1. BASIC LINK BUDGETS SATELLITE LINKS
ALLOCATING THE AVAILABLE SATELLITE RESOURCES TO ACCOMODATE THE PARAMETERS OF THE TX & RX EARTH STATIONS ...
A COMMS SATELLITE IS A RELAY FOR COMMS BETWEEN 2 OR MORE E/S’s. THIS RELAY HAS CERTAIN RESOURCES, SPECIFICALLY POWER & BW.
THE COMMS ENGINEER MUST USE THESE RESOURCES EFFICIENTLY & ECONOMICALLY.
USUALLY THE CUSTOMER STATES THE TYPE & QUALITY OF TRANSMISSION REQUIRED.
THE COMMS ENGINEER TRANSLATES THESE REQUIREMENTS INTO THE TYPE OF COMMS SIGNAL BEST SUITED TO THE LINKS & REQUIRED SIGNAL QUALITY.
THE QUALITY CAN BE EXPRESSED IN SEVERAL DIFFERENT WAYS...

2. C (dBW) CARRIER POWER RECEIVED IS DEFINED BY :
C = PtAe / [4pi(radius^2)] (WATTS)
where,
4pi(radius^2) = SURFACE AREA OF A SPHERE
Pt = ISOTROPICALLY SPREAD Tx POWER
Ae = EFFECTIVE AREA OF THE Rx ANTENNA
WHEN A DIRECTIONAL ANTENNA IS USED :
C = PtGtAe / [4pi(radius^2)]
Gt = Tx GAIN
THE MOST BASIC CALCULATION OF DETERMINING THE AMOUNT OF PWR REC’D STARTS BY ASSUMING ISOTROPIC RADIATION.
ISOTROPIC RADIATION CAN BE DESCRIBED BY IMAGINING THE SOURCE OF RADIATION AT THE CENTER OF A BALLOON. WITH THE ILLUMINATION (POWER PER UNIT AREA) IS CONSTANT.
THE Rx ANTENNA (WITH A CERTAIN EFFECTIVE AREA) WILL INTERCEPT A PORTION OF THE Tx POWER. THIS VALUE IS LATER REPLACED BY Gr (Rx GAIN).
AS, DIRECTIONAL ANTENNAS ARE USED, THE Tx GAIN IS FACTORED INTO THE EQUATION TO THE ACTUAL PWR LVL SPREAD UNIFORMLY OVER THE Rx ANTENNA. NOTE THAT THE GAIN VALUE USED HERE IS AN ABSOLUTE VALUE.
THE QUANTITIES Ae & S^2 MUST BE IN THE SAME UNITS (USUALLY METERS^2).
C(dBW) = 10logP + 10logGt + 10logAe - 20logS - 10log4pi
TRUE PERFORMANCE IS LATER MEASURED BY FACTORING IN THE ADDITIONAL LOSSES & NOISE PRESENT 2

3. pi A CONSTANT OF PROPORTIONALITY EQUALS 3.16 TIMES THE RADIUS SQUARED
(USEFUL IN SOLVING FOR THE AREA OF A CIRCLE)
THE EGYPTIAN RULE FOR FINDING THE AREA :
EQUALS 3.16 TIMES THE RADIUS SQUARED
WHICH WAS CLOSER TO THE TRUTH THAN THE BABYLONIAN VALUE OF 3
(BASED ON THE BIBLE)
IN ACTUALITY, THE MATHEMATICAL VALUE OF pi IS AN IRRATIONAL NUMBER
USEFUL IN SOLVING FOR THE AMOUNT OF WHEAT IN A FIELD, OR THE AREA TO BE TILED IN A CIRCULAR ROOM, etc
BABYLONIA OCCUPIES A REGION WHICH IS NOW MODERN IRAQ
EUCLIDS THEORY IS THAT THE RATIO OF THE AREA OF ANY CIRCLE TO THE SQUARE OF THE RADIUS IS THE SAME FOR ALL CIRCLES. THUS, AREA/RADIUS^2 = k (A CONSTANT)
THE QUANTITY “k” IS AN IRRATIONAL NUMBER. NOT RADILY COMPUTED & CAN BE REPRESENTED BY A FRACTION ONLY APPROXIMATELY. THUS, THE DEFINITION OF IRRATIONAL NUMBERS :
THOSE WHICH ARE NOT WHOL, OR FRACTIONAL VALUES.
ONE OF ARCHIMEDES’ GREAT ACHIEVEMENTS (WHICH ILLUSTRATES THE INTEREST IN QUANTITATIVE KNOWLEDGE), IS HIS DETERMINATION THAT pi LIES BETWEEN 3-1/7 AND 3-10/71. 3

