Introduction to GNSS and RNSS
Global Navigation Satellite Systems (GNSSs) and Regional Navigation Satellite Systems (RNSSs) are satellite based positioning systems. The most well known GNSS is the NAVSTAR Global Positioning System (GPS). The coming decade will bring a proliferation of GNSSs that are likely to revolutionize society in the same way as the mobile phone has done. The promise of a broader multi-frequency, multi-signal GNSS “system of systems” has the potential of enabling a much wider range of demanding applications compared to the current GPS-only situation.
In order to achieve the highest accuracies, one must exploit the unique properties of these systems and their transmitted carrier signals. These properties include the multi-satellite system tracking, the mm-level measurement precision, the frequency diversity, and the integer ambiguities of the carrier phases.
GNSSs involve satellites, ground stations and user receiver equipment and software to determine positions anywhere around the world at any time. Current and prospective providers of GNSS systems are the USA, Russia, European Union and China. The USA is modernizing its dual-frequency GPS. A third civil frequency will be added, with expected 24-satellite full constellation capability (FOC) around 2015. Russia is revitalizing its GLONASS system, from a current only partially functioning system to 24-satellite FOC reached by 2010.
The European Union is developing a complete new multi-frequency GNSS, called Galileo, which is currently in orbit validation phase and which will have its 30-satellite FOC by 2012. China is developing its own 30-satellite GNSS, called Compass, of which the first satellite was launched in April 2007.
RNSSs are GNSS augmentation systems, both India and Japan are currently developing a RNSS. India’s 7-satellite IRNSS (Indian Regional Navigational Satellite System) is expected operational in 2012 and Japan will soon launch its first of three QZSS (Quasi-Zenith Satellite System) satellites. QZSS is designed to increase the number of satellites available at high elevation angles over Japan.
The primary purpose of GNSSs is to provide positions. Different methods of positioning with GNSS exist with the achieved positioning accuracy varying from 10 meter to the millimetre level. Generally we distinguish between the usage of GNSS code observations and GNSS code and phase observations.
GNSS positioning with code observations
Single point positioning
Once a distance to a single satellite is measured the user’s position can be anywhere on a sphere with the radius of the measured distance around the satellite. A second distance creates another sphere that intersects with the first sphere, the intersection has the shape of a circle. Adding a third observed distance creates another intersection that gives the user’s position.
Time synchronization between the satellites time system and the user’s GNSS receiver is essential to obtain distances to the satellites. The GNSS satellites are equipped with atomic clocks and the navigation messages contain information about the stability and time offset for each satellite clock so that the satellites are all in the same time system. The receiver usually has a unstable clock that needs to be synchronized with the satellite time system. The time offset of the receiver creates a common bias in all observed distances to the tracked satellites by the receiver. In order to determine the time offset of the receiver and correct for the bias in the observed distance, a fourth distance to a satellite is necessary to obtain the correct user position. The observed distances are called ‘code-pseudoranges’ because of the bias in the observed distance obtained from the code observation due to the receiver clocks time offset. The positioning technique described here is known as single point positioning and gives a positioning accuracy of approximately 10 metres.
GNSS signals are affected by various errors while broadcasted and travel from the satellite to the receiver. The errors can be satellite dependent (satellite biases, clock and orbit errors), line-of-sight dependent (atmospheric delays, multipath) or receiver dependent (receiver biases, clock error). The satellite dependent errors and the atmospheric delays are similar for two receivers that are close to each other, because the signal comes from the same satellite and travels through the same atmosphere. If one of the receivers has a known position the errors in the distance measurement can be estimated for that station for each satellite. Applying these estimated errors as correction to the observations of the second station reduces the effects of the satellite dependent errors and atmospheric effects significantly. Using this technique, which is known as D-GPS when it applied to GPS only, with code pseudoranges gives a position accuracy at the metre level.
GNSS positioning with code and phase observations
Precise point positioning
Precise point positioning (PPP) is based on the observations of a single receiver. The code and phase observations are used as distance measurements to the satellites. Due to phase offsets at the satellite, known as satellite biases, and other errors that encounter the carrier phase while it travels from satellite to receiver, the unknown number of full cycles can not be fixed to an integer number, but can be estimated as a real valued number. Estimating the real valued number of ambiguities in PPP is known as initialisation. Adding external information such as more precise satellite orbits, atmospheric models, earth rotation and tidal information decreases the initialisation time and gives a more accurate estimation of the real values ambiguities. PPP can give accuracies ranging from centimetres for static receivers to decimetres for a kinematic receiver.
For more information read our research topic on Real Time PPP.
As explained in the section about differential positioning with code observations some of the errors in the received satellite signal are similar for two receivers that are close to each other. By taking the difference between satellite signals from one satellite, the satellite and the line-of-sight dependent errors cancel out. By taking the difference between two satellite signals observed by one receiver, the receiver dependent errors cancel out. Since satellite and receiver dependent biases in the phase observations can be cancelled out by taking the differences, the resulting ambiguities in the carrier cycles remain integer and we can obtain positioning accuracies at the millimetre level for static receivers and accuracies at the cm-level in real time for kinematic receivers. Real-time differential positioning with code and phase observation for kinematic applications is referred to as Real Time Kinematic GNSS (RTK-GNSS). Differential positioning can be done with a baseline between two receivers or using a network of receivers.
For more information read our research topic on CORS-RTK.