chap2.html

INDEX

2.1 Introduction

This chapter provides a brief overview of the spacecraft , the telescope system including the Science Instrument Module (SIM) , operations , and mission planning.

A number of observatory parameters are given in Table 2.1.

Table 2.1: Spacecraft Parameters

Chandra "dry" mass (incl. reserve)	4790 kg
Loaded Propellant	40 kg
Electrical Power	3 NiH₂ batteries
	Two 3-panel solar arrays
Nominal Operating Power	800−1100 W
Optical bench length	∼ 10 meters
SIM focus adjustment range	±0.4 inches
SIM focus adjustment accuracy	±0.0005 inches
SIM Z-position adjustment repeatability	±0.005 inches
Solid-state recorder capacity	1.8 Gb × 2
On-board command storage	5400 command words
Nominal command storage period	72 hours
Observatory telemetry data-rate	32 kbps
Telemetry playback downlink rates	1024, 512 and 256 kbps
Nominal ground contact periods	45 to 75 minutes per 8 hours
SI telemetry rate	24 kbps
Telemetry format 1 major frame	= 32.8 sec
	= 128 minor frames
Clock error	< 100μs
Clock stability	1:10⁹ per day
Clock frequency	1.024 MHz

2.2 Spacecraft

An outline drawing of the Observatory was shown in Figure 1.1. The spacecraft equipment panels are mounted to, and supported by, a central cylindrical structure. The rear of the spacecraft attaches to the telescope system.

The spacecraft includes six subsystems :

Structures and Mechanical Subsystem . This subsystem includes all spacecraft structures, mechanisms (both mechanical and electro-mechanical), and structural interfaces with the Space Shuttle. Mechanisms, such as those required for the sunshade door, are also part of this subsystem.
Thermal Control Subsystem . Thermal control is primarily passive, using thermal coatings and multi-layer insulation blankets. On-board-computer-controlled electrical heaters augment these passive elements to maintain sensitive items such as the HRMA at nearly constant temperature.
Electrical and Power Subsystem . This subsystem includes all hardware necessary to generate, condition, and store electrical energy. Power is generated by solar cells mounted on two solar array wings (three panels each), sized to provide a 15% end-of-life power margin. Electrical power is stored in three NiH₂ batteries. These batteries provide spacecraft power during times when either the Earth or Moon partially or completely blocks the Sun. Even so, the battery capacity requires that certain non-critical items, including science instruments, be powered down during eclipses. These eclipses occur infrequently due to the particular nature of the Chandra orbit.
Communication, Command, and Data Management (CCDM) Subsystem . This subsystem includes all the equipment necessary to provide ranging, modulation, and demodulation of radio frequency transmission of commands and data to and from the Deep Space Network (DSN) NASA Communication System (NASCOM). The CCDM includes two low gain antennas, providing omni-directional communications, an on-board computer (OBC), a serial digital data bus for communication with other spacecraft components, the spacecraft clock, and a telemetry formatter which provides several different formats.
Pointing Control and Aspect Determination (PCAD) Subsystem . This subsystem includes the hardware and control algorithms for attitude determination and for attitude and solar array control. The solar arrays can be rotated about one axis. The PCAD subsystem also includes hardware for safing the observatory. Specific details of the PCAD subsystem especially relevant to scientific performance are discussed in Chapter 5.
Propulsion Subsystem. This subsystem consists of the Integral Propulsion System (IPS) and the Momentum Unloading Propulsion Subsystem (MUPS). The IPS contains the thrusters and fuel for control of the orbit and spacecraft orientation during orbit transfer. This system was disabled once the final orbit was achieved for observatory safety reasons. The MUPS provides momentum unloading during normal on-orbit operations. Given current usage rates there would be sufficient MUPS fuel to support ∼ 50 further years of operation.

2.3 Telescope System

The principal element of the telescope system is the High Resolution Mirror Assembly (HRMA, Chapter 4). The HRMA, comprised of four concentric grazing incidence X-ray telescopes, focuses X-rays on the selected detector located in the Science Instrument Module (SIM, Section 2.4). The grating assemblies are also attached to the HRMA module.

