In space systems a tell-tale is a single-bit status indicator that is included in
telemetry
Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", and ' ...
or is used within the spacecraft's on-board software to signal conditions that must be tracked or acted upon, especially when the status changes. A tell-tale may continually change as the status it is tracking changes, or, it may change once upon change of status and then remain at that value until deliberately cleared. The latter type of tell-tale is known as a "sticky-bit" because its value "sticks", that is, remains constant, once it has been set.
The
Phoenix spacecraft contains another type of tell-tale, developed by the
University of Aarhus
Aarhus University ( da, Aarhus Universitet, abbreviated AU) is a public research university with its main campus located in Aarhus, Denmark. It is the second largest and second oldest university in Denmark. The university is part of the Coimbra Gr ...
in Denmark, as part of its ''Meteorological Station''. It is a small tube that is deflected by the
martian
Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. It became the most popular celestial object in fiction in the late 1800s as the Moon was evidently lifeless. At the time, the pre ...
wind, similar to a
sailing tell-tale. The science payload's
stereo camera
A stereo camera is a type of camera with two or more lenses with a separate image sensor or film frame for each lens. This allows the camera to simulate human binocular vision, and therefore gives it the ability to capture three-dimensional ...
recorded images of its motion to be used to determine wind direction and speed.
References
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Spacecraft communication