The Telemetry, Tracking and Command (TTC) system and, more specifically, the transponder form an essential part of most spacecraft. Current transponders, which were developed in the 1980s, are being updated to provide improvements in all areas - cost, flexibility, power consumption, mass and size. ESA is developing a number of standard models to meet the needs of future missions. For near-Earth missions, there are several concepts, depending on the method of communication and the size of the user: for satellites using direct to ground communication, the Compact Standard Transponder (CST) is being developed, while for those using a data-relay satellite, the Small User Transponder (SUT) is being developed for smaller users, and the Dual-Mode TTC Transponder (DMT) for larger users. For deep space missions, a Deep Space Transponder (DST) is envisaged.
Telemetry, Tracking and Command (TTC) are vital functions of a spacecraft. They allow data to be communicated between the ground and the spacecraft for spacecraft control and command. The communication is through a telecommunication link established between the control station on the ground and the satellite. The TTC transponder on the spacecraft plays the role of radio frequency interface with the ground.
Virtually all spacecraft - and certainly all ESA spacecraft - are equipped with TTC transponders. It has been recognised since the early days of spacecraft development that all spacecraft require such a type of control from ground, and standards for TTC interfaces between spacecraft and ground are well established. ESA and other space agencies, including NASA, DLR, CNES, and NASDA, have collaborated in their definition in order to ensure compatibility for efficient use and cost-effective cross-support.
With the TTC transponder now a standard feature of almost all ESA and European national programmes, ESA has a function dedicated to the design and development of TTC transponders: the RF Systems Division of the Technical Directorate. After in-depth surveys of future needs expressed by ESA's Programme Directorates, studies and developments are performed in the frame of the technology programmes of the Technical Directorate (the Technological Research Programme, and General Support Technology Programme), or of the Programme Directorates (the Advanced Systems Technology Programme). The objective is to design equipment that will be reusable for the maximum number of spacecraft with the minimum recurring cost. However, the great variety of spacecraft developed under the Agency's programmes does not allow for the definition of a single, unique and universal TTC transponder.
It is essential that a reliable communication link between the ground station and the spacecraft is maintained throughout the satellite's different phases of operation.
During the Launch and Early Orbit Phase (LEOP), ground control sends the required mission commands, such as to fire the booster rockets for orbital correction, to deploy the antenna or solar array, or to fire the apogee boost motors. Some of these operations must happen at precise times, while others can take place during a window of time.
During the lifetime of the mission, which is generally four to ten years, the satellite receives daily the commands required to reconfigure functions according to requirements at the time. Earth observation satellites, such as SPOT or ERS, receive instructions for their next orbits, such as the region of interest of the Earth to observe, the direction of view, or the spectral band to use. A data-relay satellite, such as Artemis or DRS, receives daily commands to inform it of its low Earth orbiting clients; it receives the necessary data for pointing one or more of its antennas towards that satellite and following its path while data relay communication is required.
The three functions of telecommand, tracking and commanding are also essential.
The telecommand link is used to upload commands to the spacecraft, particularly when mission characteristics are not defined until after launch. The Giotto probe, for example, having successfully encountered Halley's Comet and being surprisingly only slightly damaged despite several collisions with cometry debris, could be deployed for another mission, to encounter a second comet, Grigg-Skjellerup. Giotto's flight plan was completely redefined while the satellite was in orbit and the reconfigured data was then telecommanded from ground.
Telecommanding is of particular importance to deep-space probes. Their distance from the Earth creates communication problems (although the probes have a high degree of autonomy to overcome those problems). Firstly, the signals reaching the probe from the ground are so weak that the amount of data that can be transmitted is limited. Secondly, it can take up to several hours for the radio signal from Earth to reach the probe if the probe is at the edge of the solar system, which makes controlling the probe extremely difficult.
The telemetry link is equally important to the success of a satellite mission. Telemetry is the data received from the spacecraft, generally about the status of its systems. For scientific satellites, including deep-space probes, the telemetry link also carries payload data. During launch and early orbit, telemetry allows ground technicians to check that commands are being carried out correctly, e.g. that boosters are being fired or that the antennas or solar panels are being deployed. Throughout the mission, it enables the mission control centre to survey the 'insides' of the satellite, its configuration, its status, and in the case of failure, it provides the basis for the decisions that have to be made.
TTC's third function is tracking and ranging. The transponder demodulates the ranging signal contained in the uplink and remodulates it onto the downlink. Thus, by measuring the return propagation time, the distance between the ground station and the satellite can be estimated. Moreover, the transponder has the ability to generate a downlink carrier phase coherent with the uplink carrier, allowing precise estimations of orbit and speed from measurements of Doppler offset and rate of the downlink frequency at the ground station.
