As the Size of Satellites Is Reduced, the Need for Smaller System Components in the Satellite

Nanosatellite Communication and MEMS Technology

Nick Pohlman, Jeremy Opperer and Patrick Schubel

Department of Mechanical Engineering

Northwestern University

Evanston, IL 60208

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Project Summary

This paper explores the growing segments for potential use of

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MEMS devices. In particular, we focus on space-based applications in which MEMS have yet to play a significant role. If the performance of particular MEMS devices can match those of macro-sized components, the same system objectives can be achieved while significantly reducing the overall weight – usually the driving factor in spacecraft design. Of course, modifications in spacecraft architecture will hopefully evolve as MEMS components prove their performance in the space environment.

Examples are given for future distributed satellite missions showing that a greater quantity of smaller satellites with individualized capabilities can help reduce production cost as well as add robustness to the entire mission. Control of position and velocity are natural requirements of the distributed architecture. In order to trade sensor information effectively, remote communication must play a key role.

Therefore, the remainder of this report focuses on MEMS RF communication devices. Examples of phase shifters, signal filters, switches and antennas are given. The modeled performance of the switch and antenna are derived as well as the processes used for fabrication. Finally, an example is given of a MEMS RF-switch tested in orbit. The results indicate that MEMS structures can be cheaper to place in orbit and achieve improved performance over macro-sized devices.

Keywords: MEMS, RF Communication, satellites, space-based communication systems, MEMS fabrication processes, MEMS modeling

Table of Contents

Project Summary 1

Introduction 2

Satellite Missions and MEMS:

Past, Present and Future 2

Satellite Systems 4

Nanosatellite Communications

Systems and MEMS 4

Signal Filters 5

RF-MEMS Switches 5

Antennas 8

Phase Shifters 9

HCFA 10

Picosatellite Experiment 12

Conclusions

and Recommendations 13

References 14

Biography 16

Introduction

When NASA Administrator Daniel Goldin introduced the new catch phrase, “Faster, Better, Cheaper” he may not have envisioned where spacecraft technology would head over the next decade. The defining parameter in most spacecraft is payload weight for launching from the earth’s surface to outer space [1, 2]. As expected there is a direct relationship between cost of launch costlaunch and vehicle weight of the vehicle. If the overall system weight can be reducedlowered, or distributed, the potential for reducing satellite launching costs can be significantly reducedimproved.

Naturally, the development of Micro-Electro-Mechanical Systems (MEMS) devices has expanded into the space sector as well. By cutting the mass size of components onboard the space vehicle, the launch costs and hopefully the overall budget for production can be reduced. Furthermore, other features of MEMS devices, aside from other thantheir simplesmall size, can benefits other system components, such as power. By converting from solid-state electronics to mechanical systems, the power consumption of a device can be significantly lowered. That power reductionThis increase in efficiency could help reduce the battery power, size, and charge necessary to operate the satellite. Similarly, solar panel sizes could be smaller again bringing down the overall mass and power consumption of the satellite.

Two key advantages arise when considering MEMS devices in space applications. The first advantage is realized by lowering launch cost. Currently, launching a spacecraft into low Earth orbit (LEO) costs about $10,000 per kilogram, and placing a craft into a higher geosynchronous Earth orbit (GEO) costs about $50,000 per kilogram [3](PAT7).. Obviously, by reducing mass, designers stand to gain in reducing total project cost. The second advantage is the devices’ resistance to radiation and vibration. Cosmic radiation can upset the operation of solid state components, but MEMS structures can withstand radiation. In addition, because MEMS devices have such a low mass, potentially damaging inertial and vibration forces are minimized. A rocket launch can induce very high accelerations, but with a robust MEMS design the possibility of damage is low [4].(MEMS in Space article). Nanosatellites, roughly classified as satellites weighing 1-10 kg, are some of the most promising spacecraft being designed today. Below 1 kg, satellites are classified as picosatellites. MEMS can revolutionize their design when applied to the communications, data processing, navigation, and propulsion systems.

Satellite Missions and MEMS: Past, Present and Future

thenOne must consider what scientific objectives could be accomplished on such a small platform. One of the driving factors of programs such as NASA’s New Millennium Program (NMP) is distributing satellite capability and costs amongst across separate vehicles rather than placing all focus on a single, monolithic device. In shrinking the size, the cost of replacing a damaged or failed component can be drastically altered. Budgets for failed single satellite missions have cost billions of dollars without any capability for rectifying the problem.

