NERS User Guide

Introduction

NERS library interface provides 6 public routines. Routine ners_init is called before any other NERS functions. It specifies NERS control file and the time range of intended EOP use. Time in NERS library is the interval elapsed since 2000.01.01_00:00:00.0 in TAI time scale. Units of time are seconds. Instead of time, events are often get tags of UTC function. In order to convert UTC tag to TAI time, NERS provides function ners_get_utcmtai that returns function UTC minus TAI on the specified timetag UTC. Routine ners_init initializes internal data structure of NERS object that is defined in ners.i or ners.hinclude block. This object is then passed to any other NERS function. NERS provides either the vector Earth orientation parameters on the specified moment of time or the table of the Earth orientation series for the specified range of time and the specified time step. At the end, function ners_quit releases memory acquired by ners_init.

The Earth orientation can be described either as a 3×3 rotation matrix that transforms a Cartesian vector from the rotating terrestrial coordinate system to the inertial non-rotating celestial coordinate system or as parameters on which this matrix depends. For practical needs the rotation matrix and its time derivatives are sufficient to perform astronomical reduction for Earth's rotation. However, NERS also provides the parameters that describes the Earth's rotation on which the rotation matrix depends as well. These parameters are empirical corrections to a deterministic model. Since the Earth's rotation depends on motion of the hydrosphere and atmosphere, it cannot be described with a deterministic model with the accuracy comparable with accuracy of observations and should be continuously monitored using space geodesy observations. For historical reasons several alternative Earth orientation parameters were used. For instance. For instance the rate of change of the angular variable along the axis 3, i.e. the axial motion can be described as Euler angle 3, UT1 rate, or the length of day. NERS provides many alternative Earth orientation parameters. These parameters are not independent.

Use cases

Description of NERS public functions


This web page was prepared by Leonid Petrov ()
Last update: 2017.05.10_07:07:43