# Presentation: 532322

## Super Star Tracker

**ISAL Study**
**28 January - 8 February 2002**

## Customer’s Goals of this Study

- Get a good handle on mass/power/cost/size for input into an IMDC study for SI or MAXIM Pathfinder, where 30 microarcsec line-of-sight knowledge is needed.
- At least better than the WAG we have made in the past IMDC runs
- Define “other” requirements- jitter, thermal,..
- Understand the scaling laws to make this eventually work for a more complex mission needing 30 nanoarcsec line-of-sight knowledge
- Identify required technologies:
- 1) to make possible
- 2) to make cheaper
- See what studies are currently under way (eg look through NASA Technology Inventory,...)  Where does this fit in?
- Eg. This should also be a part of the VISNAV system
- Bring some awareness of these types of issues for our more challenging imaging missions to GSFC people.
- Better define the problem-  the usual “scientist does not quite know how to describe the needs to engineer” difficulty....

## Requirements

## Alignment

- 10 deg FOV
- State of the art pointing
- Arc sec Star Tracker (Science Instrument requirement)

- Lateral Knowledge
- Lateral control
- Along axis

- (Done on optics s/c)

- 10 deg FOV
- State of the art pointing
- Arc sec Star Tracker (Science Instrument requirement)

- Laser
- (redundant)
- (issue laser life!)

- 5 arc sec FOV for telescope
- (PSF ~ 1.3 arc sec)

- Laser divergence
- 1 arc min
- 30 micro arc sec beacon

- Lateral Knowledge _+_ 7.5 micrometer
- Lateral Control _+_ 10 cm

- Knowledge _+_ 5 cm
- Control _+_ 5 m

- All fine control by detector s/c

- Beacon telescope #2

- telescope #1

- Derived Tilt specs
- Knowledge 30 micro arc sec
- Control > 30 micro arc sec
- (derived from beacon & gyro/accelerometer)
- Roll 1 arc sec

- Super Star Tracker

- Star Tracker
- Arc sec to inertial reference
- 10 deg looking at beacon

- 10E20 photons/sec
- Laser

## Trade Studies

**1)  Similar to Hubble Space Telescope:  **
- Super Star Tracker
- Centroids on beacon
- FOV big enough to catch stars
**2)  Similar to Gravity Probe-B (GP-B):**
- Telescope only centroids on beacon
- Gyro provides inertial reference frame
**3) Similar to GP-B but use of accelerometers:**
- University of MD like accelerometer (GEOID ESSP proposal using superconducting gravity gradiometer)
- Use fancy accelerometers with beacon
- May be linear or angular
**4)**  **Kilometric Optical Gyro**
- Uses Sagnac Effect with light- precision ~/(Area/perimeter)
- 	For   = 630 nm, 4 km perimeter should be adequate
- Proposed for StarLight, but cost and technical issues
**5) Space interferometry Mission - previously ruled out**
**6) Use of distance between S/C for very long focal length –ruled out**

## Option 1:  Hubble-Like

**Super Star Tracker**
**Centroids on beacon **
**FOV big enough to catch stars**
**Advantages**
- Simple concept
- Aberration correction automatic
- Roll
- Possible use of superconducting accelerometer
- Chopper wheel to get small pixels fight 1/f noise
- Analog solution?
**Disadvantages**
- FOV vs. resolution
- 15 magnitude stars => 1/4(arc min)2
- Detector pixel size, capacity
- F-number = huge
- Integration time = huge

## Option 2: Gravity Probe-B-Like

**Telescope only centroids on beacon **
**Gyro provides inertial reference frame**
**Advantages**
- Gyros exist!! 1/3 micro arc sec/day
- No need to find stars
- Just a beacon tracker telescope
- Cryo-cooler:  TRL 5 by 2005  ($2-5M for cryo-cooler (flight model only), FM + EM $3 to $7 million, mass about 20kg
- Launches on Con-X ~ 2010
- Gyro was to launch this year (Oct 2002)
- $10-100M for both cooler & gyro
**Disadvantages**
- Must know aberration Delta-V to 3cm/sec (Landis checks)
- GP-B = expensive
- Cryogen or coolers/vibrations

