How to compute the Instantaneous Power Spectrum ???

I'm looking for a means of computing the instantaneous power spectrum (IPS) of a signal, as described in the master's thesis of K.A. Hagerman ( http://www.ntis.gov/search/product.aspx?ABBR=ADA257504 ). For the signal, s(t), the ISP is the derivative of the running energy spectrum:

rho(t,w)=d/dt S_t(w)^2, where

S_t(w)=Integral of s(tau)*exp(-i*w*tau) from tau=-infinity to tau=t.

[Rick Rosson]

What have you tried so far?