ISIS1 Swept-Frequency Sounder

The ISIS 1 (International Satellites for Ionospheric Studies) Swept-Frequency Sounder measured the
ionospheric electron density in the altitude range 300 to 3500 km, and provided correlative data for the other
ISIS 1 experiments, particularly those measuring ionospheric parameters. The ionosonde was a radio transmitter
that recorded the time delay between a transmitted and returned radio-frequency pulse. A continuum of frequencies
between 0.1 and 20 MHz was sampled every 19 or 29 s, and one of six selected frequencies was also used for sounding
for a few seconds during each 19- or 29-s period. In addition to the sweep- and fixed-frequency modes of operation,
a mixed mode was available in which the transmitter frequency was fixed at one of six possible frequencies while
the receiver swept. Several virtual-range (delay-time) traces resulting from ground reflections, plasma resonances,
birefringence of the ionosphere, nonvertical propagation, etc., were normally observed.

国际电离层研究卫星系统（ISIS 1）的扫描频段声纳对300至3500公里的电离层电子密度进行了测量，并为其他ISIS 1实验提供了相关性数据，尤其是那些测量电离层参数的实验。电离层 sounding仪是一种无线电发射器，它记录了发射和返回的射频脉冲之间的时间延迟。在0.1至20 MHz的频率范围内，每隔19或29秒采样一个连续的频率序列，在每个19-或29秒周期内，还使用六个选定的频率之一进行几秒钟的sounding。除了扫描和固定频率的操作模式外，还提供了一种混合模式，在这种模式下，发射频率固定在六个可能频率中的一个，而接收器进行扫描。通常观察到由地面反射、等离子体共振、电离层的双折射、非垂直传播等引起的几个虚拟范围（延迟时间）轨迹。