The Space Development Agency (SDA) is developing the Proliferated Warfare Space Architecture (PWSA) – a constellation of hundreds of satellites in low earth orbit delivering space-based capabilities to the joint warfighter. The PWSA is a mesh network of optically connected satellites providing low-latency data transport and missile warning/tracking capabilities. SDA capitalizes on a unique business model that values speed and lowers costs by harnessing commercial development. The Optical Communications Terminal (OCT) standard was created to provide optical interoperability specifications, enable a strong marketplace, and to drive advancements in optical communication capabilities to terrestrial, maritime, and airborne warfighting elements. As part of the spiral development process, the OCT standard evolves with PWSA deployment phases. SDA has incorporated feedback as well as advancements to the OCT standard, resulting in the release of version 3.1.0. In this paper we discuss key aspects of the OCT standard, such as wavelength, modulation, data rates, polarization, link distance, error correction coding, pointing, acquisition and tracking, and position, navigation, and timing.
Free Space Optical links suffer from atmospheric effects where turbulence causes the beam to break up (scintillation), which could increase the variance in the signal at the receiver and ultimately worsening the optical link. Various techniques to reduce scintillation exist to alter how the atmosphere will affect the beam, one of them being wavelength diversity of the optical source. Diversifying the wavelength can reduce the scintillation of the optical beam due to the wavelength dependence on the refractive index of the atmosphere. An experiment was conducted comparing a broadband laser source and a monochromatic source over an instrumented 13.5km path. Beam profile and scintillation measurements were conducted along with BLS-2000 Cn2 measurements. This experiment investigates the effects of a short-coherence length / broad-band nature of a source and its ability to reduce scintillation in turbulent atmosphere. This paper will discuss the experimental setup, analysis, and conclusions of this novel experiment.
We explore the impacts of adverse weather on the propagation of a pulsed 1.5um laser source over a 1km maritime channel. The propagation path along this channel is well-instrumented with sensors to measure standard weather conditions (wind, temperature, humidity and rainfall), visibility and atmospheric turbulence. Data collected to characterize the propagation path are used to initialize channel modeling and predict performance of 1.5um propagation. A high-speed detector and a camera located at the target board recorded temporal and spatial effects of rain on the propagated laser beam. The data are analyzed for pulse width, beam profile, beam wander as a function of rainfall and compared to the channel model.
Propagation of laser beams through a turbulent atmosphere over extended ranges can cause significant beam scintillation and wander which can degrade the effectiveness of a Free Space Optical (FSO) link. The use of a spectrally broadband laser light source, with a high spatial coherence and short temporal coherence, could lead to improved performance in one or both of these areas. This experiment investigates the effect of temporal coherence on the far-field turbulence induced effects on the beam. Narrow linewidth coherent sources were compared against a broadband source over a 13.5 km slant-path. The path was instrumented with a path averaged turbulence monitoring device during data collection along with a range of other meteorological parameters to predict atmospheric parameters. Target board beam profile data was collected to measure the spatial statistics due to atmospheric turbulence along with silicon detectors to measure the temporal statistics of the atmospheric turbulence effects. This data is analyzed and compared to full diffraction wave propagation simulation results. Our analysis shows the benefit that the broadband source does not suffer as many scintillation effects as the narrow-linewidth sources.
Free space optical communication utilizing modulating retro-reflectors (MRR) can greatly reduce the complexity of a system in both pointing requirements as well as the necessity for a laser transmitter at both ends of the link. Retroreflectors are susceptible to the same atmospheric turbulence effects of scintillation and beam wander of any laser communication system. An MRR link using an array (N>1) of retroreflectors is affected by self-interference of the return beams. This self-interference can create additional fluctuation that compound to increase the apparent scintillation of the received signal. Data were collected over a 1km outdoor path on the interference pattern returned from a pair of 7mm and 12.5mm retro-reflectors, with multiple spacing distances, in varying turbulence regimes with a 1550nm and 1070nm laser. The interference data of the retroreflectors were correlated with Cn2 data collected simultaneously over the same 1km horizontal path. Under weak turbulence, the self-interference fringes matched diffraction theory, under stronger turbulence regimes the self-interference fringes were either visibly reduced or completely destroyed. We also analyze the contrast of the interference fringes as a function of wavelength for varying turbulence regimes as well as the ability to measure Fried’s parameter from the retroreflector spacing and the returned self-interference pattern.
