GLAST CDR

1
GLAST Large Area Telescope Science Requirements
and Instrument Design Concepts Steven
Ritz Goddard Space Flight Center LAT Instrument
Scientist steven.m.ritz_at_nasa.gov
2
Outline

• Context flow of requirements, definitions,
  design drivers
• Instrument overview
• Simulations
• Performance calculation results review and status
• Design modifications since PDR
• Failure modes modeling and science impact
• Summary

3
From Science Requirements to Design

• Flow of instrument requirements from science
  goals and requirements was developed as part of
  the LAT proposal.
• LAT will meet or exceed requirements in GLAST
  Science Requirements Document (433-SRD-0001).

4
Simplified Flow
Science Requirements Document (SRD)
Mission System Specification (MSS)
Mission Assurance Requirements (MAR)
LAT Performance Specifications
Interface Requirements Document (IRD)
Design Trade Study Space
LAT Subsystem Requirements
5
Aside some definitions
Effective area

(total geometric acceptance)
(conversion probability) (all detector and
reconstruction efficiencies) Point Spread
Function (PSF)
Angular
resolution of instrument, after all detector and
reconstruction algorithm effects. The
2-dimensional 68 containment is the equivalent
of 1.5? (1-dim error) if purely Gaussian
response. The non-Gaussian tail is characterized
by the 95 containment, which would be 1.6 times
the 68 containment for a perfect Gaussian
response.
68
95
6
Science Performance Requirements Summary

• From the SRD

Parameter SRD Value
Peak Effective Area (in range 1-10 GeV) gt8000 cm2
Energy Resolution 100 MeV on-axis lt10
Energy Resolution 10 GeV on-axis lt10
Energy Resolution 10-300 GeV on-axis lt20
Energy Resolution 10-300 GeV off-axis (gt60º) lt6
PSF 68 100 MeV on-axis lt3.5
PSF 68 10 GeV on-axis lt0.15
PSF 95/68 ratio lt3
PSF 55º/normal ratio lt1.7
Field of View gt2sr
Background rejection (Egt100 MeV) lt10 diffuse
Point Source Sensitivity(gt100MeV) lt6x10-9 cm-2s-1
Source Location Determination lt0.5 arcmin
GRB localization lt10 arcmin
7
Experimental Technique

• Instrument must measure the direction, energy,
  and arrival time of high
• energy photons (from approximately 20 MeV to
  greater than 300 GeV)
• - photon interactions with matter in GLAST
• energy range dominated by pair
  conversion
• determine photon direction
• clear signature for background rejection

- limitations on angular resolution (PSF)
low E multiple scattering gt many
thin layers high E hit precision
lever arm
must detect ?-rays with high efficiency
and reject the much larger (1041) flux of
background cosmic-rays, etc. energy
resolution requires calorimeter of sufficient
depth to measure buildup of the EM shower.
Segmentation useful for resolution and
background rejection.
8
Science Drivers on Instrument Design
Background rejection requirements drive the ACD
design (and influence the calorimeter and tracker
layouts).
Effective area and PSF requirements drive the
converter thicknesses and layout. PSF
requirements also drive the sensor performance,
layer spacings, and the design of the mechanical
supports.
Field of view sets the aspect ratio (height/width)
Energy range and energy resolution requirements
bound the thickness of calorimeter
Electronics
Time accuracy provided by electronics and
intrinsic resolution of the sensors.
On-board transient detection requirements, and
on-board background rejection to meet telemetry
requirements, are relevant to the electronics,
processing, flight software, and trigger design.
Instrument life has an impact on detector
technology choices. Derived requirements (source
location determination and point source
sensitivity) are a result of the overall system
performance.
9
IRD and MSS Constraints Relevant to LAT Science
Performance

• Lateral dimension lt 1.8m
• Restricts the geometric area.
• Mass lt 3000 kg
• Primarily restricts the total depth of the
  CAL.
• Power lt 650W
• Primarily restricts the of readout channels
  in the TKR (strip pitch, layers), and restricts
  onboard CPU.
• Telemetry bandwidth lt 300 kbps orbit average
• Sets the required level of onboard background
  rejection and data volume per event.
• Center-of-gravity constraint restricts
  instrument height, but a low aspect ratio is
  already desirable for science.
• Launch loads and other environmental constraints.

