Steve Parkes

Trends in the use of verbal protocol analysis in software engineering research

This article reviews the technique of verbal protocol analysis and gives a profile of its use within software engineering research over the last two decades. An overview is given of the procedures used in verbal protocol analysis, and commonly-found difficulties in the application of the technique by researchers are described. The article reports on published efforts to develop tools to automate the procedures. A review of the literature shows trends in the use of the verbal protocol analysis in software engineering research from the 1980s to the present. Recurring themes of its purpose within software engineering research are identified, including the comparison of the behaviours of subjects with differing levels of expertise and the identification of effective software comprehension strategies. Advances and problems with the development of a general-purpose encoding scheme for verbal protocol analysis appropriate to a range of domains within software engineering are described.

SpaceFibre: A multi-Gigabit/s interconnect for spacecraft onboard data handling

SpaceFibre is a spacecraft onboard data link and network technology being developed by University of Dundee for the European Space Agency (ESA), which runs over both copper and fibre optic cables. Initially targeted at very high data rate payloads such as Synthetic Aperture Radar (SAR) and multi-spectral imaging instruments, SpaceFibre is capable of fulfilling a wider set of spacecraft onboard communications applications because of its inbuilt QoS and FDIR capabilities and its backwards compatibility with the ubiquitous SpaceWire technology. SpaceFibre operates at 2.5 Gbits/s providing 12 times the throughput of a SpaceWire link with current flight qualified technology and allowing data from multiple SpaceWire devices to be concentrated over a single SpaceFibre link. This substantially reduces cable harness mass and simplifies redundancy strategies. The innovative QoS mechanism in SpaceFibre provides concurrent bandwidth reservation, priority and scheduled QoS. This simplifies spacecraft system engineering through integrated quality of service (QoS), which reduces system engineering costs and streamlines integration and test. Novel integrated FDIR support provides galvanic isolation, transparent recovery from transient errors, error containment in virtual channels and frames, and “Babbling Idiot” protection. SpaceFibre enhances onboard network robustness through its inherent FDIR and graceful degradation techniques incorporated in the network hardware. This simplifies system FDIR software, reducing development and system validation time and cost. SpaceFibre includes low latency event signalling and time distribution with broadcast messages. This enables a single network to be used for several functions including: transporting very high data rate payload data, carrying SpaceWire traffic, deterministic delivery of command/control information, time distribution and event signalling. SpaceFibre is backwards compatible with existing SpaceWire equipment at the packet level allowing simple interconnection of SpaceWire devices into a SpaceFibre network and enabling that equipment to take advantage of the QoS and FDIR capabilities of SpaceFibre.

Image Processing for Near Earth Object Optical Guidance Systems

A feature tracking algorithm is described, which is designed to support optical navigation of autonomous spacecraft. The algorithm is based on an existing system designed for use in planetary entry, descent, and landing (EDL) scenarios and includes several extra processing steps aimed at improving the performance. The algorithm is tested against the original by processing synthetic image streams of an asteroid and a lunar-type surface, taking care to use realistic surface reflectance models. The tracked features are processed to extract estimates of the camera motion between frames, and the results are compared with the ground truth to assess the robustness and accuracy of the feature tracking.

SpaceFibre: Adaptive high-speed data-link for future spacecraft onboard data handling

SpaceFibre is a high-speed data-link technology being developed by the University of Dundee for ESA to support spacecraft onboard data-handling applications. SpaceFibre operates at 2.5 Gbits/s, can run over fibre optic or electrical media, provides galvanic isolation, includes Quality of Service (QoS) and Fault Detection Isolation and Recovery (FDIR) support, and provides low-latency signalling. It operates over distances of 5m with copper cable and 100 m or more with fibre optic cable. SpaceFibre supports multiple virtual channels running over a single physical link. QoS capabilities built into the SpaceFibre hardware allow the bandwidth and priority of each virtual channel to be specified. Traffic flow over each virtual channel then adapts automatically taking into account virtual channels that have data ready to send and available buffer space at the far end of the link, along with link bandwidth and priority allocation. The novel QoS mechanism is simple but powerful and also allows the automatic detection of “babbling idiots” and virtual channels that are sending much less data than expected. After a brief introduction the SpaceFibre QoS and FDIR capabilities are explained. The approach taken in validating the SpaceFibre protocols and current status of the SpaceFibre development activities are then described.

