Jennifer L. Alvarez

Constellations, clusters, and communication technology: Expanding small satellite access to space

The trend toward small-sized spacecraft continues in government applications and is even increasing in commercial space endeavors that are funded by venture capital. Small spacecraft, including nanosatellites, microsatellites, and small satellites (smallsats), are an attractive alternative to traditional, larger spacecraft due to reduced development costs, decreased launch costs, and increased launch opportunities. A significant disadvantage, however, of a small spacecraft is its reduced or limited capabilities. The physical size of the small spacecraft reduces the size of the payload and/or the number of payloads that it can host, its propulsion capabilities, and its power. Small spacecraft are most commonly used in low Earth orbit, limiting the number of observation opportunities for a particular area of the Earth or space and the number of ground station downlink opportunities for stored data. These constraints affect the complexity and types of applications that small spacecraft can serve. Using multiple spacecraft that work together can overcome many of these limitations and expand the utility of small spacecraft. Two concepts for cooperative groups of spacecraft are constellations and clusters. A key technical challenge for small spacecraft, constellations, and clusters is communication of data. Communication challenges exist for accommodating varying numbers of users, serving high user densities in a given geographical area, and providing a consistent quality of service for different types of applications (e.g., Internet access, voice communication, machine-to-machine). This paper explores concepts for emerging communication technologies for space-ground and inter-satellite communication, while citing examples of existing and new constellation and cluster concepts. Several aspects of the communication systems are examined in terms of frequency bands, data rates, multiple access methods, and accommodation on spacecraft.

Avionics of the Cyclone Global Navigation Satellite System (CYGNSS) microsat constellation

The Cyclone Global Navigation Satellite System (CYGNSS), which was recently selected as the Earth Venture-2 investigation by NASAs Earth Science System Pathfinder (ESSP) Program, measures the ocean surface wind field with unprecedented temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of wind speeds experienced in a tropical cyclone (TC). The CYGNSS flight segment consists of 8 microsatellite-class observatories, which represent SwRIs first spacecraft bus design, installed on a Deployment Module for launch. They are identical in design but provide their own individual contribution to the CYGNSS science data set. Subsystems include the Attitude Determination and Control System (ADCS), the Communication and Data Subsystem (CDS), the Electrical Power Supply (EPS), and the Structure, Mechanisms, and Thermal Subsystem (SMT). This paper will present an overview of the mission and the avionics, including the ADCS, CDS, and EPS, in detail. Specifically, we will detail how off-the-shelf components can be utilized to do ADCS and will highlight how SwRIs existing avionics solutions will be adapted to meet the requirements and cost constraints of microsat applications. Avionics electronics provided by SwRI include a command and data handling computer, a transceiver radio, a low voltage power supply (LVPS), and a peak power tracker (PPT).

CYGNSS command and data subsystem and electrical power subsystem phase A and B developments

The Cyclone Global Navigation Satellite System (CYGNSS), which was selected as the Earth Venture-2 investigation by NASAs Earth Science System Pathfinder (ESSP) Program, measures the ocean surface wind field with unprecedented temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of wind speeds experienced in a tropical cyclone (TC). The CYGNSS flight segment consists of 8 microsatellite-class observatories, which represent SwRIs first spacecraft bus design, installed on a Deployment Module for launch. The microsatellites (microsats) are identical in design but provide their own individual contribution to the CYGNSS science data set. Within the first year of the CYGNSS program (Phase A and B), the design has been analyzed, vetted, and reviewed which culminated in a succession of updates and design improvements. This paper will discuss relevant updates to the electrical systems of the microsat, specifically the command and data subsystem (CDS) and the electrical power subsystem (EPS). For the CDS, more detailed analysis of the communication link and maturation of the transceiver module design are presented. The link budget was reviewed by the team and verified via simulation. The transceiver module has moved to a dedicated box consisting of a radio frequency (RF) board and a digital signal processing (DSP) board. For the EPS, a detailed power analysis of the mission has led to an update in the solar array configuration. The details of the power analysis, which is performed in STK and Matlab, are presented.

Frequency and waveform agile receiver covering the ultra high frequency band

Southwest Research Institute® (SwRI®) has developed a radio frequency (RF) receiver that is agile in frequency and waveform over the ultra high frequency (UHF) band. The software defined receiver is CubeSat-scale in size and power consumption, and it provides a flexible, adaptable solution for a number of RF application areas including communications, tracking and locating, and signals intelligence. Based on SwRI internal funding and previous developments for the U.S. Government, the receiver, which is currently at a technology readiness level of five, enables pre-mission and on-orbit reconfiguration plus RF agility from 300 to 3,000 MHz with a bandwidth of up to 30 MHz. This paper describes the hardware architecture of the agile receiver and provides an application example of the geographical location of a terrestrial RF transmitter. In this example, geolocation performance is simulated using performance parameters of the receiver modeled on two or three low Earth orbit spacecraft to estimate the location of the transmitter. A number of spatial separations of the three spacecraft configuration are considered. Further, two RF frequencies are simulated, one for an RF transmitter in the lower portion of the UHF band and another for an RF transmitter in a higher portion of the UHF band. Results show that this small, low power technology, which is suitable for a CubeSat payload, provides a capability for geolocating terrestrial transmitters.

