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In the Very Long Baseline Interferometry (VLBI) space geodetic technique, various station-specific error sources corrupt the observable VLBI delay. An antenna axis offset (AO) model is applied in the VLBI data analysis for antennas with non-intersecting rotational axes, such as the 26-m and 15-m antennas for the Hartebeesthoek Radio Astronomy Observatory (HartRAO). The apriori AO values recommended by the International VLBI Service for Geodesy and Astrometry (IVS) for use in geodetic VLBI data analysis are taken, where possible, from values measured in ground surveys. The apriori AO values used for the HartRAO antennas in geodetic VLBI analysis have been identified as possible sources of error. The a priori AO value of 6695.3mm for the 26-m antenna originates from a 2003 co-locational ground survey, conducted before a major bearing repair in 2008, which could have changed the AO. The a priori AO value of 1495.0mm for the 15-m antenna was determined in 2007 in only a preliminary GPS survey. In this study, the respective AO values of the HartRAO 26-m and 15-m antennas were estimated from a VLBI analysis using the Vienna VLBI and Satellite Software (VieVS) and compared with measurements from co-locational ground surveys. It was found that the VLBI estimated values do not agree within the formal margins of error with the ground survey values, in that they differ by up to eight millimetres (8mm) for the 26-m antenna and up to five millimetres (5mm) for the 15-m antenna. As the ground survey values are considered to be more accurate than the VLBI estimated values, a further investigation of the site-specific error sources that may be contaminating the accuracy of VLBI results is required.

This study analyses the meteorological impact of the variability of precipitable water vapour (PWV) retrieved from ground-based global navigation satellite system (GNSS) stations over Nigeria from 2013 to 2014; these measurements represent the foremost probe of GNSS PWV distribution and variability over Nigeria. In this study, GNSS PWV daily estimates were grouped into monthly and seasonal averages; the variations in the monthly and seasonal estimates of GNSS PWV were characterized and correlated with different weather events that are regarded as good climate change indicators. The results revealed that the spatiotemporal changes in PWV content are largely subjugated by the effects of latitude, topographical features, the seasons and the continental air masses. Our study shows that there is a very strong seasonal interplay among the GNSS PWV, relative humidity, rainfall and cloud estimates. In addition, GNSS PWV and total electron content (TEC) estimates show an opposite relationship; the semi-diurnal relationship between GNSS PWV and TEC is stronger than the seasonal relationship. The seasonal relation among GNSS PWV, temperature and wind speed appears weak, while very strong interplay exists among the GNSS PWV, sun spot number and total solar radiation estimates. Our results confirm that GNSS PWV is a good pointer for weather forecasting/monitoring and fit for climate monitoring if available on a longer time scale. Finally, we recommend the densification of the GNSS network in Nigeria, as this will enable 3D profiling of PWV, thereby providing more information on GNSS PWV time series, which is needed for long-term climatology.

Precipitable water vapour (PWV) retrieved from ground-based Global Navigation Satellite System (GNSS) stations over Nigeria from 2013 to 2014 is compared with PWV from a satellite remote-sensing te...

This study presents an in-depth analysis to comprehend the spatial and temporal variability of zenith tropospheric delay (ZTD) over Nigeria during the period 2010–2014, using estimates from Global Navigation Satellite Systems (GNSS) data. GNSS data address the drawbacks in traditional techniques (e.g. radiosondes) by means of observing periodicities in ZTD. The ZTD estimates show weak spatial dependence among the stations, though this can be attributed to the density of stations in the network. Tidal oscillations are noticed at the GNSS stations. These oscillations have diurnal and semi-diurnal components. The diurnal components as seen from the ZTD are the principal source of the oscillations. This upshot may perhaps be ascribed to temporal variations in atmospheric water vapour on a diurnal scale. In addition, the diurnal ZTD cycles exhibited noteworthy seasonal dependence, with larger amplitudes in the rainy (wet) season and smaller ones in the harmattan (dry) season. Notably, the stations in the northern part of the country reach very high amplitudes in the months of June, July and August at the peak of the wet season, characterized by very high rainfall. This pinpoints the fact that in view of the small amount of atmospheric water vapour in the atmosphere, usually around 10%, its variations greatly influence the corresponding diurnal and seasonal discrepancies of ZTD. This study further affirms the prospective relevance of ground-based GNSS data to atmospheric studies. GNSS data analysis is therefore recommended as a tool for future exploration of Nigerian weather and climate.

