Multi-Constellation and Multi-Frequency
The ability of a GNSS receiver to handle multiple frequencies from multiple constellations in the calculation of position is essential to optimal error resolution.
Using multi-frequency receivers is the most effective way to remove ionospheric error from the position calculation. Ionospheric error varies with frequency so it impacts the various GNSS signals differently. By comparing the delays of two GNSS signals, L1 and L2, for example, the receiver can correct for the impact of ionospheric errors.
The new and modernized wideband signals in the L5/E5a band provide inherent noise and multipath mitigation capabilities. When receivers combine L5/E5a capabilities with the ability to remove ionospheric error using dual-frequency, significant improvements in both measurement and positioning accuracy can be achieved.
Multi-frequency receivers also provide more immunity to interference. If there is interference in the L2 frequency band around 1227 MHz, a multi-frequency receiver will still track L1 and L5 signals to ensure ongoing positioning.
As described previously, a multi-constellation receiver can access signals from several constellations: GPS, GLONASS, BeiDou and Galileo for example. The use of other constellations in addition to GPS, results in there being a larger number of satellites in the field of view, which has the following benefits:
- Reduced signal acquisition time.
- Improved position and time accuracy.
- Reduction of problems caused by obstructions such as buildings and foliage.
- Improved spatial distribution of visible satellites, resulting in improved dilution of precision.
When a receiver utilizes signals from a variety of constellations, redundancy is built into the solution. If a signal is blocked due to the working environment, there is a very high likelihood that the receiver can simply pick up a signal from another constellation—ensuring solution continuity. While extremely rare, if a GNSS system fails, there are other systems available.
To determine a position in GPS-only mode, a receiver must track a minimum of four satellites. In multi-constellation mode, the receiver must track five satellites, at least one of which must be from a satellite in the other constellation, so the receiver can determine the time offset between constellations.