Novatel OM-20000053 Rev 2 EXTERNALCLOCK

C Commands Summary

96 MiLLennium GPSCard SW Version 4.503/4.52 Command Descriptions Manual Rev 2

EXTERNALCLOCK

Overview

The EXTERNALCLOCK and EXTERNALCLOCK FREQUENCY commands allows the MiLLennium GPSCard

to operate with an optional external oscillator. The user is able to optimally adjust the clock model parameters of

the GPSCard for various types of external clocks. The three-state clock model on GPSCards having access to this

command is different from that used on the other GPSCards.

NOTE: The EXTERNALCLOCK command will affect the interpretation of the CLKA/B log.

There are three steps involved in using an external oscillator:

1. Follow the procedure outlined in your GPSCard’s installation/operation manual for connecting an

external oscillator to your GPSCard.

2. For the chosen oscillator type, use the EXTERNALCLOCK FREQUENCY command to select the

operating frequency – either 5 MHz or 10 MHz.

3. Using the EXTERNALCLOCK command, select a standard oscillator or define a new one; the effect is

to define h0, h-1, and h-2 in the expression for Sy(f) given below.

Steps #2 and #3 define certain parameters used in the clock model for the external oscillator

Theory

An unsteered oscillator can be approximated by a three-state clock model, with two states representing the range

bias and range bias rate, and a third state assumed to be a Gauss-Markov (GM) process representing the range bias

error generated from satellite clock dither. The third state is included because the Kalman filter assumes an

(unmodeled) white input error. The significant correlated errors produced by SA clock dither are obviously not

white and the Markov process is an attempt to handle this kind of short-term variation.

The internal units of the new clock model’s three states (offset, drift and GM state) are meters, meters per second,

and meters. When scaled to time units for the output log, these become seconds, seconds per second, and seconds,

respectively. Note that the old units of the third clock state (drift rate) were meters per second per second.

The user has control over 3 process noise elements of the linear portion of the clock model. These are the h0, h-1,

and h-2 elements of the power law spectral density model used to describe the frequency noise characteristics of

oscillators:

where f is the sampling frequency and Sy(f) is the clock’s power spectrum. Typically only h0, h-1, and h-2 affect the

clock’s Allan variance and the clock model’s process noise elements.

Usage

Before using an optional external oscillator, several clock model parameters must be set. There are default settings

for a voltage-controlled temperature-compensated crystal oscillator (VCTCXO), ovenized crystal oscillator

(OCXO), Rubidium and Cesium standard; or, the user may choose to supply customized settings.