intersected orbits. Note that with the transition to the new instrument mix, the decline in the scheduling
rate occurred solely in the SAA intersected orbits. Finally, note that over 50% of times identified as north point weeks occur
when there is a local deep minimum in the number of scheduled orbits. The spike in the number of orbits scheduled in June
1998, was the result of scheduling many NICMOS observations that required less than one orbit (HST visit duration is
counted as integer orbits). The peak in January 2000 was caused by the scheduling of many short duration Servicing Mission
Observatory Verification observations. Finally, the deep reductions in scheduled orbits in May and December 1999, were due
to vehicle safing events where three days and 6 weeks of observations were lost respectively.
3.2.2.
Working Around the SAA
About four years ago the SPIKE planning system was enhanced such that it would identify and assign appropriate plan
windows to visits at times when SAA hiding is maximized paying particular attention to the north point weeks. The system
gives the desired result when this criteria is applied to visits that are free of restrictions due to visit structure and other user
specified timing constraints. This implies that the inability to more effectively schedule around the SAA is mainly due to the
restrictions placed on visits through the structure of their observing sequences or required absolute and relative timing
dependencies. Many of these restrictions are legitimate and necessary to achieve the desired scientific results. When
unnecessary restrictions are discovered, recommendations may be made to observers for their relaxation or complete
removal.
3.3.
Sun & Moon Avoidance
A 50-degree cone of avoidance around the sun and a 9-degree cone around the moon define limits within which observations
are not normally planned or executed. This prevents stray light contamination and damage to the inside of the telescope from
those bright sources. These restrictions are something most observatories are forced to work around and are in fact the least
restrictive of HST’s constraints.
3.4.
Spacecraft Roll Restrictions
HST'
s solar panels rotate around only one axis. The resulting single degree of freedom combined with depth of battery
discharge restrictions translates into roll limits of the spacecraft about the target-telescope line of sight. Additionally, the
'
belly'of the telescope is not designed to be exposed to direct sunlight (thermal radiators, and fixed-head star trackers are
located there), and the aperture door acts as a sun-shade for the tube interior over only a restricted roll range. Solar arrays
also are not allowed to be shadowed by any part of the telescope. The combination of these restrictions results in spacecraft
roll limits that are effectively a function of the angle between the target, telescope and the Sun. Therefore, the allowed normal
roll range of the telescope at a given pointing depends on the sidereal date of the observation. By itself, this constraint only
restricts viewing of most targets at Sun-telescope angles below 90º where the normal roll tolerance from the nominal position
is only ±5 degrees. At Sun angles between 90º - 178º there is a range of roll limits from ±21º to ±30º. Between 178º - 180º
the roll is unrestricted. As will be discussed later, the roll constraint plays the most havoc when combined with other
restrictions.
3.5.
Guide Star Availability
Nearly all observations require one or two guide stars to enable HST to maintain stable pointing during long exposures. The
use of one guide star only provides X and Y axis control and allows the telescope to drift in roll about the guide star position
at a rate of about one milliarcsecond per second of time. The magnitude of the target’s motion in the science aperture
increases with the exposure time and the length of the moment arm between the guide star and target position. Therefore,
most observations require two guide stars to provide the precision three axis control needed for observing at long exposure
times. Three Fine Guidance Sensors (FGS) use pickle shaped areas lying at the outer edge of the primary field of view to
locate and lock onto the necessary guide stars. Because of roll restrictions described in the previous section and acentric
locations of science apertures, guide stars move in and out of the FGS fields of view over time and may not be available at
all times for a given target. Since telescope position dictates guide star availability, restrictions occur either because there are
no guide stars at the appropriate time or none at all to support an observation. The only remedy in both cases is to reposition
the telescope in such a manner that guide stars will appear in the FGS fields of view. This may not be feasible if
repositioning forces the desired target and science aperture out of the alignment. In general, lack of guide star availability
does not create restrictions which significantly affect observatory efficiency. On the other hand Long Range Plan stability is
affected when use of an individual FGS must be restricted in some way. To date, two FGSs have been replaced and some
performance problems have placed restrictions on how one FGS could be used. Since guide stars are identified as part of the
implementation and planning process on the ground, plan windows for affected visits may be reassigned due to lack of
alternate guide stars in the original timeframe.
