Calhoun

iniQiuiic^iul Ar{hiv« of tilt Mil vdl Poii^roduiit School

Calhoun: The NPS Institutional Archive □Space Repository

Theses and Dissertations

1. Thesis and Dissertation Collection, all items

2007-09

Ground segment preparation for NPSATl

Koerschner, Luke E.

Monterey, California. Naval Postgraduate School

http://hdl.handle.net/10945/3261

Downloaded from NPS Archive: Calhoun

DUDLEY

KNOX

LIBRARY

htt p://w ww. n ps. e du/l ib ra ry

Caflwuo is the Naval Postgraduate School's public access digital repository for research mate rials and institutiional putilicatiaos created by the NPS community. Calhoun is named for Professor of Mathematics Guy K. Caftiouo, NPS's first appointed and putJlished schoteily author.

Dudley Knox Library / Naval Postgraduate School 411 Dyer Road / 1 Univefsity Circle Monterey, California USA 93943

NAVAL

POSTGRADUATE

SCHOOL

MONTEREY, CALIFORNIA

THESIS

GROUND SEGMENT PREPARATION FOR NPSATl

Thesis Advisor:

by

Luke Koerschner

September 2007

James A. Horning

Second Reader:

David Rigmaiden

Approved for public release; distribution is unlimited

THIS PAGE INTENTIONALLY LEET BLANK

REPORT DOCUMENTATION PAGE

FormApprovedOMBNo^ 0704-018^^^^ Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instruction, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188) Washington DC 20503.

I. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

September 2007 Master’s Thesis

ATITLEANDSUBTITLE^GroundSegmentPreparationforNPSAT^^^^^^^^ 5. EUNDING NUMBERS

6. AUTHOR(S)

7. PEREORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PEREORMING ORGANIZATION

Naval Postgraduate School REPORT NUMBER

9. SPONSORING /MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING N/A AGENCY REPORT NUMBER

II. SUPPLEMENTARY NOTES The views expressed in this thesis are those of the author and do not reflect the official policy

or position of the Department of Defense or the U.S. Government. _

12a. DISTRIBUTION / AVAII.ABIITTY STATEMENT 12b. DISTRIBUTION CODE

Approved for public release; distribution is unlimited

13. ABSTRACT (maximum 200 words)

Most satellites rely on a ground control station to command their payloads and through which they download data from their payloads. The Naval Postgraduate School’s satellite (NPSATl) is no exception. The spacecraft’s payloads, which include the Coherent Electromagnetic Radio Tomography (CERTO), Langmuir probe, Configurable Eault Tolerant Processor (CETP), as well as the Visible Wavelength Imager (VISIM), all generate data that require collection on the ground through a radio frequency downlink. Telemetry from NPSATl ’s unique attitude control system, which uses only MEMS angular rate sensors, magnetic coils, a magnetometer and a GPS could aid in the development of improved or more economical attitude control systems. The goal of this thesis is to ready the ground control segment for operation for collection of data from and command of NPSATl immediately after launch.

Included is a description of the spacecraft to ground calculation, bidirectional, link budget and the operation and testing of the ground antenna pointing control system. Euture space systems students and faculty will use the ground control segment to harvest the data and reap the knowledge of the experiments that will orbit inside NPSATl. What better way to test the pointing of the antenna than to use it to track the Midshipman Space Technology Applications Research Program’s first satellite (MidSTARl).

14. SUBJECT TERMS Ground Segment, NPSATl, MidSTARl 15. NUMBER OE

PAGES

_ 77 _

16. PRICE CODE

17. SECURITY 18. SECURITY 19. SECURITY 20. LIMITATION OE

CLASSIEICATION OE CLASSIEICATION OE THIS CLASSIEICATION OE ABSTRACT

REPORT PAGE ABSTRACT

_ Unclassified _ Unclassified _ Unclassified _ UU _

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. 239-18

1

THIS PAGE INTENTIONALLY LEET BLANK

11

Approved for public release; distribution is unlimited

GROUND SEGMENT PREPARATION FOR NPSATl

Luke E. Koerschner Major, United States Army B.S., North Carolina State University, 1990

