Nanosat Science Instruments for Modular Gravitational Reference Sensor (MGRS)

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1 Microgravity White Paper Decadal Survey on Biological and Physical Sciences in Space Fundamental Physics Sciences (FPS) Applied Physical Sciences (APS) Nanosat Science Instruments for Modular Gravitational Reference Sensor (MGRS) Ke-Xun Sun 1, Turki Al-Saud 2, Mohammed Almajed 2, Haithem Altwaijry 2 Saps Buchman 1, Robert Byer 1, Dan DeBra 1, John Goebel 3 1 Stanford University 2 King Abdul Aziz City for Science and Technology 3 NASA Ames Research Center Contact: kxsun@stanford.edu, voice , fax Ginzton Lab N119, Stanford, CA Abstract A gravitational reference sensor (GRS) measures the position of a proof mass moving in space only under the influence of the gravity field. The GRS is the core scientific instrument in recent earth gravity field mapping missions such as GRACE and GOCE, and the planned gravitational wave detection missions such as LISA, BBO, and DECIGO. We have designed Modular Gravitational Reference Sensor (MGRS) [1] to achieve simpler architecture, higher performance, and lower cost, and have made many progresses in MGRS development The space tests of scientific instruments have been very complex and expensive. Frequently a designated space mission is flown as a pathfinder with high cost and years of development itself. Based on the recent development of small, micro, and nano satellites, we proposed to test MGRS robustness using several nanosats, which only weighs 5-30 kg, and measures 25 cm to 50 cm in maximum dimensions. Each nanosat carrying a key MGRS instrument will be built as a piggy back package co-launched by a larger mission. As such, MGRS can be mostly tested for a small fractional cost of a stand-alone pathfinder mission. Nanosats provide an excellent training ground for young engineers and graduate students. Currently we are developing two key MGRS scientific instruments --- the UV LED charge management system, and the grating angular sensor. We expect the effective development time will be no longer than 24 to 36 months. The total development cost of the entire series of MGRS instruments will be a very small faction of larger spacecraft approach for the comparable instrument. This white paper will describe MGRS concepts, its nanosat series, and the two science instruments under development. The continued support from microgravity fundamental science community and R&D sponsors will be needed to fully take advantage of nano satellites. 1

2 Nanosat Scientific Instruments for Modular Gravitational Reference Sensor (MGRS) 1. Introduction Microgravity is one of the most valuable properties the space can offer, and is available to space instruments regardless the size and power of the vehicles. The small, micro and nano satellites provide the most cost-effective way of using the space. A gravitational reference sensor (GRS) measures the position of a proof mass moving in space only under the influence of the gravity field. The GRS is the core scientific instrument for recent earth gravity field mapping missions such as GRACE and GOCE, and the planned gravitational wave detection missions such as LISA, BBO, and DECIGO. We have designed Modular Gravitational Reference Sensor (MGRS) [1] to achieve simpler architecture, higher performance, and lower cost, and have made many progresses in MGRS development Space flights are the preferred methods of GRS testing. However, this can be very complex and expensive. A separated large space mission may be needed as pathfinder with high cost and years of development itself. However, a divide-and-conquer approach based on small, micro, and nano satellites, can accelerate the development and test while lowers the cost. We propose to test MGRS robustness using several nanosats, which only weighs 5-30 kg, and measures 25 cm to 50 cm in maximum dimensions. Each nanosat carrying a key MGRS instrument will be built as a piggy back package co-launched by a larger mission. As such, most MGRS technologies can be tested for a small fractional cost of a single large mission. With reduced complexity, each nanosat provides an excellent training ground for young engineers and graduate students. 2. MGRS concept MGRS was originally proposed for the gravity-related precision missions beyond LISA. However, it has beneficially impacted LISA architecture. Until recently, the baseline design for LISA GRS had been direct illumination of the PM as published in 1998 [2, 3]. Beginning in 2003, we have revisited the GRS design and proposed the MGRS in 2004, a multi-layer architecture containing several suggestions: 1) the laser beam from the remote spacecraft does not directly illuminate the PM, but illuminates the GRS housing surface. 2) Only use one proof mass. 3) Multiple internal optical sensors are used to measure the center of mass. MGRS represents the next generation of technology for space gravitational wave detection and an array of precision experiments in space. Since then, the LISA and BBO baseline designs have changed substantially. The BBO has moved to a single PM. In the new LISA 2006 baseline, the laser beam from the remote spacecraft no longer directly illuminates the proof mass, but instead measures the separation between remote GRS housings. As such, the MGRS architecture has been adopted partially in LISA and BBO. Figure 1 shows a schematic overview of the MGRS architecture [1]. The laser light from the remote spacecraft is heterodyned external to the MGRS housing and does not illuminate the proof mass (PM) directly. The internal distance measurement is relayed to an external reference via the housing wall. The spacecraft follows the movement of the PM in a pure gravitational 2

3 field. The optical bench and GRS housing are mounted on the spacecraft. Two independent measurements are conducted among three targets, namely the PM, the incoming laser beam, and the position of the housing. In the MGRS, measurements are naturally made from the PM to the housing wall, and from the housing wall to the incoming laser phase front. This measurement sequence achieves the shortest possible optical path length. Multiple optical sensors are used in MGRS for precise readout of proof mass center relative to the housing. We have developed concept of two layer sensing and control. Interferometric optical sensing enables for high precision picometer level science measurement. Incoming Laser Beam Outgoing Laser Beams Two Step In-field Interferometry adjusting telescope MGRS Housing Reflective Multi Sensor External Optical Readout Interferometr Beams y Large gap Optical (>2 cm) Shadow Sensing Proof Mass Figure 1. Modular Gravitational Reference Sensor (MGRS) 3. MGRS Nanosat series Several key subsystems of MGRS are: 1) Optical displacement sensing for proof mass center of mass measurement, including lasers, fiber optical transport, diffractive optical cavities, and optoelectronics. 2) Optical angular sensing for proof mass orientation measurement, similarly including lasers but at different wavelengths, diffractive optical sensors, and optoelectronics. 3) UV LED charge management systems for disturbance reduction, including deep UV LED source at 255 nm, temperature control, voltage probe, and electronics drivers. 4) External laser interferometry system, including infrared laser at 1064 nm, and second harmonics generator for 532 nm wavelength, two-color optics, and photodetectors. 5) Housing and caging mechanism for proof mass shielding from external disturbances, and securing proof mass from launch vibrations. 6) Microthrusters for drag-free control. The thrusters will be selected from MEMS, or FEEP based approach. 3

