The user of the GPS navigator is always interested in the real accuracy of the GPS navigation system and the degree of confidence in its readings. How close can you get to any navigation hazard, relying only on the GPS receiver? Unfortunately, there is no single answer to this question. This is due to the statistical nature of GPS navigation errors. Let’s consider them in more detail..
GPS positioning accuracy and causes of GPS errors.
The ionosphere and troposphere, ionospheric and tropospheric refraction affect the speed of propagation of radio waves. This is the main source of errors after disabling SA. The speed of radio waves in the void is constant, but changes when the signal enters the atmosphere. For signals from different satellites, the time delay is different. Delays in the propagation of radio waves depend on the state of the atmosphere and the height of the satellite above the horizon. The lower the satellite, the greater the path its signal travels through the atmosphere and the greater the distortion. Most receivers exclude the use of signals from satellites with an elevation of less than 7.5 degrees above the horizon.
In addition, atmospheric interference depends on the time of day. After sunset, the density of the ionosphere and its effect on radio signals decreases, a phenomenon well known to short-wave radio operators. Military and civilian GPS receivers can autonomously detect atmospheric signal delay by comparing delays at different frequencies. Single-frequency consumer receivers make an approximate correction based on the forecast transmitted as part of the navigation message. The quality of this information has recently increased, which further increased the accuracy of GPS navigation.
To preserve the advantages of high accuracy for military GPS navigators, the access restriction mode SA (Selective Availability) was introduced in March 1990, which artificially reduces the accuracy of a civil GPS navigator. When the SA mode is activated, an error of several tens of meters is added in peacetime. In special cases, errors of hundreds of meters may be entered. The US government is responsible for the operability of the GPS system to millions of users, and it can be expected that re-enabling SA, and even more so, such a significant decrease in accuracy, will not be introduced without sufficiently serious reasons..
The coarsening of accuracy is achieved by a random shift of the transmission time of the pseudo-random code. Errors arising from SA are random and equally probable in every direction. SA also affects heading accuracy and speed on the GPS. For this reason, a stationary receiver often shows slightly varying speed and course. So you can evaluate the degree of impact of SA by periodic changes in course and speed on GPS.
Errors in ephemeris data during GPS navigation.
First of all, these are errors associated with the deviation of the satellite from the calculated orbit, clock inaccuracies, signal delays in electronic circuits. Correction of these data is made from the Earth periodically, in the intervals between communication sessions errors accumulate. Due to its small size, this group of errors does not matter for civilian users..
It is extremely rare, but larger errors can occur due to sudden interruptions in the information in the satellite’s memory devices. If such a failure is not detected by means of self-diagnostics, then until the error is detected by the ground service and the command about the malfunction is transmitted, the satellite may transmit incorrect information for some time. There is a so-called discontinuity or how often the term integrity is translated, navigation integrity.
The effect of the reflected signal on the accuracy of GPS navigation.
In addition to the direct signal from the satellite, the GPS receiver can also receive signals reflected from rocks, buildings, passing ships, the so-called characterizing multipath propagation (multypath). If the direct signal is closed from the receiver by the superstructures or rigging of the vessel, the reflected signal may be stronger. This signal travels a longer way, and the receiver “thinks” that it is farther from the satellite than it actually is. As a rule, these errors are much less than 100 meters, since only closely located objects can give a sufficiently strong echo.
Satellite geometry for GPS navigation.
Depends on the location of the receiver relative to the satellites, which determine the position. If the receiver has caught four satellites and all of them are in the north, satellite geometry is bad. The result is an error of up to 50-100 meters or even the inability to determine the coordinates.
All four dimensions are from the same direction, and the area of intersection of the position lines is too large. But if 4 satellites are located evenly on the sides of the horizon, then the accuracy will increase significantly. Satellite geometry is measured by the geometric factor PDOP (Position Dilution Of Precision). The ideal satellite arrangement corresponds to PDOP = 1. Larger values indicate poor satellite geometry..
Suitable for navigation are PDOP values less than 6.0. In two-dimensional navigation, HDOP (Horizontal Dilution Of Precision) is applied, less than 4.0. The vertical geometric factor VDOP, less than 4.5, and the temporary TDOP, less than 2.0 are also used. PDOP serves as a factor for accounting for errors from other sources. Each pseudorange measured by the receiver has its own error, depending on atmospheric interference, errors in the ephemeris, SA mode, reflected signal, and so on..
So, if the estimated values of the total delay of the signal for these reasons, URE User Range Error or UERE User Equivalent Range Error, in Russian EHP – the equivalent range-finding error, totaling 20 meters and HDOP = 1.5, then the expected location error will be equal to 20 x 1.5 = 30 meters. GPS receivers provide different information for accuracy assessment using PDOP.
In addition to PDOP or HDOP, GQ (Geometric Quality) is used, the inverse of HDOP, or a quality score. Many modern receivers display EPE (Estimated Position Error) directly in distance units. EPE takes into account the location of the satellites and the forecast of the error of the signals for each satellite depending on the SA, the state of the atmosphere, the errors of the satellite clocks transmitted as part of the ephemeris information.
Satellite geometry also becomes a problem when using the GPS receiver inside vehicles, in dense forests, mountains, near tall buildings. When signals from individual satellites are blocked, the position of the remaining satellites will determine how accurate the GPS position will be, and their number will show whether the position can even be determined. A good GPS receiver will show not only which satellites are being used, but also their location, azimuth and elevation, so you can determine if reception of a given satellite is difficult.
Based on materials from the book All About GPS Navigators.
Naiman V.S., Samoilov A.E., Ilyin N.R., Sheinis A.I..