Small Aperture Telescope Augmentation Study
R. Lambour, E. Pearce, S. Ferner, E. Rork, P. Trujillo, A. Decew, P. Hopman
MIT Lincoln Laboratory
244 Wood St. Lexington, MA, 02420 USA
Abstract
Ground–based optical sensors are routinely used by the United States and other
nations to track objects in deep space orbits due to their inherent sensitivity advantage
over radars for this task. However, these sensors are subjected to the variations of
atmospheric weather, which can result in the inability to track or observe objects as
frequently as desired, which in turn may lead to difficulty re-acquiring the objects at a
later date. One concept for reducing the impact of weather on ground-based sensors is
rather simple: increase the number of sensors and disperse them geographically
to decrease the chances that all of the sensors will be simultaneously weathered out.
This paper presents results from a recent study that examined the use of smallaperture and relatively inexpensive optical sensors to augment the existing Ground-Based
Electro-Optic Deep Space Surveillance (GEODSS) network. Starting with assumptions
about the operational mission of the sensors, we derive system level requirements for the
sensors that flow down to a preliminary design for the sensor. This paper discusses the
requirements definition process and the resulting sensor design.
1. Introduction
Ground-based optical sensors are routinely used in the United States and other
nations to track man-made objects (satellites) in deep space orbits due to their inherent
advantage in sensitivity over radars for this task. The United States utilizes the GroundBased Electro-Optic Space Surveillance (GEODSS) network for this task. The GEODSS
network consists of three sites distributed globally, each of which has 3 1-m aperture
telescopes. These sites are located at Socorro, New Mexico, Maui, Hawaii, and Diego
Garcia. A fourth dedicated space surveillance site exists in Moron, Spain with a single
smaller telescope system (0.56-m aperture) referred to as the Moron Optical Space
Surveillance (MOSS) system. All of these sites track low earth orbit and deep space
satellites obtaining metric and photometric space surveillance data. Being ground-based
sensors, all are subjected to the variations of atmospheric weather, which can result in the
inability to track objects as often as desired, which in turn can lead to difficulty reacquiring the objects at a later date. The four sensor sites are distributed in a roughly
uniform fashion longitudinally, so that poor weather can render a significant fraction of
the geosynchronous belt unobservable for one or more nights at a time. One relatively
simple solution to reduce the susceptibility of ground-based sensors to weather is to
increase the number of sensors and disperse them geographically to reduce the
probability that all sensors will be weathered out simultaneously. This concept is referred
to as geographic dispersion.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
To address the utility of this concept, MIT Lincoln Laboratory (MIT/LL) was
tasked to formulate broad system-level requirements for a low cost, sustainable, smallaperture, and autonomous telescope system that could be used cooperatively with the
GEODSS network. This sensor is referred to as the SATA sensor throughout this paper.
The goal of the study was to produce the following products. First, system level
requirements and supporting technical analysis were developed for the functional
capabilities of the SATA sensor (e.g., required sensitivity and surveillance rate). Second,
broad concepts for system operations, maintenance, security, and communications were
developed and are presented. We were instructed not to consider specific sites in detail,
so a high-level examination of site requirements was conducted. These system-level
requirements and system concepts were flowed down to develop derived requirements on
some sensor subsystems. Finally, a notional system design that meets the system-level
design and performance expectations was developed after conducting an analysis of the
current state of the market in commercial off-the-shelf (COTS) system components and
an analysis of the current state of the art in the various system components. Some of the
more interesting study products will be described in this paper.
2. SATA Sensor Operational Concept
The study process began with development of an operational concept for the
SATA sensor. That concept is that the SATA sensor will be a search-based system rather
than a traditional task-and-track system, but will be capable of limited task/track
operations and will be capable of interfacing with the command and control system for
GEODSS as well as independent operation. The primary reason for this concept is the
desire not to have to command the sensors remotely unless absolutely necessary; thereby
simplifying routine sensor operations and reducing sensor operational costs. The primary
mission of the SATA system will be to augment current GEODSS deep space
surveillance capabilities by assuming the catalog maintenance task from the GEODSS
system. The SATA sensor(s) is envisioned to operate autonomously, without need of a
human operator or human intervention during normal operations. We interpret this
operational concept as a definition of mission requirements.
