The objective of this assignment is to demonstrate an initial
understanding of different payload classes and how they drive the
design of the satellite bus and the overall space system and mission
In this class, we've talked about different types of space missions,
with four of the primary categories being Navigation spacecraft,
Observation spacecraft (which may look at the Earth or out into space),
Communication spacecraft, and Exploration spacecraft.
In this assignment, you will work on your own to learn more about
these systems in order to supplement the presentations made during
NAVIGATION - During lecture, we will spend little time
discussing the ways that navigation satellites work. These questions and
their references will introduce you to satellite navigation technology.
You should definitely work your way through the outstanding Trimble
Navigation's GPS Tutorial; additional references include the Wikipedia
SatNav Overview, the Global
Positioning System Overview, and the Wikipedia
Overview of GPS. Other references are certainly available via the
1. First, you should certainly be aware that satellite-based
navigation is only one method of navigation; many others exist and have
been used for far longer. List three navigation methods that do not rely on spacecraft.
Landmarks, celestial, ded reckoning, LORAN
radio navigation, etc.
2. As for satellite navigation services, you are certainly aware of
GPS. But there are others, and it is worth being aware of at least
a little information about them:
i) What was the name of the first operational satellite navigation
system, who used it, and over what time period did it operate? What type
of orbit were the satellites in? What was the approximate accuracy
of the system? Briefly describe how a navigation solution
TRANSIT, US Navy, 1960-1996. 5-10 satellites in polar
LEO ~ 600 nm. 1 satellite had to be in view to get a position fix.
Often, none were in view.100-200 meters accuracy when available.
To obtain a position fix, the Doppler s-curve was obtained by listening
to the satellite, and the shape was analyzed to determine position on
ii) What country runs GLONASS? How many satellites are in the
system, and in what orbits are they? How accurate is the system's
Standard Precision (SP) service?
Russia. ~24 in 3 MEO (12-hour period) orbit
planes. ~70 meters.
iii) What country(ies) is (are) developing Galileo?
When will this system become operational? How accurate is the system
intended to be? Why is this system being developed given the global
availability of GPS?
European Union. Slated for operation
in 2020. Accurate to a meter. Developed in order to have a positioning
system independent of US in case of political disagreement.
3. The original operational GPS system used 24 satellites in 6
different orbital planes (although more satellites are now operational).
i) Why are so many satellites used and deployed in different planes?
In order to achieve complete global coverage
with multiple satellites (5 or 6) in view of any location at one
ii) GPS receivers use "trilateration" in order to determine
their location in space. Ideally, 3 trilateration measurements would be
required to fix a point in space. GPS, however, uses 4 such
measurements. What is the primary reason for doing this?
4 measurements provide an accurate fix even
with inexpensive clocks in the receivers (needed to make them practical
in terms of cost).
iii) One technical demand of the GPS system is high precision
maintenance of the orbits of the individual spacecraft (which also
requires high precision sensing of their position using techniques other
than GPS). Explain why this is critical by describing how this
requirement leads to good performance in terms of a GPS receiver being
able to determine its position.
Trilateration measurements are from the
"known" locations of the satellites. Any errors in these
locations lead to solution errors. So, high orbits (away from affects of
drag) are used, satellites are maintained in precise orbits through
precision tracking and use of propulsion, and any errors from their
predicted locations are immediately distributed to the receivers via the
GPS signals that are transmitted to the ground.
iv) What is GDOP and why does the geometric
relationship between the receiver and the transmitters make a difference
in the precision of the receiver's position estimate?
Geometric Dilution Of Precision
There are errors in using time to
determine the actual distance from a satellite. The manner in
which the errors from multiple satellites add together depends on the
different directions from which the signals arrive, and therefore the
geometry of the receiver/satellite system. If more than 4
satellites are in view, choose the satellites that give the best
relative geometry... the lowest "dilution" of precision.
4. To quantitatively explore the concept of
multi-lateration, let's do a few simple 2-D "bi-lateration" planar examples. Imagine
that there are two signal sources, each broadcasting a signal that moves
at 1 unit per second. One source is at the origin of the Cartesian
plane, (0,0). The second source is at (10,0).
i) Your receiver states that you are 7.07 seconds
away from both signal sources. What are your possible locations
in the Cartesian plane?
