Eppley's precision spectral pyranometer ͑PSP͒ is used in networks around the world to measure downwelling diffuse and global solar irradiance at the surface of the Earth. In recent years several studies have shown significant discrepancy between irradiances measured by pyranometers and those computed by atmospheric radiative transfer models. Pyranometer measurements have been questioned because observed diffuse irradiances sometimes are below theoretical minimum values for a pure molecular atmosphere, and at night the instruments often produce nonzero signals ranging between ϩ5 and Ϫ10 W m Ϫ2. We install thermistor sondes in the body of a PSP as well as on its inner dome to monitor the temperature gradients within the instrument, and we operate a pyrgeometer ͑PIR͒ instrument side by side with the PSP. We derive a relationship between the PSP output and thermal radiative exchange by the dome and the detector and a relationship between the PSP output and the PIR thermopile output ͑net-IR͒. We determine the true PSP offset by quickly capping the instrument at set time intervals. For a ventilated and shaded PSP, the thermal offset can reach Ϫ15 W m Ϫ2 under clear skies, whereas it remains close to zero for low overcast clouds. We estimate the PSP thermal offset by two methods: ͑1͒ using the PSP temperatures and ͑2͒ using the PIR net-IR signal. The offset computed from the PSP temperatures yields a reliable estimate of the true offset ͑Ϯ1 W m Ϫ2 ͒. The offset computed from net-IR is consistent with the true offset at night and under overcast skies but predicts only part of the true range under clear skies.
In this paper we present a general analysisof frequency domain SAR processing based on the relationship between the phase of the two-dimensional Fourier transform of a point response to its range-time history. The paper demonstrates how this provides an appropriate basis for the design of a coherent strip-mode processor, free of geometric or phase distortion and artefacts, and without excessive computational cost. In consequencethe paper is highly relevant to the real-time, precision, processing of SAR data. The paper comments on the relevance of the analysis to ambiguity estimation and the processing of very long integration time SAR data. IntroductionSynthetic aperture radar (SAR) is a well-understood technique for achieving highresolution radar imagery from moving platforms. The technique involves the coherent transmission, reception and analysis of pulses and echoes at points along the platform track; the inter-pulse phase coherence allows (roughly speaking) the echoes to be analysed as though acquired concurrently from an extended, sampled, antenna aperture.The generation of a high-resolution image from the echo data involves (in addition to any pulse compression activity) a correlation, or sequence of coherent phase weighted sums, of the elements of echo data with a function (the azimuth processing replica) which is determined by the variation of the relative geometry between the surface to be imaged and the platform, over the time when data is acquired by the synthetic aperture. The imaging process in azimuth is analogous to the focussing of a lens; the surface to be imaged lies in the near field of the synthetic aperture.SAR processing in azimuth is essentially a matched-filter process; an image point is readily generated from the (baseband) echo data by a process of:
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