Plan separation and orbit changes

PSLV-C37 carried 104 satellites in February 2017: a Cartosat-2 series spacecraft and 103 co-passengers. A large satellite count is not a payload-mass unit. Mission planning still has to establish which bodies separate, in what order and under what conditions. A successful launch photograph cannot demonstrate that every separation occurred.

Imagine two dispensers with intersecting release paths. Even when all payloads share a nominal orbit, releasing them together may be unsuitable. The design needs verified geometry, timing and relative motion. A commanded release and an independently observed object in the intended orbit are different pieces of evidence.

On September 26, 2016, PSLV-C35 first released SCATSAT-1 in an orbit about 724 kilometres high. The fourth stage continued with seven passengers. Two later firings, separated by a coast, moved it through an intermediate elliptical orbit to a roughly 669-kilometre circular orbit for the remaining deployments. The first payload did not accompany those later stage maneuvers.

This illustrates why shutting down and restarting an upper stage can be useful. A burn changes velocity and hence the orbit; its location and direction matter. Coasting between burns lets the stage reach another part of the trajectory. Simply firing twice at arbitrary times would not reproduce the demonstrated delivery plan.

Follow the payload handoffs

An orbit specifies motion around Earth, not merely a height above the ground. For an original diagram exercise, distinguish a 700-by-700-kilometre circular orbit from a 700-by-500-kilometre ellipse. Both reach 700 kilometres, but only the first stays at that altitude in the idealized model. Neither pair of altitude numbers tells you the orbital plane.

In a same-plane idealization, slowing tangentially at the high point lowers the opposite side of an orbit. A later maneuver can establish a lower circular orbit. Real operations also track attitude, propellant, payload clearances and navigation uncertainty. The teaching sketch explains the sequence; it is not a trajectory solution or a launch command plan.