The telescope system also includes:

Optical Bench Assembly
Spacecraft Support Structure Assembly
Fiducial Transfer Optical Components
Spacecraft to Telescope Support Struts
Forward and Aft HRMA Contamination Covers
Magnetic Baffle Assembly
Stovepipe Baffle

The Optical Bench Assembly is primarily the long composite structure separating the HRMA from the SIM. The Spacecraft Support Structure Assembly includes the ring to which the spacecraft is mounted. The Fiducial Transfer Assembly Optical Components are discussed in Chapter 5. The forward and aft contamination covers were opened on-orbit and cannot be closed. The forward contamination cover also serves as the sunshade.

The Magnetic Baffle Assembly was designed to prevent low energy (up to about ∼ 100 keV) electrons (reflecting through the X-ray optics) from reaching the focal plane. More details about these baffles may be found at https://wwwastro.msfc.nasa.gov/xray/spectops.

The stovepipe baffle, located inside the optical bench and at the entrance to the SIM, includes tantalum coated plates to prevent X-rays, other than those passing through the telescope, from reaching the focal plane. There are several such baffles inside the optical bench. Details of the baffles may be found at the WWW address above.

2.4 Science Instrument Module (SIM)

The SIM , shown schematically in Figure 2.1, is a movable bench on which the focal-plane X-ray detectors are mounted. Kinematic mounts (flexures) and thermal isolation are provided between the SIM and the telescope optical bench. A graphite epoxy support structure houses the translation stage.

Figure 2.1: A schematic of the Science Instrument Module.

2.4.1 SIM Motions

The focal-plane instruments are positioned by the SIM Z-axis translation stage with a repeatability to ±0.005 inches over a translation range of 20 inches. The SIM X-axis motion sets the focus to an accuracy of ±0.0005 inches over a range of 0.8 inches. The fine-focus adjustment step is 0.00005 inches.

2.5 Electron Proton Helium Instrument (EPHIN)

The local particle radiation environment was monitored by the EPHIN detector until 2018, when it was depowered due to erratic behavior related to increasing on-board temperatures. EPHIN consists of an array of 5 silicon detectors with anti-coincidence shielding. The instrument is sensitive to electrons in the energy range 150 keV - 5 MeV, and protons/helium isotopes in the energy range 5 - 49 MeV/nucleon. The field of view is 83 deg and the instrument is mounted on the Sun side of the spacecraft near the HRMA. Prior to 2013 the EPHIN data rates were monitored by the OBC to activate commands to safe the ACIS and HRC during periods of high radiation such as a solar flare. Due to spacecraft heating, the EPHIN no longer produces reliable data and thus on-board radiation monitoring data are now provided by the ACIS instrument.

The forerunner of the Chandra-EPHIN was flown on the Solar and Heliospheric Observatory (SOHO) satellite. Information is available at http://www2.physik.uni-kiel.de/et/ag-heber/costep/ephin.php. The EPHIN instrument was built by the Institut fur Experimentelle und Angewandte Physik at the University of Kiel, Germany. Drs. Reinhold Muller-Mellin and Hoarst Kunow are the Co-Principal Investigators.

2.6 Operations

2.6.1 Launch and On-orbit Verification

Chandra was launched on-board the Space Shuttle Columbia from the Kennedy Space Center in Florida on 1999-Jul-23 at 12:31:00:04 a.m. Eastern Daylight Time (EDT). The Observatory was deployed from the Space Shuttle a few hours later at 8:45 a.m. EDT. Two burns of the IUS (Inertial Upper Stage) took place an hour after Chandra was released. A series of five burns of the Integral Propulsion System (IPS) over the period 1999-Jul-24 to 1999-Aug-7 took Chandra to its final orbit.

Once in final orbit, the Orbital Activation and Checkout (OAC) phase started. During this time, all systems were brought on-line and numerous calibrations were performed. After the contamination covers on the HRMA were opened, and after a few passages through the radiation belts under this condition, the front-illuminated (FI) ACIS CCDs showed signs of decreased, and spatially-dependent, energy resolution together with increased charge transfer inefficiency (CTI), consistent with radiation damage. Steps were successfully taken to prevent further damage (see Chapter 6). Due to this situation, and because of uncertainties of the long term stability of the FI chips at that time, additional ACIS calibrations were performed and emphasis was placed on observations requiring the use of the FI CCDs. Note that the back-illuminated (BI) CCDs were unaffected, and the situation is now stable in that further degradation has been slowed to match pre-launch expectations. See Chapter 6 for further details. Normal operations started in 1999-Nov.

