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[{"authors":["admin"],"categories":null,"content":"I am a lecturer in Space Geodesy in the Department of Civil, Environmental and Geomatic Engineering (CEGE) at University College London (UCL) with research interests in astrodynamics and space debris dynamics for orbit prediction and orbit determination, and also, in various aspects of global navigation satellite systems (GNSS) and related technologies. If you're still reading (👍) and want to know more, have a look at my CV. I teach Mathematical Modelling and Analysis to first and second year undergraduates in the MEng in Engineering and Architectural Design Programme and contribute to teaching on various modules in CEGE. Please reach out if you're interested in collaborating!\n","date":1626739200,"expirydate":-62135596800,"kind":"taxonomy","lang":"en","lastmod":1636329600,"objectID":"2525497d367e79493fd32b198b28f040","permalink":"https://s-bhattarai.github.io/authors/admin/","publishdate":"0001-01-01T00:00:00Z","relpermalink":"/authors/admin/","section":"authors","summary":"I am a lecturer in Space Geodesy in the Department of Civil, Environmental and Geomatic Engineering (CEGE) at University College London (UCL) with research interests in astrodynamics and space debris dynamics for orbit prediction and orbit determination, and also, in various aspects of global navigation satellite systems (GNSS) and related technologies. If you're still reading (👍) and want to know more, have a look at my CV. I teach Mathematical Modelling and Analysis to first and second year undergraduates in the MEng in Engineering and Architectural Design Programme and contribute to teaching on various modules in CEGE.","tags":null,"title":"Santosh Bhattarai","type":"authors"},{"authors":null,"categories":null,"content":"In an operational sense, satellite clock time offset prediction (SCTOP) is a fundamental requirement in global navigation satellite systems (GNSS) technology. SCTOP uncertainty is a significant component of the uncertainty budget of the basic GNSS pseudorange measurements used in standard (i.e not high-precision), single-receiver applications. In real-time, this prediction uncertainty contributes directly to GNSS-based positioning, navigation and timing (PNT) uncertainty. In short, GNSS performance in intrinsically linked to satellite clock predictability. Now, satellite clock predictability is affected by two factors: (i) the clock itself (i.e. the oscillator, the frequency standard etc.) and (ii) the prediction algorithm. This research focuses on aspects of the latter.\n","date":1461711600,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1461711600,"objectID":"9723547690e3a2bfcc5abe05ad0b6994","permalink":"https://s-bhattarai.github.io/project/phd/","publishdate":"2016-04-27T00:00:00+01:00","relpermalink":"/project/phd/","section":"project","summary":"Investigating and modelling the behaviour of GNSS satellite clocks on-orbit","tags":["phd"],"title":"Satellite Clock Behaviour Modelling","type":"project"},{"authors":null,"categories":null,"content":"","date":1524783600,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1524783600,"objectID":"a021c8ff3535e2f93a16b2c1179b643d","permalink":"https://s-bhattarai.github.io/project/radiation_force_modelling/","publishdate":"2018-04-27T00:00:00+01:00","relpermalink":"/project/radiation_force_modelling/","section":"project","summary":"High-Fidelity, Physics-based Radiation Force Modelling from Earth-orbiting Spacecraft","tags":["pod","radiation-force-modelling","ray-tracing"],"title":"Radiation Force Modelling","type":"project"},{"authors":null,"categories":null,"content":"","date":1524783600,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1524783600,"objectID":"6867f4d1570e0f05bf28a32d9ed90f1a","permalink":"https://s-bhattarai.github.io/project/satremsens_aqm/","publishdate":"2018-04-27T00:00:00+01:00","relpermalink":"/project/satremsens_aqm/","section":"project","summary":"Satellite Remote Sensing focussing on Urban Air Quality Monitoring","tags":[],"title":"Satellite Remote Sensing for Air Quality Monitoring","type":"project"},{"authors":["Santosh Bhattarai"],"categories":["Demo"],"content":"UCL SGNL FSP Visualiser v2021\n","date":1626739200,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1636329600,"objectID":"aa7513cda7566bcb1d9e0a5d03138c17","permalink":"https://s-bhattarai.github.io/post/fsp/","publishdate":"2021-07-20T00:00:00Z","relpermalink":"/post/fsp/","section":"post","summary":"This is a visulisation displaying the results of a Dstl-commisioned study to investigate what the population of man-made objects in space will look like in the +5, +10 and +25-year time horizon.","tags":["Academic"],"title":"Future Space Population Visualiser v2021","type":"post"},{"authors":["S. Bhattarai"],"categories":null,"content":"Version notes ucl_sc_geodesy_v10.html - UCL Science Centre Lecture Series, Autumn 2020, London, UK WebGL presentation slides. Runs on deck.js. Use the \u0026ldquo;m\u0026rdquo; key to open the menu for quick navigation between slides. As a template, the presentation uses the slide deck from David Lyon's \u0026lsquo;Intro to WebGL with Three.js\u0026rsquo; (Slides | Video).