This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The primary objective of this work is to create a highly accurate sensitivity analysis tool for m... more The primary objective of this work is to create a highly accurate sensitivity analysis tool for multi-stage launch vehicle trajectories. This tool is designed to assess the impact of various parameters on the trajectory and performance of multi-stage launch vehicles. To achieve this, we have developed high-fidelity simulation software that considers all translational and angular movements by modelling the six-degrees-of-freedom (6DOF) equations of motion. The validation of this software is based on experimental data. An essential aspect of this work is the utilization of the developed sensitivity analysis tool to determine how different parameters affect the trajectory of multi-stage launch vehicles. Through the sensitivity analysis conducted using the developed tool, it is possible to identify which parameters are of critical importance during the design phase. We apply a generic mission profile for the Minotaur-I launcher to obtain parametric dependencies of the flight path. Through a comprehensive parametric study, we evaluate a range of critical parameters, including gross lift-off weight, a specific impulse of each stage, pitch-over manoeuvre initial time and angle, and ignition impulse of each stage. These parameters significantly influence the trajectory, performance, and reliability of the launch vehicle for mission design and success. The results of the sensitivity analysis underscore that even minor variations in these parameters can result in substantial deviations from the nominal insertion altitude. The acceptability of errors in specific impulse changes varies across stages, with maximum changes of 6.07%, and the fourth stage showing less sensitivity at 0.13%. However, it's important to note that variations in the parameters of the first stage tend to be challenging to rectify once they occur, with maximum changes in specific impulses reaching 75.57%. Another noteworthy discovery is that the acceptability of changes in pitch-over manoeuvre initiation times depends on the rate of change; they can be deemed either acceptable or unacceptable based on this factor, with changes ranging between 17.90% and 98.38%.
2019 9th International Conference on Recent Advances in Space Technologies (RAST)
Due to the launch safety constraints placed on Space companies built microsatellites, designing a... more Due to the launch safety constraints placed on Space companies built microsatellites, designing a simple and reliable propulsion system with performance and characteristics in line with the typical requirements of microsatellites presents a significant challenge to aspiring engineers. This paper presents the design process of a low-cost, propulsion system with a two-phase high density propellant. The aim of this study is the preliminary design of a 50mN RESISTOJET system operating at low pressure. The preliminary design and the optimization of the thruster dimensions are presented.
The ixRD is the primary science payload on Sharjah-Sat-1, a 3U CubeSat expected to be launched in... more The ixRD is the primary science payload on Sharjah-Sat-1, a 3U CubeSat expected to be launched in Q4, 2022. Its main scientific goal is monitoring bright hard X-ray sources and transients in 20 - 200 keV band. The iXRD consists of a CdZnTe crystal (6.45 cm2 area, 5 mm thickness), a Tungsten collimator with square holes with an opening angle of 4.26o, readout and control electronics and power circuitry, a back-shield and mechanical structures. Some of the design elements of iXRD have been inherited from the XRD on BeEagleSat with significant improvements in terms of collecting area, X-ray background and electronic noise. In this article, the design of the iXRD is discussed in detail taking into account mechanical, electronics, control software and data handling aspects. Its expected performance is determined after ground calibration. Depending on the pixel size, the energy resolution is 4 - 7 keV at 60 keV and the minimum detectable energy is 19 - 23 keV.
During the preliminary phase of a satellite system design, a complete model of the satellite may ... more During the preliminary phase of a satellite system design, a complete model of the satellite may not be available due to the uncertainties in the design. As a remedy, a simple methodology based on system engineering approach may be used to rapidly determine pointing requirements to initiate requirement analysis phase of the satellite design. For this purpose, the effects of the pointing accuracy and stability requirements on the spaceborne image acquisition are identified for the push-broom sensor applications. Further, image guidance algorithms are identified to obtain proper attitude profiles which are used in line of sight analysis. To determine the relationship between the geometrical properties/requirements of the images and the pointing requirements, line of sight analysis is carried out and implications for the selection of the specifications of the electro-optic payload equipment are discussed. Moreover, a simplified mathematical model is established in order to determine satellite stability requirement related to the scan speed of the electro-optic payload equipment. The method is applied to an Earth observation satellite with a push-broom electro-optic sensor which has a sub-meter level ground sampling distance. The presented analysis should also be of interest to optical Earth observation satellite system mission analysis and general satellite design, particularly attitude and orbit control sub-system design.
