Dániel Hamar

Crop yield estimation by satellite remote sensing

Two methods for estimating the yield of different crops in Hungary from satellite remote sensing data are presented. The steps of preprocessing the remote sensing data (for geometric, radiometric, atmospheric and cloud scattering correction) are described. In the first method developed for field level estimation, reference crop fields were selected by using Landsat Thematic Mapper (TM) data for classification. A new vegetation index (General Yield Unified Reference Index (GYURI)) was deduced using a fitted double-Gaussian curve to the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data during the vegetation period. The correlation between GYURI and the field level yield data for corn for three years was R 2=0.75. The county-average yield data showed higher correlation (R 2=0.93). A significant distortion from the model gave information of the possible stress of the field. The second method presented uses only NOAA AVHRR and officially reported county-level yield data. The county-level yield data and the deduced vegetation index, GYURRI, were investigated for eight different crops for eight years. The obtained correlation was high (R 2=84.6–87.2). The developed robust method proved to be stable and accurate for operational use for county-, region- and country-level yield estimation. The method is simple and inexpensive for application in developing countries, too.

Yield estimation for corn and wheat in the Hungarian great plain using Landsat MSS data

Abstract Models for the forecasting of crop yields using remotely-sensed satellite data are studied intensively worldwide. After reviewing the experience gained by other researchers in this field, we selected procedures which might be suitable for the estimation of corn and wheat yields in Hungary. In order to study the relations between various remotely-sensed spectral data (and their combinations) and the actually measured final yields we investigated archived agricultural and Landsat MSS spectral data for 1984. A linear relation has been sought and found between the yield data for 47 corn and 55 wheat fields in Hajdu-Bihar county and various weighted and summed spectral quantities. Among the vegetation indices derived from satellite data and corrected for atmospheric effects the most promising were the spectral indices sensitive to the green biomass. The latter, summed over a certain period in the growing season, exhibited a regression of 45-86 per cent, depending on the crop and the period of summatio...

Fine structure of whistlers recorded digitally at Halley, Antarctica

Abstract Two whistlers recorded digitally at Halley, Antarctica (L = 4.3) were analyzed by matched filtering with 10 Hz frequency resolution. For the construction of the matched filter, a more realistic description of the whistler waveform than has been used for lower latitude whistlers was applied. The ( ƒ, t ) pairs obtained, together with the corresponding magnitudes, gave a high resolution dynamic spectrum which revealed the fine structure of the whistlers. Whistlers which appeared as well defined, discrete, single traces on a conventional spectrogram, turned out to be composed of several components covering various parts of the spectrum. Further analysis of the strongest and longest component of a whistler resulted in high resolution travel time residual curves similar to those obtained by the ray tracing method or by averaging of numerous occurrences of whistlers in the same duct. The matched filter method of analysis can provide new insights into the details of ducted whistler propagation. It also yields the amplitude variation vs time (or vs frequency) along the whistler trace, thus potentially giving information about the spectrum of the source spheric, and/or the frequency dependence of magnetospheric amplification due to wave-particle interactions.

Trace splitting of whistlers: A signature of fine structure or mode splitting in magnetospheric ducts?

Previously, we reported on the discovery of fine structure in whistler data received on the ground at Halley, Antarctica. This structure was not apparent in conventional spectral analysis but was revealed by the technique of digital matched filtering. We have now examined a larger data set, and a commonly observed phenomenon is that single whistler traces become split into two, over various frequency ranges. Examples are presented in the form of time-transformed spectrograms in which reference model whistlers are represented as vertical lines. The splitting is typically 5–15 ms (about 0.5% of the total whistler travel time) and extends over frequency ranges of a few hundred hertz which may occur anywhere between the upper and lower cutoff frequencies of the whistler. The splitting may be either symmetrical or unsymmetrical with respect to the unsplit trace. The effect is unlikely to arise in the spectrum of the lightning source or from propagation under or through the ionosphere. It may, however, be a signature of field-aligned fine spatial structure in plasmaspheric density, and hence refractive index, in the whistler duct. For simple longitudinal propagation, electron density fluctuations of the order of 1% and spatial scale sizes of the order of 50 km in the equatorial plane are implied. It seems possible that the observations could also be interpreted in terms of the mode theory of ducted propagation, assuming the excitation of two modes with group velocities differing by a few tenths of a percent.

