ISO contained several on-board clocks: one for each instrument and one for the satellite itself. Several time keys derived from these on-board clocks, and also from ground-based clocks, are present in the ISO data ( ISO Architectural Design Document, [85]). The most prominent ones are the UTC (Universal Time(Coordinated)), the UTK (Uniform Time Key) and the ITK (Instrument Time Key). Also in widespread use is the Ground Station Time (GST), the time when data were received. Note that the abbreviations UTC and GST are often used interchangeably and that GST does NOT refer to Greenwich Sidereal Time.
The GST is contained in each record of the Telemetry Data Format
(TDF) archive; it is the time of reception of the start of the
format by a ground station. This time is expressed in the Universal Time
Coordinated (UTC) scale as two
I*4 integers; the first holding the whole number
of seconds since midnight at the beginning of 1989; the second holding the
remainder in units of 
seconds. In some other data structures, the
UTC is expressed in other ways, namely `yydddhhmmss'
and the Modified Julian
Date offset in days from 2000.0. These are all exactly equivalent in physical
meaning within the given precision; in particular all of them `stand still'
during a leap second.
The spacecraft on-board time is contained in frame zero of each TDF format. It is read from the same oscillator that drives the telemetry encoding, and therefore increments by a fixed amount per TDF format. Experience with other missions has shown that although ideally the spacecraft clock is a very convenient quantity for a time scale and easy for software to manipulate, it may be subject to discontinuous jumps or resets, and therefore it cannot be used where it is necessary to label data with a unique time, or where a uniformly increasing time is required. The on-board time is thus not used within the Off-Line Processing (OLP) software.
As an alternative that does not suffer such disadvantages, an artificial on-board time called the uniform time key, or UTK, is derived from the GST. It is the UTK that is used to index all products derived from spacecraft telemetry as distinct from instrument telemetry.
The uniform time key (UTK) is defined as follows:
A continuously increasing time scale is also necessary for labelling instrument telemetry records. While each of the four instruments had its own way to synchronise data with spacecraft telemetry, these have been unified by defining for each an Instrument Time Key (ITK) as follows:
The four ITKs have been defined as follows:
seconds
(approximately
seconds) and is calculated
as a scaled difference of two UTK times. The LWS ITK is unique
over a revolution (ISO orbit).
seconds and
is calculated by combining the UTK and the times contained in
PHT floating blocks. The PHT ITK is unique over a revolution.
seconds. The SWS ITK is unique over the whole mission.
For CAM and SWS it is thus possible to label data records with a time key that is unique for the whole mission; for LWS and PHT, the time key should be used in combination with the TDT number.
The relationships between (i) UTC and UTK, and (ii) UTK and ITK are both established for every observation at the beginning of data processing and recorded in the Compact Status file. DERIVE_ERD, which reformats an observation's raw data into Edited Raw Data (ERD), makes the appropriate clock calibrations easily available in FITS header keywords and labels all ERD records with an instrument time key.
Attention is drawn to possible irregularities in the relations between UTC and UTK and ITK over one observation. The UTC-UTK relationship may not be entirely stable or constant, for one or more of the following reasons:
Therefore, the following should be borne in mind:
Users should also be aware that a small fraction of data was lost due to regular if infrequent small gaps in the telemetry stream, although most observations escaped such losses.