Minimize INSPIRESat-1

INSPIRESat-1 (International Satellite Program in Research and Education Satellite-1)

Sensor Complement   Spacecraft   Launch    References

Initiated by the CU/LASP (University of Colorado/Laboratory for Atmospheric and Space Physics) in 2015, the INSPIRE (International Satellite Program in Research andEducation) is a multinational consortium of universities collaborating to develop a constellation of small satellites for cutting edge space and earth science research, a supporting global ground station network, as well as research and educational programs covering spacecraft design, space systems engineering, operations, and data analysis. 1)

INSPIRESat-1 is a 9U CubeSat mission carrying the DAXSS (Dual Aperture X-ray Solar Spectrometer) instrument. The INSPIRE consortium participants involved in the mission and spacecraft design of INSPIRESat-1 include CU/LASP, IIST (Indian Institute of Space Science and Technology), Thiruvananthapuram, Kerala, India, and NCU (National Central University), Taoyuan City, Taiwan.

Since late 2015, graduate and undergraduate students at the aforementioned institutions have been engaged in developing parallel mission concepts, systems definitions, and preliminary designs for INSPIRESat-1. Building on past experience in scientific payload design and aeronomy research, the INSPIRESat-1 student design team at NCU conducted a semester-long feasibility study during Winter 2015 with guidance from the Taiwan NSPO (National Space Organization). The objective of this study was to formulate the mission concept and subsystem requirements for INSPIRESat-1, in order to determine whether such requirements can be met using COTS (Commercial Off The Shelf) components. This feasibility study was then subjected to a SDR (Systems Definition Review).

The initial COTS-based SDR design was subsequently revised during Spring 2016, incorporating updated DWTS payload requirements, more advanced thermal and structural analysis, as well as the preliminary design of laboratory prototype subsystems. This work culminated in a PDR (Preliminary Design Review) in June 2016.

The Middle and Upper Atmosphere:

The Earth's middle and upper atmosphere is defined as consisting of the region spanning the stratosphere (~20 – 60 km), mesosphere (~60 – 90 km), thermosphere (~90 – 1000 km). Unlike the troposphere (~0 – 20 km), which is home to nearly all human activity, water vapor, and what is traditionally thought of as "weather", the middle and upper atmosphere are dry, with densities rapidly decreasing over an extremely large vertical range. The neutral atmosphere of this region has also been extremely difficult to study on a global scale in the past, with in-situ instruments limited by the horizontal and vertical range of their balloon or sounding rocket launch vehicles.

Despite this, the middle and upper atmosphere are known to play an important role in the Earth's atmospheric environment, climate, and weather, as well as having significant influences on atmospheric and space operations and technology. The structure and stability of the stratospheric polar vortex is known to play a large role in influencing tropospheric pressure gradients and the jet stream, manifesting in wintertime weather as the Arctic Oscillation.

Incorporation of assimilated stratospheric observational data into weather forecast models has become recognized as being crucial in forecasting such weather phenomena. On a climatological scale, photodissociation of stratospheric molecular oxygen by solar middle and far ultraviolet radiation is the formation mechanism for the ozone layer, whose variation is strongly influenced by temperature dependent chemical processes, as well as transport by stratospheric winds such as the Brewer-Dobson Circulation.

Further aloft between 90 – 1000 km, the thermosphere forms what is considered to be the outermost layer of the Earth's atmosphere, extending across altitudes covering much of LEO (Low Earth Orbit). Despite its high altitude, the thermosphere, as well as the ionosphere formed from photodissociation of thermospheric air by solar extreme ultraviolet radiation, have long been known to exert a significant influence on space operations and wireless communications technologies, while also serving as a key interface in the solar-terrestrial system. 2) 3)

The neutral atmospheric density of the thermosphere produces satellite drag on spacecraft in LEO, forming one of the major orbital perturbations for operational spacecraft in this region and rendering orbits with altitudes less than 300 km unstable for missions longer than one year. Monitoring and forecasting of thermospheric drag conditions and variability have become crucial for LEO operations, using empirical models such as the Jaccia-Bowman (JB) series and more recently physics-based models such as the Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM).

Photoionization of the neutral thermosphere by solar extreme ultraviolet radiation forms the plasma of the ionosphere, which acts to refract and reflect radio waves from terrestrial and satellite transmitters. Such ionospheric refraction acts to produce scintillation effects on satellite communications, while also affecting the propagation of long range HF radio communications.

