The Colorado Space Grant Consortium

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The Colorado Space Grant Consortium

At University of Colorado at Boulder

Citizen Explorer I:

A Creative Full Duplex Radio Link for a University Satellite Mission

Colorado Space Grant Consortium’s

Undergraduate Space Research Symposium

Written By:

David Zachary Allen and Jonathan Nikkel

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Table of Contents

1.0Abstract…………………………………………………………………….…..2

2.0Introduction……………………………………………………………….…...3

3.0General Subsystem Overview…………………………………………….…..3

3.1Purpose of Communications Subsystem……………………….…….....3

3.2How the Comm. System Achieves It’s Purpose………………………..3

3.3End-to-End Communications Structure………………………….……..4

4.0Technical Overview……………………………………………………...……4

4.1Ground Station………………………………………………………...4

4.11 PPP Computer……………………………………...………….4

4.12 Terminal Node Controller…………………………………… .5

4.13 Radio Transceiver……………………………………………..5

4.14 Transmit/Receive Antenna Array………………………….….5

4.15 Tracking Computer……………………………………………6

4.16 Tracking and Tuning PC Card………………………………...6

4.2Satellite…………………………………………………………………6

4.21 Receive Antenna………………………………………………6

4.22 Radio Receiver………………………………………………..6

4.23 Terminal Node Controller……………………………………..6

4.24 Radio Transmitter……………………………………………..7

4.25 Radio Phasing Device and Transmit Antenna Array…………7

4.3Edu-Station…………………………………………………………….7

4.31 Receive Antenna………………………………………………7

4.32 Radio Receiver………………………………………………..7

4.33 Terminal Node Controller……………………………………..7

4.34 Communications Protocol……………………………………..7

4.35 Computer Operating System…………………………………..8

4.36 Edu-Station Java Program……………………………….…8

5.0Conclusion……………………………………………………………..………8

Appendix A: Diagrams…………………………………………………………….9

Appendix B: Glossary of Terms…………………………………………………12

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1.0 Abstract:

A Creative Full Duplex Radio Link for a University Satellite Mission

By: Jonathan Nikkel and David Zachary Allen

The Citizen Explorer-1 (CX-1) Small Satellite Mission is being designed and built by students at the University of Colorado, Boulder (CU, Boulder), who are primarily undergraduates. The project exists to educate K-12 students around the world about Ultra Violet radiation and Earth’s Ozone Layer. CX-1 will use an onboard photometer and a spectrophotometer to measure reflected Ultra Violet radiation from the earth’s atmosphere. Schools will be able to purchase Edu-Stations that are made by CU students, to download Ozone data in real-time as CX-1 orbits overhead. Edu-Stations consist of a radio receiver and a Terminal Node Controller (TNC) to receive packets sent by the satellite.

Citizen Explorer consists of nine different subsystems including: the Attitude Determination and Control System (ADCS), responsible for attitude control of the Spacecraft; Command and Data Handling (C&DH), responsible for the flight computer operations; Communications (COMM), responsible for all radio communications and Radio Hardware; Power (POWER), responsible for the spacecraft electrical system; Science, responsible for flight and ground instrumentation and data analysis; the Solar Panel System (SOLAR), responsible for the solar panel subsystem; the Structural System (STRUCTURE), responsible for flight mechanical systems and launch vehicle interface, the Software System (SOFTWARE), responsible for end-to-end mission operations system (EEMOS), and the THERMAL System (THERMAL), responsible for spacecraft thermal analysis and design.

Following the launch of Citizen Explorer, College students will command the satellite from a control-room at the University of Colorado in Boulder, Colorado. The data-link to the satellite will be accomplished using a full-duplex, 9600 baud link on Amateur Radio Bands. The uplink will reside on the Amateur radio 2-meter band, and the downlink will reside on the Amateur radio 70-cm band. During nominal communications with the CU groundstation, Citizen Explorer will utilize the Point-to-Point Protocol (PPP), in conjunction with the Tele-Communications Protocol (TCP) and the Internet Protocol (IP). In order to stay within Federal Communications Commission guidelines, all packets appearing on the radio link will be framed in the Amateur X.25 protocol. Using PPP and TCP/IP is a first for university satellites. Citizen Explorer will have an IP address, allowing for seem-less network integration into CU ground station computers. Sending a file to Citizen Explorer will be as simple as issuing an ftp command to the satellite’s IP address.

