ANDE Project
The Naval Research Laboratory
06-19-06
Title: ANDE Spin Vector Measurements using Polarimetric Photometry
Period of Performance: December 2006 – November 2008 (2 years)
Co-Principal Investigator: Eileen V. Ryan, Ph.D.
New Mexico Institute of Mining and Technology
Magadalena Ridge Observatory
801 Leroy Place
Socorro, New Mexico 87801
(505) 835-6803
(505) 835-6807 (FAX)
eryan@mro.nmt.edu
Co-Principal Investigator: Mark Pesses, Ph.D.
SAIC
1911 N. Fort Myer Dr., Suite 1100
Arlington, VA 22209
(703) 797-3003
pessesm@saic.com
Co-Investigator: William H. Ryan, Ph.D.
New Mexico Institute of Mining and Technology
Magadalena Ridge Observatory
801 Leroy Place
Socorro, New Mexico 87801
(505) 835-6646
bryan@nmt.edu
INTRODUCTION
The purpose of the Naval Research Laboratory’s (NRL) Atmospheric Neutral Density
Experiment (ANDE) is to improve precision orbit determination and prediction by
monitoring total atmospheric density at 400km1. Our objective is to augment the mission
deliverables through a ground-based campaign to monitor the spin rate and orientation of
the spacecraft in the context of polarization, and to determine how these parameters are
affected by solar activity.
The spin motion of ANDE will have some affect on the spacecraft’s orbit, just as the spin
of a curveball has an affect on a baseball’s trajectory. We propose to measure the spin
vector of ANDE using polarimetric photometry. The spin period of ANDE could be
determined without the use of polarimetry. The albedo variation on the surface of the
satellite (see Figure 1) will manifest itself in peaks and troughs as the object passes
overhead, and those variations will produce a changing “lightcurve”, obtained from timeseries photometry, similar to that shown in Figure 2. However, polarimetry, which affects
the amplitude of the ANDE light curve, provides a significantly more sensitive method to
measure the direction of the spin vector. This is based on the fact that the polarimetric
signature of a surface is usually much more dependent on look-angle variations than is
the color signature. In our case, the look-angle is the angle between the ANDE spin-axis
in the spacecraft hemisphere pointing towards Earth and the look direction of our
telescope. The amplitude of the ANDE light curve should be a minimum near zerodegree look-angle, where its spin axis is aligned with a telescope that does not resolve the
satellite, and a maximum near 90-degree look-angle, when the spin axis is perpendicular
to the telescope’s line of sight.
Figure 1. The ANDERR MAA Sphere, 19-inches in diameter.
A.C. Nicholas at al., “ANDE Risk Reduction Flight observing opportunities and mission update”,
Proceedings of the 2005 AMOS Technical Conference, Maui HI.
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Figure 2. Representative lightcurve of the Mock ANDE Active Sphere, generated using
an IDL-based forward model simulator. Note that shading was chosen to enhance
demonstration of the qualitative features, not for quantitative accuracy.
This investigation will be undertaken to also determine how solar activity influences the
spacecraft’s orbital characteristics. Figure 3 illustrates how a geomagnetic storm can
change the orbits of resident space object unexpectedly, depicting the loss of spacecraft
positions during the March 1989 super-storm. Magnetic storm changes are due to orders
of magnitude increases in the upper atmospheric density as a result of heating in the
lower atmosphere.
Figure 3. Effects of the March 1989 magnetic super-storm. During major magnetic storm,
all auroral effects may extend far to the south.
We will monitor solar “weather”, and when NORAD posts a high activity alert, and after
the ANDE spacecraft position has been re-determined, we will attempt to obtain
lightcurves to solve for the spin rate and orientation, and analyze any changes.
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The following sections detail our methods utilizing polarimetric photometry, and describe
the work structure and contributions from each of the participants. Expected deliverables
and the accompanying budget are also outlined.
POLARIMETRIC PHOTOMETRY AND SPIN RATE
The temporal brightness variations (i.e., “lightcurves”) of unresolved targets such as
artificial satellites can be used to develop a powerful tool for general characterization
studies. This information is obtainable with modest instrumentation, including off-theshelf commercial telescopes. Analysis of these temporal signatures permits the extraction
of a rotation period and pole orientation, and may also be an indicator of the general
health status of a space object.
