Robert E. Zee

A Feasibility Assessment for Low-Cost InSAR Formation-Flying Microsatellites

Multistatic interferometric synthetic aperture radar (InSAR) is a promising potential payload for a small satellite constellation. CanX-4 and CanX-5 are a pair of formation-flying nanosatellites launching in 2009; once formation flight has been demonstrated, a future multistatic InSAR constellation of low-cost microsatellites can exploit subcentimeter intersatellite baseline knowledge, with digital elevation map height errors on the order of 1 m in the flat-terrain case. This paper evaluates the feasibility of such a mission, using case studies of commonly proposed configurations: the Interferometric Cartwheel, the Cross-Track Pendulum, and the Cartwheel-Pendulum (Car-Pe) configuration. In each case, several SAR transmitters are considered: L-, C-, and X-band transmitters with parameters mirroring existing ldquolargerdquo satellite SAR missions, and a theoretical X-band microsatellite transmitter. The available interferometric baselines, ground coverage, and image resolutions are evaluated in each scenario. The X-band ldquomicrordquo transmitter is feasible, but the low transmit power severely limits the ground coverage. The X-band ldquolargerdquo transmitter provides the largest ground coverage and the highest resolution along with the X-band ldquomicrordquo option. The resolutions are wavelength dependent and remain relatively constant among the configurations. The operating areas of the pendulum demonstrate the largest degree of overlap, while the longer along-track baselines of the cartwheel result in a smaller overlap. Both two-receiver (pendulum and cartwheel) configurations demonstrate baseline characteristics that may be optimal for different applications, while the three-receiver Car-Pe demonstrates the advantages of both the pendulum and cartwheel.

Maritime surveillance with synthetic aperture radar (SAR) and automatic identification system (AIS) onboard a microsatellite constellation

New developments in small spacecraft capabilities will soon enable formation-flying constellations of small satellites, performing cooperative distributed remote sensing at a fraction of the cost of traditional large spacecraft missions. As part of ongoing research into applications of formation-flight technology, recent work has developed a mission concept based on combining synthetic aperture radar (SAR) with automatic identification system (AIS) data. Two or more microsatellites would trail a large SAR transmitter in orbit, each carrying a SAR receiver antenna and one carrying an AIS antenna. Spaceborne AIS can receive and decode AIS data from a large area, but accurate decoding is limited in high traffic areas, and the technology relies on voluntary vessel compliance. Furthermore, vessel detection amidst speckle in SAR imagery can be challenging. In this constellation, AIS broadcasts of position and velocity are received and decoded, and used in combination with SAR observations to form a more complete picture of maritime traffic and identify potentially non-cooperative vessels. Due to the limited transmit power and ground station downlink time of the microsatellite platform, data will be processed onboard the spacecraft. Herein we present the onboard data processing portion of the mission concept, including methods for automated SAR image registration, vessel detection, and fusion with AIS data. Georeferencing in combination with a spatial frequency domain method is used for image registration. Wavelet-based speckle reduction facilitates vessel detection using a standard CFAR algorithm, while leaving sufficient detail for registration of the filtered and compressed imagery. Moving targets appear displaced from their actual position in SAR imagery, depending on their velocity and the image acquisition geometry; multiple SAR images acquired from different locations are used to determine the actual positions of these targets. Finally, a probabilistic inference model combines the SAR target data with transmitted AIS data, taking into account nearest-neighbor position matches and uncertainty models of each observation.

Registration of multi-frequency SAR imagery using phase correlation methods

The advancing development of low-cost small spacecraft platforms enables Earth observation constellation missions using a variety of imaging methods, including multistatic (single-transmitter, multiple-receiver) interferometric synthetic aperture radar (InSAR). Image registration is a necessary step preceding interferometric analysis of multistatic or repeat-pass SAR imagery. An effective image registration method for a potential InSAR constellation mission must not incur errors from distortions caused by imaging geometry and temporal changes in the terrain, and should as well be applicable to multi-frequency SAR registration and potentially multi-modal registration with optical imagery. Furthermore, the ability to register and process images onboard the spacecraft is desirable due to power and downlink limitations on a small platform. Herein we demonstrate a frequency-domain registration method on single-frequency, repeat-pass, and dual-frequency SAR imagery, for use in a future small satellite remote sensing constellation.

