I was thinking about the same thing as you do for the past two weeks. Anyway, according to DCS website, they removed the radar altimeter scale from HUD. Perhaps the real F15C don't have a radar altimeter. However, I read somewhere on the internet the F15E does have a radar altimeter.See the attached info below. You can press f10 and click on the airport if you want to find the elevation of the airport. I guess during bad weather with minimum visibility in real life, the pilot have to radio the airport to find out the elevation of the airport.
New F-15C Cockpit and Updated Cockpit Systems
Perhaps the most noticeable new features with Lock On: Flaming Cliffs 3 is the new, beautiful cockpit. This cockpit is fully rendered in every detail and provides 6 degrees of freedom of movement within the cockpit. This new cockpit includes both day and night lighting, an improved HUD that is focused to infinity, re-worked HUD symbols and many improvements and new functions for the radar and radar warning receiver:
– New flap position lights
– Flaps position lights logic was updated.
– AI & SAM warning lights are functional now.
– Active jamming is indicated by open X in the center of TEWS screen. Flashing X - standby, steady X - active.
– CHAFF/FLARE, and MINIMUM lights are operational now.
– Marker beacon light was made functional. MKR display was added on HUD.
– Added CSET display on HUD - is displayed for the first 10 seconds after ILS mode selection.
– Steer mode select knob was animated.
– Fixed HUD Integrated Flight Director indication.
– Added bank steering bar on HUD.
– HUD altitude scale is always barometric now.
– Removed radar altimeter scale from HUD.
– Fixed AoA scale and numeric indication on HUD.
– Adjusted HUD display logic of Mach number and current/maximum G displays.
– ADI light was made functional.
– Fixed unstable radar lock at certain ranges in the vertical scan mode.
– Radar scan zone in TWS is limited by 60 degrees now.
– DTWS scan zone automatic centering is implemented.