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MECH 372 / ENGR 372
Space Systems Design and Engineering II
 
Assignment #7 - Payloads
 
 
Objective: 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 architecture. 

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 lecture.

 

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 web.

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.

 

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 was obtained.

 

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? 

 

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?

 

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? 

 

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?

 

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.

 

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?

 

  

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?

 

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?

 

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?

 

   

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.

 

5. How do high-performance infrared sensors drive the thermal design of a satellite, and why is this the case?

 

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.

 

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!!!

  1. What is its radiated power per m2 of external area? 

 

  1. What is its wavelength of peak emission?

 

  1. 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:

  1. 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 the satellite?

 

  1. How would the communication subsystem be impacted by a mission that required large quantities of high resolution, realtime imagery?

 

  1. Would you expect a high precision remote sensing spacecraft to use a spin-stabilization attitude control technique? Why or why not?

 

 

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 C. 

  1. 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?

 

  1. 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.

 

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.

where

  • 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).

  1. 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)?  
  2. 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 mission?
    • the transmit power  
    • the data rate  
    • the satellite antenna area (you will need to understand the relationship between antenna area and gain)  

 

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.  

For 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:

  1. Power generation  
  2. Attitude sensing and control  
  3. Communications systems, both on-board and on the ground  
  4. Thermal  
  5. Structures