USING SMALL SATELLITES TO ANSWER BIG QUESTIONS IN SOLAR-TERRESTRIAL … · 2020. 1. 1. · There is no shortage of big questions in solar-terrestrial research ๏ What are the effects

USING SMALL SATELLITES TO ANSWER BIG QUESTIONS

IN SOLAR-TERRESTRIAL SCIENCELara Waldrop

University of Illinois at Urbana-ChampaignDepartment of Electrical and Computer Engineering

HAO 75th Anniversary

There is no shortage of big questions in solar-terrestrial research

๏ What are the effects of solar variability on the solar-terrestrial environment, including changes on decadal time-scales?

• Radiative vs. non-radiative impacts• Distinguish from lower atmosphere variability• Response to geomagnetic storms

๏ How are small-scale structures produced in the ionosphere and thermosphere? How do they affect global dynamics?

• Equatorial plasma bubbles• Gravity waves• Polar cap patches

adapted from HAO Strategic Plan 2011-2015


For example:


Many space weather investigations benefit from space-based observations

๏ in-situ sampling• solar wind speed and composition • magnetospheric ion density• precipitating electron flux

๏ large-scale earth observation • [O]/[N2] via UV airglow emission • geocoronal density via H Lyman alpha

emission • auroral boundary localization

๏ stereoscopic remote sensing• solar coronal emissions • ring current via ENA imaging

CubeSats being deployed from

the Space Station


Small satellites are a low-cost platform for space deployment

CubeSat protocol:

Standardized size and mass: one unit (U) = 10x10x10 cm, < 1.33 kg can combine as 2U, 3U, 6U, etc.

Standardized deployment as secondary payloads on existing launch vehicles

<1 kgMicrosatellite Nanosatellite Femto- and (CubeSat) Picosatellite

10 kg - 1 kg100 kg - 10 kg


Sensor and hardware development continues to push size and performance limits

PAYLOAD:

๏ photon detectors (spectrograph, polarimeter, photometer, ...)

๏ particle detectors (ion/neutral mass spectrometer, ...)

๏ field detectors (magnetometer, electric field probe, ...)

SATELLITE BUS:

๏ power systems

๏ attitude determination and control systems

๏ communication systems

๏ electronics


Science requirements can drive sensor development and mission design

For example, knowledge of neutral atmospheric composition is required for numerous space weather investigations, such as:• thermospheric response to geomagnetic storms • ion-neutral coupling and H+ continuity: H+ + O H + O+ • tides, gravity waves• Joule heating• photochemical and charge exchange reaction rates• photoelectron flux calculation

To date, long-term, global measurements are notoriously sparse.Direct sensing has not been available since the era of Dynamics Explorer...

Electronics Time of flight measurement

Mass binning and memory mapping HVPS

Top Hat ESA

Top Hat ESA


PI: John Noto, Scientific Solutions, Inc. Team: NASA/GSFC (sensor), CalPoly (bus), UIUC, ERAU, Arecibo, Univ. of Wisc.

PAYLOAD: dual-aperture, gated time-of-flight mass spectrometer mass = 560 g, size = 1.5 U

The NSF EXOCUBE mission was inspired by a novel sensor for in-situ density measurement


H

H2 He

H2O

N2

O2

OH

O

N

0 128 256 384 512 Channel # Time of flight ~ sqrt(M)

OBSERVABLE: density of all major ion and neutral species in the upper thermosphere: H, H+, He, He+, O, O+, N2, N2+

• peak power = 1.6 W (full filament), = 0.6 W (ions only)

• mass resolution: M/dM ~ 12is limited by uncertainties in energy dispersion, time resolution and time of flight path.

• ionizing filament voltage governs sensitivity to neutral density

The NSF EXOCUBE mission was inspired by a novel sensor for in-situ density measurement

M

q= 2

E

q

TOF2

d

ram di

rectio

n

nadir


Use of COTS hardware enables rapid, low-cost fabrication and allows for student-led development

POWER: Lithium-ion batteries at start-up 8 solar panels generate 2.6 W per orbit

ATTITUDE DETERMINATION AND CONTROL: +/- 5o pointing knowledge along ram direction Solar array sensor on nadir panel + magnetometer Gravity gradient booms provide passive control Sinclair momentum wheel on pitch axis Magnetotorquer coils on each axis Kalman filter control

COMMUNICATION: Monopole antenna deployed via burn wire 437 MHz amateur band (one receiver initially) ~2 MB per day downlink capability

๏ power budget and downlink accommodates ~ 600-1000 samples/day

๏ sampling period of 1 s chosen to ensure sufficient spatial resolutionfor science goals

๏ maximizing science return demands onboard processing,target-of-opportunity sampling, and duty cycling

๏ operational design must be flexible and responsive to launch opportunity once known


Mission design must optimize science operations within communication and power limits


EXOCUBE served as a valuable proof-of-concept for direct neutral sensing...