The growth of optical communication has created a need to correctly characterize the atmospheric channel. Atmospheric
turbulence along a given channel can drastically affect optical communication signal quality. One means of
characterizing atmospheric turbulence is through measurement of the refractive index structure parameter, Cn2. When
calculating Cn2 from the scintillation index, σΙ2,the point aperture scintillation index is required. Direct measurement of
the point aperture scintillation index is difficult at long ranges due to the light collecting abilities of small apertures.
When aperture size is increased past the atmospheric correlation width, aperture averaging decreases the scintillation
index below that of the point aperture scintillation index. While the aperture averaging factor can be calculated from
theory, it does not often agree with experimental results. Direct measurement of the aperture averaging factor via the
pupil plane irradiance covariance function allows conversion from the aperture averaged scintillation index to the point
aperture scintillation index. Using a finite aperture, camera, and detector, the aperture averaged scintillation index and
aperture averaging factor are measured in parallel and the point aperture scintillation index is calculated. A new
instrument built by SSC Pacific was used to collect scintillation data at the Townes Institute Science and Technology
Experimentation Facility (TISTEF). This new instrument’s data was then compared to BLS900 data. The results show
that direct measurement of the aperture averaging factor is achievable using a camera and matches well with groundtruth
instrumentation.
The usage of long-range optical systems for tracking applications encounters regions of deep turbulence throughout propagation. Such conditions lead to the inability to remain on target for a tracked object due to scintillation. To mitigate this issue, a double pass optical system is utilized as a means of tracking enhanced backscatter (EBS) and thus keeping alignment while characterizing turbulent conditions. EBS is detected through image processing algorithms that capture the returning constructive interference from the target. This paper evaluates EBS optical systems using a retro-reflector at a 1 kilometer distance in order to validate theoretical models that typify atmospheric turbulence regarding low-ground propagation. Meteorological conditions are also included in the empirical data obtained for the analysis of atmospheric conditions that contribute to non-homogenous turbulent conditions along the path.
Laser beam speckle resulting from atmospheric turbulence contains information about the propagation channel. The number and size of the speckle cells can be used to infer the spatial coherence and thus the Cn2 along a path. The challenge with this technique is the rapidly evolving speckle pattern and non-uniformity of the speckle cells. In this paper we investigate modern blob counting techniques used in biology, microscopy, and medical imaging. These methods are then applied to turbulent speckle images to estimate the number and size of the speckle cells. Speckle theory is reviewed for different beam types and different regimes of turbulence. Algorithms are generated to calculate path Cn2 from speckle information and path geometry. The algorithms are tested on speckle images from experimental data collected over a turbulent 1km path and compared to Cn2 measurements collected in parallel.
The Navy is actively developing diverse optical application areas, including high-energy laser weapons and free- space optical communications, which depend on an accurate and timely knowledge of the state of the atmospheric channel. The Optical Channel Characterization in Maritime Atmospheres (OCCIMA) project is a comprehensive program to coalesce and extend the current capability to characterize the maritime atmosphere for all optical and infrared wavelengths. The program goal is the development of a unified and validated analysis toolbox. The foundational design for this program coordinates the development of sensors, measurement protocols, analytical models, and basic physics necessary to fulfill this goal.
The authors have recently developed an optical transmissometer device used for estimation of the visibility and atmospheric extinction coefficient along a horizontal or slant terrestrial path of ranges from 500m out to 6 km. This is a bistatic device using a modulated LED beacon transmitter and an 8” (200mm) primary receiver lens with a silicon (Si) photodetector. We discuss how this device can be used to simultaneously obtain an estimate of the atmospheric turbulence characteristics along the same propagation path, using the optical intensity scintillation effect, without requiring any hardware modifications to the existing device. Device principles of operation are presented, followed by the results of a preliminary proof-of-concept field test which yielded encouraging results showing validity of the basic system design but indicating that additional engineering work is required to resolve some implementation details, and further field testing needed to verify and validate the system.
