ection 3
Keplerian elements make it possible to describe a satellite’s orbit and locate a satellite on
its orbit at a particular time. In addition to furnishing a universal language for chroni-
cling and pinpointing satellites, these elements provide the information necessary to
predict the passage of specific satellites. That ability is essential to users of direct read-
out from polar-orbiting satellites.
NOAA and METEOR-series polar-orbiting environmental satellites continuously transmit
low-resolution imagery of Earth as an AM signal corresponding to the reflected radia-
tion of Earth as observed by sensors. This results in a strip of image as long as the trans-
mission is received and as wide as the scanning instrument is designed to cover (typi-
cally 1700 miles in width). A normal reception period is approximately 14 minutes.
However, not every one of a polar-orbiting satellite’s 14 daily passes will be within
reception range of a particular ground station; nor will every receivable pass be in opti-
mal reception range. Limited reception occurs because, in order to provide global cov-
erage, satellites in polar-orbits provide imagery in slightly-overlapping swaths (see satel-
lite chapter for polar orbiter coverage). Ascending satellites move westward with each
orbit, descending satellites move eastward with each orbit.
Ephemeris data (a collection of data showing the daily positions of satellites) is provid-
ed by NASA, NOAA, and electronic bulletin boards (pages 114–115). The data can be
inserted into satellite tracking programs or used to manually calculate satellite positions.
The next page describes the composition of the NASA two-line orbital elements.
figure 57.
OR B I TA L DATA
North
satellite (r, θ)
Ζ
descending node
equatorial
plane
orbital
plane
apogee
χ
vernal
equinox
Ω
ascending node
line of
nodes
perigee
ω
.ι
r
θ
S