Submitted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE IN SPACE SYSTEMS OPERATIONS

from the

NAVAL POSTGRADUATE SCHOOL September 2007

Author: Luke Koerschner

Approved by: James A. Homing

Thesis Advisor

David Rigmaiden Second Reader

Professor Rudolf Panholzer

Chairman, Space Systems Academic Group

THIS PAGE INTENTIONALLY LEET BLANK

IV

ABSTRACT

Most satellites rely on a ground control station to command their payloads and through which they download data from their payloads. The Naval Postgraduate School’s satellite (NPSATl) is no exception. The spacecraft’s payloads, which include the Coherent Electromagnetic Radio Tomography (CERTO), Eangmuir probe, Configurable Eault Tolerant Processor (CETP), as well as the Visible Wavelength Imager (VISIM), all generate data that require collection on the ground through a radio frequency downlink. Telemetry from NPSATl ’s unique attitude control system, which uses only MEMS angular rate sensors, magnetic coils, a magnetometer and a GPS could aid in the development of improved or more economical attitude control systems. The goal of this thesis is to ready the ground control segment for operation for collection of data from and command of NPSATl immediately after launch.

Included is a description of the spacecraft to ground calculation, bidirectional, link budget and the operation and testing of the ground antenna pointing control system. Euture space systems students and faculty will use the ground control segment to harvest the data and reap the knowledge of the experiments that will orbit inside NPSATl. What better way to test the pointing of the antenna than to use it to track the Midshipman Space Technology Applications Research Program’s first satellite (MidSTARl).

