  In this work, we propose a forward-modeling method to study the trajectory
and speed of the interplanetary (IP) Type-III radio burst exciter. The model
assumes that the source of an IP Type-III radio burst moves outward from the
Sun following the Parker spiral field line. Using the arrival time of the radio
waves at multiple spacecraft, we are able to determine the trajectory of the
radio source in the Ecliptic plane, and its outward speed, as well as the
injection time and longitude of the associated electron beam near the solar
surface that triggers the Type-III radio burst. For the application of this
method, we design a system to gather the arrival time of the radio wave from
the radio dynamic spectra observed by Solar Terrestrial Relations Observatory
(STEREO)/WAVES and Wind/WAVES. Then the system forward models the trajectory
and speed of the radio burst exciter iteratively according to an evaluation
function. Finally, we present a survey of four Type-III radio bursts that are
well discussed in the literature. The modeled trajectories of the radio source
are consistent with the previous radio-triangulation results, the longitude of
the associated active region, or the location of Langmuir waves excited by the
electron beam.
