PPS

WARNING: The filter needs a valid PPS to initialize and to process measurements. In almost all cases, the DISABLED and GENERATED options in the Source field of the PPS Source (0x0C,0x28) command should not be used. Additionally, GNSS antennas, if used, need to have a clear view of the sky and be receiving GNSS signals for a valid PPS. Temporary GNSS outages can be tolerated.

The pulse-per-second (PPS) is a timing signal that can be used to synchronize the 3DM-GQ7 with other systems.

The 3DM-GQ7 can accept an external PPS or generate one as an output derived from the internal clock or from one of the on-board GNSS receivers. Both input and output signals use a configurable digital control line available through the GPIO pins on the Main/Aux connectors.

IMPORTANT: the PPS source and the assigned GPIO pin are separate configurations.

The PPS time sync status can be monitored by using the Time Sync Status (0xA0,0x02) data descriptor. This message tells the user when a PPS lock has been achieved.

Once the GPIO and PPS source have been configured, the user can set the whole number of seconds using the Time Broadcast Command (0x01,0x72). The user must send a separate instance of this command to set the week number. See shared GPS Timestamp (0xFF,0xD3) for more information.

As shown in the figure below, this must be done within 250 milliseconds after the PPS line goes high. This command can be sent every second, or just once at initialization.

NOTE:  If the time command is sent only once, the time will be kept synchronized by the PPS input. If the PPS signal stops, the 3DM-GQ7 will continue tracking time using its internal clock. In this case, accuracy will be degraded.

IMPORTANT: If a glitch occurs on the PPS line, the time command should be sent again to ensure seconds were not double-counted or lost.

To receive data timestamps referenced to the PPS (whether input or output), use the following messages from the Shared Data (0xFF) set: GPS Timestamp (0xFF,0xD3), Delta Time (0xFF,0xD4) , External Timestamp (0xFF,0xD7), External Time (0xFF,0xD8).

TIP: Time synchronization via command or data packets is not very accurate due to transmission and processing delays. Typical delays might be on the order of a few to tens of milliseconds, or even longer. The 3DM-GQ7's PPS offers a very precise, very low-latency alternative.

 

Outputting a PPS

When configured to provide a PPS output, the 3DM-GQ7 will generate a square wave pulse with a 50% duty cycle and a period that defines one second. The rising edge of the pulse signals the beginning of a new second. When the PPS source is an internal GNSS receiver and that receiver has a valid navigation solution, the pulse edge will align with start of a new second of GPS System Time with a 3-sigma error of 60 ns (see Electrical).

Use the PPS Source (0x0C,0x28) command to set the PPS source. To generate a PPS from the 3DM-GQ7's internal clock, set source to generated. To generate it from an internal GNSS receiver, set source to either receiver_1 or receiver_2.

The 3DM-GQ7's GPIOs must be configured to provide a PPS through the GPIO Configuration (0x0C,0x41). See User GPIO for a list of GPIOs that support PPS output and instructions on GPIO configuration.

 

Inputting a PPS

When configured to receive a PPS input, the user should provide a square wave signal with a period that defines one second and whose rising edge signals the start of a new second.

Set the PPS source to GPIO using the PPS Source (0x0C,0x28). Configure the appropriate GPIO pin as a PPS input using the GPIO Configuration (0x0C,0x41) command. See User GPIO for more details.