多普特效應(yīng)、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(主動(dòng)輻射信號(hào))
2. 無源（不主動(dòng)輻射信號(hào)，只接收已有的信號(hào)）
GNSS-R
GNSS-R 作為一種遙感技術(shù)，需要相應(yīng)的信號(hào)接收和處理設(shè)備，與傳統(tǒng) GNSS 接 收機(jī)不同的是， GNSS-R 接收機(jī)需要對(duì)直射信號(hào)和反射信號(hào)分別進(jìn)行采集和處理，數(shù) 據(jù)采集平臺(tái)可以為岸基，機(jī)載，星載等條件下搭載的接收機(jī)。由于信號(hào)來源方向的不 同以及經(jīng)過反射面的反射衰減和傳播路徑衰減后反射信號(hào)變得非常微弱，接收機(jī)需要 架設(shè)方向和增益均不同的兩副天線，用來接收直射信號(hào)的低增益右旋圓極化（ RHCP ） 天線朝上，用來接收反射信號(hào)的高增益左旋圓極化（ LHCP ）天線朝下。
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.
