Commonly-Used Fields - Novatel WAAS G-III User Manual

Reference receiver

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Chapter 5

5.2.2

Commonly-Used Fields

Some of the more commonly-used fields are discussed in this section.

5.2.2.1

Message Time Stamps

All NovAtel format messages generated by the G-III Reference Receiver have a GNSS time stamp in the

header. GNSS time is referenced to UTC with the zero point defined as midnight on the night of January 6,

1980 at 0:00 hours. The time stamp consists of the number of weeks since that zero point (0 to 1023) and the

number of seconds since the last week number change (0 to 603,799). GNSS time differs from UTC time since

leap seconds are occasionally inserted into UTC but GNSS time is continuous. In addition, a small error (less

than 1 microsecond) can exist in synchronization between UTC and GNSS time.

The data in synchronous logs, like the RANGEB log, are based on a periodic measurement of satellite

pseudoranges. The time stamp on these logs is the receiver estimate of GNSS time at the time of the

measurement. When setting time in external equipment, a small synchronous log will be accurate to a fraction

of a second. A synchronous log with trigger ONTIME 1 can be used in conjunction with the 1 PPS signal to

provide relative accuracy better than 250 ns.

Other log types (asynchronous and polled) are triggered by an external event and the time in the header may

not be synchronized to the current GNSS time. Logs that contain satellite broadcast data have the transmit time

of their last subframe in the header. The header of a polled log, like VERSIONB, gives the approximate GNSS

time when its data was generated.

5.2.2.2

Locktime

The maximum allowable satellite locktime is 65,535 seconds. GEO satellites are stationary and therefore, the

possibility exists for a locktime longer than the maximum.

If the locktime is larger than 65,535 s, the GEO locktime rolls back to 32,768 s.

Locktime is reported in the RANGEB (see Section 5.5.12 on Page 115) and MEASUREMENTDATAB (see

Section 5.5.11 on Page 112) logs.

WAAS G-III Reference Receiver User Manual Rev 1

Data Logs

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