4. THIS IS THE HEART OF THE LINK BUDGET
C/T (dBW/K)
CARRIER-TO-THERMAL NOISE
where,
C = EIRP - LOSSES + Gr
and,
C/T = EIRP - LOSSES + G/T
THIS IS THE HEART OF THE LINK BUDGET
ONE CRITERION OF LINK PERFORMANCE IS CXR PWR-TO-NOISE TEMP. CXR PWR = EIRP - L + Gr (dBW)
TO CALCULATE C/T, THE TERM 10logT IS SUBTRACTED FROM EACH SIDE OF THE EQUATION. THE RESULT IS :
C/T = EIRP - L + G/T
where, GAIN IS IN dBi, AND SYSTEM TEMP IN DEGREES KELVIN
THE NOISE TEMP IS THE SUM OF ANTENNA NOISE, LNA & THE SUM OF ALL RESISTIVE ELEMENTS BETWEEN ANTENNA & LNA.
THIS FIGURE OF MERIT CAN BE IMPROVED BY INCREASING THE EFFICIENCY (AND, THUS GAIN) OF THE ANTENNA (OR, INCREASING ITS SIZE), OR, DECREASING THE NOISE.
WHICH, AGAIN, IS DECREASED BY VIRTUE OF A MORE NARROW BEAMWIDTH (A RESULTANT OF GREATER SIZE/EFFICIENCY) 4

5. (WITH BOLTZMANNS CONSTANT k)
C/kT (dBHz)
CARRIER-TO-THERMAL NOISE DENSITY
(WITH BOLTZMANNS CONSTANT k)
C/kT = C/No = C/T
where,
kT = No = N/B = N (dBW/Hz)
(IN A 1Hz BANDWIDTH)
ALL A PHYSICAL TEMP GENERATE ELECTROMAGNETIC RADIATION. PART OF THIS ENERGY WILL uWAVE FREQS & PRESENT IN THE Rx SYSTEM.
THE NOISE PWR (N) OF RADIATION WITHIN A BANDWIDTH (B) IS :
N = kTB
where, k = BOLTZMANNS = x 10^ (WATT/Hz-KELVIN)
THE NOISE DENSITY (No) IS THE NOISE PWR IN A 1Hz BW & IS uWAVE FREQS.
IF THE SIG HAS NOT BEEN DEMODULATED, OR THE BW IS UNKNOWN, A MEASURE OF SYS PERFORMANCE IS THE RATIO OF CXR PWR TO NOISE DENSITY (C/No) 5

6. C/N (dB) C/N = C/kTB CARRIER-TO-NOISE IN BANDWIDTH B
where,
C/kTB = C/kT - 10log(BW)
and,
C/kT = C/No
A FILTER IN A RCVR USUALLY BLOCKS MOST OF THE NOISE. AND, ONLY THE FREQ BW NEEDED FOR COMMS IS ALLOWED TO PASS. THE C/N = C/kTB = C/kT-10logBW
MOST COMM LINKS HAVE A C/N OF 10dB OR MORE.
THE ACTUAL GOAL IS SET BY THE SIG QUALITY DESIRED, OR, REQUIRED BY THE Rx EQUIPMENT.
C/N OF 16+ IS NOT UNCOMMON FOR ANALOG TV
C/N OF 11 IS REQ’D FOR 25MHz DIGITAL (TO ATTAIN AN Eb/No OF 8 OR BETTER).
The ratio of the level of the carrier to that of the noise in the intermediate frequency (IF) band (as close as possible to the carrier level you’re measuring) before any nonlinear process, such as amplitude limitation and detection, takes place. 6

