SAA-hiding Implications of Restrictive < 3-Gyro Modes
Abstract:
Two classifications of SAA-hiding are defined and discussed in terms of the
potential impact on the HST LRP for restrictive (~80-degrees of ecliptic longitude
availability) 2-Gyro modes. Strict geometric hiding ability of SAA-impacted
orbits is shown to be dramatically impacted on top of the normal reduction in
opportunities caused by the global shrinkage of the constraint windows. If
heuristic SAA-hiding is impacted in the same proportion, then an approximate
28-orbit/week drop in calendar scheduling rates can be expected due to this
effect in the SAA-impacted part of the day—alone--unless major changes to
candidate selection is implemented. Even if candidate selection is modified,
SAA-impacts during orbital viewing will eliminate usability of a significant
fraction of SAA-impacted orbits. Large programs (> 100 orbits) on a single target
must be relatively unrestricted by the users (e.g., relaxed orient ranges) and have
a significant fraction of visits schedule as Sched-100 if they are to allow other
programs time aboard the telescope.
Introduction:
Approximately half of all HST orbits are impacted by the SAA (intersect an SAA
model contour). Being able to use these orbits in some fashion for science or
calibration is critical for maximizing efficient use of HST whether under nominal
3-Gyro or sub-3-Gyro operations. "SAA-hiding" is an important concept in HST
Science planning, so it is useful to briefly review and clarify terminology.
What is "SAA-hiding"?
One common way of stating the geometry of SAA-hiding is that the HST passage
through the SAA is hidden in occultation. However there are other useful ways
of describing the same occurrence. "Prime" SAA-hiding during a given orbit
occurs when the spacecraft passes through the SAA and the observatory is
pointing somewhere along the nadir-zenith circle (straight down, in the simplest
case). A bit cruder way of stating this geometry is that the earth-longitude of an
SAA-passage is approximately aligned with an RA opposite (180-degrees away
from) of the target at which HST is pointing.
"SAA-hiding" is often used in two contexts: "geometric SAA-hiding" is when the
SAA passage occurs completely within the time of target occultation; and
"heuristic SAA-hiding" or "LRP SAA-hiding” refers to how a visit schedules in
the planning and scheduling system, such as in SPIKE or SPSS. To be a heuristic
SAA-hider, a visit must stay within its TRANS estimated orbits while scheduling
through SAA-impacted orbits. The distinction is one of whether the SAA can be
geometrically hidden, i.e., completely contained within target occultation, for a
given target, versus whether a visit--which may not completely fill the allotted
orbital viewing, for example--can schedule compatibly with the SAA and not
expand in total duration. The latter type of SAA-hiding does not have a unique
proper name but we will refer to it as "heuristic SAA-hiding". In the every-day
vocabulary of the LRP group, this type is usually referred to as simply "SAAhiding", but here we preserve an explicit distinction. The difference is important
and its implications play a role in the discussions that follow.
Visualization of Geometric SAA-hiding
Each geometrical conceptualization of geometric SAA-hiding, as first introduced,
is similar, and which ever is used, the same implication is derived when
examining SAA-hiding for successive SAA-impacted orbits. With each
revolution of HST around the earth, a finite amount of time passes, and
correspondingly the earth rotates beneath. Approximately 96 minutes pass for a
sidereal orbit--15 full sidereal orbits per day, with 24-degrees of earth longitude
movement with respect to the stars. Just as important, about 103 minutes
separate longitude passages--approximately 14 revolutions corresponding to
about 25.7 degrees of rotation out of 360.
Geometric SAAHiding
SAA Passage
Target
Occultation
SAA Passage occurs completely
during target occultation.
Figure 1: An illustration of Geometric SAA-hiding. Geometric SAA-hiding occurs when the
entire SAA passage is in target occultation.
