An Attitude Control System for a Low-Cost Earth Observation Satellite with Orbit Maintenance Capability

UoSAT-12 is a low-cost minisatellite built by Surrey Satellite Technology Ltd. (SSTL), it is amongst other objectives also a technology demonstrator for high performance attitude control and orbit maintenance on a future constellation of earth observation satellites. The satellite uses a 3-axis reaction wheel configuration and a cold gas propulsion system to enable precise and fast control of its attitude, for example, during orbit manoeuvres. Magnetorquer coils assist the wheels mainly for momentum dumping. This paper describes the various attitude control modes required to support: 1) the initial attitude acquisition phase, 2) a high resolution imager payload during pointing and tracking of targets, 3) the propulsion system during orbit manoeuvres. The specific attitude controllers and estimators used during these control modes are explained. Various simulation and in-orbit test results are presented to evaluate the performance and design objectives. To improve the control and estimation accuracy, on-board calibration and alignment procedures for the sensors and actuators are utilised. Some calibration results and the resulting improvement in accuracy from these procedures are shown

In-Orbit Modelling and Calibration of the Sun Sensors on UoSAT-12 and Tsinghua-1 Satellites

The modelling relations and calibration techniques of the miniature analogue sun sensors, developed at Surrey Satellite Technology Limited (SSTL), are discussed in this paper. Two model equations, an algebraic model based on a multi-variable polynomial algebraic curve-fitting procedure and a physical model based on analytical geometry relations, were developed. Parameter sensitivity analyses were conducted for the physical model, and an in-orbit calibration approach is proposed and was implemented with the sun sensors on-board UoSAT-12 and Tsinghua-1. A sequential batch filter algorithm was developed, based on the principle of the least-square estimation for regression models, to handle the large amount of in-orbit data in a sequential way. Satisfactory performance improvements have been achieved through the in-orbit calibration analyses for both satellites

In-Orbit Estimation of the Inertia Matrix and Thruster Parameters of UoSAT-12

This paper describes a Recursive Least Square (RLS) procedure for use in orbit to estimate the inertia matrix (moments and products of inertia parameters) of a satellite. To facilitate this, one attitude axis is disturbed using a reaction wheel whilst the other two axes are controlled to keep their respective angular rates small. Within a fraction of an orbit three components of the inertia matrix can be accurately determined. This procedure is then repeated for the other two axes to obtain all nine elements of the inertia matrix. The procedure is designed to prevent the build up of momentum in the reaction wheels whilst keeping the attitude disturbance to the satellite within acceptable limits. It can also overcome potential errors introduced by unmodeled external disturbance torques and attitude sensor noise. The results of simulations are presented to demonstrate the performance of the technique. The paper also describes an RLS procedure which can be used to estimate the thruster coefficients for thrust levels and alignment of the cold gas thrusters used for attitude control on UoSAT-12. A general on-line method is presented which uses a three axis reaction wheel system to accurately determine the relationship between the commanded and actual torque produced by the thrusters. The calibration algorithm is designed to be robust against external disturbance torques, inertia matrix modelling errors and attitude sensor noise. The results of both simulations and successful in-orbit tests are presented, illustrating the effectiveness of the technique

Effects of ad libitum and restricted diets in different feeding regimes on growth and carcass attributes of boars of a selected genetic line

