SSC04-V-4

Pedtke 1 18th Annual AIAA/USU Conference on Small Satellites

The Modular S-Band Radio Suite

Dan Pedtke, Mark LofquistAeroAstro, Inc.

20145 Ashbrook Place, Ashburn, VA 20147; 703-723-9800dan.pedtke@aeroastro.com, mark.lofquist@aeroastro.com

Kimberly KohlheppAeroAstro, Inc.

12 Farnsworth Street, 4th Floor, Boston, MA 02210; 617-451-8630kim@aeroastro.com

ABSTRACT: Radios are a necessity for every spacecraft mission, yet there is no existing family of radiosspecifically designed for nanosatellite-class spacecraft. Conventional spacecraft radios typically try to address awide range of missions with a single design. As a result, they often tend toward higher complexity, mass, powerconsumption and cost, making them less than ideal for use in small spacecraft, where mass, volume and powerconstraints are typically severe. AeroAstro’s Modular S-Band Radio Suite (MSBRS) is intended to specificallyaddress this otherwise neglected market. The MSBRS is a set of very small, inexpensive radio modules designed toenhance cost, volume, and mass efficiencies for 1 to 30 kilogram nanosatellites.

AeroAstro is developing the MSBRS for nanosatellites using a cost-effective mixture of commercial-off-the-shelfcomponents with proven design methods and innovative satellite engineering. Through its modular design, theMSBRS provides significantly more flexibility to tailor the radio to the needs of each particular mission. Forexample, if a mission needs more transmitted power to meet its link budget, then the appropriate high-poweramplifier module is incorporated into the “stack” of modules comprising the complete radio. MSBRS provides onlywhat the mission requires and doesn't burden the spacecraft with unnecessary and unused capabilities.

BACKGROUND

One of the primary obstacles for nanosatellites is a lackof suitable miniature components. Given the fact thatthere are limited resources available to develop newcomponents, there is a strong desire to focus on thosethat are most common among a variety of missions andwill therefore provide maximum benefit to the spacecommunity. Every space mission – nano, micro orother – has a need to communicate: send data to theground, receive commands, or both. In other words, allsatellites need a radio, yet no suitable radio subsystemsexist for nanosatellites.

The problem with conventional spacecraft radios, andspacecraft systems in general, are that they are designedto try and fit the largest number of vehicle types andmission scenarios as possible. In an attempt to be “allthings to all situations” the systems tend toward highermass, higher power consumption, higher complexity,higher cost, and less capability. While conventionalspacecraft designs can often tolerate these hits inperformance, they are show stoppers for nanosatellites.

A more elegant approach is that of focused performancethrough modular design – functional modularity.

Instead of thinking of the radio as a large monolithicdevice, it can instead be thought of as an ensemble ofwell-defined modules. Based on a mission'srequirements, the spacecraft designers can simply buildan appropriate radio using modules that meet individualperformance needs. For example, if one needs moretransmitted power to meet a specific link budget, thenall that need be done is to select the correct high-poweramplifier (HPA) module and integrate it with the rest ofthe radio components. One can even envision modulesthat operate at different frequencies, say S-Band and X-Band. In one instance an all S-Band radio may bepreferred; in another it may be desirable to have an S-Band receiver and an X-Band transmitter. With themodular radio concept, converting between one radioconfiguration and another is as simple as removing onemodule and replacing it with another.

AeroAstro is progressing toward making the modularradio for nanosatellites a reality with the developmentof the Modular S-Band Radio Suite (MSBRS).AeroAstro's MSBRS is a set of very small radiomodules specifically intended to address the needs ofnanosatellites, but capable enough to accommodate theneeds of larger spacecraft as well. By taking themodular approach instead of developing a radio for a

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specific set of requirements, AeroAstro is creating adesign solution flexible enough to suit both existing andfuture nanosatellite concepts.

