Hybrid Optical RF Airborne Communications

INV ITEDP A P E R

Hybrid Optical RFAirborne CommunicationsThese communication systems provide high throughput for mobile long-range

networks by combining the reliability of radio frequency links with the

high capacity and low cost of optical links.

By Larry B. Stotts, Fellow IEEE, Larry C. Andrews, Senior Member IEEE,

Paul C. Cherry, Member IEEE, James J. Foshee, Member IEEE,

Paul J. Kolodzy, Senior Member IEEE, William K. McIntire, Malcolm Northcott,

Ronald L. Phillips, Senior Member IEEE, H. Alan Pike, Brian Stadler, and

David W. Young, Senior Member IEEE

ABSTRACT | The use of hybrid free-space optical (FSO)/radio-

frequency (RF) links to provide robust, high-throughput com-

munications, fixed infrastructure links, and their associated

networks have been thoroughly investigated for both com-

mercial and military applications. The extension of this

paradigm to mobile, long-range networks has long been a

desire by the military communications community for multi-

gigabit mobile backbone networks. The FSO communications

subsystem has historically been the primary limitation. The

challenge has been addressing the compensation of propaga-

tion effects and dynamic range of the received optical signal.

This paper will address the various technologies required to

compensate for the effects referenced above. We will outline

the effects FSO and RF links experience and how we overcome

these degradations. Results from field experiments conducted,

including those from the Air Force Research Laboratory

Integrated RF/Optical Networked Tactical Targeting Network-

ing Technologies (IRON-T2) program, will be presented.

KEYWORDS | Communication system field trials; free-space

optical communications; gigabit communications; hybrid com-

munication; long-range communications; optical turbulence

compensation; radio-frequency (RF) communications

I . INTRODUCTION

There is a need for high-capacity communications networks

for high-throughput applications [1]. The military typically

transmits and receives video, voice, chat, and other

important information simultaneously among various dis-

mounted, maneuver force elements, airborne and maritime

assets, and the upper echelons. New, unallocated spectrum

must be utilized if the projected future military requirementsare to be met, given the oversubscription of VHF-UHF-L

band (30 MHz–1.55 GHz frequencies). The spectrum

regimes that appear to fit these criteria are free-space optical

(FSO) and radio frequencies (RF) in higher bands such as

Ku and Ka. Fig. 1 illustrates the U.S. Department of Defense

view of military networking that is critically reliant on

high-capacity infrastructure-free networking. The Defense

Advanced Research Projects Agency (DARPA) has beenfunding the development of the requisite technologies for

the next-generation Global Information Grid (i.e., global

fiber optic network), e.g., Dynamic Multi-Terabit Core

Optical Networks (CORONET) program [2]. These are all

deployable Bsystems.[ None provides a backbone capability

that is cited in the airborne and space layers illustrated in

Fig. 1, i.e., theater backbone for brigade and below.

Manuscript received July 2, 2008; revised December 1, 2008. Current version

published May 13, 2009.

L. B. Stotts is with the Defense Advanced Research Projects Agency, Arlington, VA

22203 USA (e-mail: [email protected]).

L. C. Andrews and R. L. Phillips are with the University of Central Florida, Oviedo,

FL 32765 USA (e-mail: [email protected]; [email protected]).

P. C. Cherry and W. K. McIntire are with L-3 Communications, Salt Lake City, UT

84416 USA (e-mail: [email protected]; [email protected]).

J. J. Foshee and B. Stadler are with the Air Force Research Laboratory,

Wright Patterson AFB, OH 45433 USA (e-mail: [email protected];

[email protected]).

P. J. Kolodzy is with Kolodzy Consulting, Centreville, VA 20120 USA

(e-mail: [email protected]).

M. Northcott is with AOptix Technologies, Campbell, CA 95008 USA


H. A. Pike is with Defense Strategies & Systems Inc., Front Royal, VA 22630 USA


D. W. Young is with the Applied Physics Laboratory, The Johns Hopkins University,

Laurel, MD 21029 USA (e-mail: [email protected]).