Free-space laser communications are subjected to performance degradation when heavy fog or smoke obscures the line of sight (high-loss optical media). On the other hand, it has been demonstrated that laser-induced plasma filaments (LIPF) can propagate for long distances (up to a few kilometers) through clouds and/or turbulent (lossy) atmosphere. Here we propose to use LIPF to improve and/or restore laser communication in adverse, high-loss and/or denied conditions. This work is focused on demonstrating the proof of concept and is dedicated primarily to gaseous, optically transparent media.
The characterization of atmospheric effects on a propagated laser beam is important to applications ranging from free-space optical communications to high-energy laser systems for ship defense. These applications are frequently developed for a dynamic propagation environment in which either one or both ends of the optical link are moving. The instruments are often constrained by size, weight, and power limitations due to the platforms on which they will be installed. The dynamic nature of the optical link induces several difficulties in link-path instrumentation: turbulence statistics on a continuously changing path are hard to interpret, and the optical instruments must be designed to maintain a high-quality link between beacon and receiver. We will review some of the scintillometer designs and we examine the associated data produced by these different instruments.
The growth of optical communication has created a need to correctly characterize the atmospheric channel. The measurement of turbulence, due to its ability to drastically effect signal quality, is an important part of this characterization and can be partially accomplished via calculation of the scintillation index. However, proper calculation of the scintillation index requires that the background (specifically the diffuse solar background) be accurately subtracted from the transmitted signal. While there are many methods to remove this background we introduce a hardware based method which seeks to overcome the weaknesses of traditional approaches while adding its own strengths. The corrected signal is allowed a greater dynamic range and atmospheric background variations are accounted for during transmission. We begin by discussing the scintillation index and traditional means of background subtraction followed with an introduction of our proposed optical design. We provide details of the experimental setup, data collection over a maritime location in San Diego, and analysis. Finally, we compare scintillation index calculations using our new method and a traditional method of background subtraction. Our results ranked our method favorably alongside common methods of background subtraction.
Current transmissometer designs can be physically bulky, electronically complex, and susceptible to background light; ultimately limiting performance. We describe a novel transmissometer design based upon a modulated LED source and an AC-coupled receiver to improve upon the aforementioned shortcomings. The design aims to reduce both complexity and SWAP through the use of a high frequency modulation technique, while ultimately improving SNR and measurement range over a variety of atmospheric conditions. The instrument is a dynamic atmosphere and range transmissometer (DART). First we discuss the theory associated with our technique; particularly addressing how the effects of atmospheric turbulence are handled. Next, we describe the radiometry and calibration procedures for the transmitter and the receiver. We describe the instrument hardware and how the DART was built and tested in the laboratory. Finally, we discuss the field experiment to test the DART against a commercial unit over a 700m coastal path in San Diego. The processed data are compared with concurrent measurements from the Optec LPV-3 commercial transmissometer. Transmission data from the DART tracks the commercial instrument very well over varying atmospheric conditions.
Obtaining accurate, precise and timely information about the local atmospheric turbulence and extinction conditions and aerosol/particulate content remains a difficult problem with incomplete solutions. It has important applications in areas such as optical and IR free-space communications, imaging systems performance, and the propagation of directed energy. The capability to utilize passive imaging data to extract parameters characterizing atmospheric turbulence and aerosol/particulate conditions would represent a valuable addition to the current piecemeal toolset for atmospheric sensing. Our research investigates an application of fundamental results from optical turbulence theory and aerosol extinction theory combined with recent advances in image-quality-metrics (IQM) and image-quality-assessment (IQA) methods. We have developed an algorithm which extracts important parameters used for characterizing atmospheric turbulence and extinction along the propagation channel, such as the refractive-index structure parameter C2n , the Fried atmospheric coherence width r0 , and the atmospheric extinction coefficient βext , from passive image data. We will analyze the algorithm performance using simulations based on modeling with turbulence modulation transfer functions. An experimental field campaign was organized and data were collected from passive imaging through turbulence of Siemens star resolution targets over several short littoral paths in Point Loma, San Diego, under conditions various turbulence intensities. We present initial results of the algorithm’s effectiveness using this field data and compare against measurements taken concurrently with other standard atmospheric characterization equipment. We also discuss some of the challenges encountered with the algorithm, tasks currently in progress, and approaches planned for improving the performance in the near future.