GOALS AND ASSUMPTIONS

There are a wide variety of missions for whichnanosatellites are being considered. They can be in anynumber of orbital altitudes – low-earth-orbit (LEO),medium-earth-orbit (MEO), geostationary transfer orbit(GTO), or even interplanetary and extra-terrestrialmissions (e.g., a Venus Lander). And the missions canrequire a single satellite or perhaps as many ashundreds of satellites that must communicate amongstthemselves. To accommodate the variety of potentialmissions of interest, one must be able to provide severaldifferent communications capabilities:

• LEO-to-Earth Communications• MEO-to-Earth Communications• GTO-to-Earth Communications• Inter-satellite Communications

AeroAstro's goal in developing the MSBRS is toprovide a single radio architecture that encompasses allof these different forms of spacecraft communications.The basic assumptions for each of the four differentcommunications types include:

• LEO-to-Earth Communications: a circular800km altitude orbit with a 3-meter groundstationdish

• MEO-to-Earth Communications: a circular20,000km orbit with a 10-meter groundstation dish

• GTO-to-Earth Communications: an ellipticalorbit, with 600km perigee and 40,000km apogee,with a 10-meter groundstation dish

• Inter-satellite Communications: a circular 800km(LEO) orbit for one spacecraft and a GEO-stationary orbit for the other spacecraft

While there are a number of groundstation standards,they are all typically based on those recommended bythe Consultative Committee for Space Data Systems(CCSDS), so this was assumed to be the baseline forgroundstation compatibility. CCSDS is a near-universalstandard whose origins are based upon the older SpaceFlight Tracking and Data Network (STDN) standardused by NASA and the Deep Space Network (DSN).Additionally, there are the Air Force Satellite Control(AFSC) standards, which are also largely compatiblewith the CCSDS standards. The notable exceptionrelates to encryption, but this is at the “data” or “byte”level and the radio systems described here work at the“bits” level, so encryption can be performed externallyif required.

At the bit level, only modulation format need beconsidered. Standard modulation schemes such asBinary Phase Shift Keying (BPSK), Quadrature PhaseShift Keying (QPSK) and Offset QPSK (OQPSK) areall possibilities. Phase modulation and sub-carriers arealso possible, but are typically used for uplink only forlow-rate telecommand. So with CCSDS as thebaseline, and working only at the “bits” level, it thenbecomes possible to design radio systems that arebroadly compatible with almost any groundstation inthe world.

In terms of general mission parameters, several goalswere set to ensure that the MSBRS is applicable acrossa wide variety of missions. These goals are shown inTable 1.

Table 1. Mission Parameter Goals.

Specification Value

Lifetime 0 to 10 years

Data Throughput 0 to 100Mbps

Bandwidth 0 to 100MHz

Electrical Interface RS422 (typical)

Mechanical Interface Modular

Vibration Levels 20g rms (90 sec/axis)

Shock Levels 500g

Single Event Upsets(SEU's)

Tolerant

Single Event Latch-ups(SEL's)

Autonomous detectionand mitigation

Total Ionizing Dose(TID)

5krad (component level)

Note that radiation tolerance can, of course, beimproved significantly by putting aluminum around theelectronics. With such small modules, the mass impactof this approach is acceptably small, increasing the“box-level” TID tolerance to much higher levels.

There are essentially only three RF bands of generaluse, namely:

• S-Band (2 to 2.3 GHz)• X-Band (7 to 9 GHz)• K-Band (12 to 20 GHz)

Spacecraft communications in each of these three RFbands can be accomplished by using a set of core S-Band receiver (RX) and transmitter (TX) modules, andtransmit converters that heterodyne S-Band to X-Bandor K-Band as required. High-power amplifiers, or otheroptional components for each of the respective bandscan also be used as required for the particular mission.

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Based on all of these assumptions, AeroAstroperformed a set of analyses to create a set of basicspecifications for the radio modules.

DESIGN APPROACH

AeroAstro’s approach in development of the MSBRSarchitecture relies on the combination of several keyfeatures that will ultimately produce a nanosatelliteradio system of unprecedented utility, mass/volumeefficiency and cost effectiveness, including:

• Modularity• Small and lightweight• Low power consumption• Latch-up detection and mitigation (LDM)• Optional DC/DC converter module• Programmable transmit and receive frequencies• Fly-lead interfaces• Elimination of the need for a diplexer• Use of commercial technology

The modular design of the radio provides maximumversatility and adaptability to accommodate widelyvarying mission requirements and constraints. Eachmodule is 3” x 2” x 1” and weigh about 200g. Thereceiver module consumes only 1Watt of power (thenearest competitor uses 4W minimum). Some moduleshave LDM circuitry, so that if an SEL occurs, theelectronics are shut-down to prevent damage, allowingvery high-performance complementary metal-oxidesemiconductor (CMOS) circuits to be used inappropriate applications. An optional DC/DC convertermodule is available to accommodate a variety of busconfigurations and bus voltages. All frequency settingcontrols are programmable via embedded PIC®microcontrollers for ease of programming, eliminatingthe need for custom crystals. Further volume and masssavings can be made by eliminating connectors andusing fly-leads for connections.