2.6.2 The Ground System

The Chandra "Ground System" is comprised of facilities required to operate the spacecraft, receive and analyze the spacecraft telemetry and provide scientific support to the user community. The ground system includes the following elements:

Deep Space Network: (DSN). The DSN is used for communicating commands to the spacecraft and receiving telemetry.
NASA Communications: (NASCOM). NASCOM provides communications links between the DSN and the Operations and Control Center (OCC) and between the OCC and other ground facilities.
Operations and Control Center: (OCC). The OCC is responsible for operating the observatory. This includes activities such as preparing command loads, processing telemetry, attitude determination, monitoring health and safety, etc. OCC personnel utilize two major software environments, the On-line System (ONLS) and the Off-line System (OFLS). The ONLS deals primarily with real-time operations such as receiving telemetry and sending commands through the DSN. The OFLS deals with functions such as mission planning and supporting engineering analysis. The Software Maintenance Facility (SMF) which maintains the flight software is operated by Northrop Grumman Aerospace Systems (NGAS) and is located at the OCC.
Chandra X-ray Center: (CXC). The CXC is the focal point for service to the scientific community. The CXC is contracted to issue the Call for Proposals (CfP; https://cxc.harvard.edu/proposer/CfP/) and organize peer reviews. The CXC assists prospective observers in developing proposals, generates an observing plan from the proposals that are selected, and supplies data products to observers. The CXC performs on-orbit calibration and maintains the calibration database, produces response functions, etc. The CXC is responsible for providing limited assistance to observers, including software, for analyzing data. The CXC is also responsible for archiving Chandra data.

2.6.3 Commanding

All normal Chandra operations are preplanned. The OFLS divides the mission schedule into approximately one day segments and generates spacecraft and instrument commands to be executed that day. Once a day, this command load is uplinked to the spacecraft and stored. Three consecutive daily segments are loaded to assure autonomous operation for 72 hours. Stored command loads can be interrupted if necessary, and updated either because of an emergency or to accommodate Targets of Opportunity (TOOs). The interruption process may require up to 24 hours to complete depending on numerous factors including the availability of ground contact. In a true emergency, ground contact can almost always be scheduled.

2.6.4 Telemetry

The telemetry is formatted into major frames and minor frames - a major frame lasts 32.8 sec and includes 128 minor frames. Each minor frame contains 1019 bytes of science and engineering data plus a 6 byte header (yes - 1025, not 1024, total bytes!) that includes a 3-byte minor frame counter - the Virtual Channel Data Unit (VCDU) counter - which rolls over every 49.8 days.

During normal science operations, telemetry data is generated on the Observatory at a rate of 32 kbps, of which 24 kbps are devoted to the "science stream" data from one of the focal-plane instruments and the remainder allocated to other systems, including 0.5 kbs to the "next-in-line" instrument. The data is recorded on one of two solid state recorders for subsequent transmission. Each solid state recorder has a capacity of 1.8 Gbits equivalent to 16 hours of operation.

The recorded data are transmitted through one of the low gain antennas to the ground at 1024 kbps, (or 512 kbps, or 256 kbps) during scheduled Deep Space Network contacts every eight hours (nominally). Contacts last 45-75 minutes. The ground stations, in turn, transmit the data to JPL which then transmits the data to the OCC.

2.6.5 SI Science Data

There are individual telemetry formats for HRC and ACIS data. The 24 kbps data is collected by the CCDM subsystem from each instrument as a sequence of 8-bit serial-digital words through a Remote Command and Telemetry Unit (RCTU). An additional small amount of housekeeping telemetry is always collected from each instrument independent of the selected format.

