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54 EuroPak-15a Receiver User Manual Rev 5
Chapter 5 Commands
5.3.7 EXTERNALCLOCK
Overview
The EXTERNALCLOCK command allows the EuroPak-15a 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. This command will affect the interpretation of the CLOCKMODEL log, see Page 77.
2. If the EXTERNALCLOCK command is enabled and set for an external clock (TCXO, OCXO,
RUBIDIUM, CESIUM, or USER) and the CLOCKADJUST command, see Page 50, is
ENABLED, then the clock steering process will take over the VARF output pins. If clocksteering
is not used with the external oscillator, the clocksteering process must be disabled by using the
CLOCKADJUST DISABLE command.
There are three steps involved in using an external oscillator:
1. Follow the procedure outlined in Chapter 2, Installation starting on Page 23 of this manual to
connect an external oscillator to your EuroPak-15a enclosure.
2. Using the EXTERNALCLOCK command, select a standard oscillator and its operating
frequency.
3. Using the CLOCKADJUST command, disable the clocksteering process if external
clocksteering is not used.
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 25 on Page 55. Or, the user may
choose to supply customized settings.
S
y
f()
h
2–
f
2
-------
h
1–
f
------- h
0
h
1
fh
2
f
2
++++=