Effect of leap seconds on space physics coordinate transformations

The future use of leap seconds in the definition of Coordinated Universal Time (UTC) is being investigated by an URSI Commission J Working Group. This issue may be of interest to many in the space physics community because it impacts on the co-ordinate transformations that we use. This arises because Universal Time (UT1) is a measure of the spin phase of the Earth. Thus it is a key factor in transformations from inertial co-ordinates (as used to represent spacecraft orbits) to co-ordinate systems spinning with the Earth, e.g. geocentric and geodetic co-ordinates plus any system based on the geomagentic dipole - such as geocentric-solar-magnetospheric and solar-magnetic co-ordinates. (But note that it does not impact on the transformations from inertial co-ordinates to non-spinning non-geomagnetic systems - such as geocentric-solar-ecliptic coordinates.)

Leap seconds are used to keep UTC (based on atomic clocks) within 0.9 seconds of UT1 (measured from the spin phase of the Earth). Given that the Earth rotates by 13.5 seconds of arc in 0.9 seconds, the difference between UT1 and UTC amounts to a spin phase error of less than 6.5 X 10-5 radians, which is equivalent to a position error of less than 3 km at geosynchronous orbit. Thus we have been able to neglect the difference between UTC and UT1, and substitute UTC for UT1 in space physics co-ordinate transformations.

If use of leap seconds were to cease (and there may be significant pressure for this because leap seconds cause problems for important applications such as global positioning systems), the difference between UTC and UT1 would then increase at about 0.7 seconds per year. Thus the space physics community would gradually need to include the correction from UTC to UT1 in its co-ordinate transformations.

Last updated 7 January 2000 by Mike Hapgood (Email: M.Hapgood@rl.ac.uk)

RAL Space Plasma Group logo Logo for coordinate systems