U.S. patent application number 13/538655 was filed with the patent office on 2013-01-03 for apparatus, system and method for spacecraft navigation using extrasolar planetary systems.
Invention is credited to George William Hindman.
Application Number | 20130006449 13/538655 |
Document ID | / |
Family ID | 47391409 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130006449 |
Kind Code |
A1 |
Hindman; George William |
January 3, 2013 |
Apparatus, system and method for spacecraft navigation using
extrasolar planetary systems
Abstract
The present invention provides an innovative apparatus, system
and method for onboard spacecraft location determination and
navigation by employing the observation of extrasolar planetary
star system motion. In one apparatus embodiment a gas absorption
cell is placed between a sensor and the light from a reference star
system with at least one exoplanet, such that the sensor can detect
the spectrum through the gas absorption cell. Radial velocities can
be calculated via Doppler Spectroscopy techniques and incorporated
into a spacecraft navigation solution. The present invention can
enable and enhance significant mission capabilities for future
manned and unmanned space vehicles and missions.
Inventors: |
Hindman; George William;
(Austin, TX) |
Family ID: |
47391409 |
Appl. No.: |
13/538655 |
Filed: |
June 29, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61571554 |
Jun 30, 2011 |
|
|
|
Current U.S.
Class: |
701/13 ;
356/614 |
Current CPC
Class: |
B64G 1/36 20130101; G01C
21/025 20130101; B64G 2001/1064 20130101; G01J 3/0264 20130101;
B64G 1/361 20130101; G01C 21/24 20130101; G01J 3/0205 20130101 |
Class at
Publication: |
701/13 ;
356/614 |
International
Class: |
G01B 11/14 20060101
G01B011/14; B64G 1/36 20060101 B64G001/36; G01C 21/24 20060101
G01C021/24 |
Claims
1. A spacecraft extrasolar planetary star tracker apparatus,
comprising: a sensor to detect a spectrum from a star system with
at least one exoplanet; and a gas absorption cell placed between
the sensor and the star system with at least one exoplanet such
that the sensor can detect the spectrum from the star system with
at least one exoplanet through the gas absorption cell.
2. The apparatus of claim 1, wherein the detected spectrum is used
to calculate radial velocity via Doppler spectroscopy.
3. The apparatus of claim 1, wherein the detected spectrum
measurements are used to calculate spacecraft position.
4. The apparatus of claim 1, wherein the detected spectrum
measurements are accumulated and used to calculate a filtered
estimate of spacecraft position.
5. The apparatus of claim 1, wherein the star system with at least
one exoplanet is used to calculate spacecraft attitude.
6. A spacecraft navigation system using extrasolar planetary star
motion comprising: a sensor located on a spacecraft to detect a
spectrum from a star system with at least one exoplanet; a gas
absorption cell located on the spacecraft placed between the sensor
and the star system with at least one exoplanet such that the
sensor can detect the spectrum from the star system with at least
one exoplanet through the gas absorption cell; a computer located
on the spacecraft that is connected to the sensor by a data bus; a
software algorithm located in the computer that can calculate
radial velocities from the detected spectrum via Doppler
spectroscopy techniques; and a software algorithm located in the
computer that can calculate spacecraft position using the
calculated radial velocities from the detected spectrum.
7. The system of claim 6, wherein the computer has an additional
software algorithm that is used in the process of controlling the
velocity of the spacecraft.
8. The system of claim 6, wherein the calculated radial velocities
are accumulated and used to calculate a filtered estimate of
spacecraft position.
9. The system of claim 6, wherein the software algorithm that
calculates spacecraft position uses a Kalman filter.
10. The system of claim 6, wherein the software algorithm that
calculates spacecraft position includes additional navigation
sensor measurements.
11. The system of claim 6, wherein the star system with at least
one exoplanet is used to calculate spacecraft attitude.
12. The system of claim 6, wherein there is more than one sensor
for the purposes of detecting different star system spectrum
simultaneously.
