Real Time Kinematic satellite navigation is a technique used to enhance the precision
of position data derived from satellite-based positioning systems, being usable in conjunction
with GPS, GLONASS and/or Galileo. It uses measurements of the phase of the signal′s
carrier wave, rather than the information content of the signal, and relies on a single
reference station to provide real-time corrections, providing up to centimetre-level accuracy.
With reference to GPS in particular, the system is commonly referred to as Carrier-Phase Enhancement,
or CPGPS. It has application in land survey and in hydrographic survey.
Overview Background
Normally, satellite navigation receivers must align signals sent from the satellite to an
internally generated version of a pseudorandom binary sequence, also contained in the signal.
Since the satellite signal takes time to reach the receiver, the two sequences do not initially
coincide; the satellite's copy is delayed in relation to the local copy. By increasingly
delaying the local copy, the two copies can eventually be aligned. The correct delay represents
the time needed for the signal to reach the receiver, and from this the distance from
the satellite can be calculated. The accuracy of the resulting range measurement
is essentially a function of the ability of the receiver's electronics to accurately process
signals from the satellite. In general receivers are able to align the signals to about 1%
of one bit-width. For instance, the coarse-acquisition code sent on the GPS system sends a bit every
0.98 microsecond, so a receiver is accurate to 0.01 microsecond, or about 3 metres. The
military-only P(Y) signal sent by the same satellites is clocked ten times faster, so
with similar techniques the receiver will be accurate to about 30 cm. Other effects
introduce errors much greater than this, and accuracy based on an uncorrected C/A signal
is generally about 15 m. Carrier phase tracking
RTK follows the same general concept, but uses the satellite signal's carrier wave as
its signal, ignoring the information contained within. The improvement possible using this
signal is potentially very high if one continues to assume a 1% accuracy in locking. For instance,
in the case of GPS, the coarse-acquisition code changes phase at 1.023 MHz, but the L1
carrier itself is 1575.42 MHz, over a thousand times more often. The carrier frequency corresponds
to a wavelength of 19 cm for the L1 signal. A ±1% error in L1 carrier phase measurement
thus corresponds to a ±1.9 mm error in baseline estimation.
The difficulty in making an RTK system is properly aligning the signals. The navigation
signals are deliberately encoded in order to allow them to be aligned easily, whereas
every cycle of the carrier is similar to every other. This makes it extremely difficult to
know if you have properly aligned the signals or if they are "off by one" and are thus introducing
an error of 20 cm, or a larger multiple of 20 cm. This integer ambiguity problem can
be addressed to some degree with sophisticated statistical methods that compare the measurements
from the C/A signals and by comparing the resulting ranges between multiple satellites.
However, none of these methods can reduce this error to zero.
Practical considerations In practice, RTK systems use a single base
station receiver and a number of mobile units. The base station re-broadcasts the phase of
the carrier that it observes, and the mobile units compare their own phase measurements
with the one received from the base station. There are several ways to transmit a correction
signal from base station to mobile station. The most popular way to achieve real-time,
low-cost signal transmission is to use a radio modem, typically in the UHF band. In most
countries, certain frequencies are allocated specifically for RTK purposes. Most land survey
equipment has a built-in UHF band radio modem as a standard option.
This allows the units to calculate their relative position to within millimeters, although their
absolute position is accurate only to the same accuracy as the computed position of
the base station. The typical nominal accuracy for these systems is 1 centimetre ± 2 parts-per-million
horizontally and 2 centimetres ± 2 ppm vertically. Although these parameters limit the usefulness
of the RTK technique for general navigation, the technique is perfectly suited to roles
like surveying. In this case, the base station is located at a known surveyed location, often
a benchmark, and the mobile units can then produce a highly accurate map by taking fixes
relative to that point. RTK has also found uses in autodrive/autopilot systems, precision
farming, Machine Control systems and similar roles.
The Virtual Reference Station method extends the use of RTK to a whole area of a reference
station network. Operational reliability and accuracy depend on the density and capabilities
of the reference station network. A Continuously Operating Reference Station
network is a network of RTK base stations that broadcast corrections, usually over an
Internet connection. Accuracy is increased in a CORS network, because more than one station
helps ensure correct positioning and guards against a false initialization of a single
base station. See also
Differential GPS Galileo positioning system
Global Positioning System European Geostationary Navigation Overlay
Service GLONASS
References
External links SmartNet Aus A network of continuously operating
reference stations for receiving RTK corrections A comprehensive online tutorial for Network
Real time Kinematic by Tekmon Geomatics Magellan RTK page
Trimble Ag GPS, section explaining RTK Product Survey on RTK DGPS receivers for hydrographic
use Satel.com Radio technology for RTK
GPSPP.sakura.ne.jp RTKLIB: An Open Source Program Package for GNSS Positioning
North GPS High-End RTK Components for custom applications.