## Option 2: Gravity Probe-B-Like (cont.)

**Deltas on GP-B**
- Since 1/3 micro arc sec /day is not required, then we may be able to back off on this capability
- Cryo-coolers mean normal conductivity launching
- If negligible magnetic field @ L2, simplifies magnetic shielding design
- Requirements on magnets in s/c
- Neutralize cosmic-ray charging
- Respinning up gyro? (dynamic range)
- Cryo getters less important?
- Proof mass? Yes-maybe
- Squids will be better
- Venting vs. cooler mechanism
- Mass, power,size,cost,other req’ts (mag,jitter,thermal)
- Need to integrate over 10 sec you get 10E-13radians????
- ConX, NGST Cooler specs:  150W BOL, 250W EOL, 10 yr lifetime, 20-30kg include electronics, heat syncs 1@100K, 1@room temp, produces 7.5mWatts 6K
- Selection in March 2002
- Cryo cooler mating to Adiabatic Demagnetization Refrigerator (ADR) starts in 2005. Temperature 2 K or less. Estimate 30 watts.

## Option 2: Gravity Probe-B-Like (cont. 2)

**FYI (Estimates from M. DiPirro):**
- GP-B cost, size, weight TBD
- GP-B Reproduction without dewar and spacecraft
- 	Cost = $20 to $100 million
- 	Size: 0.5 meters diameter by 2 meters length
- 	Mass: about 100 kg
- LHe Dewar
- 	TBD values for comparison
- 	$25 to $50 Million
- 	size 2 meters diameter by 2 meters tall
- 	mass about 600 kg
- 	not recommended

## Option 3: Super Accelerometers

**University of MD like accelerometer (GEOID ESSP proposal using superconducting gravity gradiometer)**
- Laboratory model measured 10E-15 m/s2 acceleration
**Use Super Accelerometers with beacon and beacon tracker**
- Unable to project, but 1st look is the angular accelerometers will not work.  Perhaps looking at angular displacements.
**Advantages**
- No need to find stars
- No spin up
- Less sensitive to magnetics than GP-B option
- Squids will be better than GP-B
**Disadvantages**
- Cryogen ?
- No flight design for accelerometers
- Delta-V to 3 cm/sec (aberration correction)
- charging

## Option 3a: Super Accelerometers

**University of MD like accelerometer (GEOID ESSP proposal using superconducting gravity gradiometer)**
**Linear gravity gradiometer.**
**Measure ********2****R (centripetal)**
** ********2****)**** ****=10E-14 rad****2****/sec****2****/sqrt(Hz)**
- Noise spectral density of 2
-   = 2 /2 for >>
-   = sqrt(2 )/sqrt2 for ~
- We would operate with ~

## Option 4: KOG & Beacon

**Kilometric Optic Gyro    **
- Resolution ~ lambda/(area/perimeter)
**Advantages**
- Beacon star tracker only
- No need to find stars
**Disadvantages**
- Extra s/c ?
- Understand “cost/technical” issues for StarLight de-selection
- Range control ?

## Possible Techniques for Tracking 

**Koesters Prism Interferometer**
**Interferometer**
- Move detector in know pattern and AC detecting input signal.  Then move s/c
- Fixed detector - look at fringes (DC).  Then move s/c
**Centroiding**
- Several (up to 5) defocus strips to reduce aberration and clarify point spread functions
- One centroid
**Quad cells**
- Optically split beam
- PSF on 4 detectors
**Position Sensitive Diodes (for beacon) @ 633 nm (some of 3 and 4)**
**Spatial heterodyne interferometric tracker (moiré pattern).**

## First Cut at Hardware Approach for Option 2

- 1. Use state-of-the-art star trackers for “coarse” pointing
- 	- currently one arc second
- 	- proposed Air Force milli arc second ST
- 2. Use beacon on optics spacecraft and beacon detector on detector spacecraft to
- maintain alignment or alignment knowledge of the two spacecraft to within 30
- micro arc seconds. Translate optics or detector spacecraft (decision based on
- control design) based on beacon detector data .
- 3. Use Gravity Probe-B type gyros on detector spacecraft to maintain knowledge
- of inertial reference. If necessary, pitch or yaw spacecraft based on gyro data.
- 4. Once the frequency of the disturbances are known for the beacon detector,
- a decision could be made on whether or not to include state-of-the-art linear
- accelerometers (recommended by Eric Stoneking) in each spacecraft.  These
- accelerometers would respond to any high frequency disturbances beyond the
- capability of the beacon detector.