Three dimensional parabolized Navier-Stokes and potential flow solvers have been developed to obt... more Three dimensional parabolized Navier-Stokes and potential flow solvers have been developed to obtain numerical results for cascade and ducts. The axisymetric Ni bump channel, the 2-D supersonic wedge cascade and the Squire and Winter 90 0 bend chatmel, have been used as test cases. The subsonic PNS scheme solves the non-conservative set of governing equation using the line successive over-relaxation technique, the Poisson equation is also used in evaluating the cross flow velocity correction and the pressure field. The supersonic PNS scheme is a space marching, conservative, non-iterative, implicit finite difference analog to the time marching method of Schiff and Steger. The full potential solver used to calculate cascade flow field solution is a non-linear, 2nd order accurate, non-conservative finite difference scheme. Computations illnstrate the established flow symmetry in a subsonic Ni-bump channel, the shock-boundary interaction in supersonic Ni-challllel and wedge cascades and finally, the secondary flow within a 90 0 bend channel. The results from each solver were compared and agreements as well as disagreements have heen commented on.
zet TÜRKSAT 3USAT uydusu, İstanbul Teknik Üniversitesi öğrencileri tarafından TÜRKSAT için gelişt... more zet TÜRKSAT 3USAT uydusu, İstanbul Teknik Üniversitesi öğrencileri tarafından TÜRKSAT için geliştirilen ve Türkiye'nin ilk yerli haberleşme uydusu olma özelliğini taşıyan bir 3U küp uydudur. Uydunun alt sistemlerinin çalışabilmesi için gerekli elektrik enerjisinin karşılanması amacıyla İTÜ Uzay Sistemleri Tasarım ve Test Laboratuarı (USTTL) bünyesinde Elektrik Güç Sistemi (EGS)tasarlanmıştır. Bu bildiride tasarım sürecinde yapılan çalışmalar ve üretilen deneme sistemlerinin sonuçları hakkında detaylı bilgi verilecektir. GİRİŞ TURKSAT 3USAT uydusunda haberleşme sistemi(HBS), elektrik güç sistemi(EGS), yönelim belirleme kontrol sistemi(YBKS), uydu bilgisayarı ve yörünge düşürme sistemi(YDS) olmak üzere yedekleriyle birlikte 5 temel sistem bulunmaktadır. Bu temel sistemler dışında uydu üzerinde güneş panelleri, algılayıcılar, görüntüleme sistemi, anten açılma sistemi, süper kapasiteler gibi alt birimler bulunmaktadır. TURKSAT3USAT uydusunun yapım amacı sadece yerli bir haberleşme u...
European Framework 7 (FP7) kapsamında oluşturulan QB50 projesinde İstanbul teknik Üniversitesi ve... more European Framework 7 (FP7) kapsamında oluşturulan QB50 projesinde İstanbul teknik Üniversitesi ve Hava Harp Akademisi tarafından geliştirilen BeEagleSat küp uydusu yer alacaktır. QB50 projesi yaklaşık olarak 50 küp uydunun birlikte alçak yörüngeye (340 ila 400 km arası) gönderilerek, eşzamanlı atmosferik ölçümler yapılmasını amaçlamaktadır. 40 tanesi 2 birimlik (2U -10x10x20cm ve 2kg) ve 10 tanesi 3 birimlik (3U -10x10x30cm ve3kg) olmak üzere üzerlerinde bilimsel faydalı yük taşıyan küp uydular olacaklardır. Sabancı Üniversitesi ve İstanbul Teknik Üniversitesi tarafından geliştirilen ek bilimsel faydalı yük de projede yer almaktadır. BeEagleSat uydusunda belirli alt sistemler, ülkede uydu projeleri ve alt sistemleri üzerine yetkinlik ve tecrübe kazanılması amacıyla yurt içinde geliştirilecektir. Uydudaki diğer alt sistemler de hazır alınarak tüm sisteme entegre edilecektir.
