PERFORMANCE OF THE ASTROMESH® DEPLOYABLE MESH
REFLECTOR AT KA-BAND FREQUENCIES AND ABOVE
Authors Geoffrey Marks, Edward Keay, Steven Kuehn,
Michael Fedyk, Peter Laraway
Astro Aerospace – Northrop Grumman Aerospace Systems
6384 Via Real, Carpinteria, CA, USA
phone +1 805 684 6641
fax +1 805 684 3372
Lead Author: geoff.marks@ngc.com
Abstract
Over the past few years Astro Aerospace, a business unit of Northrop Grumman Aerospace Systems,
has been developing the capabilities of the AstroMesh® deployable reflector to perform at Ka-Band
frequencies and above. This paper presents the results of this research and development effort which
included RF performance tests on a 5-meter reflector at two independent test sites plus evaluation of
thermal distortion effects performed in the large solar simulator at the Jet Propulsion Laboratories.
The results show that this technology is real and ready to enable the next generation of TBPS KaBand satellites.
Introduction
The AstroMesh® family of deployable mesh reflectors has evolved over more than 15 years of
continuous development to become the most advanced and reliable (100% success) unfurlable
reflector technology available. A new configuration of the AstroMesh reflector has been developed for
high frequency Ku & Ka-Band large aperture applications based on this successful flight heritage.
This advancement in the state of the art of deployable reflectors is real, tested and ready to fly.
The AstroMesh® track record of success can be attributed to its uniquely efficient reflector structure
that is: exceptionally light, with an incredibly accurate & repeatable deployed surface shape,
extremely high deployed stiffness and a robust deployment mechanization that leads to an inherently
more reliable reflector on-orbit. The AstroMesh® design has been the topic of previous publication
and is not the subject of this paper, but it is helpful to understand the AstroMesh® design evolution
and flight heritage to appreciate its ideal suitability applicability to higher frequency large aperture
applications.
The AstroMesh® reflector was developed with high frequency applications in mind. Original design
goals during the conceptual development phase included: scalability up to 100-Meters, accuracy &
reflectivity up to 40 GHz, with low mass. Shown in figure 1 the basis of this development was a
perimeter truss concept with back-to-back geodesic structures, called nets, formed from composite
fiber ribbon, supported at the truss edges and tensioned by ties. This drum-like structure is extremely
stiff and light weight. The reflective surface is formed by a flight heritage gold-plated molybdenum
mesh stretched behind the geodesic structure “front net.” The facets formed by the net fix the shape
of the reflective surface with great precision.
Early AstroMesh® work focused on applications up to 2GHz, primarily L and S-Bands. The first flight
opportunity was for Thuraya Telecommunication Company aboard the Boeing GEM bus as part of
their L-Band mobile telephone service. The Thuraya reflectors are 12.25-Meter diameter. Three have
been successfully launched between 2000 and 2008.
Evolving the design, Astro upgraded the AstroMesh® into a slimmer package for stowage on the side
of a standard spacecraft bus. Referred to as AstroMesh-1 (AM-1), it achieved a 40% reduction in
stowed diameter with the same stowed height (resulting in approximately 60% reduction in volume). A
12-Meter AM-1 configuration was used on the MBSat S-Band mobile direct broadcast satellite
launched aboard a Space Systems/Loral LS-1300 bus and successfully deployed in March 2004. All
three INMARSAT 4 satellites utilize 9-Meter AM-1 reflectors aboard Astrium’s Eurostar 3000 bus; all
have successfully deployed on-orbit. INMARSAT 4 is an L-Band Broadband Global Area Network
application, with the last launch in August 2008 completing the constellation.
Astro continues to evolve and improve the AstroMesh® reflector family with the addition of a
configuration optimized for the 3 to 6-Meter aperture range. This innovative variant, called
AstroMesh-Lite (AM-Lite) was developed for use on smaller spacecraft or high frequency application.
AM-Lite development hardware currently in test is the primary subject of this paper. Besides verifying
the reflector structure and deployment kinematic performance of AM-Lite the development hardware
has been utilized to demonstrate the excellent RF performance at Ka-Band and higher frequencies.
Two development AM-Lite reflectors are discussed. The first is a fixed truss (non-deployable) 5-Meter
circular aperture offset reflector with Focal length/diameter (F/d)=1.0 and a 30 Openings Per Inch
(OPI) mesh surface. This reflector is being used to characterize AM-Lite RF performance. The
second reflector is a fully functional qualification unit. This reflector is a 5.2-Meter circular aperture
offset reflector with F/d=0.67 and a 40 OPI mesh surface. The test program and results are
discussed further in the body of this paper.
