this function calculates the doppler shift of a signal
caused by the earth's ionosphere.  the answer is returned in
hz/hz.  the dummy variables in the order in which they appear
are;
1. theta - the zenith angle of the source in radians
2. dmx - the peak electron density in electrons /c.c.
3. amx - the altitude above the surface of the earth of
the peak density in km.  if it is negative, a value of 350 km.
is used
4. freq - the frequency of the source signal in hz
5. derthe - the derivitive of theta in radians/sec
6. derdmx - the derivitive of dmx in electrons/sec
if theta is less than 0 or greater than pie, theta is
assumed to be 0.