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2.0Introduction

The Citizen Explorer-1 (CX-1) Small Satellite Mission is being designed and built by students at the University of Colorado, Boulder (CU, Boulder), who are primarily undergraduates. The project exists to educate K-12 students around the world about Ultra Violet radiation and Earth’s Ozone Layer. CX-1 will use an onboard photometer and a spectrophotometer to measure reflected Ultra Violet radiation from the earth’s atmosphere.

Following the launch of Citizen Explorer, College students will command the satellite from a control-room at the University of Colorado in Boulder, Colorado. The data-link to the satellite will be accomplished using a full-duplex, 9600 baud link on Amateur Radio Bands. The uplink will reside on the Amateur radio 2-meter band, and the downlink will reside on the Amateur radio 70-cm band.

The originality of the CX-1 communications system lies in the fact that it makes use of Point-to-Point Protocol (PPP), along with Tele-Communications Protocoll (TCP) and Internet Protocol (IP), in all communications between the satellite and the CU Mission Operations Control Center. Using PPP and TCP/IP is a first for university satellites. Citizen Explorer will have an IP address, allowing for seem-less network integration into CU ground station computers. Sending a file to Citizen Explorer will be as simple as issuing an ftp command to the satellite’s IP address.

The CX-1 mission is also unique in that it makes possible an unprecedented opportunity for K-12 students to participate in a real-world satellite experiment. Schools around the world will have the opportunity to purchase “Edu-Stations”, which are low cost communications ground stations that consist of a personal computer interfaced with a Terminal Node Controller and an Eggbeater Receive Antenna. Purchasing these stations will enable schools to receive data from CX-1 in real time, giving teachers the opportunity to make use of real first-hand data from a satellite, and giving students an opportunity to participate in the mission itself, by sending the data they receive to the CU Mission Operations Control Center via the Internet.

3.0General Subsystem Overview

3.1Purpose of the Communications Subsystem

The communications subsystem provides the Citizen Explorer satellite with a digital wireless link to the ground using the Amateur-X.25 (AX-25) protocol on the amateur radio bands.

3.2 How the Comm. System Achieves its Purpose

Citizen Explorer will utilize two types of flight-to-ground links. The first link will consist of communications between Citizen Explorer and the CU Mission Operations control center (CU-MOCC) and will utilize Point-to-Point Protocol (PPP) over the amateur radio link. Direct communication between CU-MOCC and the satellite will be strictly used for command and control of the satellite. The second flight-to-ground link will occur through a series of strategically placed Edu-Stations which are located at K-12 schools throughout the world. Science data sent through Edu-Stations will utilize only the AX.25 protocol, without a PPP connection.

All communications originating on Earth will be sent to Citizen Explorer on the VHF amateur radio band (2-meter band), and all communications originating on Citizen Explorer will be sent to Earth on the UHF amateur radio band (70-centimeter band). Separate transmit and receive frequencies are necessary to accommodate the full-duplex nature of the PPP protocol in which data is sent and received simultaneously.

All Edu-Station transmissions from Citizen Explorer are one-way: Edu-Stations are only receivers. Therefore, Citizen Explorer enters “Edu-Station” mode, in which the PPP connection normally used to connect to the CU-MOCC, is no longer used. In Edu-Station mode, Citizen Explorer sends many science-data packets to the Edu-Stations. These data packets are then compiled, and sent to CU-MOCC via the Internet.

3.3End-to-End Communications Structure

(Refer toDiagram A1)

All ground transmissions originate from the CU-MOCC computers. Commands are sent via Ethernet to the ground station. The ground station computer (Linux PC) passes the commands to the ground station TNC (Terminal Node Controller), which sends them to the transceiver. The transceiver sends the data feed up to the satellite, via the 2-meter amateur radio band.

The radio signal sent from the ground station is then received by Citizen Explorer through its 2-meter receive antenna. The signal then passes through the receiver, to the on-board TNC. Finally, the data is passed to the satellite C&DH (Command and Data Handling) system to be processed.

All flight data and replies originate from the satellite C&DH system. They are sent through the TNC to the transmitter. The transmitter then sends the data feed to the ground.

All science data that is collected by Citizen Explorer is transmitted to Edu-Stations using the AX.25 protocol. The Edu-Stations operate in receive-only mode. When Citizen Explorer transmits a science data feed, the feed is received by an Edu-Station through an Eggbeater antenna. The data feed then passes through a TNC, which de-packetizes the data, to the Edu-Station computer. The computer then processes the data feed, and sends it via the Internet to CU-MOCC.