Wavelet-based despeckling for onboard image processing in a small satellite SAR maritime surveillance constellation

New developments in small spacecraft capabilities will soon enable formation-flying constellations of small satellites, capable of performing remote sensing missions at low cost. One such mission concept under investigation involves a maritime surveillance microsatellite constellation. Two or more small microsatellites will follow a large synthetic aperture radar (SAR) transmitter, each carrying a SAR receiver antenna and one carrying an automatic identification system (AIS) antenna. Voluntary broadcasts of vessel position and velocity via AIS are received and decoded, and compared with SAR observations to verify the data and identify potentially non-cooperative vessels. Limited transmit power and downlink time will dictate that data be processed onboard the spacecraft. Herein we demonstrate a wavelet-based speckle removal method from the novel perspective of the maritime surveillance microsatellite mission, comparing wavelet thresholding methods and demonstrating that the resulting filter effectively removes speckle for improved target detection, reduces the size of the imagery data, and preserves detail necessary for registration and target evaluation.

The Generic Nanosatellite Bus: From Space Astronomy to Formation Flying Demo to Responsive Space

With the increasing number of services on the Internet, it has become a great challenge to help users find services according to their demands. Personalized recommendation technology is an effective way to solve the problem. Existing service recommendation approaches make recommendations among services with same or similar functionalities to meet the non-functional requirements of users, while the functional requirements for service are seldom taken into account and new services that satisfy the needs of users are difficult to be recommended. Therefore, in this paper, we introduce social tags to the process of service recommendation and build service functionality oriented social tags model to describe user preference for service functionality, then we present a personalized service recommendation algorithm for service functionality (PSR-SF). The proposed algorithm first discovers the neighbors of a target user according to services use frequency of users, and then clusters services that have been used by the target user and his neighborhoods by using the functional characteristic vector of services which based on service functionality oriented social tags model. Finally, target user preference for service classes are generated by using service functionality tags use information of users to make recommendations. The experiment results show that the performance of PSR-SF algorithm is better than those existing recommendation algorithms in terms of service recommendation precision, recall and F values.In many pervasive applications like the intelligent bookshelves in libraries, it is essential to accurately locate the items to provide the location-based service, e.g., the average localization error should be smaller than 50 cm and the localization delay should be within several seconds. Conventional indoor-localization schemes cannot provide such accurate localization results. In this paper, we design an adaptive, accurate indoor-localization scheme using passive RFID systems. We propose two adaptive solutions, i.e., the adaptive power stepping and the adaptive calibration, which can adaptively adjust the critical parameters and leverage the feedbacks to improve the localization accuracy. The realistic experiment results indicate that, our adaptive localization scheme can achieve an accuracy of 31 cm within 2.6 seconds on average.The Space Flight Laboratory (SFL) at the University of Toronto Institute for Aerospace Studies develops missions using spacecraft measuring 20 by 20 cm in its cross section and up to 40 cm in length. Each spacecraft can weigh up to 15 kg with up to 9 kg of payload. One of the three SFL operational missions uses the Generic Nanosatellite Bus (GNB) form factor and was conceived, built, and delivered into orbit within seven months from project inception. This nanosatellite precedes an operational 75 kg microsatellite mission by demonstrating the payload technology. Other technologies currently in orbit include reaction wheels and propulsion system, which will be used in follow up missions. Of the five nanosatellites currently under construction at SFL, two are intended for performing astrophysics investigation, two are intended for carrying out formation flying technology demonstration, and one is intended for performing preoperational duties as a way to fast track the readiness of new technologies that are slated for larger, operational missions; the latter is currently slated for launch in Q3 2009. In addition, SFL is also providing a number of critical subsystems for an operational microsatellite mission. These spacecraft build upon a set of common components and technologies that are shared across multiple missions and implement an architecture that is directly expandable to larger, operational missions. The development of these missions follows the microspace approach for managing risks and ensuring rapid development, which maintains cost-effectiveness and responsiveness to new missions. Typically each spacecraft implements multiple on-board computers, high data rate radios, sensors and actuators. The system implements a number of redundancies to mitigate failures. The subsystem complement and the complexity of the spacecraft can be tailored to meet various mission needs, from a passively stabilized spacecraft using permanent magnets to a three-axis stabilized platform with reaction wheels with optional propulsion system. The spacecraft can also accommodate fixed appendages such as booms, antennas, and additional solar panels. SFL also builds its own separation systems called “XPODs” and arranges, on a regular basis, shared launches for nanosatellite developers worldwide through its Nanosatellite Launch Service (NLS) program.

