Re: Про GPS
А теперь официальная информация:
"B. Accuracy
GPS provides two services for position determination, SPS and PPS. Accuracy of a GPS fix varies with the capability of the user equipment.
1. Standard Positioning Service (SPS)
SPS is the standard specified level of positioning and timing accuracy that is available, without restrictions, to any user on a continuous worldwide basis. SPS provides a predictable positioning accuracy of 100 meters (95 percent) horizontally and 156 meters (95 percent) vertically and time transfer accuracy to UTC within 340 nanoseconds (95 percent). Decisions to change operational modes of GPS to include degrading GPS accuracy to civil users will be made by the NCA.
2. Precise Positioning Service (PPS)
PPS is the most accurate direct positioning, velocity, and timing information continuously available, worldwide, from the basic GPS. This service is limited by the DOD to users who are specifically authorized access. P(Y) code capable user equipment provides a predictable positioning accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters vertically and time transfer accuracy to UTC within 200 nanoseconds (95 percent).
C. Availability
Provided there is coverage as defined below, SPS will be available 99.85 percent of the time.
D. Coverage
GPS coverage is worldwide. The probability that 4 or more GPS satellites are in view anywhere on or near the earth (over any 24-hour period) with a PDOP of 6 or less, and with at least a 5 deg mask angle, is 99.90 percent.?E. Reliability If the conditions on coverage and service availability are met, the probability that the horizontal positioning error will not exceed 500 meters is 99.97 percent.
F. Fix Rate
The fix rate is essentially continuous, but the need for receiver processing to retrieve the spread-spectrum signal from the noise results in an actual users fix rate of 1-20 per second. Actual time to a first fix depends on user equipment capability and initialization with current satellite almanac data.
G. Fix Dimensions
GPS provides three-dimensional positioning and time when four or more satellites are available and two-dimensional positioning when only three satellites are available."
--Federal Radionavigation Plan 1999