[LEAPSECS] Schedule for success

Zefram zefram at fysh.org
Mon Dec 22 06:47:55 EST 2008

James Cloos wrote:

>Obviously GPS needs to provide quicker almanac sync.

There used to be a notable problem with GPS that all the satellites
transmitted the almanac in unison. This is highly redundant, and
means that the receiver has to wait the full 12(?) minutes to compile a
complete almanac. There was a proposal that the satellites could instead
transmit the almanac at offset phases, so that a multi-channel receiver
could compile a complete almanac, by combining the pieces received from
different satellites, in one or two minutes. Anyone know whether this
has been implemented?

>But I have to ask: why should navigation systems care about time of

>day? Shouldn't seconds since an epoch be enough?

A good point. GPS time lacks leap seconds precisely in order to make
it easy to convert to a linear scale for the purposes of computation.
I suspect that much of the need for UTC in navigation systems comes
from adherence to an established dumb design/standard rather than from
fundamental requirement. GLONASS notably makes gratuitous use of UTC,
and suffers operational glitches at every leap second. (GPS is not
immune from dumb design either: consider the signal format for GPS time,
and, for that matter, the concept of representing GPS time as a date
and time-of-day in the first place.)


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