Download Airglow photometers on board sounding rockets

An Introduction to Space Instrumentation,
Edited by K. Oyama and C. Z. Cheng, 25–31.
Airglow photometers on board sounding rockets
B. R. Clemesha, H. Takahashi, A. Eras, N. B. Lisboa, and D. Gobbi
Instituto Nacional de Pesquisas Espaciais, INPE, Brazil
Airglow consists of optical quantum emissions from excited atmospheric species produced by photochemical
and ionic reactions in the mesosphere and thermosphere. Atomic and molecular oxygen, hydroxyl, metallic atoms
and ions are some of the typical emissions. These emissions have been measured by ground-based photometers
for many years, and have provided important information on atmospheric processes occurring in the mesosphere
and lower thermosphere. Although providing much useful information, ground-based measurements suffer from
the fundamental limitation that they do not provide information on the vertical distribution of emission intensity.
It is possible to overcome this limitation by launching photometers on board sounding rockets. In this way it is
possible to measure the emission profile as the rocket passes through the emission layer. This article describes
the techniques used for this type of measurement and provides some technical details of the instrumentation used.
Key words: Airglow, emission, photometer, rocket.
1.
Introduction and Aim of the Instrument
In the decade between 1960 and 1970, many experiments
were carried out to measure the airglow emission profiles
and to investigate the emission mechanisms (Furuhata et al.,
1966; Baker and Waddoups, 1967; Greer and Best, 1967;
Kulkarni, 1976).
In the decade of 1980, investigations of atomic oxygen
chemistry in the MLT region were significantly advanced
by simultaneous observations of oxygen related airglow
and atomic oxygen concentration using an oxygen fluorescence lamp (Greer et al., 1986) The oxygen related emission mechanisms have been studied and clarified by this
technique (McDade and Llewellyn, 1986; McDade et al.,
1987).
Airglow sounding rocket measurements make it possible to obtain the volume emission profile as a function of
height. Satellite observation (limb sounding) is also possible to retrieve vertical profiles, but with a horizontal resolution of about 500 km extension in the line of sight. A great
advantage of satellite sounding is that it can measure along
the satellite orbit covering on a global scale. Ground based
photometric observations measure integrated emission rates
along a line of sight. No information of the emission profile
is available. Such measurements are useful to monitor temporal variations of the emission intensities and molecular
temperatures (Shiokawa et al., 2007)
The present work describes airglow photometers designed for use on board sounding rockets. Photometer block
diagrams, subsystems, mechanical and electronic design are
presented. A significant difference between the rocket photometer and ground based instruments is in the launch conditions (acceleration and vibration) and space environment
(vacuum). The photometer design should take into account
such severe conditions. Vacuum, thermal and vibration tests
are necessary before payload integration in order to guarantee reliable operation of the photometer. A number of
practical problems which can arise during the design, construction and testing of the photometers will be discussed.
Airglow consists of optical quantum emissions from
excited atmospheric species produced by photochemical
and ionic reactions in the mesosphere and thermosphere.
Atomic and molecular oxygen, hydroxyl, metallic atoms
and ions are some of the typical emissions. According to
the emission processes involved, the emission forms a layer
with a vertical width of around 5 to 10 km at a given altitude. A well known emission, the atomic oxygen green line
at 557.7 nm, hereafter OI5577 where OI indicates neutral
atomic oxygen, is normally located between 90 to 105 km
of altitude with a peak emission height at around 97 km and
a half intensity width of 7 to 10 km (Greer et al., 1967). The
emission profile depends on the atomic oxygen and neutral
atmospheric density as well as the local ambient temperature. Therefore if one measures a vertical profile of the
volume emission rate, for example, one can estimate the
vertical density profile of the excited species such as atomic
oxygen or hydroxyl.
Atomic oxygen in the mesosphere and lower thermosphere (MLT) region plays a major role in photochemical processes in this region, calling the special attention of
aeronomers (Bates, 1988). In the mesosphere from 80 to
100 km, there are several emission layers. Figure 1 shows
volume emission profiles of these emissions in the MLT region observed by a rocket experiment carried out in 1990
(Takahashi et al., 1996). These profiles relate to the photochemical and dynamical properties of the upper mesosphere
and lower thermosphere at the time of the experiment. The
atomic oxygen density profile has been calculated by using
the observed emission profiles (Melo et al., 1996).
