The SNAP Reference Spacecraft

1
The SNAP Reference Spacecraft

• February 2005

Competition Sensitive
2
Mechanical Overview

• Rigid Observatory
• No flexible appendages
• Stable thermal attitude
• Minimal thermal snap
• Minimize number of mechanisms
• More reliable
• Avoid image jitter
• Passive thermal design
• More reliable
• Less vibration
• Cryo-coolers are not mature

3
SNAP Observatory Cross Section
4
Electrical System Overview

• No single point failures
• Block redundant spacecraft
• Redundant Instrument Control Units
• Instrument Detectors and Mechanisms modularized
  for failure tolerance
• Industry Standard 1553 interface between
  instrument and spacecraft
• Wide band downlink controlled directly by
  Instrument
• Memory Capability incorporated into the
  Instrument
• No requirement for SSR in spacecraft
• Spacecraft consists of 3 independent modules
  connected by a 1553 bus
• Power system
• ACS System
• CDH System

5
Observatory Block Diagram
6
Focal Plane Features

• All detector electronics on focal plane
• Runs at 140 K (low power)
• CCD readout using CRIC (Preamp/ADC) CLIC
  (clocking) ASIC
• HgCdTe readout using SIDECAR ASIC
• Each sensor chip has its own chain of electronics
• Redundancy
• Prime interface can power any subset of sensors
• Backup interface can power any subset of sensors
• Detector interface reduces number of wires from
  focal plane to ICU

7
Focal Plane Block Diagram
8
Thermal Overview

• Requirements
• Optics and metering structure at 280K
• Focal plane at 140K
• Solar array lt 350K
• Upper and inner baffles lt 220 K
• Stable temperatures
• Method
• L2 orbit eliminates earth load on radiator
• Array rejects heat from front only using optical
  solar reflectors
• Low emissivity silver mirrors
• Extensive use of MLI blankets
• Adiabatic spacecraft to instrument interface
• Actively heated optics and metering structure via
  networked thermal controllers

9
Thermal Model

• Completed 13 node thermal model
• Set heater sizes
• Use to validate detailed model
• Observatory Thermal Desktop model in progress
• 2900 nodes
• 220 active heaters
• Steady state Heater power 140 Watts
• Power on focal plane (including heaters) 27
  Watts
• Large radiator to add margin to focal plane
• Results
• Mirror gradients (radiative heaters)
• Primary lt 1.3 C
• Secondary lt 0.2 C
• Tertiary lt 0.3 C
• Fold-Flat lt 0.4 C
• Detector
• Gradients lt 0.05 C
• Can run down to 130 K

10
Thermal Results
Metering Structure
Detectors
Fold-Flat Mirror
Focal Plane
11
L2 Launch

• Launch vehicle
• Atlas V or Delta IV
• Direct injection into L2 requires C3 -0.71
  km2/sec2
• C3 V2- Escape Velocity2 (Measure of energy from
  vehicle)
• Smallest Delta IV (4040-12) can launch 2780 kg
  into L2
• Smallest Altas V (401) can launch 3495 kg into L2
• Designing SNAP to fit with a launch mass of 2780
  kg (including margin, reserve and fuel)

12
L2 Mission
Top View ( is day after launch)
Sample Mission Timeline
Side View ( is day after launch)
13
Delta V budget

• An L2 orbit requires onboard fuel to maintain the
  orbit
• A Delta V budget is the way of keeping track of
  fuel
• Hydrazine is the fuel of choice
• Isp for Hydrazine is 220 sec

• CBE mission mass 1501 kg, with reserves 1839 kg
  (dry)
• Needed fuel 108 kg (5 year mission)
• Fuel for maximum launch capability 151 Kg (5
  year mission)

14
Mass Budget

• Mass Allocation has a 40 margin with the 2780 kg
  direct to L2 Capacity
• NASA expects 25-30 margin

15
Power Budget

• Instrument average Power
• 275 W during science,
• 235 W during downlink,
• 675 W during bake,
• Instrument Peak Power
• 911 Watts
• Given reference spacecraft, need array EOL output
  of 750 watts

16
SNAP Operations
Science Ops
Scheduler
NASA Science Support Center
Mission Ops
Command and Control
SN database
D S N
SN trigger
Public data archive mirror
Telemetry
Image processing pipeline
User support
SNAP collaboration
User support
SNAP computing engine
Science community
17
Reference Spacecraft Summary Table
18
Reference Spacecraft Summary Table
19
Reference Spacecraft Summary Table
20
Tall Pole 1 Attitude Control
Patrick Jelinsky
21
ACS system

• ACS Simulation Activities
• Control Strategies and Data Filtering Schemes
  are being Examined
• Models now include published Sensor Noise
  Drift, as well as Momentum Wheel Rumble
• Future Versions will Include (FEM) Flexible Body
  Mechanics and Fuel Slosh

Sensor Unit
Actuators
Controller
TTL Star Tracker
Hydrazine Thrusters
ACS controller
Ball 602 Star Trackers
Reaction Wheels
Northrup SIRU
SNAP ACS Block Diagram
22
ACS Control Modes
23
TTL Star Tracker

