Fig 1 Components of the AstroMesh Reflector

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1 Application of the AstroMesh Reflector to Astrophysics Missions (Zooming in on Black Holes) Authors Geoff Marks - Chief Engineer at Astro Aerospace, a Strategic Business Unit of Northrop Grumman Aerospace Systems Dr Charles Lillie Senior Systems Engineer at Northrop Grumman Aerospace Systems, Redondo Beach, CA Steve Kuehn Systems Engineer at Astro Aerospace, a Strategic Business Unit of Northrop Grumman Aerospace Systems Abstract: It has long been a goal in the Radio Astronomy community to launch a Space to Ground Very Long Baseline Interferometry (VLBI) mission using large aperture antennas operating efficiently at frequencies up to 86 GHz. This is driven by the need for high resolution imaging of radio sources with baselines of 60 to 80 thousand kilometers. To date this goal has been impeded by the technical challenge of manufacturing reflectors with apertures greater than 20 meters with the surface figure accuracy required for this purpose. This paper explains some of the background work which supports the fact that such missions are now possible using the AstroMesh reflector technology developed at Astro Aerospace. but the tension in the network of webs is much higher so the mesh has no influence on the shape of the reflector. The accuracy of the reflector surface is solely determined by the dimensions of the webs and the facets are essentially flat with negligible pillowing error. The tensioned webs cause the rim structure to be compressed and eliminate all free-play so it is possible to create an accurate structure using free running bearings in the truss hinges. This provides for a trouble free deployment. 1.0 ASTROMESH DEVELOPMENT 1.1 Flight History The AstroMesh Reflector concept was developed in the early 1990s, initially in response to a Space Radar application but requirements rapidly changed, driven by the needs of the evolving Mobile Communication Systems. From the earliest phases of the development the Reflector was therefore designed to meet the most difficult requirements from that market High Stiffness and Thermal stability Low Areal Density Electrical requirements: such as low Passive Intermodulation Products coupled with adequate grounding for electrostatic discharge The high stiffness and stability are derived from the reflector s deep structural shape. As shown in Figure 1 the structure consists of a set of frames around the perimeter and two networks of webs across the aperture which are tensioned back to back by a set of springs. The reflective mesh is shaped by the front set of webs into a series of triangular facets which approximate the shape of the desired parabola. The mesh is in tension Fig 1 Components of the AstroMesh Reflector The reflector development has been very successful. The first m flight unit launched in 2000 on the Thuraya 1 spacecraft and there have been 6 subsequent launches of various sizes. All flight deployments were perfect with no anomalies, making this design the only 100% successful large aperture unfurlable reflector for space applications. These reflectors are assembled and flown without any surface adjustment. They are all capable of operation at C-Band but are being used at L- Band and at S-Band frequencies. All spacecraft report excellent performance to date. The flight history is shown in Figure 2

2 Fig 2 AstroMesh Reflector Launch History Two additional Reflectors are being prepared for launch. Fig 3 Planned Launches The first is an 11-m for the INMARSAT Alphasat program and the second is a 6-m reflector for a unique science application on JPL s SMAP spacecraft where the reflector is spinning above the spacecraft at 14.7 RPM as part of the Radar Scatterometer and Radiometer payload. This is the first application of the smaller frame size suitable for apertures between 4-m and 8-m. This smaller truss is the focus of current IRAD developments at Astro where it is being used to develop the reflector capabilities at higher frequencies. 1.2 Development for High Frequency Application As discussed above, the surface accuracy of the reflectors built to date has been better than required for the mission. This was without any penalty because the accuracy is determined by a number of features which have other driving requirements.

