A data logger device for in-situ measurement of one or more physical parameters comprising a power source； one or more sensors for measuring the one or more physical parameters； a data store for storing representations of at least some of the measured values of the one or more physical parameters； control logic arranged to write the representations of at least some of the measured values to the data store and arranged to read data from the data store during data transmission； an antenna； and a transmitter coupled to the antenna and configured to transmit the stored data by passive transmission. The physical parameter may be temperature. A processor may estimate basal body temperature by extrapolating from a plurality of temperature readings during a sleep period. The estimated basal temperature may be used to form an indication of ovulation.