First flight spectrum of ions

ORBIT: Launch on Jan. 31, 2015, via ELaNa-X SMAP Delta II inclination = 98o

sun-synchronous (dawn/dusk) elliptical (440-675 km)

Flight spectrum of neutrals (during outgassing)


EXOCUBE served as a valuable experience for “lessons learned”...

Next time:

๏ De-scope dependence on curriculum-based student involvement

๏ Prioritize more testing in the development timeline

๏ Incorporate component redundancy everywhere possible

๏ Prepare a ground-station contingency plan in the event of communication problems

๏ Better prepare for potential launch delays


Mission design can also be inspired by the launch opportunity itself

For example, the Exploration Mission 1 is the first planned flight of the Space Launch System and the second uncrewed test flight of the Orion spacecraft.

• Launch projected to occur in September, 2018

• Orion spacecraft will perform a circumlunar trajectory

• EM-1 will host several secondary CubeSat payloads for deployment along the sun-earth line near 5.5 Re into translunar, heliocentric orbits (assuming no propulsion)

The EM-1 CubeSat trajectory is ideal for investigating the Earthʼs neutral hydrogen geocorona...

He2+ + H He+ + H+

On+ + H O(n-1)+ + H+

H + h! H+ + e

H+ + H H+ + H* SW

H + H+ H* + H+

H + h! H* + h!

H + O* H* + O*

Nonthermal

H + O H + O Thermal

Exosphere

H+ + O H + O+

H + h! H+ + e

H + O+ H+ + O

Escape via

Solar Wind

Polar Wind

Mid Thermosphere / Ionosphere H + O+ H+ + O

Ionosphere Polar Cap

Plasmasphere and

H

H

H2O + h! H + OH

OH + h! H + O

MLT

H

Figure 1: Flow diagram for hydrogen in the exosphere

• Ionization of neutral H is the primary source of TEC in the topside ionosphere

• H readily escapes from Earth due to thermal evaporation and solar wind pickup

• Charge exchange between H and H+, O+ ions generates energetic neutral atoms (ENAs), which dissipate magnetospheric energy after solar storms and enhance gravitational H escape

• Solar radiation pressure perturbs gravitationally bound H and creates an extended “geotail”


What drives the distribution, energization, and transport of atomic hydrogen?

Routine estimates are limited to ~90 km and from 3-8 RE • The low altitude estimates (via NASA/TIMED/SABER) depend on assumptions regarding MLT chemistry and are insufficient for establishing the exospheric H source

• The high altitude estimates (via NASA/TWINS/LAD) are typically derived by assuming a parametric form for the exospheric distribution and require knowledge of the solar source emission brightness and interplanetary background emission


Sparse empiricism limits our understanding of the thermospheric and exospheric H population

Available data are notaccurate or comprehensive enough to resolve data-model discrepancies or to advance physics-based model development 10010

equatorial equatorial

meridional meridional

10 0 10geocentric distance [RE]

10

1010

10

0

0

geocentric distance [RE]

geoc

entri

c di

stan

ce [

RE]

geoc

entri

c di

stan

ce [

RE]

30 50 75 100 150225

337

30 50 75 100 150225

337

adapted from Zoennchen et al., [2013]

adapted from Bailey and Gruntman [2011]

ONE DAY SEASONAL AVERAGE

(adapted from Bailey and Gruntman [2011] and Zoennchen et al. [2013])

H contours derived from TWINS data

-50 0 50 100 150XGSE [RE]

-50

0

50

100

150

200

250

YGSE

[RE]

3D HLAP lines-of-sight projected onto the ecliptic (5oresolution shown)

lunar orbit 0 50 100

0

50

100

150

200

XGSE [RE]

Y GSE

[RE]

-50 0 50 100 150XGSE [RE]

-50

0

50

100

150

200

250

YGSE

[RE]

tomographic sensingvia 3-axis stabilized rotation and nutation of the field-of-view

opticallythick region< 3 RE 1 day

after orbitinsertion

2 days

110 h

210 h

SAMPLING MODES:

0-110 hours: 10 Hz sampling

110-210 hours: 4 Hz sampling (M1) 10 Hz sampling (M2)

210+ hours: 4 Hz sampling


Multi-directional measurement of H airglow emission intensity enables H density estimation

PAYLOAD: H 121.6 nm (Lyman alpha) photometerwhose FOV scans in 3Dvia controlled rotation andnutation of satellite bus

The Tomographic Hydrogen Emission Observatory (THEO)Team: UIUC, SRI, SSI, UC Berkeley, ERAU, Arecibo

