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Can-Sat

Midterm Design Review

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March 8, 2002

The Department of Aerospace Engineering and Engineering Mechanics

The University of Texas at Austin

Austin, Texas 78712

Team CanSat

ASE 463Q

University of Texas at Austin

March 8, 2002

Dear Dr. Stearman,

The following paper is the midterm report for our CanSat project. Our main goal is to create a pico-satellite the size of a 12 ounce coke can with transmitting and receiving capabilities. Our team has broken down our objectives into three main systems for our satellite: structure, data telemetry, and sensors. The work we have completed thus far in each of these subsystems is thoroughly discussed here, as well as the work intended to be completed in the future.

If you have any questions regarding this report or the CanSat project in general, please feel free to contact us in WRW 407, or at our individual email addresses listed below.

Sincerely,

David Dominguez

ddavid@mail.utexas.edu

Robert Harpold

q_kumber@mail.utexas.edu

Shaun Stewart

w.sam@mail.utexas.edu

Abstract

This report summarizes the progress of the Can-Sat project, as an initial satellite design initiative at The University of Texas Satellite Design Lab (UTSDL). The main objective of this project is to demonstrate that small, inexpensive satellites can be built, tested, and launched in the span of a year. A major obstacle in designing satellite missions is the ability to implement a reliable, inexpensive data telemetry system for control and data relay communications. This is mainly due to the fact that aerospace engineering undergraduate students are generally not adequately trained for the design of digital packet communication systems. In order to allow for the establishment of an ongoing satellite design program at The University of Texas, the main goal of this project is to develop and document a small, inexpensive satellite data telemetry system that future groups can incorporate in improved satellite designs. This report summarizes the details of that design.

Acknowledgments

The CanSat team would like to acknowledge the following people for their help with our project.

• Dr. Noel Clemens

• Dr. Takuji Ebinuma

• Dr. E. Glenn Lightsey

• Dr. Ronald O. Stearman

Table of Contents

Index of Tables 5

Index of Figures 6

1.0 Introduction 7

1.1 Problem Statement 7

1.2 Project Background 7

1.3 Objectives 10

1.4 Previous Missions 10

1.5 Can-Sat Group Task Assignments 11

2.0 Background Theory 13

2.2 Communication System Design 14

2.3 Amateur Radio Protocol 15

2.4 Signal Structure 15

Analog vs. Digital Sensor Theory 16

3.0 Current Progress and Future Goals 17

3.1 Circuit Design 17

3.2 Sensor Testing 17

3.3 Structural Design 17

Requirements 17

Material 18

Current Design 18

Other Considerations 20

Antenna Choices 21

4.0 Cost Analysis 22

5.0 Project Schedule 23

6.0 Conclusions 24

7.0 References 25

Appendix 26

Index of Tables

Table 1: Communications Summary………………………………………………….11

Table 2: Power Requirements………………………………………………..……….15

Table 3: Comparison of Commonly Used Alloys…………………………………….20

Table 4: Temperature Limits for Drag-Sat………………..…………………………25

Index of Figures

Figure 1: Contraction of Orbit Under Drag……………………………………………5

Figure 9: Preliminary Satellite Design…………………………………………..……21

Figure 10: Internal Frame Design………………………………………………….…22

Figure 11: Drag-Sat Functional Interfaces………………………………………...…23

1.0 Introduction

This report presents the preliminary design results of the Can-Sat project. The design is a work in progress and is scheduled for completion and launch by July 2002.

1.1 Problem Statement

Design a small, can-sized satellite with command and control capabilities that will transmit basic analog and digital measurements (made on the satellite) to the Earth on request. The design should be as inexpensive and simple as possible.

1.2 Project Background

1.2.1 University of Texas Satellite Design Laboratory (UTSDL)

The motivation for this project came from an initiative by Dr. Glenn Lightsey to create a satellite design lab at The University of Texas at Austin and to use student-built spacecraft to perform experimental measurements in space. Once the Can-Sat project is underway, Dr. Lightsey plans to oversee the construction of a permanent ground station at the W. R. Woolrich Laboratories Building at the University of Texas at Austin. This ground station will include a steerable antenna array on the roof of the building with an observation station in the satellite design lab. Once an inexpensive, compact satellite design is built and tested, the systems developed can be extended to experiments for both the Shuttle GAS program and the Cube-Sat program. These programs allow students to conduct experiments in space.