3 The accuracy of the web structure is determined by the material properties (Coefficient of Thermal Expansion (CTE) and modulus) of the webs and the accuracy to which the webs are made. The web material is a Twaron/thermoplastic composite mainly chosen for durability but easily meets the accuracy requirements. The accuracy of the web surface is also determined by the facet sizes and the facet size has been chosen based upon other criteria than accuracy. The accuracy of the rim truss is determined by the material properties and manufacturing accuracy of the parts. The materials were chosen for strength and cost reasons, not for maximum accuracy. It can be seen that there is a lot of room for improvement and the current Internal Research and Development (IRAD) at Astro is establishing the true capabilities. As part of the IRAD program we have built two models. The first was a 5-meter diameter exploratory model non-deployable which was used to verify that improved materials and construction of the reflector surface could achieve the required surface accuracy. This model was tested at multiple frequencies and demonstrated good performance and correlation with predictions achieving directivities of 62.4 db at 31 GHz and 67.6 at 65 GHz, demonstrating a reflector efficiency of greater than 80% at its design frequency of 30 GHz. The second was a fully deployable version which has been used to verify both its mechanical capabilities and its RF capabilities. This model has also been used in two collaborative test programs with NASA. In the first test the RF performance was verified in the horizontal test range in NASA s Glenn Research Center in Cleveland. The test configuration, shown in Figure 4, suspended the reflector from three points on its rim truss and tested performance at its design point of 33 GHz but also at 49 GHz. Performance was again excellent, as shown in Figure 5, with directivities of 62.8 and 65.2 db at the two frequencies. Figure 4 5-meter Reflector in Test at NASA GRC Grating Lobes Figure 5 Far Field Elevation and Azimuth pattern at 49 GHz (Directivity = 65.2 db) These tests verified the measurements performed on the original model and confirmed our ability to design the reflector to a given frequency requirement. They also demonstrated the extreme stiffness of the basic structure since the reflector held its shape despite being supported in 1G at only three points. The remaining question was the ability to predict performance under thermal extremes on orbit. This question had two components; Can temperatures of the reflector components be accurately predicted?

4 Can the shape resulting from these temperature excursions be predicted? These questions were answered by the second collaborative program with JPL. In this test the reflector was suspended in the large solar simulator at JPL (shown in Figure 6) and illuminated at different sun intensities with a specific shade pattern on its surface (simulating on-orbit conditions when the spacecraft or its solar arrays shadow the reflector). The test chamber was equipped with a photogrammetry system which enabled measurement of the reflector surface under the test conditions. good. Figure 6 Thermal Test This test has been fully reported in [1]. Solar Thermal Vacuum Testing of Deployable Mesh Reflector for Model Correlation. The conclusions of the test were that there were lessons to be learned in the thermal modeling aspect of the test but that, when these lessons were applied, the thermoelastic predictions were very The reflective mesh used in the development program was specially developed for the process and is a knitted molybdenum/gold wire mesh of very high density. It has been tested over a large frequency range and will provide acceptable performance at the required 86 GHz where losses will be no more than 1.2 db. The results of the development programs accomplished to date have given the confidence to be able to predict the performance of AstroMesh reflectors at the frequencies required for the Space to Ground VLBI mission. In addition a further method of surface enhancement has been demonstrated in a portion of our test antennas. This technique provides shaping of each facet so that the facet count is effectively quadrupled. There is a consequent reduction in surface error. This feature has been incorporated and measured and allows an overall reduction in the number of surface control features. For example the number of tension ties in the IRAD reflector as built is This number can be cut in half as a result of this enhancement and so this feature will be used for all future applications since it is enables a reduction in complexity, cost and weight. 2.0 SCIENCE REQUIREMENTS FOR VLBI MISSIONS Radio telescopes are used to study the naturally occurring radiation from stars, galaxies, quasars and other astronomical objects at wavelengths between 1 mm (300 GHz) and 10 meters (30 MHz). This radiation comes from thermal processes such as black body radiation, free-free emission and spectral line emission as well as from non-thermal process such as synchrotron radiation, pulsars, and masers (microwave amplification by stimulated emission of radiation). Astronomical masers occur naturally in molecular clouds around proto-stars and the envelopes of old stars from molecules such as OH, SiO and H 2 O. Maser action amplifies otherwise faint emission lines at specific frequencies, as shown in Table 1. Masers rely on an external energy source, such as a nearby, hot star, to pump the molecules back into their excited state. The Doppler shift of water masers orbiting around the super-massive black hole in NGC 4258 has been used to determine the black hole s mass, and the observed size of the masers orbit provided a trigonometric distance measurement for the galaxy and a calibration of the cosmic distance scale. The frequencies bands of the radio astronomy receivers at Molecule Band (GHz) L OH C Ku H 2 0 Ka Q SiO W Table 1. Astronomical Masers the Very Large Array (VLA) in Socorro, New Mexico and at other radio observatories cover these maser emission frequencies.

