Project

Orekit

Goal: Orekit is a low level space dynamics library written in Java and distributed as Free and Open-Source Software (FOSS) under the permissive Apache V2 license. It has gained widespread recognition since its release in 2008.

Updates
0 new
1
Recommendations
0 new
1
Followers
0 new
32
Reads
1 new
724

Project log

Bryan Cazabonne
added a research item
Earth orbital space suffers from the ever increasing count of space objects, including operational satellites and space debris. Space system operations rely on the management of vast catalogs of objects to avoid any damaging collision. NORAD (North American Aerospace Defense Command) and NASA (National Aeronautics and Space Administration) both maintain a database for a large quantity of orbiting objects. Data are stored as Two Line Elements (TLE) and used along with specific analytical propagation models. Operation centers need Orbit Determination methods to accurately compute conjunctions and collision probabilities. With more and more flying objects, computations must be fast enough to ensure satellite safety. Mixing Orbit Determination and TLE analytical propagation models appears to be an effective way to grant security in space. This paper presents an open-source solution for an Orbit Determination method based on TLE propagation models. The method was implemented and validated inside the Orekit space mechanic library. It was then confronted with a classical numerical Orbit Determination on a GNSS test case.
Bryan Cazabonne
added a research item
The paper presents an open-source orbit determination application based on the Draper Semi-analytical Satellite Theory (DSST) and a recursive filter, the Extended Semi-analytical Kalman Filter (ESKF). The ESKF reconciles the conflicting goal of the DSST perturbation theory (i.e., large step size) and the Extended Kalman Filter (EKF) theory (i.e., re initialization at each measurement epoch). Validation of the Orekit ESKF is demonstrated using simulated data. Both the satellite’s state vector estimation and the measurement residuals are used as comparison metrics.
Bryan Cazabonne
added a research item
Earth orbital space suffers from the ever increasing count of space objects, including operational satellites and space debris. Space system operations rely on the management of vast catalogs of objects to avoid any damaging collision. NORAD (North American Aerospace Defense Command) and NASA (National Aeronautics and Space Administration) both maintain a database for a large quantity of orbiting objects. Data are stored as Two Line Elements (TLE) and used along with specific analytical propagation models. Operation centers need Orbit Determination methods to accurately compute conjunctions and collision probabilities. With more and more flying objects, computations must be fast enough to ensure satellite safety. Mixing Orbit Determination and TLE analytical propagation models appears to be an effective way to grant security in space. This paper presents an open-source solution for an Orbit Determination method based on TLE propagation models. The method was implemented and validated inside the Orekit space mechanic library. It was then confronted with a classical numerical Orbit Determination on a GNSS test case.
Bryan Cazabonne
added a research item
Space agencies generally use numerical methods to meet their orbit determination needs. Due to the ever increasing number of space objects, the development of new orbit determination methods becomes essential. DSST is an orbit propagator based on a semi-analytical theory. It combines the accuracy of numerical propagation and the speed of analytical propagation. The paper presents an open-source DSST orbit determination application included in the Orekit library. Accuracy of the DSST orbit determination is demonstrated by comparison with a numerical method. Both the satellite's state vector estimation and the measurement residuals are used as comparison metrics.
Luc Maisonobe
added a research item
Orbit Determination is a technique used to estimate the position of a satellite from its observable measurements. Missing or incorrect modeling of troposphere and ionosphere delays is one of the major error source in space geodetic techniques such as Global Navigation Satellite Systems (GNSS). Accurate computation of these two delays is a mandatory step to cope with accuracy needs which are close to centimeter or millimeter levels. This paper presents the different steps of development of estimated tropospheric and ionospheric models. All these models are included in the Orekit open-source space flight dynamics library. Adding estimated tropospheric and ionospheric models into an orbit determination process can be a difficult procedure. Computing and validating measurement derivatives with respect to troposphere and ionosphere parameters are critical steps. To cope with this constraint, we used the Automatic Differentiation technique to avoid the calculation of the derivatives of long equations. Automatic Differentiation