Given this operational concept, we have developed system level performance
requirements for the SATA sensor within the context of the current GEODSS network
and with expectations that space-based space surveillance sensors will be deployed in the
future. Our performance requirements are also consistent with the realization that the
SATA sensor will be primarily a search sensor, not a task-and-track sensor. We have
utilized forward-looking operational, maintenance, and force structure concepts in the
development of the system requirements, in particular: 1.) Geographical dispersion to
minimize weather outages, 2.) Autonomous, unattended operation, 3.) Streamlined
acquisition and maintenance, and 4.) Use of Commercial Off The Shelf (COTS)
components to lower costs if appropriate; COTS was used as a means and not an end in
the sensor design. We have analyzed the design trade space and have developed a
conceptual sensor design as a benchmark for sensor performance evaluation and cost
estimation.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
3. SATA Requirements Definition and Supporting Analysis
As mentioned previously, the SATA sensor is envisioned to be an autonomous
search sensor that will assume the GEODSS catalog maintenance task. Therefore SATA
must be able to detect deep space (DS) targets over its entire field of regard (FOR)
throughout any dark period. This concept, and the brightness of the DS targets it must
catalog will be the primary drivers for the requirements definition.
We propose a set of functional requirements for the sensor based on our
operational concept. These functional requirements in turn flow down and specify a set
of derived requirements that more completely specify the sensor requirements. These
functional requirements are listed and defined in Table 1. Representative derived
requirements are listed in Table 2. In the following subsections, we identify the issues
impacting the system design and requirements definition; define each requirement and
present supporting analysis for the definition.
Table 1. Functional Requirements for the SATA Sensor
Requirement
CONOPS
Surveillance Rate (deg2/hour)
Sensitivity (sensor magnitude)
Site Requirements
Number of Sites
Seeing (arcsec)
Sky Brightness (mag/arcsec2)
Power
Communications
Weather Protection
Security
Operations and Maintenance
Deployment options
Sustainability
Maintenance
Reconstitution
Definition
Concept of Operations
Surveillance capability under specific operational assumptions
Limiting apparent magnitude of target detectable by sensor under specific operational
assumptions
Based on desired deep space coverage capability and weather dispersion
Limit of sensor spatial resolution due to atmosphere
Brightness of night sky background at site
Sensor electrical power requirements
Ability to communicate observations, health and status to remote command facility
Ability to sense changes in local weather that may impact operation
Ability to monitor security of sensor and site
Concept for deployment of sensors in field
Ability of sensor hardware/software to be sustained/maintained for desired operational lifetime
Preventative and post-malfunction repairs
Ability to reconstruct sensor after physical destruction
Table 2. Representative Derived Requirements for the SATA Sensor
Requirement
Optics
Aperture (m)
Design
Mount
Camera
CCD size
Pixel pitch (mm)
Metric Accuracy (arcsec)
Readout time (seconds)
Dark noise (electrons/pixel/sec)
Readout noise (electrons)
Definition
Physical size of the telescope aperture
General class of telescope optical design
Type of telescope mount (polar, equatorial)
Diagonal dimension of focal plane
Physical size of CCD pixels
RMS accuracy of metric observations
Time needed to read single frame from CCD
Signal from unilluminated pixel
Readout noise from CCD and camera electronics
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
3.1 Surveillance Rate Requirement
The surveillance rate (search rate) of an electro-optic sensor and its limiting
sensitivity are interdependent quantities. The surveillance rate of an electro-optic sensor
can be defined simply as:
FOV
[deg2/hr]
(1)
S
t field
where FOV is the field of view of the sensor in deg2, and tfield is the amount of time spent
taking data in a single field. The amount of time spent taking data in a single field
depends on the number and length of the exposures, as well as other factors such as the
length of time to actuate a shutter, and the time needed to slew to the next field of
interest.
The mission of the SATA sensor is catalog maintenance via search operations;
therefore we assume the sensor is intended to find as many DS objects as possible. Using
the sensor to search regions of the sky with high object density facilitates the mission.
