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:
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.
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:
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:
Log File Name and Location
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":
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.
The data will be updated on the set date and time and again every day at the set time.
The data will be updated on the set date and time and again once per week on the set day of the week.
The data will be updated on the set date and time and again once per month on the set day number of the month.