To first approximation then, during each orbit or revolution of HST, the Right
Ascension of a given longitude shifts by 1.6 hours (sidereal orbit) or 1.7 hours
(HST revolution). Likewise, the RA of the SAA longitudes also shifts by this
same amount in the direction of earth's rotation (increasing RA). From this, the
prime-SAA hiding is seen to advance in RA throughout the day, orbit-by orbit,
by about 1.7 hours of RA at a time. In order to use targets which provide
"geometric SAA-hiding" throughout the SAA-impacted orbits, the scheduling
system could sequence visits with approximate separations of 1.7 hours RA (if
each visit were 1-orbit in length) throughout the SAA-impacted orbits. SAAhiding in this context—geometric SAA-hiding—depends upon the time of day as
well as the target RA. Since SAA-impacted orbits constitute approximately half
of all orbits, the geometric SAA-hiding target RAs sweep through half of the total
sky (one hemisphere) each day.
Heuristic SAA-hiders: a super-class of SAA-hiding
All by itself, this would imply that one could not get "geometric SAA-hiding" for
more than a few orbits consecutively for a given target. However, from a
scheduling and planning perspective, the more important class of visits is
"heuristic SAA-hiders". These visits are sometimes more than 1 or 2 orbits in
length and can schedule within their TRANS estimated orbits during time
windows that can extend several days or more. The reasons for this extended or
"heuristic SAA-hiding" are multiple: 1) visits do not always fill orbital visibility
during each orbit, 2) the geometry of orbital visibility is more complex in that
orbit plane orientation, earth-sun season, and target declination are factors in
whether or not a visit can schedule while the target is above the horizon, and 3)
the alignment structure (indivisible scheduling segments) of a visit may allow
scheduling "around" an SAA passage, allowing passages to occur during target
visibility.
Heuristic SAA-hiding
Geometric SAA-hiding
Figure 2: Venn diagram illustrating that geometric SAA-hiders are a subset of the more
general LRP- or Heuristic SAA-hiders.
The location of the hemisphere of geometric-SAA-hiding targets shifts with the
orientation of the HST orbit-plane. HST's orbit precesses "backward" (opposite
to Earth's rotation) with a sidereal period of 56-days. Correspondingly, with
each passing day, the hemisphere of prime-hiding RAs rotates on the sky by
about 6.4 degrees in a direction of decreasing RA. Over the course of one month,
the selection of geometric SAA-hiding RAs sweeps over the entire sky, and after
2 months the hemispherical coverage repeats. After 5/6ths of a precession
period, targets in an RA band about 4 hours wide (with higher RA values)
become available as geometric SAA-hiders that were not available as geometric
SAA-hiders during the previous precession cycle. Similarly, targets in the lower
4-hours of the geometric SAA-hiding band "drop out" of being able to
geometrically hide the SAA. The reader should keep in mind that this
characterization is true only for "geometric SAA-hiding", but geometric SAAhiding is not as important as "heuristic SAA-hiding" for telescope scheduling
purposes.
Review of Major “Restrictive” 2-Gyro Conditions.
Under 2-gyro operations, the required geometries for maximum effective use of
orbital visibility for science activities is such that targets are generally on the
"West" side of the sun (instantaneous ecliptic longitudes lying in the "leading"
hemisphere in the direction of earth's motion about the sun. Essentially, this
required geometry instantaneously eliminates nearly half of the sky at any point
during the year. These segments of the sky become available for target viewing
only after earth has travelled around the sun such that a target once again is in
the leading hemisphere of earth in its travel around the sun.
The implications for the impact of 2-Gyro operations on the ability to schedule
HST efficiently are quite dire if the worst-case projected restrictions come to pass.
The 10-degree pointing uncertainty produces a number of further restrictions.
1) Operations within 90-degrees of the sun become impossible. Since roll
uncertainty could also be as high as 10-degrees, violation of the +/-5 degrees of
off-nominal role could occur while viewing a target within 90-degrees of the sun,
therefore nearly another quarter of the sky becomes accessible at any given time
of the year. Further, since HST would not be allowed to come closer than 10degrees to the 90-degree sun-line, the actual restriction would be no closer than
100-degrees, further reducing the viewing season for a given target.