13. A method for onboard spacecraft navigation using extrasolar
planetary star systems, the method comprising the steps of: having
an initial estimate of a spacecraft position in an inertial
reference frame; selecting a reference star system with at least
one exoplanet from an onboard software database; detecting a
spectrum from the reference star system with at least one exoplanet
through a gas absorption cell onboard the spacecraft; using the
detected spectrum from the reference star system with at least one
exoplanet to calculate radial velocity via Doppler spectroscopy;
and incorporating the radial velocity calculations and the initial
estimate of spacecraft position into a filtered estimate of
spacecraft position.
14. The method of claim 13, wherein the means for filtering include
a Kalman filter.
15. The method of claim 13, wherein the filtered estimate of
spacecraft position includes additional navigation sensor
measurements.
16. The method of claim 13, wherein the filtered estimate of
spacecraft position includes Global Positioning System
measurements.
17. The method of claim 13, wherein the filtered estimate of
spacecraft position includes Deep Space Network measurements.
18. The method of claim 13, wherein onboard spacecraft navigation
is used in the process of controlling the velocity of the
spacecraft.
19. The method of claim 13, wherein the reference star system with
at least one exoplanet is also used to calculate spacecraft
attitude.
20. The method of claim 13, wherein onboard spacecraft navigation
is used in the process of controlling the attitude of the
spacecraft.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn.119 to U.S. Provisional Patent Application No.
61/571,554 filed Jun. 30, 2011, the entire contents of which are
hereby expressly incorporated by reference for all purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is an innovative apparatus, system and
method for spacecraft navigation employing the use of extrasolar
planetary system motion. Spacecraft navigation can generally be
described as, but not limited to, the determination of a
spacecraft's position, velocity and attitude at certain times as
well as the determination of orbital parameters and trajectories.
Extrasolar planetary systems are star systems other than the Sun
that have planetary companions. The present invention relates to
several different fields including spacecraft hardware, software,
navigation, astronomy, Doppler spectroscopy methods and astrometric
techniques.
[0004] 2. Description of the Related Art
[0005] Precise determination of spacecraft position and velocity is
necessary in order to achieve mission success for operations of
near Earth and interplanetary missions. Onboard flight technologies
can provide spacecraft position, navigation and timing (PNT). Areas
of related art include traditional spacecraft navigation hardware
and software, tracking such as NASA's Deep Space Network (DSN), the
Global Positioning System (GPS), X-ray navigation and extrasolar
planetary detection.
[0006] Space navigation traditionally relies on initial spacecraft
position, velocity and attitude estimates that are regularly
updated by onboard inertial measurement unit (IMU) data. An IMU is
a device that measures a spacecraft's velocity changes and
orientation using a combination of accelerometers and gyroscopes.
Spacecraft orientation can also be aided by a star tracker, which
is an optical device that measures the relative position(s) of
star(s) against the celestial background using photocells or a
charged couple device (CCD) camera. Additional components such as
horizon or sun sensors are also traditionally employed.
[0007] Methods of onboard orbit and position determination involve
accurate updates to the spacecraft's navigation state matrix ("Nay
State"). Periodic updates from external signals can be processed by
onboard software algorithms and filters. As an example, in low
Earth orbit (LEO), the Nay State can be refined by employing Kalman
filtering and data from terrestrial navigation aids such as C band
radar tracking or the GPS. There are various ways to implement
these software filtering capabilities, one of which is NASA's GPS
Enhanced Onboard Navigation Software (GEONS).
[0008] GEONS supports the acceptance of many one way forward
Doppler, optical sensor observation and accelerometer data types.
GEONS was designed for autonomous operation within the limited
resources of an onboard computer. It employs an extended Kalman
filter (EKF) augmented with physically representative models for
gravity, atmospheric drag, solar radiation pressure, clock bias and
drift to provide accurate state estimation and a realistic state
error covariance. GEONS incorporates the information from all past
measurements, carefully balanced with its knowledge of the physical
models governing these measurements, to produce an optimal estimate
of a spacecraft's orbit. GEONS' high-fidelity state dynamics model
reduces sensitivity to measurement errors and provides
high-accuracy velocity estimates, permitting accurate state
prediction.