4 With further considerations in technical complexity, we have accordingly planned the following nanosat series as charted in Figure 2. In addition to the component system nanosats, we planned a system tests for ATC control, and another final MGRS stem test. These may be micro satellites with larger dimensions. Figure 2. Small satellite series for MGRS. Each Nanosat (green and cyan color) will carry a key subsystem for space test. A Microsat (purple color) will carry the small scale MGRS for system test. Stanford, KACST and Ames will collaboratively on the MGRS nanosat series. A nanosat may measure 12 cm x 12 cm x 28 cm, weighing ~ 2-5 kg. A small satellite may weigh 10~20 times more. An intermediate version is perceived with maximum dimension of 50 cm. Most key components of MGRS will fit into nanosats, and a sub-scaled MGRS could fit into a microsat. We have perceived a series of seven Nanosats and one microsat for space tests of key component technologies of MGRS, and finally a microsat to space test MGRS system performance. All the nanosat and microsat modules will be designed for ride-along flight as the secondary payloads, and the launching costs of MGRS series nanosat and microsat will be minimized. Currently we are working on the scientific instruments for UV LED nanosat [4, 5] and the grating angular sensor nanosat [6, 7]. In the following we will illustrate the instrument design and engineering. 4

5 4. UV LED Instrument UV LED charge management system [4, 5] is an important part of MGRS. Short wavelength (~255 nm) UV light removes the electrical charge the proof mass generated by cosmic rays, thus minimizes the disturbance force due to electromagnetic and electrostatic interactions. Compared with mercury lamp, UV LEDs weighs less, and consumes much less power. The UV LED has passed lifetime, radiation, vibration, and thermal stress tests. UV LED is an ideal candidate for nanosat instrument test. Figure 3 shows the tentative design for the UV LED instrument. A metal-coated photoelectric structure with UV light entrance hole is located in the center. Two modules containing UV LEDs and driver electronics are mounted on the two sides of the photoelectric structure. Voltage probe modules, environmental control modules are added for complete experiments. The total power consumption is estimated 3~5 W, the total weight is estimated 3~4 kg. Photoelectric structure UV Module 1 UV Module 2 Upper Deck Top View Thermal Environment UV LED Module 1 Upper Deck UV LED Module 2 I/O Bus Voltage Probe 1 Lower Deck Thermal Controller Voltage Probe 2 9 cm Thermal Environment 18 cm Figure 3: UV LED scientific instrument for nanosat. 5

6 4. The Grating Angular Sensor Instrument The grating angular sensor [6, 7] was invented specifically for GRS proof mass orientation sensing. Assisted by grating angle magnification and diffracted beam cross section compression, the grating angular sensor has achieved very high sensitivity (0.2 rad/hz 1/2 ) using a mere 5 cm working distance. Figure 4 shows the tentative design the grating angular sensor instrument. Two identical units containing the grating, laser, and electronics components are built into a small volume. Due to the sizes of the lasers, the instrument will need slightly larger enclosure, but can be housed by a 30 cm cubic nanosat, or a hexagonal disk shape nanosat, with maximum dimension of 50 cm. Data processing electronics Grating Actuator Servo Electronics Laser and Isolator assembly #1 Laser and Isolator assembly #2 I/O interface Thermal Controller 5. Conclusion Figure 4: Grating angular sensor instrument MGRS promises higher performance and lower cost for an array space science missions. MGRS nanosat series will enable critical robustness qualification. We are in the process of demonstrating first two MGRS instruments. References [1] Ke-Xun Sun, G. Allen, S. Buchman, D. DeBra, and R. L. Byer, "Advanced gravitational reference sensor for high precision space interferometers", Class. Quantum Grav. 22 (10), S287 (2005). [2] K. Danzmann et al, LISA technology concept, status, prospects, Class. Quantum Grav. 20 (2003) S1 S9 [3] S. Kawamura et al., The Japanese Space Gravitational Wave Antenna DECIGO, Amaldi 6 Conference, Okinawa, 2005, Classical and Quantum Gravity, Class. Quantum Grav. 23 S125-S131 [4] Ke-Xun Sun, Brett Allard, S. Williams, S. Buchman, and R. L. Byer, LED Deep UV Source for Charge Management for Gravitational Reference Sensors, Class. Quantum Grav. 23 (2006) S141-S150 [5] Ke-Xun Sun, Nick Leindecker, Sei Higuchi, John Goebel*, Sasha Buchman, Robert L. Byer, UV LED Operation Lifetime and Radiation Hardness Qualification for Space Flights, Journal of Physics: CS 154 (2009) [6] Ke-Xun Sun, Saps Buchman, and Robert L. Byer, Grating Angle Magnification Enhanced Angular and Integrated Sensors for LISA Applications, Journal of Physics CS, 32: , [7] Ke-Xun Sun, Patrick Lu, and Robert Byer, A Robust, Symmetric Grating Angular Sensor for Space Flights, LISA 6th International Symposium, AIP Proceedings, 873 (2006) p