3.6.
User Timing and Orient Restrictions
Over 75% of all HST science observations have one or more timing or orient restrictions which limit when they can be
executed. The restrictions can be on a single visit or they can relate more than one visit to each other. Typical timing
restrictions force a visit to occur either before, between, or after certain dates and times. Timing linksets can also be defined
with minimum and maximum separations to sequence or group observations in time. Timing of observations to occur close
to a certain phase of a given periodicity are also possible.
Visits are also time restricted implicitly through the use of absolute and relative orientation restrictions. Because the HST
instruments are fixed with respect to the spacecraft, the entire telescope must be rolled if an observer needs to orient a target
with respect to a science aperture. Since allowable roll angles are tied to the telescope-Sun angle, this type of observation is
restricted to times when the requested angle or angles do not violate the spacecraft roll constraint. Visits can be restricted to
single or multiple orientation ranges and therefore single or multiple observing windows. Orient linksets are used to specify
the orientation of observations with respect to each other. Two or more visits may be restricted to occur at the same
orientation or at offset angles from a reference visit within the set.
User requested timing or orient restrictions are applied to visits unless they cannot be supported because of conflicts with
previously mentioned constraints. This is true even when these restrictions may force visits to be scheduled at less than
optimal viewing times. While legitimate science requirements justify their use, anecdotal evidence from operational
experience indicates that many of the timing restrictions that are applied may not be necessary or at the very least could be
relaxed. Unfortunately, it is difficult to identify and measure this due to the time and cost required to review individual
science programs and the decisions observers make about them. Instead, each Cycle we attempt to improve HST user
education concerning the impact of these restrictions on overall observing efficiency and encourage them to be viewed as a
limited resource.
3.7.
Enfolded Constraints, Reduced Opportunities, and their Net Impact
The constraints mentioned to this point combine in various ways to create the best and worst in terms of observing efficiency.
Intersected constraints may result in observing windows at the most efficient or least efficient time for the given target, or no
observing suitability at all. Suitability refers to the possible times or orientations at which an observation can take place for a
given constraint. Because of the exclusive nature of constraints, an intersection of the suitability from individual constraints
yields the resulting possible scheduling opportunities, or the net suitability. If any of the individual suitabilities are mutually
exclusive, then the visit is unschedulable and cannot be assigned a plan window until a solution is found which resolves the
conflicts between the individual constraints. Restrictions or non-suitability in one visit is propagated to all members of a
timing or orient linkset.
Figure 2. Example of how constraints can combine to restrict observing opportunities
Figure 2 gives an example of how various multiple constraints can lead to very narrow observing opportunities. The
timelines at the top and bottom of the graph use a two digit year, decimal point, and a three digit day of year date format.
The horizontal blocks on the various lines show when the given constraint can be met, sometimes in combination with
other constraints, and sometimes by themselves. The top line shows the net result of intersecting all constraints. Note
the constraints of this example in the following order: a) Guide star availability prevents over half of all conceivable
opportunities from being possible; b) meeting the "SAME ORIENT" observer specified requirement between separate
observations further narrows [slightly] the observing opportunities; c) orbital viewing, is a combination of target
visibility, bright-earth limb constraints and visit structure, and again further reduces the potential opportunities; d) in
this case, because of the high target declination, neither the sun nor moon ever interfere with viewing opportunities [a
minority occurrence]; e) "USC" or user specified constraints, which in this particular case is the result of testscheduling the observation in the HST operational environment over a subset of the planning interval; these constraints
refine or verify those produced by the planning software; f) "Links To 01, etc." shows observer specified inter-visit
timing restrictions [sequence within 1-day, in this case] further reduce scheduling opportunities to a few days during the
year.
3.7.1.
