Capstone 2012-2013PolarCube PDR

October 16, 2012

Table of Contents

1. Overview and Project Motivation

2. Scientific Theory and Radiometry

3. Overview of Polar Cube Payload and Physical Constraints

4. Radio Frequency Receiver

5. Diplexer

6. Intermediate Frequency System

7. Operational Calibration

8. Digital Board

9. Sampling and Power Constraints

10.Milestones

11.Acknowledgements

Project Motivation

• Our payload will measure air temperature at various altitudes with a surface spot size of 15 km and temperature accurate to within 2 Ko This level of accuracy allows for the data to be used

for meaningful weather prediction

• Our primary science mission is to track polar ice retreat

• The payload will be smaller, lighter, and cheaper than current weather satellites

• A reproducible satellite bus allows for the possibility of a fleet of low cost satellites which will increase the amount of useful data collected

Previous works

• The ALL-STAR satellite bus, which will carry our payload, was designed by an interdisciplinary engineering team working with the Colorado Space Grant Consortium

• We will work closely with members of COSGC to integrate our payload with their satellite bus.

• Team TeamWork, an EE capstone group from fall 2011, created a proof of concept radiometer, which we will further miniaturize and integrate with the ALL-STAR bus

• Scientific guidance regarding radiometry and remote sensing will be provided by Dr. Al Gasiewski

ALL-STAR Satellite Bus

Microwave Sounding:Frequency vs. Zenith Opacity

Courtesy of Dr. Gasiewski

• Airborne compounds selectively pass certain bandwidths of electromagnetic radiation while attenuating others

• PolarCube will use 118.75 GHz, a resonance frequency of diatomic oxygen, as a center frequencyo This frequency was

chosen because it is stable and well known

Weighting Function Profile for 8 Channels

Courtesy of Lavanya Periasamy, University of Colorado

• PolarCube will measure 8 channels above the center frequency

• Each channel has a corresponding weighting function which gives that channel's weight on atmospheric temperature as a function of altitude

• This graph shows the weighting function of all 8 channels at nadir (facing downwards at the earth)

ALL-STAR Payload Physical Constraints

• �The PolarCube will be designed by implementing the 3U CubeSat specifications to be connected to the primary ALL-STAR system housing

• �The length of the PolarCube must be 16.50 cm

• �The volume of the PolarCube payload is 1277 cm3

• �Mass must not exceed 2000 g (including 81.19g structure)

• �Center of gravity must be within 2 cm of the geometric center of the payload.

�

Block Diagram of the Bus and Payload

Block Diagram of the Radiometer

RF Receiver

RF Receiver

IF System

*

IF System

Courtesy of Space Grant Consortium, University of Colorado

• The IF circuit boards will be assembled into the two sides of the PolarCube payload.

• The diplexer connected directly with the IF higher frequency circuit board, located in the right hand box.

• The IF low frequency board will be located in the left hand box

*

Function of Components• Amplifiers:

two signals out of the diplexer will go through two different amplifiers. They will amplify the power of the two different frequencies.

• Two splitters:

equally divide the power to 4, then feed

to each individual channel• Eight filters:

the band pass filters will choose the frequency interval that we want.

• Diode detector & RC circuit:

transform the signal to DC.• 8 Amplifiers: amplify the DC signal.

Courtesy of Space Grant Consortium, University of Colorado

IF System To-Do List

• Diplexer design

• Amplifier and power splitter for both signal bands

• Diode detector & RC circuit design

• Downsize the PCB boards in order to match the mechanical requirement

Reason Why We need a Diplexer

• The diplexer splits the signal from RF Receiver into two bands

• The diplexer is required because the amplifiers and splitters after the diplexer are not designed to work with such a wide band of frequencies

Microstrip

• Microstrip is a electrical transmission line made using a printed circuit board

• The bottom layer is a conductive ground plane covered by a dielectric substrate. Above this is a conductive trace used for the signal

Advantages of using Microstrips

• Microstrip transmission lines are commonly used for microwave-frequency signals to reduce crosstalk

• We will use them for both signal traces and implementing low-pass and high-pass filters

Low Pass Filter

Courtesy of Kyuil Hwang

• Bandwidth: 0.1GHz~1.3GHz

• Cutoff Frequency: 1.3GHz

High Pass Filter

• Bandwidth: 1.3GHz~6.5GHz

Courtesy of Kyuil Hwang

Signal detection

• Bandpass Filter(BPF CF/BW MHz) CF: the center frequency of the frequency channel BW: the bandwidth of the frequency channel

• The temperature weighting function

• On the frequency domain: The BPF will ideally pass all frequency between two limits frequencies, which is the lower and upper cutoff frequency, and bar all other signal with frequency that does not within the limits.