2.6.6 Event Timing and the Spacecraft Clock

The CCDM subsystem provides prime and redundant 1.024 MHz clocks, and the (1/1.024μs) pulses are utilized by the two focal-plane instruments for timing. Each instrument has electronics that counts the elapsed time since the beginning of the current telemetry major frame. The time of events recorded on Chandra are given in Terrestrial Time which differs from Coordinated Universal Time (UTC) by about a minute. (See https://aa.usno.navy.mil/faq/TT for a discussion.) The accuracy of the time relationship is 100 microsec. The spacecraft clock is stable to better than one part in 10⁹ per day.

2.7 Mission Planning

2.7.1 The Long-Term Schedule

The Chandra scheduling process seeks to maximize the fraction of time on-target while minimizing risk to the spacecraft. Once the list of approved target observations for a new cycle has been finalized and targets have been reviewed in detail by the Observer/Principal Investigator (PI) via User Interface staff (https://cxc.harvard.edu/cdo/observation_scheduling.html#usp), they are scheduled by the Science Mission Planners into a Long Term Schedule (LTS). LTS observations, scheduled into bins typically one week in length, generally do not fully occupy the time available for science scheduling; a reserve of unconstrained observations are kept in a pool and used to fill in short-term schedules (STSs).

Once a new LTS is populated at the start of a Cycle, Mission Planners begin the process of scheduling into short-term bins (usually one week in duration). As the Cycle goes on, the remaining LTS is amended and posted on-line at https://cxc.harvard.edu/target_lists/longsched.html. Observers should note that the predictive fidelity of the LTS generally decreases farther into the future. The placement of the unconstrained pool targets can change at any time. As the LTS is revised, non-pool targets may also be reassigned for a variety of reasons including multi-telescope coordination. Observations may be bumped or not completed because of high radiation or targets-of-opportunity (TOOs). Although most observations occur during the calendar year corresponding to the cycle, it can take up to two years to fully complete an observing cycle, with a roughly one year overlap between cycles.

Both the LTS and the STS web pages show sequence numbers for every observation that are hyper-linked to descriptive target pages. The STS is available on-line at https://cxc.harvard.edu/target_lists/stscheds/index.html. Each target page further contains a link to a plot that displays the roll, pitch, and visibility for the target for the duration of the Cycle. The target page also contains links to images of the appropriate Chandra instrument superposed at the correct roll on 2 deg images of the sky available from NASA SkyView. Any time an observation is reassigned to a new STS bin or scheduled precisely within a STS, a revised set of images is posted.

The LTS takes into account the intrinsic target visibility (based primarily on minimum Sun, Earth and Moon angles; see Section 3.3.2), additional target constraints approved by the Peer Review, and thermal and momentum limitations of the spacecraft. These additional constraints are described in Chapter 3. While user-imposed constraints can significantly enhance the science return of an observation, proposers should be aware that limitations are imposed on the number of constrained observations that may be accepted at Peer Review (see the CfP). Additionally, all constraints effectively translate into time constraints that may affect the number of STS bins available for scheduling the observation. Schedules may be interrupted unpredictably by the space radiation environment or TOO observations. This inevitably means that the next opportunity to meet all the observing constraints can be significantly delayed if those constraints are stringent.

2.7.2 Selecting Candidates for Short-Term Scheduling

For each STS, the Mission Planning and Flight Operations Teams construct an Observation Request (OR) list. The list is composed of a combination of LTS and pool targets chosen to meet both the science requirements of the observations and the constraints of the observatory. The OR is a "short list" of targets that can be scheduled: not all of them will be scheduled. Well before construction of the OR list, all observing parameters must be finalized. An overview of the process follows.

the observer is contacted before the cycle begins to confirm that observation parameters most critical to mission planning (such as coordinates and constraints) are correct.
the target is placed in the LTS or in the pool list
the observer verifies correctness of all observing parameters after a second contact from the CXC.
the target is made available for scheduling
the target appears in an OR list as a candidate for short-term scheduling
the target is either scheduled for a specific time in that STS, returned to be placed in a later STS during the revision of the LTS or returned to the list of pool targets.
the target is observed in the scheduled time or bumped to a later schedule

Some targets may be assigned to several OR lists before they are finally scheduled. Observers are contacted by CXC personnel if their targets appear in an approved schedule and then subsequently not observed, due, for example, to a radiation shutdown or a TOO.