Digital Object Identifier: 10.1109/JPROC.2009.2014969

Vol. 97, No. 6, June 2009 | Proceedings of the IEEE 11090018-9219/$25.00 �2009 IEEE

To meet that void, research and development has been

ongoing into airborne high-speed backbones for tactical

applications [3], mostly sponsored by the U.S. Air Force,

e.g., the Air Force Research Laboratory, Sensor Director-

ate (AFRL/SN) [4]–[7]. Those efforts were primarily im-

plementing FSO communication links and eventually

developing hybrid FSO/RF communication links [8].In the 1960s, the Air Force initiated efforts to develop

FSO communications. Field tests of FSO links between a

high-flying aircraft and a ground-based receiver were

conducted over ranges on the order of 100 km. The HAVE

LACE effort began in 1983 with the objective of determining

the feasibility of air-to-air laser communications between

two aircraft utilizing small, solid-state optical communica-

tion transceivers for data rates less than 1 Mbps [4]. HAVELACE was successful in meeting its technology objectives

and demonstrated that air-to-air laser communications was

possible using small off-the-shelf components. AFRL/SN

conducted a limited test using equipment to evaluate two

slightly modified terminals designed for space applications.

This test utilized terminals sighted on Haleakala, Maui, HI

and at Mauna Loa, HI, located 150 km away. Those tests

indicated limited capabilities.The follow-on program to HAVE LACE demonstrated at

Hawaii data rates of 1.1 Gbps full duplex communications

and then utilized an RF channel to begin the communica-

tion acquisition process. The hardware demonstrated in

this test formed the foundation for what became a follow-

on program to develop a suitable air-to-air system.

The Recc-Intel Cross Link (RICL) effort began in

late 1995 as an AFRL/SN program to develop an air-to-aircrosslink system capable of communicating between two

aircraft operating at 40 000 ft with separation ranges of

50 to 500 km. Objective data rates were 1 Gbps with a bit

error rate (BER) of 10�6. Unfortunately, the RICL effort

ran into development problems, and only limited ground

testing occurred with the system in 2004 and 2005. The

system utilized 810- and 850-nm-based components and

had fewer suppliers since commercial telecom compo-

nents had shifted to 1550 nm. Another problem with theRICL system was the excessive size and weight of the

gimbal system. The turret assembly alone for the RICL

weighed approximately 150 lbs. The Ultra-Wideband Laser

Communications for Intelligence, Surveillance and Recon-

naissance effort was to integrate a 1550-nm-based laser

communication system into a gimbal that was capable of

2.5 Gbps full duplex and a BER of 10�6 and operation at

40 000 ft and at ranges of 100 km. Limited ground testingof these terminals occurred and uncovered deficiencies in

the optical train that had to be rectified.

Thus the concept of FSO communications has been

around since the late 1960s [9]–[22]. Lasers offered the

potential for small transmitters and receivers with very

high antenna gain (that is, small transmitter apertures)

[23]–[27]. Specifically, FSO communication systems could

be much more efficient and provide orders of magnitudegains in data rate compared to an RF system of the same

size. Unfortunately, in the 1970s and 1980s, much of the

potential gain in efficiency was lost because of poor

electrical-to-optical efficiency, poor optical detector effi-

ciency, and the increased transmitter apertures necessitat-

ed by transmitter pointing-error limitations [28]–[30].

Also, the lifetime of laser transmitters and some key

subsystems available at the time were not good enough for apractical space system. So when all the dust cleared during

those decades, the advantages of FSO communications over

Fig. 1. Notional network centric architecture for the military.

Stotts et al . : Hybrid Optical RF Airborne Communications

1110 Proceedings of the IEEE | Vol. 97, No. 6, June 2009

RF communications were never realized, and the latterbecame the technology of choice for the past 40 years.

