Page 90 - Satellite Communications, Fourth Edition

70 Chapter Two

TABLE 2.5 Tiros-N Series Orbital Parameters
833-km orbit 870-km orbit
Inclination 98.739° 98.899°
Nodal period 101.58 min 102.37 min
Nodal regression 25.40°/day E 25.59°/day E
Nodal precession 0.986°/day E 0.986°/day E
Orbits per day 14.18 14.07
SOURCE: Schwalb, 1982a and b.

a, e,and i. These can be selected to give the required regression of
0.9856° east per day. The orbital parameters for the Tiros-N satellites
are listed in Table 2.5. These satellites follow near-circular, near-polar
orbits.

2.11 Standard Time
Local mean time is not suitable for civil time–keeping purposes because
it changes with longitude (and latitude), which would make it difficult
to order day-to-day affairs. The approach taken internationally is to
divide the world into 1-h time zones, the zonal meridians being 15°
apart at the equator. The Greenwich meridian is used as zero reference
and in the time zone that is ±7.5° about the Greenwich meridian the civil
time is the same as the GMT. Care must be taken, however, since in the
spring the clocks are advanced by 1 h, leading to British summer time
(BST), also known as daylight saving time. Thus BST is equal to GMT
plus 1 h.
In the first zone east of the GMT zone, the basic civil time is GMT +
1 h, and in the first zone west of the GMT zone, the basic civil time is
GMT−1 h. One hour is added or subtracted for each additional zone east
or west. Again, care must be taken to allow for summer time if it is in
force (not all regions have the same summer time adjustment, and
some regions may not use it at all). Also, in some instances the zonal
meridians are adjusted where necessary to suit regional or country
boundaries.
Orbital elements are normally specified in relation to GMT (or as
noted in Sec. 2.9.2, UTC), but results (such as times of equatorial cross-
ings) usually need to be known in the standard time for the zone where
observations are being made. Care must be taken therefore to allow for
the zone change, and for daylight saving time if in force. Many useful
time zone maps and other information can be obtained from the Internet
through a general search for “time zones.”