This is a breakout board for the SkyTraq Venus838LPx-T Timing GPS receiver. It's "mostly" pin compatible with the AdaFruit Ultimate GPS breakout board.
Timing modules operate differently from navigation modules. The Venus838LPx-T module performs a survey to determine its 3D location and then operates under the assumption that its location is fixed. Because of that, it can optimize its solutions for accuracy of time, providing precise timing information even with less than optimal satellite reception.
The pinout of the 9 pin connector was chosen to be largely compatible with the AdaFruit Ultimate GPS module. The differences are:
The breakout board provides a 9 pin .1" SIP header. The header provides LVCMOS (3.3v) level serial I/O for the NMEA data and command stream. The receive data pin is 5 volt tolerant (unlike the bare module). The power supply input can accept 5 volts and will generate 3.3v with an on-board LDO. The 3.3v pin can supply a small amount (perhaps up to 100 mA) of 3.3v power. Alternatively, if you wish, you can leave the Vin pin disconnected and supply regulated 3.3v power on the 3.3v pin.
The antenna is supplied 3.3 volt power. An active antenna is recommended.
There are two versions of the breakout board available. One comes a supercap to provide short-term retention of the almanac and time to facilitate warm-starting. The cap will allow the module to warm-start if power is lost for less than about an hour.
The onboard LED is on when no fix is available and blinks at 1/2 Hz once a fix is established (this is in contrast to the LED driven by the PA6H module, which blinks at 1/2 Hz when there is no fix and is mostly off when there is one).
There is a test point on the board connected up to the "P1PS2" pin of the module. This pin can be configured for a 10 MHz output nominally phase locked to GPS time (be aware, though, that the stability of this output is not comparable to a proper GPS disciplined oscillator).
The Venus838LPx-T module will provide PPS quantization error correction messages, allowing you to remove the quantization error present on the PPS output. Without correcting this error, the PPS will have approx. ±6 ns of jitter. With correction, the jitter can be reduced and the PPS accuracy can be as good as GPS can provide. The Allan deviation of the PPS output has the expected characteristic for a GPS receiver: at tau 1s it is about 10E-8 and the graph is a nearly straight line at a decade per decade reduction from there (10E-9 @ 10s, 10E-10 @ 100s, etc).
Navigation modules can provide excellent timing results, but require optimal satellite reception to do so. When a navigation receiver is presented with less than perfect conditions, the timing accuracy will suffer even more than the navigational accuracy. Timing receivers with their different optimization can retain timing accuracy even under conditions where navigational accuracy would suffer.
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