Sound film and GPS (satellite navigation): what they have in common

1. Step 1. Sound film — uses — Photovoltaic effect. The optical soundtrack was read by a photocell.
2. Step 2. Photovoltaic effect — is used in — Solar panel. The photoelectric effect in a semiconductor.
3. Step 3. Solar panel — is part of — Artificial satellite. Powering satellites.
4. Step 4. Artificial satellite — is part of — GPS (satellite navigation). A constellation of satellites.

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