A key innovation of the architecture is the eliminationof the need for the antenna-diplexer, saving significantmass, volume, complexity and cost – prime measures ofmerit for nanosatellite applications. The elimination ofthe diplexer is achieved through careful radio designand the use of a small component known as a“circulator”.

The antenna-diplexer connects a single antenna to twomodules – the TX and the RX – and provides electricalisolation between the RX and TX so that RX input doesnot overload when the TX is transmitting. AeroAstro iseliminating the antenna-diplexer by using distributedfiltering (isolation) in conjunction with a circulator. Acirculator affords a basic level of isolation between TX

and RX, as the antenna-diplexer does, but the majorityof the isolation is derived from within the RX and TXmodules themselves. Distributed filtering in the RXand TX uses very small filters at carefully selectedpoints within the radio circuitry where the losses havelittle negative effect, but enough of the necessarypositive effects to work together with the circulator toeliminate the diplexer and thereby reduce the size andmass of the overall nanosatellite radio subsystem.While a typical antenna-diplexer is at least 6” x 3” x 1”in size and has a mass of nearly 0.5 kg, the AeroAstrocirculator is nearly an order of magnitude smaller –much more suitable for nanosatellite radio.

In this age of wireless communications, there is anabundance of high-performance, mass-producedcommercial-off-the-shelf (COTS) integrated circuits(IC’s) utilized in a wide variety of miniature wirelessdevices. These COTS IC’s are manufactured by themillions, and exhibit very high reliability. AeroAstro istaking advantage of these new technologies, carefullyapplying appropriate COTS components rather thanusing expensive custom technologies to achieve furthersystem-level enhancements.

S-BAND RECEIVER

The S-Band Receiver is the most important part of thesystem, as it is on nearly 100% of the time, and DCpower consumption is ideally very low. Figure 1 showsa provisional block diagram of the S-Band Receiver.

Frontend Filtering

The RF input (RF i/p) is filtered and amplified by ahighly selective bandpass filter and low-noise amplifier(LNA). Use of such careful filtering makes it possibleto eliminate the bulky S-Band diplexer unit, requiringonly about 20dB of isolation, that can be afforded eitherby using physically separate antennas, or by using acirculator and a single common antenna. This approachis referred to here as distributed filtering.

Downconversion

The RF signal is then heterodyned or “mixed” down toan Intermediate Frequency (IF) near 170MHz, wherethe signal is then filtered by a Surface Acoustic Wave(SAW) filter for out-of-channel filtering.

I/Q Demodulator

The output is then amplified and applied to an I/Q (In-phase/Quadrature) demodulator, which corresponds tothe baseband output signal representations that the unitproduces when used inside a phase-locked loop (PLL).

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AGC/CarrierDemodLoop

BPFAMP

BPSKsub-

carrierDemod

BitSync

(4kbps)

Diff.Decod

LDMcct.

+V

DCccts

Rx DataO/P

RFi/p

Rx CLKO/PPIC

S-bandPLL

SAW

UHFPLL

VCXO(REF)

RF i/p

LO Verror

I/Q

MIX

BPFTone Ranging O/P

Figure 1: S-Band Receiver Block Diagram.

Carrier Demodulation Loop

The In-phase is a finite DC voltage, and the Quadratureoutput is at 0V when locked to a Continuous Wave(CW) signal. This forms the basis of the carrierdemodulation loop – an error amplifier/integrator drivesthe local oscillators, S-band and UHF PLLs, until theQuadrature voltage is zero. The loop parameters areadjusted so as to be able to accommodate and lock to anSTDN-compatible uplink, swept at up to 30kHz/secondto acquire lock. The carrier PLL demodulation circuitmust acquire lock and hold lock under Dopplerconditions.

Automatic Gain Control Loop

The I output corresponds to the phase-modulated carrieramplitude, and this is applied to an automatic gain-control (AGC) circuit error amplifier that adjusts theoverall RF-gain so that the amplitude remains constantindependent of input level. The loop parameters areadjusted so as to be fast enough to respond to signalfades, but not so fast as to introduce excessive noiseinto the loop.

Sub-Carrier Demodulation Loop

Next, once the RF carrier is suitably locked, the phasemodulation at 16kHz can be detected on the Q output asan AC content at 16kHz and is amplified and band-passfiltered with a bandwidth commensurate with themodulation data rate. The 16kHz sub-carriers areBPSK, and detection is achieved by heterodyning alocal 16kHz carrier that is phase locked to the samesignal. A 1-bit analog-to-digital converter (ADC) anddigital squaring loop is used to generate a pilot-tone forthe PLL to lock to.

