AIAA-2003-1651
ADVANCED PRECIPITATION RADAR ANTENNA
SINGLY CURVED PARABOLIC ANTENNA REFLECTOR DEVLEOPMENT
John K. Lin*†, George H. Sapna III†, Stephen E. Scarborough*†
Bernardo C. Lopez*‡
Abstract
NASA and JPL have recognized space inflatable and
thin film technologies as the enabling innovations for
the advancement of the next generation of Advanced
Precipitation Radar Antenna (APRA).1,2 However, the
utilization of these Gossamer technologies in this type
of application (i.e. a large thin film parabolic cylindrical
reflector deployed and supported by a space inflatable
structure) has never been done.3,4 This paper is the
result of a feasibility assessment study focused on the
development of a thin film reflector and its achievable
accuracy. The purpose and goal of the study was to
conduct reflector membrane development and surface
accuracy testing to determine the feasibility of
achieving the reflector shape accuracy requirements and
to define the path forward to meet the required accuracy
in a follow-on program. In this study effort, the halfscale Precipitation Radar Antenna 2 (PRA-2) reflector
membrane test unit, the supporting test fixture, and the
fabrication mandrel were designed and fabricated. The
mandrel and membrane were tested for surface
accuracy. The research performed in this phase was
successful in that it demonstrated the feasibility of a
Singly paraboloid cylindrical reflector and identified
areas that needed further development and research.
The result of the mandrel surface accuracy testing
(RMS = 0.2022 mm) showed promise in meeting the
accuracy requirement (machining operation is required
for improvement). The results of the membrane surface
accuracy testing (RMS = 4.3014 mm before tuning and
RMS = 1.2770 mm after final tuning) showed promise,
but more work must be done in areas of material
selection, design, analysis, and fabrication to achieve
the required accuracy. Based on design evaluation,
accurate shape guide strips are needed for a thin-film
reflector membrane, but using accurate strips alone will
not meet the membrane accuracy requirement. In order
to meet the required accuracy on orbit, tuning capability
is suggested until a better passive system can be
developed. From the results of the surface accuracy test
it is evident that the major factor causing the surface
*
Member AIAA
ILC Dover, Inc., Frederica, DE
‡
Jet Propulsion Laboratory, Pasadena, CA
†
Copyright © 2003 by ILC Dover, Inc.
Published by the American Institute of Aeronautics
and Astronautics, Inc., with permission
inaccuracy are axial wrinkles. The phenomenon of
axial wrinkling parallel to the tensioned direction is the
result of a slack membrane stress in the orthogonal
direction. These wrinkles must be reduced to achieve
the required accuracy. The result of this study showed
a definite promise in using the Gossamer technologies
for this application. However, it also showed that more
work must be done in design, analysis, and fabrication
of thin film reflectors before this emerging technology
can become a viable alternative to rigid metal structure.
Introduction
The purpose of this study effort was to develop the
reflector membrane and to determine the feasibility of
meeting the accuracy requirement by the thin film
technology. Although it was a limited risk mitigation
study, which is to ensure that the research is moving
down the right technical path, the approach of the study
was to begin by investigating a system level concept
trade study. A total of thirteen concepts were generated
including several hybrids. These concepts could be
categorized into three basic types: (1) precision frame
with simple suspension attachment, (2) low precision
frame with adjustable suspension attachment, and (3)
low precision frame with high precision suspension
attachment.
The reflector membrane test unit designed for the
reflector development study was half scale in size
(Figure 1). It consisted of four major subassemblies:
(1) The Reflector Membrane Assembly, (2) The
Adjustable Suspension System, (3) The Test Support
Structure, and (4) Precision Composite Shape Guide.
The design of the reflector membrane assembly utilized
the positive attributes of the best concepts but
implemented them in a general fashion so as to remain
applicable for all the primary concepts at this stage of
development. The reflector membrane assembly is a
thin film parabolic cylindrical reflector defined by y =
x^2/(4*p) with a focal length (p) of 0.9275 meter. Two
precision composite edge reinforcement shape guides
govern the shape of the reflector. The membrane
material, due to its availability, was selected to be 1.0
mil Kapton® HN with a 300 angstrom (b) vapor
deposited aluminum (VDA) coating. The reflector was
designed to be tensioned in one direction with the
adjustable suspension system.
