# Presentation: 863628

## SNAP Spacecraft Overview

**Henry Heetderks**
**Space Sciences Laboratory**
**12 November, 2003**

## SNAP Spacecraft Overview

**Programmatic Considerations**
**Delta IV Launch Vehicle Options**
**Mechanical Requirements**
**Electrical Requirements**
**Attitude Control Requirements**
**Downlink Telemetry Requirements**
**Propulsion Requirements**
**Integration and Test Requirements**
**Summary**

## Programmatic Considerations

**Until now our planning for SNAP as a DOE mission has been based on PI Mode mission management**
**LBNL would be responsible for procurement of the spacecraft**
**To facilitate this we had planned to have extensive discussion and studies with potential spacecraft vendors during the R&D period**
**The recently released JDEM briefing specifies a GSFC managed mission with GSFC selection of a “prime contractor” who would manage spacecraft development**
**A large optics based instrument such as SNAP requires very close integration between the development processes for the instrument and those of the spacecraft. **
**To insure the development of an instrument conceptual design which is compatible with available spacecraft technologies we will study requirements placed on the spacecraft by the instrument with spacecraft vendors during the R&D phase**
**This process will be the same independent of how the spacecraft is procured**

## Programmatic Considerations (continued)

**UCB/SSL is well placed to define the SNAP spacecraft**
**SSL performed the payload system engineering for the EUVE space observatory**
**The UCB Space Sciences Laboratory (SSL) bought and managed the spacecraft development for the highly successful RHESSI and CHIPS missions**
**SSL is now managing construction of the revolutionary THEMIS constellation (Five spacecraft in different orbits on a Midex budget!)**
**Cost and Risk Management will be early key elements of the overall program strategy**
**Define and document all top-level requirements and requirements flow-down as early as possible.**
**Once defined, don't change requirements or add new requirements without a clear programmatic understanding of the impact of those changes.**
**Keep interfaces as simple as possible, especially between hardware elements being provided by different organizations.**
**Use existing flight-proven or flight-qualified parts and components wherever possible.**
**Identify technical and programmatic risk areas as early as possible,  develop risk mitigation approaches for each risk, and track the risks carefully.**
**Develop a modular commoditized spacecraft concept which meets SNAP needs and facilitates control of cost growth**

## Key Aerospace and Optics Expertise on SNAP

| Engineer / Scientist | Major Programs  |
| --- | --- |
| Bester, Manfred | FAST, IMAGE, RHESSI, THEMIS |
| Besuner, Robert | FAST, IMAGE, ROCSAT2, RHESSI |
| Bigelow,  Bruce | IMACS,  PhD in Optics Engineering |
| Heetderks, Henry | ISEE, EUVE, FAST, IMAGE |
| Jelinsky, Patrick | EUVE, FUSE, GALEX, SOHO, COS |
| Kahn, Steven | HEAO-2, Chandra, XMM/Newton |
| Krim, Michael | HST, Classified Programs |
| Lampton, Michael | EUVE, EURD, Space Lab, FUSE, Faust, CHIPS, IMAGE |
| Pankow, David | ISEE, Keck, FAST, RHESSI |
| Sholl, Michael | Classified Programs, Airborne Laser, CHIPS |
| Kramer, William | NASA S/W, NERSC |

## Orbit Options Trade Study

## Study: SNAP Orbital Properties

-  3 day synchronous orbit,
-  Perigee = 2.56 Re (geocentric), eg. 10,000 km altitude
-  Apogee = 24.94 Re (geocentric)
-  This orbit is in the plane of the moon and is stable against lunar perturbations.  Also,  	simultaneously maximizes solar, lunar, and earth avoidance angles.
-  During perigee SNAP is not observing, rather performing data dump.
- 	This corresponds to an 86% operational efficiency.
-  Total proton dose from belts negligible

## Study: SNAP Orbital Properties

-  Orbit perigee remains over Berkeley without adjustment.
-  5.2 hour ground pass over Berkeley
-  ~300 Mbit/s downlink requirement
-  375 Gbytes storage requirement
-  Requires 0.5m 6W Ka-band Xmit, with 10m ground station

## Launch Vehicle Options

- Launch vehicle [Delta IV 4240] is capable of lifting 2020 kg to that orbit with significant mass margin held in Perigee, Apogee, SC-propulsion.  Can use equivalent Delta III, IV, Atlas V, or Sea Launch.