A Nanosatellite-Based System for High-Availability Maritime Observation

The Automatic Identification System Satellite no. 1 (AISSat-1) was launched on July 12, 2010 into a 635 km sun synchronous orbit by an Indian rocket from the Satish Dhawan Space Centre in Andhra Pradesh, India. AISSat-1 is a six kilogram nanosatellite based on the Generic Nanosatellite Bus (GNB) satellite platform, and was designed, built, and commissioned in orbit by the University of Toronto Institute for Aerospace Studies, Space Flight Laboratory (UTIAS/SFL). The second satellite in this series, AISSat-2, is slated for launch in 2013.

Evolution of Multi-Mission Nanosatellite Ground Segment Operations

The Space Flight Laboratory (SFL) at the University of Toronto Institute for Aerospace Studies (UTIAS) has been a pioneer in nanosatellite technologies since the first 1-kg “CubeSat” nanosatellites were conceived and designed. Since the launch of its first nanosatellite in 2003, SFL has launched three more nanosatellites, ranging in size from 3 to 7 kg and spanning different generations of technology. At least ten more nanosatellite missions will be launched by SFL over the next several years.

Antarctic Broadband: Ka-Band Communications for the Bottom of the Earth

www.antarcticbroadband.com High bandwidth communications is the largest sector of the commercial satellite industry. While micro- and nanosatellites are yet to service this market commercially, it is expected that such spacecraft will play an increasing role in the communications industry, with initial applications likely to be in niches which cannot be readily or easily addressed by traditional service providers. Antarctica is one such niche. Communication needs in the Antarctic are increasing rapidly, with a high rate of growth in several activities across the continent. Traditional space and terrestrial communication solutions will not be able to meet these needs in the near-future, due to the inherent limitations of coverage from geostationary orbits and the remoteness and harsh environment of the Antarctic. The Antarctic Broadband program is intended to establish a high-quality communications service for the international research community in Antarctica. The initial project phase, supported under the Australian Government’s Australian Space Research Program, has defined a satellite communications service optimized to meet the current and future data transfer needs of the entire Antarctic community, and has tested a number of important technologies notably including a high-efficiency Ka-band transponder designed for the nanospacecraft scale. The initial Antarctic Broadband mission—a nanosatellite demonstrator which may potentially serve a quasi-operational role—is currently ready for manufacture, with the two-satellite operational constellation to follow. This paper presents the proposed Antarctic Broadband system, with an emphasis on the demonstration and operational spacecraft and the Ka-band technologies which enable them.

Formation and Attitude Control for the CanX-4 and CanX-5 Formation Flying Mission

Abstract Formation and attitude control strategies for the CanX-4 and CanX-5 nanosatellite formation flying mission are discussed. An innovative strategy for managing reaction wheel momentum build-up using planned formation control thrusts is presented, and an attitude control method to maximize GPS signal acquisition is reviewed. Fine formation keeping mode, formation reconfiguration mode, and station keeping mode controllers are reviewed. High fidelity simulations with representative control and estimation errors show that the proposed attitude and formation control strategies are effective and meet performance requirements in all cases. It was found that errors in the relative velocity estimates have the largest effect on formation control errors.

Design of a Electrodynamic Tether Nanosatellite Mission

Electrodynamic space tethers (EDT) have the unique capabilities to acquire science data which would otherwise not be achievable and can provide cost-effective demonstrations of innovative concepts. From propellantless propulsion for orbit management, to power generation, to end-of-life de-orbiting of spacecraft, an electrodynamic tether provides several capabilities that are key to any satellite mission. This paper will cover the mission concept study, mission objectives, nanosatellite design, hardware selection, and operation. The basic concept involves a nanosatellite and a picosatellite that will begin the mission with the picosatellite stored into the payload bay of the nanosatellite. The picosatellite will host the EDT, its deployment mechanics and communicate with the nanosatellite for data transfer between two satellites. Following the commissioning phase, the picosatellite will be deployed providing the initial velocity to unspool the tether. The resulting gravity gradient between the nanosatellite and picosatellite will deploy the remainder of the 500-meter long aluminum EDT. The proposed mission concept study is to perform a pioneering mission demonstrating deployment and stabilization of an EDT with an end-mass, current collection and satellite de-orbiting using EDT technology. The design will employ heritage satellite design with minimal modification in order achieve a high level of fidelity, thereby minimizing potential risks. Furthermore, the tethered nanosatellite flying provide a costeffective mean to acquire science data which would otherwise not be achievable to help radio scientists understanding how a coherent backscatter (CBS) radar in the high-frequency SuperDARN system can work to improve the interpretation of convective motion of the Fregion ionosphere at high latitudes, and also detect the radiation by a conducting tether for the development of plasma electromagnetic theory..