Two SomeTwo missions that can be compared and contrasted are the Hubble Space Telescope, and the and the Chandra X-Ray Observatory [5]. It has been well documented of Hubble’s original failures due to an out of focus lens have been well documented. Fortunately, with a significant investment in time and man-powermanpower, the problem was fixed thereby allowing the space telescope to operate with its original purpose. It is interesting to note that the entire Chandra X-Ray Observatory Satellite project was nearly cancelled due to worries of similar problems experienced with Hubble.

With an orbital perigee of approximately 6,000 statute miles, Chandra, unlike Hubble, is well beyond the capability orbital range that can to be reached by the Space Shuttle. Conversely, Hubble, with a closer orbit of only 350 miles could be andreached and repaired by a human crew eliminating any possibility for repair. Any component failure aboard Chandra, from mirror lenses having a speck of dust to or a failed switch on the inability of the camera to simply turn on, could have caused the $1.5 billion project to be a complete failure without any means to rectify. If either of these satellite systems could have been distributed over a network of smaller satellites, the potential repair (and possibly production) costs could be more reasonable for a successful space mission. Hopefully with the added robustness of distributed satellite systems, NASA and other space agencies can avoid budget catastrophes such as those experienced with the Mars Surveyor in 1999.

One outcome from initial NMP research has shown that Formation Flying of multiple satellites can help distribute the single satellite payload capability. Many future NASA and military missions are basing their design on such capabilities, for example the Terrestrial Planet Finder [6] (NASA/JPL, ref: http://planetquest.jpl.nasa.gov/TPF/tpf_index.htmland TechSat 21 [7](AFRL,ref: http://www.vs.afrl.af.mil/factsheets/TechSat21.html) shown in figure 1. By expanding the number of satellites in a formation to more than one, the total aperture size can be increased allowing for improved resolution without adding cost for connecting the components with structural hardware. Without a rigid connection, the relative motions of payload components must be maintained with tight navigation and control tolerances.

Previous missions such as Earth Observing 1 (ref: http://eo1.gsfc.nasa.gov/) have shown the capability to maintain relative orbital motion or to conduct autonomous maneuvers with standard solid state electronic hardware and macro-size propulsion and control systems [8]. These types of proof of concept experiments have taken place orare even planned on for microsatellite missions.

Future NASA missions like Space Technology 5 & 6 (figure 2) (ref: http://nmp.jpl.nasa.gov/st5/) hope to be one of the first satellites to use actual MEMS devices in design and production [9]. The Air Force and DARPA are supporting expansion into miniaturization with their University Nano-SatelliteNanosatellite Program [10]. (ref: http://www.afosr.af.mil/pages/january00.htm).). While some microsatellites will use standard components, with one component necessary for the entire mission payload, the overall size of these satellites will begin to decrease in size, making MEMS more important features for space craft production.

Figure 1. TechSat 21 Mission distributed satellite radar system from AFRL [7]

Figure 2. Space Technology, part of New Millennium Program to use MEMS technology for system devices [9]

Satellite Systems

System components can roughly be segregated into three major categories: propulsion, navigation and communication. Micro-thrusters have been explored to help reduce the size of propulsion components on the vehicle making attitude control cheaper by using smaller devices and less fuel. Furthermore, navigational aids such as GPS receivers and gyroscopes are able to reduce the overall size of the navigation payload due with to MEMS technology. Another important aspect that must be included in all motion control is feedback from sensors. Naturally on a monolithic satellite, each component will be directly hardwired to an overall communication bus allowing information to be exchanged quickly. This The new architecture of distributing satellite capabilities across a fleet of separated vehicles presents a new problem for the distributed satellite systemsoverall communication design.

With the new architecture, reliable and adaptable communication systems are going to need to be developed for remote communication between devices. Algorithms and communication standards are being developed and shared to enhance capabilities of remote satellites not only with the ground, but also amongst their own fleet [11]. (ref: http://satjournal.tcom.ohiou.edu/). For example, if only one satellite in a multi-vehicle fleet has a star sensor to determine orientation, it must communicateits the measurementfinding to the other members of the fleet in order for all pieces to move as a cohesive unit. Autonomous capability to reconfigure networks and remote communication will be necessary for fleets to maintain continuity for operation. Occlusions to communication could happen as satellites cross paths or distances become too large. Potentially, some satellites’ primary purpose would be simple relays of information between vehicles separated over too great a distance. If the remote communication devices were based on MEMS technologies, many different communication capabilities could be carried on a single microsatellite allowing for potential to overcome single-point failure or allow the overall system to re-configurabilityconfigure relatively easily. The remainder of this report will not explore the details of specific communication protocol or information, which are generally– those will usually be mission specific, – but rather indicate devices that have potential in space applications for remote communication between separated vehicles.

