How does New Shepard Crew Capsule sense proximity to trigger retropropulsion just before impact?

Question

How does the New Shepard Crew Capsule sense proximity to the ground in order to trigger retropropulsion just before impact?

GIFs from Blue Origin NS-10: New Shepard launch & landing, 23 January 2019

For example, Soyuz uses a gamma ray altimeter for several reasons:

• How does a gamma ray altimeter work? What kind of sensor is used?
• Soyuz landing ground detection

For reference:

GIF from Soyuz MS-08 landing