Estimation and validation of receiver antenna codephase variations for multi GNSS signals

Besides antenna phase center corrections (PCC) for carrier phase measurements, which have to be considered for precise GNSS application, also codephase variations (CPV) exist. These are antenna dependent delays of the code which vary with azimuth and elevation. Such variations are not provided operationally in the antenna exchange format (ANTEX) at the moment. Previous studies in our working group show, that CPV should be taken into account when using code-carrier combination. Depending on the antenna type they can amount up to some dm. At Institut für Erdmessung (IfE), a concept to determine the CPV has been established. This procedure uses a robot that rotates and tilts the antenna under test precisely in the field. Real world modulated signals from the satellites are used, which is challenging in anechoic chamber procedures. Time differenced single differences are used to estimate PCC and CPV as spherical harmonics (8,8) in a post-processing approach. In this contribution we present the concept CPV of Galileo signals for several kinds of receiving antennas (mass market and high grade). In addition, we discuss the repeatability and stability of CPV for those antenna. Typical values of the CPV reaches up to 500 mm. The RMS of patterns resulting from multiple calibrations are 80 mm for Galileo C1X and 48 mm for GPS C1C

Determination of Phase Center Corrections for Galileo Signals

GNSS are widely used for positioning, navigation and timing (PVT). The quality of results depends on the antenna in use and the capability to take antenna specific effects into account. The most prominent corrections are the direction dependent phase center corrections (PCC), which include corrections for the phase center offset (PCO) and the phase center variations (PCV). These corrections range between a few up to several millimeters for carrierphase observations and up to some decimeters for code observations. In addition, the magnitude of the error depends on the used antenna type and can differ even for different antennas of the same type and manufacturer. The frequency-dependent PCC are either determined in an anechoic chamber or in the field using a robot (so-called absolute field calibration). Both methods have their advantages and drawbacks. In the Antenna Exchange Format (ANTEX) from the International GNSS Service (IGS), which is widely used, currently only PCC for L1- and L2 frequencies for GPS and GLONASS are officially published. Absolute field calibrations values for new signals like Galileo or GPS L5 are missing. Only some chamber calibration results are available in the European Permanent Network (EPN). The Institute für Erdmessung (IfE) is one of the the IGS accepted absolute field calibration institutions and provides PCC using the so-called Hannover-Concept. In this approach a robot is used to precisely rotate and tilt the antenna under test. This concepts has now been extended to an experimental approach. The PCC of new signals are estimated in post-processing as spherical harmonics using time differenced single differences. First results show both – a high repeatability of the estimated pattern and an improvement on the observation domain. In this contribution the theoretical background as well as the extended concept are described. Moreover, patterns for Galileo signals and GPS L5 will be shown and discussed. After a short introduction into the method and the extended Hannover-Concept the robot model and the adjustment concept will be presented. The contribution will show that the estimation of PCC for Galileo signals is feasible with the developed method. This can be described by the root mean square (RMS) of differential pattern (of different calibrations). This indicator for the repeatability show RMS values for the EL1X signal under 0.6 mm for the NOV703GGG antenna and under 0.4 mm for the LEIAR25.R3. The RMS for the EL5X signal is maximal 0.6 mm for the NOV703GGG or 0.65 mm for the LEIAR25.R3. Furthermore, the obtained patterns will be presented and discussed for several antennas typical to IGS stations. For instance the PCV of the LEIAR25.R3 show values in a range of -4 to 7 mm for the EL1X frequency, whereas the Up-component of the PCO is approximately 60 mm. If these PCC are taken into account, the RMS of the single differences (SD) of a short baseline, common clock experiment at the Physikalisch-Technische Bundesanstalt (PTB) can be improved

Column-integrated aerosol optical properties from ground-based spectroradiometer measurements at Barrax (Spain) during the Digital Airborne Spectrometer Experiment (DAISEX) campaigns.

The Digital Airborne Imaging Spectrometer Experiment (DAISEX) was carried out for the European Space Agency (ESA) in order to develop the potential of spaceborne imaging spectroscopy for a range of different scientific applications. DAISEX involved simultaneous data acquisitions using different airborne imaging spectrometers over test sites in southeast Spain (Barrax) and the Upper Rhine valley (Colmar, France, and Hartheim, Germany). This paper presents the results corresponding to the column-integrated aerosol optical properties from ground-based spectroradiometer measurements over the Barrax area during the DAISEX campaign days in the years 1998, 1999, and 2000. The instruments used for spectral irradiance measurements were two Licor 1800 and one Optronic OL-754 spectroradiometers. The analysis of the spectral aerosol optical depth in the visible range shows in all cases the predominance of the coarse-particle mode over the fine-particle mode. The analysis of the back trajectories of the air masses indicates a predominance of marine-type aerosols in the lower atmospheric layers in all cases. Overall, the results obtained show that during the DAISEX there was a combination of maritime aerosols with smaller continental aerosols

The Stability of the Suggested Planet in the nu Octantis System: A Numerical and Statistical Study