Initial Development of High Accuracy Reflector Surface
Development of the high accuracy surface started in 2003. The first goal that was set was to achieve
a deployable mesh surface capable of surface accuracies of λ/50 at Ku-Band. Since the deployable
structure was of high heritage being derived from the flight-proven AstroMesh® reflector program, it
was decided a fixed (non-deployable) structure (5-meter aperture) would serve as the perimeter truss
and the development focus would be on the mesh support structure and the performance of mesh
itself. The mesh support structure has two components as shown in Figure 1. This illustrates the basic
structure of an AstroMesh®.
Fig 1 AstroMesh Components
The mesh support structure consists of the front and rear nets and the tension ties. The front net is
composed of a number of webs (composite tapes) that stretch between the nodes of the perimeter
truss. The intersections of the webs are pinned together at precise positions so that the resulting
structure, when correctly tensioned by the spring tension ties forms the required shape. The mesh is
stretched behind the front net into a series of triangular facets and so the “required” shape is that
shape which best fits the array of facets to the desired parabolic shape per the optical prescription for
the antenna. The flat facets then become a source of error relative to the desired shape. It is evident
that the following determines the accuracy of the surface.
• The size of the facets (smaller facets can provide a better approximation to the desired
surface)
• The accuracy of position of the facets (determined by manufacturing accuracy and structural
stability – thermal, material creep, dynamic loads etc.)
The basic AstroMesh design addresses some of the problems of stability by having a perimeter
structure with near net-zero thermal expansion properties and high stiffness derived from its basic
structure. The effort in the development program focused on
• Web materials – a new low CTE graphite composite web was developed
• Mesh material – a denser mesh knit was developed to perform at Ku-Band
• Smaller facets which were built in to the development model
• Manufacturing Accuracy - This really addressed the need for adjustment of the as-built shape.
Previous AstroMesh designs for L and S-Band applications could be built to the required
accuracy without any adjustment. Analysis of the higher frequency structures suggested a
need for adjustment of the tensions in the ties. This was implemented in the model and
proved simple and effective. A few iterations between photogrammetry measurements and
tension adjustments (determined by Finite Element Model analysis) arrived at the correct
surface accuracy
The test results from the Ku-Band testing are not discussed here but were so encouraging that it was
decided to change the nets and mesh again to increase the surface accuracy. This was to address
the, perceived, developing demand for large aperture Ka-Band reflectors.
The reflector was rebuilt on the same frame with smaller facets and another tighter mesh knit. It was
then installed in our Near-Field Test Range and performance tested first at Ka-Band frequencies and
then beyond that up to 60GHz. The results were very good and are shown in Figure 2 below. (Note
that the reflector is supported on only three stands without any gravity offloading, demonstrating the
stiffness/stability of the basic structure.) Antenna efficiencies of 70% were achieved at Ka-Band.
80%
70%
60%
40%
30%
20%
Efficiency
50%
Ruze Trend Line
10%
Fixed Truss Test Data
0%
25
30
35
40
45
50
55
60
65
70
Frequency GHz
Fig 2 RF Test of Fixed Truss Reflector
This same reflector has been used in conjunction with a Rotman Lens fed feed (ref 1) which utilizes
the reflector more efficiently and has shown antenna efficiencies of 75% at 44 GHz. These tests are
the subject of a future paper.
Another important result of this series of tests is that it demonstrated the correlation between the RF
performance contours predicted by the GRASP modelling software using the measured facetted
shape (using photogrammetry) with those resulting from the test. This is illustrated in the charts
below.
Predicted
Measured
Fig 3 Comparison of Predicted v Measured RF Contours
Development of the AM-Lite Deployable Mesh Reflector
Based on the succcess of the fixed truss development it was decided to implement the high frequency
surface as a component of the development of our new lightweight smaller (3 to 8 meters) aperture
reflector called AM-Lite.
The goal has been to implement a Ka-Band Surface in the modified deployable truss, then
demonstrate the RF performance and stowability to the desired package and then to take the reflector
through a qualification program.
During the build phase of this reflector discussions with our colleagues at NASA’s Glenn Research
Center (GRC) and the Jet Propulsion Laboratories (JPL) generated enough interest that both
establishments generously offered Test Facilities and support.