Using a three-aperture scintillometer system (TASS) to measure irradiance fluctuations along a
slant path, it is possible to create a Cn2 profile model as a function of altitude up to (and possibly
beyond) the maximum altitude of a laser beam along the propagation slant path. This technique was
demonstrated recently in June 2011 on a beacon beam transmitted between Hollister Airport in
California and Fremont Peak at a slant range of 17 km. Although the primary experiment was to
test a hybrid optical RF communication system (FOENEX), the beacon signal at the transmitter
was intercepted by the TASS from which weighted path-average values of Cn2, inner scale l0, and
outer scale L0 were determined. Path-average values were then entered into an algorithm that
determines the parameters of the HAP Cn2 profile model (a variation of the HV profile model). In
this paper we report on these recent measurements and how this method of constructing the HAP
model can be used over other propagation paths.
Irradiance data were collected over an air-to-ground path using several different sized receiving apertures. The
data were collected from the Optical RF Communications Adjunct (ORCA) tracking beacon. The receiver system
consisted of three telescopes of sizes 51 mm, 137 mm, and 272 mm. Probability of fade, number of fades per
second, and mean fade time was computed for various intensity levels for irradiance data collected on all three
telescopes. These measured statistics are compared to fading models derived from lognormal and gamma-gamma
probability density function (PDF) models. Discussion is centered on the viability of these models under various
conditions and on the presence of aero-optic effects. The gamma-gamma and lognormal model are found to be
insufficient to model all fading statistics.
In this paper we show evidence of aero-optic effects on the measured beacon beam as the gimbal angle of a nosemounted
turret changes from 0 to 90 degrees and greater with respect to the line of flight. Data from the beacon beam
was collected with a new technology 3-aperture scintillometer over a 82km to 104km air-to-ground downlink during
field testing of the ORCA system in Nevada in May 2009. In this paper we present data analysis on the impact of an
aero-optic boundary layer on a laser link between an aircraft and a ground-based stationary node. Particularly we look at
the impact of an aero-optic boundary layer on the mean, variance, scintillation, probability density function (PDF),
power spectral density (PSD), and fading of the received irradiance. We find that the most compelling argument for the
presence of strong aero-optic effects comes from calculating the PSD of the received beacon intensity. We also find the
cumulative effect of the aero-optic boundary layer differs depending on the transmitted beam parameters, i.e. collimated
or divergent.
Irradiance data were collected over a 1km horizontal terrestrial path using several different sized receiving apertures.
The data were collected under moderate-to-strong turbulence conditions. The receiver system consisted of a 154mm
(6") refracting telescope outfitted with several removable apertures. The path was instrumented with three 3-axis
anemometers and three scintillometers, two of which were capable of measuring the inner scale of turbulence in addition
to Cn2. Histograms were formed with the data and compared to the Log-Normal and Gamma-Gamma PDF models. As
expected, neither PDF model was applicable under all conditions of aperture averaging. Hypotheses are made as to why
the models were unable to completely capture the effects of aperture averaging on received irradiance data.
Evaluation of the methods developed by Bendersky, Kopeika, and Blaunstein1 to predict the refractive index structure
parameter from the direct measurement of macroscopic atmospheric conditions were investigated. Measurements of
ground-level temperature, relative humidity, wind speed, solar flux, and aerosol loading taken by the University of
Central Florida weather station were compared against concurrent measurements of the refractive index structure
parameter made by Scintec SLS-20 scintillometers positioned near the weather station. Wind measurements were
obtained by three, three-axis sonic anemometers (capable of resolving a three-dimensional wind vector) positioned at
heights of 1, 1.5, and 2.5 meters above the ground. Temperature measurements were taken at ground level, and at heights
of 1 and 1.5 meters. Data were collected for two days atop Antelope Peak, NV. Collection times covered both daytime
and nighttime measurements.
