M

11.2 Telemetry, Tracking, and Command

Douglas Kirkpatrick, United States Air Force Academy Adapted from SMADII, Sec. 11.2 "Communications," by John Ford

The telemetry, tracking, and command (TT&C) or communications subsystem provides the interface between the spacecraft and ground systems. Payload mission data and spacecraft housekeeping data pass from the spacecraft through this subsystem to operators and users at the operations center. Operator commands also pass to the spacecraft through this subsystem to control the spacecraft and to operate the payload. We must design the hardware and their functions to pass the data reliably for all the spacecraft's operating modes. For a discussion of how we collect and manipulate housekeeping and payload data, see Sec. 11.3, Chap. 9, and Chap. 16. Chapter 13 discusses the communication link design, and Morgan and Gordon [1989] provide a wealth of information on spacecraft communications.

The subsystem functions include the following:

• Carrier tracking (lock onto the ground station signal)

• Command reception and detection (receive the uplink signal and process it)

• Telemetry modulation and transmission (accept data from spacecraft systems, process them, and transmit them)

• Ranging (receive, process, and transmit ranging signals to determine the satellite's position)

• Subsystem operations (process subsystem data, maintain its own health and status, point the antennas, detect and recover faults.)

Table 11-18 presents specific subfunctions to accomplish these main functions. Subsystem designers must ensure that all of these functions operate reliability to accomplish the spacecraft mission.

As part of carrier tracking, most satellite TT&C subsystems generate a downlink RF signal that is phase coherent to the uplink signal. Phase coherence means that we transmit the downlink carrier so its phase synchronizes with the received phase of the uplink carrier. This process is the coherent turnaround or two-way-coherent mode. Tbe coherent turnaround process creates a downlink carrier frequency precisely offset from the uplink carrier by a predefined numerical turnaround ratio. This is the ratio of the downlink carrier frequency to the uplink carrier frequency .This operational mode can only exist when the transmitter phase-locks to the received uplink carrier. For a given uplink signal, the downlink signal has a constant phase difference. For NASA's GSTDN-compatible transponders, the receiver downcoverts the uplink carrier, and creates a voltage such that the receiver's voltage-controlled oscillator runs at precisely 2/221 times the uplink carrier frequency. The oscillator frequency goes to die transmitter which multiplies it by a factor of 120. Therefore, the composite transmitter downlink is 120 x 2/221 = 240/221 times the uplink frequency, which is the turnaround ratio for NASA-compatible transponders. The turnaround ratio for transponders compatible with SGLS is 256/205. The two-way-coherent mode allows the ground station to know more exactly the downlink signal's frequency and to measure the Doppler shift, from which it computes the range rate or line-of-sight velocity between the spacecraft and the tracking antenna. This knowledge allows operators to

TABLE 11-18. What a TT&C Subsystem Does. These are the functions of a communication subsystem connecting the satellite to the ground or other satellites. In a broai sense the communications subsystem receives signals from Earth or anothe satellite and transmits signals to Earth or another satellite.

Specific Functions

• Carrier Tracking

- 2-way coherent communication (downlink frequency is a ratio of the uplink frequency)

- 2-way noncoherent communication

- 1-way communication

• Command Reception and Detection

- Acquire and track uplink carder

- Demodulate carrier and subcarrier

- Derive bit timing and detect data bits

- Resoive data-phase ambiguity if it exists

- Forward command data, clock, and in-lock indicator to the subsystem for command and data handling

• Telemetry Modulation and Transmission

- Receive telemetry data streams from the command and data handling subsystem or data storage subsystem

- Modulate downlink subcarrier and carrier with mission or science telemetry

- Transmit composite signal to the ground station or relay satellite

- Detect and retransmit ranging pseudorandom code or ranging tone signals '

- Retransmit either phase coherently or noncoherently

• Subsystem Operations

- Receive commands from the subsystem for command and data handling

- Provide health and status telemetry to the C&DH subsystem

- Perform antenna pointing for any antenna requiring beam steering

- Perform mission sequence operations per stored software sequence

- Autonomously select omni-antenna when spacecraft attitude is lost

- Autonomously detect faults and recover communications using stored software sequence scan fewer frequencies and thus, acquire the spacecraft more quickly. Deep-space imaging, data collection, and low-Earth orbit spacecraft best illustrate this advantage. These spacecraft typically have large volumes of data and a short field-of-view time to the ground station. To receive maximum data at the ground station on a direct downlink at the spacecraft's highest rate, operators must acquire the downlink signal in the minimum time. Also, if they use ranging for navigation, they can calculate range-rate information from the Doppler shift of the coherent signal.

