EXTERNALCLOCK

Sets external clock parameters

Platform: OEM729

The EXTERNALCLOCK command is used to enable the OEM7 card to operate with an optional external oscillator. You are able to optimally adjust the clock model parameters of these receivers for various types of external clocks.

 

  1. This command affects the interpretation of the CLOCKMODEL log.

  2. If the EXTERNALCLOCK command is enabled and set for an external clock (TCXO, OCXO, RUBIDIUM, CESIUM or USER) and the CLOCKADJUST command is ENABLED, then the clock steering process takes over the VARF output pins and may conflict with a previously entered EVENTOUTCONTROL command. If clocksteering is not used with the external oscillator, the clocksteering process must be disabled by using the CLOCKADJUST disable command.

  3. When using the EXTERNALCLOCK command and CLOCKADJUST command together, issue the EXTERNALCLOCK command first to avoid losing satellites.

There are three steps involved in using an external oscillator:

  1. Follow the procedure outlined in External Oscillator to connect an external oscillator to the OEM7.

  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.

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

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.

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: Clock Type. You may alternatively choose to supply customized settings.

 

The EXTERNALCLOCK command configures whether the receiver uses its own internal temperature-compensated crystal oscillator or that of an external oscillator as a frequency reference. It also sets which clock model is used for an external oscillator.

To force the OEM7 to use the internal oscillator, use the EXTERNALCLOCK disable command and physically disconnect the external oscillator input. Do not use the EXTERNALCLOCK OCXO, CESIUM, RUBIDIUM or USER parameters if there is no external oscillator connected to the OEM7.

Message ID: 230

Abbreviated ASCII Syntax:

EXTERNALCLOCK clocktype [freq] [h0 [h-1 [h-2]]]

Factory Default:

EXTERNALCLOCK disable

ASCII Examples:

EXTERNALCLOCK USER 10MHZ 1.0167e-23 6.87621e-25 8.1762e-26

EXTERNALCLOCK TCXO 5MHZ

Field

Field Type

ASCII Value

Binary Value

Description

Format

Binary Bytes

Binary Offset

1

Command header

-

-

EXTERNALCLOCK header

This field contains the command name for abbreviated ASCII or the message header for ASCII (see page 1) or Binary (see page 1).

-

H

0

2

clocktype

See Table: Clock Type

Clock type

Enum

4

H

3

freq

5MHz

1

Optional frequency. If a value is not specified, the default is 5 MHz

Enum

4

H+4

10MHz

2

4

h0

1.0 e-35 to
1.0 e-18

Optional timing standards. These fields are only valid when the USER clocktype is selected. Do not use h values with VCTCXO, OCXO, CESIUM or RUBIDIUM clock types. The h values for these options are fixed, see Table: Pre-Defined Values for Oscillators (default=0.0)

Double

8

H+8

5

h-1

1.0 e-35 to
1.0 e-18

Double

8

H+16

6

h-2

1.0 e-35 to
1.0 e-18

Double

8

H+24
Clock Type

ASCII

Binary

Description

DISABLE

0

Turns the external clock input off, reverts back to the on-board VCTCXO. When used in a binary command, use the parameter defaults (i.e. freq=1, h0=0, h-1=0, h-2=0).

TCXO

1

Sets the predefined values for a VCTCXO

OCXO

2

Sets the predefined values for an OCXO

RUBIDIUM

3

Sets the predefined values for a rubidium oscillator

CESIUM

4

Sets the predefined values for a cesium oscillator

USER

5

Defines custom process noise elements
Pre-Defined Values for Oscillators

Clock Type

h0

h -1

h -2

VCTCXO

1.0 e-21

1.0 e-20

1.0 e-20

OCXO

2.51 e-26

2.51 e-23

2.51 e-22

Rubidium

1.0 e-23

1.0 e-22

1.3 e-26

Cesium

2.0 e-20

7.0 e-23

4.0 e-29