Consumption of wheat is widespread and increasing in South Africa. However, global wheat production is projected to decline. Wheat yield forecasting is therefore crucial for ensuring food security for the country. The objective of this study was to investigate whether the anthesis wheat growth stage is suitable for forecasting dryland wheat yields in the Central Free State region using satellite imagery and linear predictive modelling. A period of 10 years of Normalized Difference Vegetation Index data smoothed with a Savitzky–Golay filter and 10 years of wheat yield data were used for model calibration. Diagnostic plots and statistical procedures were used for model validation and assessment of model adequacy. The period 30 days before harvest during the anthesis stage was established to be the best period during which to use the linear regression model. The calibrated model had a coefficient of determination of 0.73, a p-value of 0.00161 and a root mean squared error of 0.41 tons/ha. Residual plots confirmed that a linear model had a good fit for the data. The quantile-quantile plot provided evidence that the residuals were normally distributed, which means that assumptions of linear regression were fulfilled and the model can be used as a forecasting tool. Model validation showed high levels of accuracy. The evidence indicates that use of Moderate Resolution Imaging Spectroradiometer data during the anthesis growth stage is a reliable, cost-effective and potentially time-saving alternative to ground-based surveys when forecasting dryland wheat yields in the Central Free State. Significance: Developing a cost-effective technique based on satellite imagery for wheat yield forecasting is vital for food security planning in South Africa.

The Hartebeesthoek Radio Astronomy Observatory of South Africa is currently developing a lunar laser ranger (LLR) system based on a one metre aperture telescope in collaboration with National Aeronautics and Space Administration and the Observatoire de la Cote d’Azur. This LLR will be an addition to a limited list of operating LLR stations globally and it is expected to achieve sub-centimetre range precision to the Moon. Key to this expectation including the overall telescope operational performance is thermal analysis of the telescope structure, based on the thermal properties of component materials and their interaction with the environment through conventional heat transfer mechanisms. This paper presents transient thermal simulation results of the telescope’s optical tube and one metre primary mirror in terms of thermal variations and consequent structural deformations. The results indicate that on a non-windy, cloud-free and winter day, the temperature gradients on the structure could be within 1 °C with respect to the temporal ambient air temperatures at the site when these are between 9 and 23 °C. Furthermore, these gradients were coupled with thermally-induced total deformations that vary between 2.9 and 40.7 μm of the assembled telescope components. In overall, these findings suggest that both the tube and especially the mirror may respond very slowly to ambient temperatures; however, correcting for structural thermal variations is imperative in maximizing the pointing accuracy of the telescope thereby increasing the chance being on-target with the retroreflectors located on the Moon surface.

This contribution reassesses progress in the development of satellite laser ranging (SLR) technology and its scientific and societal applications in South Africa. We first highlight the current global SLR tracking stations within the framework of the International Laser Ranging Service (ILRS) and the artificial satellites currently being tracked by these stations. In particular, the present work focuses on analysing SLR measurements at Hartebeesthoek Radio Astronomy Observatory (HartRAO), South Africa, based on the MOBLAS-6 SLR configuration. Generally, there is a weak geometry of ILRS stations in the southern hemisphere and the SLR tracking station at HartRAO is the only active ILRS station operating on the African continent. The SLR-derived products – such as station positions and velocities, satellite orbits, components of earth’s gravity field and their temporal variations, earth orientation parameters – are collected, merged, achieved and distributed by the ILRS under the Crustal Dynamic Data Information System. These products are used in various research fields such as detection and monitoring of tectonic plate motion, crustal deformation, earth rotation, polar motion, and the establishment and monitoring of International Terrestrial Reference Frames, as well as modelling of the spatio-temporal variations of the earth’s gravity field. The MOBLAS-6 tracking station is collocated with other geodetic techniques such as very long baseline interferometry and Global Navigation Satellite Systems, thus making this observatory a fiducial geodetic location. Some applications of the SLR data products are described within the context of earth system science.

No abstract available.

High accuracy space geodesy relies on accurate and stable frequency standards, time interval counters, event timers and measured time offsets relative to UTC. These requirements are becoming more stringent as the global networks of space geodesy stations strive towards higher accuracy. This invariably leads to continual instrumentation upgrades and instrumental developments. In addition to the station frequency standard high stability and accuracy requirements, several relativistic considerations are required during data processing to reach optimal measurement accuracy. These aspects of time are discussed and quantified for each of the techniques of VLBI, GPS, SLR and LLR. It is proposed that fundamental space geodesy stations be equipped with the new generation of frequency standards that will allow chronometric geodesy. This will allow time determination within a stable and defined terrestrial reference frame, linked to the celestial reference frame via the Earth orientation parameters, and will also provide a link between the relativistic and classical geoid at these stations.