Variability in both the thermosphere and ionosphere is strongly affected by the wind and temperature fields of the thermosphere and mesosphere, whose dynamics are dominated by both vertically propagating waves and tides excited from the lower atmosphere, as well as in-situ forced effects. Such dynamical drivers of thermosphere and ionosphere variability can include the cumulative effects of composition changes driven by breaking atmospheric waves in the thermosphere, modulation of the ionospheric wind dynamos tidal/planetary wave winds, and propagation of such waves and tides into the thermosphere from the mesosphere.

It is apparent that the need exists for global scale observations spanning the entire vertical domain of the middle and upper atmosphere. This observational need is complicated by the fact that the atmospheric tides driving middle and upper atmospheric dynamics have periods that are harmonics of a solar day and zonal wavelengths that are harmonics of the Earth's circumference. The tides can therefore be aliased with background zonal mean zonal winds and stationary planetary wave fields if proper longitude/local time sampling requirements are not satisfied. Unambiguous resolution of the global structure of these tides requires global scale sampling over 24 hours of solar local time in all longitude zones, ideally from pole to pole. Spacecraft in Sun-synchronous orbits, as well as low inclination orbits are therefore, respectively, unable to meet these local time and latitude sampling requirements. Furthermore, these two sampling requirements are mutually contradictory, as the high inclination orbits required for broad latitudinal sampling possess a slower local time precession rate due to nodal regression. For example, the 74.1° inclination, 625 km circular orbit occupied by the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) spacecraft requires 60 days of observations to satisfy the sampling requirements necessary to resolve the tides, severely limiting the resolution of sub-seasonal scale variability.

 


 

Sensor complement: (CIP, DAXSS)

The science aim of the mission is twofold. First is to use CIP to study the ion composition, ion densities, ion velocities, and ion temperature of the ionosphere at mid altitude regions and secondly use DAXSS to measure the soft X-ray (SXR) spectrum of the sun during a quiescent period through solar maximum. With this data we can study solar flares, solar cycles, and try to understand the process of the sun's coronal heating. Beside the scientific aim, the goal of the mission is to develop a small spacecraft by collaboration among different institutes all around the world. The mission statement is as follows: 4)

• Improve the understanding of Ionosphere dynamics through observations of ion temperature, composition, density and velocity. Improve our understanding of the sun's coronal heating processes by measuring the Soft X-Ray spectrum of the sun.

 

CIP (Compact Ionosphere Probe)

CIP will take in-situ measurements of ion density, composition, temperature, velocity, and electron temperature. The CIP is a smaller version of the Advanced Ionosphere Probe (AIP, both developed in Taiwan) currently operating onboard the FORMOSat-5 mission. This instrument is capable of sampling the ionosphere at 1 and 8 Hz.

The focus of CIP data collection is on eclipse time and the transition time from sunlight to eclipse time while that of The Dual Aperture X-ray Solar Spectrometer (DAXSS) is at sunlight side.

The CIP, developed by Space Payload Laboratory at NCU (National Central University) of Taiwan, was selected as a science payload in the 1st International Satellite Program in Research and Education (INSPIRE) Workshop held at NCU in 2016. The CIP, a successor of Advanced Ionospheric Probe, is an all-in-one plasma sensor that uses a single instrument to perform multiple sensor functions in a time-sharing mechanism. 5)

CIP performs in-situ measurements of the ionospheric plasma compositions, ion concentrations, velocities, and temperatures to explore the terrestrial ionosphere.

The data collected by the CIP will give insight to ionospheric phenomena like plasma bubbles and mid-night temperature maxima. The plasma bubbles can cause scintillation in the GPS signals. The ion density will identify the plasma bubbles since the satellite will pass through these bubbles at some angle which will be indicated by the sudden drop in the ion density measurement of the CIP. The Figure 1 shows the plasma bubbles identified by the data collected by the IVM (Ion Velocity Meter) instrument.6

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Figure 1: Plasma bubbles identified in C/NOFS data (image credit: CIP Team)

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Figure 2: Illustration of the CIP mechanical structure (image credit: CIP Team)

The CIP instrument contains three circuit boards:

1) Analog Preprocessing Unit (APU): It transduces currents from the sensor into digital signals.