1
American Institute of Aeronautics and Astronautics
This paper details the research effort with a
quantitative assessment of the reflector accuracy and
recommendation for further study and improvements.
Test Support Structure
Surface Accuracy
Reflector surface accuracy was the primary focus of
the current risk assessment study. The requirement
stated that the reflector surface shall not deviate from
the theoretical cylindrical paraboloid by more than 0.17
mm RMS over the center 1/3 of the reflector area (as
shown by the orange oval area in Figure 3), and 0.25
mm RMS in the remaining area.
Reflector Membrane Assembly
1.0 mil Kapton with VDA
Adjustable Suspension
System
y
Precision Composite
Shape Guide
Figure 1. Advanced Precipitation Radar Antenna
Reflector Membrane Test Unit
Requirements
The APRA reflector requirement document was
generated by Jet Propulsion Laboratory (JPL). From
this document, the requirements and goals of the
current phase of study were established cooperatively
between ILC Dover, Inc. and JPL. The requirements
and goals listed below are the relevant sections from the
JPL requirements document.
Reflector Membrane Requirements
The material for the reflector membrane substrate is
Kapton®. The required minimum thickness of the
substrate is 1.0 mil. The coating requirement is
aluminum or copper (non-corrosive) with 1.0
micrometer (10,000 b) of minimum thickness. For the
current study effort, due to material availability for a
timely result, the coating thickness requirement was
relaxed to 300 b thick.
Reflector Shape Specifications
The shape of the reflector surface is mathematically
defined as a parabolic cylinder with the following
parameters (Figure 2): (1) the profile of the parabolic
cylinder is defined by y = x^2/(4*p). (2) The focal
length (p) equals to 0.9275 meter. (3) The reflector
width (x) equals to 2.65 meters. Finally, (4) the length
(z) of the parabolic cylinder equals to 2.65 meters.
z
x
Figure 2. Coordinate System for Mechanical Design
A/3
scan
Height
or
depth
x
y
z
length
width
Figure 3. Area of 0.17 mm RMS Surface
Accuracy
2
American Institute of Aeronautics and Astronautics
Concept Development and Trade Study
The objective of this study effort was to develop the
reflector membrane and to determine the feasibility of
utilizing the thin film technology in achieving the
accuracy requirement. During the concept development
phase, a total of thirteen concepts were generated
including hybrid concepts. These concepts could be
categorized into three basic types: (1) precision frame
with simple suspension attachment, (2) low precision
frame with adjustable suspension attachment, and (3)
low precision frame with high precision suspension
attachment.
After the system concepts were generated, a list of
trade categories were developed and defined with
assigned weights for selecting the most promising
design. The list of trade categories was generated based
on the requirement document. A team of engineers
conducted the system level trade study. The individual
results were compared and analyzed through several
technical discussions. Finally, the updated results were
compiled and the top concepts identified.
Trade Category Definition
The APRA engineering team selected a list of
important design discriminators (i.e. trade categories) to
aid in the selection process of various concepts for
prototype fabrication. In this study, the definition of
trade categories governed the analysis. Based on the
relative importance of each category, a weighting factor
is given, with heavier weight given to the more
important categories. The trade categories are as
follows:
Adjustability on Orbit – It is desirable for the design
to have the ability of adjustment on orbit for “fine
tuning” the antenna to receive the best possible signal.
Ground adjustability is also desirable with respect to
taking measurements before the antenna is launched.
Mass Impact – The impact of higher system mass is
a potential higher launch cost. Therefore, mass is an
important design consideration for later optimization.
Manufacturability – In a manufacturing environment
it is important to minimize the complexity and increase
the feasibility of the design. The design should
minimize assembly complexity in the fabrication
process of the antenna. This category forces the
engineers to evaluate the concepts in comparison with
the state-of-the-art fabrication process.
Structural Stiffness – The frame or structure that
supports the antenna should be stable and stiff enough
to meet the frequency requirement.