Figure 1 illustrates and defines those high-density regions. The figure shows that two
regions, the geosynchronous belt (GEO belt) and the Molniya apogee region (Molniya
ring) present a large number of potential targets for a ground-based sensor. We use
surveillance of those regions as a metric for developing system surveillance rate
requirements. The orbits of other DS objects must cross the equatorial plane at some
point; therefore if surveillance region covers enough area, the other deep space objects
can be detected during GEO belt surveillance operations.
Geosynchronous Belt
180 -330° E coverage
E
90
330
180
120
60
80
190
200
60
210
150
30
220
40
230
20
240
S
250
180
80
60
0
40
260
20
0
0
20
40
60
N
80
20
270
280
40
290
210
330
300
60
310
320
240
80
300
330
270
Molniya Apogee Ring
~60° declination
W
Deep Space Search Region
Low-inclination GEO Belt region
Full GEO Belt region
Molniya apogee region (Molniya ring)
Definition (mid-latitude, northern hemisphere site)
±5º centered on GEO belt covering ~150º arc; ~1500 deg2
±15º centered on GEO belt covering ~150º arc; ~ 4500 deg2
Constant declination region of 58º - 63º covering ~270º arc; ~1350 deg2
Figure 1. Location of deep space population as seen from a mid-latitude, northernhemisphere site. The GEO belt is represented by the solid line, and is labeled with east
longitude. The circles represent the instantaneous positions of the visible GEO belt
objects. The black squares represent the positions of Molniya objects at 30-minute
intervals over the course of a 24-hour period.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
A single sweep of the full GEO belt and the Molniya ring once per evening
requires a surveillance rate of 760 deg2/hour (for the stressing case of a 7.7 hour dark
period). When weather is considered, the actual available time to perform the sweep can
decrease, requiring an increase in sensor search rate to accomplish the mission in a single
night. In addition, previous work suggests that multiple, rapid sweeps of a search area
reduces object leakage through the search area [1]. Minimizing object leakage is a useful
goal for the SATA sensor as it reduces the number of routine objects that would be tasked
to the GEODSS sensors, and facilitates the SATA catalog maintenance mission
requirement. If we assume two repetitions of the search area mentioned previously, and
that the weather is clear only 50% of the time, the surveillance rate requirement increases
to > 3120 deg2/hour. Note that data collection during partly cloudy weather could reduce
this rate requirement somewhat by allowing a longer period of data collection. Also note
that a sensor placed in the southern hemisphere would not be able to observe the Molniya
ring and could perform it’s mission with a lower surveillance rate. However, requiring
the sensor to perform initial orbit determination (IOD) on previously uncatalogued
objects potentially increases the surveillance rate requirement due to the need for at least
3 observations of the object [2]. We have proposed the following requirement: Each
SATA site should be capable of sidereal-track surveillance rates approaching or
exceeding 3500 deg2/hour. The objective rate should meet or exceed that needed to
support IOD on previously uncatalogued objects (>4700 deg2/hr).
3.2 Sensitivity Requirement
The sensitivity requirement defines the faintest target that the sensor should be
capable of detecting. Determining this parameter requires understanding the brightness
of deep space targets as well as the impact of noise sources such as the sky brightness and
camera/CCD noise on the overall signal to noise ratio. The SATA sensor is envisioned to
operate autonomously searching large portions of the sky in sidereal-stare mode meaning
that objects will not be scheduled for observations at times when phase angles are low,
and conditions favor them being bright and easily detected. Instead, the sensor will have
to detect targets at unfavorable phase angles, in unfavorable sky conditions (e.g., when
the moon is up), and in unfavorable sky locations (e.g., close to the horizon) during
routine operations.
Given these constraints, we note that previously published observations suggest
that large geosynchronous satellites easily reach exoatmospheric absolute magnitudes as
faint as ~15 at large phase angles (~80º - 90º) and distances of 36,000 km [3]. Slant
ranges can be > 36,000 km, meaning that the observed apparent magnitude can be fainter.
Atmospheric extinction further reduces the apparent magnitude by approximately 0.2 –
0.6 magnitudes depending on the zenith angle of the observation. We have concluded
that the SATA sensor should be required to observe targets down to an apparent
magnitude of 16 at all elevations greater than 20º.
