Communications Subsystem Specification Sheet

Greg Merritt
gmerritt@mail.utexas.edu
August 1995

Functions of the Communication Subsystem

Carrier tracking
- 2-way coherent communication
- 2-way non-coherent communication
- 1-way communication
Command reception and detection
- Acquire and track uplink carrier
- Demodulate carrier and subcarrier
- Derive bit timing and detect data bits
- Resolve 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 Earth
Ranging
- Detect and retransmit ranging and pseudorandom code or ranging tone signals
- Retransmit phase
Subsystem operations
- Receive commands from the subsystem for command and data handling
- Provide health and status telemetry to Computer subsystem
- Perform antenna pointing
- Perform mission sequence operations
- Select omni antenna when spacecraft is lost
- Detect faults and recover communications using stored sequences

Subsystem Requirements

Requirement	Alternative/Considerations	Comments
- Data Rates - Command - Health and Status telemetry - Mission/Science	1000 bps typical 2400 bps common Low = 10-1000 bps Med = 1000 - 100 kbps High = 100 kbps to Gbps	Mission dependent
Data Volume	Record data and transmit during longer windows	Data rate vs. transmission duration - Shorter duration increases data rate - May require compression
Data storage	Tape recorders 75 * 109 bits Solid state recorders 128 * 106 bits Bubble memory 128 * 106 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 (FCC dependent)	This is set by the FCC and other national/international organizations.
Bandwidths	Max theoretical data rate which can be transmitted over a transmission channel with bandwidth B Rmax = Blog2(1 + C/N)	C/N is the average Carrier to Noise ratio for the channel
Power	Use larger antennas/higher efficiency amplifiers	S/C power may limit size of comm system transmitter
Mass	Use TWTAs (traveling wave tube amplifiers) for higher rf power output to reduce antenna size	S/C comm system mass allocation may limit this
Beamwidth	Depends on antenna selection	Ground coverage area requirements or ground footprint Antenna pointing error
EIRP (Effective Isotropic Radiated Power)	For constant EIRP, as antenna size is increased, the transmitter power requirement decreases	EIRP (dB) = transmitter power + antenna gain - front end losses Min EIRP required = space loss + atmospheric loss + antenna pointing loss - receiver antenna gain - receiver sensitivity

Communication Subsystems Interface Sheet

Subsystem	How comm affects it	How it affects comm
NGC	- Antenna pointing requirements for gimabaled antennas - Pointing requirements of the lesser of 1/10 of antenna beamwidth or .3 deg (established by World Admin. Radio Conference) - May require auto-tracking for cross-links	- S/C pointing and attitude knowledge for fixed antennas may impact antenna beamwidth requirements - Uncertainty for attitude and pointing knowledge lead to pointing loss in the link budget - Bar-B-Q mode and pointing during transfer
Command and Data Handling (Computers)	- On-board storage and processing may be necessary sometimes - Clock, bit sync, and timing requirements - 2way comm requirements (must be able to send and receive simultaneously)	- Command and telemetry data rates requirements - Fault detection and recovery (should recognize failure and make necessary corrections) - Bypass computer system
Power	Distribution requirements on power (Centralized or Decentralized power conversion; TWTA's require specialized voltage levels, so centralized power conversion is not very common with these)	Amount and quality of power may affect operation
Structure and Thermal	- Need heat sinks for TWTA - Heat dissipation for all active boxes - Location of comm subsystem should be as close as possible to antennas - Clear field of view for antenna and easy movement for gimbaled antennas	- Temperature uncertainty on non-oven-controlled frequency sources result in some uncertainty
Payload	- Specific requirements for storing data	- Max data rates for mission or science telemetry - Max data volume
Propulsion	None	None

Parabolic Antennas

Gain is the energy radiated or received by a particular antenna compared to that of a reference antenna (isotropic radiator with a gain of 1). Most antenna work to concentrate the energy radiated into a smaller region, called the beamwidth, which increases the amplitude of the signal in the bandwidth. Energy is radiated from parabola to focal point (antenna). The reflector is not turned to any particular frequency and can be used at any frequency.

In general,

Gain G = p^2 Eap D^2 / l^2, where

G = gain in W/W
Eap = aperture efficiency, (typical .5-.7)
D = antenna diameter
l = wavelength in m

So if the aperture efficiency is 0.6, then

G = 66*10^-18 f^2 D^2, where f = frequency in Hz

As it turns out, the equation for bandwidth is just

BW = 21*10^9 / f D, where

BW is in degrees
f = hertz frequency
D = meters

According to Wertz, the frequency ranges for the DSN are 2.025 - 2.120 GHz and 7.145 - 7.190 GHz, and 2.2 - 2.3 GHz and 8.4 - 8.5 GHz for the downlink. For a bandwidth of 1.9 degrees from the Earth to the Moon, we'd expect an antenna diameter in the 1.3 m range.