The first airglow observation on board a rocket was carried out in 1950 (Packer, 1961; Gulledge and Packer, 1966).
c TERRAPUB, 2013.
Copyright 25
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B. R. CLEMESHA et al.: AIRGLOW PHOTOMETERS ON BOARD SOUNDING ROCKETS
Fig. 4. Transverse photometer with integrated electronics package.
Fig. 1. Airglow volume emission profiles in the mesosphere observed by
the MULTIFOT rocket experiment (Takahashi et al., 1996).
Fig. 5. Side-looking photometer.
Fig. 2. Longitudinal photometer—exploded view showing internal components.
Fig. 6. Derivation of emission profile.
Fig. 3. Longitudinal photometer.
2.
Principle of Instrument
The purpose of the photometer is to measure the volume
emission rate for airglow emissions from the upper atmosphere. These emissions come from the mesopause/lower
thermosphere region, 80 to 110 km, and from the ionospheric F-region, 250–300 km. Two types of instrument
have been used, longitudinal and side-looking. The longi-
tudinal photometer views the sky above the rocket in the
direction of its motion and the side-looking instrument is
aligned with its optical axis orthogonal to the rocket spin
axis. The longitudinal instrument (see Figs. 2 and 3) has
the advantage that data analysis is very straight forward, but
it requires an ejectable nosecone. The side-looking instrument (see Figs. 4 and 5), on the other hand, can view the
sky through a transparent window in the payload structure
B. R. CLEMESHA et al.: AIRGLOW PHOTOMETERS ON BOARD SOUNDING ROCKETS
Fig. 7. Side-looking photometer signal.
Fig. 8. Block diagram of photometer.
but data analysis is much more complex and, to make best
use of the data, requires accurate payload attitude information. Both types of photometer contain an interference filter
to define spectral response, an objective lens and field stop
to define the angular field of view, and a photomultiplier
tube as a photon detector. They also contain a rotating disc
which acts as a shutter and calibration source. This disc is
made to complete one revolution when the rocket is close
to apogee and makes it possible to measure the photomultiplier dark current. The calibration source, consisting of
a small radioactive source which excites a phosphor, pro-
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vides a relative calibration of the pmt which can be compared with a preflight calibration, thus providing a check on
in-flight performance of the photometer and its associated
electronics. An integrated hybrid preamplifier amplifies the
photon pulses from the pmt, and converts them to TTL levels. The photon pulse rate, proportional to the photometer
signal, is determined by counting in 4 mS intervals at a repetition rate of 250 Hz, the sample counts then being sent to
the PCM telemetry encoder.
The longitudinal photometer views the integrated airglow
from above the rocket. As the rocket passes through the
airglow layer the signal drops from its initial value, proportional to the integrated emission from the entire layer,
to a minimum value when the payload is above the layer.
If there is no contaminating emission from above the layer
then the minimum will be zero. Differentiating the measured signal with respect to height gives the volume emission rate as a function of height, as illustrated in Fig. 6. Precession of the rocket spin axis complicates the analysis and
must be known and allowed for. On the other hand, providing the coning angle is not too large the accuracy with
which the precession parameters need to be known is not
too difficult to achieve.
The side-looking photometer scans the emission layer
with the rotation of the rocket. Since the rocket rotational
axis is not normally vertical this results in a repetitive scan
in the tangent height of the photometer field of view. In
principle this scan makes it possible to determine the emission layer profile with the payload below, within or above
the emission layer, depending on the zenith angle of the
rocket axis and the thickness of the emission layer. An
onion-skin type analysis is then used to determine the volume emission rate as a function of height. An example of
the signal from a side-looking photometer is given in Fig. 7.
As in the case of the longitudinal photometer, precession of
the rocket spin axis complicates the analysis and must be
known and allowed for.
Comparing the two types of photometer, the longitudinal instrument has the advantage of simpler data analysis
and fairly minor dependence on payload attitude, the sidelooking photometer provides more information, including
information on horizontal gradients in volume emission
rate, but the data analysis is more complicated and accurate information on the time history of the payload attitude
is essential.
3.
Instrument Details
The instrument described in this document is a low light
level single channel photon counting photometer for rocketborne airglow measurements. The device has been produced in two versions, a longitudinal photometer, normally
aligned with the payload axis, and a transverse instrument,
normally mounted perpendicular to the payload axis.
