—This paper presents a tutorial on the Tracking, Telemetry, and Command (TT&C) for spacecraft and satellite missions. In particular, it provides a thorough summary of the design of the TT&C, starting from elementary system aspects and going down to the details of the on-board TT&C subsystem design, its units, and the physical layer. The paper is then complemented with a description of emerging TT&C techniques and technologies, the standardization framework, and with practical examples of actual spacecraft design of European space missions. The tutorial here presented is thought for professionals (also in other telecommunication engineering fields) that want to face the challenges and state-of-the-art of the TT&C, and know more about this fundamental function that allows us to daily control and monitor our spacecraft. Index terms —TT&C, PDT, Telecommand, Telemetry, Ranging, Transponder, Satellite I. I NTRODUCTION The TT&C is a critical function of the spacecraft that ensures the flying system operates correctly. As depicted in the sketch of Figure 1, the TT&C is done by means of radio-frequency (RF) links between the space element and the ground. As the name suggests, it performs three fundamental tasks: Command (also known as telecommand , TC), Telemetry (TM), and Tracking (also denoted, with abuse of naming, simply as ranging , RNG). The TC, trivially, is the function that allows to send commands and thus control the spacecraft: the mission operation center (MOC) sends commands through an Earth-to-space data burst transmission. Since commands are simple data that trigger specific spacecraft actions, TC data rates are usually in the order of some kbps (or even bps). The TM is instead the monitoring function, i.e., the retrieval of the whole recordings and status of the spacecraft. This information is downloaded to the ground through a space-to- Earth link and then redirected to the MOC. Depending on the number and size of the spacecraft subsystems, TM data rates can range from kbps to Mbps. Finally, tracking is a function that aids the determination of the spacecraft’s flight dynamics. Namely, an RNG signal is sent from the TT&C ground station to the spacecraft that, in turn, transposes it back to the ground (i.e., it does a RNG turn-around ). In this way, the TT&C ground station can estimate the round-trip time and thus Andrea Modenini is with the European Space Agency at European Space Research and Technology Centre (ESA/ESTEC), 2201AZ Noordwijk, The Netherlands (email: andrea.modenini@esa.int). Barbara Ripani is with Politecnico di Torino, Department of Electron- ics and Telecommunications (DET), I-10129 Torino, Italy (email: bar- bara.ripani@polito.it). The view expressed herein can in no way be taken to reflect the official opinion of the European Space Agency. TT&C Ground StationTC TM Mission Operation CenterPayload Data CenterPayload TMRNG Payload Ground StationFigure 1. Sketch of the TT&C system: the ground segment can operate and monitor the spacecraft by means of the TC and TM links. The payload data can be instead retrieved by means of a dedicated payload TM link (if the TM link is not sufficient for performing such a function). The RNG is instead performed by means of a dedicated signal that is transposed back to the ground. determine the spacecraft distance. Additionally, it can adopt the Doppler measure in the downlink to predict the spacecraft radial velocity w.r.t. the ground station. Theoretically, the just described TT&C links cover all directions for guaranteeing the mission data flow. However, spacecraft often embark a payload (e.g., scientific instruments) that produces a lot of information. Thus, the TM link could be inadequate for downloading such an amount of data. Hence, the TT&C is often complemented with a dedicated payload data transmission (PDT) link, from the spacecraft to a dedicated payload ground station that, in turn, redirects the received stream to the payload data center. Clearly, the TT&C is a critical element for the success of any space mission. However, whoever tries to review the available TT&C literature can quickly realize that the topic is a “niche field” and that getting familiar with this domain can be very complicated: only a small bunch of companies have TT&C expertise, and public material is rather limited, papers are usually quite advanced (clearly meant for those already expert in the field), and academic material is almost not existing. Hence, the objective of this paper is to provide a TT&C tutorial by reviewing the general concepts and state-of-the-art. The authors wrote Section II and III to be comprehensible to any professional or student in the telecommunication field for understanding the TT&C basics. In the following sections, the paper goes more in-depth by providing mathematical formulas, tables, discussion about trade-offs, and a large set of refer- ences, easing the transition for the reader to more advanced papers. In support of the explanation, several examples will 2 be given, with particular reference to European technologies, standards, and European Space Agency (ESA) missions. Nev- ertheless, the here-provided content is also applicable to non- European space missions of major space agencies such as NASA, JAXA, and CNSA, since the standards and engineering described here are agreed upon at inter space-agency level. On the other hand, this paper will not provide basics on general space engineering, such as satellite orbits, electromag- netic compatibility, space qualification, technology readiness, etc., although these are also relevant for TT&C engineering. In the authors’ opinion, these topics are well covered by the existing literature and thus out of the objective of this paper. When these will be mentioned,