SpaceWire for Adaptive Systems

SpaceWire is a network for connecting instruments and other equipment into the payload data-handling system onboard a spacecraft. It provides high-speed (2-200 Mbits/s), bi-directional communications over point-to-point links and network capabilities using routing switches. One of the key features of SpaceWire is its simplicity resulting in low gate count implementations enabling SpaceWire interfaces to be readily included in FPGAs and ASIC devices. Spacecraft systems have to be able to adapt to failures either autonomously or under control from Earth. SpaceWire provides support for implementing fault tolerant links and networks. This paper introduces SpaceWire, considers the need for autonomy and adaptive systems in spacecraft and then explores how SpaceWire can support that autonomy.

SpaceWire protocol ID: what does it means to you?

SpaceWire is becoming a popular solution for satellite high-speed data buses, because it is a simple standard that provides great flexibility for a wide range of system requirements. It is simple in packet format and protocol, allowing users to easily tailor their implementation for their specific application. Some of the attractive aspects of SpaceWire that make it easy to implement also make it hard for future reuse. Protocol reuse is difficult because SpaceWire does not have a defined mechanism to communicate with the higher layers of the protocol stack. This has forced users of SpaceWire to define unique packet formats and define how these packets are to be processed. SpaceWire also has two different packet formats that may be presented to the end user, one with a destination address and one without. This further complicates the system design, as both options cannot be supported. Each particular mission writes its own interface control document (ICD) and tailors SpaceWire for its specific requirements making reuse difficult. Part of the reason for this habit may be because engineers typically optimize designs for their own requirements in the absence of a standard. This is an inefficient use of project resources and costs more to develop missions

SpaceWire and SpaceFibre on the Microsemi RTG4 FPGA

SpaceWire is a spacecraft on-board data-handling network which connects instruments to the mass-memory, data processors and control processors, which is already in orbit or being designed into more than 100 spacecraft. SpaceFibre is a new, multi-Gbits/s, on-board network technology, which runs over both electrical and fibre-optic cables. SpaceFibre is capable of fulfilling a wide range of spacecraft on-board communications applications because of its inbuilt quality of service (QoS) and fault detection, isolation and recovery (FDIR) capabilities. The Microsemi RTG4 is a new generation radiation tolerant FPGA. It has extensive logic, memory, DSP blocks, and IO capabilities and is inherently radiation tolerant, having triple mode redundancy built in. The RTG4 has a flash configuration memory built into the device. In addition the FPGA incorporates 16 SpaceWire clock-data recovery circuits and 24 multi-Gbits/s SerDes lanes to support high-speed serial protocols like SpaceFibre. STAR-Dundee has implemented SpaceWire and SpaceFibre IP cores using the Microsemi RTG4 Development Kit. The flight proven SpaceWire IP core was initially run at over 200 Mbits/s and the SpaceFibre IP core at 2.5 Gbits/s. With a little more work it is expected to reach 300 Mbits/s and 3.125 Mbits/s respectively. The SpaceWire IP core takes around 1% of the FPGA and the SpaceFibre IP core around 3-5% depending on the number of virtual channels supported. The use of the RTG4 with the SpaceWire and SpaceFibre IP cores provides a powerful platform for future spacecraft on-board instrument control, data-handling, and data processing. Furthermore the advanced QoS and FDIR capabilities of SpaceFibre make it suitable for a wider range of spacecraft onboard applications including integrated payload data-handling and attitude and orbit control networks and launcher applications where deterministic data delivery is required. This paper reports on the implementation and testing of the SpaceWire and SpaceFibre IP cores in the Microsemi RTG4 FPGA.

