About GNSS

GNSS

The Global Navigation Satellite System (GNSS) is a constellation of satellites, transmitting signals used for several types of navigation and positioning applications, anywhere on the surface of the earth. There are currently two Global Navigation Satellite Systems in operation: the U.S. Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS). A third system, Galileo, is currently being developed in Europe.

How it works

The GPS satellites transmit low power radio signals on multiple frequencies. L1 and L2 are the two basic carrier frequencies that contain the navigation signals. The L1 frequency is 1575.42 MHz in the UHF band while the L2 frequency is 1227.6 MHz. The signals travel by line of sight (the signal passes through clouds, glass, plastic and other lightweight materials but doesn’t go through buildings, mountains or other enclosed spaces). The L1 signal contains two pseudorandom signals: the P (Protected) code and the C/A (Coarse/Acquisition) code. The P code is also modulated on the L2 signal.

Augmentation Systems

A Satellite Based Augmentation System (SBAS) provides system error corrections, which brings positioning accuracy to the level comparable to DGPS and also provides integrity information of the positional solution. SBAS consists of a network of ground reference stations and a set of geostationary satellites, which transmit correction data to user receivers. The correction data is modulated onto the GPS-like ranging signal in the L1 band.

DGPS vs RTK

Standalone GPS positioning relies only on the receiver range measurements and is limited to an accuracy of a few meters. The error budget of range measurements includes components, which are correlated for all the receivers operating in the same area (e.g. satellite clock and orbit errors and atmospheric delays). These errors can be removed if a static receiver with a known position (base station) transmits information about these errors to a user receiver (rover). The category of positional techniques based on this principle is called “differential positioning”.

Attitude Determination

Attitude is determined from multiple GPS antennas installed on a rigid platform, and is expressed as the orientation of this platform relative to the local geographical axes (East, North, Up). Attitude determination with GPS is based on the use of highly precise carrier phase measurements in a way very similar to RTK: the orientation of antenna baseline vectors defines the platform attitude. Because GPS measurements of multiple antennas are involved, typical GPS-based attitude systems include multiple GPS receivers.