The construction of the distance scale ladder is a process of building of a chain of objects with
well determined distance. The bottom of this chain is the determination of the scale of objects in
the Solar System. This is done through radar ranging, where a radio pulse is reflected off of the
various planets in the Solar System.
The most important value from solar system radar ranging is the exact distance of the Earth from
the Sun, determined by triangular measurement of the Earth and terrestrial worlds. This allows
an accurate value for what is called the Astronomical Unit (A.U.), i.e. the mean Earth-Sun
distance. The A.U. is the ``yardstick'' for measuring the distance to nearby stars by parallax.
The parallax system is only good for stars within 300 light-years of the Earth due to limitations
of measuring small changes in stellar position. Fortunately, there are hundreds of stars within this
volume of space, which become the calibrators for secondary distance indicators.
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