This paper describes the design, testing, and lessons learned during the development of the Advanced Topographic Laser Altimeter System (ATLAS) Beam Steering Mechanism (BSM). The BSM is a 2 degree-of-freedom tip-tilt mechanism for the purpose of pointing a flat mirror to tightly control the co-alignment of the transmitted laser and the receiver telescope of the ATLAS instrument. High resolution needs of the mission resulted in sub-arcsecond pointing and knowledge requirements, which have been met. Development of methodology to verify performance was a significant effortadvancement. The BSM will fly as part of the Ice, Cloud, and Elevation Satellite 2 Mission (ICESat 2), which is scheduled to be launched in 2017. The ICESat 2 primary mission is to map the earths surface topography for the determination of seasonal changes of ice sheet thickness as well as vegetation canopy thickness

Armani, Nerses V.

Blumenstock, Kenneth A.

Bonafede, Joseph

Cramer, Alexander K.

Gostin, Alan B.

Hakun, Claef F.

Haney, Paul G.

Hinkle, Matthew R.

Jackson, Molly I.

Lee, Kenneth Y.

Lugo, Carlos F.

Matuszeski, Adam J.

Morell, Armando

Steigner, Peter J.

Stromsdorfer, Juan J.

NASA Technical Reports Server

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Beam Steering Mechanism (BSM)
Lessons Learned
Kenneth A. Blumenstock, Alexander K. Cramer, Alan B. Gostin, Claef F. Hakun, Paul G. Haney, 
Matthew R. Hinkle, Kenneth Y. Lee, Carlos F. Lugo, Adam J. Matuszeski, Armando Morell, 
Nerses V. Armani, Joseph Bonafede, Molly I. Jackson, Peter J. Steigner, & Juan J. Strömsdörfer
NASA Goddard Space Flight Center, Greenbelt, MD
Stinger Ghaffarian Technologies Inc., Greenbelt, MD
Proceedings of the 43rd Aerospace Mechanisms Symposium
NASA Ames Research Center
May 4-6, 2016 
https://ntrs.nasa.gov/search.jsp?R=20160005838 2019-08-31T03:24:25+00:00Z
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Agenda
• ICESat-2 ATLAS Overview
• BSM Overview
• Three Lessons Learned
• Conclusion
2
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
ICESat-2 ATLAS
3
Primary Mission Goals - Collect altimetry 
data of the Earth's surface optimized to 
measure ice sheet elevation change and sea 
ice thickness, while also generating an 
estimate of global vegetation biomass.
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
±2 mRad On-Orbit Range
5 µRad/sec rate
AMCS - Alignment Objective
Surface <80% reflectance
ATLAS RTA FOV
(500 km) 
ATLAS 
Instrument
RTA FOV
BSM
Transmitted 
Beam
Return 
Image of 
Ground Spot 
Pattern
•Maintain return “spots” on and within RTA FOV
•Maintain alignment over mission as defined in TAR10
5 mR
6.61 mR
+ X
5 mR
6 Receiver FOV 83.6Ø 
Fibers
6 Ground Return Spots
Nom. 20 µR Ø, r (1/e2)
4 TAMS Spots
5 mR+ X
AMCS - Alignment Monitoring & Control System
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
• Necessary to maintain alignment between the 
transmitter and receiver
• MOLA, GLAS IR channel, MLA, and LOLA depended on 
structural stability
• GLAS green channel had ground-commandable steering 
mirror in receiver
• ATLAS has the smallest transmitted beam, smallest 
receiver FOV, and smallest alignment margin of GSFC 
space-borne laser altimeters
• With such small margin, only an active alignment system 
can guarantee the required signal capture
LOLA MLA
GLAS green
GLAS IR
MOLA
Transmit/Receive Alignment
800 µR 500µR
500 µR 100160
90
400 µR 80
400 µR 100
20 
ATLAS
83 µR
5
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
ATLAS Instrument
6
Beam Steering 
Mechanism (BSM)
Earth
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Assembly components:
Mirror           
Wave 
Washer
Compression 
Plate
Moving Plate
Mirror Mount
Mount
Flexure & 
Counterweight
Fixed Plate
Cap
BSM Views 1
Flexure
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Views 2
8
Flexure Unit
Counterweight
Actuator
Field
Sensor
Counterweight 
Shaft
Actuator 
Bobbin
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Voice Coil Actuators
9
High performance flight actuators designed and built in-house
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Mechanism Control Electronics MCE
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Driving Requirements
• Range of motion
– Flexure configuration
• Position stability
– High bandwidth
– Structural modes
• Position knowledge
– Accuracy
– Sensor  selection
• Very high mirror reflectivity at laser wavelength
– Mitigates heating and distortion of mirror
– Size, weight, coating, figure
• Environmental 
– Vibe:  Avoid need for a launch lock if possible
– Thermal Stability
11
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Noteworthy Design Points
• Mirror
– Dielectric coating resulted in heavy glass mirror
– Allowed high reflectivity and tight figure 
• Flexure
– Significant engineering effort
– Optimized to trade structural modes
• Actuators
– Custom design for high performance
– High damping for pointing performance
– No launch lock needed due to high damping
• Sensors
– Blue Line DIT sensors selected; aluminum targets
– Optimization of performance parameters
• Thermal Stability
– Material selection
12
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Lesson 1 – Pointing Verification
• Very challenging major focus.
• No single metrology solution.
– Zygo 2-axis interferometer
– Leviton Inter-target Differential Electronic Autocollimator 
(IDEA)
– Leica T3000 Theodolite
• Needed high resolution, high accuracy, and absolute 
position verification over the BSM mirror’s full range 
of motion.
13
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Pointing Verification Troubles
• The Zygo wavelength was not compatible with the 
dielectric coating rendering one axis useless due to 
severe non-linearity and requiring characterization of 
the non-linearity of the axis used so that it could be 
removed by means of post processing.
• Synchronization of internal sensor data and Zygo
sensor data was troublesome.
