Why?

A useful way to spot signs of Tropospheric propagation!

Normally 1090Mhz signals are line-of-sight limited by local geography, height of target and curvature of the Earth.

During lift conditions the signals tend to reveal target aircraft further than the regular limit.

Also aircraft and ground stations may appear at elevations less than zero degrees.

N.B. Provding the altitude of the site's antenna above sea level is configured > 0 it would be expected to see negative elevation angles under normal atmospheric conditions.

However during periods of tropospheric ducting the elevation angles will be observed more negative than under "flat" conditions.

The elevation angle will (in theory!) report an angle of 90.000° if an aircraft really is on top of you. The E35L above was pretty close!