5 Ground-based radio observations must contend with atmospheric absorption at wavelengths less than 1 cm, and with scintillations due to irregularities in the ionosphere at wavelengths longer that 20 cm. At wavelengths longer than about 10 meters, the ionosphere becomes opaque to incoming signals. Fi gure 7 shows the zenith atmospheric opacity due to oxygen and water vapor, and the frequency bands used for VLBI. Molecular oxygen lines make the atmosphere opaque at 60 and 120 GHz, while water vapor has an absorption feature at 22 GHz plus absorption increasing at ~0.4% per GHz toward higher frequencies. A space-based antenna is free from these atmospheric effects and can observe these astrophysically important features. In addition, the data from a radio antenna in Earth orbit can be combined with data from ground-based antennas to create images of radio sources with spatial resolutions hundreds of times greater than the largest optical telescopes. The spatial resolution ( ) of an electro-optical system is directly proportional to the wavelength (λ) of the radiation and inversely proportional to the diameter (D) of the aperture of the collector, i.e.: λ/ D (1) Thus, a radio telescope must be hundreds or thousands of times larger than an optical telescope to achieve the same spatial resolution. A radio telescope observing the 21-cm (1.42 GHz) hydrogen line would require an aperture of 420 kilometers to achieve the spatial resolution of a 1-meter optical telescope operating at 0.5 microns. Even at 86 GHz (0.35-cm) the highest radio frequency currently in use, the telescope aperture would have to be 7 km in diameter. Radio astronomers have overcome this limitation by creating large virtual apertures by combining the phase and amplitude data from multiple radio telescopes separated by a few to thousands of kilometers. In this case D is the maximum separation (baseline) between the telescopes. These radio interferometers have achieved spatial resolutions much higher than the largest optical telescopes. The Very Long Baseline Array (VLBA) operating at 86 GHz with its maximum baseline of 8,611 km can achieve 0.10 mill-arcsecond resolution for a compact radio source like the accretion disk of a black hole, more than 500 times the resolution of the Hubble Space Telescope. The baseline (and resolution) of a ground-based radio interferometer is limited by the finite diameter of the earth, so adding an antenna in earth orbit to a ground based antenna array seemed the logical path to higher spatial resolution. The ground-based antennas would provide the collecting area and multiple baselines required to fill the virtual aperture of the array and the space-based antenna would provide the longer baselines required for higher spatial resolution. The pursuit of Space VLBI missions began in the late 1970s when the technology appeared to be within reach. An earlier mission concept, QUASAT, emerged as a joint project involving US and European scientists following a NASA-ESA workshop in Gross Enzerdorf, Austria in The Soviet Union also began planning a mission intending to launch it as soon as possible, and in 1985 the Soviets formed an international study team for their RadioAstron mission. At the 1984 QUASAT workshop the Japanese also indicated they were exploring the possibility of a Japanese mission. The feasibility of Space-to-Ground Very Long Baseline Interferometry was demonstrated from 1986 to 1988 by a series of Space VLBI demonstrations using one of the two 4.9-m antennas on a Northrop Grumman Tracking and Data Relay System (TDRS) satellite as a radio antenna, while the other antenna was used as part of a closed-loop phase tracking system. The Deep Space Network (DSN) antenna at Tidbinbilla, Australia and the ISAS antenna at Usuda, Japan made up the Space to-earth interferometer. Following successful S-band tests at 2.3 GHz, a 15 GHz (Ku-Band) experiment was successfully conducted in The 2.3 GHz and 15

