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GPS

What is GPS?

GPS stands for Global Positioning System. It is a group of government satellites made by the U.S. to provide accurate information on global/geographical positioning and navigation accessible from anywhere on the Earth.

History

How does it work?

Components

Space Segment - satellites

  • GPS uses 24 satellites (3 of them are extras and are not active) that complete their orbit every 12 hours. Their orbits are situated so that at any point on the Earth's surface there are always 5 to 8 satellites above the horizon.

Control Segment - ground stations

  • We have a total of 5 ground-based control stations. The main one is the Falcon AFB Colorado Springs These ground stations control the satellites and "talk" to them.

User Segment - receivers

  • This component is referred to as the "user". often where we receive the information sent from the satellites. GPS receivers and other users who compute velocity, time, and position using the satellite signals. The resulting information is often used for navigational purposes.

So How Does It Work? Triangulation!

GPS is a ranging system that does triangulation. Because it is triangulating, it requires reference systems in the form of satellites that move at 4 km/s. 

  1. The user (receiver) listens and detects 1-way signals from several satellites. Each transmission is a coded signal containing information along with the time-of-transmission and the satellite position. Signals are usually sent continuously (it is always sending something).
  2. The user compares the time-of-arrival of the signal and the time-of-transmission.
  3. The difference in time (delta T) is multiplied by the speed of light to obtain the range (or distance).
  4. Every range from a satellite in space puts us in a sphere around the satellite of a certain radius. This radius is the range. Intersecting several of these spheres allows the user to determine its position. Accuracy can be increased by increasing the number of satellites, and subsequently increasing the number of spheres.
  • Three satellites place us at one of two potential location points. To fix a three-dimensional location using satellites, we need 4 satellites to eliminate one of the points.

Note on Distance Measuring

Distance = Rate x Time. 

Rate is the speed of light (186,000 miles/second). Time is the time it takes for a signal to travel from the satellite to the GPS receiver.

Remember: the entire system is dependent on Time!

Measuring time on a satellite is done by SV clocks. There are 2 cesium and 2 rubidium clocks in each satellite ($100,000-$500,000 each lol!)

Measuring time at the GPS receiver does not need cesium or rubidium clocks. Receiver clocks are similar to a quartz watch. There are always errors between the satellite and receiver clocks, and that is Delta T! This is another reason why 4 satellites are ideal: they are required to solve x, y, z, and Delta T.

Signals

Usually, two L-band carriers are used to broadcast GPS signals in a Carrier Phase GPS.

  • L1 uses 1575.42 MHz and is modulated using both C/A-code and P-Code. It contains navigational messages and SPS code signals
  • L2 on the other hand uses 1227.6 MHz, and only the P-Code is used to modulate it. While unsophisticated satellites track only L1 using simple correction models, both L1 and L2 are required to filter out ionospheric delay by calculating the difference between the carriers (L1-L2).

Different from Carrier Phase GPS, Code Phase GPS primarily bases itself on C/A codes.

The C/A Code

C/A stands for coarse acquisition. A C/A code is a series of 1 MHz pseudo-random noise (PRN) codes - series of binary numbers - that repeats itself every 1 millisecond. 

The Almanac

The almanac is a collection of additional information broadcasted by a satellite about all the other satellites. When the receiver system is powered up, the almanac provides initial information the user can use to see where the other satellites will be located as well as their condition status. And because satellites are in a fixed orbit and are thus their locations are easily predictable, an almanac can be valid for months. However, despite sacrificing some accuracy in the presented format, an almanac is still a large file that can take several minutes (up to 12.5 minutes) to complete its transfer.

Time is Distance

If we are able to calculate the difference in time between code transmission and arrival, by looking at the difference between the code received and the code generated by the receiver, we can determine the distance between the satellite and the receiver. The greater the difference in time is, the greater the distance between the satellite and the receiver.

So if you want to make it harder for an outsider to gather accurate information from your satellites, the best way is to include noise (that we can easily identify) into the codes. This can be done using special encrypted chips.

GPS Errors

Sources of GPS Positional Error can include:

  • Noise is typically the case. This can lead to slight variations in the location data, and this can be used to assess the accuracy of the data. 
  • Bias
  • Blunders/mistakes - putting in the wrong coordinates into your recording system, user error.

 

Multi-Path Errors

Multi-path errors can occur because a satellite sends out radio waves in all directions. Anything can happen to these signals before they reach the GPS receiver.

  • Signals are not pervious or penetrable through all objects. Buildings, trees, walls, and mountains can prevent transmitted signals from reaching you.
  • Signals can get reflected off of certain objects, including water, buildings, metal walls, etc. As a result, any signal received from the GPS receiver around this area may contain inaccurate data.

How to Reduce Errors

  • Do it in an open area where the sky is not blocked.
  • Avoid places with materials that might prevent or disrupt the signals, including buildings and tall trees.
  • Take multiple measurements to check.
  • Have patience, because satellites are always moving. With time, non-visible satellites may come into view and allow you to gain better measurements.

GPS and GIS