Chapter
3:
Spacecraft
Here is the work-script for
chapter 3 (slides)
8 October 2021
3.1 Power systems
We talked
about solar cells and batteries and derived the solar
constant.
Click here for an animation how solar panels are
deployed. Here
is
a link to a description of a new type of battery.
18 October 2021
We looked at the variation of
illumination and made sure that we understood every variable
in the equation.
3.2 Attitude and
orbit control
We then focused on the control
system for keeping the satellite in a particular
orientation (attitude). We talked
about the sources of rotational torque. We learned
about Earth, Sun and star sensors.
20 October
2021
Midterm exam
22
October 2021
We learned how an erroneous attitude is
corrected. We then focused on the spin
stabilization. Nutation and precession are
problems in the stabilization efforts. Nutation dampers
are used in spinner satellites. For the so-called box
car satellites, three-axis stabilization is used where momentum
packages, magnetic
torquers and/or jets are important components. Click
also here for some explanation from NASA.
We looked at the engineering model of the RadioAstron
spacecraft. We also saw the focus
cabin, the
flywheels, the gas
tanks and the gas
jets. We covered issues
related to the orbit control and compared the fuel allocations
for various maneuvers.
25
October 2021
3.3
Telemetry, tracking and command (TT&C)
We talked about the measurement of the orientation, the
environment and the health of a satellite
and how these measurements are transmitted. Then we
covered issues related to the tracking
of a satellite. We talked about how commands are executed.
We learned about allocated
satellite bands:
3.4 Transponder
We learned about uplink and downlink.
27 October
2021
We
interpreted the block diagram of a transponder.
29 October 2021
We discussed the TWTA and
compared it to the amplification in the magnetic field
of a pulsar. We learned about solid-state power
amplifiers, FETs and HEMTs. We discussed
intermodulation noise, how it is generated and how it affects
the spectrum.
1 November 2021
We covered three types of antennae, the wire
antenna or Hertz dipole, the horn antenna and the
reflector antenna. The Hertz dipole was the antenna
Heinrich Hertz used to discover radio waves. Click here for a
description of the dipole and how an EM wave in space
pushes electrons in a dipole back and forth and
generates a voltage between the two elements of a dipole
and a current through them. The most famous horn
antenna is the one Penzias and Wilson used to improve
satellite communications and found by chance the cosmic
microwave background radiationwhich is the
strongest evidence we have for the Big Bang theory.
Small versions of the horn antenna are similar to feeds,
or feed horns, which
are used on satellites for a broad coverage and also in
the focal area of reflector antennas.
3 November 2021
Reflector
antennas are used on satellites to
direct information to small areas of the Earth.
Large reflector antennas are shown here and here .
We talked
about spot beam technology and the radiation pattern or beam
pattern or
the beam of a telescope. We also learned
about the gain of a telescope and the degradation of the gain
as a function of the rms surface variations with respect to a
perfect paraboloid.
Click here for a 3-dim
beam pattern of a paraboloidal antenna. Click here for figures
of beam patterns as a function of gain and angle.
We looked at the gain
of an antenna and, in particular, considered the
roughness of the surface of the reflector and how
that influences the gain.
That finished Chapter 3.