Most receivers calculate a horizontal position after they acquire three satellites.
To do this they must make presumptions on their altitude. Usually the last calculated altitude is used. If the altitude assumed by the receiver is in error, the accuracy of the horizontal position is impacted. For example, if the receiver assumed mean sea level and it was at 10,000 feet, a horizontal error of more than a mile could result. On the other hand, if the assumed value is very accurate the horizontal error may be less than if four satellites are tracked.