[0009] Interplanetary missions typically employ tracking services
from NASA's DSN, which provides radiometric ranging, Doppler and
plane-of-sky angle measurements. For spacecraft ranging, a signal
is sent from one of the DSN stations on Earth to the spacecraft,
which in turn sends a signal back to Earth. The round trip transit
time is measured to determine the line of sight slant range.
Two-way Doppler tracking also uses a signal sent to and from a
spacecraft; by looking at the small changes in frequency, the
spacecraft velocity along the line of sight can be determined.
[0010] In general, angular measurements can be made using multiple
DSN ground stations that receive spacecraft transmissions
simultaneously during overlapping viewing periods. An additional
method used by DSN is delta differential one-way range (ADOR). This
is a Very Large Baseline Interferometry (VLBI) technique that uses
two ground stations to simultaneously view a spacecraft and then a
known radio source (such as a quasar) to provide an angular
position determination.
[0011] Unfortunately, DSN resources are limited and its accuracies
degrade over large distances. Onboard spacecraft navigation systems
that can reduce tracking requirements for the DSN are currently
needed. Furthermore, GPS satellites orbiting the Earth are of
limited use for deep space missions. Thus, hardware and software
systems and methods that provide precise navigation solutions using
a methodology that is independent of Earth based systems are not
only innovative and novel but are currently needed for spacecraft
navigation.
[0012] Some recent research and development with autonomous deep
space navigation has examined the use of pulsed X-ray radiation
emitted by pulsars. Such investigations designate X-ray millisecond
pulsars as a potential signal source to be observed by a
spacecraft. However, the specific characteristics of pulsars are
limiting and very different from main sequence stars such as our
sun. The current invention uses the properties of main sequence
stars and their associated extrasolar planets.
[0013] In the past 15 years or so, over 700 extrasolar planets (or
exoplanets) have been discovered orbiting around 560 main sequence
stars (some stars have multiple detected exoplanets). These stars
are evenly distributed throughout the celestial sphere and most are
within several hundred light years (ly) of Earth. Some potential
exoplanet reference stars include, but are not limited to, Epsilon
Eridani (10 ly away), Gliese 86 (36 ly), 47 Ursae Majoris (43 ly),
55 Cancri (44 ly), Upsilon Andromedae (44 ly), 51 Pegasi (48 ly)
and Tau Bootis (49 ly). All have well known characteristics and are
even visible to the naked eye.
[0014] Before the discovery of exoplanets, the only planets known
to exist were those in our own solar system. The motion of the
Earth about our Sun is well understood and the whole solar system
in fact rotates around a common center of mass, known as the
barycenter. Astronomers, in order to detect possible planets around
stars other than our Sun, had to separate known and unknown stellar
motion to determine the motion of other stars about their own
barycenters. The initial theory postulated that if exoplanets did
exist, their orbits would cause their parent star to wobble by a
small amount. This motion was indeed detected, yielding numerous
exoplanet discoveries. The measurements to date have produced now
well known patterns of highly stable, predictable exoplanetary
system stellar motion with respect to our own solar barycenter.
This exoplanetary system stellar motion can be used to determine
the location of a spacecraft both within and outside of our solar
system. This is the methodology employed by the present
invention.
SUMMARY OF THE INVENTION
[0015] The present invention is an apparatus, system and method for
spacecraft location determination and navigation employing
extrasolar planetary system motion. The apparatus, system and
method provide onboard orbit or location determination and
navigation capabilities during spacecraft operations through the
use of specialized reference stars that have exoplanet companions.