We were able to utilize the Horizontal Near field range at GRC to give an independent assessment of
the RF performance and executed a complex Solar Simulation Test at JPL to validate our thermal
and finite element models which predict on-orbit performance.
RF Test at NASA’s Glenn Research Center
The AMLite reflector was shipped to GRC in Cleveland where it was deployed and then suspended in
the horizontal range as seen in Figure 3. It was supported at its base by its primary attachment point
and then suspended at two points by offloading lines. The surface accuracy was measured at 0.18
mm RMS. Tests were performed at 30-38 GHz and at 49-50 GHz using a specially built dual band
feed. The repeatability of the RF performance was checked by restowing and redeploying the reflector
and repeating the tests. The same performance was achieved.
Far Field Elevation and Azimuth pattern at 33 GHz
(Directivity = 62.8 dB, > 60% efficiency)
Fig 4 RF Test at NASA GRC
The tests were performed with a dual band feed supplied by GRC and demonstrated the expected RF
performance with antenna efficiencies of > 60% at 30 GHz and > 50% at 50 GHz.
Solar Simulation Test at JPL
Introduction
A solar simulation test (in a thermal vacuum environment) was conducted on a fully deployed AM-Lite
reflector in the 25 foot Space Simulator at Jet Propulsion Laboratories (JPL) in Pasadena, CA. The
test was conducted between 8 September, 2008 and 10 September, 2008. The test was performed
as a joint effort between Astro Aerospace and the Advanced Deployable Structures Group and
Laboratory at JPL (under the IPP program).
Description of Test Set-up
A fully deployed 5 meter AM-Lite reflector was suspended horizontally in the solar simulation chamber
and subjected to varying solar loads while under vacuum and with chamber walls at LN2
temperatures (approx. -180ºC). An Multi Layer Insulation (MLI) blanket was suspended above the
reflector (between the solar beam source and the reflector) in order to shadow half the reflector
(shadow line being along the major axis of the reflector). Just below the MLI blanket was a circular
truss that was attached at the perimeter walls of the thermal chamber on which the photogrammetry
camera traversed. The camera travelled to pre-set locations on the truss (approximately every 30
degrees) and at each location, a snap-shot of the reflector was taken. A diagram showing the test
set-up, test article and chamber configuration is provided in the figure below.
Thermocouples were mounted throughout the reflector for purposes of monitoring and recording
temperatures during test. Thermocouples were attached to the truss members (aluminium nodes and
composite tubes) as well as on selected sections of web on both the front and rear nets. The surface
of each thermocouple bead and copper tab was prepared in such a manner so as to mimic the optical
properties of the surface being instrumented. The thermocouples were positioned such that
approximately half of the total thermocouples were located on the sunlit portion of the reflector while
the other half of thermocouples were located on the shaded portion of the reflector.
25’ Space Simulator
Photogrammetry
camera
MLI Sun Shade
Circular truss
AstroMesh Reflector
Solar Footprint
Fig 5 Test Configuration in JPL Solar Simulator
Photogrammetry targets were also placed at all the tie points on the front net as well as on all the
aluminium nodes on the front rim. These reflective targets were illuminated by the camera flash
whenever a picture was taken of the reflector (by the photogrammetry camera).
Transmissivity measurements were taken both prior to and after completion of test to measure the
amount of transmittance through various locations on the net. These measurements were taken at
the center of the net, as well as at the perimeter of the net near the truss structure.
Various test cases were run in which the solar intensity varied from full sun, half sun, 1/4 sun and no
sun (full sun being equivalent to an intensity of 1420 W/m2). The collimation half angle of the beam in
the chamber is (was) 1º. In all test cases, reflector temperatures were allowed to stabilize (i.e. steady
state test condition) before photogrammetry images were taken.
Thermal Vacuum Solar Simulation Test
The thermal vacuum test took approximately 36 hours to complete once pump down was initiated.
Temperature data was recorded throughout the test (with the exception of a small period of
approximately 2 hours when the data acquisition system had experienced an anomaly at which time
data collection ceased).
Photogrammetry measurements were taken at four different test
configurations/cases. The sequence of the test configurations/cases were as follows: no sun
condition, 1/2 sun condition, full sun condition and 1/4 sun condition.