1.2.2 Earth-Orbiting Experiments

Get-Away Special

It was recently revealed that the University has had reserved, since the 1970’s, a canister on the Space Shuttle for experiments in space. The Get-Away Special (GAS) canister gives the University an opportunity to run an experiment at 280 kilometers altitude. This program provides an opportunity for exposing potential satellite designs to the harsh environment of space before actually launching an Earth-orbiting satellite mission.

Cube-Sat

The Cube-Sat program allows university students to launch 10 cubic centimeter satellites missions into orbit. The cost of launching a satellite through this program is $50,000. The ultimate goal of UTSDL is to implement student-built spacecraft experiments in Earth or Earth-moon orbit. However, a reliable and robust design is required before committing to the expense of launch into space.

In order to successfully implement a satellite design in space, thermal control, attitude control, and power generation become major concerns. In all these cases, the spacecraft requires command and control capability for housekeeping purposes such as antenna and solar panel pointing, station-keeping, and power consumption control. Even if the expense associated with launch is subsidized, such as in the GAS program, it is imperative that the communication system onboard the satellite be reliable, robust, and tested so that these minimum control requirements are possible.

The Can-Sat sounding rocket program has been chosen to test the individual subsystems for these eventual spacecraft, since the cost of launch is minimal ($600). Through this program the satellites are launched to an altitude of 12,000 ft for a fifteen-minute descent. This test will subject the electronics to loads similar to those for launch into space, and will allow for an extended test at high altitude.

3 Can-Sat

The Can-Sat program was initiated in 1998 to give students hands-on experience in the design of spacecraft and to determine if small satellites can perform useful functions. Several universities have participated in the project since its beginning, including Stanford, The University of Texas at Austin, The University of Tokyo, and Tokyo Institute of Technology.

The Can-Sat program allows university students to launch 12-ounce coke-can-sized satellites from a sounding rocket complex in New Mexico with minimal launch costs. This project allows for the design and testing of a compact, inexpensive data telemetry system under extreme launch loads and for conditions near 12,000 feet altitude. This project will provide not only a premier challenge for UTSDL, but also a platform for the future design and implementation of Earth-orbiting spacecraft.

2. Objectives

The Can-Sat mission objectives are listed below in order of significance:

• Collect and transmit data.

• Demonstrate command and control capability.

• Show that an inexpensive design will survive launch loads.

• Create prototype data telemetry system and signal design for future implementation and improvement.

The main objective of the initial Can-Sat project is to show that the design of a small, inexpensive data telemetry system is possible. The system will have both receiving and transmitting capabilities and will be incorporated with a complement of basic analog and possibly digital sensors. Although the sensor data is important for verification of the telemetry objectives, achieving individual sensor precision is not a primary goal of this Can-Sat. The success of this project at UT will hopefully pave the road for the funding of annual Can-Sat design projects with improved sensor packages and mission objectives.

1.4 Previous Missions

Several universities have completed CanSat projects in the past, each with different missions. The University of Tokyo’s project, Gekka-Bijin, consisted of three CanSat’s that attempted to rotate a satellite, gather temperature and pressure data, and to use a camera. They have also designed a satellite to test systems that will later be used in a CubeSat project. Other universities such as Arizona State University and Kyushu University have designed satellites to test a tracking system and to collect temperature data.

Most of these projects are used simply to give students experience in a hands-on project. They must deal with ordering equipment, designing systems, making reports, and working in a group. This will give them an idea of what to expect when they enter the work force.

UTSDL has considered some of these previous designs when working on our own project. The structure for our satellite will closely resemble some of the other designs. We are also researching components that other projects have used to determine if any will be appropriate for our satellite design.

1.5 Can-Sat Group Task Assignments

To handle the workload required for this project, the responsibilities were split between the three group members. Shaun is in charge of programming the chips to work with each other, David is in charge of handling the components (such as the sensors and the circuit board), and Robert is in charge of the structure of the satellite. The members are not restricted to working only in their area, but they specialize in one area and are responsible for making sure that area meets its requirements.

