Re: How is it possible to bounce a signal off of the troposhere?

Greetings:

Reference:
Australian Government's Ionospheric Prediction Service (IPS) web site
"Introduction to HF Radio Propagation"

http:// www.ips.gov.au/papers/richard/hfreport/webrep.htm

Radiowaves with wavelengths greater than one meter (3 feet) are reflected and refracted
in varing degrees by layers or regions the ionosphere. (Refraction is a bending
type of reflection. At microwave wavelengths less than about 30 centimeters (12 inches),
the signals travel through the ionosphere into space. This is one of the reasons why we use
microwaves to communicate with spacecraft.

The reflection of radio waves from the earth's ionosphere is dependent on
several factors including the time of day, the time in the eleven year
solar sun spot cycle, the strength of geomagnetic storms on the sun and
the radio frequency and wavelength being used.

In a region of the atmosphere extending from a height of about 50 km (31 miles)
to over 500 km (312 miles), where the atmospheric pressure is very low, some
of the molecules of gas are ionized by radiation from the sun to produce an
ionized gas. We can sometimes observe this ionization in the night time as the
Northern Lights. This region of the earth's atmosphere is called the Ionosphere.
The reference has drawings showing the various regions in the ionosphere.
Ionization is the process in which electrons, which are negatively charged,
are removed from (or attached to) neutral atoms or molecules to form positively
(or negatively) charged ions and free electrons. It is the ions that give
their name to the ionosphere, but it is the much lighter and more freely moving
electrons which are important in terms of high frequency (HF: 3 to 30 million cycles
per second, or 3 to 30 Megahertz.) radio propagation. Generally, the greater
the number of electrons, the higher the frequencies that can be reflected.

During the day there may be four regions present called the D, E, F1 and F2 layers
or regions. Their approximate height ranges are:

- D region 50 to 90 km; (31 miles to 56 miles)
- E region 90 to 140 km; (56 miles to 88miles)
- F1 region 140 to 210 km; (88 miles to 131 miles)
- F2 region over 210 km. (131 miles)

Quote from the reference:
During the daytime, sporadic E is sometimes observed in the E region, and at
certain times during the solar cycle the F1 region may not be distinct from the
F2 region but merge to form an F region. At night the D, E and F1 regions become
very much depleted of free electrons, leaving only the F2 region available for
communications; however it is not uncommon for sporadic E to occur at night.
Only the E, F1, sporadic E when present, and F2 regions refract HF waves. The D
region is important though, because while it does not refract HF radio waves,
it does absorb or attenuate them (section 1.5).
The F2 region is the most important region for high frequency radio propagation
as:
- it is present 24 hours of the day;
- its high altitude allows the longest communication paths;
- it usually refracts the highest frequencies in the HF range.

The lifetime of electrons is greatest in the F2 region which is one reason why
it is present at night. Typical lifetimes of electrons in the E, F1 and F2 regions
are 20 seconds, 1 minute and 20 minutes, respectively.

End Quote

As you can see, the reflection and refraction of radio waves from the atmosphere is very important
for long distance radio communications. For this reason the height of the radio
reflecting layers are continuously monitored by several University and Government
laboratories using Atmospheric Sounding which is a radar like technique.
Each day a radio propagation map is made for each continent. You can view these
maps and solar storm and sun spot information on the following web site provided
by the Australian Government:

http://www.ips.gov.au/

Best regards, Your Mad Scientist
Adrian Popa