Modeling cratered surfaces with real and synthetic terrain for testing planetary landers

The autonomous guidance of a spacecraft lander requires extensive testing to develop and prove the technology. Methods such as machine vision for navigation and both vision and LIDAR for hazard avoidance are being studied and developed to provide precise, robust lander guidance systems. A virtual test environment that can simulate these instruments is a vital tool to aid this work. When available, terrain elevation models can provide a base for simulation but they frequently contain artifacts, gaps, or may not have the required resolution. We propose novel techniques to model heavily cratered surfaces for testing planetary landers by combining crater models and fractal terrain to create a multiresolution mesh for simulating a spacecraft descent and landing. The synthetically enhanced models are evaluated by comparing enhanced terrain based on Clementine/RADAR data with higher resolution terrain models from the Selene and the Lunar Reconnaissance Orbiter (LRO) to show that the artificial models are suitable for testing planetary lander systems.

Asteroid Modeling for Testing Spacecraft Approach and Landing

Spacecraft exploration of asteroids presents autonomous-navigation challenges that can be aided by virtual models to test and develop guidance and hazard-avoidance systems. Researchers have extended and applied graphics techniques to create high-resolution asteroid models to simulate cameras and other spacecraft sensors approaching and descending toward asteroids. A scalable model structure with evenly spaced vertices simplifies terrain modeling, avoids distortion at the poles, and enables triangle-strip definition for efficient rendering. To create the base asteroid models, this approach uses two-phase Poisson faulting and Perlin noise. It creates realistic asteroid surfaces by adding both crater models adapted from lunar terrain simulation and multiresolution boulders. The researchers evaluated the virtual asteroids by comparing them with real asteroid images, examining the slope distributions, and applying a surface-relative feature-tracking algorithm to the models.

CCSDS Time-Critical Onboard Networking Service

The Consultative Committee for Space Data Systems (CCSDS) is developing recommendations for communication services onboard spacecraft. Today many different communication buses are used on spacecraft requiring software with the same basic functionality to be rewritten for each type of bus. This impacts on the application software resulting in custom software for almost every new mission. The Spacecraft Onboard Interface Services (SOIS) working group aims to provide a consistent interface to various onboard buses and sub-networks, enabling a common interface to the application software. The eventual goal is reusable software that can be easily ported to new missions and run on a range of onboard buses without substantial modification. The system engineer will then be able to select a bus based on its performance, power, etc and be confident that a particular choice of bus will not place excessive demands on software development. This paper describes the SOIS Intra-Networking Service which is designed to enable data transfer and multiplexing of a variety of internetworking protocols with a range of quality of service support, over underlying heterogeneous data links. The Intra-network service interface provides users with a common Quality of Service interface when transporting data across a variety of underlying data links. Supported Quality of Service (QoS) elements include: Priority, Resource Reservation and Retry/Redundancy. These three QoS elements combine and map into four TCONS services for onboard data communications: Best Effort, Assured, Reserved, and Guaranteed. Data to be transported is passed to the Intra-network service with a requested QoS. The requested QoS includes the type of service, priority and where appropriate, a channel identifier. The data is de-multiplexed, prioritized, and the required resources for transport are allocated. The data is then passed to the appropriate data link for transfer across the bus. The SOIS supported data links may inherently provide the quality of service support requested by the intra-network layer. In the case where the data link does not have the required level of support, the missing functionality is added by SOIS. As a result of this architecture, re-usable software applications can be designed and used across missions thereby promoting common mission operations. In addition, the protocol multiplexing function enables the blending of multiple onboard networks. This paper starts by giving an overview of the SOIS architecture in section 11, illustrating where the TCONS services fit into the overall architecture. It then describes the quality of service approach adopted, in section III. The prototyping efforts that have been going on are introduced in section JY. Finally, in section V the current status of the CCSDS recommendations is summarized.