10
Overview of LAT

• Precision Si-strip Tracker (TKR) 18 XY
  tracking planes. Single-sided silicon strip
  detectors (228 mm pitch) Measure the photon
  direction gamma ID.
• Hodoscopic CsI Calorimeter(CAL) Array of
  1536 CsI(Tl) crystals in 8 layers. Measure the
  photon energy image the shower.
• Segmented Anticoincidence Detector (ACD) 89
  plastic scintillator tiles. Reject background
  of charged cosmic rays segmentation removes
  self-veto effects at high energy.
• Electronics System Includes flexible, robust
  hardware trigger and software filters.

Tracker
ACD surrounds 4x4 array of TKR towers
Calorimeter
Systems work together to identify and measure the
flux of cosmic gamma rays with energy 20 MeV -
gt300 GeV.
11
Tracker Optimization

• Converter thickness profile iterated and
  selected.
• Resulting design FRONT 12 layers of 3 r.l.
  converter
• BACK 4 layers of 18 r.l. converter
• followed by 2 blank layers
• Large Aeff with good PSF and improved aspect
  ratio for BACK.
• Two sections provide measurements in a
  complementary manner FRONT has better PSF, BACK
  enhances photon statistics.

TKR has 1.4 r.l. of material. Combined with
8.4 r.l. CAL provides 9.8 r.l. total.
12
Design Performance Validation LAT Monte-Carlo
Model
Detailed detector model includes gaps, support
material, thermal blanket, simple spacecraft,
noise, sensor responses

• LAT design based on detailed Monte Carlo
  simulations.
• Integral part of the project from the start.
• Background rejection
• Effective area and resolutions
• Trigger design
• Overall design optimization
• Simulations and analyses are all C, based on
  standard HEP packages.

Instrument naturally distinguishes gammas from
backgrounds, but details matter.
13
Monte Carlo Modeling Previously Verified
inDetailed Beam Tests
High-level performance parameters
Detailed detector characteristics
(e.g., PSF) (e.g., hit
multiplicities)
GLAST Data
(errors are 2?)
Monte Carlo
1997 SLAC beam test (photons, positrons) Demonstr
ate silicon conversion telescope
principle Published in NIM A446
1999-2000 SLAC beam test (photons, positrons,
protons) flight-scale tower Published in NIM A474
14
LAT Balloon Flight
background event candidate

• Purpose of balloon test flight expose prototype
  LAT tower module to a charged particle
  environment similar to space environment and
  accomplish the following objectives

• Help validate the basic LAT design at the single
  tower level.
• Demonstrate the ability to take data in the
  isotropic background flux of energetic particles
  in the balloon environment.
• Record events for background flux analysis.

gamma event candidate
All Objectives met by Balloon Flight on August 4,
2001 (3 hrs at 38 km float)
All subsystems performed properly. Trigger rate
lt1.5 kHz, well below BFEM 6 kHz capability.
15
Calibration Strategy

• Every LAT science performance requirement has a
  defined test.
• LAT energy range and FOV are vast. Testing will
  consist of a combination of simulations, beam
  tests, and cosmic ray induced ground-level muon
  tests.
• beam tests verify the simulation and sample
  performance space analysis using the simulation
  is used to verify the full range of performance
  parameters.
• For science performance verification, beam tests
  can be done with a few towers together. Full-LAT
  tests are functional tests.

side view front view
55º, 1 GeV g
See LAT-TD-00440 and LAT-MD-00446
16
Implemented Maximum Background Fluxes
total
orbit-max fluxes used for trigger rate
calculations
Integrates to 10 kHz/m2

• LAT-TD-00250-01 Mizuno et al
• Note by Allan Tylka 12 May 2000, and
  presentations by Eric Grove
• AMS Alcaraz et al, Phys Lett B484(2000)p10 and
  Phys Lett B472(2000)p215
• Comparison with EGRET A-Dome rates provides a
  conservative ceiling on the total rate.

17
Implemented Average Background Fluxes
Integrates to 4.2 kHz/m2
orbit-avg fluxes used for downlink and final
background rejection calculations
18
Instrument Triggering and Onboard Data Flow
Level 1 Trigger
On-board Processing
Hardware trigger based on special signals from
each tower initiates readout Function did
anything happen? keep as
simple as possible
full instrument information available to
processors. Function reduce data to fit within
downlink Hierarchical filter process first make
the simple selections that require little CPU and
data unpacking.

