CHAPTER 3:  MINIMUM PERFORMANCE CAPABILITIES

OF A GPS RECEIVER

3.1  BASIC CONSIDERATIONS

There are a set of basic performance parameters that are useful for making

comparisons between different GPS receivers.  This set of parameters, together with

others, can be used to determine what type of receiver one should choose for a

particular application.  The parameters of interest are:

a. Position accuracy

b. Velocity accuracy

c. Time accuracy

d. TTFF

3.1.1  GPS System Accuracy Characteristics

There are a number of different ways in which to express GPS accuracy. All are

expressed in statistical terms, with a probability assigned to the value given, and the

number of dimensions expressed or implied.  The two primary positioning accuracy

requirements imposed on the GPS system by the U.S. DoD are 16 metres SEP for

PPS, and 100 metres 95% horizontal for SPS. SEP represents a 50% probability.  Note

that the PPS requirement is a three dimensional requirement specified at the 50%

probability level and the SPS requirement is a two dimensional requirement specified at

the 95% probability level.  Despite this inconsistency, these are the parameters and

points on the accuracy distributions that the Control Segment has used to determine

system management policies and methods.

GPS system positioning accuracy distributions are not spherical and are not Gaussian

in the tails of the distributions.  Consequently, conversions from the system accuracy

requirements to other expressions of GPS accuracy, based on an assumption of a

spherical distribution that is Gaussian in each dimension can be inaccurate, especially

at the 95% probability level which is commonly used by NATO.

"Technical Characteristics of the Navstar GPS" gives conversions of the PPS

positioning requirement for typical GPS system operating conditions as 37 metres 95%

spherical accuracy and 21 metres 95% horizontal accuracy.  Technical Characteristics

of the Navstar GPS also provides 95% accuracy tables to facilitate comparisons of PPS

and SPS spherical, horizontal, and vertical accuracies.

GPS exhibits statistical accuracy distributions because two important parameters

determine the accuracy of the position solution.  They are User Equivalent Range

Error (UERE) and Geometric Dilution of Precision (GDOP).  Both of these

parameters are variable with time.  UERE is a measure of the error in the range

measurement to each satellite as seen by the receiver.  UERE varies because of

random variations in the satellite signal, signal propagation  characteristics, and

user measurement processes.  Over the long term (days to months) UERE closely

resembles a Gaussian distribution and is equivalent for each satellite. UERE tends

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