During the
preliminary concept selection, this evaluation is
performed qualitatively.
Accuracy of Structure – This category deals with the
ability of the frame or structure to hold the antenna
precise enough to maintain the necessary antenna
(reflector) accuracy.
For designs that do not
incorporate a high precision structure, it is important for
the assembly as a whole to be capable of maintaining
the necessary antenna accuracy. This category is
evaluated qualitatively during the concept down select
process.
Packing Efficiency – This category deals with the
packing volume of the membrane assembly and
components that interface to the framing structure.
Concepts that require less packing volume are more
desirable because they require a smaller launch vehicle
and potentially lower launch costs.
Packing Management - This category defines the
simplicity of the packing method. Concepts that
require a small number of deployment devices or less
packing time are more desirable. Also, concepts that
will not damage or distort the antenna are rated higher.
Controlled Deployment – Concepts are evaluated
based on their ability to deploy the system in a smooth
and reliable fashion while minimizing dynamic loads
induced into the spacecraft system.
Multiple Deployment Capability – This category
deals with the ability of the generated designs to
withstand numerous packing and deployment cycles
without damage to the membrane or structural
components. This is a qualitative evaluation during the
concept down select stage.
Ground Testing – It is desirable for the design to be
capable of ground testing for shape accuracy and
deployment capabilities prior to launch.
Cost Consideration – This includes the costs
associated with fabricating, testing, launching, and
operating the antenna assembly.
Scalability – The design should be capable of
scaling up or down to meet requirements for other
applications.
Membrane Interchangeability – A membrane
reflector design that is insensitive or independent to the
structural and attachment design is more desirable.
3
American Institute of Aeronautics and Astronautics
Field of View (37°) – No members of the framing or
interfacing parts should obstruct the field of view of the
membrane assembly.
Thermal Stability – This category deals with the
ability of the design to maintain accuracy under all
thermal conditions that will be experienced on orbit.
System Level Concept Development
A total of thirteen concepts were developed. Out of
the thirteen concepts one was selected for further
development. The top three concepts are presented
here.
The most promising concept is called “Hybrid Chain
Link with Inflatable Rigidizable Boom and Rib”
(Figure 4). The reflector membrane of this concept is
deployed via inflation gas and supported by an
inflatable in-situ rigidizable structure. The reflector
membrane is stretched in one direction by the inflatable
rigidizable ribs. Its shape is governed by the precision
“chain” link and can be fine-tuned by the “chain”
linkage mechanism. The major advantages of this
concept are its capability of multiple packing and
deployment on the ground for testing before launch and
capability in providing higher structural stiffness once
rigidized in space. The potential disadvantages are its
possible high fabrication cost and its relatively complex
structure and mechanism design.
Inflatable
Rigidizable
Rib
Mandrel
Precision
“ Chain”
Linkage
membrane is deployed and supported by two strained
energy precision formed U-shaped booms.
The
reflector membrane is stretched in one direction over
the precision boom surface to achieve the reflector
accuracy. The major advantages of this concept are the
elimination of inflation and rigidization systems and if
properly manufactured, no adjustment of membrane is
required. The major disadvantage is in its costly
fabrication fixtures for the deployable precision boom.
Compressive Membrane
Support that tensions the
Reflector Membrane
A
A
Feed
Spacecraft
Bus
Compressive Membrane Support
Reflector Membrane
U-Shaped
Precision Boom
SECTION A-A
Figure 5. Precision Formed U-Shaped Boom with
Compressive Membrane Supports
The third concept (Figure 6) uses a mechanical
“chain” system to deploy and support the reflector
assembly. In this concept, the reflector membrane is
Mandrel
Inflatable
Rigidizable
Boom
“ Chain”
Link with
Hinge
Reflector
Membrane
Feed
Spacecraft
Bus
Feed
Figure 4. Hybrid "Chain" Link with Inflatable
Rigidizable Boom & Rib
The second concept of choice is called “Precision
Formed U-shaped Boom with Compressive Membrane
Supports” (Figure 5). In this concept the reflector
Spacecraft
Bus
Figure 6. Concept J - Mechanical "Chain"
4
American Institute of Aeronautics and Astronautics
stretched in one direction. Its shape is governed by the
mechanical precision bonding surface on the chain
links. The major advantages for this design are its
ability to withstand multiple packing and deployment
on the ground for testing before launch, and its ability
of fine-tuning. The greatest concern is the high cost of
fabrication.