• Singly scattered (optically thin) emission intensity from a given vantage in a given line-of-sight direction is proportional to the underlying emitter density along the column:

• Model-independent estimation of unknown density requires sampling the emission intensity along many distinct, overlapping lines-of-sight, casting the integral equation is cast in matrix form:

then inverting to solve for h

log10[H

](cm

−3)

YGSE(R

E)

XGSE (RE)

SIMULATED THEO DATA:• H density distribution is based on a spherical harmonic expansion of NASA TWINS LAD data (Zoennchen et al, JGR, 2013) and discretized into 2160 pixels (in the 2D X-YGSE plane)• THEO transits 6 RE - 12 RE along the XGSE axis at 0.5 RE/hour, rotates in 2D every 5 minutes, and samples Ly-a emission at 0.5 Hz

TOMOGRAPHIC RECONSTRUCTION:• For data array y, unknown H density array h, sampling geometry matrix L, and Poisson noise n:

• Unknown h is estimated using singular value decomposition with Tikhonov regularization • Nightside reconstruction is impossible for this viewing geometry and requires off-axis sampling• Outer exosphere reconstruction will be improved by incorporating smoothness constraints

log10[H

](cm

−3)

YGSE(R

E)

XGSE (RE)

TO SUN

y = Lh+ n

Figures courtesy of David Ren (UIUC ECE)

original image

reconstructed image

I[r, n̂] = g

∫r∞[n̂]

r

N [r′] Φ[r, n̂] dr′

excitationg-factor

(measured or modeled)

emitter H density

(unknown)

scattering phase function (known)

emission intensity

(measured)

I = Lh+w


Spinning/rotating platforms are well-suited formodel-independent tomographic sensing

along track distance

line-of-sight paths from spinning orbiter:

• Multi-path viewing geometry avoids need for detector calibration

• This tomographic approach is also appropriate for photometric sensing from low-earth orbit (e.g., NSF IT SPINS CubeSat)

• For O/N2, significant global improvements in assimilated Ne emerge with 6 coplanar S/C orbiting in constellation formation


Spinning/rotating platforms are well-suited formodel-independent tomographic sensing

Alex Chartier JHU/APL Dynamical model: TIEGCM, High-lat model: AMIE, Solar flux: TIMED/SEE observations, Assimilation: DART (80 members of each)

analysis spread(TECU)

Science sampling rate data generation rate transmission strength and duty cycle power requirements instrumentoperation sampling rate

Programmed transmission rate resets are designed to occur when the link margin falls below a minimum threshold.

Trade-offs between power margin, data generation margin, and communication quality must be evaluated 10 15 20

0

!"

#"

$"

%"

10 15 200

Eb/N0 in dB

Dat

a G

ener

atio

n M

argi

n (%

)

Pow

er M

argi

n (%

)

Trade Space in Mode 2 at 110 hours MET

&#"

&!"

&""

%"

Pow

er Margin [%

]

80

60

40

20

140

120

100

10 15 20Eb/N0 [dB]

80

Dat

a G

ener

atio

n M

argi

n [%

]Figure 2R: The dynamic trade-space indata and power budgets enables theoptimization of link design during THEO’s development.

Maximum Supported Channel Capacity for Link:Allen Telescope ArrayArecibo Observatory

Transmitted Data Rate From Spacecraft

Programmed Rate Reset

Programmed Rate Reset

Programmed Data Buffer

ProgrammedRate Reset and

Buffer Download

Data Sampling Rate [Hz]:1010

44

10 dB MINIMUM SNR

M1 THRESHOLDM2 BASELINE

*

*Downlink budget

calculation valuesderived in Table 1shown here as *

28.8 9.6 2.4 28.8Transmitted Data Rate From Spacecraft [kbps]:

Figure 1R: Programmed transmission rate resets are designed to occur when the link margin above Eb/N0 = 10 dB falls to zero HAO 75th Anniversary

As always, mission operations must balance science goals with power and communication capabilities


Small satellites will play a big role in solar-terrestrial research for years to come

๏ Advancing performance in power generation, ADCS precision, subsystem efficiency, and communication links will support an increasing diversity of payloads (e.g., imaging systems)

๏ Development of low-power propulsion systems along with enhanced navigation and communication capabilities will enable solar system exploration

๏ Ongoing miniaturization of sensors and bus hardware will advance development of pico- and femto-satellites

๏ Widespread incorporation of deployment infrastructure and increased launch availability will enable low-cost orbit insertion for distributed or constellation missions

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USING SMALL SATELLITES TO ANSWER BIG QUESTIONS IN SOLAR-TERRESTRIAL … · 2020. 1. 1. · There is no shortage of big questions in solar-terrestrial research ๏ What are the effects