The Signal Filter parameter GPS, within the Advanced Settings, configure the use of the software through which the system IsoView decide whether the sign of GNSS whether or not you are fit for work, based on your current quality. When the filter is Able, if the value of HDOP is less than 2.0, the signal is considered as OK; otherwise the sign of GNSS will be cut. with the filter Disabled, the value of HDOP is not considered.
The Signal Filter GPS must be used when the user does not want the system to operate with low quality in the reception of the signal GNSS, such as in the vicinity of riparian forest, so as not to affect the accuracy of the application.

The Signal Filter setup screen GPS is shown in the figure below.