Altimeter data knowledge point 2

1. Infrastellar point trajectory
The projection point of the artificial earth satellite on the ground is called the sub satellite point , Expressed in geographical longitude and latitude .

The motion of satellites and the rotation of the earth make the points under the stars move on the earth's surface , Form the trajectory of the points below the stars .

2.footprint footprint
The area of the Earth covered by the microwave radiation from a satellite dish (transponder) is called the satellites footprint. The size of the footprint depends on the location of the satellite in its orbit, the shape and size of beam produced by its transponder and the distance from the earth.

It can be understood as ： An area that receives broadcast signals from a particular satellite ; Satellite transponder ( Transmitter or sensor ) The area of the earth's surface within the field of view .
Such as cryosat-2 antenna footprint 15km
ICESAT-2 laser footprint diameter 17m

3. Principle of satellite altimetry observation
1） Selection of altimeter frequency
Antenna diameter 0.6-1.5m Parabolic antenna , Transmitting power 2kw;

The microwave band range suitable for satellite altimetry is generally 2-18GHz,Ku Band 10.9-22Ghz

When the frequency is greater than 18GHz when , Atmospheric attenuation increases sharply , Thus, the power of the transmitted signal reaching the sea level and reflected to the altimeter is reduced ;
When the frequency is less than 2GHz when , By ground communication , Navigation, radar and other civil 、 Interference of military electromagnetic radiation , At the same time, the Faraday rotation of the ionosphere and the increase of electromagnetic radiation refraction also affect the use of lower frequencies . Therefore, the general choice is 2-18GHz.

2） Antenna gain
Antenna gain is used to measure the ability of an antenna to transmit and receive signals in a specific direction .
Under the same conditions , The higher the gain , The farther the radio waves travel .

3） Formation principle of altimeter echo

（ The picture to quote https://www.docin.com/p-1775105147.html）

4. Why does altimetry satellite need waveform resetting ？
Because satellite altimetry technology was originally designed for ocean missions , Radar altimeter is often used . Therefore, its beam is wide , The surface covers a large area , The measured altitude is the average altitude from the satellite to the region .

however , There may be some problems when it is applied to inland lakes . Inland lakes are generally small , Radar waves may touch ground objects outside the lake , The received radar echo will be contaminated , Cause distance tracking error .

Reference material ：
Principle of satellite altimetry observation ：http://www.doc88.com/p-3337851807747.html

Waveform analysis and resetting of lake water level monitored by satellite altimetry taking into account the reflection difference between land and lake .

（ Good information ） Satellite altimetry Technology ：https://max.book118.com/html/2018/0622/6214235031001202.shtm

Basic theory of satellite radar altimeter observation and Application ：
https://max.book118.com/html/2017/0919/134371226.shtm

https://www.radartutorial.eu/01.basics/Radar%20Principle.en.html