F/A-18 A/G radar EXP mode offset

[Hornet_Driver]

Why do you have to offset from a target when using EXP mode? I know it´s because of doppler shift but I can´t wrap my head around why:

1. You don´t have to offset when not using EXP mode

2. How does an offset from the target change the doppler shift?

I know this is a rather complex question but I´ll try anyway.

[dorianR666]

Radar mapping the ground simply emits the radar beam around and displays the amount of reflected energy. Light = highly reflective, dark = poorly reflective. This is how MAP mode works, no doppler filtering involved.

 

The problem is that the radar beam is fairly wide (say, 1 degree) - this is a technical limitation of the radar hardware. Which means that if you want to zoom in onto an area (EXP modes), the beam is just too wide and you wouldnt see any detail in the image. It would be just a blob. This is where Doppler Beam Sharpening (DBS) post-processing technique comes in. The radar still emits its wide beam, however when you emit the beam, say, 45 degrees to the side of the aircraft's axis, different parts of the beam will be *differently* Doppler-shifted by the ground (because the ground is moving on the axis where the plane is pointing, not on the axis where the beam is emitted). This means that if you measure the returned signal as many combinations of Doppler shifted frequency and its reflected energy, you can divide the wide beam into many tiny "subbeams" so to speak. This way you have image of much higher resolution.

 

This is why DBS doesnt work when emitting straight ahead of you. The Doppler shift within parts of the wide beam would be roughly the same for all parts of the beam. So you cant distinguish returns for each "subbeam". So you cant render EXP3 of an area in front of your plane.

Edited February 23, 2021 by dorianR666

[Hornet_Driver]

12 hours ago, dorianR666 said:

Radar mapping the ground simply emits the radar beam around and displays the amount of reflected energy. Light = highly reflective, dark = poorly reflective. This is how MAP mode works, no doppler filtering involved.

 

The problem is that the radar beam is fairly wide (say, 1 degree) - this is a technical limitation of the radar hardware. Which means that if you want to zoom in onto an area (EXP modes), the beam is just too wide and you wouldnt see any detail in the image. It would be just a blob. This is where Doppler Beam Sharpening (DBS) post-processing technique comes in. The radar still emits its wide beam, however when you emit the beam, say, 45 degrees to the side of the aircraft's axis, different parts of the beam will be *differently* Doppler-shifted by the ground (because the ground is moving on the axis where the plane is pointing, not on the axis where the beam is emitted). This means that if you measure the returned signal as many combinations of Doppler shifted frequency and its reflected energy, you can divide the wide beam into many tiny "subbeams" so to speak. This way you have image of much higher resolution.

 

This is why DBS doesnt work when emitting straight ahead of you. The Doppler shift within parts of the wide beam would be roughly the same for all parts of the beam. So you cant distinguish returns for each "subbeam". So you cant render EXP3 of an area in front of your plane.

 

THANKS

[unknown]

 

The focus in the video are satellites but it is an interesting explanation how it works.