\n","date":1561071600,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1561071600,"objectID":"aee928c4c76c7d8caaf9aa0e48177693","permalink":"https://s-bhattarai.github.io/talk/satellite_geodesy/","publishdate":"2019-06-21T00:00:00+01:00","relpermalink":"/talk/satellite_geodesy/","section":"talk","summary":"Introducing (Space) Geodesy to an audience of mainly sixth-form students and their teachers, but everyone is welcome. It's free. Refreshments are provided. Come along!","tags":["geodesy","gnss","gravity"],"title":"Satellite Geodesy: Surveying the Earth from Orbit","type":"talk"},{"authors":["S. Bhattarai","M. Ziebart","S. Allgeier","S. Grey","T. Springer","D. Harrison","Z. Li"],"categories":null,"content":"","date":1560121200,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1560121200,"objectID":"adb43212881795c9f9b8a93de947026b","permalink":"https://s-bhattarai.github.io/publication/gpsiir_rfm/","publishdate":"2019-06-10T00:00:00+01:00","relpermalink":"/publication/gpsiir_rfm/","section":"publication","summary":"This paper presents recently developed strategies for high-fidelity, analytical radiation force modelling for spacecraft. The performance of these modelling strategies is assessed using a new model for the Global Positioning System Block IIR and IIR-M spacecraft. The statistics of various orbit model parameters in a full orbit estimation process that uses tracking data from 100 stations are examined. Over the full year of 2016, considering all Block IIR and IIR-M satellites on orbit, introducing University College London’s grid-based model into the orbit determination process reduces mean 3-d orbit overlap values by 9% and the noise about the mean orbit overlap value by 4%, when comparing against orbits estimated using a simpler box-wing model of the spacecraft. Comparing with orbits produced using the extended Empirical CODE Orbit Model, we see decreases of 4% and 3% in the mean and the noise about the mean of the 3-d orbit overlap statistics, respectively. In orbit predictions over 14-day intervals, over the first day, we see smaller root-mean-square errors in the along-track and cross-track directions, but slightly larger errors in the radial direction. Over the 14th day, we see smaller errors in the radial and cross-track directions, but slightly larger errors in the along-track direction.","tags":["gps","gnss","radiation-force-modelling","pod","srp","ray-tracing"],"title":"Demonstrating developments in high-fidelity analytical radiation force modelling methods for spacecraft with a new model for GPS IIR/IIR-M","type":"publication"},{"authors":["J. A. Momoh","S. Bhattarai","M. Ziebart"],"categories":null,"content":"","date":1550534400,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1550534400,"objectID":"314a0d56de7c5afede4f77f8f8473259","permalink":"https://s-bhattarai.github.io/publication/clk_jmp_cycle_slips/","publishdate":"2019-02-19T00:00:00Z","relpermalink":"/publication/clk_jmp_cycle_slips/","section":"publication","summary":"We introduce a simple single-band receiver clock jump and cycle slip (CJCS) detection and correction algorithm suitable for a standalone single-frequency Global Navigation Satellite System (GNSS) receiver. The real-time algorithm involves using an adaptive time differencing technique for the generation of adaptive differenced sequences of single-frequency code and phase observations. The sequences are used for determining thresholds and for the detection and determination of a receiver clock jump and cycle slips. The cycle slip values are fixed by rounding-up float values obtained via weighted least squares adjustment, following the elimination of the receiver’s high-order clock drift at every epoch. The performance of this new technique was investigated with simulated cycle slip values and with different types of receiver clock jumps at millisecond and microsecond levels. It achieved 100% detection and correction of all types of receiver clock jumps; between 97 to 100% cycle slip detection; and between 96.9 to 100% cycle slip correction including cycle slips of ±1 cycle, for different rates of observations acquired by different fixed and mobile GNSS receivers. The algorithm thus facilitates precise timing and positioning on standalone low-cost single-frequency GNSS devices.","tags":[],"title":"Receiver Clock Jump and Cycle Slip Correction Algorithm for Single-Frequency GNSS","type":"publication"},{"authors":["Z. Li, M. Ziebart, S. Bhattarai, D. Harrison"],"categories":null,"content":"","date":1548979200,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1548979200,"objectID":"35640e16be0df3c0cea8d8be23751772","permalink":"https://s-bhattarai.github.io/publication/shadow_fn/","publishdate":"2019-02-01T00:00:00Z","relpermalink":"/publication/shadow_fn/","section":"publication","summary":"Accurate Solar Radiation Pressure (SRP) modelling is critical for correctly describing the dynamics of satellites. A