Telemetry is a technology that allows the remote measurement and reporting of information of interest to the system designer or operator. The word is derived from Greek roots tele = remote, and metron = measure. Systems that need instructions and data sent to them in order to operate require the counterpart of telemetry, telecommand.
Telemetry typically refers to wireless communications (i.e. using a radiofrequency system to implement the data link), but can also refer to data transfer over other media, such as a telephone or computer network or via an optical link

Telemetry is an enabling technology for large complex systems such as missiles, RPVs, spacecraft, oil rigs and chemical plants because it allows automatic monitoring, alerting, and record-keeping necessary for safe, efficient operations. Space agencies such as NASA, ESA, and other agencies use telemetry/telecommand systems to collect data from operating spacecraft and satellites.

Telemetry is vital in the development phase of missiles, satellites and aircraft because the system might be destroyed after/during the test. Engineers need critical system parameters in order to analyze (and improve) the performance of the system. Without telemetry, this data would often be unavailable.

Telemetry was a vital source of intelligence for the US and UK when Soviet missiles were tested. For this purpose, the US operated a listening post in Iran. Eventually, the Soviets discovered this kind of US intelligence gathering and encrypted their telemetry signals of missile tests. Telemetry was a vital source for the Soviets who would operate listening ships in Cardigan Bay to eavesdrop on the UK missile tests carried out there.

Telemetry has been a key factor in modern motor racing. Engineers are able to interpret the vast amount of data collected during a test or race, and use that to properly tune the car for optimum performance. Systems used in some series, namely Formula One, have become advanced to the point where the potential lap time of the car can be calculated and this is what the driver is expected to meet. Some examples of useful measurements on a race car include accelerations (G forces) in 3 axes, temperature readings, wheel speed, and the displacement of the suspension. In addition, there exists some series’ where “two way” telemetry is allowed. Two way telemetry suggests that engineers have the ability to update calibrations on the car in real time, possibly while it is out on the track.

Telemetry also is used for patients (biotelemetry) who are at risk of abnormal heart activity, generally in a coronary care unit. Such patients are outfitted with measuring, recording and transmitting devices. A data log can be useful in diagnosis of the patient's condition by doctors. An alerting function can summon nurses if the patient is suffering from an acute or dangerous condition.

Telemetry is now being is used to follow members of endangered species. Such animals are now commonly equipped with instrumentation ranging from simple tags to cameras, GPS packages and transceivers to provide position and other information to the scientists, producers, activists, regulators, or other human agencies.

At a 2005 VendTec workshop in Las Vegas, a seminar noted the introduction of telemetry equipment that would allow vending machines to communicate sales and inventory data to a route truck or to a headquarters. This data could be used for a variety of purposes, such as eliminating the need for the driver to make a first trip to see what items need to be restocked before bringing the inventory inside[1].

Retailers are also beginning to make use of RFID tags to track inventory and prevent shoplifting. Most of these tags passively respond to RFID readers (e.g. at the cashier), but active RFID tags are available that periodically transmit telemetry to a base station.

1. Sanford, Tim: VendTec Workshop Offers Overview Of Latest Telemetry, Payment Tools, Vending Times, June 2005.