Timing Corrections File

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The Timing Corrections File (UT1UTC.dat) file contains the parameters for polar drift and non-uniform Earth rotation (variation in sidereal time). The file contains correction angles for polar drift (also called polar motion) and values for UT1 and UTC time differences. The data is based on definitive and predicted data as published in US Naval Observatory Bulletin A.

 

The FreeFlyer UT1UTC.dat file contains the following data in columns from left to right:

 

UTC Julian (GSFC) Date

Definitive (I) or Predicted (P) Values

Polar Drift Correction Angle-x(arcsec)

Polar Drift Correction Angle-y(arcsec)

UT1-UTC Correction Time (sec)

 

An excerpt of the file is given below:
 

24654.5      I          0.227241       0.485388       -0.4472541

24655.5      I          0.229631       0.483272       -0.4480564

24656.5      I          0.231914       0.481156       -0.4487457

24657.5      I          0.234169       0.479158       -0.4492834

24658.5      P          0.236257       0.476893       -0.4496171

24659.5      P          0.238366       0.474646       -0.4497310

24660.5      P          0.240417       0.472232       -0.4496396

24661.5      P          0.242425       0.469692       -0.4493741

24662.5      P          0.244404       0.467060       -0.4489956

 

Note: If you'd like to make your own UT1UTC.dat file, follow the example above. At a minimum you will need three lines: a first epoch, a final epoch, and an epoch in-between marking the first predictive epoch (the switch from I to P).

 

While the UT1 and UTC time differences are always taken into account, the use of the polar motion correction angles is controlled by the ComputePolarMotion Flag set in the SolarSystem object. A value of 1 causes polar motion effects to be calculated and used in coordinate system conversions.

 

// Turn on polar motion modeling

FF_SolarSystem.ComputePolarMotion = 1;

 

FreeFlyer uses linear interpolation to calculate the UT1-UTC offset as well as polar motion terms. After the end of the timing corrections file's data span, polar motion and the UT1 correction are set to zero. In other words, UT1 and UTC will be equivalent.