1. From you host … Dr. H

2. Introduction Communications design requires us to think about the following issues: Communications design requires us to think about the following issues: Over what time durations and over what distances does the spacecraft need to communicate with Earth? Over what time durations and over what distances does the spacecraft need to communicate with Earth? What does the spacecraft need to communicate & how often? What does the spacecraft need to communicate & how often? How much command, control and decision making is local and how much is centered on Earth? How much command, control and decision making is local and how much is centered on Earth?

3. Steps in Defining a Communications Architecture 1. Identify communication links A. Define mission objectives B. Define mission requirements C. Determine the architecture 2. Determine data rates for each link A. Specify accuracy required B. Determine sampling rates, quantization levels. 3. Design each link A. Select frequency band B. Select modulation, coding C. Determine antenna size, beamwidth constraints D. Determine transmitter power constraints E. Estimate received noise, interference powers F. Calculate antenna gains & transmitter power 4. Size the comm system A. Select antenna configuration B. Calculate antenna size C. Estimate antenna masses D. Estimate transmitter masses

4. Link function Data Dissemination Architecture Point-to-pointBroadcast Tracking Telemetry & Command Data Collection Data Relay Step 1: Comm. Architecture Defined by Function SpacecraftRelay Spacecraft

5. Step 2: Determine Data Rates for Each Link What information must be communicated and how fast? Analog-to-digital conversion: Sample frequency >2.2 x (max input frequency) Divide the range of the analog signal into M =2n levels, where n = no. of bits per sample Mean-square noise power = (V)2/12, where V = Vfull-scale/M Signal-to-quantization noise power ratio = (M2-1). So you need smaller steps for weaker signals.

6. Step 2: Determine Data Rates No. of bits per sample determined by mission requirements No. of bits per sample determined by mission requirements Data rate = (No. samples/sec.) X (No. of bits/sample) Abbreviated: bps Data rate = (No. samples/sec.) X (No. of bits/sample) Abbreviated: bps Analog info. Max input freq (Hz) Sampling freq. (Hz) No. bits per sample Data rate (bps) Voice, Pulse Code Modulation (PCM) 36008000764K Voice (delta PCM) 36008000656K DS1 Multiplexer 24 voice channels ---1,544M Color Television (Commercial quality) 4.0M8.8M544M Color Television (Broadcast quality) 4.2M9.25M1092.5M

7. Step 2: Data Rates for TT&C Monitoring: Monitoring: Several hundred functions might be sampled, but at a low rate Several hundred functions might be sampled, but at a low rate Typical data rate ~ 50bps Typical data rate ~ 50bps Transmitting commands: Transmitting commands: Usually ~ 1/sec. Usually ~ 1/sec. Command message is typically ~ 48 to 64 bits Command message is typically ~ 48 to 64 bits Tracking: Tracking: Ground station measures range or range rate for computing orbit ephemeris Ground station measures range or range rate for computing orbit ephemeris For typical parameters of existing TT&C systems see table on next slide For typical parameters of existing TT&C systems see table on next slide

8. Step 2: Existing TT&C Systems Network Command (Uplink) Telemetry (Downlink) Range Measurement Frequency(GHz) Data Rate (bps)Frequency(GHz) (bps) Air Force SCN (SGLS) 1.76 – 1.84 1000, 2000 2.2 – 2.3 125 -1.024M 1 Mbps PN code NASA DSN (Near Earth) 2.025 - 2.120 7.8 - 2000 2.2 – 2.3 5.6 – 500k PN code at 1 Mbps Plus 8 Ranging Tones 8Hz to 500Hz Intelsat/COMSAT 5.92 – 6.42 14.0 – 14.5 100 – 250 100 - 250 3.9 – 4.2 12.2 or 17.7 1000 – 4800 1000 - 4800 4Ranging Tones TDRSS (user satellite below 12,000 km, Single access only) S-band2.025-2.120K-band13.775 300k max 25M max S-band S-band2.2-2.3K-band15.0034 1k to 12M 1k to 300M 3 Mbps PN Code

9. Step 2: Data Rates for Data Collection Example: A geostationary satellite with a radiometer which scans the entire Earth in 20 min. with 1 km resolution Example: A geostationary satellite with a radiometer which scans the entire Earth in 20 min. with 1 km resolution Parameter Geostationary satellite Orbit altitude (km) Ground track velocity (m/s) Ground resolution (m) Scan width (deg) In-track scan (deg) Scan time No. of samples/pixel No. bits/sample Frame efficiency Data Rate (bps) 35,78601,0001818 20 min/Earth image 1.680.95 1.42 X 10 6

10. Step 2: Data Rates for Data Relay Data relay systems typically re-transmit data with a receiver/transmitter combination called a transponder Data relay systems typically re-transmit data with a receiver/transmitter combination called a transponder Transponder bandwidths of commercial communication satellites are usually 36 MHz or 72 MHz. Transponder bandwidths of commercial communication satellites are usually 36 MHz or 72 MHz. Maximum data rate can be several times the bandwidth, depending on the modulation and the receiving station size Maximum data rate can be several times the bandwidth, depending on the modulation and the receiving station size

11. Step 3: Link Design ~Electromagnetic Signal Propagation~ r Isotropic antenna: Point source with total power, P o Total power flowing through any spherical surface of radius r remains = P o Therefore, power per unit area flowing to a receiver distance r away is proportional to 1/r 2

12. Step 3: Link Design ~Electromagnetic Signal Propagation~ But real transmit antennas have directionality. o They radiate preferentially toward the axial direction o As a result, the power per unit area transmitted along the axis is: G t x (power radiated by an isotropic antenna) And this defines the transmitting antenna gain, G t

13. Step 3: Link Design P =transmitter power P L l = transm.- to-anten. line loss x G r A r G r = receive anten. Gain A r = effective receive antenna area Noise x G t L s L a Gt = transm, anten. Gain Ls = space loss =( /4  r) 2 La = transm. path loss r

14. Step 3: Link Design – Link Equation Derivation

15. Step 3: Link Equation Derivation - Continued

16. Step 3: Link Design Equations in dB

17. Step 3: Link Design Equations in dB Typical Noise Temperatures In s/c Comm Links Noise Temperature Frequency (GHz) DownlinkCrosslinkUplink 0.22-1220600.2-2040 Antenna Noise (K) Line Loss (dB) Line Loss Noise (K) Receiver Noise figure (dB) Receiver Noise (K) System Noise (K) 1500.5352.0190375250.5354.04925521000.5354.5592727200.5358.0172817832900.5356.097012952900.5357.515041830 System Noise (dB-K) 25.727.428.632.531.132.6

18. Step 3: Detailed Procedure for Link Design

19. Step 3: Detailed Procedure for Link Design - Continued

20. Step 4: Sizing the Communications System Estimated transmit power + aperture size  System mass TypeFreq. Band (GHz) Gain (dB) Beamwidth (deg) Mass (kg) SatelliteSize (m) Quad helixL(1.5)16-19181.8Intelsat-V0.4 x 0.4 x 0.47 Conical log spiralS(2.2)0 to -32201.2FLTSATCOM Parabola (fixed)S(1.7)16-19183.9GOES I, J, K0.7 dia HornC(4)16-19183.1Intelsat-V0.3 dia, 0.65 L Parabola w/Feed ArrayC(4)21-25-29.4Intelsat-V2.44 dia Parabola w/Feed ArrayC(6)21-25-15.2Intelsat-V1.56 dia Parabola – SteerableKu(11)361.65.8Intelsat-V1.1 dia Parabola w/Feed ArrayKA(20/3045-52-47.1SUPERBIRD1.7 dia