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Drivers Manual

NMEA ZDA - 18

Description

Driver for decoding NMEA UTC Time tags for the purpose of QINSy Time Synchronization. This driver only decodes time tags and positions; further time related tasks like setting the clock and interfacing the QINSy Time Synchronization Adapter are handled elsewhere.

A single driver and serial port can be used to decode positions and time tags, this is only possible for a certain driver though. (as of April 2012).

The driver gives priority to ZDA messages over the position messages. As soon as it detects and decodes a ZDA message it will accept time tags only from this message type.

The QINSy time synchronization assumes, and therefore relies on, the reported time tag, whether from a ZDA or a GGA/RMC/GGK, corresponds to the exact moment that it was transmitted on the serial or network port, i.e. with the smallest latency possible. If for instance the GPS receiver reports a GGA message with 800 millisecond latency the time synchronization will be off by 800 msec when no Time Synchronization pulse (PPS pulse) is interfaced, and exactly one second when the Time Synchronization pulse (PPS pulse) is interfaced. Note that on modern GPS receivers the output latency is normally very low (usually less than 100 msec).

The same I/O port can be used to interface position and time tags.This is only possible for a certain driver at the moment, see related drivers.

Driver Information

Driver NMEA GGA/RMC/GGK
NMEA ZDA
Interface Type Serial / Network Driver Class Type  
UTC Driver (question) Yes Input / Output Input Executable DrvPositionNMEA.exe
DrvPositionNMEAUDP.exe
Related Systems
Related Pages  



System Configuration

Output Timing Requirement

The driver handles times reported in the message that have a non-zero milliseconds component, and can accommodate slow drift in the millisecond component as is the case with Trimble receivers. But the most important requirement is that the reported time is valid for the moment that the message is transmitted by the receiver. If this condition applies and a Time Synchronization Adapter (previously called a PPS Adapter) is used QINSy rounds off the reported time to the correct whole seconds. If the outputs of the GPS receiver can be configured and the output triggering can be set then it is safest to output the ZDA message on the whole second crossing. The preferred output rate is 1Hz but higher position updates are handled without issue.

Important

It is strongly advised to configure the GPS receiver to output the ZDA message for the time synchronization. If the GPS receiver is not capable of outputting a ZDA message then the time from a GGA/RMC/GGK position message can be used as a secondary option but be warned that this time is subject to computation latency inside the receiver and may lead to a less accurate or even wrong time synchronization of QINSy.

Note that a $GPGGA message does not contain a date, therefore your computer's date should be correct within +/- 12 hours otherwise the time synchronization will be off!

Database Setup

In the Database Setup program, add a new System of type Time Synchronization, note that only one Time Synchronization system is allowed.

Choose driver NMEA GGA/RMC/GGK if your GPS receiver is unable to output ZDA messages, or driver NMEA ZDA for serial ZDA messages, or driver NMEA ZDA (Network) for UDP messages.

Select appropriate port and communication parameters.

Press the Next button.

On this page you can select the COM port on which the QINSy Time Synchronization Adapter is installed. If you do not have a Time Synchronization Adapter because your timing requirements are not stringent you can skip this step. But bear in mind that not using a Time Synchronization adapter will limit the QINSy clock accuracy and this will influence the time tagging accuracy of raw data that do not have a UTC time tag.

Note that a $GPGGA message does not contain a date, therefore your computer's date should be correct within +/- 12 hours otherwise the time synchronization will be off!