Va(t) Vb(t)

Signal detection

• Diode detection: the function of the diode detector is the square law, which means the output voltage has the square value of the input voltage.

• Transformation:

• Output signal: in the frequency domain, we

will have the convolution of Fourier transform of the signal by itself.

VVB(t) VVD(t)=VVB2(t)

Slides from ECEN5254 Remote Sensing Signals and Systems

Signal detection• RC circuit: it has the function as a lowpass

filter. It will ideally only pass all the frequency between 0 and the particular limit frequency.

• Output signal: the out signal is filtered to a lower frequency with a bandwidth 1/τ, which is the cutoff frequency of the RC circuit. The signal with frequency that’s not within the bandwidth may also pass through it a little bit, and we call it the roll off signal.

• Comparison: the dot line is the diode detector output signal, the real line is the RC circuit output signal. Since 1/τ << B, After the RC circuit, our signal frequency range will reduce to KHz.

Signal detection

• Signal processing: the input and output signal of the whole intermediate frequency circuit board will look basically like the two graphs. And the three main function, which are BPF, diode detector, and the LPF(RC circuit) will reduce the frequency signal.

IF System

Digital Board

Operational Calibration

• RF hardware is susceptible to variance in readings due to temperature change making calibration necessary

• We will use the Y-factor method for calibration, which requires measuring two known temperatures:o The mixer using thermistorso Cold space by rotating the lens towards zenith (away

from earth)

• Noise will be be added and removed from the system using a PIN diodeo the ratio of the system with and without noise will be

used to determine the system's gain due to temperature

Spatial sampling

• Each rotation of the spacecraft will produce a single line of horizontal resolution--termed a raster

• Movement across the surface of the Earth to generate sequential raster lines will be a consequence of orbiting the Earth

• Rotation rate of the spacecraft is determined by mechanical constraints of the bus and science mission requirements

• Rotation rate will be tentatively one rotation per second

• Antenna beamwidth determines the spot size on the ground, and will be approximately 15 km2

Digital sampling specifics

• ADCs will have 24 bits of output

• Samples will be averaged to further account for noise

• Range of ADC input voltages: 0-2.5V

• Number of records in raster: 52

• Data rate: 530 kbps > ALL-STAR data rate

Sample Averaging

• Average 32 digitized samples for each channel to provide more accurate measurements and further account for noise

• For diagnostic purposes, calculate standard deviation of sample results in software and provide to ground when commanded to

v0 = <v0> + <noise>

σ/<v0> = 1/sqrt(Bτ) ≈ 0.01

τ ≈ 0.1 msCourtesy of Space Grant Consortium, University of Colorado: ALL-STAR

ICD

Power and Communication System Constraints

Power Supply

• Nominal voltages: 3.3 and 12 volt rails and unregulated battery at 8.4 volts

• Power: 4 watts with occasional peak power draw of 30 W for 15 minutes once every orbit (when the COM system does not require it)

Communication System

• Information Uplink Rate: 9.6 kbps

• Information Downlink Rate: 250 kbps

• Downlink Visibility Time: 9 min

• Total Downlink Potential Per Orbit: approx. 16.88 MB

Digital To-Do

• Decide on A/Ds and a microprocessor

• Design the PCB for the digital board

• Implement science data format

• Write software to handle commands from ALL-STAR, control data transfer to ALL-STAR

• Implement software to display/analyze data from PolarCube after its reception on the ground

Milestones

• Digital and RF/IF implementation, prototyping - Begin week of October 22nd o Begin prototyping designs for diplexer and IF carriers

• Digital design milestone - Begin week of November 29tho Demonstrate functional software and digital hardware correctly

operating in conjunction with previous project radiometry prototype

• Final architecture and requirements specification - Week of January 14tho Finalize ALL-STAR ICD and digital system commands and

modes of operation

• Integration and testing - mid-February through mid-Marcho Test digital and RF/IF systems for correct functionality,

appropriate system interoperability, and performance

Acknowledgements

Special Thanks to:Dr. GasiewskiBrian Sanders and the team at Space GrantLavanya PeriasamyKyuil Hwang

Questions?