One Of The Best Definition of Channel Group Delay

Channel group delay is defined as a time difference between transmitted RF signal (measured at phase center of transmitting antenna) and signal at the output of the onboard frequency source. 

There are three different delay parameters: Fixed/Bias group delay, Differential group delay and Group delay uncertainty in bias and differential value. 

The Fixed delay or Hardware group delay is a bias term. It is included in the clock correction parameters transmitted in the navigation data, and is therefore accounted for by the user computations of system time. The group delay uncertainty shows the variability in the path delay due to operational environment uncertainty and other factors. 

The effective uncertainty of the group delay will be in the range of 3nsec (2σ). 

Differential group delay is the delay difference between two navigation signals. It consists of random plus bias components. The mean differential is defined as the bias component and will be either positive or negative. For a given navigation payload redundancy configuration, the absolute value of the mean differential delay shall not exceed 30nsec. The random variations about the mean will be in the range of 3nsec (two sigma). 

To correct the bias component of the group delay, Tgd parameter is provided to the user in the navigation message.

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