Tech Chat | How accurate is your GNSS device? – DOP Indicators
When using navigation and positioning products, we often judge the working state of the device through some state parameters. Have you ever seen the DOP indicators in the status bar such as VDOP, PDOP and HDOP? The HDOP value is also included in #GPGGA message to judge the horizontal positioning accuracy.
In this blog, we will show you what these DOP values mean and how to judge the accuracy of your device via these values.
Above is the status bar of our SingularPad field surveying software, which lists the real-time PDOP, VDOP and HDOP to help users judge current accuracy.
We use a dilution of precision (DOP) to measure the effect of the spatial geometry distribution of the observing satellite on positioning accuracy. Accuracy decay factor (DOP) is an indicator of position quality. It is a calculation that takes into account the position of each satellite relative to the other satellites in the constellation (geometric position) to predict the position accuracy that can be obtained with that constellation.
Low DOP values indicate strong satellite geometry and high probability of accuracy. High DOP values indicate weak satellite geometry and low probability of accuracy. DOP mainly includes the following types:
- PDOP: Positioning dilution of precision, it is the open-square root of the sum of the squared errors of latitude, longitude and elevation.
- TDOP: Time Dilution of precision, which indicates the offset error of the time gauge in the receiver.
- HDOP: Horizontal dilution of precision, it is the square root of the sum of the squares of errors such as latitude and longitude.
- VDOP: Vertical dilution of precision.
- GDOP: Geometric Dilution Precision, it represents the distance vector magnification factor caused by GPS ranging errors between the receiver and the space satellite. The volume of the body sketched by the unit vector representing the positioning solution from receiver to space satellite is inversely proportional to GDOP.
The calculation relationship between them is as follows:
HDOP2+VDOP2=PDOP2
PDOP2+TDOP2=GDOP2
Since the observation result is closely related to and greatly influenced by the geometrical shape between the measured satellite and the receiver. The better the distribution of satellites in the sky, the higher the positioning accuracy.
PDOP represents a parameter of the relationship between the positioning accuracy of three-dimensional position and the geometric configuration of navigation platform, which can directly reflect the distribution of satellite. When THE PDOP is large, it indicates that the distribution of satellites in the sky is not very ideal. The perimeter of the graph formed by them is too short, and the positioning accuracy is lower. The higher the positioning accuracy is, the smaller the PDOP value is.
Similarly, the larger the value of GDOP is, the smaller the volume of the unit vector represented, which is the result of the very similar angle between the receiver and the space satellite. At this time, the positioning accuracy is also worse. Good GDOP means that its small value represents a large unit vector volume, resulting in high positioning accuracy.
You can refer to the table below to judge the accuracy of your GPS device.
| 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 | At 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. |
Therefore, when we use the receiver, we must pay attention to these parameters. If the value increases, it means that the positioning accuracy is not high, you need to change the environment for operation.
More about SingularPad field surveying software
https://www.singularxyz.com/SingularPad.html
More about SingularXYZ GNSS devices
https://www.singularxyz.com/Y1.html
https://www.singularxyz.com/P1.html
https://www.singularxyz.com/T8Pro.html
