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RTK Mapping 8
The difference between RTK and differential data collection
techniques are specifically related to the information that the receiver
(or combination of receivers) is able to interpret from the signals that
are sent by the satellites.
The following section briefly describes satellite signals and the way in
which GPS receivers use these signals, to provide basic information
for understanding the differences between RTK and differential data
collection techniques.
8.2.2 Satellite Signals and GPS Receivers
Each satellite transmits two unique codes. The first and more simple
code is called the C/A (coarse acquisition) code. The second code is
called the P (precise) code. These codes are modulated onto two
carrier waves, L1 and L2. The L1 wave carries both the C/A and the
P-code, while the L2 wave carries only the P-code.
GPS receivers are categorized as being either single-frequency or
dual-frequency. Single-frequency receivers observe the L1 carrier
wave, while dual-frequency receivers can observe both the L1 and L2
carrier waves.
Submeter differential receivers such as the Pro XR and Pro XRS are
single frequency receivers. RTK receivers such as the GPS Total
Station 4700, GPS Total Station 4800, Series 4000, and Site Surveyor
4400 track both frequencies, which lets more precise centimeter-level
positions be computed.
Code and phase measurements
There is a time difference between when the code is generated in the
satellite and when it is received at the GPS antenna. Code
measurements, as described in the Mapping Systems General
Reference manual, make it possible to record this time difference. The
measurements are multiplied by the speed of light, making it possible
to determine the distance from the satellite to the GPS receiver’s
antenna—this distance is also referred to as the range.
TSC1 Asset Surveyor Operation Manual 189