Where does the signal need to go — and for how long? Coverage connects the antenna pattern, the link budget, and the motion of the spacecraft.
Hemispherical Coverage
- Broad-beam antennas for (nearly) hemispherical coverage:
- Crossed dipoles
- Quadrifilar helix
Broad Coverage from Multiple Antennas
- Transmitter → power divider → antenna 1 and antenna 2, facing opposite directions
- The combined pattern (polar plot in dBi) covers both sides, roughly 0 to +5 dBi near 0° and 180°
- Near ±90°, where both antennas contribute, the pattern breaks into dense ripples with deep nulls — the two antennas interfere
Shaping Gain for an Earth Pass
- For a nadir-pointed spacecraft, the ground station near the limb is at a much longer slant range than one directly below
- Plot: required gain relative to nadir vs. look angle from nadir (0° → about 65°) — flat near 0 dB, rising to about +12 dB near the limb
- Longer slant range near the limb needs more gain
- A shaped pattern can reduce the received-power variation over the pass
- Assumes nadir pointing
The Earth-Science Shaped Omni
- Terra (EOS-AM) Direct Access System, 8.2125 GHz: shaped reflector with a dipole feed
- Measured gain pattern vs. boresight (off-nadir) angle:
- Mean gain about −4 dBi near nadir, dipping to about −8 dBi around 15–20° (min samples near −12 dBi)
- Rising to about +7 dBi near 60–65°
- More gain toward the limb helps compensate for greater range
- 1998 design record; the dip near 20° off nadir also constrained planned direct playback (NASA, EOS-AM Direct Access System User’s Guide, 1998, §6, Figs. 2 & 14)
- Similar: NASA, EOS Aqua X-Band Direct Broadcast IDD (2002)
Antenna Patterns and Usable Coverage
- The pattern gives the gain for each direction
- The link budget sets how much gain is enough
- Illustrative installed pattern cut from −50° to +50° (link direction in antenna coordinates):
- Peak margin = 6 dB, so the minimum gain for this link is −6 dB relative to peak
- The pattern is above −6 dB from about −40° to −5° and +5° to +40°
- A deep null (about −18 dB) at 0°
- A null creates a gap even with clear line of sight
- Nulls may arise from interference (e.g. multiple antennas, structure)
Coverage Becomes Availability over Time
Availability During a Visible Interval
- Assume the link direction sweeps from −50° to +50° in 10 minutes (10°/min)
- Clear line of sight: the whole 0–10 min
- Usable link: 1–4.5 min and 5.5–9 min (pattern null from 4.5 to 5.5 min)
State the denominator
- “Availability” always needs a defined time interval
- Mission availability also includes visibility, attitude, tracking, polarization, weather, and scheduled resources
- Capacity vs. availability (Class 1): enough capacity ≠ a path at the critical moment
What We Can Now Explain
- Pointing loss comes from the gain in the actual link direction (Antenna Pointing)
- Polarization loss comes from incomplete field coupling (Antenna Polarization)
- A pattern and a link threshold define usable directions
- Motion through those directions determines usable time
- Orbit and attitude connect antenna coordinates to mission coverage