ECE 5233 Satellite Communications

1. Prepared by: Dr. Ivica Kostanic Lecture 13: Propagation effect and link margin calculation (Section 8.1-8.3) ECE 5233 Satellite Communications Spring 2014

2. Outline • Design for reliability • Components of the atmospheric losses • Abortion losses • Cloud losses • Losses associated with rain Important note: Slides present summary of the results. Detailed derivations are given in notes.

3. Link performance and availability Key link budget equation Pr – received power EiRP – effective radiated power FSPL – free space path loss La– atmospheric losses Note: La is a random variable that changes due to condition of the atmosphere between TX and RX Two thresholds are defined Performance threshold – link’s performance above target Availability threshold – link is not available due to bad performance

4. Components of atmospherics losses • Many components of loss (green – attenuation, blue – depolarization and refraction) • Atmospheric absorption (gaseous effects) • Cloud attenuation (aerosol and ice particles) • Rain attenuation • Tropospheric scintillation (refractive effects) • Ionospheric scintillation • Faraday rotation (polarization loss) • Rain and ice crystal depolarization • Losses are frequency dependent – affects different bands in different manner • All losses are random variables with spatial and temporal distribution • Many years of careful measurements have established spatial and temporal distribution of the loss contributing components • The most significant attenuation comes from rain effects (C, Ku and Ka bands) Note: Link design is performed with a margin that ensures that the system performance and availability targets are met for desired time. The margin is referred to as the “fade margin”

5. Atmospheric absorption • Atmospheric attenuation due to oxygen and water • Oxygen absorption peaks: 60GHz and 120GHs • Water vapor absorption peaks: 22GHz and 185GHz • Attenuation is relatively small for all frequencies below 50GHz • ITU graph provides zenith attenuation. For other elevation angles Note: typically margin of about 1dB is used for atmospheric losses Range of interest for commercial satellites

6. Cloud attenuation • Become important for frequencies above 10GHz • Difficult to predict due to wide variety of cloud types • On the order of 0.1 to 0.2 dB/km • Increases with frequency and temperature • Increases with lower elevation angles • Typical margin 1-2dB at frequencies around 30GHz (smaller for frequencies below) Reference: G. W. Stimson, Airborne Radar, SciTech Publishing, 1998 Note: the length of the satellite link path through the clouds is usually quite small except for very low elevation angles

7. Rain attenuation • Most significant source of attenuation in C, Ku and Ka satellite bands • Random attenuation – needs to be dealt with using probability tools • Three basic steps in calculating rain attenuation • Step 1: determine the rain rate threshold exceeded at a given link reliability threshold • Step 2: determine specific attenuation in dB/km corresponding to the rain rate • Step 3: estimate the effective length of the path and calculate the overall rain attenuation • There are two broad categories • Stratiform rain • Convective rain • Stratiform rain • Over large geographic areas • Relatively low rain rate • Convective rain • Over small geographical areas • High intensity thunderstorm • Irregular profile of the rainfall • Satellite links reliability depends mostly on convective rains. There is usually enough margin in the link design for the low rates associated with the stratiform rains Note 1: only small portion of the satellite path goes through rain Note 2: rain intensity along the path may vary

8. Rain characterization Example CCDF curve for rain rate • Principle tool – cumulative distribution function (CDF) of rain rate • Curves accumulated over many years and are accurate on average • Significant variability from year to year – especially at low time percentages of interest to satellite design • Different sources provide different averaging time (time resolution) • ITU recommends using 1min resolution • Different sources provide different spatial averaging • ITU recommends interpolation methods Note: CCDF curves are frequently given in tabular format

9. Rain climate maps • Developed by ITU • World divided into 15 different regions (A-Q) • Rain rate CDF tabulated for each climate region • The CDF based on long term average and are within +/- 10% • Not very accurate, but simple and widely used for basic calculations ITU climate map for the US Climate map rain rate CCDF in mm/h

10. Rainfall exceedance contour maps • Developed by ITU • Provide 1 min temporal resolution • ITU REC P.837-x provides method for calculating CCDF of rain rates • Recommendation provides 0.01 % maps Note: ITU REC P.837-5 is uploaded to the website Example of ITU rainfall exceedance map (0.01%)

11. Example Determine rain rate exceeded in 0.1% of time for Melbourne, FL • Using ITU climate maps • Using ITU exceedance curves Answer: • 95 mm/hour • 80 mm/hour Note: Melbourne is close to boundary between regions M and N. Using average between the two: (65+95)/2=80mm/hour