This chapter gives a broad view of some telecommunications issues, including both spacecraft and Earth-based communication. This view is of abbreviated depth, and the subject is covered further by a separate Space Flight Operations Multi-team Training Module for employees at JPL, "End-to-end Information System" (refer to the illustration in the Introduction). Details of onboard spacecraft equipment for telecommunications are covered under Telecommunications Subsystems in Chapter 11.
Your local entertainment radio broadcast station may have a radiating power of 50 kW, and the transmitter is probably no more than 100 km away. Your portable receiver probably has a simple antenna inside its case. Spacecraft have nowhere near that amount of power available for transmitting, yet they must bridge distances measured in tens of billions of kilometers. A spacecraft might have a transmitter with no more than 20 watts of radiating power. How can that be enough? One part of the solution is to employ microwave frequencies, and concentrate all available power into a narrow beam, and then to send it in one direction instead of broadcasting in all directions. This is typically done using a parabolic dish antenna on the order of 1 or 5 meters in diameter. Even when these concentrated signals reach Earth, they have vanishingly small power. The rest of the solution is provided by the DSN's large aperture reflectors, cryogenically-cooled low-noise amplifiers and sophisticated receivers, as well as data coding and error-correction schemes.
The radio signal transmitted to a spacecraft is known as uplink. The transmission from spacecraft to Earth is downlink. Uplink or downlink may consist of a pure RF tone, called a carrier, or carriers may be modulated to carry information in each direction. Commands transmitted to a spacecraft are sometimes referred to as an upload. Communications with a spacecraft involving only a downlink are called one-way. When an uplink is being received by the spacecraft at the same time a downlink is being received at Earth, the communications mode is called "two-way."
Consider the carrier as a pure tone of, say, 3 GHz, for example. If you were to quickly turn this tone off and on at the rate of a thousand times a second, we could say it is being modulated with a frequency of 1 kHz. Spacecraft carrier signals are modulated, not by turning off and on, but by shifting each waveform's phase slightly at a given rate. One scheme is to modulate the carrier with a frequency, for example, near 1 MHz. This 1 MHz modulation is called a subcarrier. The subcarrier is in turn modulated to carry individual phase shifts which are designated to represent groups of binary 1s and 0s--the spacecraft's telemetry data. The amount of phase shift used in modulating data onto the subcarrier is referred to as the modulation index, and is measured in degrees. The same kind of scheme is also used on the uplink.
Demodulation is the process of detecting the subcarrier and processing it separately from the carrier, detecting the individual binary phase shifts, and decoding them into digital data for further processing. The same processes of modulation and demodulation are used commonly with Earth-based computer systems and fax machines transmitting data back and forth over a telephone line. The device used for this is called a modem, short for modulator / demodulator. Modems use a familiar audio frequency carrier which the telephone system can readily handle.
Binary digital data modulated onto the uplink is called command data. It is received by the spacecraft and either acted upon immediately or stored for future use or execution. Data modulated onto the downlink is called telemetry, and includes science data from the spacecraft's instruments and spacecraft health data from sensors within the various onboard subsystems.
Not every instrument and sensor aboard a spacecraft can transmit its data at the same time, so the data are multiplexed. In the time-division multiplexing (TDM) scheme, the spacecraft's computer samples one measurement at a time and transmits it. On Earth, the samples are demultiplexed, that is, assigned back to the measurements which they represent. In order to maintain synchronization between multiplexing and demultiplexing (also called mux and demux) the spacecraft introduces a known binary number many digits long, called the pseudo-noise (PN) code at the beginning of every round of sampling (telemetry frame), which can be searched for by the ground data system. Once recognized, it is used as a starting point, and the measurements can be demuxed since the order of muxing is known.
Newer spacecraft use packetizing rather than TDM. In the packetizing scheme, a burst or packet of data is transmitted from one instrument or sensor, followed by a packet from another, and so on, in non-specific order. Each burst carries an identification of the measurement it represents for the ground data system to recognize it and handle it properly. These schemes generally adhere to the International Standards Organization (ISO)'s Open Systems Interconnection (OSI) protocol suite, which recommends how computers of various makes and models can inter-communicate. The ISO OSI is distance independent, and holds for spacecraft light-hours away as well as between workstations.
Aside from the information modulated on the downlink as telemetry, the carrier itself is used for tracking the spacecraft, and for carrying out some types of science experiments. For each of these uses, an extremely stable downlink frequency is required, so that Doppler shifts on the order of fractions of a Hertz may be detected out of many GHz over periods of hours. But it would be impossible for any spacecraft to carry the massive equipment on board required to generate and maintain such stability. The solution is to have the spacecraft generate a downlink which is phase-coherent to the uplink it receives.
Down in the basement of each DSN Signal Processing Center, there looms a hydrogen-maser based frequency standard in an environmentally controlled room. This is used as a reference for generating an extremely stable uplink frequency for the spacecraft to use in generating its coherent downlink.
The resulting spacecraft downlink, based on and coherent with an uplink, has the same extraordinarily high frequency stability as does the massive hydrogen maser-based system in its controlled environment in the DSN basements. It can thus be used for precisely tracking the spacecraft, and for carrying out science experiments. The spacecraft also carries a low-mass oscillator to use as a reference in generating its downlink for periods when an uplink is not available, but it is not highly stable, and its output frequency is affected by temperature variations on the spacecraft. Some spacecraft carry an Ultra-Stable Oscillator (USO), discussed further in Chapter 16. Because of the stringent frequency requirements for spacecraft operations, JPL stays at the forefront of frequency and timing standards technology.
Most spacecraft may also invoke a non-coherent mode which does not use the uplink frequency as a downlink reference. Instead, the spacecraft uses its onboard oscillator as a reference for generating its downlink frequency. This mode is known as Two-Way Non-Coherent (TWNC, pronounced "twink"). When TWNC is on, the downlink is non-coherent.
Recall that "two-way" means there is an uplink and there is a downlink, and doesn't indicate whether the spacecraft's downlink is coherent to that station's uplink or not. However, in common usage, operations people commonly say "two-way" to mean "coherent," which is generally the case. Correctly stated, a spacecraft's downlink is coherent when it is two-way with TWNC off. When a spacecraft is receiving an uplink from one station and its coherent downlink is being received by another station, the downlink is said to be "three-way" coherent.