The interest for free space laser beam propagation has recently increased due to several experiments. These experiments
have shown that optical links through the atmosphere can be effectively established, regardless of the several deleterious
negative effects of refractive index fluctuations. The most deleterious effect is the scintillation which, consequently, has
been widely investigated. However, it has not been reported yet what influence a ground profile has on scintillation
values. In this paper we show theoretical results of the ground impact on scintillation, for a laser link between aircraft
and mountain top; Antelope peak Nevada. The link was investigated for a specific mountainous profile of the ground for
path lengths of 50 km and 200 km. The theoretical analysis shows that, if a low value is assumed for the Rytov variance
(weak turbulence condition), then the presence of high peaks or mountains along the propagation path could have a
remarkable impact on scintillation because the scintillation saturation effects do not occur; if the Rytov variance assumes
medium-high values (moderate-to-strong turbulence conditions), then the scintillation index will be close to the
saturation regime. Therefore the scintillation will be slightly affected by the mountains along the path, both for the plane
wave model and the Gaussian beam wave model.
The DARPA Optical RF Communications Adjunct (ORCA) program was created to bring high data rate networking to
the warfighter via airborne platforms. Recent testing of the ORCA system was conducted by the Northrop Grumman
Corporation (NGC) at the Nevada Test and Training Range (NTTR) at the Nellis Air Force Range near Tonopah, NV.
The University of Central Florida (UCF) conducted a parallel test to measure path-averaged values of the refractiveindex
structure parameter, the inner scale of turbulence, and the outer scale of turbulence along the ORCA propagation
path from an airborne platform to the ground at Antelope Peak. In addition, weather instrumentation was set up at
ground level on Antelope Peak to measure local conditions on the mountain top. This paper presents background
information on expected atmospheric conditions for the channel, models that were used by UCF for the measurements,
path-averaged values of the three atmospheric parameters, and a Cn2 profile model as a function of altitude.
Combined RF and optical communication within a heavily scintillated atmosphere requires special modems that can
accommodate significant signal fading. A hybrid network (10 Gbps 1550 nm FSO and RF transmission) has been
developed and a link quality parameter is used to assist the network routers with the path cost calculation algorithm.
Fading statistics, determined by field experiments, are emulated in the laboratory network by a statistically-driven VOA.
COTS hardware (FEC and a special amplifier) enable a 35 dB dynamic range. The special modem and its performance
within a multi-node network are presented.
The most commonly used altitude-dependent model for refractive index fluctuations, over long high-altitude slant paths
or ground/space links, is the Hufnagel-Valley model. For the
near-ground turbulence portion of the path, this model
uses an exponential decay term suggested by Valley to connect ground level turbulence with the original Hufnagel
model which was constructed for turbulence above 3 km. However, it has long been observed that refractive-index
fluctuations in the first few hundred meters near the ground decrease with altitude raised to the -4/3 power rather than
exponentially. Recent and some earlier measurements of
refractive-index fluctuations are presented in this paper along
with a theoretical modification of the Hufnagel model to account for low-altitude turbulence exhibiting this power-law
behavior.
In this paper we present experimental beam wander measurements and compare them with theory developed from the
Kolmogorov model for the atmosphere. Data from a collimated beam over a 1km horizontal terrestrial path were
recorded with a high-speed camera. Several sets of image data were taken before, during, and after the quiescent period
of the atmosphere in June 2006. The propagation path was instrumented with a commercial scintillometer to measure
the Cn
2 and a weather station to monitor temperature and wind velocity. The data were analyzed for centroid movement
and hot spot movement.
