Swift Gamma-ray Burst Explorer: Mission Design for Rapid,?Accurate Location of Gamma-ray Bursts

Absfracr-The Swift Gamma-ray Burst Explorer is a NASA Mid-sized Explorer (MIDEX) with the primary mission of determining the origins of Gamma-Ray Bursts (GRBs). It will be the first mission to autonomously respond to newly-discovered GRBs and provide immediate follow-up with narrow field instruments capable of multi-wavelength (UT, Optical, X-ray)- observations-The- chara6teristcs-of GRBs that are the key mission design drivers, are their non-repeating and brief duration bursts of multi-wavelength photons. In addition, rapid notification of the location and characteristics of the GRBs to ground-and-space-based observatories drive the end-mend data analysis and distribution requirements.


OVERVIEW OF THE S W a hlISSION
Swift [ 11 was selected as a NASA MIDEX mission under the Explorer Program in October, 1999. The driving requirements on the mission include: Rapid ground notification of newly discovered GRBs (seconds from burst trigger to ground notification), Accurate GRB positions (arc minute to sub-arc second), and Multi-wavelength observations. These requirements led to an autonomous observatoy that is able to rapidly locate a GRB with the BAT instrument's large field-of-view, and slew the spacecraft to the GRB to bring the narrow field instruments onto the target. This autonomy required an on-orbit decision-making process that was captured in the Figure-of-Merit @OM) software, which takes input from the instruments and spacecraft, and decides whether a newly discovered GRB warrants an interruption of the current observation. The FOM can then request a slew from the spacecraft, which calculates a safe slew path (avoiding Sun, Moon, and Earth), and rapidly re-points the observatory.
The GRB location accuracy requirements drove instrument and spacecraft desigs as well as the design of the Optical Bench (OB), which mi$ntains the co-alignment of the three instruments w i t h respect to the attitude control system reference.

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The Swift mission is managed by the GSFC, and includes an international team. with key contriiutions from the UK and

PSU.
Science archiving and data analysis centen are located at the GSFC, in the UK and in Italy.

Observatory Overview
The Swift observatory consists of a spacecraft bus and a payload module, which includes 3 instruments mounted on a high stability optical bench. See Figure l.O(a). The BAT instrument monitors 1/6 of the sky for GRBs, while the narrow field XRT q d UVOT instruments carry out planned science observations. When BAT detects a burst the spacecraft slews the narrow field instruments (NFIs) onto the burst, so that they can collect data on the GRB afterglow immediately (within -1 minute) after the burst. This rapid response is one of the key features of the Swift observatory, which will greatly enhance the information available on GRBs.  The very capable aitimde control system meets the demands for rapid response to the discovery of new bursts by providing rapid slewing and settling performance. It also includes autonomous planning software to avoid bright sources while slewing to the new target. The primary actuatoIs are six reaction wheels mounted in a hexagonal umfiguration, each capable of delivering 0.68 N-m of torque.

Burst Alert Telescope (BAT)
The BAT [2] is a coded mask imager with a detector array comprised of 32,768 individual Cadmium-Zinc-Telluride detectors, each with a separate measurement chain. See Figure l.O@). Imaging is enabled by providing a mask of lead tiles forming a pseudo-random pattern in the field-of-view of the detectors. When the detector counts exceed a threshold level, image processing on board determines the burst loca-Telescope Radiator.

Power
Control Box 2 tion by a Fourier transform imaging technique. Key instrument parameters are listed in Table 1 .O(a).

X-ray Telescope '(XRT)
The G P C. Note the Star Tracker simulators attached to the forward

Ultra-Violet/Optical Telescope (UVOT)
The UVOT provides the highest resolution images of the GRB afierglow. It is based on the W O M instrument, and consists of a 30 cm aperture, Ritchey-Chretien optical system, a six-element filter wheel, and an image intensified CCD. See Figure I.O(d). The system is capable of detecting objects as dim as magnitude 24. Internal stability is provided by an iso-thermal design that provides a constant environment for the heater controlled optical metering structure. A listing of key i n s w e n t parameters is given in

DESCRIPTION OF GRBS
GRBs were detected by the Vela4 satellite in 1 %7, although tLt uwcunxnient of thcir discovery did not occur until 1973. With this discovery, some of the basic characteristics of GRBs were revealed First was their short-lived nature, producing gamma-ray flashes that last for only seconds before decaying into the background. Another characteristic is their random distribution in the sky, indicating great distances and extremely powerful explosions. In recent years it has been discovered that the brief, initial burst of gamma rays is followed by a multi-wavelength afterglow that may be detectable for weeks afterward.

