Pointing with the Spacecraft
- Voyager’s high-gain antenna is fixed to the spacecraft (body-mounted)
- The spacecraft turns to point the antenna
- Question: what else moves when the antenna moves? (everything else on the spacecraft — instruments, arrays, the whole attitude) (not on slide)
Gimbaled Antennas
- TDRS reflectors: boom mounted, two-axis gimbals
- Pointing can change without turning the whole spacecraft
- Mechanisms add limits on motion, reliability, and (cable/RF) routing
- Third option — electronic steering with a fixed aperture: phased arrays
What Is Pointing Loss?
- Boresight: the peak-gain direction
- = angle between boresight and the direction to the other terminal
- The actual link direction has lower gain than the peak
Pointing Loss
- Loss is positive in dB
- Evaluate pointing at both ends of the link
- The actual pattern may depend on two angles
Pointing Away from the Peak
- Gaussian main-lobe illustration (relative to the full HPBW):
- So dB, using the full HPBW and consistent angular units
- Offset / full HPBW:
- ¼ → about 0.75 dB loss
- ½ → about 3 dB loss (the half-power point, by definition)
Same Error, Different Beams
- At 0.5° pointing error:
- 2° beam: 0.75 dB
- 1° beam: 3 dB
- Narrow, high-gain beams are less forgiving of pointing error
WARNING
- Actual patterns differ; this near-boresight approximation does not describe sidelobes or nulls
How an Antenna Tracks a Spacecraft
| Method | What drives it? | Practical issue |
|---|---|---|
| Program track | Predicted position vs. time | Orbit and alignment accuracy |
| Step track | Small moves to seek peak signal | Needs usable signal and time |
| Autotrack | Measured angular error | Special feed/receiver hardware |
- Prediction can acquire the target before signal-based tracking takes over
Pointing Loss in the Link Budget
- Pointing losses at each end are subtracted from the aligned received power, together with polarization mismatch: margin against a threshold
Homework 3: Pointing Loss
Homework 3, Problem 2(a)
- Transmit HPBW 3.50°, pointing error 0.875°; receive HPBW 1.40°, error 0.350°
- Total pointing loss — subtract it once
- Continued in polarization and margin