GPS and Satellites

GPS and Satellites

The Global Positioning System (GPS) is actually a collection of Earth-orbiting satellites, 27 in all, 24 in everyday use and 3 extra in case one fails. The United States military developed and implemented this network for their own use and opened it up to everybody else.

They circle the globe at about 12,000 miles (19,300 km), twice every day. The orbits are designed so that at any time, anywhere on Earth, there are at least four satellites “visible” in the sky.

A GPS receiver locates four or more satellites, works out the distanc to each, and uses this data to calculate its own location. This is based on a simple mathematical principle called trilateration. When you know the distance between 1 point to 4 satellites, then that places you at a certain point on the earth’s surface.

These devices could locate the route of your journey from the Cliffs of Moher in Clare to the GPO in Dublin, the location of a ship at sea or even the precise boundary of your home.

The above diagram illustrates that the 24 satellites on the 6 orbits at 20,000km above the Earth. These satellites are constantly moving on these planes. When a person is collecting GPS data a minimum of four satellites in their vicinity is required. But, by the time their hour survey is complete more than four satellites have helped obtain the GPS readings.

Satellites were initially used solely for military intent during The Cold War but they evolved to become used for civilian purposes.

Somewhere in Ireland Ossie is recording a GPS coordinate while standing in some town in Ireland. Giving the vastness of Ireland how can we locate Ossie? Well, it is all down to the intersection of satellites.

When GPS devices are used to record the boundary of a school building or a new road, for example, the field surveyor needs to walk around the building while taking coordinates at least at every corner. Coordinates are x and y numbers that help us locate an area on a map. While the field surveyor is recording his coordinates various satellites come in and go out of range of the GPS device. As you can see from the first diagram there are more than 20 GPS satellites orbiting The Earth. These aid the recording of GPS coordinates so that field boundaries, roads, rivers and buildings can be noted and displayed on an OSi map.

Third Level

The information that the GPS devices can record from the satellite include the date, coordinates on earth, the height of the satellite above The Earth and the time. This information is received by the GPS through wavelengths from the satellite. As these wavelengths may experience deflection from surfaces on its journey to the GPS device the incorrect time may be recorded for a coordinate. The time recorded for a GPS coordinate is very important. The time is recorded on the satellite before the wavelength is sent down to Earth. When this wavelength reaches the GPS device on Earth the time is again recorded. The differences between these two times aid the identification of a point on Earth.

As well as recording the coordinates with a GPS device, numerous base stations around Ireland are also recording the same coordinate. These base stations have more advanced technology to record the correct time when the point was recorded. This Real-time positioning GPS service is important when the most precise recordings are required. Anybody who has the location of their project, the time and day, within the past 30 days, can obtain a more accurate GPS reading for their recorded GPS coordinates here. This more accurate data is termed RINEX data.

RINEX data

The Receiver INdependent EXchange Format originated from a meeting in 1989 in relation to GPS Exchange Formats. The International Co-ordination of Space Techniques for Geodesy recommended that this should be used internationally as the standard exchange format. (http://www.navcen.uscg.gov/pubs/gps/rinex/default.htm)

The basic function of RINEX is to rectify the GPS data points that were recorded from a GPS receiver. These receivers could be the size of a laptop or smaller. These GPS receivers are used by members of the OSi field staff. They record the dimensions of a new building with the aid of at least four satellites.

The satellites possess clocks on board their crafts. This record of time keeping is very important to keep accurate as time is one of the key methods that are used to calculate the coordinates of a GPS receiver on Earth. The GPS receiver also possesses a clock in its frame. The length of time it takes the wavelength to get from the satellite to the GPS receiver is recorded. If the clock on the satellite is not in sync with the time on the GPS receiver, an incorrect reading will be recorded and subsequently incorrect coordinates for the point on Earth where Ossie is standing will result. As well as this, the wavelength may be deflected off of surfaces, such as right angles of buildings, and this too could create errors and time differences between the satellite and GPS receiver and clocks.

Imagine Magellan as a member of the OSi Field Team.

She needs a GPS receiver and at least four satellites each time she takes a reading for each of the corners of a new building, which is positioned 3metres from the R445. Each of the four satellites have a clock but they may not exactly in sync. Even if they were perfectly set to the same milisecond the day that they were put up into space, over time the four clocks would change slightly. Any slight change would alter a GPS reading. The same can be uttered in relation to the clock on the GPS receiver. If Magellan did not avail of the RINEX service on the OSi website the house may not be the shape it is on the ground or even the house could be incorrectly positioned on the R445 itself. RINEX data takes into consideration the errors that may occur when recording GPS points and recalculates them to more accurate co-ordinates.

As well as errors in the recorded coordinates occurring from clock drift other factors may influence the errors such as velocity of light, the frequency of the wavelengths being emitted from the satellite. These errors can be recalculated by browsing to the RINEX data section of this website here. There you can input a general coordinate of where you took GPS readings from as well as the date captured. Base stations around the country also capture the GPS data but more accurate calculations take place there to rectify the errors with clock drift etc. In the RINEX section choose the desired radius in which the base stations will be captured. Usually the base station closer to where the GPS reading were recorded is the best choice. After the correct RINEX data files are obtained a clear difference between them and the uncorrected GPS points can be clearly seen in the GIS software.

European Satellite-Galileo

This satellite project was built by the European Union (EU) and European Space Agency (ESA). When the project is complete 30 satellites on the Medium Earth Orbit will aid accurate GPS readings for Europeans. (http://www.aatl.net.publications/gelileo.htm) Previously surveyors using global positioning systems had to avail of the U.S. GPS or Russian GLONASS satellites. There are many countries who got involved to make this project become a reality. These countries include China, Israel, Ukraine, India, Morocco, Saudi Arabia, South Korea and 27 member states.

The US was not entirely satisfied with the existence of Galileo. They considered it a threat to their safety and their US GPS satellite. The reason for this was because the US GPS satellite is controlled by the military and in times of conflict and in the past they have been able to carry out a process called ‘selective availability’. This caused intentional inaccuracies with the GPS readings so that the US-GPS satellite could not be used against them in conflict. Seen as Galileo was initially going to use the same frequencies understandable complications would occur. This issue was resolve when the EU agreed that Galileo would transmit over a different frequency.

Aside from the frequency the main difference between the US satellite to the EU satellite is that the latter is solely for the purpose of civilian use. The two base stations for Galileo are located in Munich, Germany and Rome, Italy

(http://www.tekes.fi/galileo/Performance%20Engineer%201_NN-PERF-1.doc). The main benefit that Galileo satellite will have to OSi is that the GPS coordinates recorded by the field staff and the flight plans for the capture of aerial photographs will be more accurately recorded.