• The Zygo angular range was much less than the BSM 
range of motion.
14
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Pointing Verification Setups
15
Zygo Interferometer Setup 
on Optical Bench
IDEA Setup Outside 
TVac Chamber
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Optic and Optical References
16
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Pointing Verification Strategy
• Optical measurements during TVac were taken using 
IDEA, whose wider FOV could tolerate gross motion of 
the BSM mount due to temperature changes in the 
vacuum chamber. 
• IDEA measurements were used to make temperature-
dependent adjustments to the scale factor, determined 
before TVac. 
• Performance over the full range was quantified using 
interferometer data which had been post-processed to 
remove non-linearities. 
• Processing a combination of data from these three 
instruments verified BSM pointing performance over 
its full required range of motion and temperature. 
17
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Lesson 2 – Troubling Structural Mode
• Measurement of the frequency response revealed a very 
undesirable structural mode near 900 Hz. 
• Using FEM modal analysis, we were unable to determine the 
source of this very troubling behavior. 
• Performance was met with filtering, but consuming controller 
filtering capability for possible future structural mode 
changes.
• A brute force approach was utilized by replacing components.
• It was theorized that the counterweight shaft was lacking 
stiffness, so a ribbed shaft was designed and fabricated.  
• The stiffer shaft solved the problem.
18
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Before/After Balance Shaft Replacement
19
Before After
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
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Flight Center
Lesson 3 – Puzzling Flight Model Behavior
• Discovered in the Flight Model upon completion of 
performance testing just before environmental testing.
• Mirror motions from Point A to Point B resulted in passing the 
target and circling back at greater magnitudes than previously 
observed.
• Controls engineers believed there to be a balance problem
• Balancing was achieved by using a balance fixture to change 
the gravity vector by 180 degrees; then the counterweight 
was adjusted such that minimal motion occurs upon 180 
degree gravity vector changes .
20
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM FM in the 6-Axis Balancing Fixture
21
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Investigation
• Investigation revealed when balancing in the vertical 
(gravity) direction that our technician observed there 
was significant motion in the horizontal (non-gravity) 
direction.
• Our technician was instructed to ignore the 
horizontal axis motions when performing balancing.
• It was found that the horizontal motions were very 
significant in the Flight Model and were minimal in 
the Engineering Model.
22
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
What Happened?
• It so happened that the flexure axes were rotated 45 
degrees from the sensor axes.
• If the two flexure axes do not have their rotation axes 
located in the same place, the counterweight can balance 
one or the other flexure axis, or find a compromise.
• In this case, one flexure axis was balanced mirror light 
and the other flexure axis equally balanced mirror heavy, 
resulting in horizontal motion. 
• The problem was in the manufacturing tolerance of the 
flexure being out of spec, resulting in a 9 mil (0.009 inch) 
separation of rotation axes.
23
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Imbalance Phenomena Graphic
24
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
In Hindsight
• Had the flexure axes been aligned with the sensor axes, we 
would have realized balancing one axis results in the other 
being unbalanced.
• Had we paid more attention to our technician, we would have 
discovered the problem earlier.
• We suffered a painful schedule slip to await flexure 
remanufacture, integrate into the Flight Model hardware, and 
repeat performance testing.
• It turned out that we had previously used our best flexure for 
vibe/life testing, so we needed to remanufacture as good or 
better for the Flight Model, which was achieved.
25
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
Conclusion
• The team was very motivated to be tasked with meeting 
requirements not faced before.
• We never before developed hardware to meet sub-
arcsecond levels of pointing.
• Pointing requirements were met with margin.
• Verification of systematic error throughout mission life, 
full range of motion, and full range of temperature, was 
extremely challenging.
• As position knowledge and pointing requirements for 
similar systems in the future become more demanding, 
verifying these requirements will require not only the 
synthesis of measurements from multiple references, but 
also deep understanding of the limitations of each 
reference chosen.
26
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Pointing Stability
27
670 675 680
670
672
674
676
678
680
682
Science Step Response
AZ-axis (urad)
E
L
-a
x
is
 (
u
ra
d
)
0 5 10 15 20
660
670
680
690
sec
u
ra
d
AZ-axis
0 5 10 15 20
660
670
680
690
sec
u
ra
d
EL-axis
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Pointing Stability
• We have achieved 0.2 mRad rms pointing stability, an order of magnitude 
better than our requirement
• If using the BSM, you reflected a laser from NASA Ames Research Center 
to San Francisco International Airport, which is 40 km away, the center of 
the spot would stay on a dime.
28
40 km
+/- 0.22 mRad
Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
Goddard Space
Flight Center
BSM Integrated onto the ATLAS Bench
29


Beam Steering Mechanism (BSM) Lessons Learned

Beam Steering Mechanism (BSM) Lessons Learned

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