is equivalent to calculating the derivatives by applying chain rule without expressing the analytical formulas. Therefore, Automatic Differentiation allows a simpler computation of the derivatives and a simpler validation. This paper presents how the Jacobian measurement matrix is computed by Automatic Differentiation. It also describes the impact of using estimated tropospheric and ionospheric models. Finally, a study of different model configurations is performed in order to highlight the relevant tropospheric and ionospheric parameters to estimate. The performance of the different models is demonstrated under GPS orbit determination conditions. Both satellite state vector estimation and measurement residuals quality are used as indicator to quantify the orbit determination performance. This paper addresses that estimated tropospheric and ionospheric models are actually more accurate than empirical models to estimate satellite state vector in GNSS orbit determination. A gain of about 60% is obtained on the estimation of the satellite position when estimated models are used, without altering the computation time.
Luc Maisonobe
added 4 research items
This paper presents a new way to deal with transition matrices handling in variational equations. While working simultaneously on several problems dealing with orbit propagation: low-thrust trajectories and orbit determination, we implemented a feature allowing to add user equations to a propagator in order to solve the first problem. Then it appeared that this feature could be reused to deal with the second one. Indeed, the orbit determination problem is based on variational equations involving transition matrices, which can also be considered as additional parameters, propagated at the same time as the original state vector. This method allows a very modular implementation of both problems, with looser coupling in the equations. It has been successfully integrated in the Orekit open-source library.
Space objects catalog maintenance demands an accurate and fast Orbit Determination (OD) process to cope with the ever increasing number of observed space objects. The development of new methods, that answer the two previous problems, becomes essential. Presented as an alternative to numerical and analytical methods, the Draper Semi-analytical Satellite Theory (DSST) is an orbit propagator based on a semi-analytical theory allowing to preserve the accuracy of a numerical method while providing the speed of an analytical method. This propagator allows computing the mean elements and the short-period effects separately. We reproduced this architecture at the OD process level in order to be able to return, as desired, the mean elements or the osculating elements. Two major use cases are thus possible: fast OD for big space objects catalog maintenance and mean elements OD for station keeping needs. This paper presents the different steps of development of the DSST-OD included in the Orekit open-source library [1]. Integrating an orbit propagator into an OD process can be a difficult process. Computing and validating derivatives is a critical step, especially with the DSST whose equations are very complex. To cope with this constraint, we used the automatic differentiation technique. Automatic differentiation has been developed as a mathematical tool to avoid the calculations of the derivatives of long equations. This is equivalent to calculating the derivatives by applying chain rule without expressing the analytical formulas. Thus, automatic differentiation allows a simpler computation of the derivatives and a simpler validation. Automatic differentiation is also used in Orekit for the propagation of the uncertainties using the Taylor algebra. Existing OD applications based on semi-analytical theories calculate only the derivatives of the mean elements. However , for higher accuracy or if the force models require further development, adding short-period derivatives improves the results. Therefore, our study implemented the full contribution of the short-period derivatives, for all the force models, in the OD process. Nevertheless, it is still possible to choose between using the mean elements or the osculating elements derivatives for the OD. This paper will present how the Jacobians of the mean rates and the short-periodic terms are calculated by automatic differentiation into the DSST-specific force models. It will also present the computation of the state transition matrices during propagation. The performance of the DSST-OD is demonstrated under Lageos2 and GPS Orbit Determination conditions.
Romain Di-Costanzo
added an update
Project goal
Orekit is a low level space dynamics library written in Java and distributed as Free and Open-Source Software (FOSS) under the permissive Apache V2 license. It has gained widespread recognition since its release in 2008.
Background and motivation
I've been working on Orekit by implementing part of the semi-analytic orbit extrapolator (DSST - Draper Semi-analytical Satellite Theory)
 