## Coarse Acquisition Procedure

- Use state-of-the-art star trackers (currently 8x8 degree FOV, accuracy of one arc second) to establish rough inertial reference of the optics and detector spacecraft.
- Detector s/c star tracker sees the beacon; translate detector s/c to bring beacon into field of view of beacon tracker (15 arc seconds). This puts the beacon to within one arc second of the nominal position.
- 3. 	Find science target on science instrument by translating detector spacecraft in search pattern.
- 4. 	Use beacon tracker to control pitch and yaw of detector s/c so beacon is within 30 micro arc seconds of its most sensitive position. Reset gyros to zero at this point.
- 5. 	Go into control logic sequence for science operations.

## Science Mode Control Logic Sequence –  Version Using Science Instrument 

- Gyro Move?

- Beacon Move?

- Translate
- Detector
- Spacecraft

- X rays near
- Edge of
- Detector?

- X rays near
- Edge of
- Detector?

- Beacon Move?

- X rays near
- Edge of
- Detector?

- Translate
- and Pitch/
- Yaw
- Spacecraft

- Translate
- and Pitch/
- Yaw
- Spacecraft

- Go to Start

- Go to Start

- Go to Start

- yes

- Go to Start

- yes

- yes

- yes

- no

- no

- yes

- Go to Start

- no

- no

- Go to Start

- no

- Go to Start

- yes

- Notes:
- 1.  Beacon on optics spacecraft
- 2.  Two gyros on detector spacecraft (for pitch and yaw)
- Drift about 1/3 micro arc sec per day.  Several outputs
- from gyros, maybe to arc sec values with precision and
- accuracy of 30 micro arc seconds.
- 3. Beacon detector on detector spacecraft, 15 arc sec
- diameter field of view
- 4. State of the art star tracker on each spacecraft (8 x 8
- degree field of view.
- 5. Field of view for science instrument x ray detector
- ( about 1 arc sec by 1 arc sec).

- no

- Start

- Ranging done by time of flight laser
-  on detector spacecraft used in separate range control logic sequence.

## Beacon Move?

- Gyro Move?

- Pitch and Yaw
- Detector
- Spacecraft

- Delta Theta
- Beacon – Delta
- Theta Gyro
- = 0 ?

- Delta
- Theta B
- Working FOV
- Of Beacon?

- Gyro Move?

- yes

- no

- yes

- yes

- yes

- no

- Go to Start

- no

- Go to Start

- no

- Go to Start

- no

- Start

- Ranging done by time of flight laser
-  on detector spacecraft used in separate range control logic sequence.

- Theta B
- Working FOV
- Of Beacon?

- Translate
- detector
- Spacecraft

- no

- yes

- Go to Start

- Translate
- Detector
- Spacecraft

- Delta
- Theta Beacon
- Working FOV
- Of Beacon?

- Yes, means
- pure roll

- Go to Start

- yes

- Pitch and
- Yaw detector
- Spacecraft

- Go to Start

- no

- Go to Start

- Translate
- Detector
- Spacecraft

- Go to Start

- Theta B
- Working FOV
- Of Beacon?

- yes

- Go to Start

- no

## Science Mode Control Logic Sequence –  Version Using Independent Tight Control

- Gyro Move?

- Pitch/
- Yaw detector
- Spacecraft

- no

- yes

- Go to Start

- Start

**Science Mode Control Logic Sequence –  Version Using Independent Tight Control**

- Go to Start

- Beacon Move?

- Translate
- detector
- Spacecraft

- no

- yes

- Go to Start

- Go to Start

- Start

- Ranging done by time of flight laser
-  on detector spacecraft used in separate range control logic sequence.

