Well best guess for me would be when a number of visable satelites are closest to earth in their orbits signals would take less time to reach reciever i havent got time to reall look at this as for time of day i would need more time to poke around the US navy website
Accuracy depends on satellite geometry, which varies with time
The system time reference is managed by the U.S. Naval Observatory in Washington, DC. GPS time is measured from Saturday/Sunday midnight at the beginning of the week. The GPS time scale is a composite “paper clock” that is synchronized to keep step with Coordinated Universal Time (UTC) and International Atomic Time (TAI). However, UTC differs from TAI by an integral number of leap seconds to maintain correspondence with the rotation of the earth, whereas GPS time does not include leap seconds. The origin of GPS time is midnight on January 5/6, 1980 (UTC). At present, TAI is ahead of UTC by 32 seconds, TAI is ahead of GPS by 19 seconds, and GPS is ahead of UTC by 13 seconds. Only 1,024 weeks were allotted from the origin before the system time is reset to zero because 10 bits are allocated for the calendar function (1,024 is the tenth power of 2). Thus the first GPS rollover occurred at midnight on August 21, 1999. The next GPS rollover will take place May 25, 2019.
For surveying purposes, the most accurate positional data are desired. Because the GPS satellites orbit the earth twice each day, their position relative to a point on the ground is continually changing. In addition, GPS satellites are taken 'off-line' occasionally for maintenance or a problem (See NANUs at USCG GPS web site). Therefore the quality of a GPS-derived position can vary. This condition is called Dilution of Precision or DOP. There's a class of GPS analysis software that can help the user determine the best time for precise data collection