Satellite laser ranging (SLR) as a proven geodetic technique provides a wide and significant contribution to scientific studies of the Earth/Atmosphere/Ocean systems. In particular, modelling of the Earth and pole tides plays a very important role when analyzing SLR data. The accuracy of the determined satellite orbits is highly dependent on the models used for dynamic orbit determination. Gravity field models, which are represented by a series of spherical harmonic coefficients, have an impact on the satellite orbit and its precision. In addition, different empirical model parameterization used during SLR data analysis also has significant influence on the computed spherical harmonics, satellite orbits and their accuracies. In this contribution we investigate the impact of different SLR parameterizations on gravity field models used for precise satellite orbit determination. Data from satellites LAGEOS 1 and 2 were analysed using four different parameterization schemes namely: IERS1, IERS2 and IERS3 Earth tide models and pole tides. These are compatible Earth tide models of different complexity derived from IERS2010, a standard model of the International Earth Rotation and Reference Systems Service (IERS) and the standard IERS2010 pole tide model, and they aim to study their impacts on SLR solutions. The results indicate that the combination of IERS3, which is the most complex Earth tide model and the standard IERS2010 pole tide model, has a significant influence on the accuracy of gravity field models in precise orbit determination. In particular almost all the evaluated models give the smallest RMS values when IERS3 and pole tides are jointly selected in the analysis software during SLR data analysis, which indicates that the most complex models are also the most accurate. This work therefore validates the currently accepted IERS2010 Earth tide and pole tide models.

The Hartebeesthoek Radio Astronomy Observatory (HartRAO) is the only very long baseline interferometry (VLBI) station in Africa, its importance to the current and future network is discussed in the presented analysis. In order to quantify the contribution of the HartRAO station to VLBI products, different approaches (e.g. comparison of number of observations to sources, formal errors of EOP and baseline length repeatabilities) are analysed. Two datasets were used, CONT08 and IVS-R1 sessions from the years 2011 and 2012. The results suggest that the station is of high importance to the current and future global VLBI network. In particular, HartRAO is crucial for maintaining the ICRF on the Southern Hemisphere; it is also one of the most important stations for the estimation of polar motion and nutation (e.g. when HartRAO is excluded from different networks the formal error of nutation increases by about 50 %). The quality of the station coordinates of remote southern stations such as TIGOCONC is also heavily dependent on HartRAO.

Space geodetic techniques and satellite missions play a crucial role in the determination and monitoring of geo-kinematics, Earth's rotation and gravity fields. These three pillars of geodesy provide the basis for determining the geodetic reference frames with high accuracy, spatial resolution and temporal stability. Space geodetic techniques have been used for the assessment of geo-hazards, anthropogenic hazards and in the design of early warning systems for hazard and disasters. In general, space geodesy provides products for Earth observation, science and influences many activities (e.g., building and management) in a modern society. In order to further promote the application of space geodetic methods to solving Earth science problems, the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) was commissioned as an important geodetic infrastructure that integrates different geodetic techniques (such as Global Navigation Satellite Systems, Very Long Baseline Interferometry, Satellite Laser Ranging, Interferometric Synthetic Aperture Radar and Doppler Orbitography and Radio-positioning Integrated by Satellite), models and analysis techniques for the purpose of ensuring long-term, precise monitoring of geodetic observables vital for monitoring Earth system processes. Since its inception, there has been considerable progress made towards setting up the infrastructure necessary for the establishment of the GGOS database. While the challenges that beleaguer the GGOS are acknowledged (at least at global level), the assessment of an attuned GGOS infrastructure in the African context is necessary, yet lacking. In the present contribution, (a) the African preparedness and response to the observing system is assessed, and (b) the specific scientific and technological challenges of establishing a regional GGOS hub for Africa are reviewed. Currently only South Africa has a fundamental geodetic observatory located at Hartebeesthoek, Pretoria. Other countries in Africa have shown interest to participate in global geodetic activities, in particular through interest in the development of a unified African geodetic reference frame (AFREF). In particular interest has been shown in the proposed African VLBI Network (AVN), which will be partially based on existing ex-telecommunication radio antennas. Several countries are investigating their participation in the AVN, including Kenya, Nigeria and Ghana.

