Geoscience Reference
In-Depth Information
fIGURe 9.6
Good position dilution of precision (PDOP) (left) and bad PDOP (right).
the satellites at any given time with respect to the receiver location. Ideal satellite geometry exists
when the satellites are located at wide angles relative to each other and are evenly distributed in
azimuth and elevation angle as observed from the user's location. Poor geometry results when the
satellites are located in a line or are grouped in clusters which may happen when a part of the sky
is obstructed.
The two major factors that reflect the accuracy of GPS are (1) error in the range measurement,
σ, and (2) geometric configuration between the receiver and the satellites. The two factors combined
define the ultimate positioning error as a product of σ and the geometry factor, PDOP (position dilu-
tion of precision)—namely, standard deviation of 3D positioning = PDOP × σ.
The error in range measurement, σ, is called User Equivalent Range Error (UERE) and repre-
sents a quality of a single range measurement; it is computed as a square root of a sum of squares of
the individual error sources. For SPS, the UERE is around 25 to 33 m under SA and drops to around
5 to 8 m under no SA. For PPS using dual-frequency receivers, the UERE amounts to ~1 to 2 m.
The geometric factor, PDOP, reflects the instantaneous geometry related to a single receiver
and is determined by the position of the GPS satellites with respect to the receiver. PDOP can be
interpreted as the reciprocal value of the volume of a tetrahedron that is formed by the positions
of the satellites and the user. The best geometry corresponds to a large volume and vice versa (see
Figure 9.6). Typically, more satellites yield smaller PDOP value, and PDOP of two and less indicates
an excellent geometry; PDOP below six refers to a good geometry, and PDOP of seven and above
indicates virtually useless data. A geometric factor related to a pair of receivers working in relative
(differential) mode (e.g., a base and a rover) is called Relative DOP (RDOP). An example of varying
geometry and partial loss of signal lock on GPS positioning accuracy is shown in Figure 9.7, where
stand-alone positioning (lower accuracy, top figures) and differential (relative) positioning (higher
accuracy, bottom figures) are illustrated. PDOP can be estimated using the approximated location of
the user and the satellite broadcast ephemeris included in the satellite navigation message.
9.5.8 i n t e R f e R e n c e a n d J a M M i n g (i n t e n t i of n a l i n t e R f e R e n c e )
Radio interference can, at minimum, reduce the GPS signal's apparent strength (i.e., reduce the
signal-to-noise ratio by adding more noise) and, consequently, the accuracy, or, at worst, even block
the signal entirely. Medium-level interference may cause frequent losses of lock or cycle slips (a sud-
den jump in the carrier phase observable by an integer number of cycles), and may render virtually
useless data.
9.5.9 i n t e n t i of n a l d e g R a d a t i of n of f t h e s a t e l l i t e s i g n a l
Selective Availability (SA) is the DOD policy of denying to nonmilitary GPS users the full accuracy
of the system. It is achieved by dithering the satellite clock (called delta process) and degrading
the navigation message ephemeris (called epsilon process). The effects of SA (that are highly
unpredictable) can be removed with encryption keys or through relative positioning techniques
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