2) This restriction would then also mean that regions within 10-degrees of the
ecliptic poles would never be safely viewable.
3) Anti-solar avoidance. Under normal 3-gyro operations, HST is allowed to
point at targets in the ecliptic plane at the anti-solar point, however since the sun
must never strike HST's underside (due to the WPFC2 radiator shadowing
restriction and potentially other restrictions), HST cannot point within 10degrees of the anti-sun in 2-Gyro because a worst case pointing-excursion could
result in HST drifting in a direction to illuminate the underside.
Combining all of these restrictions, the resulting instantaneously viewable
segment of the sky resembles an 80-degree lemon slice with the top and bottom
10-degrees cut out along with a 10-degree radius "bite" out of one side at the antisun point.
Implications for the LRP and Large Program Scheduling
Until Cycle 12, the extent efficient observatory usage depended upon a relatively
uniform distribution of targets around the sky was not fully appreciated. With
the introduction of COSMOS (~270 Cycle 12 orbits) and the UDF-CDFIRGRAPES (~ 600 orbits)--all lying within 1-hour RA of each other--available
observing opportunities were oversubscribed with no way of containing all 4
programs within Cycle boundaries and allowing other TAC-approved
observations and calibrations to execute as well. With the introduction of less
than 900 orbits on two targets in a single RA band, each with orient restrictions,
the ability of HST to complete the observations of these targets within nominal
Cycle boundaries was overwhelmed. Under 2-Gyro restrictive operations, the
limits on ability to complete large programs looking at single or closely clustered
targets will become even more restricted.
The number of days in which a target is viewable with a substantial fraction of
the nominal 3-Gyro science time still usable varies largely depending upon
ecliptic latitude and HST orbit plane phase orientation, however peaks around
70-days. This target-viewing duration corresponds to the size of "lemon-slice"
referred to earlier.
If visits were nominally Sched-30 (only 30% of orbits usable), then a mere 21-day
window could be expected. Statistically, there would likely be less than a 50%
chance that the visits would be SAA-hiders during the constraint window, so
even if nothing else were to schedule within the window, a 100-orbit,
unrestricted (no links, no orients) program would consume nearly every
available SAA-free orbit unless it could schedule in SAA-impacted orbits.
Clearly, if large programs are to be completed, Sched-100 will be a requirement
for the majority of the visits. Single-field programs significantly larger than 100orbits at high ecliptic latitudes would likely need to be unconstrained in orient or
have a number of closely spaced orients at which parts are done.
Poor Geometric SAA-hiding under Restrictive 2-Gyro
The question addressed next is "how much of the SAA-impacted time can be
filled with SAA-hiders under restrictive 2-Gyro?".
At any given time during the HST precession cycle, roughly half of the sky's RAs
have targets capable of geometric SAA-hiding, while only about a 22% slice of
the sky is capable of providing targets under restrictive 2-Gyro within reach of
HST's view. Figure 1 shows the HST field of regard (green) and geometric SAAhiding (red) regions of the sky over one synodic HST precession Cycle. The field
of regard slowly increases in ecliptic longitude throughout the year (360
degrees/year) while the geometric SAA-hiding region shifts in decreasing RA
much quicker (360 degrees every 56 days).
Since the ecliptic coordinate system is tilted away from the earth’s equatorial
coordinate system by about 23.5 degrees, a crude approximation of the sky
overlaps can be obtained by a cut through some intermediate coordinate plane.
When one does this, the overlap regions of ecliptic longitude and equatorial RAs
can be plotted in a single figure with slices through the intermediate plane,
yielding Figure 3 (and 4 and 5). The fraction of the red region covered by a green
region at any time gives an estimate of the fraction of the SAA-impacted orbits
that allow geometric SAA-hiding during the day. Table 1 provides a numerical
estimate of the fraction of SAA-impacted orbits that can be geometrically hidden
under the restrictive 2-Gyro scenario over a long period of time (integral
numbers of precession Cycles).