Bit and Clock Recovery Loops

The resultant bit-stream is now filtered using a Raised-Cosine Filter, yielding an “eye-diagram” that, ifsampled optimally, can produce matched filterperformance. Another 1-bit ADC and digital squaringloop is used to generate a pilot tone equal to the bit-rate.The PLL is then locked to this, and this then optimallysamples the eye-diagram waveform for best noise-performance. The outputs are data and clock streams.

Telemetry Outputs

The telemetry outputs are Received Signal StrengthIndication (RSSI), Carrier Loop Stress (a measure ofthe carrier loop voltage), and “In-Lock” indicatoroutputs for each the carrier, sub-carrier and clockrecovery loops. The analog outputs can be either singleended or differential, while the digital outputs can beeither single ended CMOS or differential RS-422.

Latch-Up Detection and Mitigation

In most missions, the receivers are on 100% of the time,they are susceptible to latch-up when exposed to high-energy particles. Wherever possible, non-CMOSelectronics are utilized (which are essentially latch-up-proof), but space-efficient and highly miniaturizedCMOS electronics cannot be easily replacedconsidering what is available on the commercialwireless market today. It is possible, on a distributedbasis, to monitor the current consumption of CMOScircuits, and if a latch-up occurs, immediately shutdown the power to the circuit in question, hold it off fora short interval to ensure that the particle and depositedcharge have dissipated, and then reapply power. Thisshould extend the life of the electronics considerably.

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Specifications

The S-Band Receiver in conjunction with the S-BandTransmitter form the basis for the entire modularconcept, and can be considered as the IntermediateFrequency (IF) for the higher frequency applications atX or K-bands.

The nominal specifications for the receiver are shownin Table 2.

Table 2. S-Band Receiver Specifications.

Specification Value

Modulation BPSK (factory preset)

PM Modulation ±1 to ±2 Rads(user defined)

Sub-Carrier 16 kHz

RX Frequency Range 2025 MHz to 2120 MHz

Data Rate (typical) 2 kbps

Data Rate Range 1 kbps – 4 kbps

Bit Shaping Bessel Roofing

Sensitivity -110 dBm (BER=1e-6)@ 2 kbps

Lockup Time < 500msec (typical)

Noise Figure 6dB

Dynamic Range +38dB (SFDR)

Input Intercept Point -8dBm(±500kHz of channel)

TX Tolerance Level +57dBm(2200-2300MHz)

Carrier Track. Threshold -116dBm

Carrier Tracking Range ±10kHz

Spurious Response < -50dBc

Mass 180 grams

Volume 3" x 2" x 1"(LxWxH)

Current <200mA

DC Consumption <1 Watt

Supply Voltage +5V (+10%, -0%)

S-BAND TRANSMITTER

The S-Band Transmitter portion of the modular suite isthe core element for the down/return link. By carefulfiltering of the transmitter white noise at the RX band,and using a low-noise HPA, it is possible to eliminatethe S-Band Diplexer unit, when used in conjunctionwith the distributed filtering of the receiver unit.

The design for the S-Band Transmitter, shown in Figure2, is based upon the AeroAstro transmitters successfullyflown on the Canadian MOST mission. Thesetransmitters have been working as specified since themission was launched on June 31, 2003.

Frequency Generation

A common problem with the generation of BPSK andother such linear modulation formats is feedback intothe voltage-controlled oscillator (VCO) causingmodulation distortion, resulting in what appears asexcessive phase noise. One way to avoid this is to use ahigh degree of isolation between the VCO and theresultant modulation, but this requires extensive metalscreening.

Another approach is to derive the target outputfrequency by heterodyning two non-harmonicallyrelated frequency to produce the final output frequency.This completely avoids the problem and has the addedadvantage that one of the PLL’s can have a muchsmaller frequency step size than if only one PLL wasused to generate the output frequency.

The downside to this approach is the need for more DCpower and RF filtering for rejection of the unwantedmixer output products. However, with the wirelesshardware available today, this can be done very easilyusing hybrid PLL’s and ceramic filters.

Data Filtering

Downlink modulation from the command and datahandling (C&DH), or on-board data handling (OBDH),system is applied through Bessel low-pass filters for thenear-distortionless low-pass filtering, and then appliedthrough a Butterworth low-pass filter for ultimateattenuation well above the Bessel filter’s low-passcutoff frequency.