To achieve the goal of a robust high-throughput

airborne backbone communications network, innovative

hybrid networking and link technologies that exploit FSO/

RF channel diversity and synergy must be employed to

yield higher performance than either FSO or RF alone. The

first focused effort on hybrid FSO/RF communications was

the DARPA Optical RF Communications Link Experiment(ORCLE) program. Its primary focus was to research the

requisite networking and physical layer technologies for

networking radio and FSO nodes. The technology dem-

onstrations in ORCLE suggested that hybrid FSO/RF

communications was feasible and possibly could be pro-

totyped today. This recognition led DARPA to initiate the

Optical RF Communications Adjunct (ORCA) program

with AFRL, an effort to prototype the aforementionedhybrid system [8].

Since 2007, a more complete understanding of the key

attributes of an FSO communications system and the

development of advanced network routers has provided

the impetus to relook at developing a new tactical network

backbone capability [31], [32]. Besides DARPA, the AFRL

also began to investigate the efficacy of airborne hybrid

FSO/RF communications for such tactical applicationsafter the success of ORCLE. The Integrated RF/Optical

Networked Tactical Targeting Networking Technologies

(IRON-T2) program adapted the terminals developed

under the Multi-Platform-CDL program (MP-CDL) with

the development of a high-speed router and an optical

input. The optical input would allow interactive operation

with a companion FSO communications terminal. The

router was designed to accommodate short outages due toscintillation (optical). Requests for packet retransmission

would be through the RF channel, thus enabling the

combined FSO/RF wireless communications (hybrid).

Today, IRON-T2 experimentation is a key risk-reduction

activity supporting the DARPA/AFRL ORCA program.

This paper will describe the technical attributes that

motivate the development and use of hybrid FSO/RF

communication links. We shall describe recent efforts bythe IRON-T2 development team in demonstrating the

nascent FSO and network technologies. Results from

recent experiments also will be provided. These experi-

ments coupled with captive experiments have provided the

basis for a more complete understanding of the FSO

communications link. That has provided the authors to

more accurately model the FSO channel inclusive of the

application of adaptive optics. Those models are providingthe basis of understanding the reliability of airborne FSO

communication links and thus are critical for the

development of a prototype airborne hybrid FSO/RF

communications network. This effort provides risk miti-

gation for the ORCA prototype effort. It should be noted

that although this effort is motivated by military-based

communications needs, it has direct relevancy to all

mobile backbone networks under consideration for com-mercial applications.

II . MOTIVATION FORFSO/HYBRID LINKS

The future of mobile networking is reliant on the

availability of high-throughput backbone networks to

provide backhaul and/or access to the Internet core. Boththe military and commercial networks that must operate

without fixed infrastructure need to provide this capability

organically. The commercial enterprises are investing in

fixed FSO links as an alternative to optical fibers for

deployment in metropolitan-area networks (MANs) due to

their attractive characteristics. Fixed-location FSO links are

very easy to deploy and have high capacity, ease, and low

cost of deployment, making them suitable for backbones inmilitary applications and also for MANs and extension of

existing MANs. The weather attenuation dependency for

FSO links is very high. RF links are more reliable than FSO

links and can be deployed as easily as FSO links but are not

considered for backbone networks due to their low

capacity. Thus, for both military and future commercial

backbone networks, the characteristics of hybrid FSO/RF

communications architectures are highly desirable.RF communications, wireless technology characterized

by high frequencies, originated more than a century ago.

While it has the advantages of both channel stability and a

strong resistance to cloud attenuation on its side, it also faces

significant practical limitations. Chief among these is low

data rate, which renders RF systems incapable of high-speed

transmissionVan essential component for an effective

backbone network. Rain can also negatively affect the RFsystem, causing degradation in unfavorable weather condi-

tions. FSO communications offers a solution to address the

issue of low data rate presented with RF systems. As

indicated above, FSO technology is not a panacea. It is only

the combination of these two communications modalities

that can provide both the requisite high data rate and

reliability needed for backbone communications for both

military and commercial applications.Table 1 shows the basic rationales for such a hybrid

approach. FSO communications can provide high-speed

transmission because of its naturally large and easily

available optical bandwidth, which can be much greater

than 10 Gbps. However, it is susceptible to weather effects,

especially clouds. RF communications has the disadvantage

of lower data rate because of spectrum availability but has a

more stable, reliable channel and operates better in clouds,but not as well in rain. In addition, RF links suffer from

multipath. On the other hand, FSO operates better in rain.