Reflector Membrane Design and Fabrication
The reflector membrane test unit designed for the
reflector development study was half scale in size. It
consisted of three major subassemblies: (1) The
Reflector Membrane Assembly, (2) The Adjustable
Suspension System, and (3) The Test Support Structure.
The reflector membrane assembly is a parabolic
cylindrical reflector with the focal length of 0.9275
meter. It is fabricated from 1.0 mil Kapton® NH film
with 300 b VDA coating. The shape of the reflector is
governed by a set of precision composite edge
reinforcement shape guides. The reflector is designed
to be tensioned in one direction with the adjustable
suspension system.
modified storage rack with precision cord guides
installed on top and bottom of the fixture. Hole
locations for the precision cord guides are transfer
drilled from computer generated templates.
For the reflector membrane assembly, the Kapton®
film is precision wheel cut from computer generated
electronic files. Then the films are seamed together
using a standard butt-and-taped configuration. After
the films are seamed together, the reflector membrane
is tensioned by hand and attached to the edge
reinforcement guide strips. The edge reinforcement
guide strips are fabricated from a graphite composite
laminate. The graphite composite is cured on a
precision mandrel (Figure 7) under vacuum to provide
proper consolidation pressure. The finished reflector
membrane assembly is then attached to the cords of the
suspension system and mounted onto the supporting
fixture (Figure 8).
The adjustable suspension system is designed to
provide tension in the circumferential direction of the
parabolic cylinder and to provide fine adjustment for
better reflector accuracy. The adjustable suspension
system consists of mounting brackets, swivels, cord
attachment hardware, extension springs, adjustment
screw assembly, and Vectran® cord.
The test support structure is designed from a
Figure 7. Edge Reinforcement Precision Shape
Guide Mandrel
Composite Shape
Guide or
Reinforcement
Strip
Adjustment Screw
Extension Spring
Figure 8. Reflector Membrane Test Unit Assembly
5
American Institute of Aeronautics and Astronautics
Reflector Membrane Accuracy Testing and Results
Mission Research Corporation, utilizing the latest
MetricVision® Coherent Laser Radar, conducted the
surface accuracy test (Figure 9). The test equipment is
a portable coordinate-measuring machine that uses a
broadband frequency modulated infrared laser, capable
of measuring three-dimensional features with accuracy
of ± P ± 0.001 in) at 10 meters and ± 2.5 ppm
above 10 meters. The measurements were taken at less
than 10 meters distance. The equipment consists of a
mobile workstation that allows direct software control
of the measuring laser. This feature enables fast
measurement, adjustment of the reflector membrane,
and re-measurement of the same point to quantify the
effect of the adjustment.
article. The relative locations of the tuning balls were
mapped and stored for post testing evaluation and for
coordinating data of multiple maps. Next, a boundary
around the reflector was established to allow automated
data collection within the defined boundary. This
automatic feature enabled fast data collection and fast
feedback on reflector adjustment. After the system was
tuned and the boundary defined, a quick preliminary
mapping run was conducted to see where and how the
Figure 9. Surface Accuracy Testing in Progress
One of the major concerns of measuring surface
accuracy of a large inflatable on Earth is the effect of
gravity. To minimize the influence of gravity, surface
accuracy measurements were taken with the tensioned
direction (i.e. the z-direction) as the vertical direction.
The surface of the fabrication mandrel was scanned
for accuracy to determine the steps necessary to meet
the accuracy requirement in the future. Data from the
scanning (Figure 10) showed that the fabrication
mandrel has an accuracy of 0.2020 mm RMS, and that
the current low cost fabrication mandrel did not meet
the requirement of 0.17 mm RMS. However, with
additional CNC machining operation under temperature
controlled environment the required accuracy can be
met in the future.
