What effect does the number of channels have on the operation of GNSS equipment?

Questions answered by Evan Bollard, GlobalPOS

GNSS expert Evan Bollard says:

All GNSS equipment requires a sufficient number of channels to perform the required observations. With the increasing proliferation of positioning satellites the number of channels available in a receiver could be a limitation for optimal productivity or robustness.

In general terms, the more channels available on a receiver, the better precision/productivity/robustness can be achieved from that receiver. However, other considerations must also be made in relation to measurement type (code or carrier phase) processor speed, power usage and interface design when choosing a suitable receiver to use for a particular project.

Focusing on carrier phase capable receivers typically used for professional work, we can make the following generalisation: channels (basically the radio frequency front end) can either be fixed to certain capabilities based on the hardware configuration at manufacture, or can be fully controlled by the on-board firmware as to what use (or frequency range) to assign to each channel on the receiver.

As an example, the acquisition of signals can be achieved faster (during both the start and reacquisition processes if the signal is lost) if multiple channels are assigned to look for each satellite and each signal on that satellite (e.g. by slightly shifting the part of the code the receiver is trying to match on each channel, thus covering the possibilities many times faster).

As you could imagine this acquisition can be much faster than if one channel is assigned to look for each satellite code signal on a sequential basis. This speed increase improves productivity by allowing initialisation to be realised earlier.

You can see how the number of required channels needed to achieve faster acquisition can increase quite rapidly if you are using a multi constellation receiver tracking as many as 35 visible satellites (possible now with suitable receivers) with in some cases five (or more) signals available from each satellite. This is equivalent to up to 175 signals before even considering the use of multiple channels per satellite signal.

Also, additional channels can be used for functions such as interference monitoring. Multiple channels can be used to track a single signal using slightly different tracking loop parameters and thus provide more robust tracking of that signal in difficult or changing environments such as under canopy or in areas of high multipath.

The use of more channels provides more raw data to be used in the determination of the receiver’s position. The extra observations provide more redundant information and allows the statistical analysis to be performed in a more robust way by allowing for the removal of identified ‘noisy’ or outlier measurements whilst maintaining sufficient redundancy to get the correct answer for the PNT (position, navigation, time) solution.

So as a general statement in answer to the question – the more channels the better, with consideration of other factors such as those mentioned very briefly above.