GPS is a space-based radio navigation system owned by the United States government and operated by the United States Air Force. The United States military first used GPS as a navigation tool in the 1970’s, in the 1980’s the US government made GPS available to the general public free of charge. GPS system provides geolocation and time information to a GPS receiver anywhere on or near the Earth.
A- GPS components
The GPS system has three main components:
1- Space segment:
The space component consists of about 31 GPS satellites, the United States Air Force operates these 31 satellites + 3 – 4 decommissioned satellites that can be reactivated if needed. a minimum of 24 satellites are operational at any given moment, ensuring that at least four satellites are in view at the same time from almost any point on earth. in a specially designed six orbits at around 10,900 nautical miles which is around 20,200 kilometers above the earth.
Taking about twelve hours for each orbit, the orbit are inclined at 55 degrees to the Earth’s equatorial axis. each satellite carries four atomic clocks, which are compared internally with each other to provide leverage time high accuracy. accuracy is of the order of nanoseconds. the complete coverage that satellites offer makes the GPS system the most reliable navigation system in modern aviation.
2- Control segments:
The control segment is made up of a series of ground stations, it consists of worldwide monitor and control stations that is used to interpret and relay satellite signals to various receivers. Ground stations include two master control station, an alternate master control station, twelve ground antennas and sixteen monitoring stations. in simple language control stations look after GPS satellites to maintain satellites and their proper orbits, it also adjusts satellite clock and uploads updated navigational data.
3- Users segment:
The user segment of the GPS system involves various receivers from all different types of industries. National security, agriculture, space, surveying and mapping are all examples of end users in the GPS system. in aviation the user is typically the pilot who’ve used GPS data on display in the cockpit of the aircraft.
B- Frequency used
The GPS satellites transmits on two different frequencies, and the transmission is done on UHF band. first one is called the l1 band which is transmitted on 1517.42 megahertz which is called as course acquisition or CA code, this provides standard positioning service or SPS, which can be used by all users. second one is called l2 band which is transmitted on 1227.60 megahertz, which is called as P code or precise code, this provides precise positioning service or PPS which can only be used by military users only.
C- How it works?
Now let’s take the closer look our GPS system works, GPS satellites are solar-powered and fly-in medium Earth orbit and transmit radio signals to receivers on the ground, as I told you earlier that a minimum of 24 satellites are operational at any given moment, these satellites transmits the data which is called as all minute data. that is basically the specific satellite info to the user, that is their position in the orbit, time and PRN code that is pseudo-random noise code, which is basically the identification code for each satellite, which allows any user to know which satellite is transmitting.
The GPS receiver in an aircraft receives time data from the satellites atomic clocks, it compares the time it takes for the signal to go from the satellite to the receiver, and multiplies this time with speed of radio wave and to get a range position line. so by timing the signal from three satellites will determine the precise 2d or two-dimensional location, and timing the signal from at least four satellites will determine the precise 3d or three-dimensional location, which provides user with 3d fix, which gives latitude, longitude, altitude and time information.
D- GPS errors
Everything is perfect in this world but, because we see things up close we are seeing what we believe is imperfection. similarly GPS has also got the following seven errors :
1- clock time bias error: the clock on a GPS receiver might not be as accurate as the atomic clock on a GPS satellite creating a very slight accuracy problem.
2- satellite clock error: errors in the atomic clock of the satellite which gives and accuracies up to 1.5 minutes.
3- satellite ephemeris error: satellite may not be where it is saying, it is due to disturbance from the planned orbit due to which orbit calculations can be inaccurate causing ambiguity in determining the satellites exact location.
4- selective availability: the error deliberately introduced by the US to protect its own interest by dithering atomic clocks.
5- ionospheric refraction: the signal from the satellites actually slows down as it passes through the Earth’s atmosphere, it causes the waves to bend a little bit it is also called ionospheric group play. GPS technology accounts for this error by taking an average time, which means the error still exists but is limited.
6- interference: it is due to interference caused from signals on similar frequency, as the frequency received by receiver from the satellite is very weak, it is caused due to distance between the satellite and the receiver.
7- geometric dilution of position between satellites giving position is around 60 degree but, when the satellites give a poor cut at a very large angle there is a dilution of the accuracy of the derived position. the best cut is when one is overhead and other three are at 120 degree apart, so it’s all for the GPS working.