6 GHz experiments demonstrated that Space VLBI was technically feasible and the strength of the fringes that were observed confirmed the hypothesis of bulk relativistic motion in radio-loud quasars was correct. In 1988, despite the demonstrated feasibility of Space VLBI, NASA and ESA found that although the QUASAT studies had shown no serious technical problems were anticipated, the mission cost was beyond their budget allocations and the program was cancelled. The RadioAstron program forged ahead despite experiencing continuous launch delays due to economic constraints in Russia. The Japanese radio astronomers were actively engaged in VLBI studies from an early stage, including participation in the TDRS experiments. In 1987 they proposed a Japanese-led VLBI Space Observatory Program (VSOP, aka HALCA and Muses- B) [2]. HALCA (Figure 8) Figure 9 illustrates the unprecedented spatial resolution of Space VLBI achieved with HALCA and the VLBA. Early Chandra X-ray Observatory observations of PKS resulted in the surprising detection of an X-ray jet in the object. It shows the milli-arcsecond-scale HALCA/VLBA image of the core of this quasar in comparison with the arcsecond resolution of Chandra (pixels) and the Australia Telescope Compact Array (lower contours) images[3]. The X-ray jet in PKS , observed for the first time by Chandra, is a dramatic example of a cosmic jet that has blasted outward from the quasar into intergalactic space for a distance of at least 200,000 light years! was launched February 12, 1997 from the Kagoshima Space Center into an orbit with an apogee of 21,400 km, a perigee of 560 km, an inclination of 31 degrees, and a period of about 6.3 hours. This orbit provided good (u,v) plane coverage and high resolution for imaging of celestial radio sources with the space and ground based antennas. Observing at 1.6 GHz and 5 GHz with an 8-m antenna, HALC A produced high dynamic range images of unprecedented resolution despite the loss of its highest frequency band (22 GHz) during launch. Although designed for a 3-year mission, HALCA operated until the attitude control system failed in HALCA operations officially ended in November The international collaboration for VSOP was led by ISAS and backed by Japan's National Astronomical Observatory, NASA's Jet Propulsion Laboratory, the US National Radio Astronomy Observatory (NRAO), the Canadian Space Agency, the Australia Telescope National Facility, and the European Joint Institute for Very Long Baseline Interferometry. Figure 9. The Core of Quasar PKS The jet s presence means that electromagnetic forces are continually accelerating electrons to extremely high energies over enormous distances. Chandra observations, combined with radio observations, provide insight into this important cosmic energy conversion process. After more than 20 years of development, RadioAstron (aka Spektr-R) [4] was launched on July 18, 2011 onto a highly elliptical orbit with an apogee of 334,727km, a perigee of 1,248 km and an inclination of 51.8 degrees. RadioAstron s four receivers cover the standard astronomical wavelengths of 92-cm (0.32 GHz), 18-cm (1.66 GHz), 6-cm (4.8 GHz) and 1.3-cm (22 GHz). RadioAstron's 10-meter space radio telescope will work as part of a VLBI network with ground-based radio telescopes. The focal ratio of the telescope is 0.43 and its surface accuracy is ± 0.5 mm. Surface figure accuracy is the primary challenge for radio telescopes designed to operate at frequencies greater than 20 GHz. High aperture efficiency is required for a successful Space VLBI mission,

7 however; and the design goal should be efficiencies 55% at all frequencies to obtain the desired resolution. 3.0 CONFIGURATIONS AND PROPERTIES Two sample configurations have been evaluated, one a 15-meter aperture and a 25-meter aperture. The results are presented below. The potential performance of each version is shown in tables which consider the complete antenna system and includes errors resulting from numerous effects. This table was adapted from an earlier NASA/JPL study for an Advanced Radio Interferometry Mission between Space and Earth (ARISE) [5]. The conclusion of that study was that the surface accuracy of the reflector would be the limiting performance factor since at that time an RMS surface accuracy of no better than 1mm was envisaged. The tables are updated with the new knowledge of reflector and mesh performance. Feed effects such as spillover are not included. 