MAE 495/589 – Spacecraft
Environment & Interactions
Lecture 13: The Neutral Environment
• Spacecraft Glow
• Communications
1
Housekeeping Items
Typo in lecture 10
• HW3 – Due 2/23
𝑚
𝛸 𝑐 = 4𝜋
2𝜋𝑘𝐵 𝑇
3
2
2
−𝑚𝑐
𝒄𝟐𝑒𝑥𝑝
2𝑘𝐵 𝑇
• HW4- Will be assigned 2/21, due 3/2
– Note: Due by 3pm
• Exam 1 – becomes available at noon on 3/4, due
3/9 at 3pm
– Virtual office hours (just for this class) 10-noon on 3/4
– Exam is take home; I will only answer questions
clarifying the problem statements during the exam
availability period
– No class 3/7
2
Spacecraft Glow
• Starting in the early-mid 1970s several
spacecraft, including Atmospheric Explorer C
and E, reported anomalous light emission as
measured by onboard spectrometers
– Early observations indicated strong correlation
between brightness intensity and altitude that closely
followed scale height of AO
– Brightness observed primarily in the ram direction
– This has been called the spacecraft ‘glow’ phenomena
3
Spacecraft Glow
• Visual observations of this phenomena were
obtained starting with STS-3
– Seen on shuttle surfaces in the ram direction
– Also associated with thruster firings
4
Spacecraft Glow
• Physical mechanism
understood
for
glow
not
well
– Different apparent mechanisms for large vs. small
spacecraft and different materials
– Does appear to be a correlation with temperature
– Source of shuttle glow thought to be due to
recombination of NO2
• Excited states produce photons
– Overall, seems to be related to presence and also
impact energy of AO
– Dearth of research mainly due to the fact that, despite
the shocking appearance, there’s not really any
significant negative effects
5
Spacecraft Glow
• We can estimate the brightness of the glow
phenomena as a function of altitude using a
correlation based on historical measurements:
𝑙𝑜𝑔𝐵 = 7 − 0.0129ℎ
𝐵 − 𝑏𝑟𝑖𝑔ℎ𝑡𝑛𝑒𝑠𝑠 𝑅𝑎𝑦𝑙𝑒𝑖𝑔ℎ𝑠
ℎ − 𝑎𝑙𝑡𝑖𝑡𝑢𝑑𝑒 𝑖𝑛 𝑘𝑚
1 𝑅𝑎𝑦𝑙𝑒𝑖𝑔ℎ =
𝜋
𝑝ℎ𝑜𝑡𝑜𝑛𝑠
× 1010 2
4
𝑚 ∗ 𝑠 ∗ 𝑠𝑟
6
Spacecraft Glow
• What the heck is a steradian?
– Unit of solid-angle measure
– Analogous to radian, but for 3D geometry
7
Spacecraft Glow
• For purposes of spacecraft glow, the brightness
equation tells us the flux of photons per unit
solid viewing angle
• Some comparisons:
– The night sky: 250 Rayleighs
– Auroras: 1000 kRayleighs
8
Spacecraft Glow
• Example: Calculate the brightness of the glow
associated with a spacecraft in a 200 km circular
orbit.
9
Spacecraft Glow
• Primary effect of glow is on remote sensing observations
from optical payloads
– Brightness interferes with optical measurements
• Since glow has been found to be dependent on materials,
select materials not susceptible to glow
– Unfortunately materials not susceptible to glow are susceptible
to AO attack and vice versa
• Generally want to avoid AO attack more than you want to avoid
glow
• Typically, just place sensors in the wake region rather
than in the ram direction and/or accept some loss of
resolution
10
Communications
• Communications are a major part of spacecraft
operations
– Commercial
– DoD
• Communications
are
electromagnetic waves
carried
out
via
– Troposphere is 75% of the mass of the atmosphere
and is electrically neutral and non-dispersive for
frequencies up to 30 GHz
11
Communications
• Neutral atmosphere changes the propagation
velocity of electromagnetic radiation from that
of vacuum
– Propagation velocity is less in air than in vacuum, so
a signal from space to ground is delayed to some
extent
– The refractive index, n, of a medium is defined by:
𝑛≡
𝑐
𝑉
𝑚
c−𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑝𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑣𝑎𝑐𝑐𝑢𝑚 [3𝑥108 ]
𝑠
𝑉 − 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑝𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚
– And the refractivity, N, is defined by
𝑁 ≡ 106 (𝑛 − 1ሻ
12
Communications
• Tropospheric excess time (or time error) is defined as the
difference in time that it takes for a signal to travel along
the electromagnetic path in the medium and the
geometric Euclidean distance in vacuum
• The excess path length (or length error) is determined
by multiplying the vacuum speed of light time the
excess time:
∆𝑟 = 𝑐
න
𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐
𝑝𝑎𝑡ℎ
𝑉 𝑠
−1 𝑑𝑠
