Slane CUC Jan2005

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Observing Constraints and Their Impact on Mission Planning: 

Observing Constraints and Their Impact on Mission Planning

Chandra Spacecraft: 

Chandra Spacecraft +X +Z +Y

EPHIN Temperature Limits: 

EPHIN 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 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 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

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” EPHIN Temperature Constraints

Propulsion Line Temperature Limits: 

Propulsion Line Temperature Limits 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) 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 MUPS Thrusters Fuel lines throughout this area

Propulsion Line Temperature Restrictions: 

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.

Where (and How Long) Can We Look?: 

Where (and How Long) Can We Look? +X -Z HRMA <50 ks per orbit <60 ks per orbit <60 ks per orbit EPHIN Prop Lines Pitch angle restrictions limit observation of targets based on sun angle:

How Significant Are the Limitations?: 

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

Example: Crab Monitoring Campaign: 

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

Example: sub-ms Pulsars in 47 Tuc: 

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)

Example: Mosaic with Roll Constraints: 

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

Summary (from POG): 

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

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