Absolute Power Calibration Research Articles

An Algorithm for Detecting Coherence in Cyclone Global Navigation Satellite System Mission Level-1 Delay-Doppler Maps

An algorithm for detecting coherence in Cyclone Global Navigation Satellite System (CYGNSS) mission delay-Doppler maps (DDMs) is presented. Because CYGNSS DDMs report only the observed power without phase information, the algorithm uses estimates of power “spread” within the DDM to flag coherency. Since the estimate used is a ratio of the powers in differing portions of the DDM, it is less sensitive to absolute power calibration and to the GPS C/A code type observed, and is applied to CYGNSS Level-1 uncalibrated DDMs. The basic detector formulation is described along with modifications to improve performance in lower signal-to-noise ratio (SNR) situations. The required detection thresholds are determined using matchups with CYGNSS “Raw I/F” mode measurements for which the DDM phase can be computed and used to identify coherence more precisely. Application of the final detector over a large CYGNSS data set suggests that approximately 8.9% of all inland returns are coherent. Inland regions persistently identified as coherent were found largely to be associated with the presence of water bodies. A smaller set of desert locations apparently having very low surface roughness were also found to be associated with persistent coherence. The detector was also applied to a set of ocean measurements, with the results showing that persistent coherence is limited to areas with sheltered waters. Ocean tests avoiding such regions indicate that the detector's false-alarm rate is approximately 0.0012% for the detection threshold used.

Track-Based Cyclone Maximum Wind Retrievals Using the Cyclone Global Navigation Satellite System (CYGNSS) Mission Full DDMs

Cyclone maximum wind retrievals are demonstrated using tracks of full delay-Doppler map (DDM) measurements of the Cyclone Global Navigation Satellite System (CYGNSS) constellation. The method is based on the production of a library of simulated DDMs for a synthetic storm model, as the parameters of the synthetic storm are varied. A matched filter approach between normalized simulated and measured DDMs is then applied to retrieve storm parameters. Because the method uses normalized DDMs in the retrieval, it is insensitive to the absolute power calibration of the DDM and instead relies on the DDM shape. Results from the method are shown for CYGNSS full DDM overpasses of Hurricane Irma as a case study. The maximum wind speed estimates obtained were associated with a retrieval root-mean-square error (RMSE) of 6.89 m/s and an unbiased RMSE 5.20 m/s with a mean difference of 4.52 m/s compared to reported National Hurricane Center Best Track forecasts.

Collective scattering system for transport study on NSTX

The problem of degraded energy confinement due to excessive transport across the magnetic field remains one of the critical issues for magnetically confined plasma. Radial transport of ion energy is relatively well understood, but that of the electrons has remained as anomalous, greatly exceeding the neoclassical prediction. It has been suggested that the anomalous electron thermal transport is explained by an electron gyro-scale turbulence driven by the electron temperature gradient (ETG) instability. A 280-GHz collective Thomson scattering system has been employed to address the electron gyro-scale fluctuations in National Spherical Torus Experiment (NSTX) plasmas. The spatial resolution of the targeting wavenumber is greatly affected by the configuration of the magnetic field since the radial fluctuation is perpendicular to the local magnetic field line. The effect of the toroidal field curvature and magnetic shear on the spatial resolution of the scattering system is investigated numerically. An absolute power calibration was performed to determine the power response of the heterodyne detection system. These spatial resolution studies and absolute power calibration were applied to estimate the normalized density fluctuations from the measured scattering signals in NSTX plasmas.

Technique for calibration of meteorological radarsusing differential phase

Meteorological radars that are capable of making both polarisation diversity and phase measurements require an accurate method for absolute power calibration to achieve their full potential. A technique is described which uses a combination of differential reflectivity and differential phase measurements in rain to achieve this. The method is demonstrated using data obtained with the 3 GHz Chilbolton radar, which was recently upgraded to measure differential phase and Doppler velocity. The sensitivity of this technique to the assumed drop size distribution is estimated, and shown to have a minor effect on the accuracy of the calibration.