7. Eb/No (dB) Eb/No = C/No - 10log(R) ENERGY PER BIT - NOISE DENSITY
where,
R = BIT RATE (BITS/SECOND)
PERFORMANCE OF DIGITAL CIRCUITS IS OFTEN MEASURED AS A SPECIFIC BER.
WHICH IS RELATIVE TO THE Eb/No.
DIGITAL SYSTEMS NEED A FURTHER CONVERSION TO FIND THE RATIO OF ENERGY/BIT TO THE NOISE DENSITY.
THE PERFORMANCE OF DIGITAL CIRCUITS IS OFTEN MEASURED AS A SPECIFIC BER. WHICH, THRU EMPIRICAL DATA, IS RELATIVE TO THE (MORE EASILY MEASURABLE) Eb/No.
AN Eb/No OF 8 ROUGHLY EQUALS A BER OF 1 to 9 x 10^-3
AN Eb/No OF 10 IS ROUGHLY 1 to 9 x 10^-5 7

8. W = 1/2[c(PERMITTIVITY)] x E^2 (W/m^2) Z = 1 / [c(PERMITTIVITY)]
E (dBuV/m)
ELECTRIC FIELD STRENGTH
(POWER PER UNIT AREA)
W = 1/2[c(PERMITTIVITY)] x E^2 (W/m^2)
W = [1/2(E^2)] / Z (W/m^2)
where,
Z = 1 / [c(PERMITTIVITY)]
W = 2E (dBW/m^2)
E = 1/2(W ) (dBuV/m)
THE ILLUMINATION LEVEL FOR RADIATION FROM A SATELLITE IS NORMALLY MEASURED IN INCIDENT POWER PER AREA (W/m^2).
IN OTHER RADIO & TV APPLICATIONS (WITH LOWER FREQS), THE STRENGTH OF A SIGNAL IS SPECIFIED AS AN ELECTRIC FIELD STRENGTH (uV/m).
c = VELOCITY OF LIGHT ( x 10^8 m/SEC)
PERMITTIVITY OF FREE SPACE = x 10^-12 FARAD/m
E = THE ELECTRIC FIELD STRENGTH IN VOLTS/METER
Z = 1/(SPEED OF LIGHT x PERMITTIVITY) = 377 ohms
NOTE : THE TERMS W & E ARE IN ABSOLUTE VALUES (NOT dB)
THE CONSTANT = 10log(1/2c x PERMITTIVITY x 10^-12)
NOTE : THE 10^-12 IS INCLUDED TO CHANGE THE FIELD VALUE FROM V/m TO uV/m.
THIS IS 1 OF THE FEW DECIBEL EQUATIONS WITH MULTIPLICATION BECAUSE THE POWER IS PROPORTIONAL TO THE SQUARE OF THE ELECTRIC FIELD.
THE FIELD STRENGTH FOR THE EDGE OF THE EARTH :
E = 1/2(EIRP - LOSSES )
BASED ON A DISTANCE OF 41,679 km 8

9. EIRP (dBW) EIRP = PGt (WATTS) EIRP = 10log(P) + 10log(Gt) (dBW)
EQUIVALENT ISOTROPICALLY RADIATED POWER
EIRP = PGt (WATTS)
EIRP = 10log(P) + 10log(Gt) (dBW)
TYPICAL VALUES OF EIRP RANGE FROM :
0-90 dBW FOR EARTH STATIONS
20-60 dBW FOR SATELLITES
TRANSMITTER POWER IS NORMALLY GIVEN IN WATTS, ANTENNA GAIN IS VIRTUALLY ALWAYS STATED IN dBi.
SO, THE ADDITIONAL DESIGNATION OF THE 10log FUNCTION MAY NOT REALLY BE NECESSARY. BUT, ALWAYS BE SURE OF THE TERMS & HOW THEY’RE STATED. 9