The current modern commercial pig is an animal that has evolved through deliberate breeding programs, controlled environmental influences and nutrition to yield a highly efficient feed converter and fertile animal. The objective of this study was to measure the growth performances and carcass characteristics of entire male grower – finisher pigs which were subjected to different seasonal variations, nutrient dense diets, feeding regimes and group situations under South African circumstances. Period 1 was conducted in the winter from 6 June 2008 to 13 August 2008 and Period 2 in the summer from 3 October 2008 to 10 December 2008. The sire lines that were selected for the experiment had the same genetic breeding values (Topigs Selection Index value), of which two sire lines were the same in both Period 1 and Period 2 and one of two different sires lines was used either in Period 1 or Period 2. The animals were fed two different rations, a high (FH) and a low ration (FL), with the low ration’s specifications being 95 % of the high ration. The animals were randomly allocated three different feed regimes throughout the trial; restricted single feeding (RSF), ad libitum single feeding (ASF) and ad libitum group feeding (AGF). In the winter animals had a greater growth response compared to the summer, with end weight and average daily gain being significant (P<0.05) higher. A significant (P<0.05) improvement in average daily gain, feed efficiency and protein deposition rate were observed when animals were fed a higher energy and protein content in their diet, especially during summer. A significant (P<0.05) improved feed conversion was observed for restricted animals, but end weight, average daily gain and average protein deposition rates were significant (P<0.05) lower compared to ad libitum group and individually fed animals. In conclusion; the impact of decreasing the nutrient density of the diet for growing pigs through incremental changes in diet composition had a variable impact on overall growth performance and carcass quality. Feeding the high energy and protein ration improved growth performance during summer, but also in the initial stages of growth when feed intake capacity was limited. The objective when formulating diets should be to provide the essential amino acids and energy in amounts needed to support maximal and efficient growth. Using growth models estimated optimal feed intake curves will not deliver optimal results. Only when measuring and calculating the actual feed intake and protein deposition rates optimal performance levels will be reached. Measurements of feed intake and growth performance data derived from pigs penned individually should be adjusted before they can be applied to commercial situations or research conditions in which pigs are penned in groups.Dissertation (MSc(Agric))--University of Pretoria, 2010.Animal and Wildlife Sciencesunrestricte

A multi-mode attitude determination and control system for small satellites

Thesis (PhD)--Stellenbosch University, 1995.ENGLISH ABSTRACT: New advanced control techniques for attitude determination and control of small (micro) satellites are presented. The attitude sensors and actuators on small satellites are limited in accuracy and performance due to physical limitations, e.g. volume, mass and power. To enhance the application of sophisticated payloads such as high resolution imagers within these confinements, a multi-mode control approach is proposed, whereby various optimized controller functions are utilized during the orbital life of the satellite. To keep the satellite's imager and antennas earth pointing with the minimum amount of control effort, a passive gravity gradient boom, active magnetic torquers and a magnetometer are used. A "cross-product" detumbling controller and a robust Kalman filter angular rate estimator are presented for the preboom deployment phase. A fuzzy controller and magnetometer full state extended Kalman filter are presented for libration damping and Z-spin rate control during inactive imager periods. During imaging, when high performance is required, additional fine resolution earth horizon, sun and star sensors plus 3-axis reaction wheels are employed. Full state attitude, rate and disturbance estimation is obtained from a horizon/sun extended Kalman filter. A quaternion feedback reaction wheel controller is presented to point or track a reference attitude during imaging. A near-minimum time, eigenaxis rotational reaction wheel controller for large angular maneuvers. Optimal linear quadratic and minimum energy algorithms to do momentum dumping using magnetic torquers, are presented. A new recursive magnetometer calibration method is designed to enhance the magnetic in-flight measurements. Finally, a software structure is proposed for the future onboard implementation of the multi-mode attitude control system.AFRIKAANSE OPSOMMING: Nuwe gevorderde beheertegnieke vir die oriëntasiebepaling en -beheer van klein (mikro-) satelliete word behandel. Die oriëntasiesensors en -aktueerders op klein satelliete het 'n beperkte akkuraatheid en werkverrigting as gevolg van fisiese volume, massa en kragleweringbeperkings. Om gesofistikeerde loonvragte soos hoë resolusie kameras binne hierdie tekortkominge te kan hanteer, word 'n multimode beheerbenadering voorgestel. Hiermee kan 'n verskeidenheid van optimale beheerfunksies gedurende die wentelleeftyd van die satelliet gebruik word. Om die satellietkamera en -antennas aardwysend te rig met 'n minimale beheerpoging, word 'n passiewe graviteitsgradiëntstang, aktiewe magneetspoele en 'n magnetometer gebruik. 'n "Kruisproduk" onttuimellings beheerder en 'n robuuste hoektempo Kalmanfilter afskatter is ontwikkel vir die periode voordat die graviteitsgradiëntstang ontplooi word. 'n Wasige beheerder en 'n volledige toestand, uitgebreide Kalmanfilter afskatter is ontwikkel om librasiedemping en Z-rotasietempo beheer te doen gedurende tydperke wanneer die kamera onaktief is. Gedurende kamera-opnames word hoë werkverrigting verlang. Fyn resolusie aardhorison, son en stersensors met 3-as reaksiewiele kan dan gebruik word. 'n Volledige oriëntasie, hoektempo en steurdraaimoment Kalmanfilter afskatter wat inligting van bogenoemde sensors gebruik, is ontwikkel. 'n “Quaternion” reaksiewiel terugvoerbeheerder waarmee die satelliet na verwysings oriëntasiehoeke gerig kan word of waarmee oriëntasiehoektempos gevolg kan word, word behandel. 'n Naby minimumtyd, "eigen"-as reaksiewielbeheerder vir groothoek rotasies is ontwikkel. Optimale algoritmes om momentumontlading van reaksiewiele met lineêre kwadratiese en minimumenergie metodes te doen, word afgelei en aangebied. 'n Nuwe rekursiewe kalibrasietegniek waarmee 'n magnetometer outomaties gedurende vlug ingestel kan word, is ontwikkel. Ten slotte, word 'n programstruktuur voorgestel vir aanboord implementering van die nuwe multimode beheerstelsel

