Vidojevic and Maksimovic 2009:
have analyzed 88 type III bursts, using STEREO WAVES data in the 125 kHz to 16 MHz band, from November 2006 to September 2007.
Type III bursts are caused by electron beams streaming away from the Sun along magnetic field lines, at near relativistic speeds. The 16 MHz radiation originates from roughly 1 solar radius above the photosphere. The 20 kHz radiation originates at roughly 1 Astronomical Unit from the Sun.
One of their conclusions is that the electron beam speed is almost constant with distance from the Sun.
They find an expression for the frequency drift rate, noting that the maximum flux density (watt per meter^2 per hertz) occurs near 0.8 MHz, a result that needs to be explained.
Calibration is performed using the Galactic background signal:
Dulk et al. 2001: http://adsabs.harvard.edu/abs/2001A%26A...365..294D