V

THIS PAGE INTENTIONALLY LEET BLANK

VI

TABLE OF CONTENTS

I. INTRODUCTION . I

A. STATEMENT OF THE PROBLEM . I

B. NPSATI OVERVIEW . I

II. NPSATI GROUND SEGMENT OVERVIEW . 3

A. GENERAL . 3

1. Frequencies . 3

2. NPSATI Antennas and Pointing . 3

3. NPSATI Passes . 5

B. COMMAND PATH (UPLINK AND DOWNLINK) . 5

1. Computer and Software . 5

2. Digital Telemetry Receiver with Tracking . 8

3. Controller . 9

4. Enclosure . 12

III. NPSATI LINK BUDGET . 15

A. COMMUNICATIONS LINK BUDGET . 15

1. Margin . 16

2. Slant Range . 16

3. Bit Error Rate . 18

4. Antenna Gains . 18

a. Ground Antenna . 18

b. NPSATI Antennas . 18

5. Pointing Error . 19

6. Efficiency . 21

7. Noise Temperature . 21

8. Wavelength . 22

9. Beam Width . 22

10. Atmospheric and Rain Losses . 23

11. Free Space Path Loss . 23

12. Pointing Error Loss . 24

13. Effective Isotropic Radiated Power . 24

14. Propagation & Polarization Loss . 25

15. Link Budget . 25

B. TEST LINK BUDGET . 28

IV. TESTS, INSTALLATION, & CALIBRATION PROCEDURES . 31

A. FEED HORN . 31

B. TEST EQUIPMENT . 36

C. PROCEDURES . 36

I . Sources of Error . 36

a. Timing Errors . 36

b. Satellite Orbital Ephemeris . 36

vii

c. Antenna Location . 37

d. Pointing Calibration . 37

2. Initial Assembly and Checkout . 38

3. Slewing Initial Checks . 38

4. Aiming Point Tests . 39

a. Close Aiming Point . 39

b. Medium Aiming Point . 39

c. Distant Aiming Point Tests . 39

D. WINDPROOFING . 40

V. COMMUNICATIONS CONTINGENCIES . 51

A. REDUNDANT GROUND STATIONS . 51

B. NPSATI CONTROL . 52

VI. CONCLUSION AND RECOMMENDATIONS . 55

LIST OF REFERENCES . 57

BIGLIOGRAPHY . 59

INITIAL DISTRIBUTION LIST . 61

viii

LIST OF FIGURES

Figure 1. Horizon Fade . 4

Figure 2. Uplink Frequency Mixing . 6

Figure 3. Communications Block Diagram NFS ATI . 7

Figure 4. Connections on back of RC2800 PRK Dual Rack Mount Controller . 10

Figure 5. Antenna Deck Spanagel Hall . 10

Figure 6. Minimum & Maximum Elevations . 12

Figure 7. Controller Pointing Resolution . 19

Figures. Test Link . 28

Figure 9. Feed Horn Placement . 32

Figure 10. Feed Horn Mounted Inside Support Arms . 33

Figure 11. Feed Horn Signal Measurement . 35

Figure 12. Ballast Roof Mount . 41

Figure 13. Antenna Base . 42

Figure 14. Wind Loading Perpendicular to Antenna Aperture . 44

Figure 15. Wind Loading Parallel to Antenna Aperture . 45

Figure 16. NPSATl Communications Contingencies . 53

THIS PAGE INTENTIONALLY LEET BLANK

X

LIST OF TABLES

Table 1. NPSATl Uplink Budget, Short Form . 15

Table 2. Link Budget . 27

Table 3. Feed horn position final tests . 34

Table 4. 25G BRM Allowable Antenna Areas . 49

Table 5. Communications Parameters Comparison . 51

THIS PAGE INTENTIONALLY LEET BLANK

ACKNOWLEDGMENTS

I would like to thank my second reader, David Rigmaiden, for the hands on work that he did to make the antenna setup a reality. Thanks also to Professor Billy Smith of the U.S. Naval Academy for his invaluable assistance. The morning he spent showing me his ground control segment saved me weeks of work with the Nova software. LTC Lawrence Halbach directed my initial self directed study of the ground segment. Glenn Harrell’s work machining the feed horn mount and creating a measurement tool to check that the feed horn was in the center of the parabolic dish was much appreciated. Dr. James Newman had the idea of using an anemometer to park the antenna in the safe position during periods of high winds allowing us to use a commercial ballast mount. I would also like to thank Mr. David G. Brinker P.E., S.E. of the Rohn Products Division of Radian Communications Services Inc. for permitting me to publish Rohn figures in this thesis. MAJ Steve Moseley mounted the feed horn cover. Professor Rudolph Panholzer suggested moving the azimuth motor lower and closer to the ballast mount to improve stability following azimuth changes. Einally I would like to thank my thesis advisor, Jim Homing, for the software scripts he wrote for my thesis work with the controller and for his tireless proofreading of this thesis.

THIS PAGE INTENTIONALLY LEET BLANK

XIV

I.

INTRODUCTION

A. STATEMENT OF THE PROBLEM

Most communications satellites are in geostationary (GEO) orbit allowing terrestrial transmitters and reeeivers to point their antennas to fixed elevation and azimuths indefinitely. Other military dishes are designed to traek geosynchronous satellites by dithering toward the strongest signal strength to follow the minor ehanges in azimuth and elevation of their geosynehronous target. Many low earth orbiting (LEO) satellites relay data to GEO satellites whieh pass that information down to terrestrial receivers. NPSATl is a LEO satellite without the benefit of a relay satellite. Data from NPSATl experiments will only be available if telemetry ean be requested and reeeived by a ground segment. The ground antenna’s four degree spot beam will require a high degree of pointing aeeuraey from the eontroller. Other eonsiderations arise from a student satellite with a finite design life. With a limited lifespan it is desirable to establish eommunieations with the satellite immediately after launeh; ideally the ground eontrol segment should be fully operational prior to launeh.

B. NPSATl OVERVIEW

The student and faeulty built NPSATl is a LEO satellite whieh is designed to be a seeondary payload on a military or government launch. It incorporates an Evolved Expendable launeh vehiele Secondary Payload Adapter (ESPA) for mounting as a seeondary payload under the Orbital Express primary payload whieh was to be launehed on an Atlas V roeket. That launeh was missed, so arrangements are being made to launeh in 2009 on a Minotaur roeket with an ESPA seeondary payload suite. NPSATl uses eommereial, off the shelf, lithium ion batteries. The eylindrieal polygon shape of NPSATl has solar panels mounted on eaeh of its twelve faees, and will ineorporate on orbit testing of a triple junction solar cell design. The telemetry and eommand patch antenna design is deseribed in detail by Erel (2002). Testing of these mierostrip antennas was doeumented by Gokben (2003). Two sets of transmit and reeeive antennas are found

1

on the satellite. The primary transmit/receive (TX/RX) antennas are on the nadir pointing side of the satellite and the back up antennas are on the zenith pointing side. The transmit antenna is an elliptical patch measuring 5.66 cm across the short axis and 6.6 cm across the long axis. The receive antenna is a larger elliptical patch measuring 7.293 cm across the short axis and 8.509 cm across the long axis. Naval Research Laboratory (NRL) experimental payloads include the coherent electromagnetic radio tomography (CERTO), and a Langmuir probe. Naval Postgraduate School (NPS) experiments consist of a three axis micro-electromechanical (MEMS) rate sensor combined with magnetic coils to implement a magnetic attitude control test and a visible wavelength imager (VISM). The Solar cell Measurement System (SMS) experiment will test the new solar cell technology that will orbit on the satellite. Additionally, the CETP is a Naval Postgraduate School (NPS) designed payload that will orbit on NPSATl. A CETP is currently in use on MidSTARl. Results from MidSTARl telemetry show that the CEPT is experiencing single event upsets over the South Atlantic Anomaly (SAA) region. The SAA is a region in space over Brazil where the magnetosphere has a decrease in strength. The magnetosphere protects the Earth and LEO spacecraft from most solar high energy radiation particles which are strong enough to change a bit in a processor. More in depth reports of the CETP voting circuit operation will be included in NPSATl telemetry.

