ACSER ION/IAC/ASRC Rehearsal Seminar
ACSER staff and students will be attending 3 international conferences in the coming weeks. These talks will preview the content they will be presenting. Please feel free to join us for one, some or all of the presentations. Each talk should run for approximately 20 minutes.
GNSS Data as Court Evidence: Lessons from Remote Sensing
Prof Andrew Dempster, Director, ACSER | ION GNSS+ 2018, Miami USA
Digital forensics is a relatively well established discipline but GNSS forensics has not attracted a lot of interest, at least in the published arena. The opposite is true for remotely sensed data, which has extensive literature. This paper looks at the requirements of digital forensics applied to GNSS and the lessons that a GNSS expert witness can gain from the work done for remote sensing.
On the Liability Requirements of The Australian Space Legislation
A/Prof Elias Aboutanios, Deputy Director, ACSER | Australian Space Research Conference 2018, Gold Coast
In this work we will examine the risk to the Australian Government arising from the activities of Australian space players. This issue has recently come to the fore as part of the reform of the Australian Space Activities Act 1998 (the Act-1998). Whereas the Act-1998 stipulates that applicants for an authorisation for a space activity must indemnify the Government for the lesser of the Maximum Probable Loss (MPL) or $750M, the proposed new bill has capped this financial requirement at $100M. Furthermore, the new bill provides for the actual insurance requirement to be set in the legislative instruments, which are adaptable. This brings the Australian requirements in line with international best practice and provides a more favourable environment for Australian space actors. Previously, we have analysed the risk to the Australian government under the Act-1998 and showed that the insurance requirement is excessive. We have also shown that reducing the requirement will not lead to excessive risk to the Australian Government. In this talk we will discuss the liability of the Australian Government associated with Australian space activities and present modelling of the additional risk to the Government that arises from the change in the legislated insurance requirement. Furthermore, we will discuss methods for setting the actual insurance requirement in the legislative instruments.
Investigating The Sensitivity of Delay Doppler Maps to Wind Direction Using Ambiguous Stare Processing
Ben Southwell, ACSER PhD Student | ION GNSS+ 2018, Miami USA
The application of Global Navigation Satellite System - Reflectometry (GNSS-R) to remote sensing of the ocean surface has received much attention in recent years. The launch of TechDemoSat-1 and later, the CYGNSS constellation, saw the first dedicated GNSS-R instruments configured as scatterometers flown in space. Processing Delay Doppler Maps (DDMs) to estimate wind speed has been done by averaging the received signal power surrounding the specular point and sometimes over multiple contiguous DDMs. Such averaging results in the smearing of the spatial footprint of the window due to the receivers high velocity in low Earth orbit. Recently, stare processing has been applied to DDMs collected by TechDemoSat-1. This method tracks a fixed point on the surface as it moves through the delay-Doppler domain providing multiple looks at the point at various incidence angles. The stare point is selected so that it lies on the ambiguity free line to improve the achievable resolution of the method. However, as shown in this paper, this results in a sampling of the surface slopes PDF that is unable to resolve both wind speed and direction.
Sensitivity of GNSS-R measurements to wind direction supports the retrieval of wind vector estimates. This has not yet been demonstrated using spaceborne platforms. Whilst sensitivity to wind direction has been demonstrated for airborne platforms, the global coverage provided by spaceborne platforms make them ideal for oceanography applications. The capability of estimating the direction of the wind will support more complicated and better performing applications. Thus, it is of interest to develop processing methods that will facilitate the estimation of wind direction using GNSS-R receivers on spaceborne platforms.
In this paper, it is proposed to track a number of fixed points simultaneously. All but one of these points are chosen so that they do not fall on the ambiguity free line. Whilst degrading the resolution, the ambiguity provides sufficient diversity in the sampling of the surface slopes so that sensitivity to wind direction is achieved. Ambiguities in the wind direction are introduced due the nature of the bivariate uniform distribution used to model the surface slopes. Additional ambiguities are observed when analyzing simulated stare profiles which are found to be heavily dependent on the direction of the receiver velocity, and, for some scattering geometries, are not present. Finally, the impact on the achievable resolution is investigated when combining observations from multiple stare points. It is shown that the achievable resolution is worse when observing multiple stare points but a larger portion of the resolution cell contributes to the measured signals.
Vision based state estimation using a graph-SLAM approach for proximity operations near an asteroid
Arunkumar Rathinam, ACSER PhD Student | International Astronautical Congress 2018, Bremen, Germany
With the growing interest in small body exploration missions, interest in achieving autonomous navigation near small bodies is gradually increasing. This is mainly driven by the high accuracy demands to perform mission critical operations near or on the surface of a small body with a weak gravitational field coupled with dynamic perturbations. To achieve autonomous navigation, the spacecraft must possess good knowledge of its state as well as the asteroid's state, the geometry (map) and dynamic characteristics, and an ability to predict the future states. The problem of navigation and mapping an unknown environment is interdependent and can be solved using the Simultaneous Localization and Mapping (SLAM) framework.
This presentation highlights a factor graph SLAM approach to estimate the trajectory of a spacecraft orbiting a uniformly rotating asteroid and reconstruct the asteroid's shape using distinct landmarks on the asteroid's surface. The algorithm uses a dynamic model of the spacecraft and asteroid to derive the motion factors and a sensor model along with the registered images from the navigation camera to construct the measurement factor. During each key-frame, the factors are assembled in a graph, and iterative optimization methods are used to solve for the maximum-a-posteriori estimates of the spacecraft's pose, asteroid's pose and the landmark positions. Experimental results from simulation suggest that the proposed method can accurately estimate the states and reconstruct the 3D map of the asteroid using the landmark positions.
UAV Reflectometry for Sea State Estimation
Dr Eamonn Glennon, ACSER Research Associate | Australian Space Research Conference 2018, Gold Coast
Satellite navigation (e.g. GPS) signals reflected from the Earth’s surface can be used as a passive radar system to estimate sea state (wave height, wind speed, wind direction), classify land types, detect changes in ground cover and possibly other surveillance tasks such as detecting objects on the open ocean, floods on land and to track environmental incidents such as oil spills. UAV reflectometry technology being developed by Seaskip/UNSW represents a considerable improvement over existing sea condition measurement solutions currently available in terms of both spatial and temporal resolution.
This project aims to develop a working prototype GNSS reflectometry sensor payload for deployment on UAVs.
This project is part of the DMTC’s High Altitude Sensor Systems (HASS) Program, which will enhance Australian defence capability and build industrial capacity in sensor and on-board data processing technology for unmanned aerial systems and small-satellite platforms. See the DMTC website for more information: https://www.dmtc.com.au/our-activities/sensor-systems/