Satellite Communications Engineering T. Scott Parmer Lockheed Martin Space Systems Company

Who am I? BS Materials Engineering, New Mexico Tech General Engineer, Defense Contract Management Agency, Philadelphia, PA Systems Engineer, Lockheed Martin IS&GS, Philadelphia, PA MS Electrical Engineering, Villanova University Communications Systems Engineer, Lockheed Martin SSC, Denver, CO

What is a Communications Systems Engineer? Ensure that spacecraft can communicate with those back on earth There are many variants of this central tenant – more in a bit Fundamentals of RF circuit design, information theory, antenna design, statistics and other fields all play a part Most of these fields are only touched on in undergraduate coarsework

The Decibel Most link analysis is performed using decibels (dB) Allows the wide power ranges and multiplicative nature of the link to be quickly assessed Decibels are a ratio, subscripts give insight into what the divisor is 10 log10(2) ≈ 3 dB i.e. a 3 dB addition is a 2x increase in power A key measure of link performance is Signal to Noise Ratio Unit Relative to dBi Isotropic Antenna dBW 1 Watt dBm 1 milliwat dB-Hz 1/Hertz dBK 1 Kelvin

A key measure of link performance is Signal to Noise Ratio (SNR) We’ll see that noise is brought in to the system or generated internally The signal level decreases significantly when transmitted The ratio of the signal power to the noise power determines whether we can see the signal or not Spectral graphics

Time, Frequency, and Phase Domain Add frequency domain I’ll be using multiple representations of the same signal

Modulation and Waveforms Modulation is the process of taking data and turning it in to symbols fit for transmission RF carrier frequencies are higher than the data rate so the symbols are typically a modification of the sine wave carrier These modifications alter the amplitude, frequency, and/or phase of the signal Add graphic of time domain waveform and phase state diagram

Coding Prior to transmission digital data is coded to add some redundancy which improves link performance This redundancy prevents small errors from impacting the accuracy of the data transmission A simple example of this is a Cyclic Redundancy Code (CRC) Because the coding allows some errors to be accepted it allows the link to operate at lower SNR This is typically a winning proposition in that we can transmit at a lower power level and still get the data through There isn’t any free lunch however, the bandwidth efficiency decreases and some channels are bandwidth limited

Transmission and Antennas Once the signal has been generated from the underlying data it is typically amplified and transmitted Amplification takes the signal and increases the power Solid state (transistor) and traveling wave tubes (TWT) amplifiers are typically used for space applications Antennas are used to launch and direct the energy through free space They take on too many forms to cover here

Once transmitted the signal spreads out as a spherical wave Channel Impairments Once transmitted the signal spreads out as a spherical wave Free space loss is proportional to distance transmitted From geostationary orbit it can be a loss of ~200 dB for microwave COMSAT applications 10-20 power reduction In addition to free space atmospheric gasses and exoatmospheric plasmas can attenuate, delay, and otherwise mess with the signal

The larger the antenna the more signal power is collected in general Receive Antennas Again, receive antennas come in all shapes and sizes depending on the application The larger the antenna the more signal power is collected in general Larger antennas mean higher data rates in general

Noise Antennas receive more than just our intended signal The atmosphere blocks some of the incoming signal and emits power of it’s own- thermal photon emmision at microwave frequencies is prevalent The galactic background is visible at RF frequencies Other signals can jam our intended signal In addition to all of that the electronics that we use to receive the signal also generates thermal noise

Reception and Decoding After all of our efforts the resulting signal is a tiny spec of power above the noise level This small power is amplified up to a meaningful level and then typically digitized for final signal processing The received signal is run through a decoding step that interprets the received symbols (from the error correction step) as bits and these are passed on The SNR is typically set after the first amplification stage because later noise corruption is very limited

Link Budget Overview Item Value Units Notes Frequency 12.2 GHz Ku Band Data Rate 36 Mbps FEC rate 1/2 Ratio convolutional Symbol Rate 72 Msps Transmit Power 120 Watts 20.8 dBW Antenna Gain 29.2 dBi Effective Isotropic Radiated Power (EIRP) 80.0 dBmi Based on Intelsat G-16 at 99° W Slant Range 40000 km free space loss -206.2 dB Atmospheric Loss -2 Received Signal Strength Isotropic (RSSi) -128.2 Receive Antenna Gain 40 Antenna Temperature K @ 10° Elevation LNA Noise Figure 2 LNA Noise Temp 169.6 System Temperature 205.6 Receive Site Figure of Merit (G/Tsys) 16.9 dBi/K Boltsmann's Constant -198.6 dBm/HzK Signal Bandwidth MHz SNR 8.7 Eb/N0 11.7

Links That I’ve Worked On GPS- Signal power is very important to military users who have to deal with jamming and mountainous terrain COMSAT- Commercial satellite services relay information from site to site. Mostly integrated with internet traffic as another form of backhaul TT&C- Basic satellite Telemetry, Tracking, and Control. Getting basic information from vehicles and telling them what to do.

Other Related Fields Encryption- Number and information theory centric Space and terrestrial physics- Channel impairments are due to underlying physical phenomena Electrical Engineering- Microwave circuit design is just high frequency analog design Optical Engineering- Similar principals, higher frequencies Radar- Two way link budging and different signal processing