2

II. NPSATI GROUND SEGMENT OVERVIEW

A. GENERAL

The ground segment consists of those components on the ground that allow control of and communications with the spacecraft. The NPSATI ground segment includes a 10 foot parabolic dish antenna which is operated through a general purpose computer that sends commands to the controller which steps the azimuth/elevation motors. The uplink to NPSATI, and downlink from it are handled with two frequencies and those signals are passed through a modulator/demodulator (MODEM) between the computer and the antenna. An overview of the ground segments components is depicted in Figure 3. This section covers components of the ground segment in more detail.

1. Frequencies

A single ground relay antenna is used to transmit to the NPSATI at 1767.565 MHz L-Band and receive transmissions from NPSATI at 2207.3 MHz S-band. Doppler shift is compensated for in the high and low frequency synthesizers. The separation of these two frequencies allows full duplex communications without interference between the two frequencies.

2. NPSATI Antennas and Pointing

Communications with NPSATI is not contingent upon the proper functioning of its Attitude Control Subsystem (ACS). Normally the ACS keeps transmit and receive antennas pointed toward nadir. The zenith pointing antennas act as a backup to the Nadir pointing antennas in the event the spacecraft looses pointing capability and begins to tumble. The tolerance of NPSATl’s nadir pointing via its ACS is estimated to be +/- 10 degrees. NPSATI uses hemispherical patch antennas with half power beam widths determined by Erel (2002) to vary between 60.1 and 79.5 degrees at the uplink frequency and between 65.6 and 74.3 degrees at the downlink frequency (p. 42, 46). The average uplink half power beam width is 69.8 degrees, and the downlink average beam width is

3

69.5 degrees. The fact that they are omni directional allows them to transmit and received at much wider beam widths if the link is strong enough. For the purpose of calculating the link budget the rounded average beam width of 70 degrees was used for both uplink and downlink from NPSATl.

NPSATl’s sister satellite MidSTARl, which was built for Naval Academy payloads, also contains a CFTP that was designed at NPS. The same design will be employed on NPSATl. MidSTARl does not have an attitude control system so it experiences roll fades. A roll fade is a drop in radio frequency signal strength that occurs when the satellite rolls from one omni directional antenna to another. Roll fades on MidSTARl can cause the temporary loss of communications when combined with pointing error losses. This is mentioned because NPSATl will also experience roll fades if its attitude control system fails. NPSATl’ s attitude control system points it to nadir not directly toward the ground antenna. As a result of the nadir pointing antenna on NPSATl fades in signal strength will be experienced at low elevation angles even when the attitude control system is working. These fades can occur because the antennas on NPSATl will not always have the ground antenna in their half power beam width. This concept of “horizon fade” is best understood by Figure 1 which is conceptual and obviously not drawn to scale, because the four degree spot beam has an arc length of 149 km at 10 degrees of elevation.

4

3.

NPSATl Passes

Depending on the inclination of the spacecraft’s orbit there should be at least four good opportunities to communicate with NPSATl each day. The original launch inclination would have yielded four daily satellite ground passes high enough above the horizon to permit time for downlink and uplink. Professor Smith of the Naval Academy has had good success with low grazing passes too, and if transmission and reception initiates below 10 degrees elevation then the system may have six usable passes daily. Presently MidSTARl’s orbit offers six good passes a day. Since MidSTARl has an inclination higher than the latitude in Monterey, CA it can pass directly over, or to the North of, the antenna at NPS. Those passes may be associated with loss of connectivity near zenith as the azimuth is changing faster than the antenna controller can receive commands and send status updates to the computer. This topic is discussed in more detail in this chapter (Section B3).

B. COMMAND PATH (UPLINK AND DOWNLINK)

I. Computer and Software

One software component of the computer is the orbit propagator. Since orbital ephemeris is only down linked once a day, software must predict the satellite’s position over time with mathematical algorithms. The propagator that was tested for this thesis was embedded in Northern Lights Software’s Nova program. Satellite Toolkit (STK) was also used to propagate orbital data in early tests that used software written for an operating system shell to send commands to the controller. Both propagators worked well but the Nova software communicates directly to the controller while STK requires that the pertinent orbital data be exported and requires more programming.