Conducting single frame orthorectification on satellite images to create an ortho-image requires four basic components, namely an image, a geometric sensor model, elevation data (for example a digital elevation model (DEM)) and ground control points (GCPs). For this study, orthorectification entailed the use of a single scene Pléiades primary panchromatic image, applying the Pléiades rigorous geometric model, utilising a high-quality 2 m DEM and using GCPs that were acquired from two different collection methods. The application of these different GCPs to the execution of orthorectification encompassed the aim of this paper, which was to investigate and compare the positional accuracies of ortho-images under two scenarios. Firstly, GCPs were manually collected through fieldwork utilising a Trimble GeoExplorer 6000 series handheld GPS device and secondly, by utilising TerraSAR-X based GCPs that were acquired from Airbus Defence and Space. The objective of this study was to determine the geolocation accuracy of a high-resolution satellite ortho-image when different types of ground control are used. This required the execution of two orthorectification tests where only the type of GCPs differed. The results of these tests were interesting since it highlighted the difference in positional accuracy when utilising various sources of ground control to perform orthorectification on satellite imagery. The comparison results showed that utilising the manual GCPs produced a better positional accurate ortho-image as opposed to using the TerraSAR-X based GCPs. Nonetheless, the TerraSAR-X based GCPs still produced a sub 2 m accurate ortho-image, which is more than sufficient for the production of most geospatial products.Keywords: orthorectification, digital elevation model (DEM), ground control point (GCP), high-resolution satellite imagery, TerraSAR-X based GCPs, WorldDEM™, Airbus Defence and Space

Accounting for multipath in Global Navigation Satellite Systems (GNSS) is a difficult task and an important one, especially during the pre-investigation phase for the installation of a permanent GNSS station for positioning or timing applications. Sites with a high level of multipath can cause positioning errors or timing errors resulting in the quality of GNSS products (position or timing) becoming degraded by several metres or nanoseconds. We investigate and attempt to map multipath as part of the site investigation for the installation of the timing antenna for lunar laser ranging applications at the Hartebeesthoek Radio Astronomy Observatory (HartRAO). A high-resolution wavelet power spectrum and a standard deviation parameter are used to map multipath in both the time and frequency domain as well as spatial variations on the sky plot. The high standard deviation values on the sky map are attributed to reflections due to shrubs or trees on the site, while smaller standard deviation areas are attributed to bare soil or less vegetated as this would give constant reflection over time provided the ground has constant moisture. We conclude that the site is suitable for installation of the timing antenna and that a mask of 15°-20° elevation angle will be applied to the timing antenna to minimise multipath at lower elevations.

The establishment of a new fundamental space geodetic station in South Africa has been proposed. A Satellite Laser Ranging System (SLRS) utilising a 1 m optical telescope enhanced for Lunar Laser Ranging (LLR) will form part of the geodetic instrumentation to be located at the new fundamental space geodetic station. The Tracking and Mount Control (T&MC) subsystem of the ranging system will require development of high-precision steering and pointing control for the telescope. A working model using National Instruments (NI) LabVIEW and an NI CompactRIO interface will be developed. We present an overview of this component of the ranging system and sketch the way forward. Envisaged outcome of this project is to achieve arc-second accurate steering and pointing of the telescope.

This paper addresses foundation requirements to construct a new Geodetic/Geophysical Observatory at approximately five kilometers south of Matjiesfontein, in the southwestern Karoo. The observatory will host a Lunar Laser Ranger, Satellite Laser Ranger, and other collocated geophysical instruments, such as seismic and gravimeter stations, to be located in a specially designed vault. Field and laboratory tests and surveys show that the site is stable and capable of supporting the 5 ton weight of the Lunar Laser Ranger (LLR) structure that includes a one meter diameter optical telescope and mount to conduct laser-ranging experiments. The foundation requirements for auxiliary buildings and calibrating beacons required for the instrument and the envisaged experiments were also included in this study.

The author wishes to express his gratitude to the Inkaba yeAfrica project for providing funding for a multicoreprocessor computer. This work is based on research supported by the National Research Foundation under grant IFR2011041500034.

