Fermi Gamma-ray Space Telescope
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Announcement of Opportunity for GLAST

Gamma-ray Large Area Space Telescope (GLAST)
Science Requirements Document - Final
July 9, 1999

CONTENTS

1. Introduction 1
1.1 Purpose 1
1.2 Scope 1
1.3 Applicable Documents 2
1.4 Background 2
2. Science Objectives 4
2.1 Active Galactic Nuclei 4
2.2 Isotropic Diffuse Background Radiation 6
2.3 Gamma-Ray Bursts 7
2.4 Molecular Clouds, Supernova Remnants and Normal Galaxies 10
2.5 Endpoints of Stellar Evolution (Neutron Stars andBlack Holes) 11
2.6 Unidentified Gamma-Ray Sources 12
2.7 Dark Matter 13
3. Summary of Requirements 14
3.1 Table 2: Instrument Requirements 15
3.2 Table 3: Mission Requirements 17

Signature page showing signatures of Co-chairs: Neil Gehrels and Peter Michelson, SEU Theme Director: Alan Bunner, Project Scientist: David Bertsch, and Project Formulation Manager: Scott Lambros

1. Introduction

The Gamma-ray Large Area Space Telescope (GLAST) mission is a high-energy gamma-ray observatory designed for making observations of celestial sources in the energy band extending from 20 MeV to 300 GeV and higher. This mission will:

  1. Identify and study nature's high-energy particle accelerators through observations of active galactic nuclei, pulsars, stellar-mass black holes, supernova remnants, gamma-ray bursts, and the diffuse Galactic and extragalactic high-energy radiation.
  2. Use these sources to probe important physical parameters of the Galaxy and the Universe that are not readily measured with other observatories.
  3. Use high-energy gamma rays to search for a variety of fundamentally new phenomena, such as particle dark matter, quantum gravity, and evaporating black holes.

The GLAST mission's scientific objectives require an instrument with large collecting area, imaging capability over a wide field of view, ability to measure the energy of gamma rays over a broad energy range, and time resolution sufficient to study transient phenomena. The instrument must also achieve sufficient background discrimination against the large fluxes of cosmic rays, Earth albedo gamma rays, and trapped radiation that are encountered in orbit.

1.1 Purpose

This document defines the science objectives and corresponding measurement requirements for the GLAST mission. It has been prepared by the GLAST Science Facility Team (http://glast.gsfc.nasa.gov/science/resources/gfst/), co-chaired by Peter Michelson and Neil Gehrels.

1.2 Scope

The measurement requirements are those mission and instrument capabilities that are needed to achieve the stated science goals. Technologies have been identified and are under development that will meet the measurement requirements. In some cases, these technologies offer the prospect of exceeding the stated measurement requirements. In those areas where exceeding the measurement requirements appears to be feasible within the overall scope of the mission resources and would enhance the discovery potential of the mission, measurement goals have been stated as well.

1.3 Applicable Documents

Documents that are relevant to the development of the GLAST mission concept and its requirements include the following:

  1. "Recommended Priorities for NASA's Gamma Ray Astronomy Program 1996-2010", Report of the Gamma Ray Astronomy Program Working Group, April 1997.
  2. "The Evolving Universe: Structure and Evolution of the Universe Roadmap 2000 - 2020", roadmap document for the SEU theme, NASA Office of Space Science, June 1997.
  3. "The Space Science Enterprise Strategic Plan: Origins, Evolution, and Destiny of the Cosmos and Life", NASA Office of Space Science, November 1997.
  4. "Gamma Ray Large Area Space Telescope Instrument Technology Development Program", NRA 98-217-02, NASA Office of Space Science, January 16, 1998.
1.4 Background

High-energy gamma-ray astronomy is currently in a period of discovery and vigor unparalleled in its history. In particular, the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO) has moved the field from detection of a small number of sources to detailed studies of several classes of Galactic and extragalactic objects. The CGRO/EGRET discoveries of gamma-ray blazars, pulsars, high-energy gamma-ray bursts, and a large class of unidentified high-energy sources have given us a new view of the high-energy gamma-ray sky, while raising fundamental new questions about the origin, evolution, and destiny of nature's highest energy sources of radiation.

