Operation OEM6 Cards and Enclosures
Chapter 4
OEM6 Family Installation and Operation User Manual Rev 12
68
4.9
Transferring Time Between Receivers
The
ADJUST1PPS
command is used as part of the procedure to transfer time between receivers. The number
of Pulses Per Second (PPS) is always set to 1 with this command. It is typically used when the receiver is not
adjusting its own clock and is using an external reference frequency.
The TIMESYNC log is also used to synchronize time between receivers. It contains a time status field that
may show COARSE or FINE, among others. For a complete list of the time status values and definitions,
refer to the
GPS Time Status
section of the
OEM6 Family Firmware Reference Manual
(OM-20000129).
Section 4.9.3, Procedures to Transfer Time
on page 69 provides details on the time transfer procedure.
Terms used in the procedure are defined in
Section 4.9.2, Time Definitions
on page 68. Refer also to the
ADJUST1PPS
command and the TIMESYNC log descriptions in the
OEM6 Family Firmware Reference
(OM-20000129).
4.9.1
GPS to Receiver Time Synchronization
Receiver time synchronization with GPS time does not occur until the receiver locks onto the 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 six seconds.
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 (TOW).
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
When at least four satellites are 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 appears as 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 OEM6 family receiver tracking satellites and has a FINE or FINESTEERING receiver clock state.
Cold Clock
An OEM6 family receiver that needs to have its clock synchronized with the Fine receiver. It may have any
clock state (except FINE or FINESTEERING) that includes UNKNOWN.
The terms PPS and TIMEMARK are synonymous.
