What is modelled. Satellites fly one Walker shell from SpaceX's public FCC filings, with J2 drift. They are not the live constellation, so a given satellite number is not a real spacecraft. The downlink's channel plan, OFDM numerology and frame timing follow independent published measurements (Humphreys et al., 2023). The sync sequences, pilots, antenna element count, satellite EIRP, noise temperature and the scheduler's rules are not public, so they are modelled, and adjustable where they matter.
Sky view and obstructions
Every satellite above the horizon, seen from the dish, with zenith at the centre and the horizon at the edge. The terminal only uses satellites above its minimum elevation (dashed ring). Click or drag on the sky to paint obstructions like trees or a roof edge; satellites behind them are unusable.
Sky
Estimated over the next hour at the scheduling interval.
Scheduling and handoffs
The network reassigns each terminal to a satellite on a fixed schedule, reported to be every 15 s. This simulator's scheduler picks the highest satellite that stays clear of obstructions and above the minimum elevation for the whole interval. Each change of satellite costs a short outage while the beam re-points and the receiver re-syncs.
Last two minutes
Events
Phased-array antenna
The dish is a flat array of small antennas. To point the beam at a satellite, each element's signal gets a phase shift so they add up in that one direction; nothing moves. Tilting the beam away from straight up costs gain (scan loss), and spacing the elements more than half a wavelength apart lets copies of the beam (grating lobes) appear.
Element weights
Colour is each element's phase shift; brightness is its amplitude taper.
Beam pattern
Gain over every direction the array can see, as direction cosines (east, north). The circle is the horizon; the cross marks the satellite.
Cut through the beam
Link budget
From the satellite's transmit power to the SNR the receiver sees. Path loss is set by the slant range, which grows as the satellite gets lower. Rain attenuates Ku-band strongly, and a low-elevation path spends longer in the rain.
Downlink signal structure
The downlink uses eight 240 MHz channels on a 250 MHz grid from 10.7 to 12.7 GHz. Each channel carries OFDM: 1024 subcarriers 234 kHz apart, a 32-sample cyclic prefix, and frames 1/750 s long of 302 symbols. Each frame opens with a primary sync symbol (repeated short sequences, for timing and coarse frequency) and a secondary sync symbol (known on every subcarrier, for channel estimation).
Channel plan
One frame, 1333.3 µs
302 symbols × 4.4 µs = 1328.8 µs; the remaining 4.5 µs is a frame guard.
Received spectrum
Average power per subcarrier over the captured data symbols, after frequency correction. Edge subcarriers are guard bands.
Receiver: synchronization and demodulation
A captured frame, received at the current SNR with the serving satellite's Doppler shift and the dish's oscillator error. The receiver finds the frame start from the repeated PSS sequences, estimates and removes the frequency offset in two steps, measures the channel from the SSS, then equalizes each data symbol and tracks the residual phase.
Frame timing metric
Correlation of the capture with itself 128 samples later. It plateaus while the window sits on the PSS's repeats.
Channel estimate
Before equalization
After equalization and phase tracking
Data subcarriers carry random QPSK here. The real downlink adapts modulation and coding, which isn't public, so the throughput in stage 4 is a modelled 60% of the Shannon capacity.