What is GPS?

GPS is the acronym for Global Positioning System which is a satellite-based navigation system comprising of about 24 satellites. It is able to work under any weather conditions, in any part of the world, throughout the day without subscription or setup charges. Satellites are put into orbit for military use by the U.S. Department of Defense (USDOD) although they were available for civilian use in the 80s.

How GPS works

GPS satellites usually revolve around the Earth two times a day in a particular orbit with each satellite transmitting unique and orbital parameters allowing GPS devices decode and calculate the precise location of the satellite. This information and trilateration are used by receivers for calculating their exact location. In short, the receiver measures the distance to each satellite by the amount of time a transmitted signal is received. The receiver is able to determine a user’s position and display it electronically with distance measurements from some more satellites.

If you are trying to calculate your 2-D position (i.e. longitude and latitude) and track movement, the GPS receiver has to be locked onto at least 3 satellites’ signals. To determine your 3-D position (i.e. longitude, latitude, and altitude), 4 or more satellites would be needed. A GPS receiver normally can track 8 or more satellite but this depends on which time of the day and your location on the earth.

When your position is determined, the GPS unit will be able to calculate other information like Sunrise and sunset, distance to your destination, trip distance, track, bearing, and speed.

Accuracy of GPS

GPS receivers of today are very accurate due to their parallel multi-channel design. Receivers lock onto satellites quickly when the device is turned on and maintain a tracking lock in dense tree cover or in urban settings with tall buildings.  The accuracy of GPS receivers can be affected by factors like weather and other error sources.

What’s the signal?

GPS satellites transmit a minimum of 2 low-power radio signals which travel by line of sight which means they can pass through clouds, plastic, and glass but won’t go through most solid objects like mountains and buildings. However, there are modern receivers that can track through houses because they are more sensitive.

There are 3 types of information that a GPS signal has:

  1. Almanac data – this tells the GPS receiver where each of the GPS satellite should be at any time of the day and shows the orbital information of the satellite and other satellite in the system.
  2. Ephemeris data – this is used for determining a satellite’s position and gives vital information about a satellite’s health, current time and date.
  3. Pseudorandom code – this is an I.D. code for identifying which satellite is transmitting information. You get to see which of the satellites you are getting their signals on your device’s satellite page.

GPS Signal Error Sources

There are several factors that affect the signal and accuracy of GPS which include:

  1. Satellite Geometry/shading – Signals of satellites are more effective whenever the satellites are at wide angles relative to each other instead of in a line or tight grouping.
  2. Orbital errors – The satellites reported may not be relatively accurate.
  3. Signal multipath – GPS signals may reflect off solid objects like tall buildings or large mountains before it gets to the receiver which increases its travel time and may cause errors.
  4. Receiver clock errors – The built-in clock of a receiver may have a slight timing error because it is not totally accurate than the atomic clocks on GPS satellites.
  5. Ionosphere and troposphere delays – Satellite signals slow down when they pass through the atmosphere. However, the GPS system has a built-in model which corrects this error partially.
  6. Number of visible satellite – The accuracy of the receiver depends on a higher number of satellites it sees. If a signal is blocked you may get position errors or even no position reading at all. Normally, GPS units do not work underground or underwater but receivers of today which are high-sensitivity can track some signals in such places.

 

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