Schedule science beyond ground contact

The International Satellites for Ionospheric Studies program joined Canadian spacecraft development with American launch and scientific contributions. Alouette 2 was followed by Isis 1, launched in 1969, and Isis 2, launched in 1971. Canadian companies built much of the hardware while universities and government laboratories supplied experiments. A program is a coordinated scientific effort, not simply a collection of similarly named objects.

NASA dates the second launch to April 1, 1971; the archive’s launch ledger records 02:57:07 UTC. That instant falls on March 31 in California. The CSA overview describes a March launch without specifying its time convention. Comparing time zones can explain this difference; it does not suggest two launches. The photograph shows the earlier Isis 1 on launch hardware indoors, before flight.

A receiving station sees an orbiting spacecraft only during suitable passes. An onboard clock and recorder allow an experiment to operate between contacts and transmit the stored record later. This changes where observations can be collected. It also introduces scheduling, storage and timekeeping requirements that a real-time-only system does not have.

NASA describes a one-hour tape recorder on Isis 2. That is an instrument-system capacity, not evidence that every hour of the mission was retained. Observations must fit the selected recording mode and available capacity; power, command schedules and later contact opportunities also limit what can be acquired and returned.

Acquisition and reception are different events

In an original digital analogy, reserve 240 megabits of usable storage for a 100-kilobit-per-second stream. Using decimal units, 240,000,000 bits divided by 100,000 bits/s gives 2,400 seconds, or 40 minutes. A 45-minute session would need 270 megabits. These are invented values for practicing unit conversion, not specifications of the historical tape recorder.

If headers, error protection or other instruments also use storage, allow for them before scheduling the observation. A faster downlink can empty the recorder sooner during contact, but it does not increase how much it holds between contacts. Keep acquisition time separate from the later transmission time when matching observations from multiple instruments.