## Mechanical Requirements -- Structural

**The SNAP Instrument is large compared with the spacecraft volume required to provide necessary avionic functions**
**The spacecraft must provide the structure between the instrument and the Delta IV Payload Attach Fitting  (PAF)**
**This,  along with constraints set by the geometry of the instrument optics bench,  restricts the spacecraft to a particular size and shape**
**This suggests that a custom spacecraft structure is probably the best approach**

## The SNAP Instrument with Spacecraft Attached

## SNAP with Single 1.2 Meter Antenna

## The SNAP Instrument Separated from Spacecraft

## The SNAP Spacecraft

## Mechanical Requirements on the Spacecraft

**Provide the Structural connection between the PAF and the Telescope**
**Must meet Delta IV requirements on mechanical modes with instrument attached**
**Provide mounting area for spacecraft avionics and the Instrument OCU and Instrument Mass Memory boxes**
**Provide Thermal and Stray Light control as detailed in the following viewgraphs**

## Thermal Requirements on Spacecraft

**Spacecraft is thermally isolated from the instrument to buffer the telescope from temperature changes driven by different spacecraft power modes**
**Two high power dissipation instrument components, the Observatory Control Units (OCU)  and the Instrument Mass Memory Units are also mounted on the spacecraft deck to mitigate thermal effects on the instrument**
**The spacecraft must collect heat conductively from deck mounted components and radiate it from the bottom side of the deck**

## Contamination Requirements on Spacecraft

**Like any telescope,  the SNAP instrument is very sensitive to contamination**
**This situation is exacerbated by the stringent requirements on photometric absolute accuracy imposed by the supernova measurement  and by the  need to operate the focal plane at 140 degrees K**
**The instrument is sealed to the maximum extent practical to ameliorate this**
**As a second line of defense we will impose strict controls on materials and processes and contamination levels of the spacecraft components,  structure,   and harness which will be verified by instrument QA witness and monitoring of TQCM during spacecraft thermal vacuum testing**

## Stray Light Requirements on Spacecraft

**The SNAP instrument is extremely sensitive to stray light**
**Since the observatory must operate in full sun light,  the design of the light baffles and evaluation of possible stray light sources is a challenging System Engineering task**
**In addition to extensive baffling in the instrument,  we require a light tight skirt to seal the gap between the instrument and the spacecraft,  and we will levy a requirement on the light transmission of the spacecraft deck.**

## Simplified Observatory Electrical Block Diagram

## Electrical Requirements on the Spacecraft

**Supply two switched +28 volt power services to each of two OCU’s**
**Provide up to 500 watts average instrument power**
**Provide Ground Commands via the 1553 bus**
**Provide spacecraft engineering and telemetry functions**
**Accept Instrument engineering packets via the 1553 bus for S-Band downlink**
**Provide a 300 Mhz band width Ka Band downlink**
**Provide a single wire OCU processor soft reset line**
**Accept ACS Star Guider Packets from the telescope via the 1553 bus**

## Detailed Observatory Electrical Block Diagram

## Requirements on the SNAP ACS System(defined in drawing  00007-PP02-A)

**      ****Operation of the ACS system will typically consist in keeping the telescope pointed at a fixed target for a period of time and then moving to a new target.**
**1)   For large changes of pointing direction (i.e. of any size),  we require the ability to drop the target into 3x3 Arcsec box with a 98% confidence .  **
**Change of the telescope pointing direction will be done with the science instrument shutter closed.**
**2)   The time allowed to make the pointing direction change of item 1)  is a function of the step size:**
**                                          ****Step Size              Settling Time**
**                                       ****3 Arcminutes      30 Seconds**
**                                       ****1 Degree                2 Minutes**
**                                     ****15 Degrees              6 Minutes**
**                                   ****180 Degrees            60 Minutes**
**3)   Once pointed,  the shutter on the science instrument will open and the direction must be held constant for a ****1000 ****second exposure per the following jitter specification.  Full stabilization must occur within 2 seconds of opening of the shutter:**
**                                          ****Requirement                        Goal**
**          ****Yaw and Pitch         ****.02**** Arcsec RMS                   ****.01**** Arcsec RMS**
**                            ****Roll         ****.80**** Arcsec RMS                   ****.40**** Arcsec RMS**

## Requirements on the SNAP ACS System

**To perform dithering we will change the telescope pointing direction by an angle between 0.25 and 1.0 Arcsec.  For steps limited to this range we require a relative pointing ability which is tighter than that of item 1) above:**
**                                                      ****Requirement                                  Goal**
**          ****Yaw and Pitch            ****.25**** Arcsec RMS                               ****.025**** Arcsec RMS**
**                            ****Roll          ****8.0**** Arcsec RMS                                ****0.8**** Arcsec RMS**
**a)  The instrument shutter will be closed for approximately 30 seconds during the dither step.**
**b)  Within 2 seconds following the opening of the shutter,  the telescope must be pointed to  within the requirements of item 3) above.**