Thus the FSO should be enabled on clear/light haze/rainy

periods to provide the highest data rate, and the RF operate

when it is cloudy to provide the required high reliability. The

high directionality of FSO links substantially reduces the

probability of multipath. The table also cites another


Vol. 97, No. 6, June 2009 | Proceedings of the IEEE 1111

potential advantage for a hybrid approach: size, weight andpower (SWaP). Yet, there still are some challenges for both

FSO and RF link that must be addressed before a hybrid

FSO/RF network can be a reality. In other words, just having

them operate under the above atmospheric conditions is

necessary but not sufficient to ensure that the backbone

meets the robust system connectivity requirement.

The FSO tests in 2006 and the IRON-T2 Program in

2007, discussed earlier, demonstrated that many of thechallenges to accomplish reliable FSO and RF communica-

tions could be met under the appropriate conditions [33],

[34]. Its enabling technologies were sufficient to support

reliable FSO communications and provide insights on how

to significantly improve FSO performance in horizontal

links under nighttime and some daytime conditions. The

program also showed the effects of tropospheric dispersion

and multipath on an RF system’s BER. These technologies,and the resulting systems, need to be further developed to

increase performance and robustness.

III . IRON-T2 TECHNOLOGYDEMONSTRATOR

The IRON-T2 effort demonstrates the viability of the

requisite technology components for hybrid FSO/RF com-munications links. It incorporates high-speed adaptive

optics, a single-mode fiber optical modem, optical frequency

conversion, a simple forward error-correcting (FEC) code,

and optical automated gain control (AGC). The adaptive

optics provides more light into the fiber for improved signal-

to-noise ratio (SNR). The single-mode fiber-optical modem

provides an improved noise figure. The optical AGC

mitigates up to 50 dB of signal variation due to scintillation.These combine to provide a high-capacity 274 Mbps

X/Ku-band RF communications system.

The results from testing on the IRON-T2 development

system provide the basis for the new development shown in

this paper. The testing was performed between the

mountains of Haleakala and Mauna Loa in Hawaii (see

Fig. 2). The results indicate that high-capacity optical links

are possible but that the uncorrected time-dependentvariations in received optical power due to atmospheric

attenuation and scintillation can reduce the quality of the

communications link. Launch power needs to be increased,

optical losses in the telescope need to be reduced, and the

tradeoffs among aperture size, launch power, and eye safety

must be considered.

Table 1 Generally Complementary Channel/Hybrid Characteristics for Airborne Networking

Fig. 2. IRON-T2 2006 experiment layout.



A. FSO SystemThe FSO experiment in 2006 addressed a number of

system configurations that were evaluated, including a single-

channel 2.5 Gbps transmission, a single-channel 10 Gbps

transmission, and a four-wavelength-division multiplexed

(WDM) channel transmission for an aggregate data rate of

40 Gbps (4 � 10 Gbps). The tests used optical modems

constructed with commercial-off-the-shelf (COTS) compo-

nents, and the optical telescope used adaptive optics (AO) toimprove the optical link availability. The AO performed

high-speed tracking and aberration correction to reduce the

effects of atmospheric scintillation.

The FSO terminals utilized a unique bidirectional,

adaptive optics method of beam control to compensate in

real-time for atmospheric turbulence and control the

diameter of the beam. A tip/tilt mirror arrangement

controlled fine pointing and tracking, while a deformablemirror corrected the perturbations caused by atmospheric

turbulence. The FSO terminals used AO compensated

laser-com terminals coupled to commercial 8-in (clear

aperture) telescopes. These terminals have a greater than

1 kHz closed-loop correction of approximately 30 Zernike

aberrations including tip/tilt and focus.