Managing Observatory Constraints
Target viewing, Sun and Moon avoidance, spacecraft (roll and SAA), and guide star constraints are in essence fixed in stone.
Efficient use of HST depends on understanding them, building the right puzzle pieces, and fitting those pieces appropriately.
The HST Observation Planning Team is responsible for helping to build the pieces that both satisfy the observer’s science
goals and maximize the overall efficiency of the telescope. To do that we continually monitor and study how observer’s visit
structures and time related constraints fit into the mold created by the fixed constraints. Where appropriate, suggestions and
requests are made to change visits to better fit the mold. Likewise, adjustments must be made to the LRP if the automated
planning process fails to fit the available pieces optimally. And while an upper limit to efficient use of the telescope is
defined by the fixed constraints, the efficiencies achieved are limited by the types of visits that have been provided by the
users. Results of studies on how HST can be used more efficiently are fed back to users each observing cycle. The intent is to
allow users to make better choices, not to prevent the use of constraints which are needed to achieve quality science.
4. UNPLANNED EVENTS AFFECTING EFFICIENCY AND LRP STABILITY
4.1.
4.1.1.
Servicing Missions
Overview
Servicing Missions (SM) allow science instruments and other spacecraft components to be upgraded, repaired, or replaced.
However, while replacing, improving, or extending the capabilities of the HST, SMs and the subsequent vehicle and
instrument Observatory Verification (OV) also cause large instability to the LRP in addition to creating other planning
problems. Since initial insertion of HST into orbit there have been three Servicing Missions and Observatory Verifications
(SMOV). The HST science timeline is interrupted shortly after the shuttle is established in orbit about a day before the
shuttle-HST rendezvous to allow HST to be reconfigured and positioned for grapple. To date, the shortest SM duration has
been about five days. Once HST is released from the shuttle, OV starts during which the spacecraft and instrument
operation is verified. To avoid adhering new contaminants onto optical surfaces and to allow the contaminants to dissipate,
the telescope is pointed away from the bright earth during the first two weeks of OV. This prevents normal science operations
from resuming even if the instruments were ready. The number of activities and observations required to perform the
verification varies for each SMOV depending upon changes made to the telescope and complexity of the new instruments
installed.
While efforts are made to minimize the length of OV, there are time dependencies between the different OV activities that
prevent scheduling them as efficiently as science observations. This can extend the time for instrument (re)commissioning
and resumption of science operations. Scheduling and execution of science observations is resumed immediately following
OV checkout of the instrument or mode they are using. Typically, this has meant that the WFPC2 is first back on line as it is
the simplest instrument. The minimum total impact to the normal science observing schedule is roughly equivalent to 4-5
weeks of downtime (including SM). SMOVs affect the stability of the LRP since the large number of observations that were
originally planned to execute in those time frames must be delayed. Table 1 lists the impacts from previous SMOVs. Since
the number of orbits scheduled per week has changed between the different times, the equivalent time in weeks is also listed.
The total science impact for SMOV3A does not include the down time during the SM since the science timeline had been
suspended six weeks earlier because of a vehicle problem.
Table 1. Executed SMOV vs. Displaced Science Orbits
SMOV1
SMOV2
SMOV3A
SMOV start
January 1994
April 1997
December 1999
Executed SMOV orbits
711 orbits
772 orbits
225 orbits
Total Science Impact
840 orbits (9.5 weeks)
1000 orbits (11 weeks)
275 orbits (3.5 weeks)
4.1.2.
Attempting to Minimize Impacts Using Long Range Planning
The perturbing affects of a SMOV on the LRP are reduced by purposely assigning unrestricted observations to the anticipated
timeframe and slightly lowering the subscription level in the weeks following SMOV. This is done during the initial
planning of the Cycle that includes the SMOV period. If the SM occurs at its planned time, the impacted visits can in
principle be shifted to slightly later times without having to be moved to the end of the scheduling queue. However, while
the duration of the SMOV is known, the actual launch date cannot be known with certainty. This greatly complicates and
may defeat attempts to plan observations in the affected period. In the recent case of SMOV3A, the date was changed seven
times before launch occurred in mid-December 1999, three months later than initially planned. To minimize further
instability in the original plan, no adjustments were made to the LRP after the first major SM delay until the Space Shuttle
launch actually occurred and the SMOV start date was known with certainty.