(5, 5) and (5, -5)
ii) Your receiver states that you are 4.24 sec from
the first source and 7.62 sec from the second source. What are
your possible locations in the Cartesian plane?
(3,3) and (3,-3)
iii) Your receiver states that you are 13.42 sec
from the first source and 12.65 sec from the second source. What
are your possible locations in the Cartesian plane?
(6,12) and (6,-12)
REMOTE SENSING - These questions relate to the phenomena,
technologies and associated system sizing relationships for remote
sensing missions. A primary reference for answering these questions is
Section 11.2 of Understanding Space. Other references exist, of
course, and you are encouraged to seek them out as necessary.
4. In the context of remote sensing, briefly explain the difference
between a passive and an active sensing system.
Active provides its own energy that is
transmitted, reflected and received. A passive receives energy provided
by a different source, perhaps visible light reflected by the target or
infrared energy radiated by the target.
5. How do high-performance infrared sensors drive the thermal design
of a satellite, and why is this the case?
Sensors must be cooled to 77-120 deg K in order
to be sensitive enough to detect small amounts of infrared energy. This
is cold, and it often requires cryogenic thermal design approaches,
liquid nitrogen, active refrigeration, etc.
6. You have an option of putting your remote sensing satellite into
either an LEO orbit (altitude of 600 km) or a geosynchronous orbit
(altitude of about 35,800 km). At geosynchronous altitude, your
camera system is capable of 10m resolution. What resolution would
the same camera system achieve in the LEO orbit? Refer to Eq 11-8 in Understanding Space.
As per Eq 11-8, the ratio of resolutions is
the ratio of altitudes s.t. 450/35800 = 0.0168. So, if the geo
satellite had a resolution of 10m, the LEO satellite would have a
resolution of 0.168m (16.8 cm). You could also say that the LEO
resolution was nearly 60 times better (1/.0168=59.67).
7. Many hot objects emit infrared energy and are interesting
enough that we want to find and track them from space; things like
forest fires, rocket plumes, etc. Given the information in the
Sellers chapter as well as that from the thermal subsystem lectures
about radiation, consider a object that is 1,000 deg C., and for the
purposes of this problem assume that it is a perfect blackbody radiator.
MIND YOUR UNITS!!!
- What is its radiated power per m2 of external
J=sigma*T4 = 149 kW/m2
- What is its wavelength of peak emission?
lambda = 2898/T microns = 2.28 microns
- If you were to design an observational payload
tuned to this wavelength (from part (b)) with 5 meter resolution at
an altitude of 450 km, what aperture diameter would be required?
8. Briefly describe how the design of a remote sensing system
effects other subsystems for the following situations:
- Consider an active microwave radar. Compared to a
visible camera with a 1m instrument resolution, how does the radar
payload affect the overall mechanical design and configuration of
Wavelength is ~500,000 times greater, which
means the aperture diameter is ~500,000 times greater. This is
huge, leading to enormous radar apertures - so BIG systems - since it
isn't feasible to scale that much, radar satellites typically
operate at lower resolutions and/or use active synthetic aperture
approaches to establish an effective diameter that is much larger
than the physical size of the satellite's antenna (the effective
diameter is associated with the distance the satellite travels
during the flight time of the radar signal - in addition, images are
often created using multiple 'snapshots')
- How would the communication
subsystem be impacted by a
mission that required large quantities of high resolution,
Downloading this amount of information in
realtime is an enormous driver, affecting the data rate of the
communications link, the availability of ground stations, probably low
Earth orbit which means many stations around the world to ensure
realtime connectivity, etc.
- Would you expect a high precision remote
sensing spacecraft to use a spin-stabilization attitude control
technique? Why or why not?
Probably only with a despun platform. More
likely, it would be a 3-axis stabilized satellite with precision
COMMUNICATION - The questions below relate to major design
considerations when developing high power communication missions.
Relevant references are noted below.
9. High power communications spacecraft need to generate a LOT of
power, and they thermally dissipate a LOT of power. From the
thermal perspective, consider a thermal balance for a spherical
communication spacecraft with a 1 meter diameter in LEO. Following the white
paint example shown on Slide 42 of the Thermal Subsystem lecture, the steady state temperature
with 0W of internally dissipated power was -67 deg
- Now, assume that the satellite dissipates
20 kW of power.