2) Digital Control Unit (DCU): It controls digital to analog/analog to digital converters (DA/ADs) on APU board, monitors PCBs' temperature, and receives commands/sends science data from/to C&DH.

3) Power Management Unit (PMU): It supplies and regulates necessary powers for CIP.

The preliminary specifications of the CIP are present in Table 1. The data is transferred to the C&DH using UART over RS422 lines. The C&DH communicates with the CIP at 115,200 ±1% baud, with 1 start bit, 8 data bits, and one stop bit.

Parameter

Value

Mass

0.5 kg

Dimension

0.7U

Voltage

12 V

Power

3.45 W

Interface

UART, 115200 bps (RS422)

Connector

9-pin D-sub female

Table 1: Preliminary CIP specifications

Modes of Operation: CIP has the following operation modes:

• Planar Langmuir Probe (PLP): Measures electron temperature

• Ion Drift Meter (IDM)/Ion Trap (IT): Provides arrival angles of ion velocity and ion density.

• Retarding Potential Analyzer (RPA): Measures ion temperature, ion composition, ion density and ion ram velocity.

Each mode can either be in normal mode or fast mode. There is one data packet available every second in normal mode and 8 every second in fast mode.

The C&DH formats CIP commands into 8 bytes consisting of 2 header bytes, 1 function code byte, 4 data bytes (time tag or command), and a checksum byte computed using 3rd through the 7th command bytes. The CIP will reject commands with bad framing or checksum. The C&DH sends all commands to the CIP within the 900 ms interval of one of the one second (1PPS) pulses, as shown in Figure 3:

InspireSat1_Auto9

Figure 3: Command synchronization with the PPS signal (image credit: CIP Team)

Two kinds of science data packets are available. One is the raw science data packet (280 bytes) and the other is processed science data packet (60 bytes). The CIP raw science data packets are 280 bytes long. Each packet shall start with a 23-byte header that consist of 2 start bytes, 1 function code byte, 4-byte data time tag, and 16-byte status of health (SOH) data. The header will be followed by 256-bytes of CIP science data. The data will be followed by one-byte checksum. The checksum byte shall be constructed using the 3rd byte through the 279th byte of this data packet. The processed science data consists of 23 bytes header, 36 bytes of science data (processed) and 1 byte of checksum constructed using the 3rd byte through the 59th byte.

CIP shall transmit all data packets within the period starting at the leading edge of the C&DH PPS signal and ending at least one-half period before the following PPS. 6) 7)

 

DAXSS (Dual Aperture X-ray Solar Spectrometer)

DAXSS is a modified Amptek X123 that will observe Solar X-rays, specifically soft X-rays. Hot plasma in the sun's corona is best measured in the soft X-rays. Many emission lines for important elements (Fe, Si, Mg, S, etc) are in the soft X-ray range. Soft X-rays are always present in the sun but 100 times brighter during flares, these observations will also lend to understanding the temperature difference between the sun's corona and photosphere.

The DAXSS payload is comprised of an X-ray Spectrometer, a Solar Position Sensor (SPS), a small Solar Irradiance Measurement instrument (PicoSIM), a CDH board, an EPS board, and an interface board. The X-ray spectrometer is the commercial off the shelf (COTS) Amptek X123 Fast Silicon Drift Detector (SDD) with Si-PIN detector and beryllium filter which can detect Soft X-ray (SXR) photons with energies between 0.5-30 keV with better than 0.15 keV energy resolution. The SPS/PicoSIM package is designed in-house and the SPS can achieve solar pointing knowledge of a few arc-seconds with an 8-degree field of view. All flight boards are designed in-house and both the CDH (Command & Data Handling) and EPS (Electrical Power Subsystem) boards have flight heritage on both of the MinXSS-1 and MinXSS-2 satellites.

The SXR regime measurable by DAXSS is desirable for solar physicists studying the sun. Soft X-rays are constantly emitted by the sun but can be up to 100 times brighter during solar flares. The motivation behind looking at this portion of the solar spectrum is that it may be used to better model and predict atmospheric heating and expansion, satellite drag, communication blackouts, power grid failure, and spikes in harmful radiation. The sun's continuum is observable between 0.5-30 keV and furthermore there are many emission lines which can be used to identify the relative abundances of elements present in the sun.

