developed by DOD
initially operated in
1993 ,fully operated
intersection of spheres
why consistent, accurate clocks are
Determining Range of GPS
Dilution of precision
GPS involves THE FOLLOWING
Space segment of GPS satellites.
GPS Satellites Details.
Total number of satellites are 30,24 are
under work,6 are extra satellites.
Each satellite has 6 orbits.
3 satellites works for GPS, 4th
work for accuracy.
3 satellites for 2d fix.
Satellite orbital distance 20,000km.
Orbital speed is 14,000km/hr
60 degrees apart and 55 degrees
with respect to equatorial plane.
Use measurement of 4 satellites
Distance=travel time*speed of
Distance from two satellites defines 2 points
(in 2 dimensional space.)
The distance from a third
satellite narrows the
location to an
THREE SV RANGES KNOWN
Satellite-1) 20,000km radius
Satellite-2) 21,000km radius
Satellite-3) 22,000km radius
Located at one of these 2 points
However, one point can easily be
Eliminated because it is either not
on earth or moving at impossible
Rate of speed.
CESIUM ATOMIC CLOCKS are used in GPS
Due to general theory= 45 microseconds
special theory= 7 microseconds
overall effect= 38 microseconds
Hence GPS satellite are slowed by 38 microseconds
of a second error occurs
it introduce error of 1,860 miles.
Receiver and satellite use same code
GPS sync with each satellite by shifting the
timing of the start of an internally generated
Compare incoming code with receiver generated
Measure time difference
between the same part of code
Series of ones
and zeroes repeating
every 1023 bits. So
of bits that pattern looks
random thus called
Technique used to correct some of these errors
Referred to as “differential GPS” or DGPS
In DGPS, two GPS receivers are used
One receiver is located at an accurately
surveyed point referred to as the “base
A correction is calculated by comparing
the known location to the location
determined by the GPS satellites
The correction is then applied to the
other receiver’s (known as the “rover”)
GPS system allows you to search the local
area for nearby AMENITIES such as hotels,
GPS is very useful during adventures.
GPS devices are perfect for water navigation.
Since there are no landmarks in large bodies