Observing Constraints and Their
Impact on Mission Planning
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Chandra Spacecraft
+X
+Y
+Z
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
EPHIN Temperature Limits
• EPHIN is used in safing instruments during
episodes of high particle radiation
- located on the –Z side of the spacecraft;
illuminated by Sun directly at pitch of ~90°
EPHIN
• EPHIN is subject to degraded performance
at elevated temperatures (T > 90 - 100° F)
- +27 Volt Rail supply drops to ~half its
normal voltage during high temperatures;
makes false alarms more likely
Daily Maximum Temperatures for TEPHIN
current red = 120 degF
116.8
120
111.8
110
100
103.7
current yellow = 105 degF
90
orig red = 86 degF
85.2
80
Daily Max (degF)
96.1
73
orig yellow = 81 degF
70
60
50
40
30
20
10
0
1/1/2000
12/31/2000
12/31/2001
12/31/2002
12/31/2003
12/30/2004
• EPHIN temperature has
increased with time
- it has exceeded its
original vendor-supplied
limits of 81/86° F
- maximum temperature
in a given orbit depends
on total pointing history;
Mission Planning activity
12/30/2005
Date
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
EPHIN Temperature Constraints
• For a given pitch angle, EPHIN will
settle to a particular temperature
in a long observation
- for a large range of angles, this
settling temperature is above the
EPHIN limits
• Time required to reach settling
temperature is maximized by
cooling EPHIN first
- this is done by observing at a
“cold” pitch attitude
• For pitch angles in 60-130° F range, maximal pre-cooling still allows only
50-60 ks of observing before approaching temperature limits (and less if
temperature prior to bad-pitch observation is not low)
- best cooling is obtained for targets at highest pitch angles (i.e. in the
anti-sun direction)
- anti-sun attitudes can also be used during radzone passages to “cold soak”
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Propulsion Line Temperature Limits
Fuel lines
throughout
this area
• The Momentum Unloading Propulsion System
(MUPS) uses thrusters fed by hydrazine
- the fuel lines are subject to temperature
variations due to varying spacecraft attitude
- the same is true for fuel lines to the RCS
thrusters (which are no longer used)
MUPS
Thrusters
• Hydrazine begins to freeze at 35° F
- fuel lines are wrapped in heating coils that
are activated by thermostats
- additional thermistors provide temperature
monitoring along the lines (but not control)
• In early 2004, two propulsion line sensors
(PLINE03T and PLINE04T) began falling
below their caution low limits of 45° F
- freezing and thawing of hydrazine could
cause line to rupture
- a rupture could cause the fuel to leak and
contaminate the spacecraft
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Propulsion Line Temperature Restrictions
• The thermostats controlling the circuit
are on the -Y/-Z side of the s/c, the lines
of concern are on the +Y/-Z side
- initially, the -Y side was cooler than
the +Y side
- thermistor PLINE02T, located near the
thermostat, shows this is no longer true
• S/C attitudes with pitch > 168° F cause
lines to cool quickly; PLINE03T and
PLINE04T lag behind heater control,
becoming colder than desired
- we must avoid these attitudes
• The result of this restriction is twofold:
1. Observations at pitch > 168° F cannot be done. This is a new reduction on
sky visibility (though not a large one – only about 1.1% of a sphere, and no
sky region is permanently unobservable).
2. We cannot use this pitch region for “cold-soaking” either. This limits how
much we can pre-cool EPHIN, and lowers amount of time we can spend
at bad EPHIN pitch.
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Where (and How Long) Can We Look?
EPHIN
-Z
HRMA
Prop Lines
+X
• Pitch angle restrictions limit observation of targets based on sun angle:
Angle Range
Restriction
0-45
No observations
HRMA sun avoidance
70-120
<50 ks per orbit
EPHIN temperature limitations
60-70, 120-130
<60 ks per orbit
EPHIN temperature transition
168-180
No observations
Propulsion line temperature limitations
Chandra User’s Committee Meeting (1/25/05)
Reason
P. Slane (Chandra Mission Planning)
How Significant Are the Limitations?