A 3-Unit CubeSat, called "3USAT", is developed for voice communication in Low Earth Orb... more A 3-Unit CubeSat, called "3USAT", is developed for voice communication in Low Earth Orbit (LEO). The spacecraft was launched on 26 th of April 2013 from China Jiaquan Satellite Launch centre with a LM2D launch vehicle. The main payload of the 3USAT is a linear transponder operating in VHF/UHF. The 3USAT includes many sensors and a camera for taking pictures, as well. Most subsystems of the CubeSat have redundancy which is provided by COTS equipment and/or in-house developed systems. The two transponders are developed genuinely by the project team. The project team have gained valuable experience from developing subsystems, integrating COTS equipment's, and carrying out space simulation testing. The payload power requirements resulted in temperatures exceeding 100 o C. A remedy was developed to remove heat resulting from high power amplifier. Other problems encountered and resolved include grounding issues of electronic and RF systems, the transfer of data and power usi...
2013 6th International Conference on Recent Advances in Space Technologies (RAST), 2013
TURKSAT-3USAT is a three unit CubeSat developed for voice communication at low Earth orbit. The p... more TURKSAT-3USAT is a three unit CubeSat developed for voice communication at low Earth orbit. The payload is a VHFIUHF transponder operating at amateur bands. The transponder and all other subsystems, except the stabilization, are doubled for redundancy. Where possible, both COTS systems and in-house development is employed. The energy is provided using lithium polymer batteries together with super capacitors. Satellite stabilization is accomplished using passive magnetic attitude control system with hysteresis rods. The TURKSAT-3USAT is launched from JSLC of China, on April 26, 2013.
Proceedings of 5th International Conference on Recent Advances in Space Technologies - RAST2011, 2011
Nan o satellite structures have limited communication ability due to limited power and size. Most... more Nan o satellite structures have limited communication ability due to limited power and size. Most of satellite includes sensors, for instance camera, also image transfer requires more data transfer speed. In this project, 2.4 GHz (S Band) communication system is designed for LEO satellites (700-900 km). System includes system-on-chip transmitter device, preamplifier, band pass filter, power amplifier and microstrip antenna
A three unit CubeSat called TURKSAT-3USAT is being developed for voice communication at low earth... more A three unit CubeSat called TURKSAT-3USAT is being developed for voice communication at low earth orbit. The payload consists of two VHF/UHF transponders operating at amateur bands. All other subsystems are also doubled for redundancy. Where possible, both ...
A highly efficient, redundant electrical power system is developed in Istanbul Technical Universi... more A highly efficient, redundant electrical power system is developed in Istanbul Technical University (ITU) Space Systems Design and Testing Laboratory (SSDTL). A Hardware-in-the-loop (HIL) simulation platform is designed for testing, calibration and qualification procedures of the power system. The system is based on a MATLAB/Simulink model of space qualified solar cells. A DC/DC converter generates required power for any given load depending on the solar cell model. An interface drives the converter by the information from the solar cell model. The system is connected to developed power system, in order to measure its performance. This paper explains the methods, trade-offs and the results of the project.
HAWELSAT is a 2U CubeSat developed by HAVELSAN and Istanbul Technical University (ITU) as a part ... more HAWELSAT is a 2U CubeSat developed by HAVELSAN and Istanbul Technical University (ITU) as a part of QB50 Project [1] in Turkey. During its mission, HAVELSAT will collect electron density information in its orbit and transfer this data to the ground station (GS). In QB50 program, the aim is to use all collected e-density data from all satellites to get better thermosphere models. In this article, the developed on-board software (SW) and the development phases will be briefly explained. Moreover, a summary of difficulties faced during these phases and also lessons learnt are summarized.
