ALMA MEMO No . 535 Simulation Series of a Phase Calibration Scheme with Water Vapor Radiometers for the Atacama Compact Array
ALMA MEMO No . 535 Simulation Series of a Phase Calibration Scheme with Water Vapor Radiometers for the Atacama Compact Array
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发表时间：
2005
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通讯作者：
Y. Tamura
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作者：
Y. Asaki;R. Kawabe;K. Morita;Y. Tamura
我们已经进行了一系列的模拟阿塔卡马紧凑型阵列（ACA）1使用水蒸气辐射计（WVR）的相位校准计划。在所提出的方案中，专门用于测量对流层水汽含量的WVR连接到12米的天线。利用WVR观测数据，将高空对流层水汽变化引起的多余路径长度（EPL）拟合成一个简单的二维斜率。由于湍流水汽的干涉相位波动的每个基线获得从推断的视线EPL的差异，并从干涉相位的校正减去。我们估计的剩余均方根（RMS）相位为30米基线后，WVR测量误差，风速，系数和幂律指数的EPL的空间结构函数的各种值的校正，和WVR仪器的分布范围。从模拟中，我们发现，对于WVR没有测量误差，所提出的校准方案显示出良好的性能，为所有对流层条件下，和一个简单的关系之间的剩余RMS相位和WVR分布的程度存在：范围越接近干涉仪阵列，更有效的校准方案将是。当增加WVR测量误差时，尽管RMS相位在不稳定对流层条件（50百分位条件）下仍然得到改善，但在良好条件（25百分位条件）下可能不需要所提出的校准方案。我们发现，如果WVR的性能达到预期的水平（一个因素约3比目前的阿尔马WVR规范），该计划是相当有前途的ACA。此外，我们提供了一个简单的统计模型来解释模拟结果。模拟和统计模型的分析表明，基线为10-30 m的无线电视宁度数据对于确定干涉仪阵列周围WVR分布的最佳范围是重要的。在观测过程中，对射电视宁度和WVR测量误差的监测将有助于判断是否应在随后的ACA数据简化中使用预先校准的数据。对于ACA进行的低海拔观测，WVR的预期性能也很重要。1 ACA由12个用于干涉测量的7米天线和4个用于测量天体源总功率的12米天线组成。
We have carried out a series of simulations of a phase calibration scheme for the Atacama Compact Array (ACA) 1 using water vapor radiometers (WVRs). In the proposed scheme the WVRs devoted to measurements of tropospheric water vapor content are attached to the 12-m antennas. The excess path length (EPL) due to the tropospheric water vapor variations aloft is fitted to a simple two-dimensional slope using WVR measurements. Interferometric phase fluctuations for each baseline due to the turbulent water vapor are obtained from differences of inferred line-ofsight EPL and subtracted from the interferometric phases for the correction. We have estimated residual root-mean-square (RMS) phases for 30-m baselines after the correction for various values of WVR measurement errors, wind velocities, coefficients and power-law exponents of the spatial structure function of the EPL, and extents of the distribution of the WVR instruments. From the simulations we found that, for WVRs with no measurement errors, the proposed calibration scheme shows an excellent performance for all tropospheric conditions, and that a simple relationship between the residual RMS phase and the extent of the WVR distribution exists: the closer the extent to the interferometer array is, the more effective the calibration scheme will be. When WVR measurement errors are added, although the RMS phases are still improved in unstable tropospheric conditions (50-percentile conditions), the proposed calibration scheme may not be needed in good conditions (25-percentile conditions). We found that, if the performance of the WVR achieves the expected level (a factor of about three better than the current ALMA WVR specification), the proposed scheme is quite promising for the ACA. In addition, we provided a simple statistical model to explain the simulation results. The simulations and the following analyses with the statistical model show that radio seeing data with baselines of 10–30 m is important to determine the optimum extent of the WVR distribution around the interferometer array. Monitoring of the radio seeing as well as the WVR measurement errors during observations will be useful to judge whether pre-calibrated data with the proposed calibration scheme should be used in the following ACA data reduction or not. For observations at lower elevations made by the ACA, the expected performance of the WVR is also important. 1 The ACA consists of twelve 7-m antennas for interferometry and four 12-m antennas to measure total power of celestial sources.