Satellite Communications Engineering T Scott Parmer Lockheed Martin

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 (d. B) – 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 log 10(2) ≈ 3 d. B i. e. a 3 d. B addition is a 2 x increase in power • A key measure of link performance is Signal to Noise Ratio Unit Relative to d. Bi Isotropic Antenna d. BW 1 Watt d. Bm 1 milliwat d. B-Hz 1/Hertz d. BK 1 Kelvin

Signal to Noise Ratio • 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

Time, Frequency, and Phase 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

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

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 d. B 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

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 Frequency Data Rate FEC rate Symbol Rate Transmit Power Antenna Gain Effective Isotropic Radiated Power (EIRP) Slant Range free space loss Atmospheric Loss Received Signal Strength Isotropic (RSSi) Receive Antenna Gain Antenna Temperature LNA Noise Figure LNA Noise Temp System Temperature Receive Site Figure of Merit (G/Tsys) Boltsmann's Constant Signal Bandwidth SNR Eb/N 0 Value 12. 2 36 1/2 72 120 20. 8 29. 2 80. 0 40000 -206. 2 -2 -128. 2 40 36 2 169. 6 205. 6 16. 9 -198. 6 72 8. 7 11. 7 Units GHz Mbps Ratio Msps Watts d. BW d. Bi d. Bmi km d. Bmi d. Bi K d. B K K d. Bi/K d. Bm/Hz. K MHz d. B Notes Ku Band convolutional Based on Intelsat G-16 at 99° W @ 10° Elevation

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