1. Capstone 2012-2013 PolarCube PDR
October 16, 2012

2. Table of Contents Overview and Project Motivation
Scientific Theory and Radiometry
Overview of Polar Cube Payload and Physical Constraints
Radio Frequency Receiver
Diplexer
Intermediate Frequency System
Operational Calibration
Digital Board
Sampling and Power Constraints
Milestones
Acknowledgements

3. 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 K
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

4. 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

5. ALL-STAR Satellite Bus

6. Microwave Sounding: Frequency vs. Zenith Opacity
Airborne compounds selectively pass certain bandwidths of electromagnetic radiation while attenuating others
PolarCube will use GHz, a resonance frequency of diatomic oxygen, as a center frequency
This frequency was chosen because it is stable and well known
Microwave Sounding
Due to quantum mechanical effects of airborne compounds, primarily water vapor and diatomic oxygen, the Earth’s atmosphere selectively passes certain bandwidths of electromagnetic radiation while significantly attenuating others.
Courtesy of Dr. Gasiewski

7. Weighting Function Profile for 8 Channels
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)
Courtesy of Lavanya Periasamy, University of Colorado

8. 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 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.

9. Block Diagram of the Bus and Payload

10. Block Diagram of the Radiometer

11. RF Receiver
Zooms in on RF

12. RF Receiver
Zooms out on RF

13. IF System
Zooms in on IF

14. Courtesy of Space Grant Consortium, University of Colorado
IF System
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 YL
As we can see from the previous slides, the IF circuit boards will be assembled into the two sides of the polar cube satellite. Since the diplexer connected directly with the IF higher frequency circuit board. There will be an additional carrier next our higher frequency circuit board carrier.
Courtesy of Space Grant Consortium, University of Colorado

15. 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. YL
two signals out of the diplexer will go through two different amplifiers. They will amplify the power of the two different frequencies, which are the high band(1.3Ghz - 6.5Ghz) and the low band(.1Ghz-1.3Ghz)
Courtesy of Space Grant Consortium, University of Colorado

16. 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

17. 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

18. 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

19. 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

20. Courtesy of Kyuil Hwang
Low Pass Filter
Bandwidth: 0.1GHz~1.3GHz
Cutoff Frequency: 1.3GHz
Courtesy of Kyuil Hwang

21. Courtesy of Kyuil Hwang
High Pass Filter
Bandwidth: 1.3GHz~6.5GHz
Courtesy of Kyuil Hwang

22. 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)
Let’s take a close look at the signal passing through one channel as a demonstration, other channels are similar. As we mentioned before, our goal is to reduce the high frequency signal(GHz) to a low frequency signal(KHz) so that we are able to analyze them easily. And the intermediate frequency part of the radiometer contributions to this procedure. Basically, when the signal comes out from the power splitter, the first component it will pass is the band pass filter(abbreviation BPF). As you can see, the BPF has two values, the CF is short for the center frequency of the frequency channel we need; and BW is the bandwidth of the frequency channel. We calculate these values mostly based on the temperature weighting function that Chris mentioned before. 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.

23. 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)
After the signal was filtered, it will pass through the diode detector. The function of the diode detector is the square law, which means the output voltage has the square value of the input voltage. The input signal of the diode in the frequency domain will look closely like one of the second graph. The diode detector will provide the output signal that looks like the third graph. The transformation behind this is after the diode detector, we have v2(t) in time domain, but in the frequency domain, we will have the convolution of Fourier transform of the signal by itself, which makes the signal looks like the graph on the bottom. As you can see, the bandwidth does not change, but there is one more triangle signal appears with twice center frequency and same bandwidth as the BPF. After this diode, our signal frequency range will reduce to from 0 to B MHz.
Slides from ECEN5254 Remote Sensing Signals and Systems

24. 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.
Last part is the RC circuit, which has the function as a lowpass filter. It will ideally only pass all the frequency between 0 and the particular limit frequency. The output signal will be like the second graph. As we can see, the signal is filtered to a lower frequency with a bandwidth 1/τ. However, 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, it won’t affect our result that much. Since we have the relation that 1/tau << B, so eventually our signal will be only left like the signal in the second graph. The bottom graph is a comparison of the input and output of the RC circuit. After the RC circuit, our signal frequency range will reduce to KHz.

25. Signal detection
In conclusion, the difference of the input and output signal of the whole intermediate frequency circuit board will look like the graph here. We have a large range signal coming in as the input of the BPF, and after the three main functions we discussed in the previous slides, the output signal of the IF circuit board will be reduced to a low frequency signal. So we succeed the converting from the high frequency signal to a low frequency signal.
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.

26. IF System
Zooms in on IF

27. Digital Board
Zooms in on RF

28. 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:
The mixer using thermistors
Cold space by rotating the lens towards zenith (away from earth)
Noise will be be added and removed from the system using a PIN diode
the ratio of the system with and without noise will be used to determine the system's gain due to temperature
Calibration of the system is necessary because RF hardware is very susceptible to variance so a calibration check must be made on every rotation of the satellite. We will be using a Y-Factor test to calibrate our system. This entails measuring the voltage at two different known temperatures multiple times which for us will be the temperature of the mixer and cold space. Y-factor is a ratio of two noise power levels, one measured with the noise source ON and the other with the noise source OFF:
Y = NON/NOFF
Because noise power is proportional to noise temperature, it can be stated:
Y = TON/TOFF
The instruments mentioned above are designed to measure Y-factor by repeatedly pulsing the noise source ON and OFF. NON and NOFF are therefore measured several times, so that an averaged value of Y can be computed
These two voltages and temperatures are plotted and then the newly measured data can be plotted off this line. In this way we maintain a high standard of calibration and data collection.

29. 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

30. 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
•The digital subsystem is tasked with reporting the status of the PolarCube payload as well as processing sensed data and transferring it to the ALL-STAR bus for transmission to ground.
•The main functional components of the digital system consist of a microcontroller and two ADCs which are read by the microcontroller.
•Data provided to the ALL-STAR bus by the payload will consist mainly of averaged over-sampled data from the ADC, which converts the filtered output of the IF system.
•Status information that will need to be provided by the payload include reporting that digital systems are functional and operating (OK status) to the ALL-STAR bus when queried in accordance with the ICD. Additionally, for the purpose of diagnosing issues regarding payload-provided data and reporting when data is reflecting an error condition, diagnostic data points will occasionally be reported to ground.

31. 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 ms
Courtesy of Space Grant Consortium, University of Colorado: ALL-STAR ICD

32. 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 MB

33. 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

34. Milestones
Digital and RF/IF implementation, prototyping - Begin week of October 22nd
Begin prototyping designs for diplexer and IF carriers
Digital design milestone - Begin week of November 29th
Demonstrate functional software and digital hardware correctly operating in conjunction with previous project radiometry prototype
Final architecture and requirements specification - Week of January 14th
Finalize ALL-STAR ICD and digital system commands and modes of operation
Integration and testing - mid-February through mid-March
Test digital and RF/IF systems for correct functionality, appropriate system interoperability, and performance

35. Acknowledgements Special Thanks to: Dr. Gasiewski
Brian Sanders and the team at Space Grant
Lavanya Periasamy
Kyuil Hwang

36. Questions?