• About 18% (by time) of Cycle 6 observations are at continuously bad pitch
• In a typical orbit, we obtain about 160 ks of observing time
- this varies, depending upon radzone details and slew profiles
• The maximum “bad pitch” time in an orbit is 50-60 ks, or about 35%
of the usable orbit (i.e. of about 160 ks outside of radzones)
- thus, half of our budget for bad pitch is used up simply by distribution
of targets on the sky
- the remainder is available for targets whose constraints force observation
at bad pitch (including TOOs, for which scheduling is often not flexible)
• Examples of constraints that force observations at bad pitch angles:
- observations coordinated with XMM, RXTE, and some others
- monthly monitoring of targets
- many roll and window constraints
Such constrained observations need to be limited
• Future technical reviews will flag such observations
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Example: Crab Monitoring Campaign
Proposal:
Monitor Crab
Nebula for 10 ks
at 30-d intervals
Problems:
• Restrictions from
prop. lines and
HRMA constraint
prohibit this exact
sequence
• Exposure counts
against EPHIN
limits for most
weeks
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Example: sub-ms Pulsars in 47 Tuc
Proposal:
800 ks observation
of 47 Tuc to search
for sub-ms pulsars
Problems:
• Requires that
observation is
interrupted only
by radzones in
order to maintain
timing coherence
This is impossible
(but would not
have been earlier
in the mission)
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Example: Mosaic with Roll Constraints
Proposal:
1.4 MS mosaic
of survey field
• Four acceptable
roll ranges, each
with +-8 degree
tolerance
Problems:
• Pitch angle
prohibits more
than 50-60 ks
per orbit in this
region
New restrictions
make it impossible
to get full amount
of time for this
program as it is
specified
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)
Summary (from POG)
Pitch angle constraints - Changes in the thermal properties of the spacecraft with time are introducing additional
restrictions in the solar pitch angles that can be observed. The pitch restrictions are of two kinds:
(a) EPHIN is subject to degraded performance at elevated temperatures that may affect its use in safing the science
instruments from high levels of particle radiation. During long observations at pitch angles between ~65 - 125 degrees
the EPHIN may reach temperatures that trigger anomalous performance depending on its prior thermal history.
(b) Due to excessive cooling in propellant lines, [there is] a restriction against pointing at pitch angles greater than 168
degrees. We urge that you carefully consider how to configure your observation such that it does not require a pitch
angle greater than 168 degrees, for example, by making it completely unconstrained, or by checking that constraints
can be satisfied with the target at angles between 45 and168 degrees from the sun (subject to EPHIN-related pitch
constraints). Even if accepted by the peer review, it may subsequently turn out that observations which can
only be accommodated at pitch angles greater than 168 degrees simply will not be done.
These spacecraft constraints have several implications for proposers:
• Observations in the 65-125 degree pitch zone can be done, but if long will be broken into shorter durations, which
may be separated by a day or more. The maximum continuous duration may be in the vicinity of 30 ks, but is
dependent on the preceding pitch angle history of the observatory. If such observations have roll constraints, the
observations must either be brief or the roll constraints must be generous enough to allow multiple segments at
their different, time-dependent roll angles. Constraining roll angles to be constant for multiple segments is
discouraged, as achieving off-nominal roll angles may present thermal difficulties.
• Observations at pitch angles outside but near near the 65-125 degree zone may be segmented if the EPHIN thermal
prediction model shows a possibility of overheating.
• Simultaneous longer-duration observations with telescopes (such as XMM-Newton) with a preferred pitch angle
in the 65-125 degree range may be very difficult, or even impossible, to schedule.
• Targets near the ecliptic poles (such as the Magellanic Clouds) are especially affected since their pitch angles are
always close to 90 degrees
Chandra User’s Committee Meeting (1/25/05)
P. Slane (Chandra Mission Planning)