This report describes a method for estimating ionospheric signal delay for pseudorange GNSS measurements made with a single-frequency receiver. To reduce the influence of the instability of the time scale of the receiver, we determine the difference between magnitudes of ionospheric delay of signals from two satellites at one time. To eliminate random fluctuations from the difference, we average data over a period from 30 to 60 minutes. Numerical simulation is carried out to determine the influence of a change in the critical frequency and height of the ionospheric maximum on pseudorange. It is shown that under favorable conditions variations in the ionospheric delay difference can be used to correct the ionospheric model that reflects hourly variations in its parameters. We study a method for determining current values of critical frequencies of the ionosphere for a regional correction of its model.