Sep 1, 2023
GEOMETRIC DILUTION OF PRECISION (GDOP) – Understanding Network and Security for Far-Edge Computing
GDOP is a calculated value that combines the impact of several factors related to the angle at which the ground station can reach the satellites into a single coefficient that expresses how accurate a calculated position is.
Referring back to the previous figure, we can see an example of good geometry of the satellites involved. They are spread across the sky in all three axes. Contrast that with the following situation. In this case, the user is in an area surrounded by mountains. The terminal has no choice but to use samples from satellites that are closer together in the sky, and the calculated position will be less accurate as a result:
Figure 3.43 – Poor geometry due to obstructions
Other sources of GNSS inaccuracy
Atmospheric refraction is when a satellite’s signal is bent a little while traveling through the upper layers of the atmosphere. Sunspot activity can cause interference. Lower-quality receivers are more susceptible to measurement noise, which can happen even under perfect environmental conditions. A clock error of 1 nanosecond (a billionth of a second) can introduce as much as half a meter (1.5 feet) of imprecision.
Urban environments pose a particular challenge to GNSSs. Not only is the geometry compromised by buildings, but the signals the user can receive are often reflected off of them – causing unwanted multipath propagation as previously discussed. If you’ve ever requested a ride from an app on your phone and wondered why the driver thinks you’re at a restaurant two streets away, these are likely culprits.
Global Positioning System (GPS)
The first satellite for what we now know as GPS was launched in 1978 by the United States Air Force. At first, only the US military had access to the system.
In 1983, pilots of a commercial flight from Alaska to Korea made a navigational error that took their aircraft over the Kamchatka Peninsula near Japan. In response, a Soviet SU-15 interceptor shot down the Boeing 747, killing all 269 civilians onboard. To prevent future incidents, the US opened GPS for civilian use.
As of 2020, GPS is operated by the United States Space Force and remains open for anyone to use. At the time of writing, it has 32 satellites in a semi-synchronous21 medium Earth orbit (MEO) with an altitude of 20,200 kilometers (12,600 miles). Each orbit has a different inclination, providing global ground coverage.
21 A semi-synchronous orbit is one in which the spacecraft passes over a given point on the Earth twice per day.
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