The hardware integration and testing will be the responsibility of all group members. In addition, everyone contributes to the written and oral reports.

2.0 Background Theory

The main theoretical background required for the design of the CanSat involves circuit theory. A firm understanding of the relationship between current and voltage and the role of capacitors, resistors, and inductors in circuit design is imperative for the completion of this type of project. There are also issues with converting a radio frequency signal to digital data and programming microchips to carry out those conversions. It requires understanding of how binary data is stored in data registers, the different types of registers, the implementation of interrupt logic, and amateur radio protocol for transmitting data. Most of these topics are more closely related to electrical engineering than aerospace engineering and are thus outside the bounds of an aerospace undergraduate preparation. There has been a large learning curve associated with these topics and given below is a brief synopsis of those topics and recommendations as to what future CanSat groups may need to research before attempting to improve on this CanSat design.

2.1 Circuit Theory

It is assumed that any student working on the CanSat project has at least a minimal understanding of circuit theory. The circuitry for the satellite will involve inductors, capacitors, resistors, and op-amps among other electronic components. Each of the sensors require 5 volts (V) each to function properly, and therefore a step-down transformer will change the output of 9V batteries to provide the correct amount of voltage to each.

2.2 Communication System Design

Figure 2.1 shows a basic schematic of all the major components required for the data telemetry system. For simplicity, the interfacing resistors and capacitors have been omitted. The data sheets for each chip, as well as a complete circuit schematic, are provided in the Appendix. This telemetry system is much like the modem on a computer. The radio signal can be thought of as sound tones from a telephone port. Sound tones are transmitted from the ground and received by the antenna on the satellite. A decoder chip (CM8870 in this case) converts the sound tones into four-bit binary numbers and sends them to the micro-controller. Pre-programmed definitions for the appropriate command signals will be continually compared against all incoming signals, so that errant signals on the appropriate frequency cannot switch off any of the satellite subsystems. The details of the signal structure will be discussed in the amateur radio protocol and signal structure sections.

After the micro-controller interprets the signal, it will begin taking sensor data (analog and digital in this case), convert the inputs to 8-bit binary numbers, and send them to an encoder chip. The encoder chip converts the binary numbers to their corresponding sound tones and then sends them to the radio-frequency (RF) transmitter for transmission to the ground station. Future CanSat projects may opt to store data on the CanSat itself in memory for analysis after recovery of the satellite. This design has reserved pin locations for that purpose even though data storage will not be an objective for this mission. Once the transmitted data reaches the Earth and is received by the ground station receiver, the data then needs to be decoded once again so that it can be interpretted on a PC. Depending on the quality of the receiver purchased, this may or may not require the design and programming of a second decoder chip.

2.3 Amateur Radio Protocol

In order for a commercially built receiver to receive and interpret RF data signals, amateur radio protocol, AX.25, must be followed. There is a predetermined manner in which AFSK (Audio Frequency Shift Keying) packet communications must be designed so that commercial amateur radio receivers and transmitters will recognize a signal. The specifics of these requirements remain to be determined; however, they will be included in the final report.

2.4 Signal Structure

There are 16 possible tones for RF communication. Each sound corresponds to a particular 4-bit number that can be used to compose words, much like the letters of the alphabet. The tones are the same as the 12 available on a telephone, except there are four extra, named: A, B, C, and D. Currently, we are planning to use three-tone command words. The commands will begin with ‘*’, [1 0 1 1], and end with ‘#’, [1 1 0 0]. If the satellite receives a 3-tone signal on the appropriate frequency starting with ‘*’ and ending with ‘#’, then the middle tone will be implemented as a command. This provides control safety in that stray RF signals, which may correspond to commands, will have to be validated in this manner to before implementation. Future groups may expand on this design, but this is the minimum required to demonstrate command signal validation.

2.5 Analog vs. Digital Sensor Theory

At this time, our CanSat only has analog sensors. Once these sensors have been integrated, we will consider employing digital sensors as well. The difference between these types of sensors is discussed below.