Luc Maisonobe
added 2 research items
This paper presents a new way to deal with attitude modeling. It is designed to be able to handle highly accurate models with very complex settings while remaining simple to use, even for non attitude specialists. The representation is split in several layers, for greater flexibility and thereby provides an extensive range of attitude profiles. The core layer is a basic attitude mode similar to what can be found in many attitude simulation tools. Additional layers can be stacked to it, so that additional constraints can be taken into account, and modify the basic attitude (offsets, spin, …) It is also possible to set up an attitude sequence containing several modes linked together, in which switching from one mode to the next is events-based. Events are triggered automatically and their occurring time is computed on the fly during simulation: the user does not need to know in advance when they are supposed to occur. Numerous predefined events exist, like eclipse entry/exit, field of view entry/exit, orbital events, etc.
Verification of the java Orekit implementation of the Draper Semi-analytical Satellite Theory (DSST) is discussed. The Orekit library for space flight dynamics has been published under the open-source Apache license V2. The DSST is unique among analytical and semi-analytical satellite theories due to the scope of the included force models. However, the DSST has not been readily accessible to the wider Astrodynamics research community. Implementation of the DSST in the Orekit library is a comprehensive task because it involves the migration of the DSST to the object-oriented java language and to a different functional decomposition strategy. The resolution of the code and documentation anomalies discovered during the verification process is the important product of this project.
Luc Maisonobe
added 2 research items
Semi-analytical propagation techniques are of great interest, as they aim to combine the accuracy of numerical techniques with the speed of analytical methods. Amongst them, the Draper Semi-Analytical Satellite Theory (DSST) stands out, with its extensive treatment of perturbations, its exibility and also its maturity. Unfortunately DSST was for long unavailable for the wider Astrodynamics community. Aiming to �ll this gap, the Orekit space ight library provides an open-source implementation of the DSST theory, with a migration from a procedurally-oriented to an object-oriented paradigm. The purpose of this paper is to discuss the validation process of this implementation in the Java language. Orekit DSST was validated against Fortran 77 DSST Standalone Orbit Propagator, developed by Paul Cefola and his colleagues. The validation has so far been focusing on mean elements equations. Particular topics of interests are: validation of complex force models such as high order central body potential, including resonance e�ects; validation over long time span, with a few orbit decay test cases. Results show great consistency between the two implementations. Thanks to this validation process, the main remaining errors in Orekit DSST were identi�ed and corrected. Now the di�erences on slow variables are low and show almost no drift, even for propagations lasting 20 years. All perturbation contributions implemented in Orekit are validated for the mean elements. This includes: zonal, tesseral geopotential and 3rd body potential for the analytically averaged contributions, atmospheric drag and solar radiation pressure for the numerically averaged contributions. This study and the development of the Java implementation were performed under contract with the European Space Agency.
Open source software tools have been gaining acceptance in the astrodynamics community for some applications, though heritage tools still dominate precision orbit determination and propagation. This paper examines recent tide modeling improvements in the open source Orbit Extrapolation Toolkit (Orekit) and compares it with the US Naval Research Laboratory's (NRL) heritage Orbit Covariance Estimation And ANalysis (OCEAN) system. First, the two tools are compared directly against each other by propagating a given state vector for Stella, a geodetic satellite sensitive to tidal variations in the geopotential. Second, orbits were fit to International Laser Ranging Service (ILRS) laser ranging data using OCEAN and orbit determination software built around Orekit so that a more useful comparison could be made. Five days of data were used to solve for orbital parameters using OCEAN and Orekit. This solution orbit is then propagated forward 25 days and compared to subsequent five day orbit solutions. This comparison between predicted and fitted orbit solutions is used as a metric to compare the quality of each piece of software's dynamic modeling capability. Results from the direct orbit propagation comparison indicate the RSS of postion difference between the OCEAN and Orekit propagated orbit grow to only 7 meters over 25 days. It is also seen that the difference between OCEAN's and Orekit's implementation of Earth tides are less than 3% of the total tidal effect. The results of the orbit determination analysis show that the Orekit orbit solution comparison is at worst on the same order of magnitude in accuracy as the OCEAN orbit solution comparison, and at best more accuate than the OCEAN orbit solution comparison. While OCEAN produces a more accurate orbit prediction than Orekit in the majority of the cases studied, more testing is need to understand the origin of the difference.
Luc Maisonobe
added 3 research items
Among existing orbital propagation techniques, semi-analytical methods are of great interest: by separating the computation of long-term evolution from one side and the short-term variations from the other side, they tend to be significantly faster than classical numerical methods while keeping similar accuracy. The implementation of the Draper Semi-analytical Satellite Theory (DSST) into the free OREKIT library offers to the astrodynamics community one of the most mature and versatile semi-analytical propagator. This paper focuses on the validation process of this implementation covering a large panel of orbits, from LEO to HEO. Comparison with legacy Fortran 77 software demonstrated a great consistency.
Luc Maisonobe
added a research item
Orekit is a library for space flight dynamics. It was released under an open-source license in 2008 and has since gained widespread recognition. It has already been used operationally, it has been selected as the basis of new generation systems in agencies, it has been used for several studies and ground systems developments by various industrial actors, and it is used for training purposes at universities. The project has gone through several phases, becoming more and more open at each step. During the first phase, Orekit started as a closed-source product. During the second phase (2008), Orekit switched to a permissive open-source license (Apache License V2) in 2008. The Orekit third phase started in early 2011. The third phase included a collaborative site, with direct public visibility of the development version control system, issue tracker, mailing lists, blog and wiki. The third phase culminated in late 2011 when the first external committer was nominated and gained write access to the source code repository. The Orekit project is now entering its fourth phase, with a completely open governance model, involving representatives from different space field actors in a Project Management Committee (PMC). The Orekit governance model follows the Apache Software Foundation meritocracy. This model is based on several roles (user, contributor, committer, Project Management Committee member and PMC chair). This paper explains the various roles and the rights that are bound to them. Everyone can go through the various roles. The rules to get access to the various roles are explained in the paper. They are based on merit previously earned within the project. Merit is gained through contributions and involvement. The first PMC of the Orekit project is officially set up as the 5 th ICATT conference is taking place. It is composed of people with different affiliations in order to meet the needs of the widest possible range of users. There are representatives of spacecraft manufacturers, academics (both European and US), satellite operators, software industry and independent experts. Representatives of space agencies are expected to join the PMC soon. This PMC will be in charge of defining the roadmap for the future evolution of the Orekit project. The rules for evolution of the PMC are explained in the paper. The role of the PMC is mainly to define global orientation. The technical low level decisions are still taken by the members of the development list where everyone can contribute to discussions, regardless of affiliation and whether they are or not members of the PMC. Orekit is a community oriented project.
Luc Maisonobe
added a project goal
Orekit is a low level space dynamics library written in Java and distributed as Free and Open-Source Software (FOSS) under the permissive Apache V2 license. It has gained widespread recognition since its release in 2008.
 
