6.1.5.2  PDOP Less Than Six

PPS users are much less sensitive to large values of DOP than SPS users.  Many PPS

users will have sufficient position accuracy using GPS as a stand alone system even if

PDOP is greater than six.  For example, for navigation missions, horizontal position

accuracy is usually a more appropriate measure than PDOP.  As a general rule of

thumb, a PDOP of six is typically equivalent to an HDOP of four (although PDOP

obviously contains a vertical component which can vary).  This means that an

approximate worst case PPS error for "normal" horizontal variations would be around

160 metres (assuming a three sigma URE of 20 for all satellites and a maximum

geometric effect of 2 X HDOP = 8).  Many PPS users of GPS can navigate safely with a

horizontal position accuracy of a kilometer or more, for example, ships in open ocean

and aircraft enroute at altitude, and can therefore tolerate much higher values of HDOP

(and PDOP).  Therefore, "areas of reduced accuracy" is often a more appropriate term

than "outage" for conditions of large PDOP, since the accuracy of the position solution

may be reduced but still adequate for the mission requirements.

This suggests that the user should evaluate the performance standards with respect to

the anticipated mission requirements.  If the mission requirements are significantly

different than the constraints used to develop the performance standards, an

independent assessment of service coverage via computer simulations may be

warranted.  One method of determining the real time effect of prevailing range errors

and satellite geometry is calculation of a FOM described in paragraph 6.3 below.  The

user can then reduce the uncertainty associated with global averages and long term

statistics by comparing the current accuracy estimate to the mission accuracy

requirements and thereby significantly improve the probability of success of the

mission.

Most military GPS users will have to contend with the possibility of GPS "outages," due

to hostile local conditions, for example, terrain masking or intentional jamming.  One

solution for some applications is an integrated navigation system.  For example, if a

GPS receiver is integrated with an inertial navigation unit, an intermittent GPS solution

can be sufficient to maintain continuous high accuracy positioning.  For other

applications, vertical aiding can be used as a pseudo satellite to enhance availability,

or differential GPS can be used to minimize range errors and correspondingly reduce

sensitivity to DOP.

6.1.5.3  Four Satellite Solution and Five Degree Mask Angle

In effect, the performance standards are based on a "model" GPS receiver that

calculates a four satellite PVT solution and is constrained by a five degree mask angle.

In evaluating the impacts of these constraints, the user must consider the type of

equipment that he is actually employing. Significant gains in service coverage can be

achieved by the use of aiding,  for example, from an altitude source or precision clock.

Similarly, significant gains in service coverage can be expected if the satellite mask

angle actually implemented by the receiver  and GPS antenna is lower than five

degrees.  Correspondingly, a higher mask angle

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