What is spillover vs ground reflection in radio astronomy?
In a radio telescope’s noise budget, spillover and ground reflection are two distinct ways unwanted radiation enters your receiver. They’re related but not the same:
Spillover
Spillover is receiver noise caused by the feed “seeing” outside the dish.
A feedhorn or dipole ideally illuminates only the reflector surface. In practice, its beam pattern extends beyond the edge of the dish. Radiation entering through those side and back lobes bypasses the reflector entirely.
What contributes to spillover noise
- Warm ground emission (biggest contributor).
- The sky outside the main beam, which includes the Galaxy, the Sun, and general sky background.
- Any nearby warm structures.
Why it matters
Spillover directly increases system temperature. In most dishes, ground-spillover can be 10–30 K of added noise unless the feed is well-matched to the reflector’s f/D.
Ground Reflection (Ground Bounce)
Ground reflection refers to radio waves from the intended source being reflected off the ground and re-entering the feed, adding an unwanted, delayed copy of the signal.
Instead of bypassing the dish (as in spillover), the reflected signal:
- Comes from below the horizon,
- Is phase-shifted and weaker,
- Can produce standing waves or ripples in the measured spectrum.
Where it shows up
- Spectral bandpass: sinusoidal ripples vs frequency.
- Power vs elevation plots: nulls and peaks.
- Interferometry: changing fringe patterns as geometry changes.
How to remember the difference
- Spillover = feed sees the wrong things.
(Noise bypasses the dish and directly enters the feed.) - Ground reflections = correct signal arrives the wrong way.
(A reflected, out-of-phase version of the sky signal adds interference.)