Find the shared radio window

SCD-1 and SCD-2 were placed in low-inclination orbits, approximately 750 kilometres above Earth with an inclination near 25 degrees. A spacecraft passes over different ground locations as Earth rotates below it. This geometry supports frequent opportunities over Brazil, but it is not the fixed view of a geostationary satellite and does not create uninterrupted contact with every platform.

For a direct relay, a useful window requires a radio path from the platform to the satellite and from the satellite to the receiving station at the same time. Hills, antenna performance, interference and receiver availability can further limit a geometrically possible connection. Orbital coverage is therefore an opportunity for delivery, not a promise that every attempted message will arrive.

Use an original scheduling example: the platform can see a relay from 10:00 to 10:08, while the receiving station can see it from 10:05 to 10:12. Their overlap is 10:05 to 10:08, lasting three minutes. Adding the two windows would be wrong because the two legs must work together. A message sent at 10:02 has only the first of the two specified paths.

INPE’s historical network description gives a typical platform configuration of about 32 useful bytes every 200 seconds. That is a description of a message scheme, not a guarantee that every platform always uses those settings. In a fictional three-minute overlap, a 200-second repeat interval does not guarantee even one transmission falls inside: its timing relative to the window matters.

Find the intersection of the radio windows

A repeated packet can improve the chance that a report is received, yet receiving three copies does not necessarily mean three independent observations. A data service needs enough identification and timing information to associate duplicates with the original measurement. Otherwise, the same report could be counted several times and distort an average.

An original four-minute window illustrates the point. Packets sent every 200 seconds might arrive at 10:05:10 and 10:08:30. If both contain the same earlier measurement, there is one observation and two delivery attempts. If the platform took a fresh measurement for the second packet, there are two observations. The packet contents and timestamp convention decide, not the count of radio receptions alone.