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Earth Rotation Parameters

There are small disparities between time as measured by the motion of the earth and time as measured by atomic clocks, which can lead to mount pointing inaccuracy if not corrected. One of these effects is due to the earth's daily rotational period becoming longer as the earth's rotation slows down; this effect is about 1ms per day at the time of writing.

Astronomers use two corrections to compensate for this effect:

  • Leap seconds - Introduced whenever required to keep Universal Time Coordinated (UTC), which is based on atomic time, within +-0.9 seconds of earth rotational time
  • Delta UT1 - The remaining difference between UTC and earth rotational time on any particular day (in the range +-0.9 seconds) .
At the present time, leap seconds need to be introduced every two to three years and the intention to change the number of leap seconds is published in the International Earth Rotation and Reference Systems Service (IERS) Bulletin C about 6 months in advance of the effective date. Predictions of Delta UT1 are published daily by the IERS Rapid Service and include values for 90 days in advance of the current date.

Previous Platforms have contained a built-in value for leap seconds, current at the time the installer was built. However, they lacked an easy mechanism to update the leap second value and contained no support for the current Delta UT1 value.

The Platform now has the capability to automatically download current and future leap second values and current Delta UT1 values from the Internet on a regular basis so that the Platform will not need to be refreshed on observatory PCs when leap seconds change. If an Internet connection is not available, current values for leap second and Delta UT1 can be entered manually through a graphical interface. These options are described in more detail below.

Earth Rotation Data Tool

A graphical user interface for managing leap second and Delta UT1 vales has been added to the Diagnostics application that can be found in the Start / ASCOM Platform 6 folder. The Earth Rotation Data management graphical interface can be found in the Diagnostics Tools menu. On installation automatic update is selected, if you want a manual or no update mode then please use the tool to select your desired data source.

Five data sources are supported:

Data Source

Leap Seconds

Delta UT1

Built-in leap seconds and predicted delta UT1

Uses the leap second value built into the Platform when the installer was created.

Calculates delta UT1 from the built-in leap second value and the built-in DeltaT prediction

Specified leap seconds and predicted delta UT1

Uses the specified leap second value

Calculates delta UT1 from the specified leap second value and the built-in DeltaT prediction

Specified leap seconds and specified delta UT1

Uses the specified leap second value

Uses the specified delta UT1 value

Manual on-demand Internet update

Uses the leap second values last downloaded by running an update with the On-Demand button. Leap seconds will automatically change when the next leap second implementation date is reached.

Uses the delta UT1 value last downloaded through the On-Demand button.

Automatic scheduled Internet update

Uses the leap second values last downloaded by the scheduled data update task. Leap seconds will automatically change when the next leap second implementation date is reached,

Uses the delta UT1 value last downloaded by the scheduled data update task.

On Demand and Automatic Update Configuration

A variety of NASA and USNO sites distribute the IERS data and these can be selected from the Internet Data Source dropdown. To protect against waiting for ever for a download to complete, a timeout can be specified, after which the download task will abort. A trace file can be turned on for on demand and automatic Internet updates. These log files will appear as:

Update Type

Log File Name and Location

On-demand Update

Documents\ASCOM\Logs-yyyy-mm-dd\ASCOM.EarthRotationUpdate.xxxx.yyyyyy.txt Where yyyy, mm, dd are numeric year, month and day numbers and xxxx and yyyyyy create a time based unique identifier.

Automatic Scheduled Update

Documents\ASCOM\EarthRotationUpdate\Log yyyy-mm-dd.txt Where Where yyyy, mm, dd are numeric year, month and day numbers.

Automatic Update Schedule

The "Automatic scheduled Internet update" source will create a Windows scheduled task that runs on the date / time set and repeats according to the specified "Repeat Frequency":

Repeat Frequency

Description

None

The data will be updated on the set date and time. Afterwards the task will remain in the task list but with no further activation set.

The task can be run manually from the scheduler task list if desired.

Repeat daily

The data will be updated on the set date and time and again every day at the set time.

Repeat weekly

The data will be updated on the set date and time and again once per week on the set day of the week.

Repeat monthly

The data will be updated on the set date and time and again once per month on the set day number of the month.

See Also