Occasionally a TT&C subsystem, operating in the two-way coherent mode, loses lock on the uplink signal. At this point, the spacecraft's transmitter autonomously changes the references for the downlink carrier from the receiver's voltage-controlled oscillator to the subsystem's master oscillator. This process creates a unique downlink frequency which is no longer synchronous with the uplink carrier. This TT&C mode is two-way noncoherent communications.

For the ranging function (i.e., determining the range or line-of-sight distance), the ground station may use the ranging method of navigation to track a spacecraft

Depending on the communication standard, the ground station modulates a pseudo-random code, tones, or both onto the command uplink signal. The TT&C subsystem's receiver detects the code or tones and retransmits them on the telemetry carrier back to the ground station. From the turnaround time of the ranging code or tones traveling to and from the spacecraft, the system determines the range. If the downlink carrier's phase is coherent with the uplink carrier (two-way coherent mode), we can measure the Doppler-frequency shift on the downlink carrier signal and thus obtain range-rate information. Pointing information from the ground station's directional antenna allows us to determine the satellite's azimuth and elevation angles.

Under subsystem operations, the TT&C subsystem performs antenna pointing for any antenna that requires beam steering. Closed-loop antenna pointing requires special autotracking equipment This equipment generates error signals for the guidance, navigation, and control subsystem, so it can point the antenna. Monopulse and conical-scan systems are the most common ways of generating pointing error signals. Monopulse systems use a monopulse feed that generates difference patterns with nulls on the axis of the azimuth and elevation planes. Conical-scan systems rotate the received beam about its axis by a small angle. The rise and fall of the received signal amplitude per revolution indicates the pointing error. By correlating the feed position with the position where the signal is at maximum amplitude, the system generates error signals for the control subsystem to point the antenna. We can use open-loop antenna pointing when we know die spacecraft antenna's position and the direction to the receiver station.

Also under subsystem operations, the TT&C subsystem may do sequences of mission commands or respond to autonomous commands, such as putting itself in a safe mode and routing the omni-antenna to the active receiver. For certain failure scenarios, the subsystem may also execute fault-detection and recovery operations through a stored software sequence.

To create a robust TT&C subsystem, we must consider and satisfy three parts of satellite design: requirements, constraints, and regulations. The requirements come from a variety of sources and form the basis for the mission in which this subsystem plays a key role. Typically TT&C requirements include:

• Type of signals (voice, television, and data)

• Capacity (number of channels and bandwidth)

• Coverage area & ground site locations Qocal, regional, national, international)

• Link signal strength (usually derived from ground terminal type)

• Connectivity (crosslinks, relay ground stations, and direct links)

• Availability (link times per day and days per year, outage times)

• Lifetime (mission duration)

See Sec. 11.2.1 for a more thorough discussion of requirements.

Constraints are limits on the TT&C subsystem from various sources. Power constraints come from sizing the spacecraft and the power source (primary batteries, solar panels and secondary batteries, or radioisotope thermoelectric generator). Mass constraints arise from the mass budget, which comes from the mission design and the chosen launch vehicle. The launch vehicle generally limits die total dimensions and mass, so individual subsystems receive their allocation within those limits. The launch vehicle choice also sets the launch vibration and acoustic environment, which places constraints on the fragility of the subsystem. The interference environment further constrains the subsystem. When we choose the orbit, we also set the surrounding interference environment The owners and developers place cost limits on the total design, which in turn limits each subsystem. These cost constraints typically determine how much new technology and subsystem margin that designers can consider. Many other constraints may arise during design, depending on the mission and the people involved.