An analog sensor records data continuously, while a digital sensor defines data in several individual “steps”. The main advantage of an analog sensor is its ability to fully represent a continuous stream of information without eliminating any information. Digital sensors, on the other hand, are less affected by unwanted noise and interference, and therefore provide a clearer signal. These types of signals are often converted from one to the other. For example, this conversion occurs every time a signal is sent over a phone line. A transmitting (encoding) modem converts digital data from a computer to analog sounds and the receiving (decoding) modem then converts the analog signal back to the original digital data for analysis. Basically, analog signals are used to transmit data over long distances and digital data is required for computer analysis.

3.0 Current Progress and Future Goals

3.1 Circuit Design

Currently, an analog-digital converter has been programmed and tested. A variable input voltage to digital output on LED port

Generate sound tones on PC and convert to 4-bit binary data with decoder chip

Output corresponding command code to LED or to microphone as tones

Objective: send 3-tone command from PC to micro-controller and output the operation to the LED or microphone

Command signal = * B #

Command operation = B = 1 1 1 0

Verify recognition of command and appropriate interpretation of command operation

At this point, it is undecided whether we will build and program the encoder chip from scratch or whether we will use a preprogrammed encoder chip (MIM module). A preprogrammed chip would be more expensive, although time constraints may not allow for a design from scratch.

3.2 Sensor Testing

As stated, one of the main objectives of the CanSat project is to create a pico-satellite that can send and receive messages. In order to test the communication system, our satellite will conduct a basic environmental analysis by taking temperature and pressure measurements with analog sensors. The accuracy of the sensors is not a major concern, as our main objective is to establish a sound telemetry system. The data gathered will be used to show that the satellite telemetry system worked properly. In order to ensure that some data is collected, even in the case of command signal failure, one of the sensors may be configured to transmit data prior to launch. The sensors selected for this project are discussed below.

Pressure: MPX4115A

The Motorola MPX4115A (Figure 3.1) is an analog silicon pressure sensor capable of measuring pressure in the range of 15 to 115 kilo-Pascals (kPa). For a 5 volt input, the output voltage conversion factor to kPa is 0.04 volts per kPa, or

P(kPa) = 25*Vout (1)

This sensor can provide a measurement accuracy of (1.5 kPa and can operate in temperatures between –40 and 125 degrees Celsius ((C). It is ideally suited for microcontroller-based systems, and therefore optimal for our project.

A basic pin diagram of the MPX4115A can be seen in Figure 3.2. The output from pin 1 will lead to the micro-controller through the setup shown in Figure 3.3. The capacitors present in this schematic are intended to eliminate some of the noise picked up by the analog sensor and provide a cleaner signal.

Since it is difficult to manually modify pressure, there is no simple way to calibrate a pressure sensor. The graph seen below is a conversion chart for this sensor. Pressure readings have been taken with this sensor at an altitude slightly above sea level, and a pressure 0.95 atmospheres (atm) was measured. Further tests will be conducted to validate the accuracy of the sensor and the conversion chart.

Temperature: LM335Z

The LM335Z, seen in Figure 3.4, is an analog temperature sensor from National Semiconductor. The output voltage can be related to degrees Celsius with the following equation:

T((C) = (Vout*100)-273.15 (2)

The LM335Z has the ability to take measurements in the range of –40 to 100(C with an accuracy of ( 1(C. It is very small and easily integrated with a micro-controller, and therefore ideal for our objectives.

The pin diagram of the LM335Z is seen in Figure 3.5. Readings from the temperature sensor will be taken with the setup seen in Figure 3.6. The value of the resistor in this circuit is 1 k-ohm.

3.3 Structural Design

A preliminary design for our structure has been completed, based largely upon previous CanSat projects from other universities. Its final dimensions will depend on the mass of its components, as the total satellite mass must be less than 350 grams. We are also considering another design that may provide an adequate amount of support while having less mass. A stress analysis remains to be conducted to determine if the alternate design is structurally adequate.

Requirements

In accordance with the rules for the CanSat project, the satellite must be able to fit inside a soft drink can and must weigh equal to or less than an unopened coke-can filled with soda. This limits the dimensions to a maximum diameter of 6.604 cm and a height of 10.3 cm. The communications antenna can slightly exceed the dimension restrictions. The mass, including that of the components, is limited to 0.350 kg.

