GPS devices calculate your position using a technique called “3-D
multilateration”, which is the process of figuring out where several spheres
intersect. In the case of GPS, each sphere has a satellite at its center; the
radius of the sphere is the calculated distance from the satellite to the GPS
device. Ideally, these spheres would intersect at exactly one point, causing
there to be only one possible solution to the current location, but in reality,
the intersection forms more of an oddly-shaped area. The device could be located
within any point in the area, forcing devices to choose from many possibilities.
Figure 2-4 shows such an area created from three satellites (using part one’s
$GPGSV sentence). The current location could be any point within the gray-colored
area. Precision is said to be “diluted” when the area grows larger, which leads
to this article’s focus: dilution of precision. The monitoring and control of
dilution of precision (or DOP for short) is the key to writing high-precision
applications.
| DOP |
Rating |
Description |
| 1 |
Ideal |
This is the highest possible confidence level to be used for applications
demanding the highest possible precision at all times. |
| 2-3 |
Excellent |
At this confidence level, positional measurements are considered accurate
enough to meet all but the most sensitive applications. |
| 4-6 |
Good |
Represents a level that marks the minimum appropriate for making business
decisions. Positional measurements could be used to make reliable in-route
navigation suggestions to the user. |
| 7-8 |
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. |
| 9-20 |
Fair |
Represents a low confidence level. Positional measurements should be
discarded or used only to indicate a very rough estimate of the current location. |
| 21-50 |
Poor |
At this level, measurements are inaccurate by as much as half a football field and should be discarded. |
Table 2-1: Jon’s interpretation of dilution of precision values.
DOP values are reported in three types of measurements: horizontal,
vertical, and mean. Horizontal DOP (or HDOP) measures DOP as it relates to
latitude and longitude. Vertical DOP (or VDOP) measures precision as it
relates to altitude. Mean DOP, also known as Position DOP (PDOP), gives an
overall rating of precision for latitude, longitude and altitude. Each DOP
value is reported as a number between one and fifty where fifty represents
very poor precision and one represents ideal accuracy. Table 2-1 lists what
I believe to be an accurate breakdown of DOP values.
Looking again at figure 2-4, three satellites created a large area of
possible solutions. This situation could be improved by two factors: adding
more satellites to the fix, and using satellites evenly distributed throughout
the sky. What would figure 2-4 look like if the situation was improved like
this? Figure 2-5 shows figure 2-4 after three more evenly-distributed
satellites have been added.