• subset of full background rejection analysis,
  with loose cuts
• only use quantities that
• are simple and robust
• do not require application of sensor calibration
  constants

complete event information signal/bkgd
tunable, depending on analysis cuts
?cosmic-rays? 1few
TKR 3 xy pair planes in a row workhorse g
trigger
OR
Total L3T Rate lt25-30 Hzgt
(average event size 8-10 kbits)
Upon a L1T, all towers are read out within 20ms
On-board science analysis transient detection
(AGN flares, bursts)
Instrument Total L1T Rate lt4 kHzgt
Spacecraft
4 kHz average without throttle (1.3 kHz with
throttle) peak L1T rate is approximately 12 kHz
without throttle and 3.8 kHz with throttle).
19
On-board Filters

• select quantities that are simple to calculate.
  Intelligent use of ACD information to preserve
  acceptance of high-energy events. Filter scheme
  is tunable.
• Filters use
• ACD info match simple tracks to selected hit ACD
  tiles, count hit tiles at low energy
• CAL info energy deposition pattern consistent
  with downward-going electromagnetic interactions.
  
• TKR info remove low-energy particles up the
  ACD-TKR gap by projecting track to CAL face and
  selecting on XY position for very low CAL
  energy, require TKR hit pattern inconsistent with
  single prong.
• filter first designed using LAT
  simulation/reconstruction, then pieces
  implemented by FSW group. Now wrapping existing
  FSW code for use in LAT simulation/recon packages
  for verification and optimization for science.

20
On-board Filters Design Results

• After all selections, average background rate is
  17 Hz.

16.5 Hz total rate
composition
5 Hz line
2 Hz line
1 Hz line
Additional margin available much of the residual
rate is due to high-energy proton and electron
events with CAL Egt5GeV -- if apply ACD selections
onboard to higher energy, rate can be cut in half
(to 8 Hz), with 5 reduction in Aeff at 10 GeV.
21
Ground-based Background Rejection Analysis

• Evaluation of the performance parameters (Aeff,
  PSF, etc.) requires the final background
  rejection analysis.
• Different science analyses can optimize final
  background rejection selections differently.
• The most stringent requirement driver is the
  extragalactic diffuse flux.
• Exercises the Science Analysis Software
  infrastructure
• generate 50M background events, with a unique tag
  for each. Machinery to cull lists of events,
  single event display, analysis, etc.
• Also generate adequate sample of extra-galactic
  diffuse photon flux and pass it through the
  analysis chain.

22
Summary of Ground-Based Background Rejection
Analysis

• The background rejection analysis meets the
  requirements and is continuing to evolve,
  becoming more sophisticated, effective, and
  efficient.
• Current selections using subsystem info
• TKR use information from track fits, hit pattern
  inconsistent with single track at low energy, PSF
  cleanup cuts, location cuts, loose consistency
  between track multiple scatter and CAL energy.
• CAL require xtal hit patterns (shower shapes) to
  be consistent with downward-going EM showers
  more precise track-CAL energy centroid matching.
• ACD very low energy events require quiet ACD
  hidden shower rejection.

23
Background Rejection Results

• Requirement lt10 contamination of the measured
  extragalactic diffuse flux for Egt100 MeV
• Residual background is 5 of the diffuse (6 in
  the interval between 100 MeV and 1 GeV).
  Important experimental handle large variation of
  background fluxes over orbit compare diffuse
  results over orbit.
• Below 100 MeV no requirement, without any
  tuning of cuts for low energy, fraction rises to
  14. This will improve.
• Peak effective area 10,000 cm2 (at 10 GeV).
• At 20 MeV, effective area after onboard
  selections is 630 cm2. Different physics topics
  will require different (and generally less
  stringent) background rejection on the ground.

Diffuse flux, after cuts, scaled to generated
background
log(E) corrected (MeV)
100 MeV 1 GeV 10 GeV
100 GeV
24
Energy Resolution

• Energy corrections to the raw visible CAL energy
  are particularly important for
• depositions in the TKR at low photon energy (use
  TKR hits)
• leakage at high photon energy (use longitudinal
  profile)

gt60 off-axis 300 GeV g (require lt6)
18
Normal-incidence 100 MeV g (require lt10)
4.3
9
uncorrected corrected E(MeV)
corrected E(GeV)
25
PSF
68 containment radius 100 MeV Requirement
lt3.5 3.37 FRONT 4.64 Total 10
GeV Requirement lt0.15 0.08 FRONT 0.115 Total
68 containment
100 MeV, normal incidence FRONT
95 containment
95/68 ratio Requirement lt3 FRONT 2.1 (BACK
2.6)
NOTE With older version of TKR reconstruction
software, the 95/68 ratio is not lt3 everywhere
(particularly at high energy). This is a
software issue, not an instrument design
issue. TKR reconstruction software under upgrade
to improve hit association and more precise
vertexing.
26
Field of View

