See Figure 6.1 for a reference model. The satellite link contains transmit stations, receive stations, and the paths that connect them.

The satellite is both a transmit station and a receive station. Many factors need to be included in the assessment of the end-to-end performance [USA199801]:

1. Link budget model Uplink and downlink − C/kT (carrier-to-receiver noise density) Satellite receive power flux density −

2. Positional data model Earth terminal to satellite slant range − Earth terminal antenna elevation and azimuth − Uplink and downlink Doppler frequency shift −

3. Benign atmosphere attenuation Clear air attenuation − Rainfall attenuation − Atmospheric signal scintillation −

4. Modulation and channel encoding Noise equivalent bandwidth − Modulation spectral efficiency − Demodulator implementation loss − Probability of detection of error − Convolution channel coding gain −

5. Earth terminal model Antenna model − Receive system model − Transmit system model − Satellite model − Uplink signal power − Downlink signal power − Transponder signal and noise power sharing − Transponder uplink power flux −

The following input items are needed (among others) to produce a link budget calculation for a spacecraft to ground station link:

Earth station latitude 7 Earth station longitude 7 Spacecraft longitude 7 Downlink frequency 7 Antenna gain 7 Antenna noise temperature 7 Low noise amplifier 7 Ortho mode transducer loss 7 Effective isotropic radiated power (EIRP) 7 Intermediate frequency (IF) receive bandwidth 7 Transmit data rate 7 Waveguide losses 7

Table 6.1 defines the major terms; many of these were already defined in previous chapters. As we saw in earlier chapters, communications measurements and calculations are often done in

decibels and units related to decibels. This is done because by using logarithms, multiplication and division are replaced by addition and subtraction, respectively. Link budgets are done largely in dB.