In this work, within the context of university, aerospace industry and SME collaboration a 3U Cub... more In this work, within the context of university, aerospace industry and SME collaboration a 3U CubeSat called ASELSAT with Ground Sample Distance of at least 30 meter payload and X-Band downlink system is designed and developed. The high power requirement of X Band transmitter and packing off all subsystems within the 3U volume are main challenges. ASELSAT will house the X-Band transmitter and patch antenna developed by ASELSAN, the NanoCam of GomSpace, 3 wheel ADCS of CubeSpace, and on board computer and communication system by ErTek among others. A 3U structure called NART Structure is specifically developed for ASELSAT by Gumush Space Ltd. Key Words: Aselsat, Cubesat, X-Band Transmitter, Camera, ADCS
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The primary objective of this work is to create a highly accurate sensitivity analysis tool for m... more The primary objective of this work is to create a highly accurate sensitivity analysis tool for multi-stage launch vehicle trajectories. This tool is designed to assess the impact of various parameters on the trajectory and performance of multi-stage launch vehicles. To achieve this, we have developed high-fidelity simulation software that considers all translational and angular movements by modelling the six-degrees-of-freedom (6DOF) equations of motion. The validation of this software is based on experimental data. An essential aspect of this work is the utilization of the developed sensitivity analysis tool to determine how different parameters affect the trajectory of multi-stage launch vehicles. Through the sensitivity analysis conducted using the developed tool, it is possible to identify which parameters are of critical importance during the design phase. We apply a generic mission profile for the Minotaur-I launcher to obtain parametric dependencies of the flight path. Through a comprehensive parametric study, we evaluate a range of critical parameters, including gross lift-off weight, a specific impulse of each stage, pitch-over manoeuvre initial time and angle, and ignition impulse of each stage. These parameters significantly influence the trajectory, performance, and reliability of the launch vehicle for mission design and success. The results of the sensitivity analysis underscore that even minor variations in these parameters can result in substantial deviations from the nominal insertion altitude. The acceptability of errors in specific impulse changes varies across stages, with maximum changes of 6.07%, and the fourth stage showing less sensitivity at 0.13%. However, it's important to note that variations in the parameters of the first stage tend to be challenging to rectify once they occur, with maximum changes in specific impulses reaching 75.57%. Another noteworthy discovery is that the acceptability of changes in pitch-over manoeuvre initiation times depends on the rate of change; they can be deemed either acceptable or unacceptable based on this factor, with changes ranging between 17.90% and 98.38%.
2019 9th International Conference on Recent Advances in Space Technologies (RAST)
Due to the launch safety constraints placed on Space companies built microsatellites, designing a... more Due to the launch safety constraints placed on Space companies built microsatellites, designing a simple and reliable propulsion system with performance and characteristics in line with the typical requirements of microsatellites presents a significant challenge to aspiring engineers. This paper presents the design process of a low-cost, propulsion system with a two-phase high density propellant. The aim of this study is the preliminary design of a 50mN RESISTOJET system operating at low pressure. The preliminary design and the optimization of the thruster dimensions are presented.
The ixRD is the primary science payload on Sharjah-Sat-1, a 3U CubeSat expected to be launched in... more The ixRD is the primary science payload on Sharjah-Sat-1, a 3U CubeSat expected to be launched in Q4, 2022. Its main scientific goal is monitoring bright hard X-ray sources and transients in 20 - 200 keV band. The iXRD consists of a CdZnTe crystal (6.45 cm2 area, 5 mm thickness), a Tungsten collimator with square holes with an opening angle of 4.26o, readout and control electronics and power circuitry, a back-shield and mechanical structures. Some of the design elements of iXRD have been inherited from the XRD on BeEagleSat with significant improvements in terms of collecting area, X-ray background and electronic noise. In this article, the design of the iXRD is discussed in detail taking into account mechanical, electronics, control software and data handling aspects. Its expected performance is determined after ground calibration. Depending on the pixel size, the energy resolution is 4 - 7 keV at 60 keV and the minimum detectable energy is 19 - 23 keV.
During the preliminary phase of a satellite system design, a complete model of the satellite may ... more During the preliminary phase of a satellite system design, a complete model of the satellite may not be available due to the uncertainties in the design. As a remedy, a simple methodology based on system engineering approach may be used to rapidly determine pointing requirements to initiate requirement analysis phase of the satellite design. For this purpose, the effects of the pointing accuracy and stability requirements on the spaceborne image acquisition are identified for the push-broom sensor applications. Further, image guidance algorithms are identified to obtain proper attitude profiles which are used in line of sight analysis. To determine the relationship between the geometrical properties/requirements of the images and the pointing requirements, line of sight analysis is carried out and implications for the selection of the specifications of the electro-optic payload equipment are discussed. Moreover, a simplified mathematical model is established in order to determine satellite stability requirement related to the scan speed of the electro-optic payload equipment. The method is applied to an Earth observation satellite with a push-broom electro-optic sensor which has a sub-meter level ground sampling distance. The presented analysis should also be of interest to optical Earth observation satellite system mission analysis and general satellite design, particularly attitude and orbit control sub-system design.