For the TT&C subsystem, international law and regulatory agencies impact design significantly. Because all spacecraft communicate with users and operators on the ground, de-conflicting frequencies, orbital locations, and power levels are critical to civilized sharing of limited resources. So, we must apply to the regulatory bodies for.

• Desired communication frequencies (depending on the mission data rate, transmission power available, and altitude)

• Orbital assignment (further than 2 deg from a satellite with the same frequency, if geosynchronous)

• Desired power flux density on surface (depending on our receiver antennas)

The main regulatory agency enforcing standards is the International Telecommunications Union (ITU), which is now part of the United Nations. Within the ITU three bodies regulate the communication allocations: the Consultative Committee on International Telephony, the Consultative Committee on International Radiocommu-nications, and the International Frequency Registration Board (IFRB). The first two organizations formulate policy and set standards. The IFRB coordinates and approves frequency and orbit requests. Because these agencies are international and the number of requests is large and growing, we must plan years in advance to get approval for our communications request.

Various other bodies exist to assist organizations in coordinating and rationalizing commercial use of the radio frequency spectrum. Three of these are the International Telecommunications Satellite Consortium (INTELSAT), the European Telecommunications Satellite Consortium (EUTELSAT), and the International Maritime Satellite Organization (INMARSAT), which assist their member nations with telecommunications planning.

11.2.1 Requirements

The TT&C subsystem derives its requirements from many sources, such as (1) the mission or science objective (top-level requirements such as architecture, orbit lifetime and environment); (2) die satellite (system-level); (3) the TT&C subsystem (internal requirements); (4) other satellite subsystems; (5) the ground station and any relay satellite (compatibility); and (6) mission operations (the satellite operational modes as a function of time). From these sources come the requirements that drive the subsystem design: (1) data rates (commands and telemetry for health and status or for mission and science needs); (2) data volume; (3) data storage type; (4) uplink and downlink frequencies; (5) bandwidths; (6) receive and transmit power; (7) hardware mass; (8) beamwidth; (9) Effective Isotropic Radiated Power (EIRP)\ and (10) antenna gain/system noise temperature. Table 11-19 shows the effects of these requirements on the TT&C-subsystem design.

TABLE 11-19. TT&C Subsystem Requirements. These are typical system-level requirements that are Imposed on the TT&C subsystem. See also Table 11-23. (Courtesy of TRW)

Requirement

Alternative/Considerations

Comments

Data Rates Command

Health & status telemetry

Mission/ science

4,000 bps typical, 8-64 bps deep spc 8,000 bps Is common

Low <32 bps Medium = 32 bps-1 Mbps High > 1Mbps-1 Gbps

Range 2,000-8,000 bps 40-10,000 bps

Mission dependent

Data Volume

Record data, compress data, and transmit during longer windows

Data rate x transmission duration

• Shorter duration increases data rate

• May require data compression

Data Storage

Solid-state recorders 128 x 10s bits

Policy may dictate all data be stored that is not immediately transmitted Mission may require that data be stored then played back later

Frequency

Use existing assigned frequencies and channels

Use systems that are compatible to the existing system

Policy set by FCC, rTU, National Telecommunications & Info. Admin. Refer to the atmospheric frequency absorption charts

Bandwidths

Use C.E. Shannon's theorem to calculate channel capacity; See Chap. 13, Eq. (13-26).

Primary driver is data rate

Secondary driver Is modulation scheme

Power

Use larger antennas, higher efficiency amplifiers

Reconsider data requirements

S/C power may limit size of TT&C system transmitter

Mass

Use TWTAs for higher RF power output to reduce antenna size, reconsider data requirements

S/C TT&C system mass allocation may limit size of antennas

Beamwtdth

See Tables 13-14,13-15, and 13-16 for various antenna types, beam shapes, and beamwidths

Ground coverage area requirements or the radiation footprint on the ground Antenna gain null requirements Antenna pointing error

0 0

Post a comment