3.1 Fifteen Meter Aperture The fifteen meter reflector that has been analyzed has 42 truss bays, an F/D (Focal Length/Diameter) of 1.5, and an edge offset of zero meters. The resulting stowed package dimensions will be 1.5 meters in diameter with a height of 2.5 meters. The reflector consists of three major structural components: the truss (tubes and fittings), the webs and the tension ties. The materials for the truss and the webs are chosen for stiffness and low coefficients of thermal expansion Reflector Surface The surface distortion is comprised of three components: the systematic error due to the facets, the manufacturing and measurement error, and the on orbit thermal-elastic distortion error. The surface of the Astromesh reflector is comprised of triangular facets which results in the systematic error. The facet is a plane defined by the vertices. The tension ties are located at the facet vertices and can be adjusted, post manufacturing, to reduce the construction surface error. Photogrammetry is used to measure the location of the facet vertices. The thermal-elastic distortions are determined using the finite element model shown in figure 10. This shows the number of webs required to control the surface. To assess thermal distortions an arbitrary case was selected. The reflector was placed in a position with the sun vector twenty degrees from the reflector rim plane normal. Figure 10 Reflector Finite Element Model The surface errors are determined by the rms (rootmean-square) hpl (half-path-length) of the BFP (best fit paraboloid). The three sources of surface error are calculated separately and then combined by the rootsum-square method. The systematic error is a property of the reflector design, the result of being constructed from numerous flat facets. The systematic error is 0.023mm for the fifteen meter reflector. Based on the accuracies achieved on a previous reflector, tension tie adjustment and the manufacturing and measurement error will be mm combined. The thermal elastic distortions result in a surface error of mm resulting in a total RMS error of mm which is included in the performance assessment below Performance for a 15-meter Aperture The predicted R.F. performance derived from the above assessment is shown in Table 2 15 Meter Reflector Efficiency 8GHz 22GHz 43GHz 86GHz RF Path Attenuation Reflector Reflectance Surface Local RMS Feed Displacement* Pointing error* Surface Ohmic Efficiency* Feed Network Loss* Total Efficiency * estimated efficiency Table 2 Predicted 15 meter Aperture Efficiency The net gain at 86 GHz is 78.1 db This size of Reflector achieves the 55% efficiency goal 3.2 Twenty Five Meter Aperture

8 The 25 meter reflector considered also has 42 truss bays, an F/D of 1.5 and an edge offset of zero meters. The resulting stowed package dimensions will be 1.5 meters in diameter with a height of 4.1 meters Reflector surface For the twenty-five meter reflector the systematic error is calculated as mm and the thermal elastic distortions surface error calculated as mm. The manufacturing and measurement error will be the same at mm. The total RMS surface error is therefore mm. This result is used in the performance assessment below Performance for a 25-meter Aperture The predicted performance is shown in Table 3. Note that the reflectance of the mesh is a large contributor to the loss, assuming use of a mesh specifically designed for 40GHz applications. Some improvement can be expected from development of a tighter mesh so that the goal of 55% efficiency can be achieved. 25 Meter Reflector Efficiency 8GHz 22GHz 43GHz 86GHz RF Path Attenuation Reflector Reflectance Surface Local RMS Feed Displacement* Pointing error* Surface Ohmic Efficiency* Feed Network Loss* Total Efficiency * estimated efficiency Table 3 Predicted 25 meter Aperture Efficiency The net gain at 86 GHz is 82.0 db 3. J. Lovell et al. VSOP and ACTA Observatory Observations of PKS , Astrophysical Phenomena Revealed by Space VLBI, eds. H. Hirabayashi, P.G. Edwards and D.W. Murphy (ISAS) pp , January RadioAstron User Handbook, Prepared by the RadioAstron Science Operation Group, Version 1.1, July 2, ARISE, Mission and Spacecraft, 2nd Ed, eds. Artur B. Chemeielewski, Muriel Noca, Richard D. Weitfeldt, JPL Publication , October CONCLUSION The work performed to develop the high frequency performance of the AstroMesh Reflector has provided the confidence that a reflector can be placed on orbit which can be part of a VLBA working at 86 GHz. The system can achieve the extreme resolution required to locate the accretion disc of a Black Hole. 5.0 REFERENCES 1. Stegman, M.D., Fedyk, M. & Kuehn, S. Solar Thermal Vacuum Testing of Deployable Mesh Reflector for Model Correlation. Aerospace Conference, 2010 IEEE 2. H. Hirabayashi, H., VSOP Current Status, Astrophysical Phenomena Revealed by Space VLBI, eds. H. Hirabayashi, P.G. Edwards, and D.W. Murphy, Proceedings of the VSOP Symposium, pp. 3 8, January 2000