NAME

Time::TT::Realisation - realisation of Terrestrial Time (base class)

SYNOPSIS

$tai_instant = $rln->to_tai($instant);
$instant = $rln->from_tai($tai_instant);

$rln1_instant = $rln0->to_realisation($rln1, $rln0_instant);
$rln0_instant = $rln0->from_realisation($rln1, $rln1_instant);

DESCRIPTION

Terrestrial Time (TT) is a platonic time scale, consisting of the time axis of a relativistic reference frame located on the Terran geoid. It is not directly accessible for practical use. Instead, it must be realised by the use of atomic clocks and other equipment. A realisation is inevitably imperfect, only approximating the true TT. Consequently there are many realisations, with differing degrees of accuracy and accessibility.

An object of this class represents a realisation of TT. The main use of such an object is to convert between realisations in order to determine the time of events with very high precision, in cases where the directly-accessible realisation is not sufficiently accurate.

This is a base class, defining the interface for realisation objects. If you already have a realisation object then this document is what you should read in order to know how to use it. If you don't have a realisation object yet, there are no constructors here: see the tt_realisation function in Time::TT or the tai_realisation function in Time::TAI. If you are implementing a subclass of realisation object, see "SUBCLASSING".

The principal realisation of TT is International Atomic Time (TAI). This is defined retrospectively, in monthly bulletins from the BIPM, by its relation to real-time approximations of TAI that are supplied in public time signals by tens of metrological agencies around the world. Better realisations of TT are defined further in retrospect, and are defined by their relation to TAI. TAI thus has a pivotal role: different realisations of TT can be related to each other by using TT(TAI) as an intermediate form.

In this interface, instants on the TT scale are represented as a scalar number of seconds since the TT epoch, as described in Time::TT. All such numbers are represented as Math::BigRat objects.

METHODS

$rln->to_tai(INSTANT)

Takes an instant expressed on the time scale represented by this object, and converts it to an instant on the TT(TAI) scale. The input must be a Math::BigRat object, and the result is the same type.

$rln->from_tai(TAI_INSTANT)

Takes an instant expressed on the TT(TAI) scale, and converts it to an instant on the time scale represented by this object. The input must be a Math::BigRat object, and the result is the same type.

$rln->to_realisation(REALISATION, INSTANT)

Takes an instant expressed on the time scale represented by this object, and converts it to an instant on the time scale represented by the REALISATION object. The input must be a Math::BigRat object, and the result is the same type.

$rln->from_realisation(REALISATION, INSTANT)

Takes an instant expressed on the time scale represented by the REALISATION object, and converts it to an instant on the time scale represented by this object. The input must be a Math::BigRat object, and the result is the same type.

SUBCLASSING

This class is designed to be subclassed, and cannot be instantiated alone. Any subclass must implement the to_tai and from_tai methods. That is the minimum required. The general to_realisation and from_realisation methods may also be implemented for cases that can be done efficiently; they have default implementations in this class in terms of to_tai and from_tai.

SEE ALSO

Time::TAI, Time::TT

AUTHOR

Andrew Main (Zefram) <zefram@fysh.org>

COPYRIGHT

Copyright (C) 2006, 2007, 2010, 2012 Andrew Main (Zefram) <zefram@fysh.org>

LICENSE

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.