The WDM transmitter channels used were 1556.1,

1556.6, 1557.2, and 1557.6 nm. The high-speed data streamswere generated by multiplexing the four channels with a

polarization maintaining (PM) 1� 4 directional coupler and

encoding them with an electrooptic modulator. The data

sent were a pseudorandom bit sequence 27�1 pattern

encoded with a nonreturn-to-zero on–off key modulation

format. The modulator was followed by an erbium dopedfiber amplifier, which amplified the optical signal to about

200 mW at the output of the FSO terminal.

The WDM receiver used an optical preamplifier to

increase the received power to proper levels for input to

the digital communications analyzer, optical spectrum

analyzer, and BER test system. A microelectromechanical

systems tunable filter then selected the optical channel to

be evaluated. The 2.5 Gbps receiver used an APD with a10�9 BER sensitivity of �34 dBm, while the 10 Gbps

receiver used a PiN photodiode with a 10�9 BER sensitivity

of 10�18 dBm [31]–[33]. These sensitivities improved to

�36 and �24 dBm at 2.5 and 10 Gbps, respectively, when

an optical amplifier was used prior to the optical filter.

B. FSO PerformanceV2006Links supporting data rates from 2.5 to 40 Gbps were

observed for a period in excess of three contiguous hours

on each test day. The testing allowed a baseline of per-

formance to be established for FSO over the specific link

and provided data to support the development of designs to

improve link and system performance.

The power in the fiber (PIF) is the indication of the

overall performance of the FSO from transmission and

adaptive optic correction through the receiver chain. Itindicates the amount the received power at the aperture

that is coupled into the optical fiber. The PIF is presented

in a waterfall diagram shown in Fig. 3. During the exper-

iment, the Mauna Loa terminal was fitted with a multi-

mode fiber receiver utilizing a 62.5 �m core diameter.

Fig. 3. Example of an FSO PIF measurement.



The Haleakala terminal was fitted with a single-mode fiber(SMF). Notice that the mean light level in the SMF is

lower than that in the multimode fiber. The SMF receiver

also shows more variability than the level in the multimode

fiber. Both the lower mean coupling and the increased

variability in the SMF case come from the reduced nu-

merical aperture and small fiber core of the SMF. Spe-

cifically, smaller angular and spatial light acceptances of

the SMF affect the received power in the fiber and anyBaperture averaging,[ respectively. Fortunately, the lower

light level of the SMF is compensated by the increased

sensitivity of single-mode detectors, the availability of an

optical AGC (OAGC), and FEC coding.

The conclusion from these experiments is that for

optical communications, we will have to address up to 30–

40 dB scintillation variations. This will be especially true

for a deployed airborne system, where the communicationdistances run from 50 to 200 km. As noted, other

techniques must be employed to supplement the output

from the AO system [3], [5], [6]. The OAGC and FEC used

for this purpose, combined in what we call an optical

modem, are described in detail in Section IV, including

their use in these Hawaii experiments.

C. FSO PerformanceV2007A similar FSO configuration was tested during the 2007

Iron T2 test. The optical modem was substantially

changedVone optical WDM channel at 3.125 Gbps was

dedicated for use with the Iron T2 hybrid link and a second

WDM channel at 10 Gb/s was used to characterize the

performance of a higher speed channel. An OAGC system

was brought to the test to address the residual optical

power variations. The performance of the hybrid channelwill be discussed later. The 10 Gb/s channel exhibited

much better performance than was seen in 2006, largely

due to the better receiver sensitivity and reduction of

phase errors in the receiver system provided by the OAGC.

The FSO link was also fully bidirectional in 2007.

Additional descriptions of the modem, the OAGC, and

their aggregate performance are in Section IV.

