Global Positioning System (GPS) is made up of a constellation of at least 24 satellites in orbit around the Earth. Currently, there are 31 operational satellites (some are spares and backups), but the government has committed to having at least 24 usable satellites 95% of the time. That was expanded to 27 satellites which comprise the core constellation. Each satellite is in a medium Earth orbit at an altitude of over 12,000 miles and circles the Earth twice a day.
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In order to determine your position on Earth (lat/long, or UTM), your receiver must receive signals from at least 3 satellites. That's called 2-D mode.
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In order to determine your elevation (or altitude) also, your receiver must receive signals from at least 4 satellites. That's called 3-D mode.
Accuracy and Dependability​
The GPS system is maintained by the US Government's Department of Defense, specifically the US Space Force and so civilian use is completely dependent upon the government. However, to date, there has been no deactivation of systems since it was started in 1995.
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Your ability to use GPS accurately depends upon whether you have “line of sight” signal access to the constellation of satellites. That is, do you have a clear shot to the satellite so GPS signals may transmit and receive freely. So if you are in a canyon or between tall buildings, you may not be able to receive signals from all of the satellites above the horizon, only those in the sky above you. Sometimes, the signal you receive may be reflected off of nearby structures (like buildings). Sometimes weather can interfere with signal strength as well. All of these issues can degrade the accuracy of your particular GPS at the time you are using it.
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Why is line of site important? Because each of the GPS satellites transmit data signals that are used by GPS receivers on Earth. The data transmitted includes information on the satellite's current position in space and the time the signal was sent from the satellite. GPS receivers on Earth read that information, and using the time differences between when the signal was sent and when it was received, the effects of gravity, and knowing the speed of the signals (speed of light) they calculate your position accurately. If the signal is interfered with between the satellite and the receiver by buildings or other structures, it could delay the signal from reaching you and give the appearance that you are further away from the satellite than you really are. That delay will end up as an error in determining your exact location.
For security reasons, they used to degrade the accuracy of the system when used for civilians. That intentional degradation is called Selective Availability (SA). That degradation was “eliminated” in May 2000 and the accuracy of the system should be within 5 m with an open sky (as stated https://www.gps.gov/systems/gps/performance/accuracy/). The US has no plans to re-implement SA in the future.
Usually GPS receivers will report the estimated accuracy with the symbol "±" (read "plus or minus"). The government has committed to average accuracy of the signals to within ± 2 meters 95% of the time. This means that 95% of the time, the GPS can get you within 2 meters of your intended target. Note that doesn't mean that your receiver has that accuracy, only that the signal sent by the satellites are that accurate. Most smartphones are accurate to only ±5 meters and that is only in an open field with good line of site. Proximity to buildings can degrade that accuracy further.
Most civilians GPS use only one radio frequency for these signals, while the military typically use two. As of 2022, some higher end civilian GPS do use 2 and 3 radio frequencies. A fourth radio frequency is planned to be added for additional information soon. The addition of these frequencies will make GPS more accurate and it has been stated by gps.gov that civilian accuracy can be as good, if not better than, military. I know, hard to believe. Advancements are planned to allow accuracy down to the millimeter level
Accuracy of your GPS receiver can be further enhanced by using systems like the Wide Area Augmentation System (WAAS). WAAS is a series of land-based receivers that receive GPS signals and calculates the error in the GPS calculation. These error corrections are then re-transmitted to a person's WAAS-capable GPS via radio once every second to enhance positional accuracy of a GPS receiver. Not all GPS receivers have WAAS augmentation. WAAS is often used in Aviation GPS. According to Garmin (garmin.com), use of WAAS can improve accuracy to < 3 meters 95% of the time. There are other enhancement services similar to this, some require internet connections for your GPS.
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Accuracy of your GPS depends upon things like:
1) Line of site of satellites.
2) Structures that interfere with signals from satellites. GPS doesn't work well indoors!
3) Your GPS's receiver frequencies.
4) Solar weather.
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Often GPS positions are overlayed on maps to give you a reference of where you are. There are potential errors in some of those maps that can create inaccuracies.
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Because GPS knows the time and your location, it can also calculate your speed and is basically a real-time indication of where you are. This allows you to use GPS to make certain you stay on course to within the accuracy of your system.
GPS can be set to read bearings based on True North or Magnetic North! Make sure you know your settings when using it.
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Remember the concept of datum? GPS is (currently) setup by using the World Geodetic System from 1984 (WGS84). If you're using GPS in combination with printed or other electronic maps, make sure they reference the same datum! Otherwise there may be further inaccuracies.
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Some GPS also have a strong dependence on the power source. Make sure you use fresh batteries and types that are recommended (lithium, alkaline, etc).
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Note: Some of you have picked-up on the accuracy spec of "95% of the time." What happens the other 5% of the time and how bad can the accuracy get during that 5%? 5% of one day is 1.2 hours. Think about that!
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GPS Terminology
Azimuth- A heading or bearing.
Constellation- The entirety of all the GPS satellites.
Coordinates- the Lat/Long or UTM values describing the position of something.
Course Deviation Indicator (CDI)- a graphical means of showing you how far off your planned course you are.
Course over Ground (COG)- The course the GPS is traveling superimposed on a map.
Differential GPS- A GPS augmentation method that can improve accuracy to the centimeter range.
Dilution of Precision (DOP)- A unit-less number indicating how accurate your position can be determined. The lower the number, the better precision your GPS receiver has.
Estimated Position Error (EPE)- An estimation of spacial inaccuracy when pinpointing your position. Usually given in feet or in meters. The smaller the EPE, the more accurate your position is.
Estimated Time of Arrival (ETA)- Arrival time to a way-point as the crow flies (straight path).
Estimated Time En Route (ETE)- Estimated travel time as the crow flies (straight path).
Initialization- When the GPS starts up, this is the initial calculation of its current position.
L1, L2, L5- These are three frequencies that GPS uses.
Time to First Fix (TFF)- The time it takes for your GPS to receive all of the satellite signals it needs to determine where it is. After your GPS loses its memory, is brand new, or if you turn it on at a distant place from where it was last used, your GPS can be “lost.” It takes a while, usually a few minutes, to re-learn its position. Also known as initialization.
Waypoint- Specific locations on a path you intend to travel. This could be your starting or ending point, or any point in-between you want to cross. Also called a “fix.” Waypoints are determined by "dropping a pin" on a map to mark a spot or by entry of global coordinates in either Lat/Long, UTM, or other grid format.
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Reference: Hodgeson (2000) & www.gps.gov
Be careful!
GPS is just a tool, and it's not perfect!