By combining DAXSS spectra with solar-atomic databases such as CHIANTI [8) 9)], a model can be formed to calculate the temperature and emission measure of the sun at the time of observation. Through comparing the intensity of emission lines, the first ionization potential (FIP) factor or relative abundance of elements in the sun can further be added to the model. With the data collected by DAXSS the scientific community moves closer to obtaining answers to important questions such as why is the temperature of the sun's corona orders of magnitude hotter than its photosphere? And why do the abundances of elements change during different solar events?

DAXSS is a continuation of the three science instruments that flew on the MinXSS-1 CubeSat 10) in 2016-2017 and the MinXSS-2 CubeSat 11) in 2018-2019. The exact instruments of DAXSS were flown and recovered from a June 2018 sounding rocket flight and provided the highest resolution X-ray solar spectrum from an X123 instrument and has enabled identification of many more elements in the solar SXR spectrum. 12)

One significant improvement of the X-ray spectrometer of DAXSS over that of the MinXSS satellites is in its dual aperture design. This novel design, shown in Figure 4 incorporates a thin Kapton filter to attenuate lower energy photons between 0.5-2keV while allowing higher energy photons between 2-30keV to penetrate through. The dual aperture enables detection of higher energy photons in greater numbers without saturating the instrument with a large amount of low energy photons. As a result, greater spectral resolution across a wider energy range is captured.

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Figure 4: The dual aperture design of DAXSS showing the two apertures, the Kapton filter, and the Beryllium filter of the X123 X-ray spectrometer (image credit: DAXSS Team)

The dimensions of the DAXSS electronics assembly, including the X123 Electronics, CDH, EPS, and interface board are 100 x 130 x 83 mm, slightly larger than a 1U form factor. The SPS and PicoSIM are packaged together in a 70 x 38 x 22 mm box. The X123 Sensor dimensions with its housing are 47 x 28 x 29 mm. The CDH of INSPIRESat-1 is the master processor in a master-slave relationship with the CDH of DAXSS. The master has the ability to turn on and off power to the DAXSS payload as a whole or commands from the master CDH can be sent to DAXSS to individually. Examples of such commands could be to turn on and off the instruments or to change various table parameters of DAXSS.

 


 

Spacecraft:

The spacecraft is a 9U CubeSat with a total mass of ~8.6 kg and a size of 290 x 200 x 160 mm. It is aimed to study ionosphere, its ion composition, plasma densities, ion velocities, and ion temperature. Furthermore, it will observe Solar X-rays that will help shed light on the temperature difference between the Sun's Corona and Photosphere. The focus of CIP (Compact Ionospheric Probe) data collection is on eclipse time and the transition time from sunlight to eclipse time while that of the DAXSS (Dual Aperture X-ray Solar Spectrometer) is at sunlight side.

To meet the mission requirements each subsystem has been either developed in-house or commercially procured. The Table 2 gives the details of each subsystem.

Subsystem

Short Description

Institute

Payload

CIP (Compact Ionosphere Payload)
DAXSS (Dual Aperture X-ray Solar Spectrometer)

NCU (National Central University), Taiwan
LASP (Laboratory for Atmospheric and Space Physics)

C&DH (Command & Data Handling)

Self - developed

IIST (Indian Institute of Space Science and Technology)

EPS (Electrical Power Subsystem)

Self - developed

IIST, NCU

Structure and Thermals

Self - developed

LASP

ADCS(Attitude Determination & Control Subsystem)

XACT, Blue Canyon Technology

LASP

Communication

TRXU: Space Quest, STX: Clyde Space

NCU

Table 2: Subsystem and responsible institute

BCT XACT is highly capable ADCS system. The power subsystem is designed to provide stable and adequate power to all subsystems. The communication subsystem incorporates UHF transceiver - TRXU from SpaceQuest and a S-band Transmitter – STX from Clyde Space. The C&DH subsystem has been developed to incorporate all the interfaces, controlling them and data storage of at least 3 months. The structure has been developed based on ISRO launch loads, strong enough to survive launch and space environments.

For interconnection between the subsystems, a standard PC104 is used. The commercial off the shelf components have a nano/micro-D type connector that is re-routed to the PC104 connectors through an interface card. The Figure 5 shows the placement of each subsystem in the structure.

