Add the changelog fragment for #1746

changelog.d/1746-satellites-every-frame.md

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+- Move satellites every frame. Points were propagated once a second and held
+  still in between, so at 1 min/s they jumped about 45 px a second; each now
+  glides from its last SGP4 sample (position, velocity, gravity and its rate
+  of change) to the world time, resampled every second and whenever time
+  moves 30 s from it, within about 10 m of SGP4 (Sameh Khamis, #1746).