Rerun the analysis and find the new steady state temperature.
Assume e=0.9. Is this a significant change that would require
a complete rethinking of the thermal design?
341 deg C = 614 deg K , yes, of
course it is a significant change!
- With white paint, the absorptivity to
emissivity ratio is already close to a practical limit.
Another strategy is to increase the area that the satellite radiates
to space in order to lower the steady state temperature. What
would the spherical diameter need to be in order to achieve a
satellite temperature of 300 deg K given the internal dissipation of
20 kW of power? Is this a significant change?
Think about what this implies for the size of satellites (and/or
their radiators) that are very high power.
4.6 meters or so.
This is a very significant impact on launch
volume, moving heat to the external radiator surfaces, structural
10. The Link Equation, shown below, relates the quality of a
communication link (in terms of signal to noise ratio) to many of the
driving contributions such as broadcast power, antenna gain, losses over
distance and through the atmosphere, etc.
- E/N is signal to noise (we want this to be large)
- Pt is transmit power
- Gt and Gr are the antenna gains of the transmit and receive
antennae, respectively, and they are proportional to the area of the
antenna for broadcasts at a given frequency
- Ls is space loss, which is proportional to 1/(distance)2
for broadcasts at a given frequency
- La is attenuation loss due to absorption in atmosphere, etc.
- Ll is line loss through transmission wires
- k=Boltzmann's constant
- Ts is the system noise temperature
- R is the data rate
More information on the Link Equation is found in the MECH
371 presentation supplement on this subject. There are many things to
learn regarding where this equation comes from and how it is used in the
design and analysis of communication systems. However, using this
equation as a rough guide, we can determine some of the big trade-offs
when it comes to designing a communications payload (as well as the
communications subsystem for a satellite with a different payload).
- If the altitude of the satellite is
cut in half, what is the
effect on E/N when the satellite is directly overhead (assuming all of the parameters remain the
same)? [increases by a factor of 4]
- If you need to double the E/N, what could you do with
each of the following parameters, and if you did this, what would
the "cost" be in terms of bus subsystems or the
- the transmit power -
double the power,
but this may require you to dramatically increase power
generation and storage; may also increase radiator size since
power dissipation goes up
- the data rate - you could cut this
by a factor of 2, but this may affect the quality of the mission and
quantity of data products
- the satellite antenna area (you will need to
understand the relationship between antenna area and gain) -
area, but this would require a larger antenna which drives
structural design, mechanical configuration, mass for
propulsion, inertia for attitude control, etc.
SCIENCE - These questions relate to a generic
planetary spacecraft, which for our
purposes represents a "science class" satellite mission.
11. As with other spacecraft, the requirements and characteristics of
planetary missions drive the design of the bus subsystems/components.
For a generic planetary mission, consider how various subsystems of the
planetary craft would differ significantly compared to those of a
satellite designed for a low Earth orbiting mission.
example, for propulsion (which we study in MECH 371), planetary
spacecraft often require significant fuel and more substantial thrust
systems (compared to an LEO mission) for achieving its interplanetary
trajectory - this increases mass, requires thermal management, etc.
For each of the subsystems listed below, provide a few
similar thoughts on the primary effects and considerations:
- Power generation -
significantly different distances from sun - so perhaps great solar
generation at Mercury or Venus, or perhaps much worse thereby
requiring RTGs other other generation mechanisms
- Attitude sensing and control -
we often use properties of the Earth for sensing (IR signature of
Earth, Earth's magnetic field, etc.) and control (magnetic field,
etc.); this would have to be modified appropriately
- Communications systems, both on-board and on the ground -
longer than normal distance requires large dishes (a la the DSN) on
- Thermal - doesn't
have the Earth albedo or IR input which often is used to help keep
things warm using passive techniques during cruise phase to planet;
direct solar would vary dramatically and depend on whether you're
getting closer or farther away from the Sun; would need to know IR
and albedo environment of the new planet once you get there
- Since you may need large antenna, RTGs, propulsion units, etc.,
there would be significantly different demands on the structure in
terms of accommodating large assemblies, etc.