  Using simulations of geosynchrotron radiation from extensive air showers, we
present a relation between the shape of the geosynchrotron radiation front and
the distance of the observer to the maximum of the air shower. By analyzing the
relative arrival times of radio pulses at several radio antennas in an air
shower array, this relation may be employed to estimate the depth of maximum of
an extensive air shower if its impact position is known, allowing an estimate
for the primary particle's species. Vice versa, the relation provides an
estimate for the impact position of the shower's core if an external estimate
of the depth of maximum is available. In realistic circumstances, the method
delivers reconstruction uncertainties down to 30 g/cm^2 when the distance to
the shower core does not exceed 7 km. The method requires that the arrival
direction is known with high precision.
