Sentinel: All you need to know about the internal Rb reference

What is holdover mode? 

An operating condition of a clock which is no longer locked to an input reference. In holdover, the clock maintains the same frequency and phase from when it was last locked to a reference.  

Holdover begins when the clock is disconnected from the input reference. On Sentinel, that means when the GPS or the external 1PPS input has been removed. 

Holdover terminates when the clock locks to a new reference input. On Sentinel, that means when the GPS or the external 1PPS input has been restored, and the Sentinel has re-locked to the input.

What is transport mode? 

A mode of Sentinel in which the internal battery is used to keep the Rubidium oscillator powered and in holdover mode when the main power supply is removed. In this mode, the rest of the instrument is powered down and the battery only provides power to the Rubidium. Transport mode is only required if a time / phase measurement is to be performed at a remote site where GPS is not available.

On Sentinel, transport mode is only available if the battery option is installed. Transport mode can be enabled when switching off the instrument. 

What performance can I expect in holdover? 

After being disciplined, the Sentinel Rubidium will typically maintain accuracy to within 100ns for 12 hours after entering into holdover state. 

Note that the actual duration depends on the status of the Sentinel Rubidium before it is put in holdover mode.  Additionally, environmental conditions such as temperature changes and magnetic field change may also impact the performance obtained. 

Is regular return-to-factory calibration required for the built-in Rubidium? 

No, regular return-to-factory calibration is not necessary for the Sentinel’s Rubidium. 

Sentinel is equipped with a GPS receiver and the built-in Rubidium can be disciplined by connecting to GPS (or an external 1PPS reference) for a period of time – see below. During this process the Rubidium disciplines to the supplied reference and the software stores the value of the disciplining process. On power up the Rubidium will always use the most recent stored value, however if transport mode was not used then reconnection to GPS is required to reacquire the phase of the 1PPS output.

How long should I discipline the Rubidium with GPS before use? 

This depends on the last time Sentinel was disciplined by a reference. 

We recommend the following minimum disciplining durations: 

  • If last disciplining was < 1 week ago – discipline for a minimum of 6 hrs

  • If last disciplining was > 1 week ago – discipline for a minimum of 12 hrs

If the recommended minimum disciplining durations are met, the Sentinel’s Rubidium will become stable and will be accurate to the product specification. If not, then its performance may not be as accurate and may be hard to predict.  

What other signals besides GPS can be used to discipline Sentinel? 

An external 1PPS signal can also be used to discipline the Sentinel Rubidium. If the 1PPS reference is from a Rb oscillator or a GNSS modeule the same minimum disciplining durations are recommended as per disciplining using GPS.  

If the 1PPS reference is from a Cs oscillator then Sentienl should be disciplined for 2 hours. 

Can I discipline Sentinel to GPS for less than the recommended duration before use, for example, two hours? 

If the above recommended minimum disciplining durations are met, we know the Sentinel Rubidium will become stable and will be accurate within the product specification. If disciplined for less than the recommended duration, then its performance may not be as accurate. It is hard to predict the frequency of the Rubidium in this instance and therefore the performance.  

Which setting should I choose for “the internal reference disciplining mode”? 

We recommend it is set to “always” while a valid reference is available and connected, for example, a GPS signal or external 1pps signal. It should be set to “never” if operating in holdover mode.  

How many satellites are needed by the GPS receiver to obtain lock? 

At least three satellites are required for the GPS receiver to obtain position fix and timing information. The status of satellite connections can be viewed on the Sentinel GUI. 

What is the “survey status” displayed on the Sentinel GUI for and how long will it take? 

The self-survey is a process for the GPS receiver to solve system parameters such as baselines, integer ambiguities, line biases, and attitude, etc. It normally takes around 30mins from cold start. When Sentinel is booted up with GPS connected, the GPS receiver starts to perform a self-survey. While the self survey is in progress the GPS icon will turn yellow and a 1PPS signal will be output to the Rb if the module is locked to at least three satellites. Once the self-survey is complete and Sentinel is locked to at least three satellites, the GPS satellite icon will turn green. 

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