LION Navigator for Transfer to GEO Using Electric Propulsion

GNSS space receivers are widely used for onboard auton-omous navigation of spacecraft platforms in low Earth orbit. Navigation by GNSS up to geosynchronous altitude was made possible through the introduction of a Space Service Volume which defines signal strength up to geo-synchronous altitude. For Galileo, similar definitions are under consideration. On this basis onboard autonomous navigation for commercial communication satellites be-came a realistic possibility, too. Transfer to geostationary orbit is still fully depending on classical RF tracking by ground station for orbit determination. With electrical propulsion, the transfer duration extends to several months. As a consequence onboard autonomous naviga-tion by satellite navigation has become of commercial interest. A GNSS navigation receiver on a spacecraft on transfer orbit has to cope with extreme signal conditions from very low (at perigee) to very high (at super-synchronous apogee) altitude, which is far above the constellation satellites. At this altitude only very rare and weak signals that spill over the limb of the earth can be used. An addi-tional difficulty is the varying spacecraft orientation which is not nadir pointing, as is commonly assumed, but is varying according to the demands of optimal attitude guidance laws and power requirements. By using both GPS and Galileo together the availability of navigation signals is increased. The paper describes the design process to determine basic parameters e.g. number and orientation of receive anten-nas, receiver parameters like C/N0 thresholds, and naviga-tion procedures. Detailed simulations are presented for selected parts of the transfer arc using verified models of the navigation receiver. Finally the geostationary transfer capabilities of the space-borne LION Navigator GNSS receiver are demon-strated in a closed-loop real time test environment under RF stimulation

GNSS transpolar earth reflectometry exploriNg system (G-TERN): mission concept

The global navigation satellite system (GNSS) Transpolar Earth Reflectometry exploriNg system (G-TERN) was proposed in response to ESA's Earth Explorer 9 revised call by a team of 33 multi-disciplinary scientists. The primary objective of the mission is to quantify at high spatio-temporal resolution crucial characteristics, processes and interactions between sea ice, and other Earth system components in order to advance the understanding and prediction of climate change and its impacts on the environment and society. The objective is articulated through three key questions. 1) In a rapidly changing Arctic regime and under the resilient Antarctic sea ice trend, how will highly dynamic forcings and couplings between the various components of the ocean, atmosphere, and cryosphere modify or influence the processes governing the characteristics of the sea ice cover (ice production, growth, deformation, and melt)? 2) What are the impacts of extreme events and feedback mechanisms on sea ice evolution? 3) What are the effects of the cryosphere behaviors, either rapidly changing or resiliently stable, on the global oceanic and atmospheric circulation and mid-latitude extreme events? To contribute answering these questions, G-TERN will measure key parameters of the sea ice, the oceans, and the atmosphere with frequent and dense coverage over polar areas, becoming a “dynamic mapper”of the ice conditions, the ice production, and the loss in multiple time and space scales, and surrounding environment. Over polar areas, the G-TERN will measure sea ice surface elevation (<;10 cm precision), roughness, and polarimetry aspects at 30-km resolution and 3-days full coverage. G-TERN will implement the interferometric GNSS reflectometry concept, from a single satellite in near-polar orbit with capability for 12 simultaneous observations. Unlike currently orbiting GNSS reflectometry missions, the G-TERN uses the full GNSS available bandwidth to improve its ranging measurements. The lifetime would be 2025-2030 or optimally 2025-2035, covering key stages of the transition toward a nearly ice-free Arctic Ocean in summer. This paper describes the mission objectives, it reviews its measurement techniques, summarizes the suggested implementation, and finally, it estimates the expected performance.Peer ReviewedPostprint (published version