D. RF System and PerformanceThe IRON-T2 RF architecture is based on the MP-

CDL airborne system, which supports up to three

simultaneous, fully compliant CDL RF data links that

allow for data rates up to 274 Mbps, depending on range

and link conditions. The system can be configured to

operate in X- and/or Ku-band and typically operates in a

line-of-sight (LOS) air-to-ground or air-to-air link config-uration. The RF links are generally configured to

accommodate a specified data rate for a given range and

rain fade, typically operating with a few decibels of RF link

margin. However, for the Mauna Loa to Haleakala data

link tests, the RF system experienced additional fading

from other sources. The combination of littoral climate,

time-varying multipath reflections from the ocean, and

tropospheric effects caused severe fading, both flat andfrequency-selective, such that significant errors were

introduced into the RF link. Adaptive equalization is an

option to be implemented at the receiver to mitigate the

effects of multipath and improve the system performance.

The nature and extent of the fading ranged from

noticeable to severe throughout the test periods, depending

on atmospheric and weather conditions. Data analysis

indicates the most likely source of these dropouts was acombination of the tropospheric effects on the LOS path and

the multipath interference created by ocean reflections.

Fig. 4 shows the geometry of the RF path for the Hawaii

testing, with RF Terminal A on the Haleakala sight at an

elevation of 10 032 ft and RF Terminal B on the Mauna Loa

sight at an elevation of 11 135 ft. The slant range between the

two terminals was 147 km (79.2 nautical miles, drawing not

to scale). The path A-O-B is approximately 114 m longerthan the direct LOS path A-B, which corresponds to a time

differential of 380 ns, or approximately 26 symbols, at a

data rate of 68.5 Msps, or approximately 104 symbols, at

symbol rate of 274 Msps. For a fixed path difference of

380 ns, the interference transitions from constructive to

destructive, with a 2.6 MHz repetition.

The effects of multipath can be seen in the received

signal spectrum. Fig. 5 shows the received intermediatefrequency (IF) spectrum captured from a spectrum

analyzer at Site A. Fig. 5(a) shows the desired spectrum:

a lightly filtered sinðxÞ=x distribution. The top trace in

Fig. 5(b) is a maximum hold trace and the bottom trace is a

minimum hold, showing the total extent of the signal

variation. Note that the receiver includes AGC circuitry,

limiting the range of the received signal.

The transmitted signal during testing was a 68.5 Mbpswaveform without RF equalization. The required system

energy per bit per noise energy (Eb=N0) for a bit error rate

of 10�9 is approximately 10.7 dB with current FEC. With

the IRON-T2 hardware configuration, operation at the

68.5 Mbps information rate allowed for approximately

10 dB of operating link margin, taking into account system

losses but not propagation effects such as fades and

multipath. For calm conditions, which was typically thecase during the early morning hours when testing was

Fig. 4. Geometry showing RF multipath. Smoother, rolling wave

causes deeper multipath interference.



performed, 9 dB of margin should be allocated to ocean

reflections. Postanalysis on captured, digitized waveforms

indicates nearly 15 dB of ripple in the received signal,

indicating even more severe multipath fading than pre-

dicted, as shown in Fig. 6. With insufficient margin to

accommodate deep fades, the RF link incurs errors. Analysisindicates that multipath and atmospheric effects such as

radio-holes and tropospheric ducting combine for RF fading

losses of nearly 15–20 dB. System modifications could

improve link performance, such as the implementation of

an adaptive equalizer, higher gain antennas, stronger

coding, and reduced RF losses between the antenna and

RF components.

E. Hybrid System PerformanceTesting was successful in terms of hybrid system

operation, which included FSO packet retransmissions.