2.7.3 The Short-Term Scheduling Process

Mission Planning assigns priorities in the OR list to emphasize constrained observations; otherwise they would rarely be scheduled for observation since they tend to have a negative impact on the observing efficiency. Whenever possible the ORs span a range of angles about the satellite-Sun line to prevent excess accumulation of momentum. In consultation with the Science Mission Planning Team, the Flight Operations Team (FOT) constructs detailed STSs and command loads for the spacecraft that combine science observations with engineering activities. Along with observing efficiency, thermal, power, momentum, and pointing constraints are all factored in, as well as minimization of maneuver error and optimal guide star acquisition. Several iterations of optimization and safety checks are not uncommon for each STS before its successful review by all teams concerned (Flight and Science Mission Planning, ACIS, HRC, ACA, PCAD, Mechanisms, Communications, Thermal, Command Management) and final approval by the Flight Director.

The CXC currently starts to prepare STSs 3 weeks before they begin execution on Chandra. Thus at any given time there may be as many as 3 STSs in various stages of preparation. Changes in any of these require a rebuild which is very labor intensive. Fast-response TOOs are currently the only allowed changes. Even small changes to a schedule typically require 2448 hours to implement. During nominal Mission Planning, the final STS is approved and ready for upload by the Wednesday or Thursday before the STS commands begin executing Sunday night or Monday morning (UTC). Hence, given the nominal planning cycle, fast ( < 1 week) turn-around TOOs can most efficiently be incorporated into the STS if they are submitted to the CXC by mid-week. Such submission/notification will reduce the amount of disruption, allow time to meet constraints for other targets, and optimize the chances that all the observing requirements for the TOO can be met.

2.7.4 Coordinated Observations

The Chandra proposal process recognizes the scientific value of joining data from Chandra with those from other observatories. Joint observations may, but need not, be coordinated. Coordinated observations are those that must be done by all participating observatories within an observer-specified time interval. Coordinated observations must be specified as constraints in the Chandra Proposal Software (CPS). Detailed instructions, requirements, and recommendations for proposing joint observations are described further in the CfP (https://cxc.harvard.edu/proposer/CfP/).

Once coordinated observations are approved by peer review the Chandra science mission planning team will initiate contact with other observatories to facilitate completion of the coordination. Observers are encouraged to provide complete information to their contacts at the coordinating observatories, after which the observatories' planners will work together to achieve an effective schedule. The joint work of the planners will establish the time intervals that meet the proposal constraints and are feasible for the participating observatories. Observation times are finally fixed when coordinated observations are incorporated into short-term schedules (Section 2.7.3).

2.7.5 The Chandra Cool Targets Program

The scheduling of Chandra observations depends on balancing the solar pitch angle-dependent heating and cooling of multiple satellite subsystems (see Section 3.3 for additional details). Accordingly, there may be a need throughout the year to find thermally-useful science targets (generally to cool particular subsystems) in limited (and time-dependent) areas of the sky. As there may be no peer-review-approved targets in a specific area of the sky, and to avoid potential loss of observing time from pointings at blank sky, the CXC perceived a need for an ample source of targets broadly distributed throughout the sky at low ecliptic latitudes (within 40 degrees of the ecliptic). A call for white papers was issued in 2018-Sep with the goal of collecting suitable catalogs of targets. In addition to avoiding the always-hot regions near the ecliptic poles, the submitted catalogs/lists of targets need to be scientifically useful under the following conditions: a limited (and initially unknown) number from a particular catalog/list are observed, limited exposure times (10 to 35 ks) per target, no constraints or preferences, and with limited observing modes.

Targets from this program are scheduled only if there are no other approved targets in the desired region of the sky, and the data are made public immediately. It is the purpose of the program to make available several thousand targets distributed as uniformly as practicable over the sky at low ecliptic latitudes to meet any future operational needs. A call for white papers was concluded on 2018-Oct-22; the results are distributed on the Chandra Cool Targets (CCTs) website: https://cxc.harvard.edu/proposer/CCTs.html.