4.0Technical Overview

4.1 Ground Station

(Refer to Diagram A2)

4.11 PPP Computer

The PPP computer in the CU groundstation is a critical link in the communications infrastructure for Citizen Explorer. The PPP computer is a Gateway 2000 computer with a 100 MHz Pentium processor, which is connected to the Internet and runs on the Linux Operating System, version 2.2.6. A software firewall on the PPP- Computer connects an Ethernet port on the PPP Computer to a serial port on the PPP computer. During a pass of Citizen-Explorer over the CU ground station in Boulder, commands from the CU-MOCC will be sent to the PPP Computer via Internet. The PPP computer will then send the commands, via the firewall, to the serial port where the TNC is connected. The PPP computer is connected to the ground station TNC at 9600 baud on an RS-232 serial link.

4.12Terminal Node Controller

The ground station Terminal Node Controller is a Timewave PK-96. During mission, the TNC will run at 9600 baud in a transparent, unconnected mode to facilitate the PPP protocol. The TNC packetizes all commands into AX.25 format, and sends them to the transceiver. Any data received from Citizen Explorer is originally framed in the AX.25 format, and is depacketized before being sent to the CU-MOCC. The TNC uses the FSK (Frequency Shift Keying) modulation scheme.

4.13Radio Transceiver

The ground station radio is a Yaesu FT-847 model radio transceiver. The Yaesu radio was chosen because of the many useful features that it boasts. The Yaesu transceiver is capable of simultaneous transmit and receive on separate bands, and has enough bandwidth to handle the 9600 baud data rate used for the data-link with citizen explorer. The Yaesu also has a CAT (Computer aided Tuning) port which allows Doppler frequency corrections to be made by an outside device. In the CU ground station, the Yaesu CAT port is connected to the tracking computer, so that the tracking computer can send all frequency correction commands to the Yaesu. The uplink frequency for Citizen Explorer will be on the VHF Ham band of 2-meters at 145.860 MHz, and the downlink will occur on the UHF Ham Band of 70-centimeters at 436.750 MHZ. The ground station transceiver has an adjustable Radio Frequency (RF) power output range from 5 watts RF to 50 watts RF. When the Yaesu radio is set to transmit at 50 Watts RF power, the power measured at the end of the 125 ft. transmit coax is 40 watts.

4.14Transmit/Receive Antenna Array

(Refer to Diagram A3)

In order to ensure a reliable radio link, the CU ground station utilizes transmit and receive antennas that have maximal gain. The VHF uplink antenna is a 22-element Yagi antenna, manufactured by Cushcraft Corporation, and is called the “22XB 2 Meter OSCAR Boomer.” The uplink Yagi has 14 dBdc gain. The UHF downlink antenna is a 38 Yagi, manufactured by the Cushcraft Corporation, and is known as the “738XB 70 cm OSCAR Boomer.” The downlink Yagi has 15.5 dBdc gain. The uplink and downlink antennas are circularly polarized.

For additional signal clarity, and to compensate for line loss in the 125 feet of coaxial cable between the downlink Yagi antenna, and the ground station receiver, a pre-amplifier is positioned on the receive antenna. As it is currently set, the pre-amplifier will amplify any received signals by 20 to 25 dB.

The transmit and receive antennas are mounted with their masts parallel to each other, at the top of a 35 foot tower, on the 3rd floor roof of the CU engineering building in Boulder. The antennas are attached to the tower by Yaesu G-5400B Azimuth/Elevation Rotators (Refer to diagram A4). The Azimuth/Elevation rotators allow the transmit/receive antenna array of the CU ground station to have a full “view” of the sky. The azimuth and elevation rotators on the tower are controlled by the Yaesu G-5400B Azimuth/Elevation rotator controller, which is in the ground station.

4.15Tracking Computer

(Refer to Diagram A5)

The tracking computer is responsible for two critical tasks: controlling the Yaesu G-5400B azimuth and elevation rotators, and Doppler shift frequency-correction of the Yaesu FT-847 transceiver. The Tracking computer is equipped with the Kansas City Tracker/Tuner card. The Tracking computer runs Windows Satellite Program (WiSP), which calculates Citizen Explorer’s position and the frequency correction for the uplink and downlink Doppler shift.

4.16Tracking and Tuning PC Card

The tracker/tuner card fits into the card slot on the motherboard of the tracking computer. The card takes commands from WiSP on the tracking computer, and then uses analog signals to control the Yaesu G-5400B azimuth/elevation rotators. The card sends digital signals to the Yaesu FT-847 transceiver CAT port to control the Doppler shift frequency-correction on the uplink and downlink radio frequencies.