The longitudinal photometer consists of two units, the
photometer assembly itself and its associated electronics
package. In the transverse version the electronics package
is included in the same rectangular housing as the photomultiplier.
Photometer unit: A block diagram of the basic photometer is shown in Fig. 8. This unit contains the narrow-
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B. R. CLEMESHA et al.: AIRGLOW PHOTOMETERS ON BOARD SOUNDING ROCKETS
Fig. 9. Electronics package.
Fig. 10. Interconnection between photometer and PCM.
Table 1. Photometer interface signals.
Signal
FREQ
FANA
MAT
TEM
LUZ
COMOT
Type
DIGITAL
ANALOG
ANALOG
ANALOG
DIGITAL
DIGITAL
Range
10 BITS
0–5 V
0–5 V
0–5 V
1 BIT
1 BIT
band filter, objective lens, shutter/calibration disc, photomultiplier (PMT) and preamplifier. The optical/mechanical
layout of the longitudinal photometer is shown in Fig. 3.
Filter: The photometer uses a 50 mm diameter interference filter. A thermistor temperature sensor is used to monitor the filter temperature during the flight.
Sample rate
200 per sec
100 per sec
10 per sec
10 per sec
10 per sec
—
Description
Photometer output
Photometer output
Hi Voltage monitor
Temperature monitor
Cal disc alignment
Calibration command
Shutter/Calibration disc: The Shutter/Calibration disc
contains an aperture which is normally aligned with the optical axis. During flight, near apogee, the disc is rotated
through 360◦ by a small DC motor. During the rotation
a small radioactive light source passes through the optical
axis, providing a calibration signal. When the light source
B. R. CLEMESHA et al.: AIRGLOW PHOTOMETERS ON BOARD SOUNDING ROCKETS
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Fig. 13. OI 557.7 nm emission profile.
Fig. 11. Payload with 6 longitudinal and 4 side-looking photometers.
The side-looking photometers view the emission layer through quartz
windows.
Fig. 12. Integrated OI 557.7 nm emission as function of height.
is not in the optical path the photometer output corresponds
to the dark noise level. A LED and phototransistor combination is used to control the rest position of the disc.
Photomultiplier: The photometer uses a 1” diameter
E.M.I. pmt with an external resistor network. Negative
E.H.T. is used so that the anode is near to ground potential, and the pulse output is amplified by an Amptek chargesensitive hybrid preamplifier which includes a threshold circuit and monostable, giving TTL output. The pmt, divider
network and preamplifier are encapsulated in silicone resin.
Electronics unit: The electronics unit, shown in Fig. 9,
contains the high voltage power supply and its associated
regulator, low voltage regulators for +/−12 V and +5 V,
signal conditioner for the disc position sensor, driver circuitry for the shutter motor, signal conditioner for the filter
temperature sensor and, optionally, a logarithmic amplifier
for monitoring the analog signal from the pmt. The pulse
output from the pmt preamplifier is not processed in this
unit. The electronics package is permanently wired to the
photometer, and the high voltage part is encapsulated in silicone resin. Interfacing between the electronics unit and
the rest of the payload is made via a 15 pin connector. Decoupling capacitors housed in a screened subdivision of the
electronics unit are used to provide immunity to EM noise.
This configuration has been found to be essential to avoid
interference from the 240 MHz telemetry transmitter.
High voltage power supply: The high voltage power
supply consists of a Venus C15 encapsulated power supply
unit driven by a regulated low voltage source. The Venus
power supply contains internal feedback circuitry to maintain a constant relationship between input and output voltages. All high voltage components and connections, including the pmt, are encapsulated in silicone rubber to prevent
corona discharge.
Motor drive and position sensor: The motor driver consists of a complementary pair driven by a common emitter
amplifier. According to the logic level applied to its input
the amplifier applies either 4.4 volts or a short circuit to the
DC motor. The short circuit state is for motor damping.
47 ohm resistors and 100 KpF capacitors at the motor provide suppression of the brush noise. The position sensor
operates by exposing a photo-transistor to the light from a
LED when a hole in the shutter disc is correctly aligned.
Signal conditioners: The signal conditioners use conventional I.C. operational amplifiers to provide outputs in
the range 0–5 V for the analog telemetry input.