We investigate the use of distribution functions to characterise the geophysical signals and noise components embedded in the geodetic Very Long Baseline Interferometry (VLBI) data sets across some of the International VLBI Service (IVS) stations. The rationale of using α-stable distributions as a tool to model the noise components in geodetic observables is due to the existence of impulsive signals/noise bursts (which often take the form of excursions with intermittent occurrences) in the data sets suggesting deviations from Gaussian distribution. A deviation from Gaussian distribution type would therefore suggest that statistical techniques such as least squares analysis, often used for analyzing the geodetic data (which are often based on Gaussian assumptions) could not be robust. In this paper, the properties of a long-range α-stable distribution with long tails and infinite moments in geodetic data are investigated by way of statistically testing their distribution using a family of stable distributions. The choice of stable distributions is based on the ease with which the statistical properties of the non-Gaussian processes are defined. Results indicate that the independent geophysical noise components reconstructed from geodetic VLBI baseline data exhibit distributions that have asymptotic power-law decay (albeit variable power indices) whose underlying process can be modelled as a long-range dependent process with an α-stable distribution (i.e., the stable varieties have small characteristic exponents).

Continuous tracking of geodetic satellites using the Satellite Laser Ranging technique has provided unprecedented opportunity in long- to medium-wavelength gravity field modelling. Numerous gravity field models have been derived from such observations and have been made freely available to the science community for research purpose. The accuracy of most of the latest gravity field models in terms of precise orbit determination is currently at cm level. Improvement in the Earth gravity field modelling is anticipated as quantitative and qualitative data (in particular from low earth orbit satellites) become available in the future. Such expectations require that the accuracy and precision of existing gravity field models be assessed. The validation of gravity field models in terms of satellite orbit determination is often based on the difference between the observed and computed range. The resulting range residuals are considered an important index when determining the accuracy of the gravity models and hence the satellite orbits. In this contribution we investigate the general improvement in gravity field modelling over a period of 15 years. The orbit accuracy of twelve gravity field models (both satellite-only and combined models) were assessed by analysing seven months of data from LAGEOS 1 and 2 using HartRAO analysis software. Results show that the gravity field models developed over the years have improved by at least a factor of 2 since 1990, considering improvement in O-C residuals.

A new Satellite Laser Ranging (SLR) analysis program is being developed at Hartebeesthoek Radio Astronomy Observatory (HartRAO) that can model satellite orbits with high accuracy utilising advanced modelling and SLR data. We introduce the background to SLR analysis to an interdisciplinary audience and present the results of processing one month of LAGEOS-1 and LAGEOS-2 data in combined and separate solutions. The inclusion of Earth-tide modelling is shown to be necessary to reduce range bias and to minimise observed minus computed (O-C) residuals. The combined solution shows a reduction in range bias when allowance is made for the SLR station displacement vector due to solid Earth-tide and the effect of the Earth-tide on the static gravity field. In order to make the comparison it is necessary to set the a-priori estimation error of unmodelled forces to a low value as it tends to absorb range biases and improve O-C residuals if included as an additional solve-for parameter. A comparison between the SLR station position perturbation resulting from solid Earth-tide and range bias indicates a correlation, which probably results from an overestimate of the Earth-tide vector and indicates that SLR will be able to differentiate between and evaluate different Earth-tide models. These are preliminary results and further improvement is envisaged due to the planned inclusion of additional advanced modelling procedures. Applications of this software will include velocity field determinations for crustal dynamics studies.

Time-series of precipitable water vapour (PWV), derived from continuous Global Positioning System (GPS) observations, are analysed for the two South African stations HRAO and SUTH. Since water vapour is a major greenhouse gas, observed changes in atmospheric PWV could be indicative of weather and climate change. Our main contribution is a realistic noise model of the PWV observations which enables one to draw correct conclusions about the significance of the derived PWV increase or decrease for given time spans longer than five years. It is demonstrated that the PWV residuals that are obtained after fitting a trend and yearly signal to the data are, due to the simple model’s exclusion of short-term scatter, much larger than the PWV uncertainties provided by the GPS analysis software. Although a better solution for the associated uncertainties is obtained by using the variance of these PWV residuals for the uncertainty rescaling, it is shown that the ARMA(1,1) noise model better represents the associated statistical uncertainties than the simple white noise model. The ARMA(1,1)-derived PWV trend uncertainties are approximately 2 times greater than those for a rescaled white noise model. Finally, it is argued that the variability of the annual signal prevents any trend estimation using time series shorter than about five years. A quantitative measure is presented to determine the minimum period of continuous GPS observational data required to measure PWV trends to a specified accuracy. As result of our study, we conclude that no statistically significant PWV trends are observed at the two GPS stations between 1998 and 2006.