The architecture of a typical satellite TTC system is shown in Figure 1. The system comprises two low gain antennas with hemispherical coverage, two transponders each with transmitter and receiver, and two command decoders. The interface with the rest of the satellite is via the On-Board Data Handling (OBDH) system.
Figure 1. Architecture of a typical satellite TTC system
The uplink carrier with the telecommand (TC) signal from the ground station is received by one of the low gain antennas and applied to both receiver inputs via the diplexer. The signal consists of a 2 GHz carrier, phase-modulated by an 8 kHz or 16 kHz subcarrier, itself BPSK-modulated by the TC data at a rate of less than 10 kbit/s. The two receivers work in 'hot redundancy' and output the modulated subcarrier at baseband to the active decoder. The decoder recovers the TC data and sends it to the OBDH.
The active transmitter generates a downlink carrier phase and frequency coherent with the uplink carrier, which allows measurement of Doppler by the ground station, aiding satellite localisation. The uplink signal also contains the ranging signal which is demodulated by the receiver and transmitted back to the ground with the telemetry (TM), using phase modulation on a single downlink carrier. The transmitter amplifies the modulated downlink signal (in the 5 W RF power range according to mission requirements) which is connected to the selected antenna via the diplexer and RF switches.
The TTC system must be operational during all mission phases even if attitude control is lost, thus the antenna system coverage must be as near as possible to omnidirectional. The hemispherical coverage antennas have low gain (LGA). If the mission requires higher data rates (for the same transmit RF power), a directional High Gain Antenna (HGA) is necessary. In this case, the LGAs are used only during LEOP and in an emergency (e.g. loss of attitude control) for TTC housekeeping data, and the HGA is used during the operational phase for high-rate transmission of payload data as well as for normal TTC communication.
The transmit and receive signals use the same antenna but they are isolated from one another by the diplexer. The receive signal is applied to the two 'hot redundant' receivers. 'Hot redundancy' minimises interruption of control in emergencies and increases reliability by avoiding power on/off cycles of the receivers. The transmitters are switchable; the selection of the transmit signal is made via the RF Distribution Unit (RFDU). This switching capability together with the redundant architecture allows a 'crossed' scheme where the transmitter of one transponder can be used with the receiver of the other. This scheme has already been implemented on several ESA satellites.
The internal architecture of a TTC transponder is shown in Figure 2.
Figure 2. Block diagram of a typical TTC transponder. It comprises:
- a receive chain which tracks the uplink telecommand signal received from ground, demodulates the command data (TC) and forwards this to the onboard data handling unit (OBDH). It also demodulates the ranging signal (RNG) which is then fed to the input of the transmitter for retransmission to ground
- a transmit chain which modulates the telemetry data (TM) and ranging signal onto the downlink car
The question of frequency bands is of fundamental importance to telecommunications systems. The use of the frequency spectrum for RF transmission by all telecommunication systems is highly regulated by the International Telecommunication Union (ITU) in Geneva and is subject to formal registration and approval by the ITU authorities. The telecommunication agencies of 145 signatory countries meet at regular intervals to discuss and decide the allocation of the frequency spectrum to the different agencies. The space agencies are usually in a minority and must dedicate much effort to defend their interests.
Until the 1970s, most satellites' TTC was performed through VHF links (130 MHz bands). This was the case for ESA's OTS satellite for instance. Since the early 1980s, all European satellites, with the exception of some commercial communication satellites and some minisatellites, use the S-band (2 GHz) for their TTC (see inset). Deep-space probes such as Ulysses and Giotto may also transmit their telemetry in the X-band (8 GHz), to be able to transmit data at a high rate and to help in the evaluation of errors on ranging measurements through the ionosphere.
With the growing number of satellites, the frequency band allocated for space applications in the S-band is becoming increasingly crowded. Therefore, in the future, near-Earth missions could use the X-band, with further investment in the ground segment being required. In addition, the increasing needs of deep-space missions in terms of the amount of data to be transmitted is leading towards the general use of the X-band for such missions. Extending to the Ka-band (30 GHz) is also under consideration for some missions.
In order to protect the scarce resources of allocated frequency spectra, space agencies have had to cooperate with each other and coordinate their efforts in order to present a unified position at the ITU. This is done, for instance, through committees such as the Space Frequencies Coordination Group (SFCG), in which ESA is an active member. This need for cooperation plus the high cost of developing and maintaining a ground network has led the major space agencies to achieve a high degree of compatibility for TTC matters.