The computer with propagation software relays to the modulator de-modulator (MODEM) which mixes the intermediate frequency with the carrier frequency and feeds that communications signal through the low frequency synthesizer. The communications signal is sent back through the modem and out the antenna. Similarly signals received from NPSATl are sent through the MODEM to the high frequency synthesizer which

5

sends the signal back to the modem and on to the computer. Figure 2 illustrates the mixing of the intermediate frequency with the local oscillator for modulating the uplink frequency.

Frequency

Synthesizer

Antenna ^577 i^hz

Similarly the downlink frequency will be demodulated in a L3 software radio card that is on order. The L3 digital TT&C will eliminate the stand alone high frequency synthesizer from the architecture as will be described in this chapter (Section B2). Both synthesizers account for the Doppler shift of the moving satellite through programmed step routines. Doppler shift, the apparent increase in radio frequency of transmissions from an ascending satellite as it approaches the ground antenna and decrease in radio frequency of the frequency of the same transmissions from the satellite on its decent, is significant given the high velocities of spacecraft in the LEO regime.

Other inputs to the computer include the weather station and may also include a digital camera, and a GPS. The weather station signals will send data to the computer through a serial port. The weather station signal of interest is the wind speed which will, in high winds, alert the computer to command the controller to elevate the dish antenna to a safe position. Digital cameras could be affixed to the antenna to provide visual

6

feedback to a remote computer being used to control the antenna over the campus network. The UHF antenna that was used for a previous NPS-built satellite had a light- sensitive diode mounted on it that allowed the ground controller to bore sight its Yagi- Uda antennas with the Sun. A GPS could be connected to the computer to keep the computer time synchronized with GPS time and consequently the satellite’s ephemeris time.

The computer is the nerve center of the entire ground communications system. It is an Intel® Core™ 2 CPU 6300 @ 1.86GHz 1.86 GHz with 1 GB of RAM. It was ordered with multiple PCI card slots to accommodate the L3 communications card on order as well as the PCI card that allows it to connect to the Frequency Synthesizer. The current setup uses Northern Lights Nova software to communicate through a single serial cable to a M RC2800PRK dual rack mount controller. The controller is described in this chapter, this section, number 3.

COMMUNICATIONS BLOCK DIAGRAM NPSAT1

Full Duplex ^ .

ANTENNAAND COMMUNICATINS & CONTROL ENCLOSURE LOCATED ON ROOF OF SPANA3EL HALL36J60 DEG N LATTITUDE 121 B8 DEG W

Figure 3. Communications Block Diagram NPSATl

7

2. Digital Telemetry Receiver with Tracking

Delivery of the L3 Communications PCI- 2070 Digital Telemetry Receiver with Tracking is not anticipated until after this thesis is written, but its capabilities will be discussed here. The L3 Technical Bulletin (2004) states the following:

Capable of accepting RF input signals from -10 dBm to -VOdBm, the PCT 2070 will receive the RF signal, condition and digitally demodulate FM,

FSK, PM, BPSK, QPSK, OQPSK data. The image frequency bandwidth is programmable from 50 kHz to 30 MHz. The AFC (auto frequency control) tracking feature compensates for Doppler shift and other transmitter anomalies by using DSP algorithms to determine if the input spectrum is centered at the programmed center frequency. If the input spectrum is not symmetrical, the digital down converter is automatically stepped to track the input frequency.

One of the biggest advantages of this digital telemetry receiver card is its tracking capability which allows it to automatically compensate for Doppler shift with its automatic frequency control (AFC). This card will eliminate the need for the high frequency synthesizer depicted in Figure 3.

The card uses a Phase Lock Loop (PLL) in conjunction with Digital Signal Processing (DSP). The phase lock loop uses one or more traditional analog oscillators in combination with DSP. This card does not use Direct Digital Synthesis (DDS) in which the oscillator waveforms are generated in a processor. Some advantages of combining PLL technology with DSP is that the card is smaller and better at reducing spurious signals. Another advantage of this hybrid signal processing card is that its clock speed does not have to be multiples faster than the frequency of the generated waveform as is required in DDS. With a true DDS card, the clock speed of its processor would have to be at least twice the frequency because as described by Reed (2002) “The Nyquist sampling theorem limits the theoretical maximum attainable output lowpass frequency to half the clock frequency...” (p. 131). It is more likely that the clock speed of a comparable DDS card processor would have to be approximately 7 GHz (1.76 GHz (4)) because Reed (2002) states “it is customary to limit Ar to Fcik/4 to accommodate non-ideal analog filters.” (p. 135). Ar represents a frequency word. Essentially a DDS card of equal

8

capability would have to have a much larger processor that would consume more power, and radiate more heat, than the computer’s two 1.86 GHz CPUs. The interface of the card to the PC is via a PCI slot using a 32 bit PCI form factor.

