Trimble Outdoors SK8 - Manual (Page 63)

Lassen-SK8 Embedded GPS Module 4-11
Operation and Performance
4.9.2 Timing Pulse Output (PPS)
A pulse-per-second (PPS), ten microsecond wide pulse is available on the Lassen-SK8
8-pin interface connector. The pulse is sent once per second and the rising edge of the
pulse is synchronized with UTC. The pulse shape is affected by the distributed
capacitance of the attached cabling and input circuit. The rising edge is typically less than
20 ns wide. The falling edge should never be used for timing applications.
*
Note – The PPS signal output by the Lassen-SK8 is a CMOS/TTL level signal. If this
signal must be furnished to a remote location, the system designer should provide an RS-
422 driver for the timing pulse.
When the Lassen-SK8 is installed on the interface motherboard (supplied in the Starter
Kit), the PPS signal is connected to an open collector circuit and the polarity of the signal
is inverted.
4.10 System Architecture
The Lassen-SK8 module (see Figure 4-1) uses eight processing channels operating on the
L1 frequency of 1575.42 MHz and using the coarse acquisition (C/A) code. The module
uses custom integrated circuitry designed by Trimble to track the GPS satellite signals.
These ICs also contain support circuitry to the navigation processor. An integrated 32-bit
microprocessor is used for tracking, computing a position, and performing the I/O
operations.
The module receives the GPS satellite signals through the antenna feed line connector,
amplifies the signals, and then passes them to the RF down converter. A highly stable
crystal reference oscillator operating at 12.504 MHz is used by the down converter to
produce the signals used by the 8-channel signal processor. The 8-channel signal
processor tracks the GPS satellite signals and extracts the carrier code information as well
as the navigation data at 50 bits per second.
Operation of the tracking channels is controlled by the navigation processor. The tracking
channels are used to track the highest eight satellites above the horizon. The navigation
processor will then use the optimum satellite combination to compute a position. The
navigation processor also manages the ephemeris and almanac data for all of the satellites,
and performs the data I/O.