The motion of these exoplanets around the reference star's
barycenter provides a stable, highly predictable natural signal
pattern. The measurements of these signal patterns are taken
onboard the spacecraft and are used with onboard software algorithm
estimation techniques to determine both spacecraft location and
navigation. The present invention enables and enhances significant
mission capabilities for future manned and unmanned space vehicles
as well as reducing DSN tracking requirements and resources.
[0016] The present invention can provide primary or secondary
navigation capabilities for space missions. It is expected to
provide positional solutions anywhere within the solar system as
well as beyond our solar system. Primary autonomous navigation can
be incorporated into spacecraft designed for geostationary,
elliptical high earth orbits, or deep space orbits or trajectories.
Back-up or secondary navigation capabilities could be available for
emergency situations in low and medium Earth orbits when primary
navigation is lost (such as in the case of denied access to GPS).
The present invention could be used for manned missions and would
be particularly useful at locations currently of interest such as
lunar orbits, asteroids, comets, libration points, Martian moons or
outer solar system planets.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A better understanding of the present invention can be
obtained when the following detailed description of the preferred
embodiment is considered in conjunction with the following
drawings, in which:
[0018] FIG. 1 illustrates Solar motion about the barycenter, from
the time period of 1960 to 2025 AD.
[0019] FIG. 2 illustrates the radial velocity of the Sun as it
orbits the solar system barycenter.
[0020] FIG. 3 illustrates a spacecraft in the space
environment.
[0021] FIG. 4 illustrates a functional spacecraft block
diagram.
[0022] FIG. 5 illustrates the components of a standard star
tracker.
[0023] FIG. 6 illustrates an exoplanetary star tracker apparatus
and gas absorption cell block diagram.
[0024] FIG. 7 illustrates the principle elements of an astrometric
interferometer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] Nay State determination through the use of extrasolar
planetary system motion data is an innovative method for onboard
spacecraft navigation. It will significantly enable and enhance
mission capabilities for future manned and unmanned space vehicles
as well as reducing the need for Deep Space Navigation resources.
Over 700 extrasolar planets have been discovered around nearby main
sequence stars within the past 15 years. The motion of these
extrasolar planets around their stellar barycenters provides a
stable, highly predictable natural signal pattern. Observations
from these star systems allow for enhanced spacecraft self
determination of orbits and position as well as navigation.
Extrasolar Planetary System Motion and Measurements
[0026] Earth based exoplanet searches have sought to identify
planetary systems by observing characteristics of the parent star
about which the potential planet is orbiting. The main
methodologies employed for such exoplanet detection have been
astrometry and Doppler spectroscopy. In celestial mechanics, the
simplest case is of a single planet orbiting around one star. The
system orbital parameters can be derived from Equation 1:
a.sup.3=(M.sub.*+m.sub.p)P.sup.2 (1)
where the masses (M.sub.*, m.sub.p) are in solar units, the
semi-major axis (a) is in astronomical units (AU) and the period
(P) is in years. The motion of the star is much smaller than that
of the associated planet. Using techniques for indirect observation
of exoplanets, the small motion of the reference star is detected,
allowing for calculations that infer the existence of the
exoplanet.
[0027] Astrometry attempts to measure the movement of a star with
respect to background stars. In cases where the movement is
apparent, parallax is being measured. If a star were seen to have
an elliptical motion, the probable explanation would be that the
wobble is due to a star orbiting about its barycenter. Using
Equation 1 and the fact that the semi-major axis can be measured as
an angle, .theta., yields Equation 2:
.theta. = m p M * - a r = m p r ( P M * ) 2 / 3 ( 2 )
##EQU00001##
where .theta. is in arcsec when a is in AU, both masses are in
solar units, distance (r) is in parsecs (pc) and P is in years. For
example, if one were to view our solar system from a distance of 10
pc, Jupiter would appear as an 11.9 year disturbance in the Sun's
motion with a 0.5 milliarcsec amplitude. FIG. 1 displays what our
solar system motion about its barycenter would look like if viewed
from the north ecliptic pole at a distance of 10 pc, with the right
horizontal axis pointing to the Vernal Equinox. Planet detection is
most sensitive to stars that are near the solar neighborhood and
have a large planet. Most of the exoplanets detected to date have
been described as "large Jupiters", with periods measured in
days.
