An altimeter apparatus for external fuselage mounting for an aircraft includes an altimeter circuit and a chassis. The altimeter circuit is coupled to an antenna system. The altimeter circuit is configured to generate a first signal and transmit the first signal via the antenna system, receive a second signal via the antenna system, wherein the second signal is a reflected version of the first signal, and determine an altitude based on the first and second signals. The chassis is configured to contain the altimeter circuit and the antenna system, wherein the chassis is connected to a fuselage of the aircraft and is located outside of the aircraft.