The magnitude of the sky background varies considerably from site to site, and also
varies spatially and temporally due to airglow and the presence of the moon. It also
varies with the phase of the moon and the relative position of the moon and the field of
observation of the telescope. We have constructed a semi-empirical model of sky
brightness from modeling results of [4, 5] and adapted them for wide CCD sensor
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
bandwidths. Our model suggests that the sky background along the GEO belt and
Molniya ring regions is fainter than 18 mag/arcsec2 more than 85% of the time over the
course of a year. The model results are shown as cumulative distributions in Figure 2 for
two airglow cases. Therefore, we suggest the following sensitivity requirement for the
SATA sensor: it should be able to detect a target with an apparent magnitude of 16 in a
sky background of 18 mag/arcsec2. The sensor should perform at this specification at
all elevations down to 20. This requirement provides a robust system capable of
operating in realistic sky background conditions and at most elevations.
Molniya Ring
Cumulative Distribution (%)
Cumulative Distribution (%)
GEO Belt
100
15%
10
1
2002 GEO BK
2002 GEO T
0.1
14
15
16
17
18
19
20
21
2
Background Magnitude (m/arcsec )
22
100
3-4%
10
1
2002 MOLY BK
2002 MOLY T
0.1
16
17
18
19
20
21
22
2
Background Magnitude (m/arcsec )
Figure 2. Cumulative distribution of sky brightness for the geosynchronous belt (left)
and the Molniya ring (right) as observed from a mid-latitude, northern hemisphere site.
The circles denote the severe airglow case (BK) [6], and the triangles denote the typical
airglow case (T) [7].
3.3 Telescope Enclosure, Deployment Options, and Transportation Requirements
This section briefly discusses the functional requirements on the SATA telescope
enclosure, as well as deployment options and transportation requirements. Some of these
requirements impact development of site requirements. Our overarching goal in the
development of enclosure and siting requirements was to minimize the amount of site
preparation needed for the SATA sensors, thereby reducing the time and cost needed to
deploy the sensor. Benefits and drawbacks of several telescope enclosure options are
summarized in Figure 3. Our recommendation for the SATA sensor is an integrated
transporter/shelter (ITS). This option requires relatively little site preparation relative to
the dome or rolling roof enclosures, and it supports a highly streamlined depot-centric
deployment, maintenance, and sustainment concept.
It also facilitates rapid
redeployment of the sensor to mitigate seasonal weather outages. The integrated
transporter/shelter would be a custom design for the SATA sensor, however, several
vendors have already designed and built trailer-mounted telescopes, and the cost is not
anticipated to be significantly different from the dome or rolling roof options [8]. This
concept increases tactical utility of the SATA sensors and has a lower life cycle cost than
the traditional deployment options of dome or rolling roof enclosures.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
Figure 3. Telescope enclosure option summary.
The depot-centric deploy and retrive deployment concept minimizes requirements
on the SATA site. Simple requirements are for a modest concrete pad, local power and
communications, a security perimeter, and relatively good astroclimactic conditions
(seeing, night sky brightness). The autonomous nature of the sensor also requires
integration of weather sensors and remote interrogation of sensor health and status. In
addition, we recommend that a cloud monitoring scheme or sensor be used at the site to
increase the overall utility of the sensor by facilitating operation on partly cloudy nights.
3.4 Derived System Requirements
The surveillance and sensitivity functional requirements detailed in the previous
sections flow down to derived requirements for the telescope optics, the CCD camera,
and the telescope mount. The high surveillance rate requirement (Section 3.1)
immediately suggests that a wide-FOV telescope is needed (along with a rapid readout
CCD camera). The wide FOV in turn suggests that the telescope should have a low focal
ratio or f-number (f/#). In order to make full use of the wide FOV at the desired
sensitivity, the optics must also have good off-axis image quality, that is, the system must
be relatively free of aberrations over the entire FOV and the optics should have minimal
vignetting.
