多普特效应、B超、GNSS-R
Deliberately bouncing signals off something to learn about it (e.g. radar, echolocation) is active sensing; sensing what is already available in the surrounding environment without changing the environment to do so (e.g. eyesight, hearing) is passive sensing. GNSS reflectometry is passive sensingthat takes advantage of and relies on separate active sources - the satellites generating the navigation signals.
1. 有源(主动辐射信号)
2. 无源（不主动辐射信号，只接收已有的信号）
GNSS-R
GNSS-R 作为一种遥感技术，需要相应的信号接收和处理设备，与传统 GNSS 接 收机不同的是， GNSS-R 接收机需要对直射信号和反射信号分别进行采集和处理，数 据采集平台可以为岸基，机载，星载等条件下搭载的接收机。由于信号来源方向的不 同以及经过反射面的反射衰减和传播路径衰减后反射信号变得非常微弱，接收机需要 架设方向和增益均不同的两副天线，用来接收直射信号的低增益右旋圆极化（ RHCP ） 天线朝上，用来接收反射信号的高增益左旋圆极化（ LHCP ）天线朝下。
The UK-DMC satellite, part of the Disaster Monitoring Constellation built by Surrey Satellite Technology Ltd, carries a secondary reflectometry payload that has demonstrated the feasibility of receiving and measuring GPS signals reflected from the surface of the Earth's oceans from its track inlow Earth orbit to determine wave motion and windspeed.[1][2]
GPS signal reflections have also been used to measure soil moisture and snow depth.