• Instrument Contains Through-The-Lens (TTL) Star
  Tracker
• Four fixed 2D-array sensors in focal plane field
  stars centroiding no moving parts.
• Signals not available during large slews (slews gt
  1 arcminute)
• Signals are available to ACS when stationary or
  during small slews lt 1 arcminute
• Signals to ACS at 10 Hz update rate
• Yaw and Pitch Accuracy better than 1
  milli-arcsecond, meets requirements
• Roll accuracy at 5 Hz better than few hundred
  milli-arcsecond, meets requirements

• See A. Secroun, et al A High-Accuracy, Small
  Field-of-view Star Guider with Application to
  SNAP Experimental Astronomy 2002
  http//snap.lbl.gov/pubdocs/secrounetal.pdf

24
Slew Mode Requirements

• Slewing must be done using spacecraft star
  tracker(s) and IRU only. TTL star tracker will
  not operate during slews.
• Each day must slew from Science Pointing to
  Communications Pointing and back.
• Maximum slew angle is 82 ? (science target
  attitude vs Telemetry Downlink attitude)
• Would like to complete slew in lt 15 minutes
  observing efficiency driver
• Therefore would like slew rate 6 ?/minute
• At end of slew, orientation must be within ½ of
  the TTL Star Tracker FOV (0.05?)
• Yaw and Pitch must be within 50 arcseconds (3
  sigma) of requested orientation
• Roll must be within 900 arcseconds (3 sigma) of
  requested orienation

25
Science Pointing Requirements Using TTL Star
Tracker

• Absolute Attitude Relative to an Absolute
  Coordinate System (celestial coordinates)
• Relative Attitude relative to a nearby (lt 1
  arcsecond) object (a star). This is needed for
  precision dithering.

26
ACS Summary Comments

• Requirements development is still in progress
  numbers may change.
• All other ACS control modes are less stringent
  than Science Pointing and Slew Modes.
• The details of the TTL Star Tracker are still
  being defined.
• See dwg 00007-MW02 for detailed ACS
  requirements.

27
Tall Pole 2 S/I Accommodation

• The Spacecraft has to share limited space in the
  Fairing with the Science Instrument
• Spacecraft must support S/I structural loads
• ACS considerations require stiff structure
  without flexible appendages
• This drives us toward a custom structure with a
  hockey puck configuration
• Baseline is 52 cm from Marman clamp to Instrument
  interface

28
Observatory Design
TMA-65 Telescope with Radiator
Spacecraft Bus
Stray Light Baffles with S/I Ring Deck
SNAP Observatory
29
Spacecraft Views

• 2.5 m Bus Illustrated
• can grow to 3.5m ?

30
Tall Pole 3 Telemetry
Detector Data
Compress
Memory
Form CCSDS Source Packets
Form CCSDS Transfer Frames
Modulate Ka Band Transmitter
Assume factor of 2 compression
17 overhead due to Reed-Solomon and packing

• Assumptions for Data Budget
• 300 second science integrations, 30 second
  readout time
• 21.5 hours/day science 90 observing efficiency
• 2 hours/day download
• Factor of 2 compression of science images
• 15 minutes slew from science orientation to
  communications and back (30 minutes total)
• 17 overhead from CCSDS packaging
• 1.09e12 bits/day to download (see next chart)
• Required onboard memory depends on where
  compression occurs
• Compression During 90 ICU idle time requires ½
  the memory
• Compression during downlink requires full memory

31
Data/Memory Budget
Science Data Sizes
Onboard Memory Budget
Sufficient memory on board to handle spectrograph
data for one missed pass
Download Budget
32
Link Margins

• Downlink (150 Mbit/sec)
• Ka band at 26 GHz
• SNAP
• 35 W TWT transmitter
• 1.2m fixed dish
• Deep Space Network (DSN)
• 34 m dish
• Uplink ( 2 kbit/sec) 6.7 dB margin
• S band at 2.2 GHz
• SNAP
• Omni antenna
• DSN
• 1000 W
• 34 m dish

33
Tall Pole 4 Integration Test

• IT Occurs in 6 Phases (see dwg 00015-ME02)
• 1) Integration and test of OTA Mirrors and
  Structure
• Verify PSF
• Perform environmental testing
• 2) Integration of Focal Plane Assembly
• Electronics integration
• Environmental testing including T/V, Vibration,
  EMC?
• 3) Integration of spacecraft
• Includes functional and selected environmental
  testing
• 4) Integration of Science Instrument
• OTA to Science Instrument Deck assembly
• Focal Plane Assembly to OTA
• Baffle Assembly to Instrument Deck
• 5) Integration of spacecraft to science
  instrument
• Functional testing only
• 6) Observatory environmental testing
• T/V, Acoustic, EMC
• End-to-End Testing with BITE during T/V

