Introduction to GNSS - GNSS Remote Sensing: Theory, Methods and Applications - page 15

1.3.1 GNSS Principle

The basic principle to estimate PVT is the triangulation. At least four satellites are
needed to compute the four unknowns, the three position coordinates and one clock
error. The user segment performs two basic measurements of the GNSS signals. It
compares the code that is received with a locally generated copy in order to compute
the transmission delay between the satellite and the receiver. This measurement
is called Pseudorange. Pseudorange use four or more satellites to determine the
position of the user after the position of the GPS satellites has been obtained
using the ephemerides from the navigation message. The second and more precise
method is to obtain the difference in phase between the received carrier signal and
a receiver-generated signal at the same frequency. This measurement is known as
the carrier phase observable and it can reach millimeter precision, but it lacks the
accuracy of the pseudorange because the phase when the tracking is started can
only be known with an ambiguity of an unknown number of times the carrier
wavelength.

1.3.2 GNSS Error Sources

GNSS User Equivalent Errors (USER) is defined as the errors in PVT computation
due to three segments of GNSS architecture. In space segment, the source of
error can be satellite clock stability, satellite perturbations or selective availability
(turned off in May 2001). The control segment causes the errors like ephemeris
prediction and the user segment contributes the major part due to Ionospheric delay,
Tropospheric delay, receiver noise, multi-path, indoor positioning or due to urban
canyons.

￿ Satellite and receiver clock errors

￿ Satellite orbit errors

￿ Atmospheric effects (ionosphere, troposphere)

￿ Multipath: signal reflected from surfaces near the receiver

￿ Antenna phase center

￿ Solar radiation pressure

￿ Satellite Geometry

￿ Relativistic Effects

￿ Receiver noise

￿ Signal Obstruction