Chapter 2
Spacecraft, Telescope, Operations, & Mission Planning
2.1 Introduction
In this chapter we provide 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" weight (incl. reserve) | 4790 kg |
| Loaded Propellant | 40 kg |
| Electrical Power | 3 NiH2 Amp-hr batteries |
| Two 3-panel solar arrays |
| Nominal Operating Power | 800-120 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 seconds |
| = 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, 30-Ampere-hour
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 NASA Communication System. 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 Subsystem (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 subsystem was disabled once the final orbit was achieved for observatory safety reasons. The MUPS provides momentum unloading during normal on-orbit operations. Given the current performance there is sufficient MUPS fuel to support ∼ 20 additional 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).
Figure 2.1: Major components of
the telescope system. The grating assemblies are also shown.
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 Spacecraft to Telescope Support Struts
are self-explanatory and are shown in Figure 2.1. The
forward and aft contamination covers were opened on-orbit and cannot
be closed. The forward contamination cover also serves as the
sun-shade.
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
http://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 2.2, 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 2.2: 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 is monitored by the
EPHIN detector. 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 degrees and the instrument is mounted on the sun side of the
spacecraft near the HRMA. EPHIN data
rates are monitored by the OBC, which activates commands to safe the
ACIS and HRC during periods of high radiation such as a solar flare.
The forerunner of the Chandra-EPHIN was flown on the SOHO
satellite. Information is available at
http://www.ieap.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 July 23, 1999 at 12:31:00:04 a.m. 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 July 24 - August 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
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 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
November 1999.
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 OCC and between the OCC and other ground
facilities.
- Operations and Control Center
- (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 Online System (ONLS) and the Offline 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 NGST and is located at the OCC.
- Chandra Science Center
- (CXC). The CXC is the focal point for
service to the scientific community. The CXC is contracted to issue
the 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 data-base, 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
weekly 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. 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 always be scheduled.
The telemetry is formatted into major frames and minor frames - a
major frame lasts 32.8 seconds 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. 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 (TT) which differs from UTC by about a minute. (See
http://tycho.usno.navy.mil/systime.html
for a discussion.) The accuracy of the time relationship is 100
microseconds. 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/PI via USINT (http://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 weekly bins, 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 weekly short-term schedules.
Once a new LTS is populated at the start of a Cycle, Mission Planners
begin the process of weekly scheduling. As the Cycle goes on, the
remaining LTS is amended weekly and posted on-line at
http://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. Each week 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).
Both the LTS and the STS (Short Term Schedule) web pages show sequence numbers for every
observation that are hyper-linked to descriptive target pages. The STS is available on-line at
http://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 weekly bin or scheduled precisely within a week in
a short-term schedule, 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 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 weekly bins available for scheduling the observation. Weekly
schedules are 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
Each week, 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 that week, returned to be
placed in a later week during the revision of the LTS or
returned to the list of pool targets.
- the target is observed in the scheduled week or bumped to a later week
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
short-term schedules 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
weekly schedule before its approval by all teams concerned (FOT and Mission
Planning, Mechanisms, Command Management, ACIS, HRC, Pointing Control and
Aspect, Flight Director). Once a final schedule is approved, the CXC updates
the LTS pool list and ObsCat accordingly.
The CXC currently starts to prepare short-term schedules 3 weeks in
advance. Thus at any given time there are 3 weekly schedules 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 24-48 hour
turnaround. 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 (GMT).
Hence, given the nominal planning cycle, fast ( < 1 week) turn-around
TOOs can most efficiently be incorporated into the short term schedule
if they are submitted to the CXC by mid-week. Such
submission/notification will reduce the amount of disruption, allow
time to meet constraints and preferences for other targets, and
optimize the chances that all the observing requirements for the TOO can be met.
Last modified:12/13/12