## Guider CCD’s on SNAP Focal Plane Support ACS

**Fine guiding sensors on the science focal plane of the 2 meter primary mirror SNAP telescope with its 22 meter focal length.  These consist of a square pattern one degree on a side of four 1K X 1K CCD’s which image and track the guide star(s).  Once a star has been acquired  the science instrument tracks it, continuously calculates the position of the centroid, and transmits the position to the ACS controller via the 1553 bus.  The update rate will be the maximum which can be supported by the photon statistics of the guide star,  up to 35 Hertz.  This system will operate continuously while the science instrument shutter is open.  This system is expected to be included in the final design.**
**Note that the shutter will typically be closed for 30 seconds during CCD readout following each science exposure.    **

## UCB PSF Jitter Modeling Activity

**Effort lead by Prof. David Auslander of the UCB Mechanical Engineering department**
**Includes a group of grad students and post docs**
**Model will include detailed mechanical model for the optics and observatory as well as models of commercial ACS components**
**UCB will develop and evaluate control software to allow a verification of vendor activities and allow immediate evaluation of changes to the instrument or spacecraft**

## BATC  ACS Study

**A study of the SNAP ACS system was done by the Ball Aerospace and Technology Corporation **
**They modified their Kepler model to match SNAP properties**
**Results are shown on the following 10 viewgraphs which are used with the permission of BATC**
**Their analysis shows that SNAP requirements are met with a model which incorporates standard ACS components and the SNAP Fine Guider system along with the SNAP mechanical model developed at Berkeley**

## BATC  ACS Concept Development & Performance Modeling

**Ball Aerospace & Technologies Corp. (BATC) performed a study of the SNAP ACS system**
**Modified their detailed Kepler ACS model to include SNAP mass properties and component performance.**
**BATC’s ACS model has been calibrated using on-orbit data from high performance systems (Relay Mirror Experiment, QuickBird, etc.) and includes detailed ACS component performance models.**
**The model also included the SNAP mechanical properties provided by Berkeley.**
**The BATC analysis shows that SNAP requirements can be met using standard ACS components and the SNAP Fine Guider system.**
**ACS model performance estimates are shown in the following 9 viewgraphs**

## BATC ACS FunctionalBlock Diagram

## FGS Performance is more than adequate to support the 20 milli-arcsec jitter requirement

**Analysis of SNAP telescope and FGS system shows FGS noise will be <0.01 arcsec 1******
**Table entries show contribution to total ACS pointing jitter for various values of FGS noise**

**Green cells indicate bandwidth and FGS noise combinations that meet SNAP jitter requirements**

## Spacecraft Jitter Analysis Predicts14 milli-arcsec Pointing

**Used a modified Kepler structural model incorporating SNAP sensor and actuator performance**
**6-DOF disturbances from four active wheels**
**Modest isolators could get us below 10 milli-arcsec 1******

- Total Pointing Jitter Prediction

- Low Frequency Jitter Prediction

## Low Frequency Steady-State Pointing Analysis 3 milli-arcsec

**Control system bandwidth is optimized given sensor and actuator noise levels**
**A responsive control system is required for shutter recovery**

- Motion within an interval versus
- control system bandwidth

- BW selection to minimize motion
- from wheels and sensor while
- retaining responsiveness

*Note:  HF jitter is still the dominant*
*component of LOS jitter and is not shown here*

## Low Frequency Control Analysis with Modes - Good

**10 Hz solar array, 2 Hz Fuel and 30 Hz instrument cause no problems**

- LOS Jitter Excluding HF Mechanical Jitter
- 0.1Hz control / 5e-6 Nm wheels / 0.01 arcsec FGS

## Agility Meets All Requirements

**Presume 4Nms and 0.09 Nm available for maneuvering**
**Small maneuvers (<30 deg) are torque limited while large maneuvers are momentum limited**

## 1 deg Maneuver EasilyCompleted in 110 sec

**Fully settled at the completion of the maneuver**
**Fully optimized feed forward torques**

## Fuel Slosh is Not a Problem

**With two 30 inch tanks, slosh frequencies are expected to be 2 Hz or higher**
**40 inch tanks get down to 1.4 Hz**
**Propellant management devices need only be used for high agility**
***Vanes or metal bellows or sponges***
**Maneuver simulations include slosh modes and pointing performance is fully acceptable**
**Inertia fraction of 5% assumed**
***Placing fuel closer to CM is better***

## Shutter Settling Offset is WellWithin Pointing Budget

*Shutter Settling Offset is Well**Within Pointing Budget*

**Control system locks in to an attitude 3 milli-arcsec ****1******** ****from where it actually is and holds there – ready to go immediately**
**Immediately overcomes any initial rates**

