A satellite link budget is an accounting of every gain and every loss in a communications path from transmitter to receiver. The output is a single number — the link margin — that tells you how much headroom you have above the minimum signal needed for a useful bit error rate. If the margin is positive, the link closes. If it is negative, you do not have a working communications system.
The structure of every link budget is the same. Start with transmit power. Add transmit antenna gain. Subtract free space path loss. Add receive antenna gain. Subtract atmospheric losses, polarization losses, pointing losses, and implementation losses. Subtract the receiver noise floor. The result is the available signal-to-noise ratio. Compare it against the required signal-to-noise ratio for your modulation, coding, and target bit error rate. The difference is the margin.
Transmit power is the easy part. It is the RF output of your transmitter in watts, expressed in dBW or dBm. A 5-watt X-band transmitter is +7 dBW or +37 dBm. The complication is that "transmit power" sometimes refers to the output of the high-power amplifier and sometimes refers to the power at the antenna feed. The difference is the loss in cables, switches, and waveguides between the amplifier and the antenna — typically 1-2 dB. Be explicit about which you are reporting.
Transmit antenna gain depends on the antenna type and pointing. An omnidirectional antenna has a nominal gain of 0 dBi. A 30-cm parabolic dish at X-band has about 25 dBi peak gain. The gain you should use in the link budget is not the peak gain — it is the gain in the direction of the receiver, which depends on pointing accuracy. A 1-degree pointing error on a 25-dBi dish at X-band can cost you 2 dB of effective gain.
Free space path loss (FSPL) is the dominant loss in most links. It depends on distance and frequency: FSPL_dB = 20 log10(d) + 20 log10(f) + 32.45, where d is in km and f is in MHz. At X-band (8 GHz), a 600-km LEO downlink has about 166 dB of FSPL. At Ka-band (32 GHz), it is 178 dB. Higher frequency means more path loss but also higher antenna gain per unit area, so the trade is not as one-sided as it looks.
Atmospheric losses come from gaseous absorption (oxygen and water vapor) and rain. At X-band, gaseous absorption is typically 0.1-0.3 dB at low elevation angles. Rain attenuation at X-band is 1-3 dB for moderate rain rates. At Ka-band, rain attenuation can exceed 10 dB during heavy rain — this is why Ka-band links are usually designed with an availability target (e.g., 99.5% of the time) rather than worst-case rain.
Polarization losses arise when the transmit and receive antennas are not perfectly aligned in polarization. Linear-to-linear with a 5-degree mismatch costs about 0.03 dB. Linear-to-circular costs 3 dB by definition. Most operational links use circular polarization on both ends to avoid pointing-dependent polarization losses.
Pointing losses are the difference between the peak antenna gain and the effective gain in the actual direction of the receiver. They depend on the antenna 3 dB beamwidth and the pointing error. For a Gaussian beam, the loss in dB is approximately 12 (theta_error / theta_3dB)^2. A pointing error half the 3 dB beamwidth costs 3 dB of gain.
Implementation losses bundle everything that is hard to model precisely: phase noise, intermodulation, filter ripple, quantization noise, and digital signal processing imperfections. Typical values are 1-2 dB at the modem level. They are not optional — leave them out and your link margin will look healthier than it really is.
The receive system is characterized by a single figure of merit: G/T, the antenna gain divided by the system noise temperature, in dB/K. A 3-meter X-band ground station might have G/T around 30 dB/K. A typical NEN station has 38-45 dB/K. The required G/T for a given data rate is computed from the required Eb/No, the data rate, and the link's effective bandwidth.
Required Eb/No depends on modulation and coding. Uncoded BPSK at BER 10^-5 needs Eb/No around 9.6 dB. Add a rate-1/2 convolutional code and the requirement drops to around 4 dB. Add Reed-Solomon concatenation and you get to about 2.5 dB. Modern LDPC and turbo codes can approach 0.5 dB. Choosing the right coding is a major lever in any link budget — it can be worth 5-7 dB.
Common pitfalls: forgetting to account for slant range at low elevation angles (a ground station tracking to 5 degrees elevation sees about 2.5x the line-of-sight distance vs zenith), using nominal antenna gain instead of pointing-loss-adjusted gain, ignoring polarization mismatch when one end is linear, and reporting margin against typical conditions rather than worst case.
A useful sanity check: most operational space-to-ground links carry 3-6 dB of margin in nominal conditions. If your link budget shows 15 dB of margin, you have probably forgotten a loss term. If it shows -2 dB, you have probably been too pessimistic somewhere — but you also need to fix it before flight, not after.
SMAD Portal includes a Link Budget Calculator that walks through every term, lets you adjust modulation and coding, and reports both nominal and worst-case margins. Pair it with the Free Space Path Loss calculator and the Doppler Shift calculator for a complete communications analysis. Each calculator is parametric — change frequency or distance and every dependent value updates immediately.