Kepler did know the orbital period of the Earth, Mars, Venus, Mercury, Jupiter, and Saturn. He had access to observations going back centuries, including Brahe's most excellent data.
What couldn't know was the absolute size of the Earth's orbit, so the best he could do was give the size of the plantets' orbits in terms of the Earth-Sun distance, 1 A.U. Nobody else could do better for centuries.
Using the transit of Venus across the face of the sun, timing the instant the edge of Venus "touches" the edge of the Sun from two different locations on Earth a known distance apart makes a parallax measurable, and the absolute distance between the Earth and Venus a matter of trigonometry. Kepler's Law lets us convert all the other relative distances into absolute distances. I believe it was 1769 when this was first accomplished.