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Figure 5: INSPIRESat-1 internal structure and coordinate axis (image credit: INSPIRESat-1 Team)

 

Analysis

Access Time and Downlink: Figure 6 shows the access time for the three ground stations of INSPIRESat-1.

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Figure 6: Overview of access time (image credit: INSPIRESat-1 Team)

The data is downlinked over S-band frequency at the data rate of 2 x 106 bit/s to LASP, while the uplink and downlink over UHF- band is provided at 9600 bit/s. Other ground stations will use UHF for the uplink and downlink of essential HK data. The Table 3 give the number of megabits that can be downlinked over different ground station.

Ground Station

Access time per day (minutes)

UHF data (Mb/day)

S-band data (Mb/Day)

LASP

36.72

-

4202.27

IIST

15.87

8.72

-

NCU

9.85

5.41

-

Total

62.54

14.13

4202.27

Table 3: Downlink data per day

Power Analysis: The power analysis has been done to find out the power generation and hence prepare a power budget. The concept of the power analysis is as follows:

1) Satellite's +X (Figure 5) points in direction of velocity during eclipse time for payload to take reading.

2) Satellite's solar panels point in sun direction during sunlit period.

3) A buffer of 5 minutes is provided for satellite at the start and end of eclipse to switch from RAM orientation to sun pointing, during this time sunlight falls at an angle on the solar panels, which is obtained from System Took Kit (STK).

4) Solar panels are operating with an efficiency of 26.6% at 65ºC (worst case).

5) Buck converters are power extracts from the solar panels with 90% efficiency.

6) Satellite does not produce any power during S-band transmission. Since the S-band patch antenna is +Y faced of the satellite (Figure 5), it is assumed that no power is generated when the +Y of the spacecraft is pointing towards the nadir direction as a worst-case scenario.

Following are the solar panel inputs:

1) Number of solar cells: 30

2) Area of each cell: 30.84 cm2

The Figure 7 shows the power generation profile in one orbit over the span of 1 year along with the eclipse time.

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Figure 7: Power generation over 1 year (image credit: INSPIRESat-1 Team)

The average power generated = 17.10 W with maximum instantaneous power of 29.61 W. The power budget has been made based on average power consumption in an orbit by all the subsystems as shown in Table 4.

*CIP standby power consumption = 1.14 W. For CIP Average power = Duty_Cycle * Peak_Power(or Nominal_Power) / 100 + (1-Duty Cycle/100) * Standby_Power.

Subsystem

Peak power (W)

Nominal power (W)

Duty cycle (%)

Average peak power (W)

Average nominal power (W)

C&DH

1.40

0.91

100

1.40

0.91

DAXSS

5.2

3.5

50

2.6

1.75

Power

1.78

1.78

100

1.78

1.78

ADCS

2.82

2.82

100

2.82

2.82

GPS

1.70

1.50

100

1.70

1.50

CIP

5.00

3.47

50

3.07*

2.30*

TRXU RX

0.25

0.25

100

0.25

0.25

TRXU TX

8.00

6.50

9.62

0.77

0.63

STX TX

5.00

5.00

2.55

0.13

0.13

Battery Heater

4.64

4.64

1.00

0.05

0.05

Total

-

-

-

14.57

12.12

Power generation

-

-

-

17.10

17.10

Margin

-

-

-

17.36%

41.09%

Table 4: Power budget

Modes of Operation: The modes of operation have been divided into 5 modes based on battery's State of Charge (SOC) and payload operation as shown in Table 5.

Subsystem

Phoenix

Safe

Charging

Science

SciC

SciD

CDH, EPS, TRXU RX

On

ADCS

Off

Coarse Sun pointing

Fine sun pointing

Fine ref in RAM direction

Fine sun pointing

CIP

Off

On

Off

DAXSS

Off

On

TRXU TX

As required

STX TX

As required

Battery Heater

As required

Table 5: Modes of Operation configuration

The CDH and EPS subsystems are always ON since they are the basic subsystems of the satellite. The TRXU is ON and by default configured in receive mode. In safe mode ADCS points the solar panels towards the sun with no constrain on any other axis this is coarse sun pointing of the ADCS. In charging and SciD mode ADCS points the solar panels towards the sun with constrain +Y of the spacecraft pointing as close as possible towards the nadir direction. In SciC mode the ADCS points +X of the spacecraft towards the velocity direction with +Y of the spacecraft pointing as close as possible towards nadir direction. The Phoenix and Safe modes' purpose are to increase the SOC of the battery and hence together are called emergency modes. The Charging and Science modes of operation together are called normal modes. Figure 8 shows the mode flow followed by the spacecraft.