4.1.3.
Reacting to SMOV and Its Effects
During OV, the LRP has to remain flexible to allow for changing events on the spacecraft. Typically instruments or
particular modes of operation for an instrument will not be enabled until after a particular series of tests have been performed.
There is a push from everyone to have science observations executing as quickly as possible. However, commissioning dates
can only move to later times as more information is gained about the science instrument. Experience has shown that this
process cannot be rushed. Science observations cannot be scheduled in the timeline until the instrument or its needed modes
have been officially enabled. Premature scheduling of unenabled modes can cause calendars to be rebuilt to remove the
science observations when a critical OV check fails. Thus, for the first few weeks of OV, the LRP is not published since
science visits are (re)added to the LRP in reaction to instruments being enabled. Finally, once the OV activities begin to
stabilize, the remainder of the OV observations are merged into the LRP and it is released to the public. In a worst case
scenario, the LRP can experience drastic changes. For example, once problems were discovered with STIS and NICMOS
during SMOV2, the number of SMOV observations doubled (+400 orbits) in the process of identifying the instrument
problems and implementing solutions. Analysis revealed that STIS MAMA observations would have to be restricted to less
than half the available orbits in a day to avoid operating in the SAA. In addition, the projected NICMOS lifetime was
reduced from five to 1.5 years. In order to approach the intended science productivity of NICMOS during its shortened
lifetime, approximately 1000 new orbits of NICMOS were accepted and placed into the LRP. These unexpected orbits were
restricted to an eight month period in the LRP and observations not using NICMOS and STIS had to be delayed. In this
instance, the LRP was completely rebuilt to incorporate the needed changes.
4.2.
4.2.1.
System Failures that Result in Lost Observations
Vehicle and Instrument Safing Events
Occasionally, a problem will be detected by the onboard software or hardware that causes the vehicle to suspend all
observations until the problem is identified, fixed, and the instruments recovered. They are caused by random events, such
as component failures, limit check failures, and human errors, and cannot be predicted with any accuracy. Depending upon
the situation, the recovery may take as little as a few days to over a week. Over the last few years these events have averaged
just over one vehicle safing per year, losing less than a week’s worth of observations each time. For planning purposes they
are ignored, however once a safing does occur, the LRP must react by replanning the lost observations.
Problems can also occur at the instrument level where only a particular instrument is off line. Since observations using the
other instruments continue, the impact to planning is less compared to a vehicle safing of the same duration. However,
instrument safings occur more frequently than vehicle safings. For example, in 1998, 10 instrument safings occurred.
Because of the higher frequency of events, observations lost because of instrument safings are grouped with other failed
observations and are taken into account during Cycle planning.
4.2.2.
Other Process and System Failures
As with any endeavor, the human factor either directly or indirectly affects HST observatory efficiency and plan stability.
This is manifested at all levels including observer interactions, proposal implementation, and the planning and scheduling
processes. Likewise, spacecraft and science instrument systems can fail in ways which do not cause a safing, but may reduce
data quality or render it unusable. A high priority is placed on identifying failure causes and making system and procedure
enhancements to prevent the same problems from happening in the future.
4.2.3.
Repeating Executed Observations Due to Process and Systems Failures
When failures occur, observers are permitted to file Hubble Observation Problem Reports (HOPRs) requesting that the
affected activity be repeated. Currently the repeat rate is approximately 3% of all executed observations. The LRP must take
this repeat rate into account by reducing subscription levels in order to minimize instability. The following chart categorizes
the types of things that contribute to this HOPR rate.