Telemetry: The photometer is normally used with a
PCM telemetry encoder which has both digital and analog
inputs. The basic word length of the PCM system is 10 bits.
Analog inputs go to a 10 bit analog to digital converter via
sample and hold circuits and analog multiplexers. Fig. 10
shows how the photometer and ratemeter interact with the
PCM encoder. Table 1 show the sampling rates normally
used.
Ratemeter: The ratemeter consists of a 10 bit counter
made from 3 7493 4 bit binary counters, the output of
which is latched and coupled to the PCM bus via 74LS244
buffers. Gate, latch and reset signals are generated by a sequence timer. The clock for the sequence timer is generated
from the PCM frame sync signal, SCLK, so that counts are
latched at an appropriate point in the PCM cycle. Note that
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B. R. CLEMESHA et al.: AIRGLOW PHOTOMETERS ON BOARD SOUNDING ROCKETS
Table 2. Photometer specifications.
Dimensions (Length × Width × Height)
Total weight (kg)
Total power consumption (watts)
Typical sensitivity (Counts/sec/Rayleigh)
Typical data rate (bits/sec) including housekeeping data
Data rate, scientific data only
this synchronization is essential for proper operation of the
ratemeter/PCM encoder combination.
Shutter/Calibration control: The shutter/calibration
control circuit consists of a simple state machine which provides a command signal to the motor drive circuit. The circuit is triggered by a pulse from a separate timer. On receipt
of this pulse the motor control output COMOT goes to the
1 level, where it stays until the shutter disc has made one
complete rotation and the LUZ signal returns to the 1 level.
A delay circuit, consisting of two CD4020 counters, ensures
a high degree of noise immunity.
Mechanical design: The photometers are constructed
from aluminium. All outside surfaces are polished and
anodized in order to minimize radiative heat transfer during flight. It is important that the photometers should be
mounted in such a way as to minimize conductive heat
transfer. For the longitudinal instrument a package of 6
photometers has been used with 9 mounting points made
from stainless steel and reinforced urea formaldehyde resin.
In the case of the transverse instruments, a mounting plate
made from glass fiber reinforced epoxy resin has been used.
3.1 Results obtained by the photometers
In this section we present one of the rocket photometer
experiments carried out in May 1992, MULTIFOT (Takahashi et al., 1996). The payload contained 6 forwardlooking (longitudinal) photometers, 4 side-looking photometers, ionospheric Langmuir and capacitance probes and
an electron temperature probe. The payload configuration
is shown in Fig. 11. The longitudinal photometers were
mounted in the first bay below the ejectable nose-cone. This
photometer unit was thermally insulated from the rest of the
payload structure in order to avoid heat conduction from the
rocket body during the flight. The 4 side-looking photometers were mounted in the second and third payload bays with
their fields of view perpendicular to the rocket axis, looking
through 2 inch diameter quartz windows inserted into the
payload skin. In the case of the side-looking photometers
thermal insulation was achieved by mounting the photometers on bulkheads made from low-conductivity composite
material.
The vehicle used for the MULTIFOT experiment was
the SONDA III sounding rocket developed by the Brazilian
Institute for Aerospace (IAE). This is a two stage vehicle
with a 30 cm second stage diameter, having a capacity to
carry a payload of approximately 100 kg to 600 km altitude.
For this experiment a shortened version of the SONDA III
vehicle was used, carrying the 105 kg payload to an apogee
of 282 km.
The photometer measures the integrated emission rate
Longitudinal photometer
97 × 78 × 385 mm
1.8
2.4
80 pps/Rayleigh at 589 nm
3720
2500
Side-looking photometer
245 × 120 × 80 mm
2.17
2.4
80 pps/Rayleigh at 589 nm
3720
2500
along its line of sight. Therefore the observed intensity depends on the photometer look angle. Because of spin and
precession of the rocket, the photometer look angle varies
rapidly, causing modulation of the photometer output. In
order to correct for the rocket axis variations, an on board
2-axis magnetometer was used as an attitude sensor. The
side-looking photometer output was also used as another
reference. The details of the data processing have been reported elsewhere (Takahashi et al., 1996). In Fig. 12 the
observed integrated emission rate for the OI 557.7 nm emission is shown as a function of height, without correction for
zenith angle variations. In Fig. 13 is shown the derived volume emission rate (photons/cm3 /sec), after correction for
zenith angle variations.