The FSO retransmission algorithm assumes that the RF

link is available for all retransmission requests. As indi-

cated, during many of the Hawaii experiments, the RF link

was operating with significant transmission errors, typi-

cally during the early hours before sunrise. The system

performance was degraded most likely due to the lost

retransmission requests sent over the RF link and resulted

in degraded system packet error rate (PER). Fig. 7 shows a

good example of this. The data are taken on one of the best

days for FSO propagation throughout the whole test butone of the worst for RF performance. Between 5:00 and

8:00 am, the FSO averaged 99.86% time above threshold

(top graph), while the second plot shows the significant

errors reported by the RF modem. With no retransmission,

the PER from the FSO would be expected to be

ð1� 98:86%Þ ¼ 1:4E� 3; with retransmission, operation

should be close to error-free. However, both the AX4000

and the HLM reported significant packet errors, primarilydue to the lost retransmission requests.

In contrast, Fig. 8 shows a portion of the data from

August 23. As seen early in the plot, the RF had no errors

and the FSO was operating at �98% above threshold. This

time, however, there were no errors reported from the

gigabit testing system, except for 34 packets that were lost

at 04:59. Even with some RF errors after approximately

05:07, there were no errors reported from the system. Thisdemonstrates that when the retransmission requests get

through, the system performance is excellent.

As demonstrated with the IRON-T2 experiments, high-

capacity hybrid FSO/RF links are achievable under

controlled conditions. The RF channel was used to provide

retransmission signaling for the high-data-rate FSO sys-

tem. Experimentation was limited to early morning oper-

ation due to limitations in addressing optical turbulenceconditions. RF system experienced 15 dB or more of losses

due to multipath and tropospheric ducting.

IV. CHALLENGES FOR FSO/RFHYBRID SYSTEM

The IRON-T2 demonstration clearly indicated that under

favorable conditions, a hybrid FSO/RF communications

Fig. 5. (a) Spectrum analyzer image showing nominal received signal (1700 MHz IF); (b) received IF spectrum zoomed in showing

2.6 MHz ripple due to ocean reflection, plus moderate frequency-selective fading.

Fig. 6. Fast Fourier transform of captured IF waveform indicates

nearly 15 dB of ripple due to multipath, much higher than anticipated.



link can provide multigigabit per second data rates. The

test configuration was to emulate the propagation envi-

ronment of an air-to-air topology with the expense ofintegration onboard aircraft. The demonstration also

indicated that there are still some challenges that remain

to realize a highly reliable backbone network. Those

challenges are in both the FSO and RF link and integrationinto a network.

Fig. 7. Data collected from August 27, 2007, FSO retransmission requests dependency on RF show FSO operations below threshold (top)

combined with low RF performance (center) creates an increase in PER for the system.

Fig. 8. Data collected from August 23, 2007, FSO retransmission requests dependency on RF show FSO operations below threshold (top)

combined with good RF performance (center) creates essentially a zero PER for the system.



A. Challenges for an Effective Free-SpaceOptical Link

To achieve the promise of the very high-speed

communication of free-space laser links over long dis-

tances, several obstacles must be overcome. In addition to

clouds, which can block any single path, atmospheric

optical turbulence over tens or hundreds of kilometers can

disrupt such laser links. In particular, turbulence will

cause a spreading, constructive/destructive interferenceand wander in the laser beam, reducing power received at

the other end of the link. Atmospheric turbulence can also

cause fluctuations in the intensity of the received beam by

more than a factor of 1000.

AO systems can improve laser link performance but have

limitations imposed by the size of its apertures, the number

and bandwidth of their phase-compensating actuators, and

the distance over which it must operate. The lowest ordercorrection, tip/tilt, of the outgoing or incoming beam can

compensate for beam wander, wherever it occurs, except for

speed of light or actuator limitations. Higher order aberra-

tions, such as focus, astigmatism, and coma, can be corrected

by higher order AO corrections, but the range of influence is

limited by receiver aperture, number of AO actuators, and

system bandwidth. As a rule of thumb, the Rayleigh range

(also known as the Fresnel distance) can be used to estimatethe range over which AO will be useful. Simply stated,

aberrations that occur beyond that range cannot be resolved

by the transmitting or receiving telescopes and will not be

compensated by attempted phase adaptation.