O rb its L o s t b y Q u a rte r & C a te g o ry
G ro u n d S ys te m
In s tru m e n t & S a fin g s
P o in tin g C o n tro l
40
35
Sum of Orbits
30
25
20
15
10
5
0
9 7 .1 2 5
9 7 .3 7 5
9 7 .6 2 5
9 7 .8 7 5
9 8 .1 2 5
9 8 .3 7 5
9 8 .6 2 5
9 8 .8 7 5
9 9 .1 2 5
9 9 .3 7 5
9 9 .6 2 5
9 9 .8 7 5
Q u a rte r
Figure 3. Number of orbits lost to various categories for observations "making it to the focal plane" is
shown by quarter between SM2 and SM3A.
Ground System refers to problems discovered in the specification, implementation or scheduling of the
observer’s visit, or software problems in the ground system.
Instrument Problems and Safings refers to data quality or instrument safings which prevents successful
execution.
Pointing Control refers to guide star acquisition failures and related guiding problems.
The final quarter of 1999 does not contain the most recent safemode event which affected six weeks of
observations. Also not included are visits which could be immediately replanned.
4.3.
Last Minute Changes
Flight calendar building begins three weeks prior to execution with the goal of maximizing the number of orbits scheduled
from the pool of visits in the LRP. Once a calendar is built, changes are limited because of the difficult technical nature of
the TDRS contact request and calendar building processes. Typically, when changes occur to an existing calendar, they are
localized to the area affected by a visit that is removed or added to the schedule. That is, the calendar is not rebuilt from
scratch even though that may provide the best opportunity to recover any net loss in orbits and efficiency that might result
from the change. In the case where visits are removed from an existing calendar, the best attempt is made to find a new
visit(s) to schedule that will replace or minimize the number of orbits lost.
These impacts are a result of late changes by observers, visit implementation problems, previously unidentified constraint
violations, target of opportunity activation or spacecraft anomalies. A change on one calendar could affect data volume and
other schedule management items on the other two unexcuted calendars. If a visit is a member of a timing or orient linkset, its
related members on other calendars may be affected as well. Visits that are removed from calendars need to be re-planned for
a future date if there is no time left or they cannot be reworked promptly for remaining time in their existing plan window.
The subscription levels in the LRP must allow for this uncertainty to minimize overall plan instability.
Avoiding last minute calendar modifications is a high priority since they are costly for calendar efficiency, LRP stability, and
calendar building resources. On average, the scheduling rate drops ~2 orbits per week between the initial and executing
calendar from 80.8 to 79.0 orbits. This corresponds to roughly a 2% decrease in efficiency from the initial calendar. The key
to minimizing this reduction lies with the calendar building timeline itself. Over the past five years, the ground system has
been refined to the point where it has been possible to reduce this timeline from eight to the current three weeks. This allows
less opportunity for previously mentioned events to disrupt the calendar building process. Efforts to further reduce the current
three week interval are ongoing, and as it becomes shorter, only unavoidable calendar changes such as quick turnaround
Targets of Opportunity (< 7 days) and spacecraft or instrument anomalies will occur. It should be noted that observer initiated
changes are not discouraged in this process. Instead, the cost to implement a change is reduced since it will more likely occur
before its intended calendar is built. In addition, the proposal development and implementation process is continually
scrutinized to find where improvements can be made to prevent problems from slipping through to the latter parts of the HST
ground system process.
4.4.
Effects of HST Ephemeris Uncertainties
Another important way that plan stability is negatively impacted comes from the inherent uncertainties in the prediction of
orbital events many weeks in advance of execution of an observation. When the tolerance of a timing constraint is less than
the accumulated uncertainty in the predicted position of HST within its orbit, it is impossible to know in advance if a highly
restricted visit will be able to schedule at the assigned time. For example, if the prediction of the HST ascending node
crossing time is in error by 3 minutes each week, then over the course of 16 weeks, the prediction of that crossing time could
be off by as much as half an orbit. Likewise, the time when a target becomes visible to HST will also differ from the
prediction. If a visit needs to begin at time T +/-15 minutes, the target may be in earth occultation at time T when an attempt
is made to schedule it, using an updated orbit model, at T-3 weeks even though the prediction at T-16 weeks indicated
otherwise. This is not considered a last minute change in the sense that it is identified before the initial calendar is built and
therefore does not contribute to calendar rework. However, since we cannot accurately predict when they will execute, these
visits may contribute to schedule conflicts and inefficiencies by displacing visits which have better target viewing efficiencies
at those times. The displaced visits may then need to be re-planned for less optimal viewing times.