Approximations

If we are to use the DSN, we will expect our uplink frequency and our downlink frequency to be in specific ranges. (See chart) Although these frequencies are assigned by the FCC, we will pick approximate values for the sake of discussion. For the uplink frequency we choose 7.16 GHz, a typical value. We expect the uplink and downlink frequencies to be in a ratio defined as DL/UL = 880/749, so our downlink frequency will be 8.4 GHz. Now, since we have an equation for the beamwidth above, and we know that the value for this beamwidth must be 1.9 degrees, we can find a value for the diameter of the antenna. This value turns out to be 1.55 m, to accommodate both the uplink and downlink frequencies. Then we can calculate the gain of the antenna, knowing its diameter (using above equations).

G = 8106 W/W = 39.1 dB
(dB = 10*log(W/W))

Typical numbers

According to page 379 of Wertz, communications in the X-band region (which is what our numbers above are in) also demonstrate the following:

Component	Qty	Mass each (kg)	Mass total (kg)	Power (W)	Dimensions (cm)
Transponder-Receiver-Xmitter	2	4.75	9.0	10.4,35.0	14x30x9
Filters/switch/diplexers/etc.	1	1.5	1.5	0.0	10x22x4
Parabolic Antenna	1	9.2	9.2	0.0	150dia x 70
Totals			22.1	45.4

Data Rates

Typical data rates may depend on the application. Information for a 1969 Lunar Logistics Vehicle put the uplink rate at 1000 bps and the downlink at 5000 bps. Listed below are the typical values for NASA DSN Network Data Rates

Uplink Frequency Range	2.025 - 2.120 GHz and 7.145 - 7.190 GHz	1.0 - 2000 bps
Downlink Frequency Range	2.2 - 2.3 GHz and 8.4 - 8.5 GHz	8.0 bps - 6.6 Mbps

The spacecraft's communication system and the ground-based receiver system must each be able to handle the required data rate.

Other options and questions to consider:

Should a micro-orbiter be included in the mission to serve as a communications relay, particularly if we decide to land on the far side? If not, communications must be line of sight at all times.

There are a variety of antenna types to consider, including:
Parabolic Reflector Center-Feed, Parabolic Reflector Cassegrain, Parabolic Reflector Offset-Feed, Parabolic Reflector Off-set shaped, Subreflector with Feed array for Scanning.

Frequency options might also be considered. While the S and X bands comprise the Deep Space Network frequencies and are a good option, they are not the only possibilities. Another option might be the experimental Ka/MM wave technology. It is not yet very advanced but has some benefits that could be useful. These waves can support higher data rates (5-10 x) over the S/X band. Also, optical frequency communication systems might be considered. Their benefits include extremely high data rates, reduced power and equipment size, increased gain, and relative immunity to jamming. Use of this kind of system would result in considerable savings in volume, mass, and power over conventional systems. Also, it can be used for navigational purposes. Sources to investigate:

Chen, CC and Scwartz, JA. "Space Station-Based Deep-Space Optical Communication Experiments," Proceedings of SPIE, vol. 885, 11-12, Jan 1988.
Lesh, JR, "Deep Space Optical Communications Development Program", Proceedings of SPIE, vol 756, 15-16 Jan 1987.
White, RE, "Manned Mars Mission Communication and Data Management Systems," Manned Mars Missions Working Group Papers, vol 2, 10-14 Jun 1985.

Below is a table summarizing what was found by the Lunar Polar Coring Lander Team in May of 1990.

	S/X	Ka/MM	Optical
Bandwidth	Few Mbps	Increased	Much increased
Antenna gain	Reference	Increase of 12 dB over X band	Increase of 60-80 dB over Ka
Immunity to Interception and Jamming	Poor	Better	Excellent
Signal Acquisition	Easy	Satisfactory	Difficult
Pointing Pointing Accuracy	Few arc min Arc	Arc Sec Sec	Arc Sec to sub Arc Sec to sub
Lifetime	Long	Long	Short Laser life
Compatibility with existing systems (1987?)	Yes	No	No
Technology Status	Mature	Immature (Development planned)	Immature (Some risk)

For signal reception at Earth, use of the TDRSS system should be considered also. The TDRSS satellites carry omnidirectional antennae and six steerable antenna arrays for K-band, S-band, and C-band communications. Since the antennae are directed toward Earth and the maximum a steerable one can be moved is +/- 30 degrees in the North-South direction, a combination of the steerable and omnidirectional antenna are necessary. Also, a satellite receiving antenna on Earth is necessary to receive the relayed signal.

Communications Subsystems References Sheet

Chetty, P.R.K., "Satellite Technology and its Applications", 2nd edition, TAB Professional and Reference Books (Blue Ridge Summit, PA:1991).

Douglas, Robert L. "Satellite Communications Technology". Prentice Hall, Englewood Cliffs, NJ, 1988.

"Lunar Lander Conceptual Design", Proposal to NASA, Eagle Engineering Inc., Houston, 1988.

"Lunar Polar Coring Lander", Oasis Lunar Systems. Report to Dr. Fowler, UT. May 1990.

Pritchard, Wilbur L. and Sciulli, Joseph A., "Satellite Communication Systems Engineering", Prentice Hall, Englewood Cliffs, NJ, 1986.

"Spacecraft Subsystems". Department of Aerospace Engineering and Engineering Mechanincs, University of Texas at Austin, 1992.

Wertz, Wiley J. and James R. Wertz, "Space Mission Analysis and Design", Microcosm, Inc., Torrance, CA, 1992.

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