Large area bolometers for millimeter-wave power calibration

An accurate monolithic power meter has been developed for millimeter-wave applications. The detector is a large-area Bismuth bolometer, integrated on a fused-Quartz substrate. It simply measures the temperature change caused by the absorption of millimeter-wave radiation. The power meter is simple to fabricate, inexpensive, and can be easily calibrated using a low-frequency network. The measured responsivity for a 50Ω bolometer, with an area of 1×1cm, at a bias of 1V. and a video modulation of 100Hz, is 1mV/W. The noise spectrum exhibits a 1/f rolloff till 1KHz, and is limited by the Johnson noise for higher frequencies. The NEP of the detector is 3μWHz−1/2 at a video modulation of 1KHz. It is possible to decrease the current NEP by fabricating bolometers with higher responsivities. Possible application areas are absolute power calibration and localized power density measurements at millimeter and submillimeter wavelengths.

Short pulse radar used to measure sea surface wind speed and SWH

A joint airborne measurement program is being pursued by NRL and NASA Wallops Flight Center to determine the extent to which wind speed and sea surface significant wave height (SWH) can be measured quantitatively and remotely with a short pulse (2 ns), wide-beam ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60\deg</tex> ), nadir-looking 3-cm radar. The concept involves relative power measurements only and does not need a scanning antenna, doppler filters, or absolute power calibration. The slopes of the leading and trailing edges of the averaged received power for the pulse limited altimeter are used to infer SWH and surface wind speed. The interpretation is based on theoretical models of the effects of SWH on the leading edge shape and rms sea-surface slope on the trailing-edge shape. The models include the radar system parameters of antenna beam width and pulsewidth. Preliminary experimental results look promising and indicate that it may be possible to design a relatively compact airborne radar to infer, in real-time, the sea surface SWH and surface wind speed.

Short pulse radar used to measure sea surface wind speed and SWH

A joint airborne measurement program is being pursued by NRL and NASA Wallops Flight Center to determine the extent to which wind speed and sea surface significant wave height (SWH) can be measured quantitatively and remotely with a short pulse (2 ns), wide-beam ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60\deg</tex> ), nadir-looking 3-cm radar. The concept involves relative power measurements only and does not need a scanning antenna, doppler filters, or absolute power calibration. The slopes of the leading and trailing edges of the averaged received power for the pulse limited altimeter are used to infer SWH and surface wind speed. The interpretation is based on theoretical models of the effects of SWH on the leading edge shape and rms sea-surface slope on the trailing, edge shape. The models include the radar system parameters of antenna beam width and pulsewidth. Preliminary experimental results look promising and indicate that it may be possible to design a relatively compact airborne radar to infer, in real-time, the sea surface SWH and surface wind speed.

A Far Infrared Spectroscopic System Used in Fusion Plasma Diagnostics

A far infrared spectroscopic system, covering the wavelength region between 50 ?m and capable of variable spectral resolution between 100 and 1000, has been built. The spectroscopic system is described in some detail. It is comprised of a monochromator, which utilizes five different echelette gratings, each of which covers one octave, of a radiometric system for absolute power calibration, and of various cryogenic and room temperature detectors. The system works in two modes: a continuous mode using the absolute power calibration, the radiometer, and continuous standard radiation sources, and a pulsed mode without the radiometer for plasma radiation studies. In this manner the power spectrum of the observed pulsed plasma radiation is absolutely calibrated. A rather new method in monochromator design utilizing ray transfer matrix method and phase space diagrams is discussed, and various optical errors in the system are evaluated. The method of performing and the evaluation of radiation studies utilizing this system is presented. The system will be used in the very near future to measure the far infrared radiation from several of our fusion experiments: from Texas Tokamak, a large experimental fusion device; from a small, adiabatically heated, symmetric, hot electron stellarator; and from some other hot plasma devices. These experiments have strong magnetic fields, ranging between 30 to 5OkG. Moreover, high electron temperatures--between 1 to 20 keV--are expected, so that the synchrotron radiation in the far infrared will be appreciable. Synchrotron radiation from Tokamak as a function of the electron temperature and density is discussed.