High-energy gamma rays probe the most energetic phenomena occurring in nature. These phenomena typically involve dynamical non-thermal processes and include the following: interactions of high-energy particles (electrons, positrons, protons, pions, etc...) with matter, photons and magnetic fields; high-energy nuclear interactions; matter-antimatter annihilation; and possibly other fundamental elementary particle interactions. High-energy gamma rays are emitted over a wide range of angular scales from a diverse population of astrophysical sources including the following: stellar-mass objects, in particular neutron stars and black holes; high-energy cosmic rays that interact with interstellar gas in the Galaxy; the diffuse extragalactic background; supernovae that are considered to be sites of cosmic-ray acceleration; and gamma-ray bursts are all copious sources of gamma rays. The Sun is also known to produce high-energy gamma rays during flaring periods. Many of the sources exhibit transient phenomena, ranging from the sub-second timescales of the fastest gamma-ray bursts to AGN flares lasting days or more. Often these sources radiate the bulk of their power at gamma-ray energies.

The basic instrument requirements are defined in a two step process. First, major science themes are identified. These themes are largely based upon the science goals for a high-energy gamma-ray mission as outlined by NASA's Gamma-ray Astronomy Programs Working Group (the GRAPWG). A summary of the GRAPWG's work can be found at http://universe.gsfc.nasa.gov/grapwg.html. Secondly, for each of the major science themes, an estimate of the basic telescope properties that are most relevant to reaching the science goals are identified. In many cases, it is natural to make direct comparisons with the EGRET instrument as the most recent and successful experiment in this energy range. All of the instrument parameters are intrinsic variables except for point source sensitivity and source localization, which depend on both the mission duration and the other instrument parameters. An overview of the science themes and the requirements they impose on the instrumentation is given in Section 2. A summary of technical requirements is given in Section 3.

2. Review of GLAST Science

The high-energy gamma-ray Universe is diverse and dynamic. Measuring the various characteristics of the many types of gamma-ray sources on timescales from milliseconds to years places severe demands on the instrument and mission. Even so, the clear and compelling science goals for the GLAST mission make definite requirements possible. The following sub-sections describe the main science goals of GLAST in seven areas of current research. In each area the most relevant instrument requirements are stated.

2.1 Active Galactic Nuclei:

To date over 60 active galactic nuclei (AGN) of the "blazar" class have been detected at high gamma-ray energies. Blazars are defined by their large and rapid variability, prominent optical polarization, and often strong, flat-spectrum, core-dominated radio emission. Gamma-ray observations have yielded interesting results on individual sources, and have initiated high-energy study of AGN as a class. The gamma-ray band has become an integral part of the multiwavelength approach to studying blazars. Despite this progress, fundamental questions about the formation of AGN jets, particle acceleration, and broadband radiation mechanisms remain. The study of gamma-ray emission from all known blazars (and possibly other AGN classes) and correlation of these observations across wavelengths will create a new understanding of the AGN phenomenon.

Greatly increased numbers of gamma-ray AGN and more sensitive observations of individual sources are the keys to answering fundamental questions about blazars: What is the global structure of the AGN jet? What are the sources of variability? Is the broad-band energy distribution pure Synchrotron Self-Compton (SSC), or are the External Compton (ECS) models where the seed photons come from the accretion disk or are scattered off broad-line region clouds more correct? Is a one-zone model adequate or is an inhomogeneous jet model required? What are the redshift dependencies of the gamma-ray emission from blazars? Do different target photons or mechanisms operate in different sources? The nature and location of relativistic particle acceleration, how the supermassive black hole is involved in creating the energy distribution, and the nature of the relationship between particles, magnetic field and electromagnetic radiation in the inner jet or near the black hole all can be addressed by GLAST observations.

To answer these questions, a sensitive high-energy instrument is a critical component for AGN monitoring. It is necessary to determine the wide-band energy distribution across a range of variability timescales. A gamma-ray telescope with a large field of view is needed to monitor many AGN, most of which flare unexpectedly. Greatly improved point-source sensitivity is crucial for class studies, which would benefit from an order of magnitude increase in the number of detected AGN.

Improved gamma-ray time variability and cross-wavelength lag studies are important for understanding blazar activity. For example, at TeV energies, the blazar MKN 421 has been shown to vary on timescales as short as 15-30 minutes. Given the sparse photon numbers and constrained detector areas, gamma-ray instruments generally require the longest observation times to detect significant source variability. Measurements of relative variability timescales and lead or lag times provide sensitive measurements of the relationship between SSC components and target photons. These measurements can also contribute to constraining bulk Lorentz factors and magnetic field strengths. It would be valuable for GLAST to be able to measure variability for bright AGN on the timescale of a few hours. Given that EGRET detects variability largely on the timescale of days for most blazars, and at the 4-6 hour timescale for the brightest flares, a large increase in effective telescope area is needed.