Nanosatellite Communication Systems and MEMS

As the size of satellites is reduced, the need for smaller system components in the satellite is greatly increased. Much potential exists for miniaturization for the on-board communication system with MEMS devices.

The communication system on any satellite consists of two basic mechanisms: a transmitter and a receiver. Satellites usually deal with signals in the microwave range, which are high frequency, short wavelength signals that can carry a large amount of information. Often, the signals are in the gigahertz (109 Hz) radio frequency (RF) range.

In order to effectively transmit and receive RF signals, satellites must have extremely sensitive signal processing equipment. Also, the signal must be able to transmitted over a large distance with very high fidelity and fast data rate. Currently, most microwave signal handling is done with solid-state electronic components. Electrical performance of microwave components is determined almost entirely by the mechanical dimensions of the devices, and precision in the manufacturing of these components is extremely important [12]. (Fiedziuszko paperPAT2). MEMS devices are very attractive to use in these applications in this respect because of the ability to accurately control their dimensions. The design of conventional solid-state microwave and RF devices is hindered by three constraints, which are power consumption, sensitivity, and size [13].(Cass device articlePAT1). MEMS can improve on conventional designs in all three of these areas, as well as offering a lower production cost. As frequencies are driven higher and higher by increased data transmission requirements, the shortcomings of conventional designs are compounded by the lethargic response of macro-sized components [14](Lubecke 1999 paperPAT3). Also, in an increasingly crowded radio spectrum, high sensitivity is crucial. By integrating a MEMS device directly on a silicon chip and semiconductor circuit, energy consumption and signal noise are reduced even further.

MEMS devices in a nanosatellite can be used as signal filters, micro-switches, and antennas, signal filters, phase shifterscomponents such as phase shifters, and micro-switches. Some examples of each of these components will be presented as well as some of the fabrication and performance parameters.

signal Signal fFilters

An RF circuit requires at least one filter to pull out a desired signal from a receiving antenna or to insert one to be transmitted. Currently, surface acoustic wave (SAW) filters are used to do this job. However, these filters are relatively large and do not work well at very high frequencies (Cass article jan 2001PAT1[13]). Digital signal processing can be done on the back-end of the process, but this consumes valuable electrical power from batteries or solar panels. MEMS designs are well suited to perform front-end analog frequency filtering, taking up less space and using less power because the device is passive. By increasing filter sensitivity, potential exists to improve communications systems. A filter could not only preselect a communications band, but also perform channel selection within that band, enabling significant improvement over existing technology.

In one such design, a passive microwave filter is deposited on a GaAs membrane and displays very low power loss and good performance [15](filter book paperPAT4). First, a 2.2 mm thick membrane is machined utilizing RIE techniques on the GaAs wafer. To fabricate the filter structure, a 0.7 mm gold layer is deposited on the membrane and conventional contact lithography, e-gun evaporation, and lift-off techniques are used (figure 3). This device could easily be coupled with other components such as antennas, capacitors, and inductors on the same substrate.

MEMS Phase Shifters

Another application where MEMS devices can improve upon current solid-state design is in phase shifters. A phase shifter is a type of phased-array antenna that can be configured to transmit or receive signals in different directions without being physically reoriented. Already extensively used in microwave satellite communication tasks with FET or diode technology, a MEMS design could drastically reduce power loss. A phase shifter works by altering the transmission path of the antenna with an array of switches, providing different amounts of phase modulation of the signal. By using a MEMS switch design, which will be covered in detail in the next section, the existing design could be improved upon while reducing the cost (Cass jan 1999 article.) Because a large amount of research has been done on the design and placement of switches in a phase shifter, a MEMS phase shifter would differ from an existing one only in the fact that the solid state switch is replaced by a MEMS switch (Rebeiz article). A schematic of a MEMS phase shifter is shown in figure xx.