Component of GPS survey in civil engineering
- GPS Ground control stations
- GPS satellites
- GPS receivers
GPS Ground control stations
Ground control components include Falcon Air Force Base, Colorado Springs, Master Control Station and Falcon AFB in Colorado, Monitoring Station in the Atlantic, Diego Garcia in the Indian Ocean and Kwajalein Island in the South Pacific. The control section uses measurements collected by monitor stations to predict the behavior of each satellite’s orbit and atomic clocks. Prediction data is linked to satellites for users to broadcast. The control section also ensures that the GPS satellite orbits stay within range and the satellites do not go too far from nominal orbits.
The space segment includes satellites and delta weapons that launch satellites from Cape Canaveral in Ky, USA. GPS satellites orbit in circular ranges at an altitude of 17,440 km, each orbit lasting 12 hours. The ranges are tilted at the equator by 55A to ensure coverage in the polar regions. The satellites are powered by solar cells to orient themselves continuously to point the solar panels towards the antennas towards the Sun and Earth. Each satellite has four nuclear clocks.
When you buy a GPS, you are really only buying the GPS receiver and make free use of the other two main components, valued at billions of dollars – praise from the United States government. In the element-map datum survey and geodesy of satellite-based surveys, a datum is a reference system or an approximation of the Earth’s surface against which situational measurements are made for computing locations. The horizontal datum is used to describe a point on the surface of the Earth in latitude and longitude or any other coordinate system.
According to satellite and receiver local clocks, any navigation satellite provided by the GNSS receiver is based on the distance of a set of satellites, by means of extracting the propagation time of the incoming signals through space according to the receiver’s speed.
Note that satellites are always in motion, so before receiving a navigation message, the satellite’s signal is detected and tracked. The functional blocks of the receiver that perform these functions are antenna, front-end, and baseband signal processing (in charge of receiving and tracking signals).
After the signal is acquired and tracked, the receiver decodes the application navigation. message and estimates the user position. The Navigation Message includes:
- Ephemeris parameters, required to compute the coordinates of the satellite.
- Time and Clock Corrections to compute satellite clock offsets and time conversions.
- Service Parameters with satellite health information.
- Ionospheres’ parameters model needed for single frequency receivers.
- Almanacs that allow calculate the position of all satellites but with a less accuracy than the ephemeris.
- The ephemeris and clocks parameters are usually updated every two hours, while almanacs are updated at least every six days.
The GPS Signal in Space is specified in the following documents:
- IS-GPS-200E: Interface between the space segment of the Global Positioning System. Navigation user segment of the GPS for radio frequency link 1 (L1) and link 2 (L2)
- IS-GPS-705A: Interface between the space segment of the Global Positioning System. The Navigation user segment of the GPS for radio frequency link 5 (L5).
- IS-GPS-800: interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for signal Ll Civil (L1C) transmitted in the frequency band of Li
Read more : Types of Surveys in Civil Engineering
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