Three dimensional parabolized Navier-Stokes and potential flow solvers have been developed to obt... more Three dimensional parabolized Navier-Stokes and potential flow solvers have been developed to obtain numerical results for cascade and ducts. The axisymetric Ni bump channel, the 2-D supersonic wedge cascade and the Squire and Winter 90 0 bend chatmel, have been used as test cases. The subsonic PNS scheme solves the non-conservative set of governing equation using the line successive over-relaxation technique, the Poisson equation is also used in evaluating the cross flow velocity correction and the pressure field. The supersonic PNS scheme is a space marching, conservative, non-iterative, implicit finite difference analog to the time marching method of Schiff and Steger. The full potential solver used to calculate cascade flow field solution is a non-linear, 2nd order accurate, non-conservative finite difference scheme. Computations illnstrate the established flow symmetry in a subsonic Ni-bump channel, the shock-boundary interaction in supersonic Ni-challllel and wedge cascades and finally, the secondary flow within a 90 0 bend channel. The results from each solver were compared and agreements as well as disagreements have heen commented on.
zet TÜRKSAT 3USAT uydusu, İstanbul Teknik Üniversitesi öğrencileri tarafından TÜRKSAT için gelişt... more zet TÜRKSAT 3USAT uydusu, İstanbul Teknik Üniversitesi öğrencileri tarafından TÜRKSAT için geliştirilen ve Türkiye'nin ilk yerli haberleşme uydusu olma özelliğini taşıyan bir 3U küp uydudur. Uydunun alt sistemlerinin çalışabilmesi için gerekli elektrik enerjisinin karşılanması amacıyla İTÜ Uzay Sistemleri Tasarım ve Test Laboratuarı (USTTL) bünyesinde Elektrik Güç Sistemi (EGS)tasarlanmıştır. Bu bildiride tasarım sürecinde yapılan çalışmalar ve üretilen deneme sistemlerinin sonuçları hakkında detaylı bilgi verilecektir. GİRİŞ TURKSAT 3USAT uydusunda haberleşme sistemi(HBS), elektrik güç sistemi(EGS), yönelim belirleme kontrol sistemi(YBKS), uydu bilgisayarı ve yörünge düşürme sistemi(YDS) olmak üzere yedekleriyle birlikte 5 temel sistem bulunmaktadır. Bu temel sistemler dışında uydu üzerinde güneş panelleri, algılayıcılar, görüntüleme sistemi, anten açılma sistemi, süper kapasiteler gibi alt birimler bulunmaktadır. TURKSAT3USAT uydusunun yapım amacı sadece yerli bir haberleşme u...
European Framework 7 (FP7) kapsamında oluşturulan QB50 projesinde İstanbul teknik Üniversitesi ve... more European Framework 7 (FP7) kapsamında oluşturulan QB50 projesinde İstanbul teknik Üniversitesi ve Hava Harp Akademisi tarafından geliştirilen BeEagleSat küp uydusu yer alacaktır. QB50 projesi yaklaşık olarak 50 küp uydunun birlikte alçak yörüngeye (340 ila 400 km arası) gönderilerek, eşzamanlı atmosferik ölçümler yapılmasını amaçlamaktadır. 40 tanesi 2 birimlik (2U -10x10x20cm ve 2kg) ve 10 tanesi 3 birimlik (3U -10x10x30cm ve3kg) olmak üzere üzerlerinde bilimsel faydalı yük taşıyan küp uydular olacaklardır. Sabancı Üniversitesi ve İstanbul Teknik Üniversitesi tarafından geliştirilen ek bilimsel faydalı yük de projede yer almaktadır. BeEagleSat uydusunda belirli alt sistemler, ülkede uydu projeleri ve alt sistemleri üzerine yetkinlik ve tecrübe kazanılması amacıyla yurt içinde geliştirilecektir. Uydudaki diğer alt sistemler de hazır alınarak tüm sisteme entegre edilecektir.