4.
Future Improvements
The photometers described in this report are reliable instruments that have been launched many times in several
different payload configurations and vehicles. Except in
cases where the rocket malfunctioned, the photometers have
always achieved their objectives. The relative advantages
and disadvantages of the longitudinal and side-looking configurations have been discussed in Subsection 2.2. Since
the time when these instruments were developed (1985–
1995) there have been considerable advances in photomultiplier technology, both with respect to quantum efficiency
and miniaturization. As a result of these developments it
would now be possible to construct photometers with the
same sensitivity but with 40% of the original dimensions
or even less, depending on the application. In many applications this could result in smaller and lighter payloads
leading to the possibility of using smaller launch vehicles,
or the inclusion of more photometers in the same payload.
References
Baker, D. J. and R. O. Waddoups, Rocket Measurement of Midlatitude
Night Airglow Emissions, J. Geophys. Res., 72, 4881–4883, 1967.
Bates, D. R., Excitation of 557.7 nm OI Line in Nightglow, Planet. Space
Sci., 36, 883–889, 1988.
Greer, R. G. H. and G. T. Best, A Rocket-Borne Photometric Investigation
of the Oxygen Lines at 5577 A and 6300 A, the Sodium D-Lines and
the Continum at 5300 A in the Night Airglow, Planet. Space Sci., 15,
1857–1881, 1967.
Greer, R. G. H., D. P. Murtagh, I. C. McDade, P. H. G. Dickinson, L.
Thomas, D. B. Jenkins, J. Stegman, E. J. Llewellyn, G. Witt, D. J.
Mackinnon, and E. R. Williams, Eton 1: A Data Base Pertinent to the
Study of Energy Transfer in the Oxygen Nightglow, Planet. Space Sci.,
34, 771, 1986.
Gulledge, I. S. and D. M. Packer, High-Altitude Profiles of the O I 5577
and 6300 Night Airglow Layers Derived from Rocket Photometry, Astro. J., 71, 163, 1966.
Huruhata, M., T. Nakamura, and H. Tanabe, Rocket Observations of Emitting Height of 6300 A Oxygen Line in Night Airglow, Rept. Ionosph.
B. R. CLEMESHA et al.: AIRGLOW PHOTOMETERS ON BOARD SOUNDING ROCKETS
Space Res. Japan, 20, 223–227, 1966.
Kulkarni, P. V., Rocket Study of 5577-A OI Emission at Night Over the
Magnetic Equator, J. Geophys. Res., 81, 3740–3744, 1976.
Mcdade, I. C. and E. J. Llewellyn, The Excitation of O(1S) and O2 Bands
in the Nightglow: A Brief Review and Preview, Canadian J. Phys., 64,
1626–1630, 1986.
McDade, I. C., E. J. Llewellyn, D. P. Murtagh, and R. G. H. Greer, ETON
5: Simultaneous rocket measurements of the OH Meinel (8, 3) band and
dv=2 sequence in the nightglow, Planet. Space Sci., 35, 1137, 1987.
Melo, S. M. L., H. Takahashi, B. R. Clemesha, P. P. Batista, and D. M.
Simonich, Atomic oxygen concentrations from rocket airglow observations in the equatorial region, J. Atmos. Terr. Phys., 58(16), 1935–1942,
1996.
31
Packer, D. M., Altitudes of the Night Airglow Radiations, Ann. Geophys.,
17, 67–75, 1961.
Shiokawa, K., Y. Otsuka, S. Suzuki, T. Katoh, Y. Katoh, M. Sato, T.
Ogawa, H. Takahashi, D. Gobbi, T. Nakamura, B. P. Williams, C.-Y.
She, M. Taguchi, and T. Shimomai, Development of airglow temperature photometers with cooled-CCD detectors, Earth Planet. Space, 59,
585–599, 2007.
Takahashi, H., B. R. Clemesha, D. M. Simonich, S. M. L. Melo, N. R.
Teixeira and A. Eras, Rocket measurements of the equatorial airglow:
MULTIFOT 92 Database, J. Atmos. Terr. Phys., 58, 1943–1961, 1996.
B. R. Clemesha (e-mail: [email protected]), H. Takahashi, A. Eras, N. B.
Lisboa, and D. Gobbi