1.
2.
Geometric SAA-hiding Equatorial RAs
during a day.
Targets Available (Ecliptic
Longitude Field of Regard.)
4.
3.
Figure 3: Simplified HST Field of Regard and SAA-hiding under restrictive 2-Gyro
operations. The geometric patters represent slices in a plane approximately mid-way between
the ecliptic and equatorial planes.
Table 1: Fraction of SAA-impacted orbits geometrically hideable.
Panel
Days
Fraction of
% orbs geom.
%age Impacted
Precession Cycle
hidden
Orbits Hidden
1.
13.5
0.28
0.44
0.123
2.
10.8
0.22
0.22
0.048
3.
13.5
0.28
0.00
0
4.
10.8
0.22
0.22
0.048
Total fraction of SAA-impacted orbits hidden:
0.22
Relaxed 2-Gyro SAA Hiding
The aforementioned restrictive viewing zones are not the only possible ones that
2-Gyro operations may allow. A relaxed 2-Gyro set of constraints could allow
observing to within 60-degrees of the sun. Rather than only an 80-degree slice of
sky being available, the wedge opens up to approximately 120-degrees. The
geometric SAA-hiding and HST field of regard for this situation are shown in
Figure 4.
1.
2.
Geometric SAA-hiding Equatorial
RAs during a day.
Targets Available (Ecliptic
Longitude Field of Regard.)
4.
Figure 4: Relaxed 2-Gyro SAA-Hiding geometry.
3.
This geometry yields, as is to be expected, a larger percentage of SAA-impacted
orbits which can be used as SAA-hiders. Table 2 provides an estimate of that
fraction.
Table 2: Fraction of SAA-impacted orbits geometrically hideable.
Panel
Days
Fraction of
% orbs geom.
%age Impacted
Precession Cycle
hidden
Orbits Hidden
1.
8.1
0.167
0.67
0.11
2.
10.8
0.33
0.33
0.11
3.
8.1
0167
0.00
0
4.
10.8
0.33
0.33
0.11
Total fraction of SAA-impacted orbits hidden:
0.33
Normal 3-Gyro SAA Hiding
How does this compare with normal 3-Gyro operations? Figure 5 and Table 3
show the same concepts, except that the field of regard is represented as a 260degree slice of the sky (50-degree solar avoidance along the ecliptic). The Table
computes in the same way the relative fraction of SAA-impacted orbits that can
be hidden. Note the stark difference: 72% hidden under 3-Gyro, while only 22%
hidden under 2-Gyro operations. Approximately 40-est_orbits are scheduled in
the SAA-impacted orbits, under current nominal 3-Gyro operations. If heuristic
hiding is reduced in proportion similar to geometric hiding (not yet shown) then
this effect will produce a 40-est_orbits * (72%-22%)/72% = 28 est_orbit reduction
in scheduling during restrictive 2-Gyro operations. Under relaxed 2-Gyro
constraints, that reduction can be expected to be about 40-est_orbits * (72%33%)/72% = 22 est_orbits.
1.
2.
Geometric SAA-hiding Equatorial
RAs during a day.
Targets Available (Ecliptic
Longitude Field of Regard.)
4
.
3.
Figure 5: Simplified HST Field of Regard and SAA-hiding under nominal 3-Gyro operations.
Compare with Figures 3 and 4.
Table 3: Fraction of SAA-impacted orbits Geometrically Hideable--3-Gyro
Panel
Days
Fraction of
% orbs geom.
%age Impacted
Precession Cycle
hidden
Orbits Hidden
1.
10.8
0.22
1.0
0.22
2.
15.5
0.28
0.72
0.20
3.
10.8
0.22
0.44
0.10
4.