10. G (dBi) GAIN OF AN ANTENNA (AS REFERENCED TO AN ISOTROPIC RADIATOR)
G = Tx PWR OF ANTENNA / ISOTROPIC Tx PWR
G (PARABOLIC) = (4pi x eff x A) / WAVELENGTH^2
G = eff{[(piD x FREQ)/C]^2}
G = 20logD + 20logFREQ + 10log(eff)
TYPICAL E/S GAIN FIGURES ARE 1-60dBi
SATELLITE GAIN FIGURES RANGE FROM 14-40dBi
THE GAIN RATIO IS REFERRED TO AN IDEAL (THEORETICAL) ISOTROPIC RADIATOR. AS ALWAYS, A REFERENCE ENTITY MUST BE DEFINED.
USUALLY EXPRESSED IN dBi
eff = EFFICIENCY, A = THE PHYSICAL AREA OF THE REFLECTOR
TYPICAL EFFICIENCY #’S ARE (A COMMON APPROX # IS .55)
THE AREA OF A CIRCLE IS piD^2/4, WHICH LEADS TO THE 3rd EQ.
THE 4th EQ. USES THE VALUES OF D (IN METERS), FREQ IN GHz, EFFICIENCY EXPRESSED AS DECIMAL #’S (.55 BEING THE USUAL APPROXIMATION), WITH THE CONSTANT OF 20.4 EQUALLING :
20log(pi/SPEED OF LIGHT)
with, THE SPEED OF LIGHT EQUALLING m/nSEC
NOTE : THE UNUSUAL UNITS OF “c” ARE DUE TO FREQ BEING IN MHz & DIAMETER IN METERS. 10

11. eff Antenna efficiency (assumed 60-70%)
Actual values range from .2 to .75
Conventially illuminated (large) Earth stations typically are 65-75%
Flat plate antennas are 75% efficient
(Superconductive surfaces on these may further increase this value)
Satellite spacecraft antennas are usually less efficient.
(40-55%, or 20-30% for multi-beam)
THE “REALISTIC” VALUE OF 55% IS OFTEN USED FOR “MODELING” (WHEN ANOTHER VALUE IS NOT AVAILABLE.
ACTUAL VALUES RANGE FROM 20% (-4.4dB), TO 75% (+1.3dB)
TO ACHIEVE THE DESIRED GAIN IN MULTI-BEAM SERVICE, THE ANTENNA DIAMETER IS SLIGHTLY OVERSIZED. WHICH CAUSES CHANGES IN BEAMWIDTH & SIDELOBE STRUCTURE ... 11

12. BASICS OF ANTENNA GAIN
A Tx SHAPED ANTENNA FOCUSES THE Tx PWR
IF NO BEAM DIRECTIVITY IS APPLIED, THE RESULT IS AN ISOTROPIC RADIATOR.
(THE SUN COULD BE USED AS AN EXAMPLE)
THEORETICAL GAIN OF A PARABOLIC IS INFINITE
(THUS, THE LIMITATION IS BASED ON WAVELENGTH)
GAIN CALCULATED BY VIRTUE OF THEORETICAL IS USUALLY CONSIDERED PEAK (ON-AXIS) GAIN.
OFF-AXIS GAIN IS ALSO A SERIOUS CONSIDERATION 12

13. ANTENNA BEAMWIDTH 13

14. Gr = Rx ANTENNA GAIN (dBi) Ts = Rx SYSTEM NOISE TEMP (DEGREES KELVIN)
G/T (dBi/K)
FIGURE OF MERIT
G/T = Gr - 10logTs
where,
Gr = Rx ANTENNA GAIN (dBi)
Ts = Rx SYSTEM NOISE TEMP (DEGREES KELVIN)
Gr IS A FACTOR OF THE EFFICIENCY, OR SIZE OF THE ANTENNA.
Ts IS THE SUM OF ANTENNA NOISE TEMP, LNA TEMP & NOISE CONTRIBUTED BY RESISTIVE COMPONENTS BETWEEN THE ANTENNA AND LNA.
ANTENNA GAIN IS USUALLY GIVEN IN dBi (SO THE 10log DESIGNATION IS ASSUMED.
THE SYS NOISE TEMP IS IN DEGREES K.
NOTE : THE NUMERICAL VALUES OF GAIN (dBi) & K CANNOT ACTUALLY BE DIVIDED (1 IS IN dB, THE OTHER IS NOT)
TO SOLVE FOR G/T YOU MUST CONVERT K TO DECIBELS, THEN COMBINED (IN THIS CASE, BY SUBTRACTION).
TYPICAL E/S G/T VALUES ARE FROM -18 to +41dBi/K
TYPICAL SATELLITE VALUES RANGE FROM -20 to 10dBi/K 14

