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GPS satellites are powered by solar energy. They have backup batteries
onboard to keep them running in the event of a solar eclipse (затмение), when there's
no solar power. Small rocket boosters on each satellite keep them flying in the correct
path.
Here are some other interesting facts about the GPS satellites:
The first GPS satellite was launched in 1978.
A full constellation (созвездие) of 24 satellites was achieved in 1994.
Each satellite is built to last about 10 years. Replacements are constantly being
built and launched into orbit.
A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across
with the solar panels extended.
Transmitter power is only 50 watts or less.
GPS satellites transmit two low power radio signals. The signals travel by line
of sight, meaning they will pass through clouds, glass and plastic but will not go
through most solid objects such as buildings and mountains.
A GPS signal contains three different bits of information — a pseudorandom
code, ephemeris data and almanac data.
Some factors that can degrade the GPS signal and thus affect accuracy include
the following:
The satellite signal slows as it passes through the atmosphere.
The GPS signal is reflected off objects such as tall buildings or large rock
surfaces before it reaches the receiver. This increases the travel time of the signal,
thereby causing errors.
A receiver's built-in clock is not as accurate as the atomic clocks onboard the
GPS satellites. Therefore, it may have very slight timing errors.
The more satellites a GPS receiver can «see,» the better the accuracy.
Buildings, terrain, electronic interference, or sometimes even dense foliage (листва)
can block signal reception, causing position errors or possibly no position reading at
all. GPS units typically will not work indoors, underwater or underground.
Текст 5
OPERATING SYSTEMS
When computers were first introduced in the 1940's and 50's, every programme
written had to provide instructions that told the computer how to use devices such as
the printer, how to store information on a disk, as well as how to perform several
other tasks not necessarily related to the programme. The additional programme
instructions for working with hardware devices were very complex and time-
consuming. Programmers soon realized it would be smarter to develop one
programme that could control the computer's hardware» which others programmes
could have used when they needed it. They created the first operating system. Today»
operating systems control and manage the use of hardware devices such as the printer
or mouse. They also provide disk management by letting you store information in
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