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Subframe 4 addresses atmospheric correction. As with subframe 1, the data there offer only a partial solution to a problem. The Control Segment’s monitoring stations find the apparent delay of a GPS signal caused by its trip through the ionosphere through an analysis of the different propagation rates of the carrier frequencies broadcast by GPS satellites, L1, L2 and L5. These frequencies and the effects of the atmosphere on the GPS signal will be discussed later. For now, it is sufficient to say that a single-frequency receiver depends on the ionospheric correction in subframe 4 of the NAV message to help remove part of the error introduced by the atmosphere, whereas a receiver that can track more than one carrier has a bit of an advantage by comparing the differences in the frequency dependent propagation rates.
As the illustration indicates, the signal from the GPS satellite appears to slow down as it goes through the ionosphere. The atmospheric correction has been uploaded to the satellite from the Control Segment. The Control Segment can quantify the slowing because the tracking station is at a known point.
The ionosphere is dispersive. Since the GPS satellite broadcasts signals at different frequencies, it is important to understand that these frequencies are affected differently as they pass through the ionosphere. Their propagation rates are slowed differently in the ionosphere. They're not slowed at exactly the same rate. We'll talk more about that in a little bit.
For the moment, it's sufficient to say that a single frequency receiver, and there are such things, of course, a single frequency GPS receiver is somewhat handicapped by the fact that it cannot model the ionosphere. If the receiver has only one frequency to work with, it can't take advantage of the fact that it could quantify how much the signals have been affected by the ionosphere for itself, rather than relying exclusively on the Atmospheric Correction in the Navigation Message. A single frequency GPS receiver must rely almost completely on the Navigation Message Atmospheric Correction. This is a bit of a handicap. The atmosphere over Kwajalien in the Pacific might be quite different than the atmosphere above a single frequency GPS receiver in the continental US. This is one reason that the Navigation Message is not as perfect as we would like it to be.