The Council for Geoscience and the Hartebeesthoek Radio Astronomy Observatory (HartRAO) conducted an investigation to determine the suitability of a proposed site for a new space geodetic observatory. The proposed site is located 5 km outside Matjiesfontein in a small depression, which will shield it from radio frequency interference. The position of the site is suitable for space geodetic observations due to many cloudless days (essential for laser ranging), clear skies and low atmospheric water vapour content, which is very common in the Great Karoo. Provisionally called the International Institute for Space Geodesy and Earth Observation (IISGEO), the station is envisaged to operate advanced space geodetic techniques as part of a global network. The proposed site is close to the geological contact between sandstones and shales of the Cape and Karoo Supergroups. Geophysical, geotechnical and atmospheric measurements were included in the survey to test the geologic structure of the site. Geophysical investigations involved magnetic, electromagnetic and seismic refraction surveys that indicated the geology of the proposed terrain is relatively homogeneous and without obvious structures such as dykes and major faults. The geotechnical investigation consisted of eight backacter test pits and a terrain evaluation. Refusal depths were between a few centimetres and 0.5 m. An atmospheric and tropospheric investigation indicated low levels of atmospheric turbulence, a prerequisite for lunar and satellite laser ranging. Data gathered indicate that the site is suitable for the proposed site development.

Permanent Global Navigation Satellite Systems (GNSS) stations that are operating within the African Geodetic Reference Frame (AFREF) are contributing data to the datum realizations at global, regional and local levels. The infrastructure supports development and administrative functions of African governments, the public and investors throughout the African continent. However, African stations with high quality and continuous data that have been acquired over several decades are limited, which result in a non-uniform network. This means that additional station investment are required in Africa for new stations to contribute to the AFREF project. An assessment of the spatial distribution and densification of GNSS stations that contribute to AFREF ensure future geometrical improvements in the network have the most impact. Established GNSS stations within AFREF network contribute data for the realization of the International Terrestrial Reference Frame (ITRF), International GNSS Service (IGS) products and local AFREF solutions.

When compared to the wide range of atmospheric sensing techniques, global navigation satellite system (GNSS) offers the advantage of operating under all weather conditions, is continuous, with high temporal and spatial resolution and high accuracy, and has long-term stability. The utilisation of GNSS ground networks of continuous stations for operational weather and climate services is already in place in many nations in Europe, Asia, and America under different initiatives and organisations. In Africa, the situation appears to be different. The focus of this paper is to assess the conditions of the existing and anticipated GNSS reference network in the African region for meteorological applications. The technical issues related to the implementation of near-real-time (NRT) GNSS meteorology are also discussed, including the data and network requirements for meteorological and climate applications. We conclude from this study that the African GNSS network is sparse in the north and central regions of the continent, with a dense network in the south and fairly dense network in the west and east regions of the continent. Most stations lack collocated meteorological sensors and other geodetic observing systems as called for by the GCOS Reference Upper Air Network (GRUAN) GNSS Precipitable Water Task Team and the World Meteorological Organization (WMO). Preliminary results of calculated zenith tropospheric delay (ZTD) from the African GNSS indicate spatial variability and diurnal dependence of ZTD. To improve the density and geometry of the existing network, countries are urged to contribute more stations to the African Geodetic Reference Frame (AFREF) program and a collaborative scheme between different organisations maintaining different GNSS stations on the continent is recommended. The benefit of using spaced based GNSS radio occultation (RO) data for atmospheric sounding is highlighted and filling of geographical gaps from the station-based observation network with GNSS RO is also proposed.

This study focuses on the break-up of the African tectonic plate into separate Nubian and Somalian blocks, based on recent Global Positioning System (GPS) data. A new, unique velocity field has been obtained by processing all available observations of permanent GPS stations on Africa since 1996. The quantity and distribution of the stations and the length of the time-series of observations exceed that of previous studies by a considerable margin, allowing one to derive a reliable estimate of the differential motion between the Nubia and Somalian plates, which are considered as a single (African) block in the prevailing global tectonic plate models. The estimated relative pole of rotation of Somalia with respect to Nubia is located at 54.8°S; 37.0°E with magnitude −0.069°/Ma, implying distinct opening in the Ethiopian Rift of magnitude ≈7 mm/year and azimuth ≈N94°E, whereas in southeastern South Africa this value is reduced to ≈2 mm/year in almost the same direction. This is in accordance with some of the independent geological and geophysical tectonic models of the Nubia–Somalia plate boundary region. However, the spatial density of the current tracking network is still not optimal to establish the exact location of the entire Somalia–Nubia plate boundary; in particular, the possible branch east of Lake Victoria and heading towards the Mozambique Channel is impossible to confirm or reject at this moment.