Figure 4. Brightness projections for the 19-inch diameter Mock ANDE Active spacecraft,
assumed to be at an altitude above the Earth of 400km.
The expected visual brightness variations of the spherical Mock ANDE Active spacecraft
as it passes overhead are shown in Figure 4. These estimates are based on a Lambertian
sphere with albedo pattern variations from 0.1 to 0.9 and viewed at 90 solar phase angle,
representative of typical conditions for a low-Earth orbit ground-based observation of a
partially sunlit object2.
We propose two methods for obtaining polarimetric photometry of the Mock ANDE
Active spacecraft: using a fast-tracking (slew rates are 10/sec) 2.4m telescope, and using
a pair of portable 0.35-meter telescopes. The performance of each is described below.
McCue, G. A., et al., “Optical Characteristics of Artificial Satellites,” Planet. Space Sci., Vol. 19, p. 851,
1971.
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Since our ultimate goal is to monitor how solar activity effects the spin characteristics of
ANDE, the portable telescopes will be used to give us maximum flexibility in
deployment to a site for which we can obtain data under high magnetic storm conditions.
The 2.4m will enable us to take high-frame rate images having a high signal-to-noise
ratio. When possible, simultaneous lightcurves of ANDE will be acquired to increase the
overall precision of the measurements.
Fast-Tracking 2.4m Telescope
The Magdalena Ridge Observatory’s (MRO) 2.4-meter telescope is located at 10,612 feet
atop the Magdalena Mountains in Central New Mexico, and first light will be in
September 2006. The 2.4-meter’s control system is designed to provide convenient and
accurate non-sidereal tracking, and the telescope is capable of rapid movement and
tracking (slew rates are up to 10 /sec). The initial astronomical instrumentation package
will consist of a 4k x 4k CCD imager and a standard broadband filter set. The imager’s
field of view extends to 11 arc minutes square and will be deployed at a site that has
demonstrated a typical median seeing of 0.7 arc seconds. We predict that this system will
be capable of doing 2% photometry on objects of V~20.5 with 60 second exposures
(assuming good seeing and dark skies) and have a 5-sigma detection limit of V~23. The
ANDE characterization effort will consist of the acquisition of much shorter exposure
time-series photometric data (i.e., “lightcurves”) using a fixed-position linearly polarized
filter while tracking on the target. For observations of ANDE using the 2.4-meter
telescope, signal is not an issue. Therefore, using the facility science camera, full-frame
readout time will be the defining factor regarding time resolution of the data. In fast
readout mode (400 kHz) via four ports and with images binned 2x2, we can expect ~3-4
sec time resolution. However, as long as the telescope tracking performs to
specifications, full- frame images will not be required. Although this will allow for subone-second readout times by utilizing a much smaller portion of the image array, it will
introduce excessive wear on the shutter. Therefore, if it is determined that higher frame
rates are required, an alternate shutterless interline CCD-based camera will be used. This
camera will have a lower quantum efficiency than the science camera, but will permit 10
MHz readout speeds. With either camera, frame rates and exposure times will be adjusted
throughout this project as the optimal balance between temporal resolution and required
signal strength is identified.
Portable 0.35-meter Telescope
In addition to the 2.4-meter telescope, a pair of portable 0.35-meter telescopes capable of
tracking fast-moving targets will be used to determine the spin vector of ANDE. One is
currently deployed at NM Tech and the second is requested as part of Year 2 funding for
this proposal. We have previously used this instrument to acquire lightcurves of highaltitude targets such as the rocket body (NORAD 08840) shown in Figure 5. The lefthand image shows the satellite’s trail in a CCD frame that was tracking at sidereal rates,
and the right-hand image shows the resulting lightcurve with the derived rotation rate
(9.63 seconds). However, since ANDE will be traveling in a low-Earth orbit, we will
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employ the same technique that was described for the 2.4-meter, namely, taking short
exposures while tracking on the target itself.