[0028] For astrometry, the motion of the star is most pronounced
when the exoplanet(s) orbiting the star are in a plane
perpendicular to the line of sight of the observation point. Any
other orientation would produce some cyclical motion towards and
then away from the observation point. Doppler spectroscopy takes
advantage of this radial motion by trying to detect the alternating
red and blue spectrum shifts that a star in this orientation would
have. This Doppler motion would create a variable radial velocity
as dictated by Equation 3:
v = 30 m p sin i ( aM * ) 1 / 2 = 30 m p sin i M * 2 / 3 P 1 / 3 (
3 ) ##EQU00002##
where .nu. is in km/sec, the masses are in solar units, a is in AU,
P is in years and i is the inclination of the orbit to the plane of
the sky. Using the previous example for astrometry, Jupiter has a
velocity variation of 13.0 msec over a period of 11.9 years. Most
exoplanets detected to date have larger velocity variations than
Jupiter, over a period of just days. FIG. 2 depicts the apparent
radial velocity shift of our Sun, primarily due to Jupiter, as
viewed from the Vernal Equinox for the same time period as shown in
FIG. 1.
[0029] Doppler spectroscopy measurements are thus exceptionally
useful, since identified stars with planetary companions have a
stable, known repeatable pattern of motion. Astrometric
measurements of parallax and stellar angular displacements also
provide valuable data. Since these stellar motions about the
barycenter are known with a high degree of precision and
consistently and reliably repeat over many cycles and years, they
make excellent reference sources. Currently there are over 500
observed exoplanet star systems. This population allows for a
viable extrasolar planetary system reference database for onboard
spacecraft navigation.
[0030] Full three dimensional absolute and relative navigation
solutions are achievable from extrasolar planetary system sources,
including position and velocity determination as well as spacecraft
attitude determination. Spacecraft navigation algorithms and
software filtering can combine onboard measurements with
exoplanetary stellar motion based models and other characteristics,
such as source declination, right ascension and proper motion to
yield a solution. Absolute position or delta updates to a position
can be calculated and blended with a spacecraft's Nay State.
[0031] Absolute positions may be obtained either by range or
wavelength phase measurements. In general, a spacecraft range
(.rho.) can be calculated from the difference in the transmit and
receive times of one source spectrum by Equation 4:
.rho.=c(t.sub.r-t.sub.t) (4)
where c is the speed of light. If the range measurement is known as
well as the unit vector for the extrasolar planetary system source,
the spacecraft range in an inertial reference system may be
computed. Absolute position can also be achieved through
simultaneous observations of several sources. Determining the range
measurements of any unique set of three extrasolar planetary
systems yields the location of a spacecraft in three dimensional
space.
[0032] Wavelength phase measurements can be thought of as a total
wavelength phase that is the sum of some integer number of cycles
plus a fraction of one cycle. These measurements and their time of
arrival can be merged and used by navigation software to determine
position by employing a process similar to GPS integer cycle
ambiguity resolution. The basic equation for GPS carrier phase
pseudorange is well known in the literature and can be written as
Equation 5:
.PHI.=[1/.lamda.].rho.+f.DELTA..delta.+N (5)
where .PHI. is the measured carrier phase, N is the phase ambiguity
integer or "integer ambiguity", .DELTA..delta. is the clock bias,
.lamda. and f are the GPS carrier phase wavelength and frequency,
and .rho. is the range. Substituting f=c/.lamda. and expressing
Equation 5 as a mathematical model yields Equation 6 and Equation
7:
.PHI..sub.ij(t)=[1/.lamda.].rho..sub.ij(t)+[c/.lamda.].DELTA..delta..sub-
.ij(t)+N.sub.ij (6)
where i and j are two points in a designated reference frame at an
epoch (t) and:
.rho..sub.ij(t)=[(X.sub.j(t)-X.sub.i).sup.2+(Y.sub.j(t)-Y.sub.i).sup.2+(-
Z.sub.j(t)-Z.sub.i).sup.2].sup.1/2 (7)
[0033] While the above equations are usually applied to GPS and its
geocentric reference frame, the same concepts are employed for the
space environment for the purposes of this invention. The
wavelength selected could be any one of many that are associated
with the stellar signature of an extrasolar planetary system and
the coordinates can be in an inertial solar reference frame tied to
the solar barycenter. Using this type of solar reference frame and
an appropriate timing model defined at a specific location,
information observed at a spacecraft can be matched with data in an
onboard extrasolar planetary system database to provide a
navigation solution.
