The physical question: how much signal power reaches the receiver?
One Direction at a Time
- One transmitter → one receiver → one direction
- The reciprocal link (e.g. the uplink for a downlink) needs its own budget — transmitter and receiver parameters can differ
- Who controls which terms: spacecraft team vs. network provider
Received Power Is Only a Start
- Here we calculate carrier power at the receiver input
- We do not yet know whether the receiver can recover the data
WARNING
- A respectable number in dBW is not yet a working communications service
Equivalent Isotropically Radiated Power
- EIRP (equivalent isotropically radiated power) combines the transmitter, its feed, and its antenna:
- Transmit power − feed loss + antenna gain (all in dB; see decibels for the bracket notation)
- Isotropic reference vs. directional antenna:
- An isotropic source spreads the same power equally in all directions
- A directional antenna concentrates it toward the receiver
- EIRP is the equivalent on-axis intensity — not a new power amplifier
- E.g. a 10 W transmitter behind an 8 dBi antenna still transmits 10 W; the antenna gain changes the on-axis equivalent (see TERPSat)
- Gain itself: Antenna Gain
Collecting the Power
- At range the radiated power is spread over a sphere of area (see why power falls with range), giving a power flux density
- Only the field crossing the receive antenna’s effective collecting area is captured
- More on : effective area
The Received-Power Equation
- Transmitter + feed + antenna → ; free-space path loss (range and frequency); receive antenna
Received-Power Equation
- Units: power in dBW, antenna gain in dBi, losses in dB
- excludes the transmit feed loss already included in EIRP
- Fully expanded (Class 3):
- is a positive loss that gets subtracted: Free-Space Path Loss
- Worked example with every term: TERPSat link
- Later refinements: pointing and polarization losses (margin against a threshold), and frequency dependence (Frequency Selection)
References
- Cited: ITU-R P.525-4, Calculation of Free-Space Attenuation (2019); JPL 810-005, DSN Telecommunications Link Design Handbook; CCSDS 401.0-B-29, RF and Modulation Systems — Part 1 (2019); Friis, “A Note on a Simple Transmission Formula,” Proc. IRE 34(5), 1946
- Optional further reading: Cheung, “The Role of Margin in Link Design and Optimization” (2015); NASA, Small Spacecraft Technology State of the Art: Communications (2026); Pratt, Bostian & Allnutt, Satellite Communications, 2nd ed.; Vallado, Fundamentals of Astrodynamics and Applications, 5th ed.