An increase in energy range and spectral sensitivity for GLAST is also required for further progress in this field. Studies of spectral evolution during gamma-ray flares and measurements of spectral breaks at both low and high energies can give important clues to particle acceleration mechanisms and the location of emission regions. It is vitally important to understand the intrinsic blazar spectrum separately from the interaction of source gamma rays with the intergalactic medium. Precise determinations of redshift vs. spectral cutoff energy allow us to measure the intergalactic infrared background radiation. These measurements will provide information on the epoch of galaxy formation, on the radiation byproducts of large-scale structure, and on dark matter candidates. A broad energy range and good spectral response is a necessity for achieving these goals. Overlap and good inter-calibration with other ground-based, high-energy gamma-ray telescopes will be important for definitive studies of spectral cutoffs.

Requirements:
  • Broad energy response from ~20 MeV to at least 300 GeV to explore the low-energy spectrum where many AGN have peak emission, measure high-energy cutoffs, and overlap with ground-based gamma-ray observations. Goal is 10 MeV to > 300 GeV.
  • Spectral resolution of 10% or better (E > 100 MeV) to facilitate studies of spectral breaks at both low and high energies.
  • Effective area of at least 8000 cm2 (approx. 6 times EGRET) over the central part of the energy range to allow for variability studies of bright sources down to the sub-day timescales.
  • Field of view at least 2 steradians (approx. 3 times EGRET) for significant sky coverage to monitor large numbers of AGN.
  • Sensitivity of better than 10-8 cm-2 s-1 to study the spectra above 100 MeV of ~1000 AGN (based on blazar logN-logS extrapolations).
  • Clean separation of ~1000 sources on the sky to minimize source confusion.
  • Flux sensitivity better than 4 x 10-9 cm-2 s-1 to constrain the AGN logN-logS function to a factor of 50 fainter than EGRET.
2.2 Isotropic Background Radiation:

Improvements in AGN studies will have a direct bearing on measurements of the isotropic gamma-ray background radiation. Deep surveys of high Galactic latitude fields are important to determine if the high-energy background is completely resolvable into point sources, or if there is a true diffuse cosmic component. The identification of a cosmic background would have profound implications on studies of the early Universe. Spatial studies of the isotropic emission and the search for anisotropies will couple nicely with AGN class studies to fully describe the diffuse radiation. It is important that the GLAST sensitivity extends to high energies since air Cherenkov instruments cannot study large-scale diffuse emission. Also, the measurements of blazar cutoffs due to pair production of gamma rays on the infrared background, mentioned in Section 2.1, is an important technique for exploring the Universe around the epoch of galaxy formation.

Requirements:
  • Background rejection ratio of at least 105:1 over the full energy range for adequate measurements of the diffuse extragalactic background radiation. This brings the typical background rate down to 10% of the isotropic diffuse component. Goal of 106:1.
  • Broad energy range from 20 MeV to 300 GeV to extend diffuse spectral measurements to energy ranges that have not been well explored. Goal of 10 MeV to > 300 GeV.
  • Broad field of view for sensitive full-sky maps.
2.3 Gamma-Ray Bursts:

Gamma-ray burst (GRB) studies have come a long way in the past few years with the detection of GRB counterparts at X-ray and optical energies, and the recognition of the cosmological distance scale for GRBs. GLAST can provide measurements over an otherwise inaccessible energy range. While EGRET has detected only a handful of bursts, with each being relatively poorly studied, a major discovery by EGRET is the existence of a high-energy burst afterglow implying particle acceleration lasting for more than an hour. This has important implications for the physics of the source region. By detecting high-energy radiation from approximately 50 to 100 bursts per year (as compared to ~1 per year for EGRET) GLAST will provide constraints on physical mechanisms for GRBs and allow studies of the relationship between GeV emission and keV-MeV emission as a function of time during the burst. This sample will allow for more thorough evaluation of the importance of the temporally extended emission found with EGRET. Do most bursts exhibit this behavior? How does the high-energy spectral form and peak energy change with time? The EGRET bursts are consistent with a spectrum to GeV energies. Measurements of intrinsic burst spectra at these energies can constrain bulk Lorentz factors of relativistic fireball models and provide measurements of cutoffs due to absorption on the extragalactic background light at energies as low as 100 GeV for large redshifts. A large field of view and effective area are important for these advances. Also important are the capability for GLAST to continue observations of a burst for long periods of time (hours) after the burst has occurred. This can be achieved by having a large field of view for the GLAST instrument and/or by rapidly (minutes) repointing the spacecraft to orient the instrument toward the burst.