With regard to the required aperture size, we present Figure 4, which
demonstrates the tradeoffs between surveillance rate and limiting sensitivity as a function
of telescope aperture. Figure 4 assumes use of the CCD camera properties listed in
Table 4. Other assumptions include: 1.) a constant focal ratio of f/1.5, 2.) a sky
brightness of 18 mag/arcsec2, 3.) a target angular rate of 15 arcsec/sec for the sidereal
track case, typical of geosynchronous targets, 4.) atmospheric extinction of 0.6
magnitudes (equivalent to a sensor elevation of about 20), 5.) limiting magnitude is
calculated for an SNR of 6, and 6.) atmospheric seeing of 2 arcsec. Therefore Figure 4
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
100000
18
10000
17
1000
16
Surv. Rate
Target track
Sidereal
CCD:
Pixel size:
QE:
QE variation (% RMS)
Readout noise (electrons):
Dark noise (e/pixel):
Dark variation (% RMS)
t(read) (sec):
t(int) (sec):
t(step) (sec):
Number frames/field
f/#:
Background (mag/arcsec2):
Atmospheric extinction (mag):
100
10
1024x1024
24 um
0.66
0.35%
18
10
2.0
0.125
0.5
2.0
3.0
1.5
18
0.6 (20 deg elevation)
15
Limiting Magnitude
Surveillance Rate (deg2/hr)
suggests the telescope aperture necessary to satisfy the requirements developed in Section
3.1. A sensor capable of satisfying the requirements would have an aperture in the 5060-cm range. A smaller aperture has a high surveillance rate but insufficient sensitivity,
whereas a larger aperture has sufficient sensitivity but insufficient surveillance rate.
14
1
13
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Aperture (m)
Figure 4. Comparison of surveillance rate and limiting sensitivity as a function of
telescope aperture for an f/1.5 telescope with a specific CCD camera. For the sidereal
track case, we have assumed a target angular velocity of 15 arcsec/sec. The remaining
assumptions are detailed in the box.
The surveillance rate and sensitivity requirements developed for the SATA sensor
suggest that high quantum efficiency, low-noise, large format, and fast readout are the
desirable requirements for the CCD camera. The pixel size is a secondary requirement,
but must also be considered, as it impacts sensitivity and overall metric accuracy. In
general silicon CCD pixel sizes fall between 10 –30 m, so a pixel size of ~15 m with
an on chip binning capability would provide the most flexibility. A comprehensive
survey of commercial CCD cameras was conducted for this study. Most COTS CCD
cameras were designed for astronomical applications and have unacceptably slow readout
times. Some existing CCDs can be read out faster, but with a noise penalty. We did not
consider vendors that produce CCDs with readout times above 10 seconds. We also
considered using state of the art custom CCD cameras such as the 80-mm format CCD
camera used for Deep STARE; however use of that CCD would require expensive
custom optics to accommodate the larger focal plane. Since some existing COTS 42-mm
format cameras can provide the required sensitivity (cf. Section 4), we see no compelling
reason to recommend utilization of large-format, custom, state of the art CCD cameras.
We also note that none of the COTS cameras we surveyed used frame transfer CCDs;
therefore a very robust shutter mechanism would also be needed.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
Many commercial vendors offer suitable telescope mounts for ~50-cm class
telescopes for customers ranging from amateur astronomers to military. The mounts vary
in quality to meet user requirements. Space surveillance operations result in more
stressing requirements on the mount than is typically encountered for astronomical or
research applications. In particular, the mounts are frequently required to slew at high
rates and operate continuously over the course of a night. The mounts are also required
to step from one location to the next and damp their motion rapidly. We recommend that
the SATA mount be required to step and settle rapidly, be robust enough for
continuous operation over extended periods (up to ~15 hours at a mid-latitude site), be
capable of rapidly slewing from one surveillance area to another, and also be capable
of tracking DS targets at typical DS target rates. A quality university or research grade
telescope mount will be suitable for SATA with minor enhancements to make the mount
more robust. The mount could be fitted with oversized motors and gears, or an
“oversized” mount could be used. Note that investment in a high-quality mount for the
SATA sensors can significantly enhance the sensor surveillance rate and is strongly
recommended.
4.0 Notional SATA Sensor Design
In this section we present a notional design for the SATA sensor. The sensor
design discussed in this section covers the optics and CCD camera in detail, but does not
represent a complete design. The principles we adopted in this design were: 1.) It would
be based on COTS components wherever possible, 2.) It will utilize open systems
computer architecture (i.e., COTS equipment), 3.) It will use the “deploy and retrieve”
concept for basing and sustainment described previously, and 4.) The system is focused
on providing adequate augmenting capability for the GEODSS network, while
minimizing life cycle cost.