**Note: ** Independent loops are coupled by dynamics

## Super Star Tracker Beacon System

## SIGNAL-TO-NOISE RATIO CALCULATION

- For the Laser Beacon we first calculate the diameter of the Laser Beam at the tracker, which is just the beam divergence in radians times the distance.  We then calculate the area of the beam at the tracker.  The ratio of this area and the telescope area is multiplied by the Laser Power to yield the received power.
- This received power is then propagated thru the system by multiplying by the next three factors, the telescope, the other optics, and the two beam splitters. The fourth factor to be multiplied is the quantum efficiency of the detector QE.  The power is just converted to photons per second by the next two factors, Lam is the HeNe wavelength of 632.8 nm.
- With a millisecond integration time, the received photon number S is computed; and the signal to noise ratio SNR is computed as the square root of S in this photon noise limited case.
- The diffraction limited performance in object space, DL, is given by 2.44 /( D*SNR), where D is the telescope diameter. For an interferometer limited by intensity noise,  the phase error is given by the inverse of the product of the SNR and the square root of the number of data samples per fringe, SQRT(NS), ( Ref: Optical Shop Testing, Malacara). The optical path  error (tilt), OPE, is given by (/2), but it is also the angle error, IP, times D.
- The final result for  IP is /(2* D*SQRT(NS)*SNR).

**SIGNAL-TO-NOISE RATIO CALCULATION**

## SNR CALCULATION

## Thermal

- Meeting a pointing requirement of 30 micro arc sec requires extreme structural stability between the attitude sensor and the instrument. Mounting the attitude sensor and the instrument to the same platform would help accomplish this; in any case, the interfaces must be extremely rigid to prevent drift. In addition, temperatures must be tightly controlled to maintain dimensional stability, even with structural materials having a very low coefficient of thermal expansion (CTE).
- Allowable temperature difference was calculated as follows:
- 1. Assume a perfectly rigid structure.
- 2. Assume the best structural material currently available (M55J composite, with a CTE = 10-7 per oC)
- Since (sin 30 micro arc sec) is 1.5 x 10-10, this represents the distortion limit.
- The temperature change producing this error is
- (1.5 x 10-10) / (10-7) = 1.5mK
- Developing a lower CTE structural material should be a high research priority!
- Otherwise, temperature control must be to about 0.1mK, which has been done only in labs on very small sample volumes. Control to 1mK has been achieved on space instruments, but again in small volumes. This is another area requiring a technology upgrade.

## Issues and Concerns

- Laser reliability for long mission
- Thermal stability of beacon detector telescope
- Uniformity of laser intensity across diameter of beam
- Maturity of cryo cooler
- Maturity and adaptability of Gravity Probe-B gyros
- Capability of PMTs
- 	(count rate is a factor of 2 above current state-of-the-art)

## Open Issues

- 1)  Hubble-like star tracker with superconducting accelerometer/aperture size trade.  Inertial reference stars with beacon.
- 2)  Gravity Probe-B like
- A)  Unavailability of CDR package (wt, power, cost)
- B)  Cryo-cooler option
- 1)  Absorption - no moving parts
- 2)  Isolated mechanical compressor
- 3)  Rotary 5000 Hz
- (two of the three will be ready 2005)
- C)  Modifications
- 1)  S/C is non-rotating
- 2)  Cold on ground
- 3)  Better squids
- 4)  Eliminate cryogen cooling
- 5)  Spin up gas (Helium is dangerous to PMTs)
- 6)  Magnetic shielding requirements @ L2
- 7)  Lifetime (10 yrs)

## Open Issues (cont.)

- D)  Strict dc mag field requirements for s/c components;
- 		no field lines thru GP-B gyro
- E)  GP-B telescope modifications
- 1)  Multiple tertiary mirrors for redundant detectors
- 2)  Alternate design using interferometer (with prism)
- 3)  PMT photon counting rate (requires improved technology)
- 3)  Linear or angular superconducting accelerometer (requires improved technology)
- 1)  0.1 Hz = 1/f knee; so can integrate f @ 10 seconds
- 2)  7x10E-15 rad2/sec2=2=linear acceleration=integration if white noise = ??
- 3)  Angular accelerometer=10E-12 rad/sec2==? , Value x time2?
- 4)  Any additional input from U of MD

## Back Up Charts

## Detectors

- Note:  Mass is 4 kg for 2 detectors