Since GRBs are the most intense and rapidly changing gamma-ray sources known, deadtime effects could hinder a true measurement of the intrinsic variability timescales which constrain the size of the emission region of the highest energy gamma rays. Low system deadtime for high rates is important.

Highly desirable is the capability to provide rapid (few seconds) notification of the burst and its position from GLAST to the ground. This will allow for rapid ground-based observations of the burst. It is also useful for burst notifications to be rapidly (few 10's of seconds) sent from the ground to GLAST. The purpose of this capability is to allow GLAST to point at gamma-ray bursts discovered by other missions.

GLAST will be making the first comprehensive observations of high-energy gamma-ray emission from bursts. Since very little is known about the relationship of the high-energy emission to the better studied low-energy gamma-ray and X-ray emission, it is desirable to have a capability to simultaneously measure the low and high-energy components. Low-energy measurements are also important for the most rapid determination of the existence of a gamma-ray burst, which can be used to provide notification to observers at other wavelengths. It is a GLAST goal to have on-board low-energy measurements by a secondary instrument for gamma-ray bursts. The specific goals for such an instrument are described in Table 1. The key objectives of the monitor are to 1) provide lower energy context measurements of the light curve and spectrum of bursts for comparison with high energy measurements of the main instrument; 2) provide positions for bursts over a wide field of view to few-degree accuracy to allow repointing of the main instrument. It would be also be beneficial (but not a requirement) for the monitor to provide burst positions of arcminute accuracy for counterpart identifications to be made from the monitor data itself. In keeping with the monitor nature of the instrument, the resources (weight, power, data rate) should be limited to a few percent of those of the main instrument.

Table 1: Gamma-Ray Burst Monitor Goals.
Quantity Goals
Energy Range low energy gamma-ray, X-ray
Field of View1 3 sr or greater
Sensitivity2 0.5 ph cm-2 s-1 or better

1 Integral of effective area over solid angle divided by peak effective area.
2 In 50-300 keV band.

Requirements (Main Instrument):
  • The ability to quickly recognize and localize GRBs.
  • Field of view > 2 sr to monitor a substantial fraction of the sky at any time. Goal of > 3 sr.
  • Spectral resolution better than 20%, especially at energies above 1 GeV, for sensitive spectral studies and searches for breaks.
  • Sustain random photon rate of 10 kHz with less than 20% deadtime for determining correlations between low energy and high energy gamma-ray burst time structure.
  • Rapid notification to the ground of transients is desirable.
  • A goal in the ability to repoint the spacecraft autonomously in < 5 minutes based on an onboard trigger in order to perform the best observations of the high-energy emission and afterglow over several hours.
  • Single photon angular resolution approaching 10 arcminutes at high energies for good source localization.
2.4 Molecular Clouds, Supernova Remnants and Normal Galaxies:

Determining the sites and mechanisms of cosmic-ray production is a fundamental problem in physics. EGRET observations of the Small and Large Magellanic Clouds have shown that cosmic rays are likely Galactic in origin. X-ray and TeV observations have demonstrated cosmic-ray electron acceleration in supernova remnants. GLAST gamma-ray mapping and energy spectral measurements should provide direct evidence of proton cosmic-ray acceleration in supernova remnants.

GLAST will further contribute to these efforts by probing the cosmic-ray distribution in dense molecular clouds and in nearby galaxies (LMC, SMC, M31) both by gamma-ray mapping and by measuring the spectrum of diffuse emission from these objects. In addition, GLAST should be able to resolve questions about the possible excess of diffuse Galactic emission at high energies and look for variations in the X-ratio (N(H2)/WCO) and the cosmic-ray/matter coupling scale. Finally, GLAST will provide important measurements to confirm the existence, and better determine the Galactic nature, of the gamma-ray halo discovered by EGRET. These efforts will benefit by a large effective area and good angular resolution to allow for fine mapping of diffuse features.

