GPS ERROR SOURCES
GNSS Errors Identified
User Equivalent Range Errors
User Equivalent Range Errors (UERE) are those that relate to the timing and path readings of the satellites due to anomalies in the hardware or interference from the atmosphere. A complete list of the sources of User Equivalent Range Errors, in descending order of their contributions to the total error budget, is below:
- Satellite clock: there are three clock sources that has to be precisely synced to achieve accurate position the atomic clock operated by the satellite, the ground stations clock and the GNSS receiver clock.
GPS position calculations, depend on measuring signal transmission time from satellite to receiver; this, in turn, depends on knowing the time on both ends.
satellites use atomic clocks, which are very accurate but can drift up to a millisecond (enough to make an accuracy difference). These errors are minimized by calculating clock corrections (at monitoring stations) and transmitting the corrections along with the GPS signal to appropriately outfitted GPS receivers.
2. Upper atmosphere (ionosphere): As GPS signals pass through the upper atmosphere (the ionosphere 50-1000km above the surface), signals are delayed and deflected.
The ionosphere density varies; thus, signals are delayed more in some places than others. The delay also depends on how close the satellite is to being overhead (where distance that the signal travels through the ionosphere is least).
By modeling ionosphere characteristics, GPS monitoring stations can calculate and transmit corrections to the satellites, which in turn pass these corrections along to receivers. Only about three-quarters of the bias can be removed, however, leaving the ionosphere as the second largest contributor to the GPS error budget.
3. Satellite orbit: GPS receivers calculate coordinates relative to the known locations of satellites in space, a complex task that involves knowing the shapes of satellite orbits as well as their velocities, neither of which is constant. The GPS Control Segment monitors satellite locations at all times, calculates orbit eccentricities, and compiles these deviations in documents called ephemerides. An ephemeris is compiled for each satellite and broadcast with the satellite signal. GPS receivers that are able to process ephemerides can compensate for some orbital errors.
4. Lower atmosphere: The three lower layers of atmosphere (troposphere, tropopause, and stratosphere) extend from the Earth’s surface to an altitude of about 50 km. The lower atmosphere delays GPS signals, adding slightly to the calculated distances between satellites and receivers. Signals from satellites close to the horizon are delayed the most, since they pass through the most atmosphere.
5. Multipath: Ideally, GPS signals travel from satellites through the atmosphere directly to GPS receivers. In reality, GPS receivers must discriminate between signals received directly from satellites and other signals that have been reflected from surrounding objects, such as buildings, trees, and even the ground. Antennas are designed to minimize interference from signals reflected from below, but signals reflected from above are more difficult to eliminate. One technique for minimizing multipath errors is to track only those satellites that are at least 15° above the horizon, a threshold called the "mask angle." Multipath errors are particularly common in urban or woody environments, especially those with large valleys or mountainous terrain, and are one of the primary reasons why GPS works poorly or not at all in large buildings, underground, or on narrow city streets that have tall buildings on both sides. If you have ever been geocaching, hiking, or exploring and noticed poor GPS service while in dense forests, you were experiencing multipath errors.
They severely effect GNSS system accuracy in the marine environment at heavy weather and low clouds.
Dilution of Precision
The arrangement of satellites in the sky also affects the accuracy of GPS positioning. The ideal arrangement (of the minimum four satellites) is one satellite directly overhead, three others equally spaced nearer the horizon (but above the mask angle). Imagine a vast umbrella that encompasses most of the sky, where the satellites form the tip and the ends of the umbrella spines.
GPS coordinates calculated when satellites are clustered close together in the sky suffer from dilution of precision(DOP), a factor that multiplies the uncertainty associated with User Equivalent Range Errors (UERE - errors associated with satellite and receiver clocks, the atmosphere, satellite orbits, and the environmental conditions that lead to multipath errors).
The calculation of DOP results in values that range from 1 (the best case, which does not magnify UERE) to more than 20 (in which case, there is so much error the data should not be used). The lowest DOP encountered in practice is about 2, which doubles the uncertainty associated with UERE.
GPS receivers report several components of DOP, including Horizontal Dilution of Precision (HDOP) and Vertical Dilution of Precision (VDOP). The combination of these two components of the three-dimensional position is called PDOP - position dilution of precision. A key element of GPS mission planning is to identify the time of day when PDOP is minimized. Since satellite orbits are known, PDOP can be predicted for a given time and location. Professional surveyors use a variety of software products to determine the best conditions for GPS work.
| DOP VALUE | RATING | DESCRIPTION |
|---|---|---|
| 1 | Ideal | Highest possible confidence level to be used for applications demanding the highest possible precision at all times. |
| 1-2 | Excellent | At this confidence level, positional measurements are considered accurate enough to meet all but the most sensitive applications. |
| 2-5 | Good | Represents a level that marks the minimum appropriate for making accurate decisions. Positional measurements could be used to make reliable in-route navigation suggestions to the user. |
| 5-10 | Moderate | Positional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended. |
| 10-20 | Fair | Represents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location. |
| >20 | Poor | t this level, measurements are inaccurate by as much as 300 meters with a 6-meter accurate device (50 DOP × 6 meters) and should be discarded. |
There are several different definitions of the dilution of precision. Within the position information of the UNI-Connect’s survey tool, you find the PDOP, HDOP and VDOP values. The different DOP values have various functions in satellite geometry.
Position DOP (PDOP)
This value describes how many satellites are spread evenly throughout the sky. The more the satellites directly above you and the less on the horizon, the lower the PDOP value is.
Horizontal DOP (HDOP)
The effect of the DOP on the horizontal position value. The more good visible satellites low in the sky, the better the HDOP and the horizontal position (Latitude and Longitude) are.
Vertical DOP (VDOP)
The effect of the DOP on the vertical position value. The more good visible satellites low in the sky, the better the VDOP and the vertical position (Altitude) are.