Whistler doublets and hyperfine structure recorded digitally by the signal analyzer and sampler on the active satellite

The signal analyzer and sampler (SAS) experiment was placed on the Active spacecraft as a collaborative effort between the Eotvos University (Budapest), the Technical University of Budapest, and IZMIRAN (Moscow). The scientific objective of the experiment was to study whistler/VLF ducted propagation, VLF duct structure, and the hyperfine structure of whistlers. Digitally sampled waveforms of several field components were transmitted in real time by the SAS telemetry system at 460.4 MHz. For the transmission 900-Hz wide bands were selected between 0.5 and 21.5 kHz or a single 5 kHz wide band was transmitted. Data were received in Budapest, Hungary and at Wallops Island, Virginia. Although the obtained data were processed for various purposes, here some results concerning the hyperfine structure of whistlers are presented. The studied whistlers, recorded as several pairs of closely spaced traces (doublets), were interpreted as ducted whistlers escaping at a high altitude from a single or two closely spaced narrow ducts and reaching the satellite directly from above or after reflection from below. The whistler traces were processed by a sophisticated matched filter technique which enabled us to obtain very high resolution dynamic (frequency-time-amplitude) spectra. The hyperfine structure of traces revealed by this technique demonstrates the complexity of whistler propagation. The observed splitting of traces may be explained, for example, in terms of a number of guided modes (waveguide mode splitting) or by the superposition of closely spaced ducting structures (duct splitting).

Surface models including direct cross-radiation: a simple model of furrowed surfaces.

Abstract In the interpretation of measured reflectance data it is important to consider those surface radiation effects which make a significant contribution to the overall irradiation pattern. A model was constructed for furrowed bare soil surfaces which includes the direct cross-radiation effect between the surface elements. According to the model computations performed the direct cross-radiation plays a significant role in the measured, reflected signal intensity. The computational method developed is suitable for including the direct cross-radiation effect in surface radiation models in the optical region.

Yield forecasting for wheat and corn in Hungary by satellite remote sensing

We have developed an advanced version of our yield estimation method [Ferencz et al., 2004, Crop yield estimation by satellite remote sensing. International Journal of Remote Sensing, 25, pp. 4113–4149], that is able to provide reliable forecasts for corn and wheat, several weeks before the harvest. The forecasting method is based on the data of the Advanced Very High Resolution Radiometer (AVHRR) instruments of the National Oceanic and Atmospheric Administrations (NOAA) Polar Orbiting Environmental Satellites (POES). The method was applied to Hungary between the years 1996 and 2000. The forecasted yield values are all within 5% reliability with respect to the actual yield data produced by classic (non-satellite based) methods and provided by the Hungarian Statistical Office, with the exception of 1997, where the absolute error is about 8%.

Automatic retrieval of plasmaspheric electron densities: First results form Automatic Whistler Detector and Analyzer Network

There is an increasing ‘demand’ for plasmaspheric electron density data for plasmasphere models in Space Weather related investigations, particularly in modeling charged particle accelerations and losses in Radiation Belts. The global Automatic Whistler Detector and Analyzer (AWDA) system network detects millions of whistlers in a year. But the analysis of the whistlers to extract the plasmaspheric electron density information has thus far proved to be slow and time consuming. A recently developed whistler inversion model opened the way for an automated process of whistler analysis, not only for single whistler events but for complex analysis of multiple-path propagation whistler groups. An automatic analyzer algorithm was also developed and a practical implementation of such a system on a PC cluster is done. The prototype of AWDA runs on the cluster in quasi-realtime mode, the analysis of a whistler group take a few minutes. In this paper we present the first results of test runs processing whistlers from our archive database. We are planning to install this algorithm on AWDANet nodes in the near future to archive the quasi-realtime mode of operation in providing plasmaspheric electron densities.

Methods of Signal Analysis

The method using whistlers (impulse-generated signals) is one of the small number of techniques which yield reliable data on morphology and dynamics of the medium traversed by the signal, e.g. of plasmasphere, on magnetospheric electric fields and of ionosphere-magnetosphere coupling fluxes of the Earth and other planets. On the other hand studying the signal waveform, their amplitude-frequency behaviour the better understanding of the wave propagating in the upper atmospheric plasma can be achieved.

Application of the Method for Different Propagation Situations

The method presented in Chapter 1 can be used not only in cases in which the media in space-segments are homogeneous between the boundary surfaces — see Fig. 1.1 —, but also in cases in which the media in these space-segments are weakly inhomogeneous (quasi-homogeneous) or inhomogeneous (e.g. Ferencz, 1995b) and the gradient of the inhomogeneous medium parameters is limited only by the validity-condition of the method (Ferencz, 1978a). In the following let us generalise the method of solution — presented in point 1.1 and applied in point 1.3 — for the cases in which the media in the space-segments are quasi-homogeneous and inhomogeneous between the boundary surfaces.