5. SUMMARY
It has been shown that the major impacts to HST observing efficiency is the spacecraft itself, the environment that it must
operate in, and observer specified restrictions. The low earth orbit of HST limits the total uninterrupted time that may be
spent on data collection depending on target declination. Earth scattered light restricts target viewing beyond the physical
occultation of the target by the earth alone. Visits structured to the minimum viewing time provide optimal planning
flexibility but may not satisfy observer requirements. The LRP can be optimized for efficiency when appropriately sized
visits are available to maximize target viewing and SAA hiding each orbit. However, the planning system may have no
alternative but to choose a less efficient observing opportunity if the observer has specified target viewing, absolute or
relative timing, or orient restrictions which force the visit to be placed at non-optimal times.
The stability of the LRP is affected by uncontrollable events such as spacecraft or instrument safings, servicing missions, and
accuracy of orbital event predictions. In these cases we must be prepared to respond as quickly and accurately as possible to
minimize disruptions to individual programs and the LRP as a whole. Other unplanned events such as observer requested
visit changes and implementation and scheduling problems also create instability which must be managed from week to
week. These instabilities can be minimized by better user support and education and process improvements at all levels of the
ground system from proposal ingest and processing through long range planning and flight scheduling. These continued
improvements allow necessary observer changes while minimizing LRP and flight calendar instability.
Good science planning also depends on good ground system support. The issues discussed in this paper must be translated
into solutions in the ground system software and procedures that are used to transform observer requests into telescope
activities. A key element in achieving this is good data collection. We strongly encourage spending time and resources up
front to define metrics for monitoring and evaluating the performance not only of the observatory, but just as important, the
ground system that supports it. The collection of these metrics should be built into the ground system itself so that the data is
available on demand and new metrics should be added as they are identified.
Absent of spacecraft problems, observatory efficiency and LRP stability is maximized to the extent that the observation
planning staff understand and are able to define valid ways to use HST around its observing constraints while satisfying
observer’s science requirements. These issues must be effectively communicated to the user community through
documentation and program development tools. With a better understanding of HST’s operating constraints, those
responsible for defining, implementing, planning and scheduling individual and community science programs can make
better choices about how the telescope can actually be used to its fullest potential.
ACKNOWLEDGMENTS
The authors wish to thank Merle Reinhart for providing his technical expertise and support in the preparation of this
manuscript. We also wish to acknowledge the contributions of the numerous individuals in the former HST PRESTO
Division and current Hubble Operations Division who were instrumental in bringing HST observing efficiency to the high
levels consistently achieved today. The maturity of the HST ground system and its ability to effectively manage the
constraints and unplanned events described in this paper is due to their hard work, dedication and willingness to explore new
ways of operating a major observatory. And where appropriate, we wish to thank our spouses, families, and friends for their
patience and endurance, not only during the preparation of this manuscript, but through all of the times past, present, and
future when it may seem that we have no life outside of HST Operations.
REFERENCES
1.
2.
3.
4.
M. Giuliano, “SPIKE suitability defined”, Internal SPIKE document: http://www.stsci.edu/apsb/doc/spike/suitability.txt
M. Giuliano, "Achieving Stable Observing Schedules in an Unstable World, Proceedings of ADASS 97
J. Biretta, W. Kinzel, M. Reinhart, B. Whitmore, and J. Younger, ITM-1999-01: "Maintaining Scheduling Efficiency in
a Two-Instrument Environment: Reducing the Competition for SAA-Free Orbits", Hubble Operations Department
Internal Technical Memoranda: http://presto.stsci.edu/itm/itm.html
More information on the SPIKE planning system can be found on the WWW at:
http://www.stsci.edu/spike/index.html#what-is-spike