In performing the reflector surface accuracy test, the
first step was the establishment of reference coordinates
in space. This was accomplished by strategically
locating four tuning balls around the test fixture, then
properly locating the infrared laser in relation to the test
Figure 10. Parabolic Composite Strip
Fabrication Mandrel Surface Scan Result,
Front View and Top View
reflector should be adjusted.
The initial run is
presented in figures 11 and 12. The initial run
produced a result of 4.3014 mm RMS and a standard
deviation of 2.8477 mm with an average value of
3.2241 mm. From the top view of the scan (Figure 12),
a global deviation similar to a sine wave is evident.
This repeating pattern showed how the reflector could
be moved by adjusting the high and low points in or
out. After several intermediate runs and adjustments,
the final run was scanned (Figures 13 and 14).
Significant improvement can be observed when
comparing the initial run with the final run. The final
run produced a result of 1.2770 mm RMS and a
standard deviation of 1.2705 mm with an average of
0.1287 mm. This final run, however, did not meet the
required surface accuracy. Due to time and funding
constraints no further adjustment was performed.
6
American Institute of Aeronautics and Astronautics
Figure 11. Initial Scan Result of the Reflector Membrane Prior to Tuning, Front View
The result of the final measurement indicated that
further adjustment along the reinforced edges would no
longer improve the accuracy of the reflector surface
about 20 cm away from the edges. Notice (Figure 13
and 14) that the directions of error are not the same
along the same vertical lines; that is, the top and the
bottom may have negative errors but the center may
have errors in the positive direction. Again, this nonrepeating wrinkle pattern showed that further tuning
may not be productive in terms of getting better
accuracy. After final tuning, two other measurements
were also made. The first measurement is the center
one third of the reflector membrane, and the other is a
detail scan of a small (i.e. 20.0 cm by 10.0 cm) but
highly wrinkled area (results are shown in figures 15 to
17).
From visible inspection by eye, the center one third
appeared to have the highest inaccuracy due to
wrinkles, and this is confirmed by the result of the scan.
The result showed an RMS of 1.4645 mm, but with an
average of 0.0535 mm. This result indicates that the
overall profile as an average is fairly accurate, but due
to deep wrinkles, the RMS value is very poor. The
final scan conducted in this effort was a detail mapping
of a 20.0 cm by 10.0 cm area with deep wrinkles at an
interval of 1.0 mm by 1.0 mm spacing (Figure 17). The
result showed that one of the worst areas of the reflector
has an accuracy of 1.7649 mm RMS and a standard
deviation of 1.2278 mm.
Figure 12. Initial Scan Result of the Reflector
Membrane Prior to Tuning, Top View
7
American Institute of Aeronautics and Astronautics
Figure 13. Final Scan Result of the Reflector Membrane after Final Tuning, Front View
Based on the physical appearance of the wrinkle
patterns, the cause of deeper wrinkles on the center one
third of the reflector seems to be fabrication tolerance
driven. The fan shaped wrinkle pattern along the
composite reinforcement is an indication of loss of
membrane tension while the edge of the reflector
membrane was being taped down to the reinforcement
guide. The deep axial wrinkles on the center one third
of the membrane and smaller wrinkles on the rest of the
membrane are the result of loss of membrane stress in
the direction 90 degrees from the wrinkle lines. The
phenomenon of wrinkling (the shower curtain effect) is
well known from experience and results from the fact
that membranes have little or no compressive
resistance. However, this phenomenon is still difficult
to predict from analysis due to its highly nonlinear
behavior. The loss of membrane stress, from the
fabrication perspective, could be due to a less than
perfect seaming technique that caused fullness of
material on the center of the membrane. Therefore,
when the orthogonal direction is tensioned the direction
that is full has the tendency to bunch together and cause
wrinkles.
