Telemetry

Telemetry transmits measurements from a remote system to a receiving system. A useful telemetry chain connects a sensor, signal conditioning, a data converter, an encoding and transmission system, and a receiver that reconstructs the reported quantities. Calibration, units and the time of measurement remain part of the scientific meaning of the received data.

Spacecraft illustrate why these stages are distinct. NASA's guide describes science and engineering measurements carried by a modulated downlink, then separated on the ground using their positions in frames or identifying information in packets. Locking a receiver onto the radio carrier is different from recognizing the data frames reliably.^[ref]

For an original rate calculation, $n$ channels sampled at $f_s$ readings per second with $b$ transmitted bits per reading require at least $nb{f}_s$ bits per second for the raw measurements. Sixteen channels with twelve-bit readings at $100$ readings per second require $19,200$ bits per second before framing and other overhead. The receiving display must also distinguish the sample time from arrival time: transport delay and missing records otherwise make a remote process appear to evolve differently from the measured one.

Radio tracking estimates the location or movement of a target using radio waves associated with it. In a basic beacon system, a radio transmitter travels with the target while a radio receiver and directional antenna provide observations from known positions. An audible received pulse can identify a transmitter and help determine its direction; it does not itself supply map coordinates.^[ref]

Radio direction finding can be combined across stations by radio triangulation. In a hypothetical horizontal Cartesian coordinate system, station $A=(0,0)$ finds a target along $y=x$, while station $B=(2,0)$ finds it along $y=2-x$, with distances in kilometres. The intersecting forward bearings give $(1,1)$. A single bearing instead leaves many possible positions along one direction. Nearly parallel bearings constrain the intersection poorly: a small angular error can then produce a large position error.

A field example is local bird tracking described by the Smithsonian Migratory Bird Center. Researchers use received signals to follow a tagged bird or combine directions from different observation points, allowing repeated observations without continuous visual contact. Receiver range, battery life and the placement of observing stations limit which movements are recorded. A missing signal therefore need not mean that the animal stopped moving or died; it may have moved beyond detection.^[ref]