15. k (dBW/Hz-K) BOLTZMANNS CONSTANT (OF PROPORTIONALITY)
k = x 10^ (W/Hz-K)
k = (dBW/Hz-K)
Pn (MAX NOISE OUTPUT) = kTB
where,
T = ABSOLUTE TEMPERATURE
B = BANDWIDTH OF INTEREST
RESISTIVE NOISE IS REFERRED TO AS THERMAL, AGITATION, WHITE OR JOHNSON NOISE.
IT IS DUE TO RANDOM MOTION OF MOLECULES, etc
IN THE STUDY OF THERMODYNAMICS,
KINETIC THEORY SHOWS THAT THE TEMP OF A PARTICLE IS A WAY OF EXPRESSING ITS INTERNAL KINETIC ENERGY, THUS ITS NOISE TEMPERATURE.
THIS TEMP IS THE STATISTICAL RMS VALUE OF THE VELOCITY OF MOTION OF PARTICLES IN THE BODY.
NOTE : THIS VALUE OF KINETIC ENERGY BECOMES APPROX THE TEMP OF “ABSOLUTE ZERO” (0 DEGREES K)
IT IS ALSO THE TEMP IN WHICH ALL MOTION CEASES.
IT IS THUS APPARENT THAT THE NOISE PWR GENERATED BY A RESISTOR IS PROPORTIONAL TO ITS ABSOLUTE TEMP, IN ADDITION TO BEING PROPORTIONAL TO THE BW OVER WHICH THE NOISE IS TO BE MEASURED.
k IS THE APPROPRIATE CONSTANT OF PROPORTIONALITY USED IN SOLVING FOR NOISE POWER AT A SPECIFIC TEMP & BW. 15

16. L (dB) FREESPACE LOSS C = (EIRP x eff x AREA) / (4pi x S^2)
G = (4pi x eff x AREA) / WAVELENGTH^2
C = EIRP x [(WAVELENGTH^2) / (4piS)^2] x Gr
L = (4piS)^2 / (WAVELENGTH^2)
C = EIRP - L + Gr
L = 20logS(km) + 20logFREQ(GHz)
A FUNCTION OF DISTANCE, BUT BY USING THE WAVELENGTH IT IS EXPRESSED AS A RATIO.
Rx PWR = ILLUMINATION LVL x THE EFF AREA OR PGt/4piS^2
MULTIPLYING THAT TIMES EFF AREA YIELDS THE 1st EQUATION.
TAKING THE 2nd EQ & SOLVING FOR EFF AREA, AND SUBSTITUTING THIS VALUE INTO THE 1st EQ YIELDS THE 3rd EQ.
THE Rx CXR PWR IS A PRODUCT OF 3 FACTORS :
EIRP (INPUT PWR x GAIN OF THE Tx ANTENNA)
Rx GAIN (DETERMINED BY EFFICIENCY & SIZE OF THE DISH
THE MIDDLE FACTOR, CONSISTING OF THE REST OF THE LINK. WHICH IS A FUNCTION OF THE WAVELENGTH & DISTANCE (S)
THE RECIPROCAL OF THE MIDDLE OF EQ #3 YIELDS THE 4th EQ
AND, IS CALLED FREESPACE LOSS. THE DISTANCE & W/L MUST BE IN COMPATIBLE UNITS. L IS A LARGE RATIO WITH NO DIMENSIONS.
AS, W/L = c/ff, AND, USING DECIBEL UNITS, THE 4th EQ CAN BE WRITTEN IN THE FORMAT OF THE 6th EQUATION.
NOTE : THE CONSTANT = 10log(4pi/c)^2,
where c = km GHz OR km nSEC
S VARIES FROM 35,788 (MAX EL) to 41,679 (MIN EL) 16

17. W (dBW/m^2) ILLUMINATION LEVEL W = PGt / [4pi(S^2)]
W = EIRP - 20logS - 71
where,
THE CONSTANT 71 = 10log{4pi[(1000m/km)^2]
THE MAXIMUM DISTANCE (S) = 41,679km
THIS CORRESPONDS TO A SATELLITE ON THE 0 DEGREES ELEVATION & MAXIMUM CENTRAL ANGLE
WITH THIS VALUE USED, THE WORST-CASE LEVEL IS :
W = EIRP
THE ILLUMINATION LVL = PWR REC’D PER UNIT AREA, OR THE PWR REC’D BY AN IDEAL 1 SQ METER ANTENNA (WHERE, eff=1)
IF THE XMITTER WERE ISOTROPIC, THE A RANGE OF “S” = P/(4piS^2). WITH AN ANTENNA WITH A GAIN :
W = PGt / (4piS^2)
THE ILLUMINATION LVL = EIRP DIVIDED BY THE TOTAL SURFACE AREA (4piS^2). THIS TERM IS CALLED THE SPREADING FACTOR.
AGAIN, BASED ON UNIFORM DISTRIBUTION OF ENERGY.
FOR GEOSTATIONARY SATELLITES, THE ILLUMINATION LVL AT THE SATELLITE RANGES FROM -162to-52
THE ILLUMINATION OF THE EARTH BY A SATELLITE RANGES FROM -142 to -102 dBW/M^2
THE MINIMUM DISTANCE (SUBSATELLITE POINT) = 35,786
THE POWER INCREASE WOULD EQUAL :
20log(41679/35,786) = 1.324dB 17