34
Integration Test of OTA

• Manufacture and test 4 mirrors separately
• Manufacture and test metering structure
• Environmental test
• Integrate mirrors into metering structure
• Align and test telescope
• Full aperture PSD test
• Vibroacoustic test of telescope
• For full details see
• ltftp//sprite.ssl.berkeley.edu/pub/snap_cm/
  1_SNAP_Controlled_Drawings_Repository/
  00015-ME02-E.xls gt

35
Integration Test of Science Instrument

• Integrate telescope to instrument
• Integrate shutter and inner baffle to telescope
• Integrate upper and lower baffles to instrument
• Test basic functionality of SNAP
• For full details see
• ltftp//sprite.ssl.berkeley.edu/pub/snap_
  cm/1_SNAP_Controlled_Drawings_Repository/
  00015-ME02-E.xls gt

36
Integration of Spacecraft Environmental Testing

• Integrate instrument to spacecraft
• Integrate Solar arrays to observatory
• Test basic functionality of SNAP
• Environmental testing
• For full details see
• ltftp//sprite.ssl.berkeley.edu/pub/snap_cm
  /1_SNAP_Controlled_Drawings_Repository/
  00015-ME02-E.xls gt

37
Baseline Configuration Summary

• Orbit L2 halo, 1.7 Mkm maximum distance
• Launch vehicle Delta 4040-12 or equivalent
• Lifetime 3 years required, 5 years consumables
• Quality level Level 2/3 EEE parts, Selective
  block redundancy
• End-of-life disposal Not required
• Sun side w/ rigid body mounted solar arrays
• Standard Hydrazine propulsion system, 1 Newton
  thrusters, 150 kg total propellant required _at_
  120 m/s for corrections and ACS.
• C.B.E. Instrument Mass 1225 Kg including reserve
• Instrument Power 275 W ave. during science, 675
  W max ave, 911 W peak
• Instrument 128 Gbits memory
• Ka band downlink _at_ 150 Mbps to ?17m dish, 100
  Mbps only slightly decreases operational
  efficiency
• Fixed 1.2m Ka-band HGA, 35W TWTA S-Band TC
  thru omnis
• Low jitter ACS, no stringent absolute pointing
  requirements
• 3-axis stabilized, 4 Reaction wheels, IRUs, no
  torquer bar
• Select small reaction wheels for low jitter
• Ithaco B-wheels (or equivalent) on isolation
  mounts

38
Example Daily Observing Plan

• Start Move to location of first science
  exposure
• Open shutter, integrate image 300
  sec, close shutter, readout
• 300s Move 0.2 arcsec based on TTL star tracker
  signal, settle 30 sec
• 330s Image 1, open shutter, integrate 300 sec,
  close shutter, readout
• 630s Move 0.2 arcsec based on TTL star tracker
  signal, settle 30 sec
• 660s Image 2, open shutter, integrate 300 sec,
  close shutter, readout
• 960s Move 0.2 arcsec based on TTL star tracker
  signal, settle 30 sec
• 990s Image 3, open shutter, integrate 300 sec,
  close shutter, readout
• 1290s Move 0.2 arcsec based on TTL star
  tracker signal, settle 30 sec
• 1320s Image 4, open shutter, integrate 300 sec,
  close shutter, readout
• 1620s Move 180 arcsec based on gyros/star
  tracker, settle 30 sec
• . Repeat 4 image sets at 330 sec
  intervals
• . Repeat 180 arcsec move
• . Continue repetitive process until
  image 144 is recorded
• 47190s Image 144, open shutter, integrate 300
  sec, close shutter, readout
• Continued.

39
Example Daily Observing Plan

• Continued
• Turn-on Spectrograph
• 47520s Move up to up to 1 degree based on
  gyros/star tracker, settle 90 sec
• 47610s Move 50 arcsec based on TTL star tracker
  signal, settle 30 sec
• 47640s Spectro Image 1, open shutter, integrate
  1000 sec, close shutter
• 48640s Readout 30 sec
• . Repeat N image sets at 1030 sec
  intervals
• . Repeat up to 1 degree move
• . Continue repetitive process until
  spectro image 28 is recorded
• 77400s Complete image acquisition
• Telemetry
• 77400s Rotate up to 82 degrees to acquire ground
  station
• 78300s Begin downlink
• 85500s End telemetry, return to science field,
  up to 82 degree rotation
• 86400s End of daily sequence

40
Major Changes from Jan 02 IMDC

• Observatory Packaging Evolution

WAS
41
Major Changes from Jan 02 IMDC

• Cassegrain Focus Shutters

WAS
42
Major Changes from Jan 02 IMDC

• Observatory Packaging Structure Evolution
  (extensive FEM Studies)
• 3 day synchronous orbit ? L2 halo orbit
• PLUS Stray light, Thermal, No shadows, Launch
  mass
• MINUS RF communication link
• TMA-63 ? TMA-65 optics prescription (modest
  positioning adjustments)
• Improved mechanical packaging clearances
• Beam waist cold stop (Instrument optical
  Thermal margins)
• New aperture cassegrain focus optical masks
• Improved stray light control
• New cassegrain shutters
• ACS guider CCDs are ALWAYS available !
• Short calibration exposures are now possible (1
  second)