Q.X-rays are not used for radar purposes, because they are not :

The ionosphere reflects radio waves back to Earth in the same way a hot layer of air near the ground bends and totally internally reflects light rays to produce a mirage.

The ionosphere is a region of the upper atmosphere where free electron density varies continuously with height. As a radio wave travels upward, it meets layers of gradually changing electron density (refractive index), and beyond a certain angle of incidence the wave bends back down towards the Earth instead of escaping into space — exactly the mechanism of total internal reflection in a medium whose refractive index varies continuously, as happens with hot air layers near a road surface producing a mirage (light bending due to a continuously varying r …

Very high frequency signals (UHF, hundreds of MHz to a few GHz) are neither reflected by the ionosphere nor follow the Earth's curvature well, so they travel essentially in straight lines as space waves (direct + ground-reflected).

Radio wave propagation depends on frequency:

• Ground waves — travel along the Earth's surface, used for low frequencies (up to ~2 MHz); heavily attenuated at high frequency.
• Sky waves — reflected by the ionosphere, useful roughly up to ~30-40 MHz (HF band); UHF frequencies are too high to be reflected by the ionosphere — they simply pass through it into space. …

X-rays are unsuitable for radar because they are not reflected by the target.

Radar works by sending a wave, having it reflect off a target, and detecting the echo. X-rays are highly penetrating and get largely absorbed by matter instead of being reflected back, so no usable echo returns. (They are genuine electromagnetic waves — that i …

Sky waves travel up to the ionosphere and are reflected back to Earth, so this mode is called Ionospheric Wave Propagation.

Sky wave propagation refers to radio waves (typically in the HF/MF range) that are transmitted upward from the antenna, travel to the ionosphere — the ionised upper layer of Earth's atmosphere — and are refracted/reflected back down to the Earth's surface. Because this bending happens due to the ionosphere, this mode of propagation is called Ionospheric Wave Propagation.

This is distinct from:

• Ground wave (tropospheric) propagation — waves that travel along/close to the Earth's surface, used for low-frequency signals. …

Sky-wave propagation depends on the ionosphere reflecting the wave back down; above a certain "critical frequency" the ionosphere can no longer bend the wave enough, and it simply passes through into space.

Reasoning

Sky-wave propagation relies on refraction/reflection of radio waves by the ionised layers of the ionosphere back toward the Earth. This reflection is possible only up to a maximum usable/critical frequency (roughly 30–40 MHz), because the degree of bending the ionosphere can produce falls as frequency rises (higher-frequency waves are refracted less). VHF, UHF and microwave frequencies (tens of MHz to several GHz) are well above this critical frequency, so instead of bending back to Earth they simply pass through the ionosphere and escape into space — hence these bands rely on space wave (line-of-sight) propa …

Radio-frequency signals reach a receiver by one of three physical routes, each favoured by a different frequency band.

The three modes

1. Ground (surface) wave propagation — the wave travels along the Earth's surface, following its curvature; used for low-frequency (LF/MF) broadcast signals.
2. Sky wave propagation — the wave is reflected back to Earth by the ionosphere, allowing it to travel very long distances; used for short-wave (HF) communication. …

Different frequency bands of radio waves use different propagation mechanisms - ground, sky, and space wave - so all three are valid ways radio signals travel.

Radio waves of different frequencies travel through the atmosphere by different mechanisms:

• Ground wave propagation: Lower-frequency waves (up to a few MHz, e.g. AM radio) travel along/close to the Earth's surface, guided partly by diffraction around the curvature of the Earth and partly by following the conducting ground.
• Sky wave propagation: Medium and high-frequency waves (roughly 2-30 MHz) are reflected back to Earth by the ionosphere, allowing them to be received well beyond the line-of-sight/ground-wave range, even around the globe via multiple reflections. …