3. Controller

The controller sends signals to two electric motors one for azimuth and the other for elevation. The controller pans across the heavens based on an open loop control scheme for elevation and azimuth of the dish antenna. In other words, once the elevation and azimuth are set off of a known point or celestial object the antenna may drift from those settings. The Naval Academy used the sun as the reference point for their antenna and they reset their azimuth and elevation calibration before every pass when possible. The motors send feedback to the controller for a closed loop control scheme. The controller has the antenna follow the predicted path of NPSATl during an overhead pass. One drawback of the Dual Rack mount controller is that it has a single 9600 baud serial port connection which has to receive separate commands for azimuth and elevation changes. The fastest update rate that can be used between the Nova software running on the computer and the controller is one second. Setting the update rate faster than that could result in the dropping of commands by the controller. Dropping commands occurs when the controller receives commands faster than it can execute them and subsequent commands are sent before the previous command has been executed, so commands are “dropped” by the controller. The RC2800PX/AZ and the RC2800PX/EL controllers were also purchased as spares. They allow the option of switching to separate elevation and azimuth controllers with individual serial port connections. Although the computer only has one 9-pin serial port, a USB port to serial cable adapter was tested with HyperTerminal to demonstrate that separate azimuth and elevation serial connections could be used. If separate controllers are used the CPU will have to send commands to both of them simultaneously through multiple RS-232 serial connection achieving more responsive antenna control. The connections to the dual rack mount controller are shown

9

below in Figure 4. The black and white wires are connections to the azimuth and elevation motors and the orange and blue wires connect to the pulse switches which give motion feedback to the controller.

Figure 4. Connections on back of RC2800 PRK Dual Rack Mount Controller

4. Ground Antenna

A mesh parabolic ground antenna is located on the roof of Spanagel Hall (8th floor) at the Naval Postgraduate School in Monterey, CA. 36.595 degree North Latitude by 121.875 degree West Latitude. Figure 5 is a sketch of the location of the antenna in relation to other equipment on that deck.

1 Side

8th Deck BLDG 232 Spanagel Hall

Figure 5. Antenna Deck Spanagel Hall

10

The uplink beam width of the ground antenna is approximately four degrees and is a function of the frequency and antenna diameter. The ground antenna’s downlink beam width is approximately three degrees. The pointing accuracy of the ground antenna must be less than or equal to two and a half degrees to maintain the down link as will be discussed in Chapter III. A two and a half degree error translates into an error arc length of 24 kilometers while pointing at NFS ATI 560 km directly overhead. The maximum path loss is 162.7 dB on the uplink and 164.6 dB on the downlink as will be calculated in Chapter III. The antenna is a 3.048 meter (10 feet) parabolic dish type reflector. The antenna reflects signals transmitted from NFS ATI onto the feed horn. The feed horn also radiates the parabolic reflector with signals transmitted to NFSATl. A minimum transmit elevation over land of 10 degrees may be used to mitigate the chance of interfering with ground receivers. Over the Monterey Bay it should be safe to transmit and receive at zero degrees elevation because there are fewer ship borne transmitters and receivers that are at risk of interference on the bay than on land.

An antenna limitation is that it cannot slew through more than 374 degrees of azimuth (14 degrees of overlap) or more than 90 degrees of elevation. Because of these limitations the antenna will not be able to continuously follow a satellite that passes directly overhead. Once the elevation of the antenna reaches 90 degrees the antenna would have to rotate through 180 degrees of azimuth before following the satellite as it descended on the through the eastern horizon. The time required to rotate would result in a temporary loss of connectivity. Antennas that have to be slewed at their maximum elevation to follow the satellite on its descending pass are said to have a “keyhole” in Air Force jargon because one has to turn the antenna just like a key. Figure 6 is helpful in visualizing this keyhole where the antenna azimuth has to be rotated once the maximum dish elevation is reached.

