ISIS 2 Topside Sounder Ionograms

These ionograms were digitized from the original ISIS-2 7-track analog telemetry tapes using the facilities of the former Data Evaluation Laboratory at the NASA/GSFC. This data restoration project is headed by Dr. R.F. Benson (NASA/GSFC). Ionograms were digitized at the rate of 40,000 16-bit samples/sec. This sample rate is higher than the Nyquist frequency of 30 kHz. The sample frequency of 40 kHz provides a measurement every 25 microseconds corresponding to an apparent range (c*t/2) interval of 3.75 km. Ionograms with this sample rate are designated as "full" ionograms because they have the full 3.75 km apparent-range resolution. The ionograms used for most analysis, and those available from CDAWeb, were produced by averaging every four samples of the sounder-receiver video amplitude output to yield an average value every 100 microseconds corresponding to an apparent-range resolution of 15 km. These ionogram files are referred to as "average" files with standard resolution. Each ionogram consists of a fixed-frequency and and a swept-frequency portion. The time resolution between ionograms is typically 14 or 22 seconds depending on the frequency sweep range.

本数据集收录的电离图，系借助美国国家航空航天局/戈达德航天飞行中心（NASA/GSFC）原数据评估实验室的设备，从原始ISIS-2七号轨道模拟遥测磁带数字化得到。本数据修复项目由NASA/GSFC的R.F.本森博士牵头负责。电离图的数字化采样率为每秒40000个16位采样，该采样率高于30千赫兹的奈奎斯特频率。40千赫兹的采样频率每25微秒完成一次采样，对应视距（c*t/2）间隔为3.75千米。采用该采样率的电离图被称为"全分辨率"电离图，因其具备完整的3.75千米视距分辨率。大多数分析所用以及可从CDAWeb获取的电离图，则是通过对测深仪-接收机的视频幅度输出每4个采样点取平均，每100微秒生成一个平均值，对应15千米的视距分辨率。此类电离图文件被称作"平均"文件，具备标准分辨率。每份电离图均包含固定频率段与扫频段两部分。电离图之间的时间分辨率通常为14秒或22秒，具体取决于扫频范围。