In order for a GPS receiver to be compativle with out DVR Express Core DVRs it needs to have two interfaces: an RS232 serial port (or USB port which appears as a COM port in Windows) and a Pulse Per Second (PPS) output signal (5V TTL).  The serial data must be NMEA 0183 format ie. $GPZDA or $GPGGA messages, continuously being output through the serial port at a 1Hz rate.  The serial data is sent to the PC that the DVR is connected to.  

Our software (CoreView or Streams 7) opens the selected COM port and receives the NMEA messages.  Simultaneously, the PPS pulse coming out of the GPS receiver must be connected to one of the TTL Inputs on the Auxiliary I/O port of the DVR.  Each time a PPS pulse is detected at the DVR, it starts a timer (offset counter).  At the same time a PPS pulse is detected, an NMEA message is transmitted to our software, but there is some latency in how long it takes to be captured, decoded, and then transmitted into the DVR via eSATA/USB3 to update the DVR’s clock.  This is where the offset counter comes in – once the time message is transmitted to the DVR, the timer value is applied as an offset, giving the DVR an accurate indication of the time of the PPS pulse.  This time is then used to update the DVR’s clock.  This process repeats every second, ensuring close accuracy.