Chapter 14: GPS and Maps
Global positioning system (GPS) is widely used today in terms of navigation, data collection, and mapping
applications because of its many advantages such advantages are; ease of use, low cost, coverage over large
areas, relatively highspeed data collection, ability to collect either two or threedimensional data, ability to
provide velocity data, and ability to provide accurate time information. The last advantage is very important
because when providing data and accuracy, time plays a vital role.
GPS receiver gives absolute position—a specific position defined by coordinates such as latitude and longitude
GPS data are usually used in geographic information systems (GIS).
A Brief Overview of GPS
NAVSTAR GPS was born after the realization of LORAN’s shortcomings. Around 12 billion dollars
were spent on increasing the technology of LORAN because it had limited accuracy, radio interference
and much more.
GPS consists of 3 segments;
Space segment (SS): It’s a constellation of GPS satellites positioned in medium earth orbit at an
altitude of around 20,000 kilometers and used for weather satellites as it’s mentioned in chapter
9. Originally, the GPS constellation consisted of 24 satellites, but as of 2008, there are now 32.
The additional satellites improve the precision of GPS positioning calculations by providing
Control segment (CS): CS of the GPS is the set of ground monitoring stations that track the
location and health of each satellite, which includes conditions such as clock error and satellite
malfunctions. Satellites are tracked by their own signals and the CS uploads estimated positions
periodically, so receiver will know where to look for the satellites.
User segment (US): US of the GPS is the receiver, which has an antenna tuned to the radio
frequencies transmitted by the satellites. The receiver also has a clock and a processor to
“decode” the satellite’s signal. However, receiver’s clock is not as accurate.
It’s the intentional degradation of the GPS signals. However, in 2000, U.S gov’t recognized the
increasing importance of GPS to commercial and civil users, decided to discontinue the
intentional degradation of accuracy of the GPS signals. Their decision for this was influenced by
a number of factors such as in 1990, US FAA began pressuring the military to turn off SA
permanently so that FAA can save millions of dollars because they were replacing their own
radio navigation systems (which required continual monitoring and maintenance). But, better
accuracy attained by postprocessing the data.
Eventually, realtime data correction became possible using GPS augmentation, in which
additional info is used along with the satellite signals to provide more accuracy of positioning.
The future of GPS The suspension of SA has allowed the GPS to be used by everybody in every sector. Other
countries are trying to develop their own international satellite navigation systems. For instance,
Russian GLONASS is attempted to be updated and be more operational. European Union is
developing a 30satellite system called Galileo. We cannot fully expect what’s waiting in the
future in terms of GPS development but without a doubt there is instigation under construction.
How GPS works—The big picture
GPS satellites transmit signals to receivers on the ground and receivers decode the message into precise
time and location info. They need clear view of skies with no obstructions, so these GPS receivers aren’t
really working well in mountainous area or obstructive area.
The accuracy of a GPSdetermined position depends on the receiver and the field conditions. Most
handheld GPS has 10 to 15 meters accuracy.
How GPS works—The details
You need to know the exact location of satellites in space in order to determine GPS location.
Each satellite makes two revolutions in one sidereal day. Each satellites orbital period is ~11
hours and 58 minutes, so that at the end of a sidereal day