The design consists of 60-cm class Ritchey telescope with a COTS mount. We
present one compact optical design that achieves excellent image quality and can support
CCD cameras with pixel sizes down to ~12 m. The design uses custom, but relatively
inexpensive and readily manufactured optical components to accommodate the wide
FOV necessary for surveillance. However, any 60 cm class telescope is “custom” built
by a telescope manufacturer; they do not exist as COTS units. A COTS 40-mm format
CCD camera is recommended as the imager. Use of a COTS product ensures
availability, and the 40-mm focal plane format does not strongly drive the cost of the
optics. The system must also have control computers and image processing software.
We assume the system uses an integrated trailer and shelter enclosure, as described in
Section 3, although we have not designed such an enclosure for the notional sensor; this
design would be left up to future bidders. In other respects we assume that the sensor
will fulfill the requirements outlined in Section 3.
The design is a 60-cm aperture f/1.25 Ritchey design. Figure 5 presents a ray
trace of this system. There are two hyperbolic mirrors and two aspherical corrector
lenses. The secondary mirror represents a modest central obstruction (25% of the aperture
area). The design is corrected for chromatic aberrations over the wavelength range of
400 – 900 nm. Figure 6 presents the accompanying spot diagram for this design. The
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
design has excellent image quality out to the edge of the focal plane, and can support the
use of a CCD camera with ~12 m pixels.
A commercial CCD camera with a 40-mm format focal plane is recommended. A
specific vendor is not recommended, but we have used properties of a commercial
camera that has rapid readout and is back-illuminated (cf. Table 4).
We assume the computer hardware and software required by the SATA sensor
will be similar to that in use at the Moron Optical Surveillance System. The relevant
MOSS computer hardware would involve several LINUX-based PCs including an
executive control computer to manage the overall sequencing of telescope operations,
communications, and interrogation of the weather sensor control computer. Other
computers would handle telescope mount control, scheduling of observations, image
processing and object correlation, and data storage.
The software system at MOSS is roughly 80-100K lines of source code (SLOC)
developed specifically for the space surveillance mission. We strongly suggest that
software for the SATA sensors be developed for its surveillance mission and in parallel
with the SATA hardware development and integration, reusing code from existing
systems when possible. We recommend against the strategy of utilizing COTS software
components integrated with custom software patches. We believe this COTS-only
strategy will complicate maintenance and sustainment of the software, as the COTS
vendors will be under no obligation to sustain software or customize software needed for
the system.
Figure 5: Optical layout of optical design for the SATA sensor.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
Figure 6. Spot diagrams for the SATA sensor.
4.1 Notional Sensor Performance
We examine the performance of the SATA sensor design, and estimate its
capability for meeting functional and mission requirements defined in preceding sections.
In order to estimate sensor performance, we have had to make assumptions regarding the
type of CCD camera in use, the performance of the telescope mount, and the performance
of the image processing software. Those properties are listed in Table 4.
Table 4: Assumptions Made in Modeling of Notional Sensor Performance
System
CCD Camera
Mount
Processing
Signal to Noise ratio of Target
Atmospheric Seeing
Notes
1024 x 1024 pixels, 24 m pixel pitch
Readout time: 0.125 sec
Readout Noise: 18 electrons
Dark Current: 10 electrons/pixel/sec RMS
Quantum efficiency: 0.66
Quantum efficiency variation: 0.35%
Dark noise variation: 2%
Step and Settle: 2 sec [a]
Number of frames/field: 3 [b]
6.0
2 arcsec [c]
[a] Achievable by research and military-grade mounts.
[b] Aggressive assumption.
[c] Recommend seeing at SATA sites of this quality or better.
The estimated limiting magnitude (i.e., sensitivity) at SNR=6 is presented as a
function of integration time in Figure 7. The figure presents limiting magnitude vs.
integration time for targets with various apparent motions across the focal plane. The
target rate of 15 arcsec/sec would be representative of a geosynchronous object; Molniya
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
objects would move at slower rates near apogee (<5 arcsec/sec). The figure assumes the
sensor is pointed at 20º elevation and an atmospheric extinction of 0.6 magnitudes has
been applied. We have also assumed a sky background of 18 mag/arcsec2, which is fairly
bright, but consistent with the sensitivity requirements. This sensor achieves a limiting
magnitude of 15.85 for a GEO object at an integration time of ~0.4 seconds. At zenith,
the sensor achieves a limiting magnitude of 16.25. This sensor meets the 16th magnitude
requirement for SATA at zenith and is slightly short of the requirement at 20 elevation;
there is no margin in the sensor under those stressing conditions. The sensor
performance is improved under darker sky conditions.
