Commands
Chapter 5
EuroPak-15ab Receiver User Manual Rev 1
57
5.3.7
EXTERNALCLOCK
Overview
The EXTERNALCLOCK command allows the EuroPak-15ab to operate with an optional external oscillator.
You are able to optimally adjust the clock model parameters of the receiver for various types of external clocks.
1.
The EXTERNALCLOCK command is permanently enabled for the EuroPak-15ab and cannot be
disabled.
2.
This command will affect the interpretation of the CLOCKMODEL log, see
Page 79
.
There are two steps involved in using an external oscillator:
1.
Follow the procedure outlined in
Chapter 2, Installation
starting on
Page 26
of this manual to
connect an external oscillator to your EuroPak-15ab enclosure.
2.
Using the EXTERNALCLOCK command, select a standard oscillator and its operating
frequency.
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 satellite 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. Namely, the h
0
, 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
S
y
(f)
is the clock’s power spectrum. Typically only h
0
, 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, which are given in
Table 24
on
Page 58
.
Or, the user may
choose to supply customized settings.
Abbreviated ASCII Syntax:
Message ID: 230
S
y
f
( )
h
2
–
f
2
-------
h
1
–
f
-------
h
0
h
1
f
h
2
f
2
+
+
+
+
=
EXTERNALCLOCK
CLOCKTYPE
[ FREQ ][ h0 [ h1 [ h2 ]]]