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Figure 8: Mode flow diagram (image credit: INSPIRESat-1 Team)

Mode oscillations are avoided by providing two thresholds values between any two modes which introduces hysteresis. The mode flow, Figure 8, along with the mode profile, Table 5, shows that as the satellite progress from phoenix mode to the science modes the complexity of the system increases step wise.

 

C&DH (Command & Data Handling)

C&DH uses System On Module (SOM) provided by Emcraft. The SOM uses a System On Chip (SOC) provided by Microsemi. The SOC contains a microprocessor embedded in a Field Programmable Gate Array (FPGA). The off the shelf SOM is integrated with the rest of the satellite by a ‘C&DH card'. The C&DH card contains PC104 to interface with other subsystems.

The C&DH is responsible for collecting House Keeping (HK) data from all subsystems and science data from the payloads. It configures subsystems and maintains a Real Time Counter (RTC) for time tagging the data and scheduling the tasks. The FPGA is used to implement more interfaces, which are controlled by microcontroller via an Advanced High-Performance Peripheral Bus (AHPB).

Electrical characteristics: Table 6 gives electrical details of C&DH.

Serial number

Parameter

Value

1

Voltage

3.3 ± 0.3 V

2

Peak power

1.40 W

3

Nominal power

0.91 W

4

Operating temperature

-40ºC to 85ºC

Table 6: Electrical characteristics of C&DH

The CDH consumes 1.4 W when the processor is running at its full potential of 166 MHz and FPGA is more than 80% utilized. In practice, we will be using 60 MHz for the processor and FPGA will be less than 50% utilized.

System On Module (SOM): SOM is an off the shelf module from Emcraft. It contains a 12 MHz quartz crystal and other clocks are derived from it by using PLLs present in SOM. It contains:

• 32-Bit Arm Cortex-M3 processor

• 64KB embedded SRAM (eSRAM)

• 512 KB embedded NVM (eNVM).

Interface Definitions: The Table 7 gives the interfaces of the C&DH with various subsystems.

Subsystem

Interfaces

CIP

UART – RS422 (x1); CIP PPS – RS422 (x1)

ADCS

UART – RS422 (x1)

EPS

I2C (x3); PWM (x3); Distribution control lines (x5); Deployment control lines (x3)

TRXU

UART (x1); GPIO lines (x6)

S-band

I2C (x1); SPI (x1); GPIO (x2)

Table 7: Interface definitions

The C&DH card: The fabricated C&DH card is shown in Figure 9.

1) The lines for RS422 and TRXU GPIO lines pass through bus transceiver to protect the SOM from fluctuations in voltage and current. The bus transceiver also corrects the logic level to 3.3 V and 0 V for logic level 1 and 0 respectively.

2) Four RS422 Converter ICs converts UART for CIP and ADCS, and PPS line to RS422 for CIP.

3) The flight board contains two SD Cards slots on single SPI bus. Using two provides redundancy if one of them fails.

4) A reset IC that contains a watchdog timer is connected to the SOM module, providing the first level of reset. It is also a clock input to a 4-bit counter that counts the number of reset from reset IC to microprocessor. If the count value goes to 8 (b'1000) then the signal for External System Reset to the EPS board. EPS cuts the power supply to all subsystems and turns the power supply ON back with a delay of 1 sec.

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Figure 9: Command & Data Handling: Top view (image credit: INSPIRESat-1 Team)

 

Interface Card

The interface card is responsible for connecting various subsystems to the PC104 stack of C&DH, EPS and STX transmitter. The Figure 10 shows the fabricated interface card. It contains military grade (MIL-DTL-M83513) connectors for ADCS & GPS, TRXU, CIP, DAXSS and GSE have been provided in the interface card.