On Small Satellites for Oceanography: A Survey

The recent explosive growth of small satellite operations driven primarily from an academic or pedagogical need, has demonstrated the viability of commercial-off-the-shelf technologies in space. They have also leveraged and shown the need for development of compatible sensors primarily aimed for Earth observation tasks including monitoring terrestrial domains, communications and engineering tests. However, one domain that these platforms have not yet made substantial inroads into, is in the ocean sciences. Remote sensing has long been within the repertoire of tools for oceanographers to study dynamic large scale physical phenomena, such as gyres and fronts, bio-geochemical process transport, primary productivity and process studies in the coastal ocean. We argue that the time has come for micro and nano satellites (with mass smaller than 100 kg and 2 to 3 year development times) designed, built, tested and flown by academic departments, for coordinated observations with robotic assets in situ. We do so primarily by surveying SmallSat missions oriented towards ocean observations in the recent past, and in doing so, we update the current knowledge about what is feasible in the rapidly evolving field of platforms and sensors for this domain. We conclude by proposing a set of candidate ocean observing missions with an emphasis on radar-based observations, with a focus on Synthetic Aperture Radar.Comment: 63 pages, 4 figures, 8 table

Satellite Formation-Flying and Rendezvous

GNSS has come to play an increasingly important role in satellite formation-flying and rendezvous applications. In the last decades, the use of GNSS measurements has provided the primary technique for determining the relative position of cooperative co-orbiting satellites in low Earth orbit

Challenges and Perspectives in Lunar Communication and Navigation Systems for Future Cis-lunar Space Exploration Missions

openWith the increasing interest in lunar exploration and potential for human habitation, the need for accurate and reliable navigation systems on the Moon has become more crucial than ever. The absence of a well-established lunar navigation infrastructure poses significant challenges to future space missions, including safe landing, trajectory planning, and autonomous operations. In this context, the development of a Lunar communication and navigation system (LCNS) capable of providing precise and real-time navigation services is of utmost importance. This thesis initially presents European space agency (ESA)’s Moonlight mission, and then goes on to describe various proposed implementations of LCNS, considering its technical feasibility, performance, and potential applications. The first stage involves Global navigation satellite system (GNSS)-only technologies and architectures, then moves into the second one where the proposals are GNSS-aided, and finally in the third one, we move on to Lunar-only implementations, thus totally independent of GNSS. Ultimately, the thesis concludes that while significant challenges exist, innovative solutions can ensure the development of robust communication and navigation systems for future cis-lunar space exploration missions.With the increasing interest in lunar exploration and potential for human habitation, the need for accurate and reliable navigation systems on the Moon has become more crucial than ever. The absence of a well-established lunar navigation infrastructure poses significant challenges to future space missions, including safe landing, trajectory planning, and autonomous operations. In this context, the development of a Lunar communication and navigation system (LCNS) capable of providing precise and real-time navigation services is of utmost importance. This thesis initially presents European space agency (ESA)’s Moonlight mission, and then goes on to describe various proposed implementations of LCNS, considering its technical feasibility, performance, and potential applications. The first stage involves Global navigation satellite system (GNSS)-only technologies and architectures, then moves into the second one where the proposals are GNSS-aided, and finally in the third one, we move on to Lunar-only implementations, thus totally independent of GNSS. Ultimately, the thesis concludes that while significant challenges exist, innovative solutions can ensure the development of robust communication and navigation systems for future cis-lunar space exploration missions

Planning of an Experiment for VLBI Tracking of GNSS Satellites

As a preparation for future possible orbit determination of global navigation satellite system (GNSS) satellites by VLBI observations an initial three-station experiment was planned and performed in January 2009. The goal was to get first experience and to verify the feasibility of using the method for accurate satellite tracking. GNSS orbits related to a satellite constellation can be expressed in the Terrestrial Reference Frame. A comparison with orbit results that might be obtained by VLBI can give valuable information on how the GNSS reference frame and the VLBI reference frame are linked. We present GNSS transmitter specifications and experimental results of the observations of some GLONASS satellites together with evaluations for the expected signal strengths at telescopes. The satellite flux densities detected on the Earth s surface are very high. The narrow bandwidth of the GNSS signal partly compensates for potential problems at the receiving stations, and signal attenuation is necessary. Attempts to correlate recorded data have been performed with different software