4.2Satellite: Communications Hardware

4.21Receive Antenna

(Refer to Diagram A6)

Citizen Explorer uses a deployable monopole antenna, which deploys shortly after the acquisition sequence. Citizen Explorer receives on the VHF 2-meter Ham band at 145.860 MHz.

4.22Radio Receiver

The radio receiver on Citizen Explorer is an FM receiver on the VHF 2-meter Ham band at 145.860 MHz. The receiver was manufactured by SpaceQuest, and was designed for use in the harsh environment of Space. The receiver was intended for use with 9600 baud digital signals.

4.23Terminal Node Controller

All incoming and outgoing communications on Citizen Explorer pass through the Terminal Node Controller. Paccomm manufactures the TNC, which is a model UP-9600. The flight TNC operates in a transparent unconnected mode; transparent because without cue, the TNC immediately frames into AX.25 any data that it sees from the spacecraft C&DH system; unconnected because the TNC does not connect to the CU-MOCC via the AX.25 link layer. Paccomm specifies the modulation scheme on the UP-9600 as Direct Frequency Modulation (DFM), which is also known as Gaussian Minimum Shift Keying (GMSK).

4.24Radio Transmitter

The radio transmitter onboard CX is manufactured by SpaceQuest. Similar to the receiver, the transmitter is designed for use with digital, 9600 baud transmissions. The transmitter produces a carrier at 436.750 MHz on the 70-centimeter UHF amateur band. The transmit audio line of the TNC is connected to the UHF transmitter.

4.25Radio Phasing Device and the Transmit Antenna Array

In order to utilize the unique, 4-element, turn-style transmit antenna structure on the bottom of Citizen Explorer, a signal leaving the transmitter first enters a phasing device. The phasing device has a one-channel input, which is the original signal leaving the transmitter, and a four channel output. Each output channel is connected to one element on the transmit antenna array. In successive order, each output channel is out of phase by 90 degrees from the last output channel. When all of these signals are sent to their corresponding antennas, the result is a circularly polarized radio signal leaving the spacecraft. The transmit antenna array, and the phasing device were purchased from SpaceQuest.

4.3Edu-Station Hardware

4.31Receive Antenna

Edu-Stations utilize a receive antenna called an Eggbeater Antenna, which is manufactured by Radioware. The maximum gain on the eggbeater antenna is 6 dB, and occurs when the satellite is directly overhead. Because Citizen Explorer’s transmit signal is left-hand-polarized, the Eggbeater antenna must be left-hand-polarized as well, in order to avoid unwanted attenuation of the signal.

4.32Radio Receiver

The radio receiver is an Icom model IC-PCR100. During an Edu-Station pass by Citizen Explorer, the receiver will be set up in FM wide-band mode, with the front end of the receiver having a 20KHz bandwidth. The 20 KHz setting is required in order to accommodate the bandwidth needed to receive the 9600 baud signal from Citizen Explorer. All settings on the receiver are controlled from a Graphical User Interface on the Edu-Station computer, which runs on the Windows operating system. The radio receiver interfaces easily with the edu-station TNC, via a 9-pin serial port.

4.33Terminal Node Controller

The Edu-Station TNC is identical to the CU ground station TNC. It is a Timewave model PK96, set to run in a transparent unconnected mode at 9600 baud.

4.34Computer

Any model of computer can be used to run the Edu-Station, but the computer must follow two guidelines: the Edu-Station computer must run the Windows operating system, and it must have a web-browser installed. The Windows operating system is required because the Icom GUI software for the receiver only runs on Windows. A web browser is required in order to run the Edu-Station software. The computer-model-flexibility allows individual schools to use computers that they already own, eliminating the need to buy new computers.

4.35Communications Protocol

Unlike the full duplex link between Citizen Explorer and the CU-MOCC, Edu-Stations operate in a receive-only mode, and therefore do not utilize the PPP protocol. Packets arrive as “raw data,” without any of the network overhead data that is associated with PPP. Therefore, the AX.25 protocol is the only protocol used between Citizen Explorer and the Edu-Stations to transport data.

4.36Edu-Station Java Program

CU students wrote the Edu-Station Java program. The program takes received ozone-data packets from Citizen Explorer, and formats them in a user-friendly style that is readable by students. The program also connects to the CU-MOCC, via the Internet, and sends the formatted ozone-data to CU.