## Downlink Telemetry Requirements

**For three day synchronous orbit,  a six watt solid state Ka Band transmitter with a 50 cm diameter steered antenna on the spacecraft can dump data at a 300 Mhz rate to an 11 meter Ka Band dish at the Berkeley Ground Station**
**SNAP full data could be dumped in one 4.2 hour pass every 3 days**
**The Observatory is visible over Berkeley for 5.5 hours every 3 days**
**For an L2 orbit,  a Ka Band transmitter driving a 35 watt Traveling Wave Tube Amplifier and a 1.2 meter diameter fixed dish on the spacecraft can dump data at a 200 Mhz rate to a 34 meter Ka Band capable dish in the Deep Space Network.  **
**SNAP full data could be dumped with a 1.8 hour pass each day**

## Ka Band Transmitter Status

**While TDRSS and other spacecraft routinely transmit Ka band telemetry,  there is currently no off-the-shelf Ka band modulator which can support a modulation rate of 200 or 300 mHz**
**We have had discussions with Space Systems Loral  (SS/L) concerning the development of a 300 MHz capable Ka Band transmitter**
**They believe that a system which meets our requirements could be developed from existing space qualified mixer and multiplier modules**
**SS/L is currently in the process of responding to a request for a cost estimate to design and breadboard a suitable transmitter**

## Propulsion Requirements

| Entries are required kilograms of hydrazine assuming a monoprop system with ISP of 220 | 10Kx153K  w/ 4240-12 | 10Kx153K w/
4040-12 and S/C propulsion | L2 with
4040-12 and S/C propulsion |
| --- | --- | --- | --- |
| Launch Dispersion Correction | 5 Kg | -- | -- |
| Orbit Insertion | 0 | 190 Kg | 54 Kg |
| Angular Momentum Unload | 4 Kg X 10 Years * | 4 Kg X 10 Years * | 4 Kg X 10 Years * |
| Station Keeping | 5 Kg X 10 Years * | 5 Kg X 10 Years * | 5 Kg X 10 Years * |
| Spacecraft Disposal | 200 Kg | 200 Kg | 0 |
| Total | 295 Kg | 480 Kg | 144 Kg |

- *  Note that JDEM specifies a 6 year base mission

## Integration and Test Flow

**Integration and Test of the system is dominated by the Instrument,  not the spacecraft**
**Have to verify proper imaging of the telescope**
**We plan a full aperture end-to-end demonstration of PSF on the focal plane**
**Have to verify that the telescope imaging capability does not degrade during later system level environmental and functional testing**
**To satisfy the last requirement above we have incorporated a unique built in test function  (BITE)  into the telescope which allows us to verify at any point in the I&T flow that the instrument optics have not degraded**
**This has never before been done on any space or ground telescope**
**This has allowed us to develop an I&T flow which satisfies all Instrument requirements as well as performing a through test of the system at the Observatory level**
**The test flow is shown in drawing 00015-ME02-C**

## Telescope I&T

## Spacecraft I&T and Instrument I&T

## Observatory I&T and Launch Ops

## Summary

**The SNAP collaboration has assembled a group engineers and aerospace professionals who are exceptionally well qualified to transform the needs of the Science into a set of well defined requirements on the spacecraft**
**The collaboration includes a strong group of professional aerospace engineers with a long history of participation in NASA programs **
**This process is abetted by a culture of close interaction between scientists and engineers in the groups developing SNAP**
**Flow-down of the SNAP mission requirements to the spacecraft has been performed and spacecraft requirements are well understood**
**The most challenging requirement, on the ACS, can be met through careful design and analysis and use of the Guider data provided by the science instrument**
**Other spacecraft subsystem requirements are straightforward and within existing capabilities broadly distributed through the aerospace vendor community**
**The spacecraft can meet the mission requirements using available flight-proven or flight-qualified components and technologies**

## Summary   (continued)

**Development of the custom structure to meet the SNAP requirements is straightforward and can be done with standard technology**
**The structure supplies sufficient space for all needed avionics**
**The SNAP spacecraft performance requirements can be met using a low-risk approach**
**Unique Instrument Built in Test Equipment (BITE) and straightforward instrument-to-spacecraft interfaces simplify observatory I&T**
**Use of available flight-proven and qualified components and technologies lowers risk**
**Thorough systems engineering design and analysis to perform trades will lead to a robust system design**
**SNAP spacecraft requirements are similar to those of the recently launched NASA SIRTF mission and the Kepler Discovery mission scheduled for launch in 2007**
**Adequate margins are available in all technical areas**