  The most promising concept for low frequency gravitational wave observatories
are laser interferometric detectors in space. It is usually assumed that the
noise floor for such a detector is dominated by optical shot noise in the
signal readout. For this to be true, a careful balance of mission parameters is
crucial to keep all other parasitic disturbances below shot noise. We developed
a web application that uses over 30 input parameters and considers many
important technical noise sources and noise suppression techniques. It
optimizes free parameters automatically and generates a detailed report on all
individual noise contributions. Thus you can easily explore the entire
parameter space and design a realistic gravitational wave observatory.
  In this document we describe the different parameters, present all underlying
calculations, and compare the final observatory's sensitivity with
astrophysical sources of gravitational waves. We use as an example parameters
currently assumed to be likely applied to a space mission to be launched in
2034 by the European Space Agency. The web application itself is publicly
available on the Internet at http://spacegravity.org/designer.