We provide a detailed theoretical study aimed at the observational finding about the nu Octantis binary system that indicates the possible existence of a Jupiter-type planet in this system. If a prograde planetary orbit is assumed, it has earlier been argued that the planet, if existing, should be located outside the zone of orbital stability. However, a previous study by Eberle & Cuntz (2010) [ApJ 721, L168] concludes that the planet is most likely stable if assumed to be in a retrograde orbit with respect to the secondary system component. In the present work, we significantly augment this study by taking into account the observationally deduced uncertainty ranges of the orbital parameters for the stellar components and the suggested planet. Furthermore, our study employs additional mathematical methods, which include monitoring the Jacobi constant, the zero velocity function, and the maximum Lyapunov exponent. We again find that the suggested planet is indeed possible if assumed to be in a retrograde orbit, but it is virtually impossible if assumed in a prograde orbit. Its existence is found to be consistent with the deduced system parameters of the binary components and of the suggested planet, including the associated uncertainty bars given by observations.Comment: 11 pages, 10 figures, 3 tables; Monthly Notices of the Royal Astronomical Society (in press

The use of satellite products to assess spatial uncertainty and reduce life-time costs of offshore wind farms

Managers of offshore wind farms make strategic decisions based on information about site wind speeds and significant wave heights (SWH) available from numerical weather predictions (NWP) or local in-situ measurements. However, the coarse resolution with which such information are available, both in space and time, introduces a high degree of uncertainty into the decision process which in turn may result in higher costs during different stages of offshore wind farm life. The current work investigates how space-borne data describing wind speeds and SWH might be used to quantify spatial uncertainties and support decisions during the design and operation of offshore wind sites. Results have revealed that due to high spatial variance of wind speed, the estimated wind power can differ from that provided by an offshore met mast up to 11%. The methodology proposed for SWH has shown how data collected from distinct satellites can be efficiently interpolated (maximum absolute error observed around 1 m) to generate high-resolute spatial information of sea water surface, regardless of satellite trajectory distributions. The work has provided insights on how the propagation of measurement uncertainty through the wind farm area can affect both management costs and wind energy production over the plant life-cycle

Sensitivity of L-band vegetation optical depth to carbon stocks in tropical forests: a comparison to higher frequencies and optical indices

Supplementary data to this article can be found online at https://doi.org/10.1016/j.rse.2019.111303.Monitoring vegetation carbon in tropical regions is essential to the global carbon assessment and to evaluate the actions oriented to the reduction of forest degradation. Mainly, satellite optical vegetation indices and LiDAR data have been used to this purpose. These two techniques are limited by cloud cover and are sensitive only to the top of vegetation. In addition, the vegetation attenuation to the soil microwave emission, represented by the vegetation optical depth (VOD), has been applied for biomass estimation using frequencies ranging from 4 to 30¿GHz (C- to K-bands). Atmosphere is transparent to microwaves and their sensitivity to canopy layers depends on the frequency, with lower frequencies having greater penetration depths. In this regard, L-band VOD (1.4¿GHz) is expected to enhance the ability to estimate carbon stocks. This study compares the sensitivity of different VOD products (from L, C, and X-bands) and an optical vegetation index (EVI) to the above-ground carbon density (ACD). It quantifies the contribution of ACD and forest cover proportion to the VOD/EVI signals. The study is conducted in Peru, southern Colombia and Panama, where ACD maps have been derived from airborne LiDAR. Results confirm the enhanced sensitivity of L-band VOD to ACD when compared to higher frequency bands, and show that the sensitivity of all VOD bands decreases in the densest forests. ACD explains 34% and forest cover 30% of L-band VOD variance, and these proportions gradually decrease for EVI, C-, and X-band VOD, respectively. Results are consistent through different categories of altitude and carbon density. This pattern is found in most of the studied regions and in flooded forests. Results also show that C-, X-band VOD and EVI provide complementary information to L-band VOD, especially in flooded forests and in mountains, indicating that synergistic approaches could lead to improved retrievals in these regions. Although the assessment of vegetation carbon in the densest forests requires further research, results from this study support the use of new L-band VOD estimates for mapping the carbon of tropical forests.Peer ReviewedPostprint (author's final draft

Assessing on-farm productivity of Miscanthus crops by combining soil mapping, yield modelling and remote sensing

Crown Copyright © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Biomass from agricultural land is a key component of any sustainable bioenergy strategy, and 2nd generation, ligno-cellulosic feedstocks are part of the UK government policy to meet the target of reduced CO2 emission. Pre-harvest estimates of the biomass supply potential are usually based on experimental evidence and little is known about the yield gap between biologically obtainable and actual achievable on-farm biomass yields. We propose a systematic integration of mapped information fit for estimating obtainable yields using an empirical model, observed on-farm yields and remote sensing. Thereby, one can identify the sources of yield variation and supply uncertainty. Spatially explicit Miscanthus potential yields are compared with delivered on-farm yields from established crops ≥5 years after planting, surveyed among participants in the Energy Crop Scheme. Actual on-farm yield averaged at 8.94 Mg ha−1 and it varied greatly (coefficient of variation 34%), largely irrespective of soil type. The average yield gap on clay soils was much larger than that on sandy or loamy soils (37% vs 10%). Miscanthus is noticeably slower to establish on clay soils as shown by fitting a logistic Gompertz equation to yield time series. However, gaps in crop cover as identified by density counts, visual inspection (Google Earth) and remote sensing (Landsat-5) correlated with observed on-farm yields suggesting patchiness as causal for reduced yields. The analysis shows ways to improve the agronomy for these new crops to increase economic returns within the supply chain and the environmental benefits (reduced GHG emission, greater carbon sequestration) and reduce the land demand of bio-energy production.Peer reviewedFinal Published versio