• Defined as the integral of effective area over
  solid angle divided by peak effective area

10 GeV, uniform flux, all cuts
10 GeV, uniform flux, all cuts
FOV
cos(q)
LAT
cos(q)
q
0 10 20 30 40 50 60 70 deg
Favorable LAT aspect ratio provides large FOV
Requirement gt2 sr LAT current 2.4 sr
27
Science Performance Requirements Summary

• From the SRD

Parameter SRD Value Present Prediction
Peak Effective Area (in range 1-10 GeV) gt8000 cm2 10,000 cm2 at 10 GeV
Energy Resolution 100 MeV on-axis lt10 9
Energy Resolution 10 GeV on-axis lt10 8
Energy Resolution 10-300 GeV on-axis lt20 lt15
Energy Resolution 10-300 GeV off-axis (gt60º) lt6 lt4.5
PSF 68 100 MeV on-axis lt3.5 3.37 (front), 4.64 (total)
PSF 68 10 GeV on-axis lt0.15 0.086 (front), 0.115 (total)
PSF 95/68 ratio lt3 2.1 front, 2.6 back (100 MeV)
PSF 55º/normal ratio lt1.7 1.6
Field of View gt2sr 2.4 sr
Background rejection (Egt100 MeV) lt10 diffuse 6 diffuse (adjustable)
Point Source Sensitivity(gt100MeV) lt6x10-9 cm-2s-1 3x10-9 cm-2s-1
Source Location Determination lt0.5 arcmin lt0.4 arcmin (ignoring BACK info)
GRB localization lt10 arcmin 5 arcmin (ignoring BACK info)
28
Updates to the Simulation/Reconstruction

• Simulation and reconstruction packages completely
  rewritten since PDR
• support flight, with more robust architecture
• very large effort, validation proceeding
• re-evaluating instrument performance using new
  reconstruction. Some major improvements already
  apparent 100 MeV, normal incidence (lt5º),
  front 68 containment 2.4º
  98 containment 7.1º VERY PRELIMINARY
• support for misalignments
• hooks into hardware databases (bad channels,
  etc.) and ability to introduce hardware
  failures easily
• Now wrapping onboard filter into recon to have
  high-fidelity assessment of performance (expected
  complete July)
• Support for science verification and calibration
  planning (see IT talk)
• Support for Data Challenges (end-to-end testing,
  see SAS status talk)

29
Design Modifications Since PDR

• Most changes are in the mechanical/thermal system
  and are outside the instrument FOV. Negligible
  impact on particle propagation.
• Deleted geographic information in TKR trigger
  signal (used only for optional ACD hardware
  throttle). Now using much simpler throttle, with
  whole tower-tile associations.
• Very small impact on instrument triggered
  FOV when throttle is turned on
• Even smaller after analysis cuts

30
Failure Modes Impacts on Science Performance

• Support for reliability calculations and design
  evaluation
• Can analyze identical events with/without failure
  turned on. Difficult part is putting awareness
  into the reconstruction (as we would if failure
  really happens).

• Highest priority has been ACD loss of a single
  tile. Potential impact on trigger rates and
  downlink volume could we operate? Needed
  primarily for micrometeoroid shield reliability
  requirements.
• TKR impact on trigger efficiency of loss of a
  layer within a tower
• thick converter layer 0.6 loss of triggered
  Aeff
• thin converter layer lt0.1 loss of triggered
  Aeff
• Effects of loss of a tower

31
Impact of ACD Tile Failures

• Main issue here is the micrometeoroid shield
  reliability analysis. Can we tolerate loss of a
  single tile? YES

Damage After L1T throttle After filters Hz After simple mitigation Hz
none 1.8 kHz 17 N/A
center top tile 1.9 kHz 27 19
corner top tile 1.9 kHz 24 17
Simple mitigation when a top tile is
non-functional require found track NOT to start
in silicon layer directly underneath. Loss of
effective area is small. Additional actions
available, if needed.
32
Impact of Loss of a Tower