The structure must also be capable of withstanding the loads it will be subjected to during flight. A maximum load of 40 g’s will occur at parachute separation, so the structure must be designed to endure this acceleration. In addition to the dimension and load requirements, the structure must be able to hold the equipment necessary to complete the CanSat’s tasks: a transceiver, an antenna, and a circuit board.

Material

Aluminum was chosen for the material because it is light (the density varies between 2600 and 2800 kg/m3), inexpensive, readily available, and has a relatively high yield strength (35-500 Mpa). While steel has a slightly higher yield strength (280-600 Mpa), it has a much greater density (7850 kg/m3) than aluminum, making it undesirable from a mass perspective. Other possible materials are much more expensive than aluminum and steel.

Current Design

The current design, which is similar to previous CanSat projects from other universities, contains a main support separating top and bottom plates. With the dimensions given in Figure 3.7, the structure can hold the equipment, fit inside a soft drink can, and withstand 40 g’s. The dimensions may, however, be changed, if the components have enough mass to push the design over the mass limit. A model has been constructed with the dimensions given in the figure.

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Figure 3.7. Structure Dimensions in cm., 1:10 scale.

The transceiver used for this satellite has dimensions of 5.6 cm x 9.4 cm x 1.36 cm, disregarding protuberances. The main support has to be at least 9.6 cm tall to enclose the transceiver and its antenna. This constraint limits the thickness of the top and bottom portions to 0.35 cm each.

Preliminary calculations have been done to determine the lowest area that would be able to withstand the maximum stress. With a yield strength of 35 MPa and a maximum acceleration of 40 g’s, the following calculations were performed:

σy=[pic] (3)

where F is the force and A is the area.

F=ma= 0.35 g * 40 * 9.805 m/s2= 137.27 N

σy=35 MPa

A= F/ σy = 3.922e-6 m2 (4)

Our can has no points with an area lower than the calculated amount, so it should be able to withstand the highest stress. This was also a conservative calculation: we used the highest stress and the lowest yield strength, so we will actually be able to have a lower area and still not have structural failure.

Another potential design is shown in Figure 3.8. This is roughly the same as the current design, but it would have a much lower mass and would also be easier to build. Rather than machining it from one piece of aluminum and drilling holes in the top of it, this design would attach several smaller pieces with nuts and bolts. This structure will be analyzed in the following week to determine if it is strong enough for the predicted loads.

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Figure 3.8 Alternate Structural Design

Other Considerations

The satellite will use a parachute to land safely. As the outer shell of the soft drink can would not be able to withstand the force of the parachute separation and will not be used for structural support, the parachute must be attached directly to the structure. To do so, a hole will be drilled through the top of the structure through which an aluminum ring will be attached with a nut and bolt. This ring will protrude from the mouth hole of the can shell.

An antenna will also need to be attached to the satellite. The method for doing so has not yet been decided. The team is considering either wrapping the antenna around the structure or attaching it to the top disc.

Two methods for inserting the satellite into the can skin are being considered. A model has been constructed to test those methods and to determine the appropriateness of the current dimensions. The first method is to cut open the soft drink heightwise and put the satellite in through the side. The second method is to cut off the top of the can at its widest point and put the satellite in vertically. Previous CanSat projects from other universities have taken off the can’s top altogether and just used an aluminum plate.

Once the design has been finalized, the dimensions will be sent to the WRW machine shop so that it can be built.

Antenna Choices

In order to communicate between the ground station and the satellite, antennas will be required. The satellite and ground station will most likely use different antenna types. As of yet, no final decisions have been made as to what type or brand to use, but some preliminary research has been conducted.

A variety of antennas are used for diverse applications. The main types under consideration are half-wavelength dipole, quarter-wavelength vertical, ground plane, whip antennas, and directional. Each one has advantages and disadvantages to separate it from the others.

Half-wavelength dipoles are composed of two equal parts, totaling half the wavelength of the signal. For radio frequencies, this size will be too long for the CanSat. This type of antenna generally depends on placing wires in the ground and so is probably not practical for our ground station, as it will not be permanent.

