Signaling and signal processing have become a pervasive technology for vehicles launched into space. Both the payload, which is designed for a specific mission, and the supporting subsystems-the bus-that keep it functioning are permeated with signal-processing circuitry.
Communication, remote sensing, meteorological functions and Global Positioning Systems are some of the common applications addressed by satellite payloads, while major bus systems include structure, thermal, propulsion, power, tracking, telemetry and command (TTC) and the attitude and orbit control system (AOCS).
Communication payloads mainly contain RF and microwave circuits. However, in recent times there is a trend toward using on-board processing. Telemetry encoders and digital demodulators-mixed-signal components-form the major signal chains used for TTC, while AOCS functions include largely analog signal chains with inertial sensors, pressure sensors and other types. The signal chains for meteorological payloads are similar to those of remote sensing, but they have low resolution and operate at lower speeds.
The TTC subsystem forms the link between the ground control station and the satellite. It accepts commands for controlling the spacecraft operation and sends satellite health data to the control station. The signal chain for the telemetry encoder is part of this system.
As part of the health monitoring, many parameters of the spacecraft hardware are broadcast to the ground. They include the power-supply voltages and currents, temperature at different locations, pressure of the fuel tanks, the on/off status of circuit blocks and operating parameters of the payloads. This information is transmitted as a PCM telemetry stream through the spacecraft communication system. It is the function of the telemetry encoder to collate this information and generate the PCM stream.
The technology used in current TTC RF transponders is predominantly analog. To ensure an accurate two-way Doppler measurement, based on frequency shifts between sent and received signals, the on-board frequency and phase error between the uplink and downlink carriers should be at a minimum. This requires what is known as "long loop" architecture, whereby the Doppler frequency error is compensated in each stage of the frequency converter. This design requires a costly and time-consuming tuning exercise, and typically suffers from drift during its operation.
To overcome these problems and to achieve higher-ranging accuracy, many components of demodulation, modulation and frequency generation are implemented in the digital domain. One such option is to design a digital demodulator with standard digital radio technology. Much of this technology is still in an experimental stage, with many uncertainties about its use.
Signal bandwidth of the uplink rarely exceeds 50 kHz. Hence, the standard technique of undersampling used in digital radio can be employed for demodulation. The incoming RF signal is converted to a suitable Intermediate Frequency. The IF output is then digitized with adequate analog/digital converter resolution. The A/D converter output feeds a DSP, which recovers the baseband signal through a D/A converter. It can also provide FSK demodulation to generate the digital command word directly. Another output of the DSP feeds back automatic gain control and automatic frequency control through a suitable D/A converter.
The A/D converter works in undersampling mode. Hence, its analog section must have a frequency response beyond the Nyquist frequency. It should also have a minimum signal-to-noise ratio of 65 to 70 dB. That rules out A/D converters with resolutions of less than 12 bits.
The typical telemetry channel operates at a bit rate of 1 to 10 kbits/s-in exceptional cases that rate may be as high as 1 Mbit/s-giving a maximum required A/D conversion speed of one conversion every 800 microseconds for 8-bit resolution. The A/D converter output is multiplexed with other digital telemetry information and routed to the on-board data-handling system for data storage, processing or transmission.
The most important selection criteria for the multiplexer is a low crosstalk specification. The A/D converter dynamic range is normally from -5 V to +5 V to accommodate monitoring of negative signal levels. The conversion speed need not be very high, so successive-approximation A/D converters provide a good cost-performance balance.
With today's military focus on commercial off the shelf (COTS), the choices for high-reliability components are diminished. Trade-offs have been commonplace in the face of this dichotomy: increased performance, yet shrinking budgets and supplier base.