Space geodetic techniques provide highly accurate methods for estimating bedrock stability at subcentimetre level. We utilize data derived from Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI) and Global Positioning Systems (GPS) techniques, collocated at the Hartebeesthoek Radio Astronomy Observatory, to characterise local plate motion and compare the solutions from the three techniques. Data from the GNSS station were processed using the GAMIT/GLOBK (version 10.4) software, data from the SLR station (MOBLAS-6)were processed using the Satellite Laser Ranging Data Analysis Software (SDAS) and the VLBI data sets were processed using the Vienna VLBI Software (VieVS) software. Results show that there is a good agreement between horizontal and vertical velocity components with a maximum deviation of 1.7 mm/yr, 0.7 mm/yr and 1.3 mm/yr between the North, East and Up velocity components respectively for the different techniques. At HartRAO there is no significant trend in the vertical component and all the techniques used are consistent with the a-priori velocities when compared with each other. This information is crucial in monitoring the local motion variations since geodetic instruments require a very stable base to minimise measurement errors. These findings demonstrate that station coordinate time-series derived with different techniques and analysis strategies provide comparable results.

Agriculture has both direct and indirect effects on the quality of surface water and groundwater and is among the leading causes of water quality degradation, mainly as a result of the excessive use of agrochemicals. Water samples were collected in a selected catchment area (Bonsma Dam) in KwaZulu-Natal and analysed for physicochemical variables. The concentrations of most of the elements and total dissolved solids, as well as the pH and electrical conductivity values, met the water quality requirements for domestic, agricultural, livestock and aquatic ecosystem uses. However, the inlet streams feeding the dam were found to be eutrophic during the wet season. Analysis of nitrate in the water body of the study area indicated that agricultural applications of manure and fertilisers may be a potential source of nitrate contamination. Most elements were more concentrated in the dam during the wet season. The overall ionic conductivity values were also higher in the wet season, while the pH was lower. The outcome of this work links the concentrations of physicochemical variables to land use, agricultural practice and local geomorphology. Seasonal patterns in the concentration of physicochemical variables occur, as land use, rainfall and farming activities change seasonally, and these concentrations should therefore be determined periodically.

Development of the 1-meter aperture Lunar Laser Ranging (LLR) telescope is underway at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) which is expected to achieve sub-centimeter range precision and accuracy to the Moon, for enhanced tests of Earth-Moon system dynamics. Key to the operational performance of the telescope is thermal analysis of the telescope composite structure including the optical mirrors. This study presents a thermal analysis on the integrated component materials comprising the LLR telescope in ANSYS, with the aim of simulating its thermal behaviour in response to site-based ambient air temperature (T A ). Results show that for a full day T A profile spanning the time period 00:00 to 23:59 the resulting range of simulated thermal variations measured at 12:00 midday and 23:59 nighttime across the telescope composite structure was found to be 9.11 oC to 10.03 oC and 9.12 oC to 9.86 oC respectively. In particular, the spider assembly and outer tube surface had the largest range of thermal variations i.e. greater than absolute 1 oC and thus, could be the main areas on the telescope where most thermal variations would occur. Furthermore, validation of the outer tube thermal variations using the 64 Resistant Temperature Detector (RTD) sensors mounted onto the test tube, showed relatable overall thermal variations of about 2 oC, at wind speeds of 0 to 0.4 km/h. In overall, these findings provide a typical expectation of the LLR telescope thermal behavior in response to T A at the site; and thus could be used as a guide to develop an RTD-based thermal monitoring system for the HartRAO LLR optical telescope.

Monitoring wheat growth under different weather and ecological conditions is vital for a reliable supply of wheat yield estimations. Remote sensing techniques have been applied in the agricultural sector for monitoring crop biophysical properties and predicting crop yields. This study explored the application of Land Surface Temperature (LST)-vegetation index relationships for winter wheat in order to determine indices that are sensitive to changes in the wheat health status. The indices were derived from Landsat 8 scenes over the wheat growing area across Bloemfontein, South Africa. The vegetation abundance indices evaluated were the Normalised Difference Vegetation Index (NDVI) and the Green Normalised Difference Vegetation Index (GNDVI). The moisture indices evaluated were the Normalised Difference Water Index (NDWI) and the Normalised Difference Moisture Index (NDMI). The results demonstrated that LST exhibited an opposing trend with the vegetation abundance indices and an analogous trend with the moisture indices. Furthermore, NDVI proved to be a better index for winter wheat abundance as compared to the GNDVI. The NDWI proved to be a better index for determining water stress in winter wheat as compared to the NDMI. These results indicate that NDVI and NDWI are very sensitive to LST. These indices can be comprehensive indicators for winter wheat health status. These pilot results prove that LST-vegetation index relationships can be used for agricultural applications with a high level of accuracy.

