Online Calibration

Time synchronization is an extremely important service in wireless sensor networks. The notion of time on a node is maintained using crystal oscillators. These oscillators are designed to oscillate at a particular frequency called the nominal frequency, but typically there is always some deviation. This deviation is caused due to a wide variety of reasons which include manufacturing variations, environmental factors such as temperature, pressure, humidity. Even aging of the crystal is a long-term factor influencing the oscillation of a crystal. Various time synchronization protocols have been developed over the years which perform message exchanges at different points in time to keep the clocks in sync. These points are called the resynchronization points, and are essential because in spite of periodic synchronization, the clocks continue to drift apart with time. One way to avoid frequent resynchronization is to locally compensate for the drifts once a synchronization is performed. Ideally, these drifts are due to a combination of various factors and we should compensate for each of them. In this report, we focus on the drift caused due to temperature(which is also the most dominant factor) and ways to compensate for it. There are various techniques which have been developed to perform compensation for temperature induced drifts. The techniques rely on creation of a look-up table which relates the different temperature ranges with the corresponding frequency drifts. Since, this table is highly specific to a crystal, we need to perform the measurements for the look-up table during the manufacturing phase of every crystal. Thus, most of these techniques turn out to be costly. In this report we try to use the communication network between nodes, to populate the entries of the look-up table online. We also exploit the known relationship between frequency drift and temperature (which is a cubic polynomial) to estimate the parameters of the polynomial and use it for local compensation.