B737NG why RA reading -4 on the gate or in the ground?

[Dominic]

Hello, please kindly help me to answer, im curious why radio altimeter on b737-800 or 900 on the ground or on the gate reading -4 ??? In my point of view, radio altimeter should be calculate distance from the plane to the ground ? But why on the ground ra reading -4?? Somebody please help me to explain to me.. thank you

[sjap]

This is because when the a/c is landing, and just as the wheels touch the ground, the altimeter is at 0
Because the a/c is flaring on that moment, the nose is approx 4 ft up in the air. And the radio alt antennas are just behind the nosewheel.
So when the main wheels touch the ground, the indication is 0. the correct alt on that moment.

When the nose lowers during the landing, then the antennas go lower as well and as soon as the nlg is on the ground the altimeter antennas are also lower and therefore read -4.