This analog implementation eliminates the need for asynchronized TX clock. This means that the data ratecan be changed dynamically and simplifies operationconsiderably. Additionally, there is no need todetermine whether the transmitter will be the masterclock or the slave clock, further reducing systemcomplexity and reducing current consumption.

If desired, it is possible to implement one of the low-pass filter stages as an analog approximation to a Root-Raised-Cosine (alpha=0.6, RRC) filter for optimumnoise performance. However, this requires a fixedtransmitter data rate.

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BesselLPF

PIC I/QMod

OscPLL BPF

AMPBPF

LDMcct.

+V

DCccts

RFo/p

DataI/P

ToneRangingI/P

ATTN

BPF

Figure 2: S-Band Transmitter Block Diagram.

Tone Ranging Option

The STDN-compatible receiver has the option of a3kHz to 110KHz audio output that can be directlyapplied to the transmitter Q-channel. In this mode, thetransmitter reference oscillator is also derived from thereceiver for coherency. By injecting a small DC offsetinto the I-channel, the result appears as a low-level PMmodulated sub-carrier. The turnaround ratio can beadjusted as required.

I/Q Modulation

The filtered bit-stream is applied to a Vector or I/QModulator, which enables BPSK or QPSK modulation.The output is band-pass filtered to further reducespurious emissions, and to reduce the wideband white-noise output, enabling the possibility of a diplexer-lesstransponder/transceiver.

Dual-Power Option

For highly elliptical orbits, it is necessary to have highand low power to establish a positive link budget atapogee, but not to exceed the ITU-stipulated maximumpower flux density (PFD) on Earth while nearingperigee. For this purpose, there is a 10dB step featurein output power to cater to this need.

Specifications

Table 3 shows the nominal specifications for thetransmitter.

Table 3. S-Band Transmitter Specifications.

Specification Value

Output Power 500mW and 20mW(adjustable)

Modulation BPSK

DC PowerConsumption

< 14W(varies w/supply voltage)

Supply Voltage +7.5V to +16V (rated)

DC to RF Efficiency 4% @ 16V, 8% @ 7.5V

Current 800mA (full power)

TX FrequencyRange

2200MHz to 2300MHz

Data Rate Programmable(76.8kbps baseline)

Data Rate Range 2kbps – 10 Mbps

Filtering Baseband roofing filter(linear phase)

Signal Quality <10% EVM rms<20% EVM pk

Clock None required

SpuriousSpecification

< -50dBc

Bandwidth < 400kHz @ -20dBc@ 76.8kbps

Mass 180 grams (total)

Volume 3" x 2" x 1"(LxWxH)

Spurious Response < -50dBc

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AMP 5W HPAO/P

HPA RFi/p

SequentialLogic

DCccts

+V

LPF

Figure 3: S-Band HPA Block Diagram.

+10VTx +28V

+5V

DC/DC

DC/DC

RECEIVERRx +28V

TRANSMITTER

Figure 4: DC-to-DC Converter Block Diagram.

S-BAND HIGH-POWER TRANSMIT AMPLIFIER

The S-Band High-Power Transmit Amplifier is anoptional modular element for the down/return link path.Figure 3 shows a provisional block diagram for theHigh-Power Amplifier transmitter module.

Gallium-Arsenide (GaAs) Amplifiers

There is an abundance of high-quality GaAs amplifiersoffering ample output power up to 5Watts. Usingcomponents with a 50 Ohm input and output isattractive as it minimizes the amount of circuit boardspace required. Gain and noise filtering are carefullyadjusted so as to minimize wideband noise transmittedat the uplink receiver frequencies.

Sequential Logic

The Sequential Logic circuit ensures that all DC circuitsare synchronized such that the highly sensitive GaAsamplifiers are not stressed during turn-on or turn-off.

Specifications

Table 4 shows the nominal specifications for thiscomponent.

Table 4. S-Band HPA Specifications.

Specification Value

Output Power 5W pep (+37dBm)

IP3 +49dBm (min)

Linearity Class Linear (class A/AB)

DC PowerConsumption

<25 Watts (varies withsupply voltage)

Supply Voltage +7.5V to +16V (rated)

DC to RF Efficiency 20% @ 16V, 40% @ 7.5V

Current 1.6A (full power)

TX Frequency Range 2200MHz to 2300MHz

Mass <200 grams (total)

Volume 3" x 2" x 1" (LxWxH)

DC-TO-DC CONVERTER MODULE

Figure 4 shows a provisional block diagram of the DC-to-DC Converter.