This compatibility has been developed, for a large part, in the frame of the Consultative Committee for Space Data Systems (CCSDS), which recommends TTC system designs. Thanks to this work, a satellite from a given agency can be supported by most stations from most other agencies.
This compatibility proves to be very practical. For example:
TTC is therefore a truly worldwide standard.
The type of mission being undertaken has a significant impact on the constraints and requirements imposed on the design of the TTC transponder. Communicating with a spacecraft in geostationary orbit is different from communicating with an interplanetary probe.
In practice, with respect to TTC transponders, there are three different types of space applications:
Generally, the absolute power is not a concern in these missions, but some of them have a highly eccentric orbit which, for the transponder, means that the signals received from the ground station when the satellite is at apogee have a low level while the signal that the transponder receives when the satellite is at perigee is proportionately considerably higher. The transponder (Fig. 3) therefore has to be able to cope with a dynamic range in the received signal that can be as high as 70 dB.
In addition, the Doppler effects (frequency shift and frequency rate of change) when the satellite approaches perigee can be extremely high, which constitutes a further constraint for the receiver which must track these frequency dynamics.
The problem for missions using DRS is that the distance between the data relay satellite and the LEO satellite is similar to the distance between the data relay satellite and the Earth (Fig. 4b). Data relay satellites are not free to radiate a large amount of power at the Earth's surface because it might cause interference to terrestrial users. (This power limitation is clearly defined by the ITU in terms of maximum power spectral density per unit surface). However, the power received by the transponder on the LEO spacecraft has to be high enough for reliable communication.
The solution is the use of a technique called spread spectrum, whereby the signal transmitted by the data relay satellite is spread over a large bandwidth so as to reduce the radiated power per unit bandwidth. It appears as noise to any other user and hence does not generate interference. However, a suitable TTC transponder with a spread spectrum demodulator can interpret the signal correctly. This feature implies an additional complexity for the design of transponders for this type of application.
Figure 4. The concept behind data relay satellites
a. Traditionally, communications are made directly between the satellite and the Earth. As a result, contact time is limited to the periods when the spacecraft is visible from the ground station.
The concept of Data Relay Satellites (DRS) which provide the communication link was introduced some years ago. The system comprises a geo-stationary satellite fixed in relation to an observer on the ground, providing a relay between the user in low-Earth orbit and the ground. This concept considerably increases the radio contact time: three relay satellites carefully positioned in GEO provide 100% coverage of possible low-Earth orbits (with the exception of polar regions). The current generation of Data Relay Satellites (DRS) is not regenerative, in that the signals received by the DRS are not demodulated, but only amplified, filtered and frequency-shifted to the required frequency before re-transmission.
b. This type of system implies two types of bidirectional link: the link between the ground station and the DRS, i.e. the feeder link, and the link between the DRS and the user satellite, i.e. the Inter-Orbit Link (IOL). The telecommand signals transmitted from ground towards the DRS on the feeder link are retransmitted by the DRS towards the user satellite. The telemetry signals transmitted by the user satellite work similarly in the opposite direction.
The frequency band used by the TTC system is dictated by the propagation, performance and regulatory requirements.
The majority of TTC systems use the S-band (around 2 GHz), which allows minimal propagation loss through the Earth's atmosphere (less than 1 dB) and data rates up to approximately 1 Mbit/s. This is usually quite adequate for Earth-orbiting missions (up to and including GEO). Deep space missions require higher frequency bands to achieve the increased performance resulting from higher antenna gain for a given size.
Non-scientific satellites sometimes use a different band for their payload. For example, commercial telecommunications satellites (usually in GEO) operate at the C-band (around 6 GHz) or Ku-band (around 14 GHz) for their payload. During the Launch and Early Orbit Phase (LEOP), they may operate their TTC in the S-band. Once they enter the operational phase, they use their payload band for TTC. Meteorological satellites (also usually in GEO) normally use the L-band (around 1.5 GHz) for their payload; their TTC however uses the S-band both in LEOP and during the operational phase. Other smaller users, such as universities, tend to use the S-band for their TTC systems.
The ground- station tracking system has a dual purpose for deep space missions. The first function, as discussed earlier, is the provision and reception of RF carriers used for telemetry and command functions. Secondly, the radio tracking system also performs radiometric functions in which information is obtained spacecraft position, the radio propagation medium, and hence some properties of the solar system. This information is essential spacecraft navigation but also provides important contribution to the scientific return deep space missions.
