# Presentation: 585604

## The SNAP Mission

- May 2006
- Henry Heetderks

**Public Domain Document**
**This information is in the Public Domain. For an electronic copy see:  *****http://snap.lbl.gov/pub/bscw.cgi/198989***

## Overview of Talk

**Pictorial Overview of Observatory**
**Fully Assembled**
**Telescope and Focal Plane Assembly only**
**View of Detector System**
**Spacecraft**
**Mass and Power**
**L2 Orbit and Required Delta V**
**Potential Launch Vehicles**
**Attitude Control System**
**Electronics Systems**
**Data Volume and Telemetry**
**Downlink Margins**
**Thermal Systems and Control**
**Integration and Testing**
**Summary**

## SNAP Observatory Cross Section

##  Components of Observatory

**SNAP Observatory**

**Spacecraft Bus**

**TMA-65 Telescope**
**with Radiator**

**Stray Light Baffles**
**with Instrument Deck**

## Focal Plane System with Radiator

## Focal Plane with Detectors

## Spacecraft

## Spacecraft

##  Mechanical Requirements on the Spacecraft

**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 structure with a ‘hockey puck’ configuration**
**Baseline is 52 cm from Marman clamp to Instrument interface**

## Mass Budget 

**Mass Margin depends on Launch Vehicle chosen**
**Mass Reduction Strategy**
**Reduce Aperture**
**Use Graphite Structure**

## 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**

## L2 Orbit

**Stable thermal environment**
**Always same distance from sun and earth**
**Radiator never “sees” the earth**
**No eclipses**
**Reduced stray light sources**
**Earth and moon always on sun side and 1.5 million km away**
**mostly “see” dark side of earth and moon**
**No disposal**
**Eject cover**
**Drawbacks**
**Larger Antenna required**
**3 months till achieve orbit**
**Forceless torques to unload wheels**
**Track using S band ranging and doppler**

**Maximum Earth and Moon Illumination from L2**

## L2 Orbit Schematic

**Large Amplitude**
**Low insertion delta V**
**No eclipses**
**In Plane Amplitude = 800,000 km x 283,000 km**
**Out of plane amplitude = 224,000 km**

## L2 Animation

- Rotating Reference Frame

- Inertial Reference Frame

## L2 Mission

**Top View (# is day after launch)**

**Side View (# is day after launch)**

**Sample Mission Timeline**

## Requirements on the Launch Vehicle

**Must accommodate SNAP with its spacecraft in the fairing**
**Need C3 of -0.5 (km/sec) ****2 **
**Need Restartable final stage with capability to do direct injection to L2**
**Observatory propulsion system allocates  74 meter/sec delta V for launch and midcourse correction**

## 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**
**I****sp**** for Hydrazine is 220 sec**

**CBE mission mass = 1583 kg, with reserves 1905 kg (dry)**
**Needed fuel:  114 kg  (5 year mission)**

## Potential L2 Launchers

**Possible Launch vehicles**
**Atlas V**
**Delta IV**
**Soyuz**
**Zenit**
**Proton**
**Direct injection into L2 requires C3 =**
**       ****-0.5 to -0.71 km****2****/sec****2**
**C3 ~ V****2****- Escape Velocity****2 ****(Measure of energy from vehicle)**
**Launch Capability to L2:**
**Delta IV (4040-12):   2765 kg**
**Altas V (401):            3480 kg**
**Proton**
**Soyuz                        **
**Zenit**
**SNAP Current Best Estimate of Mass:  2020 kg   (with reserve and fuel,  no margin)**

**Zenit**

**Soyuz**

**Proton**

## Fairing Accomodation

**Delta IV                       Soyuz-ST/Fregat**

## Observatory after Launch

## ACS system

|  | Requirement | Performance | Assumptions |
| --- | --- | --- | --- |
| Jitter  | 0.02 Arcsec RMS | 0.004 Arcsec RMS | Rigid body model,
TTL star tracker operating |
| 3 arc-min Maneuver | 30 sec Settling time | 20 sec Settling time |  |

- Sensor Unit

- TTL Star
- Tracker

- Ball 602 Star
-  Trackers

- Controller

- ACS controller

- Actuators

- Hydrazine
- Thrusters

- Northrup
- SIRU

- Reaction
- Wheels

**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**

**SNAP ACS Block Diagram**

## 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 > 1 arcminute)**
**Signals are available to ACS when stationary or during small slews < 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**

## Slew Mode Requirements

**Slewing must be done using spacecraft star tracker(s) and gyro system 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 < 15 minutes: observing efficiency driver**
**Therefore would like slew rate  6 ********/minute**
**At end of slew, orientation must be within 1⁄2 of the TTL Star Tracker field of view (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 orientation**

## Science Pointing Requirements Using TTL Star Tracker

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

## Electrical System Overview

**Avoid 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**

## Observatory Block Diagram

## Data/Memory Budget

**Onboard Memory Budget**

**Science Data Sizes**

**Download Budget**

**Sufficient memory on board to handle spectrograph data for one missed pass**

## Telemetry

**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% Instrument Data System “idle” time requires 1⁄2 the memory**
**Compression during downlink requires full memory**

**Detector Data**

**Compress**

**Memory**

**Form CCSDS Source Packets**

**Form CCSDS Transfer Frames**

**17% overhead due to **
**Reed-Solomon and packing**

**Assume factor of **
**2 compression**

**Modulate**
**Ka  Band Transmitter**

## 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**

## Thermal Overview

**Requirements**
**Optics and metering structure at 280K**
**Focal plane at 140K**
**Solar array < 350K**
**Upper and inner baffles < 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**

## 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 < 1.3 ****°****C**
**Secondary < 0.2 ****°****C**
**Tertiary < 0.3 ****°****C**
**Fold-Flat < 0.4 ****°****C**
**Detector**
**Gradients < 0.05 ****°****C**
**Can run down to ~ 130 K**

## Thermal Results

**Detectors**

**Metering Structure**

**Fold-Flat Mirror**

**Focal Plane**

## Integration & Test

**I&T 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**

## Development and Test Flow Summary

**See SNAP dwg. #00015-ME02-E for detailed I&T flow**

## Baseline Configuration Summary

**Orbit L2 halo or large lissajous, 1.7 Mkm maximum distance**
**Launch vehicle: Various **
**Lifetime: 3 years required, 5 years consumables  **
**Quality level: Level 2/3 EEE parts,  Selective block redundancy**
**End-of-life disposal: May be required**
**Sun side w/ rigid body mounted solar arrays **
**Standard Hydrazine propulsion system, 1 Newton thrusters, ~114 kg total propellant required for 120 m/s delta V corrections and ACS.**
**C.B.E. Instrument Mass: 1229 Kg**** ****including reserve**
**Instrument Power:  275 W ave. during science, 675 W max ave,  911 W peak**
**Instrument 128 Gbytes memory **
**Ka band downlink @ ~150 Mbps to  ********17m dish, 100 Mbps only slightly decreases operational efficiency**
**Fixed 1.2m Ka-band HGA,  35W TWTA; S-Band T&C thru omni’s**
**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**

## Example Daily Observing Plan

- Spect. Shutter

- Phot. Shutter

- Thruster

- Transmitter

- Slew Angle

- 10o

- 1o

- 1”