15.5
0.28
0.72
0.20
Total fraction of SAA-impacted orbits hidden:
0.72
This analysis shows that less than a third of the current geometrically hideable
SAA-impacted orbits under a simple model will be hideable under restrictive 2Gyro operations. However, it is worth noting the many drawbacks in this simple
model.
1) Distribution of targets at many declinations and latitudes is not accounted for.
This simple model only takes slices along the ecliptic for computing fractions of
impacted orbits that are geometrically hidden.
2) 2-Gyro ecliptic pole "holes" are not accounted for. Targets near the ecliptic
poles are unobservable, reducing the usability of the CVZ.
3) 3-Gyro ecliptic/orbit-pole targets are not accounted for. The solar avoidance
"bite" is not as pronounced at higher ecliptic latitudes, and disappears altogether
above |l| > 50-degrees, so the full ability to hide the north-point is not taken into
account.
4) HST orbit pole orientation and precession are not fully accounted for. This is
a more general statement of a number of the previous points.
This analysis also was predicated on geometric SAA-hiding. The more
important type of SAA-hiding from a scheduling and planning perspective is
heuristic SAA-hiding. Under 3-Gyro operations there are more heuristic SAAhiders than geometric SAA-hiders (geometric SAA hiders can actually be
thought of as a specific sub-class and subset of heuristic SAA-hiders).
The relevant question then becomes, "how will heuristic SAA-hiding change
under restrictive 2-Gyro operations?". Unfortunately, this question cannot
currently be answered easily. However, it is worth noting that under 2-Gyro
mode, many of the orbits where geometric hiding is not possible will be further
degraded in usability. For example, an SAA-impact occurring during a target’s
orbital viewing would likely require extended recovery time, effectively
extending the impact of an SAA-passage on the orbital viewing time.
Recommendations and Questions
A study to determine what the effect of restrictive 2-Gyro operations will have
on SAA-impacted orbit scheduling and examine options for maximizing their
use is indicated if the restrictive constraints are likely to be required.
Historically, maximizing the efficient use of HST has pivoted critically on the
ability to use SAA-impacted orbits. If this is to continue, there are a number of
questions that could be asked. Will 2-Gyro operations indeed drastically reduce
the usability of SAA-impacted orbits under a normal candidate pool? What are
the characteristics of the SAA-impacted orbits that will have low-usability? Will
heuristic SAA-hiders be affected to a greater, lesser, or the same degree as
geometric SAA-hiders. What kind of candidates would allow effective use of the
low-utility SAA-impacted orbits? Can modified external calibrations take up the
slack and use these orbits? Does a special selection of science candidates need to
be sought? Are there nominally enough SNAP candidates that can indeed use
these low-usability orbits? Will the pool of main science or SNAPs need to be
rationed in a special manner to avoid exhausting candidates for these orbits at
inappropriate times? Will changes to SPSS scheduling be required to enforce
this? Should targets "lower" RAs in the field of regard be biased for scheduling
over targets of higher RA? Will SPIKE handle this appropriately, or will codemodifications and a new planning procedure be required?
Conclusions
Restrictive 2-Gyro operations will dramatically reduce the amount of time
available to complete large programs--indeed any program. The unrestricted
constraint windows will shrink to roughly only 30% of that available under
nominal 3-Gyro operations (for Sched-100 visits), in addition to the reduction in
science activity time due to the added guide-star acquisition times. In addition
to the proportional reduction in constraint window lengths affecting the SAAfree and SAA-impacted orbits, the demand placed on SAA-free orbits will
increase in greater proportion because SAA-impacted orbits will have over
2/3rds fewer geometric SAA-hiding opportunities, during a given day, for their
use. An enhanced pool of targets in particular latitude or declination bands may
be required to allow these orbits to be used since their relative abundance will be
quite high, perhaps as great as 25-30% of all telescope time.
Draft Feb 8, 2004 ian jordan Update Feb 12, 2004