• Three kinds of towers, by symmetry
• corner, edge, core
• Requirement lt20 degradation in 5 years
• peak Aeffgt6,400 cm2 FOVgt1.6 sr
• Use simulation to study loss of a TKR tower (6,
  core) on triggered Aeff
• 1 GeV normal incidence 6 loss
• 1 GeV 40 off axis 4 loss
• Difficult to estimate all impacts accurately an
  actual loss will trigger more intense study of
  mitigations. Rough estimates of impact
• corner peak Aeff 9,000 cm2 , FOV 2.4 sr
• edge peak Aeff 7,500 cm2, FOV 2.1 sr
  (asymmetric)
• core peak Aeff 7,500 cm2, FOV 1.9 sr
  (asymmetric)

33
LAT Has Three Tower Location Types
Loss of a corner is not as bad as loss of an edge
is not as bad as loss of a core.
Simplifying assumption tower loss means both
CAL and TKR. ACD is still intact.
34
Summary

• LAT will meet or exceed science requirements from
  the SRD.
• Much work done over the past year on simulation
  and reconstruction. Moving forward
• first filter integration complete July
• new instrument response functions, using improved
  reconstruction algorithms September
  (collaboration science meeting, data challenge
  start)
• In addition to design optimization and
  performance evaluation, detailed simulation is
  being used as a system engineering tool, e.g.,
  supporting FMEA.
• See following talks for design details.

35
Backup Slides
36
EGRET A-dome Rates (from D. Bertsch, EGRET team)
A-dome has an area of 6 m2, so orbit max rate
(outside SAA and no solar flares) corresponds to
16 kHz/m2 This represents a conservative
upper-limit for us, since the A-dome was
sensitive down to 10s of keV. Note peak rate
is at (24.7,260)
SAA
37
Onboard Filter Development

• Filter designs done with the full simulation and
  ground-based reconstruction, in consultation with
  FSW group. Demonstration of principles, included
  in science performance evaluations.
• FSW implemented most of the filter design for
  benchmarking on the flight processor.
• filtering is hierarchical. Most important to
  implement the selections that are run first
  (highest rate, largest multiplier on CPU demand).
  More cycles/event available for remaining event
  sample after each step.
• FSW implementation is being wrapped for inclusion
  in the simulation/recon packages.
• very early functional testing of the flight
  algorithms, with high fidelity. Examine details
  (e.g., existing track finding) using full set of
  SAS tools, event display, etc.
• detailed evaluation of the filter effects on the
  science performance
• opportunity for a tuning iteration and
  optimization of the final set of selections

38
Summary of Filter and Status
Primary Info Design Selection FSW Status
ACD Tile counts (energy dependent) DONE
ACD-TKR Track match with tile DONE
CAL Simple energy selections DONE
CAL Layer ratios DONE
CAL Simple topologies
TKR-CAL Track match with energy centroid
TKR Skirt only cut DONE
TKR Simple hit pattern inconsistent with single prong at low energy
TKR-CAL Minimal tracks and CAL E, or make additional demands DONE
TKR Earth direction
TKR TKR hits consistent with a track near CAL if Egt0 DONE
39
Detector Choices

• TRACKER
• single-sided silicon strip detectors for high hit
  efficiency, low noise occupancy, resolution,
  reliability, readout simplicity. Noise occupancy
  requirement primarily driven by trigger.
• CALORIMETER
• hodoscopic array of CsI(Tl) crystals with
  photodiode readout
• ANTICOINCIDENCE DETECTOR
• segmented plastic scintillator tiles with
  wavelength shifting fiber/phototube readout for
  high efficiency (0.9997 flows from background
  rejection requirement) and avoidance of
  backsplash self-veto.

for good resolution over large dynamic range
modularity matches TKR hodoscopic arrangement
allows for imaging of showers for leakage
corrections and background rejection pattern
recognition.
40
SRD LAT Requirements (I)
41
SRD LAT Requirements (II)
42
SRD LAT Requirements (III)
43
Testing Trigger Efficiencies

• Two kinds of inefficiencies

Test this on the ground in LAT using cosmic-ray
induced muons
44
On-board Science Requirements

• Requirements on transient recognition and
  localization onboard are modest.
• Requirements specify the characteristics of the
  flux (photons, energy, time period) to be
  detected
• 10 arcmin(3 arcmin goal) GRB localization
  accuracy onboard for any burst that has gt100
  detected photons with Egt1 GeV arriving within 20
  seconds.
• 5 second (2 second goal) notification time to
  spacecraft relative to time of detection of GRB
• NO requirements on AGN flare detection onboard
  in SRD. NO mission minimum.

