User's Manual Part 1

Operation Chapter 4
OEMV Family Installation and Operation User Manual Rev 5B 71
4.9.1 GPS to Receiver Time Synchronization
Synchronization of receiver time with GPS time does not occur until the receiver locks onto its first
satellite. The GPS L1 signal has two main streams of data modulated on the carrier. These data
streams are the C/A code (1.023 MHz rate) and the P(Y) code (10.23 MHz rate). Additionally, a
navigation message (at a 50 Hz rate) contains GPS satellite data including the ephemeris, clock
corrections and constellation status. This navigation message is encoded on both the C/A and P(Y)
codes. The navigation message is transmitted via individual subframes and each subframe is 300 bits
in length. With the 50 Hz data bit rate there is a new subframe transmitted every 6 seconds.
 Although GLONASS could be used for time synchronization, OEMV receivers use GPS.
4.9.2 Time Definitions
The following are related definitions:
Coarse Time Each subframe contains the transmit time of the next subframe in seconds
of GPS time of week. After the first subframe is collected and decoded by
the receiver, an approximate calculation of the "receiver clock offset" can
be made. The receiver clock offset is the difference between GPS time and
internal receiver time. The calculation is based on subframe transmit time
and the approximate propagation time from the satellite signal to the
receiver. The position of the satellite and receiver clock offset are used to
re-initialize the seconds counter on the receiver, resulting in receiver/GPS
time synchronization. The accuracy of the receiver time is expected to be
within 30 milliseconds (ms) of GPS time. This initial synchronization is
referred to as coarse time and is indicated by COARSE in the time status
field of the TIMESYNC log.
Fine Time Once at least 4 satellites have been acquired to calculate the antenna
position, a more accurate estimate of the receiver clock offset is calculated.
The new receiver clock offset is used to synchronize the receiver clock
even closer to GPS time. This is referred to as fine time and is indicated by
FINE or FINESTEERING in the time status field of the TIMESYNC log.
Fine time accuracy is a function of the GPS constellation status. For the
Standard Position Service (SPS) the time accuracy is specified as 300 ns (1
sigma) assuming that clock steering is enabled.
Fine Clock An OEMV family receiver that is tracking satellites, and has a receiver
clock state of FINE or FINESTEERING.
Cold Clock An OEMV family receiver that needs to have its clock synchronized with
the Fine receiver. It may have any clock state including UNKNOWN.
Warm Clock An OEMV family receiver that has its clock adjusted to better then 500 ms.
Refer to the TIME log to view the clock offset.