11

MINIMUM DISH ANTENNA ELEVATION OVER LAND 10 DEGREES

MAXIMUM DISH ANTENNA ELEVATION 90 DEGREES

Figure 6. Minimum & Maximum Elevations

4. Enclosure

The outdoor enclosure has a single door with two lockable handles, which both latch. The enclosure is 24” wide, 20” deep, and 30” high. DDE is the manufacturer and model PSOD-302429FT was purchased. The purpose of the outdoor enclosure is to protect the computer, controller, frequency synthesizer, uninterruptible power supply (UPS), and transceiver card from the elements. It is located as close as possible to the

feed horn, on the antenna base, to minimize the line losses between the feed horn and the

12

transceiver card. The UPS depicted in Figure 3 will need to power all of the equipment in the enclosure and the azimuth and elevation motors for twenty minutes. None of the satellite passes will be longer than twenty minutes, so if the AC power is lost at the beginning of a satellite pass the system will still have enough battery power to track and communicate through the entire pass.

13

THIS PAGE INTENTIONALLY LEET BLANK

14

III. NPSATl LINK BUDGET

A. COMMUNICATIONS LINK BUDGET

This chapter seeks to clarify the calculations used for the generation of the link budget. The link budget is a cumulative calculation of transmitter to receiver gains and losses which determines if the link is strong enough for reliable communications. Bidirectional communications mean that this link budget must be calculated from the satellite to the ground receiver and from the ground receiver to the satellite.

The short form of the uplink budget to NPSATl is depicted in Table 1, and a carrier to thermal noise ratio is calculated using the format in Gordon and Morgan’s Table 2.5 (1993) (p. 44).

Receiving earth station location: Monterey, CA

Uplink frequency /u: 1.76757 GHz

Transmit earth station antenna diameter: 3.048 m

Satellite: NPSATl _ Uplink beam: 4 degee spot beam

Parameter _ Sign Value Units Section

Earth Station

Power at the antenna for 6.99 dBW

P* = 5_ W/carrier

Transmit antenna gain G + 32.43 dBi 4. a.

39.42 dBW 13.

162.69 dB 11.

-21.6 dBi/K

Earth station EIRP Earth to Satellite

Eree space path loss L for 5u = 1840 km Satellite

Satellite G/Ts,u -i-

Carrier/thermal noise Cu/Tu l/k {k = Boltzmann’s constant)

-144.87 dBW/K -I- 228.6 dB(W/Hz K)'‘

Cu/kTu

83.731 dBHz

Table 1. NPSATl Uplink Budget, Short Eorm

15

This short format has section numbers which correspond to the calculations that follow in this chapter. The drawback of this short format is that it does not include losses for the pointing errors of both the ground and spacecraft antennas. The short form is useful though because the high carrier to thermal noise value of 87.73 dB Hz indicates that the link should have adequate strength. This value will be compared with the carrier to thermal noise from the long uplink budget. The long link budget is a more detailed spreadsheet that is developed with information from the calculations that follow in this chapter.

1. Margin

How much margin is sufficient for reliable communications? The guidance given by Space Mission Analysis and Design (SMAD) edited by Larson & Wertz (1999) is to “Adjust the input parameters until the margin is at least 3 dB greater than the estimate value for rain degradation, depending on confidence in the parameter estimates.” (p. 568). Rainfall is sparse in Monterey and outages during the handful of days annually with heavy precipitation are acceptable. Since Gaussian Minimum Shift Keying is being used a value of 9.6 dB is extracted from Larson and Wertz’s Table 13-11 as the minimum received energy per bit over noise-density (Eb/No) (p 562).

2. Slant Range

The slant range is calculated by knowing the maximum altitude of NPSATl and the minimum elevation of the ground antenna. Presently the launch parameters of NPSATl are unknown so // = 560 km will be used because it was the maximum altitude of the Orbital Express Eaunch. The 10 degree minimum elevation that is imposed on the antenna to reduce interference from and to ground stations is also used. Work began with equation (5-24) from Earson & Wertz (1999) (p.l 13).

Re

sin p = cos A,o = - ; Or [Equation 3-1]

Re + H

sin p = - ; So

Re^H

16

6371.0003fcm .

Sin p = - .-.sin 0 = 0.919204

637 1 .0003km + 560km

p = 66.8099°

Using Equation (5-26b) from Larson and Wertz (p. 1 13)

sin t] = cos a sin p

sin 7] = cos(10°)0.919204

sin;; = 0.905239

[Equation 3-2]

.-.;; = 64.8555°

Using equation (5-27) from Larson and Wertz (p. 113)... X + a = 00°

64.8555° +;L + 10° =90°

.•.;L = 15.1445°

[Equation 3-3]

Einally slant range, D, is solved with Larson and Wertz’s equation (5-28) (p. 1 13).