18.00
17.00
<QE>:
QE Var. (% rms):
Dark current (e/pixel/s):
Dark Curr. Var. (% rms):
Noise (e/pixel rms)
Straddle factor:
Atmospheric extinction
Elevation angle (deg)
16.00
LIMITING MAGNITUDE FOR SNR=6
15.00
14.00
0.66
0.35
10
2.0
18
0.8
0.6
20
13.00
12.00
0 arc s/s
5 arc s/s
11.00
15 arc s/s
10.00
9.00
8.00
7.00
6.00
0.01
Telescope:
CCD:
# pixels:
Data rate (MHz):
FOV (deg):
Eff. aperture (m^2):
Pixel width (arc s):
Eff. Sky Bkg. (V/Sq. arc s):
30 arc s/s
0.6-m f/1.25
SITe S1003
1024 x 1024
2.2
1.9 x 1.9
0.164
6.6
18
0.1
60 arc s/s
120 arc s/s
250 arc s/s
500 arc s/s
1k arc s/s
1
10
100
INTEGRATION TIME (S)
Figure 7. Limiting magnitude of 0.6-m f/1.25 SATA sensor with camera properties as
listed in Table 4 for various integration times and target angular rates. A sensor elevation
of 20 was assumed.
A statistical examination of sensor performance is presented in Figure 8. The
figure presents the fraction cases that the sensor limiting magnitude was greater than a
certain value. The estimates were made using the sky background model output along
the GEO belt and the Molniya ring as described previously (cf. Figure 2). We used those
sky background estimates to facilitate estimation of the sensor limiting magnitude at
multiple points along the arcs of interest, including the effects of atmospheric extinction
and assuming a target SNR=6 for detection. Therefore Figure 8 represents limiting
magnitude statistics along the GEO belt and Molniya ring over the course of a year. The
realized limiting magnitude is dependent upon how the sensor is used; to derive the
statistics, we assumed an integration time of 0.3 seconds and an apparent angular rate of
15 arcsec/sec for the GEO belt and 5 arcsec/sec for the Molniya ring. This presents a
single performance point in a large parameter space. We also assume clear weather for
the year. Each plot presents results for the two known airglow cases (typical and severe).
Approximately 91% of the GEO belt cases examined have limiting magnitudes > 16, and
98.5 – 99% of the Molniya ring cases have limiting magnitudes > 16. These results
demonstrate that in general, this sensor meets the sensitivity requirements in > 90% of the
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
cases examined, and may fall slightly short (0.1-0.15 magnitudes) of meeting the
requirement in the extreme case of low elevation and a very bright sky background.
Molniya Ring
1.0
1.0
0.9
0.9
Fraction with m > mlim
Fraction with m > mlim
GEO Belt
0.8
0.7
0.6
0.5
0.4
0.3
0.2
BK Airglow
T Airglow
0.1
0.0
14.0
14.5
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
15.0
15.5
16.0
16.5
17.0
17.5
18.0
0.0
14.0
Limiting Apparent Magnitude (SNR=6)
BK Airglow
T Airglow
14.5
15.0
15.5
16.0
16.5
17.0
17.5
18.0
Limiting Apparent Magnitude (SNR=6)
Figure 8. Sensitivity statistics for the 0.6-m f/1.25 SATA sensor. (Left) Sensitivity along
the GEO belt. (Right) Sensitivity along the Molniya ring. All calculations are for a midlatitude, northern hemisphere observing site.
Search Rate (deg2/hr)
10000
1000
Step and Settle Time (sec)
1
100
2
4
6
8
10
10
0.1
1
10
100
Integration Time (sec)
Figure 9. Estimated surveillance capability of the notional SATA sensor as a function of
integration time and step and settle time. The horizontal line indicates the surveillance
requirement from Section 3.1.
