This article briefly describes ephemeris toe and time of first receipt
It is well known that the satellite ephemeris specifies the orbit of the GPS satellites, i.e. Argument of Perigee, Right ascension, Inclination angle, etc. A satellite orbit is of course specified at the Time Of Ephemeris (toe), i.e. the Ephemeris Data Validity Time.
Figure 1 shows how the data specified in the ephemeris dataset specifies where the satellite is (or more accurately, ‘will be’) at the toe. Therefore this may not be the actual satellite position depending on the actual time of week (TOW) with respect to toe. The toe data is used to extrapolate the satellite position forward/backward to the position that is valid for the current TOW. Therefore if the satellite position at toe is known the actual satellite position at time the current TOW can be determined.
The Time of First Receipt is when the ephemeris data was first broadcast/used (this is the data specified within the header of the almanac data). In other words for the period of time between the Time of First Receipt and the toe the ephemeris data does not change, i.e. toe is the same for this period.