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Global Positioning System
What is a GPS?
The Global Positioning System consists of 24 satellites, that circle the globe once every 12 hours,
to provide worldwide position, time and velocity information. GPS makes it possible to precisely identify
locations on the earth by measuring distance from the satellites. GPS allows you to record or create
locations from places on the earth and help you navigate to and from those places.
Segments of GPS:
1. The Space segment: it consists of 24 satellites circling the earth at 19,000 kilometers above
altitude. This high altitude allows the signals to cover a greater area. The satellites are arranged
in their orbits so a GPS receiver on earth can always receive a signal from at least four satellites
at any given time.
2. The Control segment: The control segment tracks the satellites and then provides them with
corrected orbital and time information. The control segment consists of four unmanned control
stations and one master control station.
3. The User segment: The user segment consists of the users and their GPS receivers. The number
of simultaneous users is limitless.
How does a GPS work?
When a GPS receiver is turned on, it first downloads orbit information of all the satellites. This
process, the first time, can take as long as 12.5 minutes, but once this information is downloaded; it is
stored in the receiver’s memory for future use. Even though the GPS receiver knows the precise location
of the satellites in space, it still needs to know the distance from each satellite it is receiving a signal
from. That distance is calculated, by the receiver, by multiplying the velocity of the transmitted signal by
the time it takes the signal to reach the receiver. The receiver already knows the velocity, which is the
speed of a radio wave or 186,000 miles per second (the speed of light).
To determine the time part of the formula, the receiver matches the satellites transmitted code
to its own code, and by comparing them determines how much it needs to delay its code to match the
satellites code. This delayed time is multiplied by the speed of light to get the distance. The GPS
receiver’s clock is less accurate than the atomic clock in the satellite; therefore, each distance
measurement must be corrected to account for the GPS receiver’s internal clock error.
Lect. Note. NRM-322, Madhab Chandra Behera, Assistant Professor, NRM, College of Forestry, OUAT, Bhubaneswar
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How accurate is a GPS?
Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design.
Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. GPS
receivers are accurate to within 15 meters on average. GPS receivers with WAAS (Wide Area
Augmentation System) capability can improve accuracy to less than three meters on average.
Differential GPS (DGPS) can correct GPS signals to within an average of three to five meters.
Sources of GPS Error
1. User mistakes account for most GPS errors: ex Holding a GPS receiver close to the body or facing to
the south etc.
2. Satellite clock errors: Caused by slight discrepancies in each satellite’s four atomic clocks.
3. Orbit errors: Change in satellite ephemeris due to gravitational pull and solar pressure fluctuations.
Orbit errors are monitored and corrected by the Master Control Station.
4. Ionospheric interference: Refraction of GPS satellite radio signals by ionosphere. Fortunately, error
caused by atmospheric conditions is usually less than 10 meters.
5. Selective Availability : The intentional degradation of the satellite signals by a time varying bias.
6. Number of satellites visible: The more satellites the receiver can “see”, the better the accuracy.
Signal reception can be blocked by buildings, terrain, electronic interference and sometimes dense
foliage. The clearer the view, to the receiver, the better the reception.
7. Satellite geometry: This refers to the relative position of the satellites at any given time. Ideal
satellite geometry exists when the satellites are located at wide angles relative to each other. Poor
geometry exists when the satellites are located in a line or in a tight grouping.
GPS Terminology
2D Positioning: The receiver is only able to lock on to three satellites which only allows for a two
dimensional position fix.
3D Positioning: Position calculations in three dimensions. The GPS receiver has locked on to 4
satellites. This provides an altitude in a addition to a horizontal coordinate,
which means a much more accurate position fix.
Real Time Differential GPS:
Real-time DGPS employs a second, stationary GPS receiver at a precisely measured spot (usually
established through traditional survey methods). This receiver corrects any errors found in the GPS
signals. A DGPS station is able to do this because its computer already knows its precise location, and
can easily determine the amount of error provided by the GPS signals.
DGPS corrects or reduces the effects of:
- Orbital errors
- Atmospheric distortion
- Selective Availability
- Satellite clock errors
- Receiver clock errors
Lect. Note. NRM-322, Madhab Chandra Behera, Assistant Professor, NRM, College of Forestry, OUAT, Bhubaneswar
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DGPS cannot correct for GPS receiver noise in the user’s receiver, multipath interference, and user
mistakes. In order for DGPS to work properly, both the user’s receiver and the DGPS station receiver
must be accessing the same satellite signals at the same time.
Lect. Note. NRM-322, Madhab Chandra Behera, Assistant Professor, NRM, College of Forestry, OUAT, Bhubaneswar