A temperature profile (throughout the test) of a few reflector components is provided in the attached
graph. Note the different levels of equilibrium temperatures for the three components (web, longeron
and aluminium node) that are located in the illuminated portion of the reflector. This is due to the
physical shape of the component coupled with the surface optical properties for the component. The
webs are flat ribbons and bare graphite composite; the longerons are long, round rods and bare
graphite composite; the aluminium nodes are complex shapes with a clear sulphuric acid anodize.
Note that no adjustments were necessary to the thermal model in the no sun condition. The reasons
being that there was no solar flux – the reflector achieved a cold soak, steady state condition in which
component temperatures were driven by the cold chamber walls (IR). In addition, the adjustments to
the thermal model that are shown in the table for a full sun condition apply to all remaining sun
conditions.
Fig 6 Temperature Profiles of Sun-Lit Reflector Components During Test
Thermal Model Correlation
A thermal geometric model (TGM) and a thermal math model (TMM) of the deployed AM-Lite reflector
in the test chamber was generated prior to test and used to derive test temperature predictions.
Certain parameters of the test (such as blockage and transmissivity) were not considered in the pretest predictions simply because these parameters were to be measured/established once the test was
completed. Therefore, in certain instances, pre-test predictions did not match up with actual test
results. However, once the data was obtained and temperature behavior examined, a correlation
process ensued in which various parameters in the TMM were adjusted in order for model predictions
to fall in line with actual test results.
The correlation process involved
1. reduction of solar flux due to transmission loss through the reflector mesh material
2. adjustments of optical properties on various surfaces (based on historical data) to more
accurately depict the surface finish
3. accountability for blockage of truss members on adjacent truss members
4. consideration of the subtending half angle on surfaces that would be blocked in the thermal
analysis software (ITAS considers solar flux rays to be infinitely parallel) but in reality, would
have increasing flux impingement the further that surface was from a shadowing surface.
Two tables comparing pre-test, actual test and post test correlated temperatures is provided below.
Along with the temperatures is a summary of changes made specifically for each reflector component.
The maximum temperature represents the component that is located in the illuminated section of the
reflector; the minimum temperature represents the component that is located in the shadowed section
of the reflector. These tables summarize results for the two extreme test cases: no sun and full sun.
Table 1. Temperature results – no sun condition
Table 2. Temperature results – full sun condition
Conclusion/Lessons learned/Application to Flight Analysis
In conclusion, the solar simulation test of the AMLite reflector in a vacuum environment was a great
success. The key conclusions that were taken from this test were:
1. A flight-like AMLite reflector system (with all of its components) was subjected to worst case
on-orbit temperature extremes with no degradation in performance or functionality.
2. A correlated thermal model was developed which represents the thermal behaviour of an
AstroMesh reflector that, in turn, will be used for all future reflector thermal analyses.
Overall, good agreement was achieved between correlated model temperature predictions and actual
test results. Adjustments made to the thermal model were anticipated following observances (during
and) after completion of the test.
Adjustments made to the test TMM can be applied to future on-orbit thermal analyses for not only
AMLite reflectors, but for all AstroMesh reflectors.
Future Plans
The AM-Lite reflector used in the above test program will next be used to qualify the reflector package
for future flight missions. (Of course it is recognized that there will be a great variety of specific
applications but the size of the model (5.2 meters aperture) is representative of the expected range
and the principles of stowed restraints during launch will remain approximately the same.) The
vibration and thermal environments have been determined from worst cases of previous AstroMesh
missions. Planned tests include:
• Release and deployment at cold extremes
• Random and Sine vibration in a simulated flight cradle.
These tests will be completed in early 2010 and it is expected that this qualification will enable a
Protoflight approach to be taken for any flight missions.
Summary of Development Program to Date
A derivative of the AstroMesh reflector family for medium apertures (3-8 meters) has been developed
which offers great potential for future applications at Ka-Band and above. It offers the following
features.
• Lightweight
− reflector weights will be in the range of 15 Kg (Boom and Spacecraft Attachments depend
upon application and may double this weight)
• High Stiffness
− deployed stiffnesses of > 2 Hz are possible
• High Efficiency – antenna efficiencies of 70% and more can be achieved at Ka-Band
− Repeatability demonstrated over multiple deployments
• RF performance of the facetted surface can be accurately predicted by modelling software
• Can fly what you test
− The reflector performance is easily verified in a gravity environment - no artificial
offloading required.
AstroMesh Unfurlable Reflectors with Ka-Band capability have been proven and are at a high
Technology Readiness Level (TRL) and are ready to fly.