Space Object Self-Tracker On-Board Orbit Determination Analysis

Due to the United States\u27 growing dependence on space based assets and the in- creasing number of resident space objects (RSO), improvement of Space Situational Awareness (SSA) capabilities is more necessary than ever. As a way to aid in this need, the Air Force Institute of Technology (AFIT) is developing the Space Object Self-Tracker (SOS) as a proof-of-concept experimental satellite for RSO precision tracking and collision avoidance system in Low Earth Orbit (LEO). Specifically, SOS will use Global Positioning System (GPS) position estimates for on-board orbit de- termination. Currently, SOS will use the Simplified General Perturbations-4 (SGP4) algorithm as its orbit determination algorithm. This research investigates the use of a modified Special Perturbations (SP) orbit determination algorithm as an alternative means for on-board orbit determination (OD) for the SOS experiment. The research is focused on evaluating performance gains and studying the effects of using GPS navigation solutions as the input observation data on the achievable accuracy of the SP algorithm. The SP OD algorithm was evaluated in testing both simulated and real world observation data. The position estimates generated by the SP algorithm from both GPS navigation solution observations and observations delivered in the J2000 inertial frame were analyzed to determine the effects of the SP algorithm\u27s achievable performance. The accuracy of position estimates generated from the SP algorithm were also compared to those generated by SGP4 algorithm. Analysis leads to the conclusion that the SP algorithm will be beneficial in providing more accurate position estimates for observed GPS navigation solutions. However, the SP algorithm will require improvements to the dynamics modeled in the SP algorithm by specifically including more perturbations such as those due to air drag

Interferometric orbit determination system for geosynchronous SAR missions: experimental proof of concept

Future Geosynchronous Synthetic Aperture Radar (GEOSAR) missions will provide permanent monitoring of continental areas of the planet with revisit times of less than 24 h. Several GEOSAR missions have been studied in the USA, Europe, and China with different applications, including water cycle monitoring and early warning of disasters. GEOSAR missions require unprecedented orbit determination precision in order to form focused Synthetic Aperture Radar (SAR) images from Geosynchronous Orbit (GEO). A precise orbit determination technique based on interferometry is proposed, including a proof of concept based on an experimental interferometer using three antennas separated 10–15 m. They provide continuous orbit observations of present communication satellites operating at GEO as illuminators of opportunity. The relative phases measured between the receivers are used to estimate the satellite position. The experimental results prove the interferometer is able to track GEOSAR satellites based on the transmitted signals. This communication demonstrates the consistency and feasibility of the technique in order to foster further research with longer interferometric baselines that provide observables delivering higher orbital precision.This work has been supported by the Spanish Science, Research and Innovation Plan (MICINN) with Project Codes TEC2017-85244-C2-2-P and PID2020-117303GB-C21 and by Unidad de Excelencia Maria de Maeztu MDM-2016-0600 financed by the Agencia Estatal de Investigación, Spain.Peer ReviewedPostprint (published version

Development And Test of A Digitally Steered Antenna Array for The Navigator GPS Receiver

Global Positioning System (GPS)-based navigation has become common for low-Earth orbit spacecraft as the signal environment is similar to that on the Earth s surface. The situation changes abruptly, however, for spacecraft whose orbital altitudes exceed that of the GPS constellation. Visibility is dramatically reduced and signals that are present may be very weak and more susceptible to interference. GPS receivers effective at these altitudes require increased sensitivity, which often requires a high-gain antenna. Pointing such an antenna can pose a challenge. One efficient approach to mitigate these problems is the use of a digitally steered antenna array. Such an antenna can optimally allocate gain toward desired signal sources and away from interferers. This paper presents preliminary results in the development and test of a digitally steered antenna array for the Navigator GPS research program at NASA s Goddard Space Flight Center. In particular, this paper highlights the development of an array and front-end electronics, the development and test of a real-time software GPS receiver, and implementation of three beamforming methods for combining the signals from the array. Additionally, this paper discusses the development of a GPS signal simulator which produces digital samples of the GPS L1C/A signals as they would be received by an arbitrary antenna array configuration. The simulator models transmitter and receiver dynamics, near-far and multipath interference, and has been a critical component in both the development and test of the GPS receiver. The GPS receiver system was tested with real and simulated GPS signals. Preliminary results show that performance improvement was achieved in both the weak signal and interference environments, matching analytical predictions. This paper summarizes our initial findings and discusses the advantages and limitations of the antenna array and the various beamforming methods