45
Broad Implications for Onboard Reconstruction

• With 100 photons, the mean per-photon pointing
  error for Egt1 GeV onboard is comfortably LARGE.
  Rudimentary gamma direction reconstruction
  onboard is anyway needed for earth albedo gamma
  rejection. The crude precision needed to meet
  the SRD requirements is not a driver.
• The expected (non-transient) gamma rate (Egt20
  MeV) is a few Hz the residual rate for downlink
  (independent of science analysis onboard) is 30
  Hz. Recognizing that gt100 photons with Egt1 GeV
  have arrived within a 20 second interval from one
  direction should not require precision
  reconstruction or background rejection better
  than that needed to meet telemetry requirements.
• Simple (low resource) prototype algorithms exist,
  but not yet in FSW environment. Attempt first to
  reuse filters track finding once embedded into
  recon environment to study performance.

46
Additional Goals (no requirement)

• AGN flare localization lt 2 for flares that occur
  within the LAT FOV at high gal. lat., with DF gt 2
  x 10-6 ph cm-2 s-1 (E gt 100 MeV) that last for
  more than 1000 secs.
• For these flare parameters, notify spacecraft lt1
  min after recognition of flare.
• This recognition and localization goal is not
  easy
• Assuming 30 Hz event rate (residual photon
  candidate rate after standard onboard filter), on
  average there will be 5 photons per square
  degree bin in the FOV after 1000s. Change in
  flux corresponds to 5 detected photons.
• gt Meeting the goal is not possible without
  additional onboard filtering (additional factor
  10 needed --- not crazy to consider).
• This is a GOAL, and is not a driver. First
  approach should be to extend GRB onboard analysis
  to longer timescales and see how well we do. A
  simple energy cut will also help here.
• The science justifies making an effort! However,
  it is lower priority than other onboard software
  tasks.

47
On-board Filters

• select quantities that are simple to calculate
  and that do not require individual sensor
  calibration constants. Filter scheme is flexible
  current set is basis for flight implementation.
• order of selections to be optimized. Grouped by
  category for presentation purposes
• ACD info match track to hit tile, count hit
  tiles at low energy

outside tile boundary
inside tile boundary
Background
100 MeV g
no tile hit
Rate after ACD selections is 180 Hz
orbit-avg (360 Hz orbit-max)
cm
cm
48
On-board Filters (II)

• CAL info most of the residual rate at this point
  is due to albedo events and other upward-going
  energy events. Require track-CAL energy centroid
  loose match, fractional energy deposit in front
  layer reasonably consistent with downward EM
  energy flow. If no CAL energy, require track
  pattern inconsistent with single-prong.
• TKR info low-energy particles up the ACD-TKR gap
  easily dealt with
• project track to CAL face and
• require XY position outside this
• band for low CAL energy,
• require TKR hit pattern
• inconsistent with single prong.

Rate after CAL selections is 80 Hz orbit-avg
(130 Hz orbit-max)
Y(cm)
X (cm)
49
Effects of Rocking Albedo Gammas
albdeo gamma
L1T with Throttle
full background flux
front
back
cos (q)
As we rock, the spike spreads in q, f
At zenith, earth horizon is at 113 degrees.
Study what happens when observatory rocks to 35
and 60 degrees off zenith.
f
f
Front
Front
Back
Back
cos(q)
cos(q)
35 degree rock
60 degree rock
50
Albedo Gamma Rates
L1T rate Hz L1T rate with Throttle Hz After filters Hz After fiducial cut Hz
zenith 250 190 2 2 (no cut)
rock 35 260 200 3 3 (no cut)
rock 60 300 250 8 1 (lt45) 3 (lt53)

• Notes
• rates for other backgrounds will be reduced
  somewhat by the same angle cut, not taken into
  account here.
• small incremental L1T rate not a problem
• calculating the gamma direction only happens at
  a relatively low rate, if needed (after other
  filters), so incremental CPU load not a problem.
• can reduce the downlink contribution to whatever
  we need with a tighter fiducial cut.