A coherent link from the ground-station the spacecraft and back to Earth can be used to observe a variety of scientific phenonema:
Figure 5. Locating spacecraft using VLBI In Very Long Baseline Interferometry (VLBI), two widely-spaced antennas on the Earth observe a single broad-band source, for example, a spacecraft transponder transmitting widely spaced tones or a quasar. The received signals are correlated with an accurate time reference, allowing the location of the spacecraft to be calculated. If, further to this, there is a second source angularly close to the spacecraft, for example a quasar with a known location, several sources of error can be reduced, in particular the 'clock' and the media through which the electromagnetic wave travels. This method is known as Differential One-way Ranging (delta-DOR).
The technology used in the transponders is primarily analogue, and the architecture of the receiver phase lock loop is 'long loop', where the Doppler frequency error is compensated progressively at each stage of the down-conversion chain. The downlink carrier is phase coherent with the uplink carrier, allowing two-way Doppler measurement. The ranging signal transmitted from ground is received by the transponder and transmitted back to ground. The traditional long-loop trans- ponder contains many analogue components requiring time-consuming and costly tuning. It is also large and inflexible.
As with all spaceborne equipment, there are demands to reduce mass, size, power consumption and cost. In order to achieve such gains, there has been a steady replacement of old technologies where appropriate and a move from analogue towards digital technology. Progress has been made in miniaturisation (MMIC, ASIC), and digitisation of the demodulation, modulation and frequency generation functions. As well as the gain due to cost and size reduction, digitisation reduces the requirement for tuning, gives better repeatability of performances, and when a microprocessor is included, gives added flexibility.
Some recently launched satellites have begun to incorporate the digital technology. ERS-2, for example, contains two transponders of different generations: one is an analogue transponder of the type developed in the early 1980s, and the other is semi-digital in that it implements digitally some of the demodulation functions.
In response to the requirements of the Columbus and Hermes programmes, which were to use data relay systems, a new type of transponder was designed, one that could communicate with ground in two ways, either directly using traditional phase modulation or via a data relay satellite using spread-spectrum PSK modulation with suppressed carrier. From the breadboard of that so-called dual-mode transponder, two models have been derived and will fly in the near future:
Such spread-spectrum transponders offer extensive programmability in areas such as carrier frequency, choice of PN spreading code, and data rate.
Figure 6. Digital receiver module of the Dual Mode Transponder (DMT) used for the Huygens mission (Photo: Alenia Spazio)
Figure 7. An Experimental S-Band Transponder (ESBT) (Photo: Alcatel Espace)
Transponders of the future
Those spread-spectrum transponders, how-ever, are too large and costly for many small spacecraft users who would like to use data relay systems but are limited in budget and do not need the flexibility of previous spread- spectrum transponders.
To meet those reduced requirements, ESA is developing a Small User Transponder (SUT). The lower mass and recurring cost expected for this transponder align with the expected growth in smaller satellites requiring DRS support.
There is a similar requirement for direct-to-ground applications. The majority of users, who are in fact in GEO or LEO, require smaller and cheaper transponders but with a similar performance. In response, the Compact Standard Transponder (CST) is currently under development. It will use improvements in technology as well as a simplification of the equipment functionality to produce a compact, lightweight and low-cost solution. The idea for the CST came from the comparison of the high cost and complexity of its predecessors (all analogue transponders used in ESA missions since the 1980s) with the commercial transponders used in telecom satellites. This complexity is partly due to the need to operate up to 2 million kilometres from Earth versus the 40 thousand kilometres for commercial transponders.
The design study for the CST will run under ESA's Technological Research Programme (TRP) during 1996, and an engineering model is planned to be developed under the General Support Technology Programme (GSTP-2) beginning in 1997.
For deep space, there would be a clear advantage, either for reasons of telemetry capacity, necessity of antenna size reduction, increased navigation accuracy or for ionospheric delay resolution, for several of the planned future ESA missions to use the X-band or the Ka-band. In particular, Mercury, Rosetta, Intermarsnet or the Moon missions would be able to take advantage of the higher frequency bands. The Ka-band has a unique advantage for missions such as Mercury as it suffers negligible signal-to- noise degradation due to signal scintillation in the solar plasma.
An engineering model of such a Deep Space Transponder (DST) (see inset) is envisaged under the GSTP programme to ensure Europe keeps a deep space transponder capability for future ESA missions.
Essential Features of a Deep Space Transponder (currently under development)
X-band and Ka-band downlink (coherent)
Low noise figure
Low acquisition and tracking thresholds at high Doppler rates
Highly modular digital design