A 3-Unit CubeSat, called "3USAT", is developed for voice communication in Low Earth Orb... more A 3-Unit CubeSat, called "3USAT", is developed for voice communication in Low Earth Orbit (LEO). The spacecraft was launched on 26 th of April 2013 from China Jiaquan Satellite Launch centre with a LM2D launch vehicle. The main payload of the 3USAT is a linear transponder operating in VHF/UHF. The 3USAT includes many sensors and a camera for taking pictures, as well. Most subsystems of the CubeSat have redundancy which is provided by COTS equipment and/or in-house developed systems. The two transponders are developed genuinely by the project team. The project team have gained valuable experience from developing subsystems, integrating COTS equipment's, and carrying out space simulation testing. The payload power requirements resulted in temperatures exceeding 100 o C. A remedy was developed to remove heat resulting from high power amplifier. Other problems encountered and resolved include grounding issues of electronic and RF systems, the transfer of data and power usi...
2013 6th International Conference on Recent Advances in Space Technologies (RAST), 2013
TURKSAT-3USAT is a three unit CubeSat developed for voice communication at low Earth orbit. The p... more TURKSAT-3USAT is a three unit CubeSat developed for voice communication at low Earth orbit. The payload is a VHFIUHF transponder operating at amateur bands. The transponder and all other subsystems, except the stabilization, are doubled for redundancy. Where possible, both COTS systems and in-house development is employed. The energy is provided using lithium polymer batteries together with super capacitors. Satellite stabilization is accomplished using passive magnetic attitude control system with hysteresis rods. The TURKSAT-3USAT is launched from JSLC of China, on April 26, 2013.
Proceedings of 5th International Conference on Recent Advances in Space Technologies - RAST2011, 2011
Nan o satellite structures have limited communication ability due to limited power and size. Most... more Nan o satellite structures have limited communication ability due to limited power and size. Most of satellite includes sensors, for instance camera, also image transfer requires more data transfer speed. In this project, 2.4 GHz (S Band) communication system is designed for LEO satellites (700-900 km). System includes system-on-chip transmitter device, preamplifier, band pass filter, power amplifier and microstrip antenna
A three unit CubeSat called TURKSAT-3USAT is being developed for voice communication at low earth... more A three unit CubeSat called TURKSAT-3USAT is being developed for voice communication at low earth orbit. The payload consists of two VHF/UHF transponders operating at amateur bands. All other subsystems are also doubled for redundancy. Where possible, both ...
A highly efficient, redundant electrical power system is developed in Istanbul Technical Universi... more A highly efficient, redundant electrical power system is developed in Istanbul Technical University (ITU) Space Systems Design and Testing Laboratory (SSDTL). A Hardware-in-the-loop (HIL) simulation platform is designed for testing, calibration and qualification procedures of the power system. The system is based on a MATLAB/Simulink model of space qualified solar cells. A DC/DC converter generates required power for any given load depending on the solar cell model. An interface drives the converter by the information from the solar cell model. The system is connected to developed power system, in order to measure its performance. This paper explains the methods, trade-offs and the results of the project.
HAWELSAT is a 2U CubeSat developed by HAVELSAN and Istanbul Technical University (ITU) as a part ... more HAWELSAT is a 2U CubeSat developed by HAVELSAN and Istanbul Technical University (ITU) as a part of QB50 Project [1] in Turkey. During its mission, HAVELSAT will collect electron density information in its orbit and transfer this data to the ground station (GS). In QB50 program, the aim is to use all collected e-density data from all satellites to get better thermosphere models. In this article, the developed on-board software (SW) and the development phases will be briefly explained. Moreover, a summary of difficulties faced during these phases and also lessons learnt are summarized.
In this work, within the context of university, aerospace industry and SME collaboration a 3U Cub... more In this work, within the context of university, aerospace industry and SME collaboration a 3U CubeSat called ASELSAT with Ground Sample Distance of at least 30 meter payload and X-Band downlink system is designed and developed. The high power requirement of X Band transmitter and packing off all subsystems within the 3U volume are main challenges. ASELSAT will house the X-Band transmitter and patch antenna developed by ASELSAN, the NanoCam of GomSpace, 3 wheel ADCS of CubeSpace, and on board computer and communication system by ErTek among others. A 3U structure called NART Structure is specifically developed for ASELSAT by Gumush Space Ltd. Key Words: Aselsat, Cubesat, X-Band Transmitter, Camera, ADCS
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