B-PPM formatting for trans-atmospheric optical communication is compared experimentally to OOK (NRZ) at a single
channel data rate of 1.25Gbps in deep fading conditions. Unlike low data rate transmission using M-ary PPM
formatting, high-speed B-PPM formatting does not benefit from the theoretical improvement that has been realized at
low data rate. Although B-PPM can indeed benefit from a threshold set to near-zero, the high speed transmission
precludes the implementation of a traditional Maximum Likelihood Detection circuit that compares the integrated power
of each slot. At high speed, one has to rely on signal strength alone within the bit period which degrades the contrast
between a "one" and a "zero." Moreover, the need for twice the bandwidth for B-PPM significantly limits available
components such as APDs. More important, however, is the fact that during deep fades clock resynchronization
dominates at high data rate. The primary question to be addressed is: Does B-PPM formatting really provide sufficient
margin compared to NRZ to merit its use in deep fading atmospheric conditions? By building a special dual transceiver
system, we have been able to propagate both B-PPM and NRZ formatted signals co-linearly on two C-band wavelengths
centered close to 1550nm. Under field testing we measured the BER, including signal resynchronization, using special
InGaAs, high-speed, multimode pigtailed, APD-based detectors in the receiver. The data were collected on fully
instrumented horizontal paths of 1km and 500m with Cn2 [m-2/3] ranging from 10-15 to 10-13.
Intensity fluctuations from a 532nm CW laser source were collected over an outdoor 1km path, 2m above the ground,
with three different receiving apertures. The scintillation index was found for each receiving aperture and recently
developed theory for all regimes of optical turbulence was used to infer three atmospheric parameters, Cn2, l0, and L0.
Parallel to the three-aperture data collection was a commercial scintillometer unit which reported Cn2 and crosswind
speed. There was also a weather station positioned at the receiver side which provided point measurements for
temperature and wind speed. The Cn2 measurement obtained from the commercial scintillometer was used to infer l0, L0,
and the scintillation index. Those values were then compared to the inferred atmospheric parameters from the
experimental data. Finally, the optimal aperture sizes for data collection with the three-aperture receiver were
determined.
We report on a set of measurements made in December 2005 by researchers from the University of Central Florida, SPAWAR's Innovative Science and Technology Experiment Facility (ISTEF), Harris Corporation, NASA Kennedy Space Center, and Northrop Grumman. The experiments were conducted on the Shuttle Landing Facility (SLF) at Kennedy Space Center (KSC) over terrestrial paths of 1, 2, and 5 km. The purpose of the experiments was to determine the atmospheric-induced beam spreading and beam wander at various ranges. Two lasers were used in the experiments. Both were a pulsed 1.06 μm laser; however, one was single-mode and the other was multi-mode. Beam profiles were recorded near the target position. Simultaneous measurements of Cn2, wind speed and direction, humidity, visibility, temperature, and surface temperature profiles were all recorded.
We report on measurements made at the Shuttle Landing Facility (SLF) runway at Kennedy Space Center of receiver aperture averaging effects on a propagating optical Gaussian beam wave over a propagation path of 1,000 m. A commercially available instrument with both transmit and receive apertures was used to transmit a modulated laser beam operating at 1550 nm through a transmit aperture of 2.54 cm. An identical model of the same instrument was used as a receiver with a single aperture that was varied in size up to 20 cm to measure the effect of receiver aperture averaging on Bit Error Rate. Simultaneous measurements were also made with a scintillometer instrument and local weather station instruments to characterize atmospheric conditions along the propagation path during the experiments.
The Shuttle Landing Facility runway at the Kennedy Space Center in Cape Canaveral, Florida is almost 5 km long and 100 m wide. Its homogeneous environment makes it a unique and ideal place for testing and evaluating EO systems. An experiment, with the goal of characterizing atmospheric parameters on the runway, was conducted in June 2005. Weather data was collected and the refractive index structure parameter was measured with a commercial scintillometer. The inner scale of turbulence was inferred from wind speed measurements and surface roughness. Values of the crosswind speed obtained from the scintillometer were compared with wind measurements taken by a weather station.
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