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Figure 10: Interface card: Top view (image credit: INSPIRESat-1 Team)

 

Flight Software

The flight software of INSPIRESat-1 is a non-interrupt software with no operating system. It is basically an infinite loop performing different tasks. The lack of interrupt and scheduler introduces challenges like timing constraints, tasks priority etc. Hence the FPGA is used to parallelize several I/O tasks which traditionally is handled by the µP using interrupts. This not only frees up the microprocessor's resources but also provides a memory space foe buffering the data coming at I/Os by using the block RAMs of the FPGA.

Since the architecture is free of interrupts, the possibility of false interrupt has been ruled out. The software provides FDRI (Failure Detection, Recovery and Isolation) which detects the failure of a subsystem to communicate with the C&DH, take some measures to reset the subsystem and if all fail then isolate the subsystem by avoiding mode.

The tasks which need the bit level of operations like AX.25 protocol etc. have been offloaded to the FPGA where it is easy to do bit operations. Following I/O and bit level tasks have been implemented on the FPGA:

1) UARTs each with buffer of size 8192 bytes (8 KB)

2) PPS generation for the CIP

3) Scrambling and descrambling of the data received by the radio over UHF band

4) NRZI encoding and decoding of the data received by the TRXU transceiver

5) High level Data Link Control (HDLC) frame formation and Frame Check Sum (FCS) calculation

6) I2C modules

7) Two real time counters, one for scheduling various tasks while the other for keeping track of the UTC time for timestamps.

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Figure 11: This figure shows the top-level architecture of the flight software developed for the INSPIRESat-1 (image credit: INSPIRESat-1 Team)

The architecture of the software has been divided into three layers with hardware layer at the bottom followed by the driver layer with application layer at the top.

At the bottom of the architecture the Hardware & Hardware Descriptive Language (HDL) layer is present which directly communicates with different subsystems over the physical layer. This layer consists of different in-house developed Verilog modules and modules which are hard coded in the FPGA like GPIOs, SPI etc. They receive & store and transmit data from & to subsystems and modules like SD card, SPI flash etc.

The next layer is called the Driver Layer because it consists of APIs which are required for μP to communicate and control various Hardware Description language (HDL) modules present in the Hardware & HDL layer. The modules present in this layer allow the microprocessor to read and write from and to the buffers available in the FPGA and control the modules to communicate with different subsystems.

The C&DH - Data Poll layer represents all the global variables and is shared among various modules of the application layer only. The data is not shared by the Driver layer. The Application Programming Interface (API) of application layer communicate with the Hardware layer via Driver layer and share the obtained data with other APIs of the application layer if required.

The Application Layer consists of modules for various subsystems and tasks related to different modules of the C&DH like flash memory, SD card, etc. The System control modules uses various application layer modules to carry out mission operations. The application layer together with the hardware & HDL layer provides the FDRI capability, with hardware & HDL layer providing the failure detection functionality while recovery and isolation is provided by the application layer.

 

Electrical Power Supply

The EPS (Electrical and Power Subsystem) of a spacecraft is a very vital and critical subsystem that is expected to provide sufficient and regulated power for the operation of the entire spacecraft. The Figure 12 shows the fabricated EPS board.

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Figure 12: The EPS architecture (image credit: INSPIRESat-1 Team)

 

Launch: A launch of INSPIRESat-1 as a secondary payload is planned for December 2019 onboard a PSLV vehicle of ISRO.

Orbit: To make mid-latitude nighttime ionosphere measurements to study ionospheric effects of mid night temperature maxima and nighttime Ionospheric plasma bubbles, INSPIRESat-1 desired orbit requirements are:

• Altitude: 500 ± 50 km

• Inclination: 55º ± 10º.

This provides an orbital period of ~ 95 minutes with 35-minutes average eclipse times. The 60 minutes of sunlit time would allow the DAXSS to take readings of the soft X-ray spectrum of the sun.

 


1) Loren C. Chang, Jude Salinas, Jack Chieh Wang, Jia-Yu Su, Duann Yi, Joe Hong, Yi-Chung Chiu, Steven C.R. Chen, Amal Chandran, Michael McGrath, Dave Fritts, Larry Gordley, John Fisher, "A Preliminary Design for the INSPIRESat-1 Mission and Satellite Bus: Exploring the Middle and Upper Atmosphere with CubeSats," Proceedings of the 30th Annual AIAA/USU SmallSat Conference, Logan UT, USA, August 6-11, 2016, paper: SSC16-WK-02, URL: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=3437&context=smallsat

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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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