shadow function is a unitless quantity varying between 0 and 1 to scale the solar radiation flux at a satellite’s location during eclipses. Errors in modelling shadow function lead to inaccuracy in SRP that degrades the orbit quality. Shadow function modelling requires solutions to a geometrical problem (Earth’s oblateness) and a physical problem (atmospheric effects). This study presents a new shadow function model (PPM atm) which uses a perspective projection based approach to solve the geometrical problem rigorously and a linear function to describe the reduction of solar radiation flux due to atmospheric effects. GRACE (Gravity Recovery And Climate Experiment ) satellites carry accelerometers that record variations of non-conservative forces, which reveal the variations of shadow function during eclipses. In this study, the PPM atm is validated using accelerometer observations of the GRACE-A satellite. Test results show that the PPM atm is closer to the variations in accelerometer observations than the widely used SECM (Spherical Earth Conical Model). Taking the accelerometer observations derived shadow function as the “truth”, the relative error in PPM atm is - 0.79 % while the SECM 11.07%. The influence of the PPM atm is also shown in orbit prediction for Galileo satellites. Compared with the SECM, the PPM atm can reduce the radial orbit error RMS by 5.6 cm over a 7-day prediction. The impacts of the errors in shadow function modelling on the orbit remain to be systematic and should be mitigated in long-term orbit prediction.","tags":["radiation-force-modelling","pod","gnss"],"title":"A shadow function model based on perspective projection and atmospheric effect for satellites in eclipse","type":"publication"},{"authors":["S. Bhattarai"],"categories":null,"content":"This presentation was prepared to support a workshop on organisation resilience to global navigation satellite system failures that might arise from severe space weather events. Further details on the project are given on the UCL Institute for Risk and Disaster Reduction's website. The accompanying report is available at UCL Discovery.\n","date":1542931200,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1542931200,"objectID":"b09fa14ca4701b93e9d7ae3adc8fc56a","permalink":"https://s-bhattarai.github.io/talk/space_weather_gnss/","publishdate":"2018-11-23T00:00:00Z","relpermalink":"/talk/space_weather_gnss/","section":"talk","summary":"A talk to raise awareness of the ways in which GNSS can be affected by space weather.","tags":["gnss","resilience","space weather"],"title":"The Vulnerabilities of GNSS to Space Weather","type":"talk"},{"authors":["Z. Li, M. Ziebart, S. Bhattarai, D. Harrison, S. Grey"],"categories":null,"content":"","date":1525129200,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1525129200,"objectID":"eb4bc1416cd598198d65911610b1cbd5","permalink":"https://s-bhattarai.github.io/publication/fast_srp/","publishdate":"2018-05-01T00:00:00+01:00","relpermalink":"/publication/fast_srp/","section":"publication","summary":"Physics based SRP (Solar Radiation Pressure) models using ray tracing methods are powerful tools when modelling the forces on complex real world space vehicles. Currently high resolution (1 mm) ray tracing with secondary intersections is done on high performance computers at UCL (University College London). This study introduces the BVH (Bounding Volume Hierarchy) into the ray tracing approach for physics based SRP modelling and makes it possible to run high resolution analysis on personal computers. The ray tracer is both general and efficient enough to cope with the complex shape of satellites and multiple reflections (three or more, with no upper limit). In this study, the traditional ray tracing technique is introduced in the first place and then the BVH is integrated into the ray tracing. Four aspects of the ray tracer were tested for investigating the performance including runtime, accuracy, the effects of multiple reflections and the effects of pixel array resolution.Test results in runtime on GPS IIR and Galileo IOV (In Orbit Validation) satellites show that the BVH can make the force model computation 30-50 times faster. The ray tracer has an absolute accuracy of several nanonewtons by comparing the test results for spheres and planes with the analytical computations. The multiple reflection effects are investigated both in the intersection number and acceleration on GPS IIR, Galileo IOV and Sentinel-1 spacecraft. Considering the number of intersections, the 3rd reflection can capture 99.12%, 99.14%, and 91.34% of the total reflections for GPS IIR, Galileo IOV satellite bus and the Sentinel-1 spacecraft respectively. In terms of the multiple reflection effects on the acceleration, the secondary reflection effect for Galileo IOV satellite and Sentinel-1 can reach 0.2 nm/s2 and 0.4 nm/s2 respectively. The error percentage in the accelerations magnitude results show that the 3rd reflection should be considered in order to make it less than 0.035%. The pixel array resolution tests show