A GPS receiver location within a wide-area GNSS network
can be estimated based on network location’s server
technology adopted to resolve the receiver’s location. This
type of technology is being used to support the United
States Federal Communication Commission’s Enhanced
911 requirement for public safety location services,
which is commonly known as E911. DGPS is generally
used for positioning purposes through static or kinematics
GPS surveys. In static GPS surveys, one receiver is
placed at a point whose coordinates are known and the
other receiver is placed over a point whose coordinates
are desired. In kinematic surveys, one receiver remains at
one point (base station) normally with known coordinates,
and the other receiver (rover) moves from point to point
in the project area. Kinematic surveys in which points
positions are computed on-the-fly are known as real-time
kinematic (RTK).
Unlike static DGPS surveys, which is often used for
precise control surveys, RTK surveys provide real-time
locations of points of interest that are typically needed in
many engineering applications such as setting up project
control, construction layout, topographic mapping, asbuilt
surveying, ect.. RTK surveying requires both the
reference and remote receivers to simultaneously acquire
dual-frequency (L1/L2) GPS observations and perform
carrier phase integer ambiguity resolution. Specifically, a
fixed solution is essential for RTK surveys for surveying
and engineering projects. It is common to initiate RTK
surveys at a known point although most of GPS systems
adopt an On-The-Fly (OTF) initialization technology,
which uses the L2 frequency. In this case, the rover can
initialize and resolve integers without a period of static
initialization. The advantage of the OTF ability is that if
a loss of satellite lock occurs, initialization can occur
while in motion and the integers can be resolved OTF at
the rover within no more than 30-seconds. This is why a
communication link is required between the two receivers
in order to obtain the real-time coordinates of the
rover’s location [6]. This communication link is normally
established through radio, cell phone, or satellite. Being a
typical DGPS method, the problem with the common
RTK is that errors especially atmospheric; increase with
the increase of baseline length. This is a big issue in a
traditional RTK network because a dense network of
reference stations is required to maintain shorter baseline
lengths. This is not an issue with wide-area GPS network
since now the network GPS data is acquired at network
server, which then processes the data, computes corrections,
and sends the corrections to the rover, which can
then estimates its position robustly.