A statistical study of the errors involved in the sampling and chemical analysis of soils and plants, with particular reference to citrus and pineapples

1. Accurate and reproducible methods for the determination of N and acid- extractable P in soils are described. The variations in N, P, and K found by intensive sampling of three different soil types are presented. 3. Suggestions for the adequate sampling of soils for various purposes arc given. 4. A study is presented of the preparative stages in leaf analysis (washing, drying, grinding and storage) as applied to citrus and pineapple leaf material. Likely errors are pointed out and procedures whereby these may be minimised are described. 5. A reliable wet-ashing procedure for obtaining leaf solutions is described. 6. Accurate and reproducible methods for the determination of total N, P,. K, Ca, Mg, Na, Fe, Mn, Zn and Cu in citrus and pineapple leaves are presented. 7. It is shown that diurnal variations in nutrient concentrations occur in citrus and pineapple leaves. 8. The variations in N, P, K, Ca, Mg, Na, Fe, Mn, Zn and Cu, occurring from tree to tree in 3 different Navel orange blocks are presented. It is shown that if all these elements are to be represented . to within 10% of the mean values on a 19 : 1 probability level, all the trees in the blocks would have to be sampled. Reasonable sampling procedures for various purposes are suggested. 9. The variations in N, P, K, Ca, Mg, Fe, Mn, Zn and Cu, occurring in 3 different Cayenne pineapple plantations are presented and described. Suitable sampling procedures are suggested

Beweerde massagrafte en grafte buite 'n formele begraafplaas by die Oranjerivierkonsentrasiekamp, Suid-Afrika