Enabling fractionated spacecraft communications using F6WICS

Southwest Research Institute® (SwRI®) has designed a wireless transceiver to provide inter-satellite communications as part of the Defense Advanced Research Projects Agency (DARPA) System F6 program. System F6 (Future, Fast, Flexible, Fractionated, Free-Flying Spacecraft United by Information Exchange) seeks to demonstrate the feasibility and benefits of a satellite architecture wherein the functionality of a traditional “monolithic” spacecraft is delivered by a cluster of wirelessly-interconnected modules capable of sharing their resources and utilizing resources found elsewhere in the cluster. SwRIs System F6 Wireless Inter-module Communication System (F6WICS) provides the data link and physical layers of the network stack that are specifically designed to meet the needs of fractionated space missions. F6WICS provides deterministic, real-time media access mechanisms that make efficient use of limited communications bandwidth over a wide range of spacecraft separation distances and network populations. The data link protocol is highly robust to module failure. The physical layer waveform provides robust communication, precision time transfer throughout the network, and continuous estimation of distance between spacecraft for use in navigation. The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. Distribution Statement “A”: Approved for Public Release, Distribution Unlimited.

Increasing the capability of CubeSat-based software-defined radio applications

CubeSats are highly accessible as Earth orbiting platforms due to their low costs of development and launch when compared to traditional small satellites. This accessibility, combined with a commensurately short development timeline, can be attributed to the use of commercial-off-the-shelf (COTS) technology. However, COTS components typically have limited inherent resilience to the space environment. As such, CubeSat usage has largely been limited to experiments or applications where high availability is not required. Several technologies are enablers for increased CubeSat performance in the environment of space. Dependable Multiprocessor (DM) technology has demonstrated the capability for high system availability and reliability with COTS processors in a space environment. DM opens many possibilities for high performance, low cost processing in space, supporting technologies such as advanced software defined radios (SDR). SDR technology allows for on-orbit reconfigurability of data management, protocols, multiple access methods, waveforms, and data protection. This paper explores how these enabling technologies hold promise for increasing the availability and capability of CubeSats, allowing CubeSats to be used in advanced applications often associated with military and commercial operations.

Data access architectures for high throughput, high capacity flash memory storage systems

We present a design framework and software tools which support the development of high performance, high capacity data storage hardware systems employing flash memory technology. This framework facilitates the design of data storage systems which provide multiple terabits of storage and access and retrieval rates of several gigabits per second. This methodology supports the design of data storage systems with widely varying functional requirements by enabling rapid exploration of the design space, providing automatic validation of functional correctness, and providing accurate quantitative predictions of performance. We present two case studies demonstrating the flexibility and scope of this approach, and describe progress toward the implementation of a prototype data storage system designed using the framework.

Performance characterization of optical module designed for space applications

Next generation space missions are planning for increasingly aggressive data collection platforms that host advanced sensor technologies. Examples are focal plane arrays and synthetic aperture radar systems producing vast amounts of high-speed data on orbit, on the order of tens of Gbps. Data collection interfaces to these advanced sensors are designed around high speed differential signaling that use copper cable harnesses to provide 40 to 50 individual copper interconnects. This density of copper cabling readily introduces Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) issues. This copper cable harness requires various levels of electrically grounded shielding that introduce complications in mechanical and electrical design to avoid ground loops in the system. Optical cabling is an attractive solution to mitigate and avoid these EMI/EMC and grounding complications while also dramatically reducing the diameter and mass of the harness. Large shielded and impedance controlled copper cables can then be replaced with thinner optical cables that can be more compactly routed through the spacecraft. Micropac Industries, Inc. has recently developed prototype miniaturized optical transceiver modules for space applications. These modules convert a differential signal into a single ended optical signal that can then be used with fiber optic cable. This approach holds promise for reduced size, mass, and EMI/EMC issues, but the technology is still being proven. This paper presents the results of experiments that characterize the performance of high rate data communication over fiber optic interfaces and electrical copper interfaces. Experiments include the assessment of bit error rate performance at multi-Gbps data rates over various lengths of fiber optic and copper cable. The results show that these optical transceivers are a viable solution for an architecture in which sensor electronics and spacecraft electronics must communicate via cables using full duplex high speed communications.

Flexibility and Extensibilty in the Design of Spacecraft Communications Systems

Southwest Research Institute® (SwRI®) has developed a wireless transceiver that incorporates a flexible and extensible design and implementation strategy, enabling deployment of the radio to numerous roles for space missions. The transceiver was architected such that the software and the hardware could be quickly repurposed based on mission need. The software is written in ANSI standard and object oriented C++, facilitating modularity and functional reuse. The control and processing hardware is reprogrammable, allowing for the same hardware to be used in numerous applications. Although the radio frequency hardware is inherently narrow band in its conversion architecture, careful consideration to the implementation allows for quick, low cost rework to migrate between frequency bands and bandwidths. While the transceiver was originally intended for CubeSat telemetry, tracking, and command, it has now been extended in design for micro-satellite space-ground communication at S-band, micro-satellite inter-satellite crosslink communication at S-band, and small-satellite inter-satellite crosslink at Ka-band. An example of rapid repurposing of the transceiver from a Ka-band crosslink to an S-band crosslink shows the benefits of modularity and hardware/software that is architected for extensibility.

A configurable timing and communications engine for radio positioning with implementations for an FPGA or an ASIC

Executing various combinations of external locating techniques provides many benefits over tracking and locating systems based on radar or GPS. These embedded radio positioning applications are built on a common set of functional capabilities. Development of a specific positioning system involves selection of a subset of these capabilities and implementation into a physical form that meets the size, weight and power requirements of the application while meeting cost, schedule and risk constraints. In this paper we present the design of a timing and communications engine, which is a highly configurable resource for radio positioning applications. We also present an analysis of the design trades in implementing this functionality in a field programmable gate array (FPGA) or as an application-specific integrated circuit (ASIC).