18. PFD (dBW/m^2) POWER FLUX DENSITY
(USUALLY DEFINED WITHIN A SPECIFIED BW)
PFD = W - 10log(B/Bccir)
where,
W = EIRP (dBW/m^2)
PFD = EIRP log(B/Bccir)
THE STANDARD CCIR BANDWIDTH = 4kHz
(FOR C & Ku BAND SYSTEMS)
FOR MEASURING INTERFERENCE WITH OTHER SERVICES THE BW IS SIGNIFICANT. AN UNMOD A SINGLE FREQ COULD BE A MAJOR SOURCE OF INTERFERENCE. HOWEVER, THAT THE SAME ILLUMINATION LVL, SPREAD OVER A WIDE BW IS HARDLY NOTICED.
TO REGULATE INTERFERENCE, LIMITS MUST BE ESTABLISHED FOR SATELLITE OPERATORS.
THE ILLUMINATION LVL IS NOT APPROPRIATE, SINCE HI LVLS ARE ACCEPTABLE IF DISPERSED.
IT IS ALSO NOT APPROPRIATE TO DIVIDE THE LVL BY THE BW, SINCE AN UNMOD CXR HAS A VERY NARROW BW.
IN PRACTICE, A SMALL BW IS SPECIFIED & THE ILLUMINATION LVL SPECIFIED WITHIN THIS BW.
NOTE : (EQ #2) IS BASED ON THE MAX DISTANCE OF 41,679km. REMEMBERING THAT SPREADING FACTOR = 4piS^2
EQ #3 CAN BE TRANSPOSED TO DETERMINE THE MAX ALLOWED EIRP. EIRP = PFD log(B/Bccir)
FOR C-BAND, PFD RANGES FROM -152 (< 5 DEG EL) to -142 (>25)
FOR Ku BAND, PFD (<5 DEG EL) = -148, AND -138 (FOR >25 DEG) 18

19. DEFINITION OF SIGNAL QUALITY
(C/T)
CXR-to-THERMAL NOISE RATIO
(C/No)
CXR-to-NOISE DENSITY
(C/N)
CXR-to-NOISE POWER
(S/N)
SIGNAL-to-NOISE POWER
C/T = THE SIGNAL REACHING THE DISH (EIRP-LOSS) + E/S G/T.
THIS IS ESSENTIALLY THE HEART OF THE LINK BUDGET
C/No = C/kT = CXR PWR-to-NOISE DENSITY
(IN A 1Hz BANDWIDTH)
A VERY USEFUL UNIVERSAL TERM THAT IS INDEPENDANT OF BANDWIDTH.
C/N = C/kTB = C/No - 10log(BW)
S/N IS MUCH MORE COMPLEX, AS IT DEPENDS ON VIRTUALLY ALL THAT WHICH MAKES UP THE DEMODULATED SIGNAL STRENGTH COMPARED TO THE NOISE LVL. 19