The South African or Anglo-Boer War (1899-1902) Orange River Concentration Camp on the farm Doornbult, close to Hopetown in the Northern Cape Province, is one of the best preserved Camps from this War. The history of the Concentration Camp at Doornbult is characterised by its occurring in two phases. At first, from April 1901 to the end of July 1901, the camp was under military control but later, from 1 August 1901 until November 1902, it was placed under civil control. It is known that several people died during the first phase. It was only during the second phase that a formal cemetery was laid out and burials occurred in an organised way. Graves situated outside the formal cemetery were investigated during May 2007 as part of an archaeological project of the Heritage Foundation. About 17 graves were found outside the perimeters of the formal cemetery, and it was clear that these were formal, or organised, burial sites. It presently still remains unclear why these burial sites are located outside of the formal cemetery. It may be connected to a custom that prematurely born or unbaptized children were buried outside cemeteries. The only grave that was completely excavated contained the remains of a prematurely born child, whose remains were buried formally. No evidence was found of existence of mass graves at the Orange River Concentration Camp at Doornbult.Die terrein van die Oranjerivier Konsentrasiekamp op die plaas Doornbult naby Hopetown in die Noordkaap Provinsie, wat uit die Suid-Afrikaanse of Anglo- Boereoorlog (1899-1902) dateer, is een van die bes bewaarde Konsentrasiekampterreine van die Oorlog. Die geskiedenis van die Konsentrasiekamp bestaan uit twee fases. Aanvanklik was die kamp onder militêre beheer (vanaf April 1901), maar later (vanaf 1 Augustus 1901 tot November 1902) is dit onder burgerlike beheer geplaas. Dit is bekend dat verskeie mense in die eerste fase in die kamp dood is. Eers in die tweede fase van die kamp se bestaan is `n formele begraafplaas aangelê en daarna het begrafnisse op ’n geordende basis plaasgevind. Grafte buite die formele begraafplaas is gedurende Mei 2007 as deel van `n groter argeologiese projek van die Erfenisstigting ondersoek. Sowat 17 grafte is buite die formele begraafplaas gevind en dit is duidelik dat die informele gedeelte wel blootgestel is aan `n formele begrafnisproses. Tans is dit onbekend waarom hierdie grafte buite die formele begraafplaas aangelê is. Die vermoede bestaan dat dit verband mag hou met `n eertydse gebruik om vroeggebore en ongedoopte kinders buite `n formele begraafplaas te ruste te lê. Die enigste oorskot wat volledig opgegrawe is, is dié van `n vroeggebore baba, van wie die oorskot formeel begrawe is. Geen aanduidings van die bestaan van massagrafte is by die Oranjerivier Konsentrasiekamp op Doornbult gevind nie.The Heritage Foundation and the National Research Foundation of South Africa (NRF).http://reference.sabinet.co.za/sa_epublication/cultuream201

Executive sponsor attributes and megaproject success

Megaprojects are failing at a rate that affects national economies as well as millions of people. The role of the executive project sponsor is merely one of the decisive factors in the success of these projects, but it is still much neglected in project management literature. This paper investigates attributes required by executive sponsors of megaprojects. The paper reports on the perceptions of 26 executives who played key roles in six recently completed megaprojects. The findings include essential attributes that an executive sponsor should have in order to improve the probability of a megaproject's success. The single most significant attribute is seniority and power.http://www.inderscience.com/jhome.php?jcode=IJPOM2022-04-23hj2021Graduate School of Technology Management (GSTM

An investigation of sponsor attributes on six megaproject cases

PURPOSE : Executive sponsors play a significant role in the success of megaprojects which, in turn, affect national economies and millions of people. However, the literature on the requisite attributes of project sponsors on megaprojects is still sparse. The purpose of the paper is to provide guidelines to company boards and executives who are tasked to appoint suitable executive sponsors to megaprojects. Thus, the paper contributes to the sparse literature on megaproject sponsors. DESIGN/METHODOLOGY/APPROACH : A total of 26 senior managers, with experience in megaprojects ranging from 8 to 15 years – and who were involved in 6 recent megaprojects with a combined value of US$13.75bn – were interviewed on the attributes of megaproject sponsors. Transcriptions of semi-structured, open-ended interviews were analysed with computer-assisted qualitative data analysis software (CAQDAS). FINDINGS : The study identified the most essential attribute as appropriate seniority, being empowered and accountable, with appropriate seniority, being empowered and accountable, with apposite credibility and with both personal and positional power. The study also uncovered 13 attributes – all components of “competence” – which have not previously been explicitly identified in literature as elements of sponsor “competence”. ORIGINALITY/VALUE : In the current study guidelines are provided for the selection and appointment of appropriate megaproject sponsors.https://www.emerald.com/insight/publication/issn/1753-8378hj2022Graduate School of Technology Management (GSTM