that the dimensions of the components have to be considered when choosing the spacing of the pixel in order not to miss some components of the satellite in ray tracing. This paper presents the first systematic and quantitative study of the secondary and higher order intersection effects. It shows conclusively the effect is non-negligible for certain classes of misson.","tags":["radiation-force-modelling","pod","gnss","ray-tracing","srp"],"title":"Fast solar radiation pressure modelling with ray tracing and multiple reflections","type":"publication"},{"authors":["S. Bhattarai"],"categories":null,"content":"","date":1427842800,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1427842800,"objectID":"422b01dbe09bdfe1673aa0f5029d9baa","permalink":"https://s-bhattarai.github.io/publication/phd_thesis/","publishdate":"2015-04-01T00:00:00+01:00","relpermalink":"/publication/phd_thesis/","section":"publication","summary":"In an operational sense, satellite clock time offset prediction (SCTOP) is a fundamental requirement in global navigation satellite systems (GNSS) technology. SCTOP uncertainty is a significant component of the uncertainty budget of the basic GNSS pseudorange measurements used in standard (i.e not high-precision), single-receiver applications. In real-time, this prediction uncertainty contributes directly to GNSS-based positioning, navigation and timing (PNT) uncertainty. In short, GNSS performance in intrinsically linked to satellite clock predictability. Now, satellite clock predictability is affected by two factors: (i) the clock itself (i.e. the oscillator, the frequency standard etc.) and (ii) the prediction algorithm. This research focuses on aspects of the latter. Using satellite clock data---spanning across several years, corresponding to multiple systems (GPS and GLONASS) and derived from real measurements---this thesis first presents the results of a detailed study into the characteristics of GNSS satellite clocks. This leads onto the development of strategies for modelling and estimating the time-offset of those clocks from system time better, with the final aim of predicting those offsets better. The satellite clock prediction scheme of the International GNSS Service (IGS) is analysed, and the results of this prediction scheme are used to evaluate the performance of new methods developed herein. The research presented in this thesis makes a contribution to knowledge in each of the areas of characterisation, modelling and prediction of GNSS satellite clocks. Regarding characterisation of GNSS satellite clocks, the space-borne clocks of GPS and GLONASS are studied. In terms of frequency stability (and thus predictability) it is generally the case that the GPS clocks out-perform GLONASS clocks at prediction lengths ranging from several minutes up to one day ahead. There are three features in the GPS clocks---linear frequency drift, periodic signals and complex underlying noise processes---that are not observable in the GLONASS clocks. The standard clock model does not capture these features. This study shows that better prediction accuracy can be obtained by an extension to the standard clock model. The results of the characterisation and modelling study are combined in a Kalman filter framework, set up to output satellite clock predictions at a range of prediction intervals. In this part of the study, only GPS satellite clocks are considered. In most, but not all cases, the developed prediction method outperforms the IGS prediction scheme, by between 10% to 30%. The magnitude of the improvement is mainly dependent upon clock type.","tags":[],"title":"Satellite clock time offset prediction in global navigation satellite systems","type":"publication"},{"authors":["J. Davis","S. Bhattarai","M. Ziebart"],"categories":null,"content":"","date":1335481200,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1335481200,"objectID":"adef4fc03c703e2c198a58d046878cf2","permalink":"https://s-bhattarai.github.io/publication/eftf12/","publishdate":"2012-04-27T00:00:00+01:00","relpermalink":"/publication/eftf12/","section":"publication","summary":"An enhanced deterministic model along with a stochastic model for describing clock noise is used to compute predictions of the time-offset of individual GPS satellites from the IGS rapid timescale. These are determined with significantly lower prediction uncertainties than may currently be obtained using the IGS ultra-rapid predictions. At prediction length of one day IGS prediction errors are commonly of the order of several ns for all GPS satellite clocks. In comparison the new techniques offer to limit prediction errors at the order of 1 ns for prediction lengths of one day in the newer generation Block IIR and IIF satellites. The factors contributing to the uncertainties in the IGS predictions are discussed. The application of a Kalman filter based prediction algorithm is shown to produce close to optimal predictions.","tags":[],"title":"Development of a Kalman filter based GPS satellite clock time-offset prediction algorithm","type":"publication"}]