[0034] Furthermore, onboard software algorithms may employ
differencing techniques for one or more extrasolar planetary
systems to remove errors. A single difference calculation could be
done between the measured spacecraft wavelength phase arrival and
the phase predicted at a model location. A double difference could
be obtained by subtracting two single differences from two
different sources. A triple difference could be calculated by
subtracting two double differences from two separate time
epochs.
[0035] It is also noted that the observed star radiates in the
entire electromagnetic spectrum, so multiple wavelengths can be
monitored at the same time. This would provide for naturally
occurring multiple frequencies from the source, similar to GPS
satellites broadcasting more than just one L band frequency.
Exoplanetary System Star Tracker Apparatus for Space Navigation
[0036] FIG. 3 depicts a partial representation of the space
environment, with the Earth 1 orbiting the Sun 2. A spacecraft 3 is
also depicted, with the disclosed inventions located onboard. An
inertial solar reference frame 4 is shown with the origin located
at the solar system barycenter. The distances to the Earth, Sun and
spacecraft in the reference frame are indicated by .rho..sub.E
.rho..sub.S and .rho..sub.sc respectively. Some extrasolar
planetary systems 5 are viewable from the spacecraft. Each
independent extrasolar planetary system 5 would have a known unit
vector in the inertial reference frame as well as a known stellar
signature.
[0037] FIG. 4 depicts a spacecraft functional block diagram of one
embodiment of the invention. A spectrum wavelength .lamda..sub.eps
from one or more extrasolar planetary system sources 6 is viewable
from the spacecraft 3. The spacecraft has an onboard computer 7
with hardware components such as, but not limited to, processor(s),
memory, storage, busses, power sources, oscillators and/or timing
sources. The onboard computer 7 also has software processing
capabilities and algorithms that perform various navigation
functions such as, but not limited to, signal processing, clock
adjustments, ephemeris and model propagation and filtering
corrections (such as least squares or Kalman) to improve position
and velocity estimates.
[0038] The spacecraft 3 also has other subsystems 8. Subsystems 8
may include, but are not limited to, navigation units such as IMUs,
star trackers, GPS receivers, horizon and sun sensors. Subsystems 8
may also include, but are not limited to, scientific instruments,
guidance units, thrusters, propulsion engines and communication
systems. A data bus system 9 connects the onboard computer 7 to the
spacecraft subsystems 8 as well as to one or more extrasolar
planetary system star trackers, depicted as 10, 11 and 12 in FIG.
4. If more than one extrasolar planetary system star tracker is
located on a spacecraft, the orientation of their axes and fields
of view may be chosen to optimize a function such as, but not
limited to, viewing different sources or redundancy. An extrasolar
planetary system star tracker or sensor may be comprised of various
components such as, but not limited to, photocells, CCDs, gas
absorption cells, processor(s), memory, storage, busses, power
sources and oscillators.
[0039] The present invention incorporates advancements to
traditional star trackers that have been used in the aerospace
industry. These star trackers have been integrated into spacecraft
platforms and most applications to date have used them for
corrections to IMU or ring laser gyro derived spacecraft attitudes.
