The term time-ordered data (TOD) indicates data arranged according to when it was received (the data typically including that time). Such data is generally applicable to astronomical observation (e.g., light curves), but the concept and term are commonly seen in CMB research. Data can be stored and/or processed according to the time collected. It can be the raw form of data from instruments scanning the sky as they receive, such as ground instruments that are using the Earth's rotation to do its scanning, and the relationship of time to the corresponding point in the celestial sphere is of interest. Some processing of the observation data depends upon the time collected, and efficient processing may make use of the data being ordered by time. Observation times can be of interest regarding noise: for example, any pattern to the noise that can be discerned can be helpful in compensating for the noise. Random noise that is changing over time (such as effects of the atmosphere) can be identified if the same region is viewed at separated times under similar atmospheric conditions: the time of each observation is a factor in viewing an astronomical emission that can be presumed to be steady.