D = i?£(sin X / sin rf)

D = 6371.0003Msinl5.1445° 70.905239) = 1838.69 /tm

[Equation 3-4]

In the interest of simplicity, this is rounded up to 1840 km. Since this study began NFS ATI missed the Orbital Express launch. Euture launch opportunities include a Minotaur with an orbital altitude of between 600 and 700 km. Eor H=700 km the above calculations are performed to obtain D= 2155 km.

17

3.

Bit Error Rate

The bit error rate (BER) is the probability of a single bit being erroneous. A probability of a bit error of 10'^ was chosen because that is a typical BER that is tolerable for telemetry and command signals. Using figure 13-9 of Earson & Wertz, with this probability of error, it is found that Gaussian Minimum Shift Keying (OMSK) yields a required energy per bit over noise ratio (Eb/No) of 9.6 dB (p. 561). With OMSK the spectrum utilization of 1 represents good use of spectrum. The bit rate for both uplink to and downlink from NFS ATI is 1 15 kbps.

4. Antenna Gains

a. Ground Antenna

The aperture of the dish is 10 feet which is multiplied by 0.03048 to convert to 3.048 meters. The uplink gain is calculated using Gordon & Morgan’s equation (6.5) (p. 140).

G = 20 log loD -r 20 log lo/ -i- lOlog lorj -i- 20 A{dBi)

G = 20 log io(3.048) + 20 log 10(1.76757) + 101ogio(0.55) + 20.4(dB/) [Equation 3-5]

G = 32A{dBi)

Similarly, a downlink gain is calculated with the above equation using the 2.207 GHz downlink frequency and the result is 31.0 dB.

b. NPSATl Antennas

The gain of the patch antennas on NPSATl can be calculated using the

same formula.

G = 20 log loD -r 20 log lo/ -r lOlog lorj -i- 20 A{dBi)

G = 20 log io(0.0612) + 20 log io(2.2073) + lOlog io(0.90) + 20 A{dBi) [Equation 3-5]

G = 2.6{dBi)

18

The receive elliptical patch antennas on NFS ATI are slightly larger with an average diameter of 0.0764 meters. The receive frequency of 1.76757 GHz must also be used in the above equation to calculate a receive antenna gain of 0.4 dB.

These values were checked with a modified version of the antenna gain equation from Larson and Wertz (13-18b) (p. 555), and yielded identical values. It should be noted that dBi refers to isotropic decibels.

5. Pointing Error

0.5 de^ee Elevation

% -

0.25 degree Azimuth

Figure 7. Controller Pointing Resolution

The pointing error of the ground antenna is more difficult to estimate. Controller tests revealed that the elevation drive only makes changes of one degree or more and that the azimuth rotor makes changes in half degree increments. The best pointing accuracy that can be hoped for is half of the hypotenuse of the pointing resolution, because the controller must wait for a 0.5 degree increase or decrease in elevation to change the

19

elevation of the dish and it waits for a 0.25 degree change in azimuth to bump the azimuth to the next closest azimuth increment. Consequently, the best possible pointing accuracy is the hypotenuse of the two values depicted in Figure 7, or 0.559017 degrees. That is the resolution of the M controller but the software being tested does not command the controller to adjust the antenna unless there is a change in elevation or azimuth of a degree. The Nova software defaulted to 1.8 degrees of azimuth or elevation difference before commanding a change, but this was lowered to one degree. The hypotenuse of 1 degree of both azimuth and elevation is the square root of two or 1.41 degrees. This does not mean that the best pointing accuracy is 1.41 degrees because the Nova software can be set to lead the satellite. The Nova software allows setting of the rotator to lead the satellite in either time or degrees. By leading the satellite the hypotenuse of 1.41 is split which gives the best theoretical pointing accuracy of 0.7 degrees. Professor Smith of the Naval Academy uses the degree settings to lead an ascending portion by +1 degree and then changes the settings at zenith so that the elevation controller leads the satellite on the descending pass by -1 degree. The satellite is not really being led by the antenna. Instead the goal is to move the antenna in concert with the satellite passage. By setting a lead time of a few seconds the ground antenna adjusts while the satellite is moving so that it will not constantly be 1.41 degrees behind the satellite. Timing inaccuracies and direction errors reduce the 0.7 degree theoretical pointing accuracy but it is estimated that the total pointing error will be at least one degree. At elevations closer to zenith, above 50 degrees, the azimuth changes very quickly and the pointing accuracy decreases, because of the one second update rate of the single serial port connection. Because of this a 2 degree ground antenna pointing accuracy