Requirements:
  • Angular resolution of better than a few degrees at 100 MeV, improving to better than a degree in the GeV range, for mapping of diffuse features. Single photon resolution goal of 2° x (100 MeV/E).
  • Point source localization less than 5 arcminutes to identify supernova remnants.
  • Spectral resolution better than 30% at low energies to explore the pion bump.
2.5 Endpoints of Stellar Evolution (Neutron Stars and Black Holes):

While only roughly 1% of known pulsars are gamma-ray pulsars, these are key to the overall understanding of the pulsar phenomenon. It is important for GLAST to significantly increase the number of detected gamma-ray pulsars in order to extend the compilation of empirical trends that EGRET made possible, such as the relationships between gamma-ray efficiency, spectral hardness, and pulsar age. An order of magnitude increase in the number of detected gamma-ray pulsars would greatly enhance our understanding of the basic structure of pulsar magnetospheres and identify the sites and nature of pulsar particle acceleration. The ability to detect Geminga type (radio-quiet) pulsars out to the Galactic Center distance will provide important new insights into the basic statistics of pulsar birthrates. This will also provide much better understanding of the pulsar contribution to the diffuse Galactic emission. Large effective area and good spectral resolution are vital for these discoveries.

Improved phase-resolved spectroscopy of new and previously known gamma-ray pulsars is important for distinguishing between various models for high-energy gamma-ray emission from pulsars. Polar cap and outer gap models can be effectively distinguished by studies of the spectral structure of the pulsed emission. Adequate low-energy response (10-100 MeV) will allow searches for breaks in the primary spectrum while the high-energy response (> 10 GeV) will allow the detection of Compton cutoffs and radiation reaction limits, and will guide TeV searches.

For Galactic black hole candidates, the increasing number of known accreting Galactic sources that exhibit relativistic jets provides an important opportunity for studying the high-energy emission from such objects. Detections of significant high-energy gamma radiation, or severe limits on emission from these objects, can be coupled with AGN studies to learn about the astrophysical consequences of scaling by black-hole mass. EGRET detects a source at the Galactic Center, which could be emission from a massive black hole, but EGRET does not have adequate angular resolution to uniquely identify it.

Requirements
  • Good spectral resolution of ~10%, especially above 1 GeV where pulsar spectral breaks occur.
  • Absolute timing to 10 μ sec to facilitate searches for pulsations from millisecond pulsars and characterization of pulse profiles of detected pulsars.
2.6 Unidentified Gamma-Ray Sources:

More than half of the sources that EGRET detects are unidentified. Determining the type of object(s) and the mechanisms for gamma-ray emission from the unidentified gamma-ray sources is a high priority for GLAST. By measuring precise positions of these sources, the possible relationship between unidentified sources and supernova remnants, radio pulsars, molecular clouds, and other candidates can be explored. Perhaps an entirely new source population is involved. Only source locations on the order of arcminutes or better can begin to answer these questions.

How many unresolved point sources are in the Galactic plane? What is the nature of the emission at the Galactic Center? What is the nature of the unidentified sources at high Galactic latitude? Exploring these questions requires significantly improved single-photon and source localization capabilities as compared to EGRET. Such localizations, coupled with the broadest possible gamma-ray energy range, will enable effective multiwavelength observations of unidentified gamma-ray sources for the first time. In addition, long exposure times and large effective area will allow for sensitive searches for gamma-ray pulsations from possible radio-quiet pulsars.

Requirements:
  • Source localization less than 5 arcminutes for sources of strength > 10-8 ph cm-2 s-1 (10 times fainter than EGRET) and less than 0.5 arcminutes for strong sources to facilitate counterpart searches at X-ray and lower energies.
  • Broad energy range to extrapolate GLAST spectra into the hard X-ray and TeV regimes to facilitate studies at other wavelengths.
  • Large field of view to allow high-duty-cycle monitoring of unidentified sources for time variability.
2.7 Dark Matter:

Aside from normal diffuse emission, GLAST will search for extended emission from possible cold dark matter clouds in the Galaxy and from galaxy clusters as a signature of unusual concentrations of unseen gas or cosmic rays. Many models of cold dark matter feature heavy supersymmetric particles whose line emission can be detected in the 10's or 100's of GeV range. Good spectral response over a broad range of energies and a wide field of view is important to look for these dark matter signatures. Another form of dark matter may be primordial black holes (PBHs). While EGRET has already set important limits on PBH production, greater sensitivity and the ability to identify and distinguish between photons arriving simultaneously in the instrument would aid in further PBH studies.

Goals:
  • Broad energy range with response to at least 300 GeV to constrain cold dark matter candidates.
  • Spectral resolution of 2% at energies above 10 GeV to identify relatively narrow spectral lines.
  • High efficiency recognition and reconstruction of multiple gamma events, i.e., events in which two or more signal gamma rays arrive within one trigger and read-out time window for sensitive searches for primordial black holes.

