A method for ascertaining and monitoring fill level of a medium in a container by means of a field device by a travel time measuring method, wherein transmission signals are transmitted in the direction of the medium and reflection signals are received, wherein received reflection signals are registered as echo signals in an echo function dependent on travel time or travel distance, wherein position and/or amplitude at least of a wanted echo signal in the echo function are/is ascertained by means of an echo search algorithm and a continuous echo tracking of changes of position and/or of changes of amplitude of the wanted echo signal in the echo function is performed in a defined search window, wherein position and/or amplitude at least of a wanted echo signal is maintained, in case no wanted echo signal is ascertained in the search window, wherein from position and/or amplitude at least of a wanted echo signal, fill level is ascertained, and wherein a measured value of fill level is output. For this, an asymmetric search window is used for ascertaining the wanted echo signal.