In most nanosatellites, space, size and power efficiencyis at a premium, and distributed DC/DC convertersimpose upon all these factors. A highly efficient,centralized DC/DC converter would be a bettersolution. Nonetheless, there will be applications wherecompliance with a 28 or 42V bus will be required, orwhere DC isolation and the prevention of ground loopsare of paramount concern.

BENEFITS

Flexibility of the Modular Approach

The flexibility of the modular radio architecture is themost important advantage the MSBRS design has overtraditional radio designs. This flexibility extends acrossseveral different parameters, offering numerous benefitsto this approach.

As shown in Figure 5, there are many possible missionorbits that are typically required of nanosatellitemissions, ranging from LEO, to GTO, to interplanetary.AeroAstro’s Modular S-Band Radio System (MSBRS)effectively forms an Intermediate Frequency (IF) core,enabling not only S-Band operations, but when used inconjunction with the modular converter modules,operation in the X-Band, K-Band and other bands isalso possible, where each band may prove desirable as afunction of orbit and groundstation availability.

In addition to the flexibility of using the same basiccomponents across a range of operating bands, themodularity of the MSBRS architecture enables thespacecraft systems engineer to tailor the radio design totheir particular mission and assemble only thoseelements required to meet mission communicationobjectives – and nothing more.

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Figure 5: MSBRS Modular Concept.

Additionally, the customer can arrange the MSBRSmodules in whatever physical configuration mostbeneficial to the overall spacecraft layout. Modules canbe placed or “strung along” in otherwise unusablespaces in the spacecraft structure. The HPA can belocated closer to the antenna to maximize efficiency,while the receiver can be kept closer to the core of thesatellite for radiation shielding or a better thermalenvironment. Alternatively, the customer can combinethe modules in a more conventional fashion into a“block” and place all the communications hardware inone place.

Beyond Modularity

The extremely small size and mass, low powerconsumption, and reduced complexity of the MSBRSdesign facilitates true “nanosatellite” spacecraft in the 1to 30kg range.

By making use of leading-edge commercial-off-the-shelf (COTS) wireless electronics for both digital andanalog elements, very small systems can beimplemented. Today’s analog op-amp technologyoffers very low power consumption while providinghigh-speed operation. Combined with ultra-low powerCMOS digital electronics, it is possible to implementSTDN, SGLS (less the encryption element) and othercommon satellite communication standards usingextremely small amounts of DC power.

Typical small spacecraft communications systemrequirements are fairly straightforward, and the radiosused to address these requirements need not be overlycomplex. Most spacecraft communications systemsutilize a well-defined modulation scheme and dataformat. Applying cutting-edge COTS components to astraightforward and simple set of requirements enablestremendous performance enhancement at minimal cost.

One can draw an analogy of the concept of using COTScomponents for space applications to that of usingtoday's PC to run a perfectly adequate DOS designprogram from 10 years ago – the speed gain isenormous, and none of the functionality is lost – onlythe “bells and whistles” not necessary for spaceapplications are missing.

This system offers high-performance at low cost, withthe flexibility of a modular system and the low powerconsumption necessary to enable nanosatellites today.

HERITAGE

AeroAstro developed two nanosatellite-sizedtransmitters for the Canadian Space Agency (CSA) andthe University of Toronto Institute of AerospaceStudies (UTIAS) Space Flight Laboratories (SFL) forthe Microvariability of Stars (MOST) mission. Thesetransmitters were the precursors for the MSBRS S-Band Transmitter Module.

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Figure 6: Detailed view of a MOST transmitter.

Both transmitters (one of which is shown in Figure 6),were built in less than three months, at low cost, and arecurrently providing on-orbit communications for the

mission. MOST was launched on June 30, 2003 fromRussia, and successfully commissioned on July 1, 2003.The downlink was determined to be 100% nominal asper pre-launch tests and continues to function in anominal fashion. The MOST mission is in a 600kmcircular orbit, with an expected lifetime of at least oneyear.

ACKNOWLEDGMENTS

The development of the Modular S-Band Radio Suite isbeing performed under DARPA Small BusinessInnovation Research (SBIR) Program funding, througha contract let by the US Army Aviation and MissileCommand (contract number W31P4Q-04-C-R088; Mr.Charles Piner, COTR).

AeroAstro would like to gratefully acknowledge thedirection and guidance provided by Dr. GordonRoesler, Program Manager in DARPA's TacticalTechnology Office (TTO).