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Appendix T. User Spacecraft Clock Correlation System
T.1 General
There are several ways of setting or correlating a clock in space with an Earth based
time standard. Use of Global Positioning System (GPS) for position and time is
becoming very common. But we need to set clocks on near Earth satellites that are
outside the GPS constellation, do not have a GPS receiver, or are in deep space. This
is commonly done by using either an RF signal that travels one way (spacecraft to Earth
or Earth to spacecraft) or for more accurate correlations, a signal that travels two ways
(Earth to satellite and back to Earth). These techniques can also be used between any
two nodes, such as a space station and another satellite. Setting a clock at one point to
agree with another one at a different location is referred to as time transfer. The one
way method is referred to as the Return Data Delay (RDD) method or the RCTD
method. Some missions use the uplink instead of the down (return) link. The principals
are the same but the higher data rate on the down/return link allows for more accurate
time correlation. The TDRSS uses PN spread spectrum for ranging, which lends itself
to an accurate two way time transfer method called the USCCS. USCCS is outlined
here and is more fully described in the USCCS User Guide [452-UG-USCCS] which
also covers the RDD methods. In this discussion we do not concern ourselves with the
various hardware delays that must be taken into account and are fully covered in the
452-UG-USCCS.
T.2 Overview
The fundamental principal of the USCCS is shown in Figure T-1. A signal, a PN epoch,
is sent from the ground to a User spacecraft via a TDRS and back to the ground. When
the signal arrives at the User spacecraft, it triggers a reading of the spacecraft clock.
We call this the Spacecraft Time and the reading is sent to the ground via the normal
spacecraft telemetry. Knowledge of when the signal left the ground, t1, and when it
returned, t3, is used to accurately calculate when it was at the spacecraft and triggered
the reading of the spacecraft clock, t2. On the ground, the reading on the clock is
compared to the time that the reading was triggered by the epoch. This results in a
measurement (not a calculation) of the spacecraft clock error from UTC. For each
transmitted PN epoch, there is a set of t1, t2, t3 but we have not used notation here to
distinguish one set from another.
It is up to the User MOC to manage the spacecraft clock. Some projects adjust the
oscillator frequency that drives the clock, some reset the clock, and some just maintain
a table of Spacecraft Time vs. UTC.
Revision 10 T-1 450-SNUG