3. Summary of Requirements

Section 2 describes a broad range of scientific goals that define the ultimate technical requirement, which the GLAST instrument must meet. Often, these requirements are difficult to quantify without referring to other parameters. For instance, point source sensitivity can be improved by increasing effective area, by increasing observation times through larger field of views, or by decreasing the point-spread function width to reduce background. Improved spectra can be achieved both by reducing intrinsic energy resolution and by increasing source statistics that come from more effective area. Although the parameters are interrelated, the stated scientific expectations can effectively guide the requirements. Table 2 is a summary of the basic requirements of the GLAST instrument based upon the science outlined in Section 2. Table 3 is a summary of the derived requirements for the overall mission. Both requirements and goals are listed.

Table 2. Summary of GLAST Instrument Requirements. (Table notes refer to GLAST, not EGRET.)
Quantity EGRET GLAST Requirement GLAST Goal Science Driver
Energy Range 20 MeV - 30 GeV 20 MeV - 300 GeV 10 MeV - > 300 GeV ALL
Energy Resolution 10% 10% (100 MeV - 10 GeV)1 2% (E> 10 GeV) ALL
Effective Area2 1500 cm2 8000 cm2 > 10,000 cm2 ALL
Single Photon Angular Resolution - 68%3 (on-axis) 5.8o (@100 MeV) < 3.5o (@100 MeV)
< 0.15o (E > 10 GeV)		 < 2o (@ 100 MeV)
< 0.1o (E > 10 GeV)		 ALL
Single Photon Angular Resolution - 95%3 (on-axis)   < 3 x θ68% 2 x θ68% ALL
Single Photon Angular Resolution
(off-axis at FWHM of FOV)		   < 1.7 times on-axis < 1.5 times on-axis ALL
Field of View4 0.5 sr 2 sr > 3 sr ALL
Source Location5,8 Determination 5 - 30 arcmin 1 - 5 arcmin 30 arcsec - 5 arcmin Unidentified EGRET
Sources, GRBs
Point Source Sensitivity6,8
(> 100 MeV)		 ~ 1 x 10-7 cm-2 s-1 4 x 10-9 cm-2 s-1 < 2 x 10-9 cm-2 s-1 AGN, Unidentifieds,
Pulsars, GRBs
Time Accuracy 0.1 ms 10 μsec absolute7 2 μsec absolute7 Pulsars, GRBs
Background Rejection > 106:1 > 105:1 > 106:1 ALL, Especially
Diffuse
Dead Time 100 ms/event < 100 μs/event < 10% instrument ave. for bursts up to 10 kHz (< 20 μs/event) GRBs
Transients     Complementary low-energy observations, Trigger and location for S/C repointing, High efficiency recognition and reconstruction of multi-γ events GRBs, Primordial BHs

1 Equivalent Gaussian sigma, on-axis.
2 Peak effective area, including inefficiencies necessary to achieve required background rejection.
3 Space angle for 68% and 95% containment. 4 Integral of effective area over solid angle divided by peak effective area. Geometric factor is Field of View times Effective Area.
5 Range: bright sources to sources of 10-8 ph cm-2 s-1 flux at > 100 MeV.
6 Sensitivity at high latitude after a 2-year survey for a 5 σ detection.
7 Relative to Universal Time.
8 Derived quantities.

Table 3. GLAST Mission Requirements Summary.
Quantity GLAST Requirement GLAST Goal Science Driver
Mission Life 5 years, with no more than 20% degradation 10 years ALL
Telemetry Downlink - Orbit Average 300 kbps 1 kbps near-realtime for notifying ground of transients in progress > 1 Mbps GRBs, AGN, ALL
Telemetry Uplink 4 kbps 1 kbps near-realtime for notifying GLAST of transients in progress GRBs, AGN, ALL
Spacecraft Repointing   < 5 min. autonomous GRBs, AGN
Pointing Accuracy1 0.5o accuracy
< 30 arcsec knowledge		   ALL
Observing Modes Rocking zenith pointing
Pointed mode		   ALL
Targeting   Point anywhere in the sky at any time, More than 1 target per orbit ALL
Spacecraft Clock Accuracy 8 μsec absolute2 1 μsec absolute2 PULSARS, GRBs
Spacecraft Position Accuracy 3 km 1 km PULSARS, GRBs
Data Loss
Data Corruption		 < 2 %
< 10-10 undetected corrupted event fraction		   ALL

1 3σ diameter.
2 Relative to Universal Time.