−
න
𝑐 −1 𝑑𝑠
𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐
𝑝𝑎𝑡ℎ
∆𝑟 − 𝑒𝑥𝑐𝑒𝑠𝑠 𝑝𝑎𝑡ℎ 𝑙𝑒𝑛𝑔𝑡ℎ
𝑑𝑠 − 𝑒𝑙𝑒𝑚𝑒𝑛𝑡𝑎𝑙 𝑝𝑎𝑡ℎ 𝑙𝑒𝑛𝑔𝑡ℎ
13
Communications
• Electromagnetic path is the path signal travels
with minimum time
• If deviation between electromagnetic path and
geometric path is small (i.e. bending is small),
we can approximate the excess path length by:
∆𝑟 ≈
න
𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐
𝑝𝑎𝑡ℎ
𝑛 𝑠 − 1 𝑑𝑠 = 10−6
න
𝑁(𝑠ሻ𝑑𝑠
𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐
𝑝𝑎𝑡ℎ
14
Communications
• Refractivity
atmosphere
is
not
constant
through
the
– Function of atmospheric temperature, pressure, and
humidity
– For Earth, tropospheric vertical excess path length is
~2.4 m and ~20m along the horizon
– Refractive index for air consists of two components:
• Hydrostatic (dry) component: depends on the amount of dry
gases in the atmosphere. Accounts for ~90% of excess path
length
• Wet component: depends on the amount of water vapor
present. Accounts for ~10% of excess path length
• Both are independent of frequency up to 30 GHz
15
Communications
• Radio refractivity for air is given by:
𝑃𝑑
𝑒
𝑒
𝑁 = 𝑘1 + 𝑘2 +𝑘3 2
𝑇
𝑇
𝑇
𝑃𝑑 − 𝑎𝑡𝑚𝑜𝑠𝑝ℎ𝑒𝑟𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑚𝑏𝑎𝑟
𝑒 − 𝑝𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟𝑡 𝑚𝑏𝑎𝑟
𝑇 − 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 [𝐾]
• Or, in terms of the wet and dry components:
𝑃𝑑
𝐷𝑟𝑦: 𝑁𝑑 = 𝑘1
𝑇
𝑒
𝑒
𝑊𝑒𝑡: 𝑁𝑤 = 𝑘2 +𝑘3 2
𝑇
𝑇
16
Communications
• If refractivity at ground level is known
(measured), we can estimate the refractivity as a
function of altitude:
−ℎ
𝑁 ℎ = 𝑁 0 ex p
𝐻
ℎ − 𝑎𝑙𝑡𝑖𝑡𝑢𝑑𝑒 𝑘𝑚
𝐻 − 𝑠𝑐𝑎𝑙𝑒 ℎ𝑒𝑖𝑔ℎ𝑡 [𝑘𝑚]
• We can then integrate the terms in the excess
path length equation to find the excess path
length
17
Communications
• Several alternative models exist to determine
excess path length
• One is called the Hopfield model
– Quartic model describes wet and dry refractivity as a
function of altitude by:
𝜑 − 𝑔𝑒𝑜𝑚𝑒𝑟𝑡𝑖𝑐 𝑙𝑎𝑡𝑖𝑡𝑢𝑑𝑒
18
Communications
• If we assume troposphere is radially symmetric
and bending can be neglected, we can integrate
the Hopfield model to find the excess path
length:
19
Communications
• Excess path length is highest when satellite is at
the horizon (E=0)
20
Communications
• Other models exist
that include higher
order effects such as
bending and changes
in water vapor as a
function of altitude
• The International
Telecommunications
Union (ITU) offers a
variety of data on
these topics
21
Communications
• Another effect of the neutral atmosphere on
communications is known as scintillations
– Scintillations are due to small-scale irregularities in
the number density in the atmosphere
– Generally correlated with the wet component of the
refractivity
– Usually
modeled
statistically
rather
than
deterministically
– ITU-R tropospheric scintillation and multipath fading
prediction model is given in ITU recommendation
P.618-11
22
Communications
• Another important effect of the
neutral
atmosphere
on
communications is absorption
– Electromagnetic waves can
be absorbed by specific
constituents of Earth’s
neutral atmosphere
• This
is
strongly
wavelength dependent
and based on species
specific properties
• Primarily caused by O2,
CO2, and H2O
23
Communications
• Due
to
absorption,
we
can’t
send
communications signals through the atmosphere
at all wavelengths of light
24
Communications
• Satellite communications done in the 1-40 GHz
range
25
Communications
• L-band (1-2 GHz)
– GPS carriers, satellite mobile phones
– Iridium, Inmarsat
• S-band (2-4 GHz)
– Weather radar surface ship radar, some commsats
– ISS and Space Shuttle, Inmarsat, Solaris
• C-band (4-8GHz)
– Satellite TV networks, raw satellite feeds
– Commonly used in areas subject to tropical rainfall
(less susceptible to rain fade compared to Ku-band)
– Telestar: first transatlantic tv signal 1962
26
Communications
• X-band (8-12 GHz)
– Primarily used by military. Radar applications,
weather monitoring, maritime traffic control, air
traffic control
• Ku-band (12-18 GHz)
– Communications, broadcast satellite services
– Astra, Starlink
• Ka-band (26-40 GHz)
– Communications, close range targeting radars
– Iridium Next, Project Kuiper, 5G networks (partial
overlap)
27