20. LINK BUDGET (COMPONENTS)
TRANSMITTER POWER P (W)
ANTENNA GAIN G (dBi)
RADIATED EIRP (dBW)
ILLUMINATION RCVR (dBW/m^2)
FREE SPACE LOSS (dB)
SYSTEM NOISE TEMPERATURE Ts (K)
RECEIVE FIGURE OF MERIT G/Ts (dBi/K)
CXR-to-THERMAL NOISE RATIO C/T (dBW/K)
CARRIER-to-NOISE DENSITY C/No (dBHz)
CARRIER-to-NOISE RATIO C/N (dB)
PFD IS THE THE PWR ORTHOGONALLY CROSSING THE UNIT SURFACE = Tx PWR+TxGAIN-10log(4piD^2)
GAIN = ________________________________________
RSL = EIRP - LOSS + RX GAIN
RSL = SPREAD LOSS + EFF. AREA
RSL = RADIATED FLUX DENSITY + EFF. AREA
FREESPACE ATTEN = 10log(WAVELENGTH/(4piD))^2
Rx PWR = EIRP - LOSSES + Rx GAIN
C/N = C/T - 10log(k) - 10log(B)
C/N = C/No - 10log(BW)
C/T = EIRP - LOSSES + G/T
C/No = C/kT
C/No = EIRP - LOSSES + G/T - k
C/No = C/N + 10log(Occ. BW) 20

21. (THERE IS NO STANDARD FORMAT)
BASIC LINK BUDGETS
COME IN VARIOUS LENGTHS & STYLES
(THERE IS NO STANDARD FORMAT)
3 KEY EQUATIONS FORM THE BASIS :
FOR MOST UPLINK BUDGETS :
EIRP = 10logP + Gt
C/T = EIRP - L + G/T
C/kT = C/T
FOR MOST DOWNLINK BUDGETS :
C/N = C/kT - 10logB 21

22. CHARACTERISTIC PARAMETERS
THE TRANSPONDER
CHARACTERISTIC PARAMETERS
THE TX/RX FREQUENCY BANDS & POLARISATIONS
THE TX/RX COVERAGE (SFD & GAIN CONTOURS)
THE TX EIRP & CORRESPONDING PFD ACHIEVED
THE RX PFD REQUIRED TO ACHIEVE THE REQ’D TX EIRP
THE G/T BASED ON THE SFD CONTOUR
NON-LINEAR CHARACTERISTICS
RELIABILITY AFTER x YEARS FOR y PERCENTAGE OR NUMBER OF CHANNELS TO REMAIN IN WORKING ORDER
THE PRINCIPLE PARAMETERS WHICH CHARACTERISE THE PAYLOAD FROM THE POINT OF VIEW OF THE LINK BUDGET ARE THE D/L EIRP & THE G/T.
ALTHOUGH THEY CHARACTERISE DIFFERENT LINKS, THEY ARE NOT INDEPENDANT.
THE U/L C/No IS PROPORTIONAL TO THE SATELLITE G/T, AND THE D/L C/No IS PROPORTIONAL TO THE D/L EIRP.
IT IS POSSIBLE TO COMPENSATE FOR AN INCREASE IN SYSTEM NOISE TEMP BY AN INCREASE IN THE EIRP. THIS REMAINS VALID FOR A LINK WITH CONSIDERABLE INTERFERENCE & INTERMODULATION NOISE.
THE CHOSEN OPERATING POINT, AS DEFINED BY THE CORRESPONDING IBO/OBO, RESULTS FROM A COMPROMISE BETWEEN THE AVAILABLE PWR & THE LVL OF IM NOISE.
A SMALL BO HAS A BENEFIT OF HI PWR, BUT ALSO HIGH IM NOISE SINCE THE TWT OPERATES IN A HIGHLY NON-LINEAR REGION. A LARGE BO LIMITS IM NOISE & REDUCES EIRP
NOTE : A ZERO INPUT BO FOR MULTICXR OPS CORRESPONDS TO AN OPERATING POINT BEYOND SATURATION FOR 1 OF THE CXRS. HENCE, THE MAXIMUM EIRP IS OBTAINED BY A NON-ZERO IBO. 22

23. TRANSMITTER POWER (P) USUALLY SPECIFIED IN WATTS
THE 1st NUMBER OF THE LINK BUDGET
(OFTEN ADJUSTED TO OBTAIN THE DESIRED PERFORMANCE)
FOR SATELLITES, Tx POWER IS LIMITED BY THE DC POWER AVAILABLE VIA THE SOLAR ARRAY. (10-200W)
EARTH STATION TRANSMITTERS RANGE FROM 1-10KW
IF LOSSES ARE SIGNIFICANT, THE Tx POWER IS THE ANTENNA INPUT FLANGE.
(LOSSES BEFORE THIS POINT MAY BE DEDUCTED FROM THE Tx PWR) 23