Luc Maisonobe
added a research item
The Orekit library is a space flight dynamics library developed since 2002 by CS. It was operationally used during the Jules Verne ATV mission. Since 2008, the library is freely available as an open-source product under the terms of the business-friendly Apache V2.0 license. Given the small size of the market and the still high need for advanced tailored solutions for space systems, the service based business model for added value is much more suited than the license based business model or its new version, the Software As a Service. There are several business models that can be used for economically sound open-source systems. Some are well suited for the space field and will be explained. Some are not adapted and the reasons for this will also been explained. Open-source is an approach that proved efficient in mainstream software industry. It does not always need a very large community as was once thought, but still needs some involvement. The return on investment increases for all contributors as the project expands and the risks decrease at the same time as more and more people use it. The model is attractive for both public entities, academics, industry and SMEs, bringing something to each one of them. It also increases the yield of public funding. OREKIT is an example of a successful open-source project initiated by private industry and operationally during ATV rendezvous. Since its inception, the OREKIT library was aimed both towards quick development for simple use cases and towards fine tuning for expert users. In order to fulfill the first goal, the programming interface provides high level features like attitude modes, automatic discrete events handling within propagation (ground station visibilities, eclipses, maneuver start/stop, altitude crossing, user defined event …), transparent handling of leap seconds, automatic transforms between all frames, transparent use of Earth Orientation Parameters and much more. In order to fulfill the second goal, several physical models are provided for many concepts (orbits, propagators, frames, events, attitudes, time scales, ephemerides …) and all of them can be extended naturally to add user specific models or change the behavior of the provided ones. Since many models offer similar user interfaces, it is possible to build applications that can be used both in a mission analysis configuration with fast models and in an operational configuration with accurate models with a single switch. The presentation will provide both a business view and a technical view of OREKIT. It will explain the benefits of open-source and business models. It will present an overview of the library features and available physical models. A focus on a few innovative concepts will be made, like for example discrete events, time scale handling, slave and master propagation modes, management of time-dependent frames, models switching or transparent handling of complex models that need lots of configuration data. Some examples of how the tool can be used in different operational contexts will be given. The roadmap for the future of the tool will be presented.