Newton’s final version (published in 1726) of Philosophiae Naturalis Principia Mathematica was a great scientific achievement of the time and contained sufficient information to allow calculation of the dynamics of terrestrial and celestial bodies; it also expounded on the absolute nature of time and space. As examples, Newton’s statements (Newton in Principia: the mathematical principles of natural philosophy, New York, 1726) that “Absolute, true, and mathematical time, of itself, and from its own nature flows equably without regard to anything external” and “Absolute space, in its own nature, without regard to anything external, remains always similar and immovable” were fundamental to Newtonian calculations. These Newtonian concepts of space and time were challenged and proven to be only approximate by Einstein through his 1905 paper on special relativity, as well as his discovery of general relativity in 1915. The implications for and applications of general relativity within the fields of space geodesy is discussed in this chapter.

In a collaborative effort with the regional sub-commissions within IAG sub-commission 1.3 “Regional Reference Frames”, the IAG Working Group (WG) on “Regional Dense Velocity Fields” (see http://epncb.oma.be/IAG) has made a first attempt to create a dense global velocity field. GNSS-based velocity solutions for more than 6,000 continuous and episodic GNSS tracking stations, were proposed to the WG in reply to the first call for participation issued in November 2008. The combination of a part of these solutions was done in a two-step approach: first at the regional level, and secondly at the global level. Comparisons between different velocity solutions show an RMS agreement between 0.3 and 0.5 mm/year resp. for the horizontal and vertical velocities. In some cases, significant disagreements between the velocities of some of the networks are seen, but these are primarily caused by the inconsistent handling of discontinuity epochs and solution numbers. In the future, the WG will re-visit the procedures in order to develop a combination process that is efficient, automated, transparent, and not more complex than it needs to be.

Advances in Geosciences is an Open Access Journal for refereed proceedings and special publications

Determining the range to a satellite in orbit around the earth utilising the technique of satellite laser ranging (SLR) was pioneered in the early 1960s. The first successful ranging experiment was reported in the 3 December 1964 issue of Flight International (Smith 1964). Dr. Henry H. Plotkin of Goddard Space Flight Centre led a NASA team to track the Beacon-B (also known as Explorer-22) satellite for ten successful sessions during the period 11 October to 13 November 1964. A team from General Electric Co. (Valley Forge, Pennsylvania) also participated from Phoenix, Arizona. Using a telescope mounted with a ruby laser, expected range accuracy was about 3 m. Current accuracy is at the level of 1–2 cm.

Tropospheric delay is a major error source in positioning by Global Navigation Satellite Systems (GNSS). Many techniques are available for tropospheric delay mitigation consisting of surface meteorological models and global empirical models. Surface meteorological models need surface meteorological data to give high accuracy mitigation while the global empirical models need not. However, most GNSS stations in the African region are not equipped with a meteorological sensor for the collection of surface meteorological data during the measurement. Zenith Tropospheric Delay (ZTD) is often calculated by the various high precision GNSS software packages by utilising standard atmosphere values. Lately, researchers in the University of New Brunswick and Vienna University of Technology have both developed global models (University of New Brunswick (UNB3M) and Global Pressure and Temperature 2 wet (GPT2w) models) for tropospheric delay correction, respectively. This report represents an appraisal of the performance of the GPT2w and UNB3M models with accurate International GNSS Service (IGS)- tropospheric estimations for fifteen IGS stations over a period of 1 year on the Africa continent. Both models perform significantly better at low latitudes than higher latitudes. There was better agreement between the GPT2w model and the IGS estimate than the UNB3m at all stations. Thus, the GPT2w model is recommended as a correction model of the tropospheric error for the GNSS positioning and navigation on the African Continent.Keywords: Global Navigation Satellite Systems (GNSS), Zenith Tropospheric Delay (ZTD), Zenith Wet Delay (ZWD), Zenith Hydrostatic Delay (ZHD), International GNSS Service (IGS), Blind Tropospheric models