51
Pointing Knowledge

• Context requirements and their origins
• Overview of pointing knowledge elements
• Details
• Experimental technique
• Intrinsic LAT measurement characteristics
• LAT mechanical/thermal distortion effects
• Observatory/instrument coordinates to sky
  coordinates
• Resulting pointing knowledge and source
  localization
• Calibration techniques and stability timescales
• Precision of corrections and uncorrectable
  effects
• Sequence of calibrations on orbit
• Required supporting ground-based calibrations

52
Context Requirements Summary

• Observatory pointing knowledge 10 arcsec
• The single photon direction measurement ability
  of the LAT is not very precise, on average, but
  the best photon LAT will ever measure will have
  an error of 30 arcsec.
• The pointing knowledge should be better than the
  best single photon error to avoid throwing away
  information. The single photon accuracy does
  not, by itself, set the scale of the pointing
  knowledge, however, since LAT will measure many
  photons from each source. (see following slide
  on localization)
• The mission assigned suballocations as follows
• LAT internal 7 arcsec
• Spacecraft (both GNC and thermal/mechanical) 6
  arcsec
• Calibration residual 4 arcsec
• Jitter 1 arcsec

53
LAT Source Localizations
SRD 0.5 arcmin 1s radius for 10-7 source
10 arcsec, 1s radius
note!
Systematics will dominate here
figure is from LAT proposal. Will be updated.
There is no critical value (original SRD
requirement was a factor 2 more stringent).
54
Overview of Pointing Knowledge Elements
Source localization
Ancillary packet info
LAT-SC calibration
Single photon direction measurement (sky coords)
Single photon direction measurement (observatory/i
nstrument coordinates)
Correction algorithm
Internal LAT mech/thermal effects
Uncorrected effects
High-level LAT measurement characteristics
Intrinsic low-level LAT measurement
characteristics
Experimental technique
55
Experimental Technique and Science Drivers on
Instrument Design
Background rejection requirements drive the ACD
design (and influence the calorimeter and tracker
layouts).
Effective area and PSF requirements drive the
converter thicknesses and layout. PSF
requirements also drive the sensor performance,
layer spacings, and drive the design of the
mechanical supports.
Field of view sets the aspect ratio (height/width)
Energy range and energy resolution requirements
bound the thickness of calorimeter
Electronics
Time accuracy provided by electronics and
intrinsic resolution of the sensors.
On-board transient detection requirements, and
on-board background rejection to meet telemetry
requirements, are relevant to the electronics,
processing, flight software, and trigger design.
Instrument life has an impact on detector
technology choices. Derived requirements (source
location determination and point source
sensitivity) are a result of the overall system
performance.
56
Tracker/Converter Issues
g
Some lessons learned from simulations
Expanded view of converter-tracker
At low energy, measurements at first two layers
dominate due to multiple scattering
At 100 MeV, opening angle 20 mrad
All detectors have some dead area if isolated,
can trim converter to cover only active area
At low energy, direction measurement is dominated
by planes closest to conversion point.
Low energy PSF completely dominated by multiple
scattering effects q0 2.9 mrad /
EGeV (scales as (x0)½) High energy PSF set by
hit resolution/lever arm
1/E
At higher energies, more planes contribute
information Energy significant planes 100
MeV 2 1 GeV 5 10 GeV
gt10
PSF
Roll-over and asymptote (q0 and qD) depend on
design
E
57
Intrinsic LAT Measurements Characteristics

• Low-level
• Strip pitch 228 mm
• Plane spacing 3 cm
• Tower size 60 cm tall, 37 cm wide
• Geometric characteristics
• hit resolution/plane spacing 2 mrad (410
  arcsec)
• hit resolution/tower height 0.1 mrad (21 arcsec)
• thus, on average, a tower rotation of 7 arcsec
  will have a very small impact on the strip hit
  distribution in a single event.
• stack diagonal angle 32 degrees
• High-level
• Photon direction determined by reconstruction
  software algorithms.
• Different categories of events and corresponding
  measurement quality.
• Kalman filter is used. Optimal algorithm to
  handle both multiple scattering and geometric
  effects.
• At low energy, first measurements dominate due to
  multiple scattering. Even for cross-tower
  events, it is the measurements in the tower
  containing the conversion that dominate the
  direction measurement.
• At high energy, it is the tower with the largest
  track path length that dominates the direction
  measurement.