DRIVING REQUIREMENTS
The Swift challenge of studying these GRBs is to provide for the rapid detection and localization of the GRB, followed up with multi-wavelength obsewations of the afterglow. Figure  3.0(a) describes the flowdown of key science objectives into system requirements. The driving requirements on the system design are to: Report coarse GRB position within 10 arcmin in 30 seconds Report X-ray GRE3 position within 5 arc seconds within 100 seconds Report UV/Optical position within 0.3 arc seconds within 270 seconds

AND VERIFICATION
The initial burst detection by BAT must be accurate enough to allow the burst to fall within the field of view of the NFIs after a slew of up to 100 degrees. The coarse GRB position requirement error budget is shown in Figure 3.0(a). The frst tier of the error budget covers BAT internal alignment stability and centroiding capability, as well as spacecraft alignment stability and attitude knowledge accuracy. BAT internal alignment stability was verified by a combination of tests and analyses, and was within the allocations.  for on-orbit perfonnance. Vibration and thermal testing verified the ability of the BAT structure to survive the launch and orbital environments without suffering alignment shifts.
One time alignment shifts due to launch and ground handlmg events will be calibrated out of the system on-orbit. This was backed up with a structuralkhexmal analysis that showed alignment was maintained through the predicted orbital ther-    ~i~ 4.o(b). ne =T design includes an he able at the Gamma ray burst Coordinates Network (GCN) thermal control system and a low CTE mhic epoxy tube within 30, 100, and 270 seconds, respectively of the initial forms the strum berween the grazing incidence BAT-detected GRB. What distinguishes Swift fkom other error was wduobservatories is its ability to autonomously re-target based during . I call*on testing at the panw x-ray on in-orbit discoveries. Current missions take hours or days beam facility in Germany, in the MI of 2OO3.

Segment
to re-point, much too long to capture the dying GRB afterglow. Table 6.qa) gives the allocation of these timing The spacecraft ACS system provides a a m t e attitude requirements to the various swift elements. A~S S the top of howledge to the inslmments once Settled on the table are the burst alert messages that are transmitted by On-orbit boresight calibration b e t w e the XRT and the star Swift on discovery of a GRB. n e rows of the table come-Trackers will make use of the most benign on-orbit condispend to the individual delays in getting the bust alert m etions, to remove all one-time launch shift effects.
sage to the ground. These include instrument and FOM An additional feature of the XRT system design is the tele-Processing time, spacecraft planning and slewing time, and scope alignment monitor, which can m m e the m i d i p -COnImUniCatiOnS delays. Each Of the time requirements starts ment between thefocaLplane, the_instxument~inte&e, and with the burst being detected by the BAT b e n t , and.
get introduced because the design of the telescope and orbiting observatories that a GRB has been discovered. includes a heat pipe, harnessing, and thermal blanket close-A breakdown of the ti-g requirements is Presented in outs that could affect pointing. These items introduce loads Table 6.0(a), showkg how each of the steps in the GRE3 intn the iinoippnrtPd aft end nf the telewnp, and each nne detection and notification process was broken down to the was treated caremly to ensure minimal impact. n e h a t level where it could be allocated to a single mission element. pipe design included flexure mounts that prevent lateral loading. A mockup of the flight harness was used to test its TO achieve these rapid X-ray and UV/Optical afterglow obsercompliance to relative motion between the telescope and the vations q u k e s a highly autonomous observatory that must harness support plate on the spacecraft. Thermal blankets evaluate whether a burst is scientifically interesting; plan a were carellly closed out by overlapping sections such that slew which avoids the Earth, Sun, and Moon; rapidly and accuthe blankets could not constrain motion of the telescope. The rately settle on the newlydiscovered burst; and provide the final results of the pointing performance will require the ground with information on the bursts so that follow-up obsercompletion of on-orbit calibration between the boresight of vations are possible. Two key aspects of the autonomous XRT and the star trackers. This is planned during the early design were its simple inmfaces and onboard slew planning orbit checkout period.
by the spacecraft command and data handling system.
xRT -miding and the cooid c-. XRT the star baseplate. The last element of the mor bud-end5wa-g meSSge E .~~G G~G t i~& g o t h e r g r o u n d The simple interface allows messaging and calculations to be kept to a minimum, and permits subsystem level testing to veri% interfaces. The other key to the design is that the spacecraft is responsible for maintaining the safety of the observatory. The N F T s could he damaged by slews acms bright objects, so this drove the requirement for on-board bright object avoidance during slews. Several issues were resolved satisfactorily during the development process, including slew speed limitations to give enough time to recover from a C&DH reboot, impact of mass growth on slew speed, and added slew time to account for additional roll for slews when crossing the ecliptic plane.

CONCLUSIONS
The driving requirements and design of the Swift mission have been described. The flowdown of requirements into the elements of the mission through the pointing and message timing error budgets defined the challenge for the team. The end-to-end verification process employed gives the team confidence that the mission will meet its requirements and science objectives as defined at the outset of the program.