For the 0.35-meter telescope, the 2% photometry limit for 1 second exposures is V~12
and V~9.5 for 0.1-second exposures. If the V filter is replaced by a polarizer, then we
estimate a +/- 1.0 magnitude variation (projected from the experience that a V filter
causes a loss of ~1 magnitude compared to an unfiltered setup, but the polarizer will
introduce a ~0 - 2 magnitude attenuation). Based on this, we should be able to do
approximately 2 - 3% photometry down to an elevation of 20 with 1 second exposures,
even allowing for a magnitude or so uncertainty in the brightness predictions. Similar
precision photometry will be possible with 0.1 sec exposures on the high albedo portion
of the lightcurve throughout the duration of the pass, but will be limited to when ANDE
is nearly overhead for the lower albedo data points. Therefore, with the USB cabled CCD
requested as part of this proposal, we should be able to acquire 1- 2 second time
resolution data on the Mock ANDE Active spacecraft using the portable 0.35 meter
telescopes. In addition, the shutterless interline CCD mentioned above for the 2.4-meter
can also be mounted on the portable telescopes. Despite its lower quantum efficiency, it
will still permit that acquisition of 5 or more frames of data per second with a
photometric precision of 2 - 3%.
Figure 5. CCD image of NORAD 08840 (trailed streak) taken with a 0.35-meter
telescope, and its derived rotational period (9.6 sec).
3-D POLARIZATION MODEL FOR SPIN ORIENTATION
Our use of polarimetric-photometry is to increase the spin-modulated amplitude of the
spacecraft light curve, and more importantly, to increase the strength of the dependence
of this amplitude on the spin axis-telescope angle.
Polarization Ground Truth
Spectropolarimetric ground truth images of both ANDE spacecraft were taken in
February 2006 at NRL. The images were obtained using a SIGMA SD1O camera with a
focal length of 28 mm, aperture value of F4.5, an ISO setting of 800, and a shutter speed
of 1/15 s. The SIGMA camera has three separate 2268 x 1512 focal plane arrays, one for
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each RGB color. The value of three Stokes parameters, S0, S1, and S2 have been
obtained using an Edmund linear polarizer with a mean cross transmission of 0.15%
between 400 and 700 nm. The polarizer crossed transmission characteristics are shown
in Figure 6. The Stokes parameters have been measured using four different Edmund
Scientific color filters (red, green, blue, and “natural sky” light blue). The transmission
curve for each filter is in Figure 7.
Figure 6. Polarizer crossed transmission characteristics.
Figure 7. The transmission curve for each filter used in measuring S0, S1, and S2.
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We will calculate the Degree of Linear Polarization (DOLP) and the Angle of
Polarization (AOP) from the Stokes parameters. We have spectropolarimetric images of
the Mock ANDE Active spacecraft for five different orientations. Using a terrestrial
globe analogy, one image is centered on the North Pole, and the other four images
centered on the Equator at 0o, 90o, 180o, and 270o longitudes. This resulted in the
spacecraft having one-half of its surface imaged from three different angles and the other
half from two.
The Mock ANDE Active spacecraft is 19.0 inches in diameter [Nicholas, et. al., 2005],
and in our images it takes up 28.0% of the camera’s 2268 x 1512 pixel focal plane array.
This gives ~ 961,000 of pixels “on target” and ~ 1690 pixels per square inch of the
imaged spacecraft surface.
We will use the above dataset to construct a 3-D spectropolarimetric model of the ANDE
spacecraft (see next section). This model and the observed time histories of S0, S1, S2,
DOLP, and AOP in each spectral region will be combined do find a least squares fit
estimate to the spacecraft’s rotation vector. Note that the direction and magnitude of the
spacecraft rotation vector must be independent of observing color.
Spacecraft Polarization Model
We assume that regions of the spacecraft (S/C) surface that have the same chemical
composition have the same index of refraction as a function of wavelength n(). We
estimate the value of n() using the observed values S0, S1, S2: the calculated values of
DOLP and AOP; the viewing and illumination geometry; and Fresnel’s equations. To do
this, without the aid of a supercomputer, we divide the S/C surface up into 15o by 15o
“bins.” This decreases the surface regions fit per color from ~ 1.8 million to 288.