This sensor concept has a FOV of 1.9 x 1.9. Figure 9 presents estimated
surveillance rates for a single sensor using the assumptions listed in Table 4 (with the
exception that we have plotted the search rate for multiple sensor step and settle times).
The horizontal line indicates the surveillance requirement outlined in Section 3.1 (3500
deg2/hr). Within the likely operational envelope of tint = 0.3 – 1.0 seconds, this system
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
meets the requirement at tint < 0.5 sec and falls below it for larger values of tint. Even so,
at tint = 1.0 seconds, the surveillance rate is still at 2415 deg2/hr.
5.0 Conclusions
MIT/LL has developed broad system-level functional requirements for a
sustainable, small-aperture, low-cost, and autonomous telescope system envisioned to
augment the current deep space electro-optical (EO) network. These requirements have
been developed within the context of the current EO network (i.e., GEODSS with the
Deep STARE upgrade) and the expectation of future Space-Based Space Surveillance
deployment. We have assumed that the SATA system will be a surveillance-based
system rather than a task/track system, but that the system must also be capable of limited
task/track capability. We have assumed that the SATA sensors will augment the
GEODSS system by adding capacity and assuming the catalog maintenance task from the
GEODSS system. We have focused on developing a sensor with significant surveillance
capability to minimize the number of objects that leak through the EO surveillance
fences.
The functional requirements have been listed in Section 3 and throughout this
report. These requirements flow down to set derived requirements for the sensor. We
envision the SATA sensor to consist of a ~60-cm aperture, wide field of view (FOV)
telescope (F/1.5 or below), a commercial 42-mm format CCD camera with high quantum
efficiency and rapid readout, and a robust research grade equatorial fork mount capable
of the continuous operations required by the space surveillance mission. We recommend
that this sensor system be housed in an integrated transporter and shelter to facilitate a
streamlined maintenance concept and also rapid redeployment, if desired. The system is
envisioned to be autonomous and thus requires an autonomous control computer system
tied to robust weather sensors, and local power and communications.
A notional sensor design was developed as a performance benchmark. The
notional sensor is a 0.6-m aperture f/1.25 system with a COTS 42-mm format CCD
camera. Tools were developed to estimate the performance of this sensor under realistic
sky background conditions, including the effects of scattered moonlight and airglow. The
analysis suggests the notional design is capable of meeting it’s sensitivity requirement in
more than 90% of the sky background conditions that exist over the course of a year.
Four sensors, distributed geographically, would provide sufficient coverage of deep space
to perform routine catalog maintenance and provide augmentation of the GEODSS
sensors.
6.0 References
1. Lambour, R. L., and E. C. Pearce, Development and Simulation of Search
Strategies for 36-cm Schmidt Class Telescopes at the ETS, 2001 Core
Technologies for Space Systems Conference Proceedings, 28-30 Nov. 2001.
2. Bate, R. R., D. D. Mueller, and J. E. White, Fundamentals of Astrodynamics,
Dover Publications, Inc., New York, NY, 1971.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.
3. Lambour, R. L., R. Bergemann, C. von Braun, E. M. Gaposchkin, Space-Based
Visible Space Object photometry: Initial Results, Journal of guidance, Control,
and Dynamics, Vol. 23, No. 1, pp. 159, 2000.
4. Garstang, R. H., Night-Sky Brightness at Observatories and Sites, Publications of
the Astronomical Society of the Pacific, Vol. 101, pp. 306, 1989.
5. Krisciunas, K., and B. Schaefer, A Model for the Brightness of Moonlight,
Publications of the Astronomical Society of the Pacific, Vol. 103, pp. 1033-1039,
1991.
6. Broadfoot, A. L., and K. R. Kendall, The Airglow Spectrum, 3100 – 10000
Angstroms, Journal of Geophysical Research, Vol. 73, pp. 426-428, 1968.
7. Turnrose, B. E., Absolute Spectral Energy Distribution of the Night Sky at
Palomar and Mount Wilson Observatories, Publications of the Astronomical
Society of the Pacific, Vol. 85, pp. 545, 1974.
8. Melshiemer, F., private communication, 2001.
This work was sponsored by the Department of Defense under Air Force Contract No. FA8721-05-C-0002. Opinions,
interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United
States Government.