Quarter-wavelength vertical antennas are relatively easy to build and inexpensive. As they are vertical, they have more powerful signals towards the horizon and are almost a manageable size. For a signal of 440 MHz, the length would be 17.5 cm, which is larger than the satellite. This might work if it were wrapped around the satellite, with only the antenna’s tip protruding from the can’s mouth hole.

Ground-plane antennas are good for mobile applications and are being considered for the ground station. They can be attached to a car roof and are omnidirectional.

Whip antennas are flexible and come in many sizes. Other CanSat projects have used whip antennas for their satellites. Since a whip antenna is flexible and is thus less susceptible to snapping, it might be a good choice for the CanSat.

Directional antennas direct most of their energy in one direction and have good gain for that direction. Some of the previous projects have used a common directional antenna, a Yagi, for ground stations.

We will make the final decision on our antenna design the week after Spring Break after consulting our advisor, Dr. Ebinuma.

4.0 Cost Analysis

One of the goals of the CanSat project is to make the satellite as inexpensive as possible. Table 4.1 shows a breakdown of the purchases made so far, and also includes those anticipated in the future. The cost to launch the satellite is the most expensive aspect of this project, at approximately $600.

Table 4.1 Cost Analysis

|Item |Cost |

|Electrical Components (resistors, capacitors, etc) |$100 |

|AVR Testing Board |$80 |

|Sensors |$20 |

|Power Supply |? |

|Soldering Iron |? |

|CanSat Launch |$600 |

|Tranceiver (in satellite) |$150 |

|Ground Station |$500 |

|Antenna |$20 |

|Parachute |$20 |

|Total |$1490+ |

5.0 Project Schedule

Table 5.1 shows the proposed schedule for the remainder of the CanSat design. It should be noted that a summer group will complete the testing of the satellite and oversee the launch. All dates are subject to change.

6.0 Conclusions

The CanSat program was developed to give students an opportunity to participate in the design of actual hardware for a real mission. Although our satellite will take temperature and pressure readings during its flight in mid-July, its main purpose is to serve as an educational experience. In addition, our project will provide a starting point for future UTSDL CanSat projects.

Much work remains to be done on the project. So far, we have tested and calibrated the sensors, worked on the design for the main structure of the satellite, and started programming the chips. In the coming three weeks, we plan to have all of our components and hardware designed and acquired. That will give us time to integrate everything in the final satellite. An extra week has been reserved for unforeseen problems associated with the integration process. The following week, the satellite will be tested on the ground by sending signals to it from our ground station components and receiving information sent by the transceiver.

7.0 References

[1] Lightsey, Glenn, Student Produced Atmospheric Research Cluster of Satellites (SPARCS), The University of Texas at Austin, September 2001.

[2] C. C. Chao, G. R. Gunning, K. Moe, S. H. Chastain, and T. J. Settecerri, An Evaluation of Jacchia 71 and MSIS90 Atmosphere Models with NASA ODERACS Decay Data, The Journal of Astronautical Sciences, Vol 45, No. 2, April-June 1997, pp. 131-141.

[3] Vallado, David A., Fundamentals of Astrodynamics and Applications, The McGraw-Hill Companies Inc., 1997.

[4]Larson, W., Wertz, J., Space Mission Analysis and Design, Kluwer Academic Publishers, Dod, 1997.

[5] Bird G. A., Molecular Gas Dynamics and the Direct Simulation of Gas Flows, Oxford University Press, 1994.

[6] , world wide web, November 2001

[7] Lu, Richard A., Modifying Off-The-Shelf, Low Cost, Terrestrial Transceivers for Space Based Applications, SSDL Stanford California, June 1 1996.

[8] Niederstrasser, Carlos Guillermo, Development of a Satellite Beacon Receiving Station, SSDL Stanford California, SSC98-II-7.

[9] , world wide web, November 2001

[10] , world wide web, November 2001

[11] , world wide web, November 2001

[12] Sarafin, Thomas P., Larson, Wiley J. Spacecraft Structures and Mechanisms, Microcosm, Inc. Torrance, CA, 1995.

Appendix

Links:

AT90S4433 Microcontroller



CM8870 DTMF Receiver



MX614 Modem



LM135 Temperature Sensor



MPX4115A Pressure Sensor



Preliminary CanSat Communication System Design

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