The result of the above is a 3-D model of the index of reflection as a function of
wavelength of the ANDE S/C surface. This model combined with an S/C illumination
model and the viewing geometry is used to simulate the ANDE light curves. With this
simulation, there are 288 possible orientations of the S/C spin axis.
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1
Intenisty
0.8
0.6
S1
n = 1.52
0.4
SO
SO
0.2
S1
0
0
20
40
Angle Look
60
80
Figure. XXX
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SO
SO
Intenisty
0.8
0.6
n = 3 + 7i
0.4
S1
0.2
S1
0
0
20
40
Angle Look
60
80
Figure XXXX.
STATEMENT of WORK
We will determine the spin rate and pole orientation of ANDE based on ground-based
photometric and polarimetric observations. The Co-PI (Dr. Pesses) has spectralpolarimetric ground truth data on both the passive and active ANDE satellite in the visual
magnitude range 0.45-0.70 µm. From this data he will construct a 3-D model of each
satellite. This will be compared to time-series polarimetric photometry obtained using the
Magdalena Ridge Observatory 2.4-meter telescope in order to determine the orientation
of the spacecraft’s spin vector. In addition, traditional unpolarized lightcurves will be
acquired and analyzed in an effort to demonstrate the higher quality of the information
contained in the polarized data.
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Polarimetric photometry will be obtained of ANDE with the 2.4m telescope for 2 nights
per month. Data will be reduced to obtain spin rate and pole orientation, and these data
will be correlated to magnetic storm activity.
Over the course of this 2-year investigation, solar activity will vary. Therefore, we also
propose to observe the effects of space weather on the ANDE spacecraft spin motion.
Magnetic storms and solar flares can increase the upper atmospheric mass density by
orders of magnitude. Following large space weather event in which an ANDE orbit has
be significantly modified, an observing campaign will be initiated using a 0.35-meter
commercially available telescope with satellite tracking drive when the 2.4m telescope
cannot see the spacecraft.
For the first year of this investigation, an existing 0.35-meter telescope (belonging to NM
Tech and located in Southwest) will be used. In the second year, an additional one will be
purchased and located on the East coast for maximum spatial coverage of solar storms.
We will select 2 events per year where these telescopes will be deployed to obtain data on
ANDE under high magnetic activity conditions. Event locations will be limited to be
within a 600 mile radius of the Co-PI’s locations. These portable 0.35m telescopes will
be equipped with a position-variable polarization filter, as well as a liquid crystal filter.
Data will be obtained using both filter types and the results compared.
DELIVERABLES
The following objectives will be realized at the conclusion of this 2-year effort:
1) Observations will be made of the Mock ANDE Active spacecraft to support NRL’s
monitoring of its spin vector and spin orientation.
2) Using ground truth polarimetric data, a 3-D model of the ANDE spacecraft will be
constructed.
3) A comparison of the results from polarimetric lightcurves vs. unpolarized lightcurve
data will be made.
4) The effect of solar activity on the ANDE spin vector will be analyzed.
5) A paper will be published and presented at an international conference.
WORK BREAKDOWN
The contributions of each of the participants are described below.
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Dr. Eileen Ryan
Dr. E. Ryan will assist in the lightcurve data collection using the 2.4m telescope, as well
as the 0.35m portable telescope located at NM Tech. She will also participate in the data
reduction and analysis of the spin rate of the spacecraft, and will take the lead in the
overall management of the project, ensuring that its deliverables are achieved.
Dr. William Ryan
Dr. W. Ryan will assist in the lightcurve data collection using the 2.4m telescope, as well
as the 0.35m portable telescope located at NM Tech. He will also be responsible for
collecting data with and without the polarization filters, and making the comparisons of
the results. Further, he will be the lead participant responsible for the bulk of the
reduction and analysis of the acquired data leading to a spin rate determination as a
function of solar activity.
Dr. Mark Pesses
Dr. Pesses has already acquired spectropolarimetric ground truth images of the ANDE
spacecraft. He will be responsible for generating the 3-D polarization model for
determining the spacecraft spin orientation as a function of solar activity.
BUDGET DETAIL
SHORT BIOGRAPHIES
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