by M.Lampton UCB SSL
I have computed a measure of the scientific return from the ISUAL limb-viewing instruments, when affixed to the ROCSAT-2 spacecraft in three alternative viewing angles. Briefly, ROCSAT-2 is three-axis stabilized with respect to the local horizon, and flies in a circular orbit of 891 km altitude. Its 97 degree inclination gives it a sun synchronous precession rate, and the launch time will be chosen to position the ascending node at local solar time equal to 22:00, or equivalently the node is 150 degrees east of the noon meridian. When viewing the 60km altitude limb, the nadir angle of the view direction is 63 degrees.
At this nadir angle, three alternative viewing azimuths were chosen for the computations that I report here: DUSK viewing the limb westward at night; TRACK viewing the limb along the orbit track either northward or southward; and DAWN viewing the limb eastward at night.
The attached figure shows the surface of the earth in local solar time, plotted for times near the summer solstice when the solar declination is +23.5 degrees. The coordinates are geographic latitude and longitude measured from the noon meridian. The white region is terrestrial daytime or twilight and is not suited for sensitive lightning detection or sprite monitoring. The gray region is terrestrial nighttime, defined by the criterion that the solar zenith angle is greater than 104 degrees; that is, the sun is more than 14 degrees below the horizontal defined at the 60km altitude level at the target position.
Of particular interest are the target tracks for the ISUAL sprite viewer, for the three mounting angles described above. DUSK is the track centered on the left or dusk portion of the figure. DAWN is the track centered on the right portion. TRACK is the sinuous great-circle track that spans the whole range of longitudes. The DUSK and DAWN tracks are separated in space from TRACK by a distance determined by the nadir angle of the limb.
The most scientifically productive portion of each orbit is when the view target lies in the latitude zone -45 < lat <+45, because in this zone the tropospheric electical activity is greatest. I have integrated the amount of viewing time per orbit for the three viewing angles, imposing this latitude constraint and also the constraint that the viewing target limb point must lie in darkness, SZA>104. The results are:
DUSK: 1155 sec/orbit
TRACK: 1365 sec/orbit
DAWN: 1695 sec/orbit
Of these there is a slight advantage of viewing the dusk direction, since thunderstorms are more frequent at dusk than closer to local midnight where the other views are concentrated. That effect is offset, however, by the somewhat shorter night observing time available on the dusk side of ROCSAT-2. There is a strong advantage (not shown by these computations) that TRACK viewing affords, namely the ability to view phenomena from a range of elevation angles thereby providing a statistical means of quantifying the otherwise indeterminate depth coordinate.