The antenna is where RF performance meets the rest of the spacecraft — everything is a trade.
Antenna Selection Choices
| Choice | Examples | What it affects |
|---|---|---|
| Radiating structure | Wire, patch, horn, reflector | Pattern, gain, bandwidth |
| Coverage control | Body, gimbal, beamforming | Pointing, power, complexity |
| Deployment | Fixed, hinged, unfurlable | Mass, packaging, reliability |
| Feed and polarization | Single/multiple feeds, linear/circular | Losses and compatibility |
- Radiating structures: Antenna Types; coverage control: body-fixed and gimbaled pointing, beamforming arrays; feeds and polarization: Antenna Polarization
Antenna Trades Across the Spacecraft
- Mass: aperture + supports + deployment + pointing hardware
- Attitude control: can we keep the beam on the ground station?
- Structures: can we package, deploy, and support the antenna?
- Power and thermal: what does the chosen architecture require?
- Operations: what link is available during acquisition or safe mode?
- Which trade would matter most for a small Earth orbiter? For a distant planetary mission?
Antenna Choices for the Mission
- “The ground station can see the spacecraft, but the link drops.” What would you check besides range?
- Possible checks: installed pattern, attitude, pointing, polarization, receiver threshold, and resource availability (see usable coverage)
- Would a broader beam help? What would it cost? (less gain in exchange for pointing tolerance and coverage) (not on slide)
- Would circular polarization help if the spacecraft rolls? What must still match at the receiver? (the sense) (not on slide)
Before we finish
- A larger antenna gives us more gain. What might prevent us from using it?
Homework 3: Margin During the Pass
Homework 3, Problem 3: Fixed vs. Gimbaled
- Requirements: nadir-pointed science and continuous communications through a 12-minute overhead pass
- Fixed antenna: gain offset (relative to peak) and extra path loss vs. time; at midpass, peak margin = +5 dB; margin ≥ 0 is usable
- Ideal gimbal: holds peak gain with no added loss
Time (min) Fixed gain (dB) Extra (dB) Fixed margin (dB) Gimbal margin (dB) 0–2 −8 4 −7 +1 2–5 −2 1 +2 +4 5–7 0 0 +5 +5 7–10 −2 1 +2 +4 10–12 −8 4 −7 +1
- Fixed: usable only 2–10 min. Gimbal: usable throughout the pass
- The gimbal restores peak gain; it does not remove the extra path loss
- Why not just rotate the spacecraft toward the station? It would violate the required nadir science attitude
Homework 3: Mass and Pointing
Homework 3, Problem 4: Mass Limit
- Same science and continuous-communications requirements; installed antenna-system mass limit = 5.0 kg (mass includes feed, supports, and mechanisms)
- Option A: 2.0 kg fixed antenna. Option B: 6.0 kg gimbaled antenna. Both have the RF performance of Problem 3
Option Installed mass Communications + science All requirements? A: Fixed 2.0 kg; 3.0 kg below limit Cannot maintain both through full pass No: loses link at pass ends B: Gimbaled 6.0 kg; 1.0 kg above limit Tracks station while science stays nadir-pointed No: exceeds mass limit
- Neither option meets every requirement as proposed
The Engineering Trade
- Reallocate at least 1.0 kg to permit the 6.0 kg gimbaled system — other hardware must give up that mass allocation
- Or redesign the gimbaled system to 5.0 kg or less while retaining RF and pointing performance (and verify the redesigned hardware)
- Mass location matters even at fixed total mass: mass farther from a rotation axis contributes more rotational inertia
- This affects slew torque and settling
- (Problem 4(c) was declared outside the course’s scope; full credit for everyone)