To good approximation, the individual tower
misalignment is also the photon location
misalignment. This also represents the
pessimistic case.
58
Overview of Pointing Knowledge Elements
Source localization
Ancillary packet info
LAT-SC calibration
Single photon direction measurement (sky coords)
Single photon direction measurement (observatory/i
nstrument coordinates)
Correction algorithm
Internal LAT mech/thermal effects
Uncorrected effects
High-level LAT measurement characteristics
Intrinsic low-level LAT measurement
characteristics
Experimental technique
59
Converting to Sky Coordinates

• Information from ancillary data packet
• Attitude information is conveyed to LAT from
  spacecraft every 200ms. Includes both attitude
  and angular velocities.
• maximum angular velocity is 30 deg/minute, or 360
  arcsec/update, so velocity info is necessary with
  a timestamp better than 0.5ms. OK.
• angular acceleration terms are negligible (lt0.25
  arcsec)
• LAT-SC calibration residual
• The error from the calibration is carried
  forward, but can be updated over time (see later
  slide on on-orbit calibrations).

60
Calibration Techniques and Stability Timescales

• Internal LAT alignment using cosmic rays
  (straight trajectories)
• standard technique for particle trackers
• no external measurements or references needed
• ground muons from cosmic ray airshowers
• on-orbit proton cosmic rays
• statistics set by detail needed
• individual sensors require higher statistics.
  Stable.
• tower-to-tower lower statistics needed. Varies
  on thermal timescales.
• LAT to Observatory
• done on orbit. Observe known sources.
  Precision/statistics source-dependent. Brighter
  sources provide 10 arcsec localization knowledge
  in 1 month.
• after initial alignment observation, all-sky
  survey data can be used for continuous
  update/refinement
• gt effects that are slow compared to 2 months
  are automatically calibrated out.

61
Sequence of On-Orbit Calibrations

• LAT SVAC Plan (LAT-MD-00446).
• First, LAT internal alignment using CRs.
• Then, initial two-week observations to know
  LAT-SC alignment to better than 15 arcsec (easily
  sufficient for most science topics).
• optimization of initial observing strategy
  (source selection, optimal orientation, etc.)
  under investigation.
• Then, proceed with sky survey and use known
  sources to reduce the error over year 1.

62
Supporting Ground Measurements (I)

• Captured in LAT survey plan, SVAC plan, IT
  survey procedures documents.
• Demonstration of technique
• introduce misalignments into the simulation and
  reconstruction
• implement muon flux to simulate the measurements,
  with LAT pointed horizontally.
• Remaining systematic offsets under study, but are
  already sufficiently small.
• The ground-based tower-to-tower alignment
  information doesnt carry forward significantly
  to flight. Launch loads will change the
  alignment, and it will anyway be re-established
  using cosmic rays on orbit.

63
Supporting Ground Measurements (II)

• The main purpose of the ground-level muon
  measurements is to test functionality and as part
  of the mechanical/thermal alignment requirements
  testing and analysis.
• LAT ground-level muon measurements will be made
  with LAT Z-axis parallel to the ground
  (sideways).
• Verified rate of tracks crossing tower pairs with
  the simulation

m
m
tower pairs
tower pairs
Current estimate sufficient statistics in one
day to do tower-to-tower verification
measurement. Good match to LAT thermal timescale.
64
Alignment Summary
Source localization
Ancillary packet info
LAT-SC calibration
Single photon direction measurement (sky coords)
Single photon direction measurement (observatory/i
nstrument coordinates)
Correction algorithm
Internal LAT mech/thermal effects
Uncorrected effects
High-level LAT measurement characteristics
Intrinsic low-level LAT measurement
characteristics
Experimental technique
65
Loss of corner tower

• easiest case to estimate
• Aeff loss 10
• FOV loss very small.

good experimental handle on-orbit can pretend
any other working corner tower is off and check
for incremental background leakage and other
systematic effects.
66
Loss of single edge tower

• corner closest to dead edge tower also
  significantly impacted FOV. For purposes of
  conservative estimate, assume only 3x4 LAT left.
• Aeff loss 25
• FOV loss 10.

good experimental handle on-orbit can pretend
any other working edge tower is off and check for
incremental background leakage and other
systematic effects.
67
Loss of single core tower

• most difficult (and painful) case. For purposes
  of conservative estimate, assume whole quadrant
  compromised. Looks like 2 LATs, each 1x2, with
  overlap.
• Aeff loss 25
• FOV loss 15. (35 loss in 1-dim in the bottom
  left and top right quadrants)

good experimental handle on-orbit can pretend
any corresponding tower is off and check for
incremental background leakage and other
systematic effects.