Figure 14. Final Scan Result of the Reflector
Membrane after Final Tuning, Top View
8
American Institute of Aeronautics and Astronautics
Figure 15. Surface Scan Result of the Center One Third after Final Tuning, Front View
Conclusions and Recommendations
In this current study effort, the half-scale PRA-2
reflector membrane test unit, the supporting test fixture,
and the fabrication mandrel were designed, fabricated,
and tested for surface accuracy. The research performed
in this phase was successful in that it demonstrated the
feasibility of a paraboloid cylindrical reflector and
identified areas that needed further development and
research. The result of the mandrel surface accuracy
testing (RMS = 0.2022 mm) showed promise in
meeting the accuracy requirement (machining operation
is required for improvement). The results of the surface
accuracy testing (RMS = 4.3014 mm before tuning and
RMS = 1.2770 mm after final tuning) showed
improvement, but more work must be done in areas of
material selection, design, analysis, and fabrication to
achieve the required accuracy. Based on design
evaluation, accurate guide strips are needed for thin
film reflector membranes, but using accurate strips
alone will not meet the membrane accuracy
requirement. In order to meet the required accuracy on
orbit, tuning capability is strongly suggested until a
better passive system can be developed. From the
results of the surface accuracy test it is evident that the
major factor causing the surface inaccuracy is axial
wrinkles. These wrinkles must be eliminated to achieve
the required accuracy. The phenomenon of axial
wrinkling parallel to the tensioned direction is the result
of loss of membrane stress in the orthogonal direction.
Figure 16. Surface Scan Result of the Center One
Third after Final Tuning, Top View
9
American Institute of Aeronautics and Astronautics
Results:
Average = -1.2680 mm
Max. Value = 1.870 mm
Min. Value = -4.846 mm
STD. DEV. = 1.2278 mm
RMS = 1.7649 mm
Scale = -4.000 to 4.000 mm
Figure 17. Detail Scan of A Deep Wrinkle, 20 cm by 10 cm Area
At the current technology level, without major
tooling and process improvements, the lowest RMS
surface accuracy achievable is probably in the order of
0.5 mm RMS.
As identified above, one of the major factors causing
surface inaccuracy is the formation of axial wrinkles.
To improve the accuracy of the reflector membrane in a
follow-on phase, it is recommended that the following
challenges be investigated:
• Identify sensitivities of areas/items causing
wrinkles.
• Improve the stability (deployed stiffness) of the
guide strip.
• Add guide strips to determine if this will reduce
wrinkles and improve surface accuracy.
• Vary reflector membrane thickness (particularly
greater than 1.0 mil) to see if it will reduce the
depth of wrinkle.
• Improve seaming technique to eliminate fullness in
the non-tensioned direction.
• Perform sub-scale prototype development of
mechanical chain concept to identify the challenges
of the chosen concept.
• Verify improvements after recommendations are
implemented.
• Evaluate feasibility of deploying a membrane and
its effect on retaining required accuracy.
Acknowledgement
The authors thank NASA and JPL for funding this
project in advancing enabling technologies.
The
authors especially thank Michael Lou and Eastwood Im
for their support in the Advanced Precipitation Radar
Antenna reflector development program.
References
[1] Eastwood Im, and Ziad S. Haddad, “ Global
Precipitation Measurement: JPL Planned Contribution
to GPM,” Jet Propulsion Laboratory, 2001
[2] E. Im, S.L. Durden, G. Sadowy, and L. Li, “ DualFrequency Airborne Precipitation Radar (PR-2)
Observations in CAMEX-4,”
Jet Propulsion
Laboratory, 2001
[3] John K. Lin and David P. Cadogan, “ An Inflatable
Microstrip Reflectarray Concept for Ka-Band
Applications,” AIAA-2000-1831, 41st AIAA/ASME/
ASCE/AHS/ASC Structures, Structural Dynamics, and
Materials Conference and Exhibit, Atlanta, Georgia,
April 3-6, 2000.
[4] C. E. Willey, R. C. Schulze, R. S. Bokulic, W. E.
Skullney, J. K. Lin, D. P. Cadogan, and C. F. Knoll, “ A
Hybrid Inflatable Dish Antenna System for Spacecraft,”
AIAA-2001-1258, 42nd AIAA/ASME/ASCE/AHS/ASC
Structures, Structural Dynamics, and Materials
Conference and Exhibit, AIAA Gossamer Spacecraft
Forum, Seattle, WA, April 16-19, 2001.
10
American Institute of Aeronautics and Astronautics