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GNSS basics

Last Updated: Dec 02, 2013 06:05PM CET

What is the difference between GPS and GNSS?

GPS means Global Positioning System. It's a satellite based navigation system and was initially developped by the USA for military purpose. Civilian applications have access to some of the GPS signals.

Nowadays, other satellite based navigation systems are also available such as the Russian GLONASS, the future European Galileo, and the Chinese Beidu. Inertial navigation systems can take advantage of these constellations to use more satellites and further improve robustness in harsh environments. This is why new generations of navigation systems are often stated as GNSS (Global Navigation Satellite based System), which can support many satellite based navigation systems at the same time.

What is the DGPS?

DGPS stands for Differential GPS. It is the GPS signal corrected by different ground reference stations (that can estimate the ionosphere error on the GPS signal). Traditionally, DGPS accuracy can go down to sub-meter level. Newer techniques called RTK allows DGPS accuracy to be at centimeter level.

What does RTK mean?

RTK stand for Real Time Kinematic. RTK algorithms make use of the carrier phase measurement to estimate accurately the distance from the receiver to the satellites. It allows an accuracy of 1-2 cm on position.

What does mean “GPS True Heading”?

GPS True Heading is achieved by using two GPS antennas on the same equipment. If we know the positions of theses antennas, we can figure out the bearing without any help of magnetometers or other reference. Note that GPS True Heading should not be confounded with GPS Course, which is the actual trajectory direction and can only be provided when the system is in motion.

What is harsh environment for a GNSS?

In order to provide a good accuracy, a GNSS system requires at least 4 satellites to be visible. In addition, high accuracy is only achieved if those satellites are well distributed in the sky so that position is well known in all three axes.

In urban environments, where large buildings are surrounding the GNSS receiver, it may be difficult to find an optimal view of sky for accurate positioning. In addition, signal reflections on buildings walls and roofs may fool the GPS receiver. This is called the "Urban Canyon" effect. Wrong position can then be provided by standalone GPS/GNSS receivers.

In such application, a high accuracy inertial system can be of a great help to maintain good positioning accuracy while the GPS is not able to maintain a good fix.

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