Individual star trackers have also been used during the approach
phase of rendezvous operations to update a spacecraft's relative
Nay State. FIG. 5 depicts a typical star tracker. Major components
usually include a light shade 13, a bright object sensor 14, a
shutter mechanism 15, a protective window 16, an adapter plate 17,
and a main assembly instrument section 18 with connectors 19.
[0040] The present extrasolar planetary system star tracker
invention could still be employed for traditional uses. However,
the greatest benefits are derived from the innovative approaches
implemented in the instrument package, namely orbit and location
determination and navigation capabilities through utilization of
Doppler spectroscopy and/or astrometry. Doppler spectroscopy is
achieved by placing a gas absorption cell or other similar device
in the star tracker field of view. Another embodiment would allow
potential astrometric data to be obtained with a photon collector
or a Michelson interferometer. A navigation solution is determined
or refined by the radial velocities produced by Doppler
spectroscopy of a reference star with exoplanets and/or astrometric
angular displacements and parallax measurements.
[0041] An embodiment of the present invention may use single
aperture and/or interferometric equipment for astrometric
measurements. Radial velocity detection for Doppler spectroscopy
may use the Fabry-Perot and/or gas absorption cell techniques. The
preferred embodiment of the present invention star tracker system
would make use of an I.sub.2 gas absorption cell. The I.sub.2 gas
absorption cell technique has been successful in the Earth based
detection of exoplanets. The main components consist of a
translucent glass cell, heaters, temperature sensors, insulation
and necessary electronics.
[0042] FIG. 6 depicts a block diagram preferred embodiment of an
extrasolar planetary system star tracker with a gas absorption cell
apparatus. Iodine gas is enclosed in a central tube 20 and the
whole cell housing 21 is placed in the path of the stellar spectrum
6 being observed. The spectrometer CCD 22 records the photons
detected in the designated wavelengths for both the stellar
spectrum 6 and the I.sub.2 gas cell spectrum 23. The electronic
package 24 may be comprised of various components such as, but not
limited to, processor(s), memory, storage, busses, power sources,
oscillators as well as software algorithms and programs. The pure
stellar spectrum template is eventually compared to the combined
I.sub.2 gas cell and stellar spectrum to derive the necessary
radial velocities
[0043] With the present invention, data could also be collected
from a potential astrometric interferometer. Most existing star
trackers are set up to detect some minimum light flux intensity and
then record the location of the light in the star tracker's field
of view. Interferometers obtain data in another manner. The present
invention apparatus may have various embodiments with an
interferometer, either within the extrasolar planetary system star
tracker apparatus itself, several devices located on the spacecraft
platform or devices located on multiple spacecraft.
[0044] Referring to FIG. 7, light from the target star is collected
by two subapertures and routed via minors to a beam splitter (a
partially reflective mirror) where the two beams are combined. This
combined beam will exhibit constructive and destructive
interference; the interference will be at a maximum if there are
equal optical path lengths from the source to the beam splitter via
the two arms. If the source direction is shifted relative to the
interferometer baseline, an additional path delay results in one
beam external to the interferometer. This path delay must be
compensated by an equal amount of path delay in the other beam
internal to the interferometer to maintain the maximum
interference. This relationship can be written as Equation 8:
X=BS+C=|B| sin .theta.+C (8)
where B is the baseline vector (essentially the vector connecting
the two subapertures), S is the unit vector to the star, C is a
constant (instrument bias) and the delay X is the amount of
internal path length necessary to equalize the path delays. Thus,
the delay X is a measure of the angle between the interferometer
baseline and the star unit vector.
[0045] The present invention apparatuses, systems and methods
disclosed in this application are envisioned to have multiple
forms, steps and embodiments. These can include, but are not
limited to, various modifications, separate and/or integrated
components, chipsets, boards, sensors and computer architectures as
well as similar or analogous hardware and software.
* * * * *