The Rayleigh range is defined as the distance along the

propagation direction of a beam from the beam waist to the

location where the area of the cross section is doubled. A

laser communications system is assumed with a beam exitinga telescope of diameter D. From diffraction theory, we know

that range R where the beam is twice the cross-section of the

exit diameter can be derived from the relationship

ffiffiffi

2p

D � �R

D(1)

where � is the wavelength of light used in the systems.

If one wants to Nyquist sample the optical field at the

range R, then the imaging system needs to sample at

resolution equal to half this diameter. This implies that

Dffiffiffi

2p � �R

D

or

R � D2

ffiffiffi

2p

�¼ 0:7D2

�(2)

where D2=� is known as the Fresnel distance or Fresnellength from diffraction theory. For D ¼ 0:1 m, as used in

some recent experiments, we have

R � 0:7ð0:1 mÞ2=ð1:55� 10�6 mÞ¼ 4:5 km:

A similar set of logic would apply to AO sensing at thereceiver. The implication of the above is that compensa-

tion for turbulent fading and other effects can be effective

near the receiver but will be ineffective in compensating

for turbulent effects past that range. Fig. 9 illustrates a

typical scenario for a �60–200 km FSO airborne link. The

AO system would also compensate for the aerooptical

boundary effects and turbulent effects within the Rayleigh

ranges at the transmitter and receiver ends. In addition, itwould eliminate beam wander from the complete path and

help focus the beam onto the receiver optical fiber. The

turbulent effects induced by the extended turbulence by

the atmosphere in between the Rayleigh ranges will have

to be compensated for by other means, as noted above and

in [5] and [6].

B. FSO Performance Using the AOptixAdaptive Optics

To illustrate the range limitations, we will examine the

results of some propagation experiments performed

recently over a 10.02 km FSO link between Campbell,

CA, and Saratoga, CA, on July 23, 2008. Fig. 10 shows a

Google Earth view of the link and its terrain cross-section.

The altitudes of the end points are 200 ft at the Campbell

site and 1842 ft at the Saratoga site. The slant angle is 2.9�

in free air but is actually much closer to ground as the

terrain slowly rises into the Santa Cruz Mountains. As is

expected, the turbulence is variable and has a strong

diurnal cycle, many times dominated by local effects at the

Campbell site in the day and the interface of the cool

marine layer to the free atmosphere at or below 2000 ft.

In the July 23 experiments, some of the authors

recorded high-speed received power at the aperture andinto a fiber and 1550 nm focal plane images with curvature

AO on/off at both ends and on/off at alternate ends. For

this experiment, the transmitter was located at the

Saratoga site and the receiver at the Campbell site. Both

the transmitter and the receiver diameter was 0.1 m, and

we have the same Rayleigh range as calculated before

R � 0:7ð0:1 mÞ2=ð1:55� 10� 6 mÞ ¼ 4:5 km:

In some recent experiments, the FSO link has a 10 cmaperture optical terminal, the same type of optical terminal

used in the 2007 Hawaii experiments [4]. The adaptive

modulation rate was 30 kHz, with a closed-loop bandwidth



of 1.2 kHz. The laser wavelength was 1550 nm. The data

collection involved measuring both the power entering the

receiver, focused on the wavefront sensor, PIB, and the PIF

at a 32 kHz sample rate, with 1-s statistics generated every

minute.

The total power on the AO wave front sensor PIB is a

reasonable facsimile of the total power entering the

telescope aperture. The power in the single-mode fiber

PIF is a reasonably accurate, albeit scaled, facsimile of the

power within the airy disk. Fig. 11 shows the received

Fig. 9. Laser communications link with adaptive optics compensation.

Fig. 10. Google Earth view of the FSO link and its terrain cross-section.



power statistics, derived from the 10:00 am sequence taken

on July 23. The white data represents the PIF measurement

and the red curve shows the PIB measurements.

Looking at the bottom data set for remote AO and local

AO both OFF, we see that the FSO link experiencessignificant fades (PIB) and poor coupling into the single-

mode fiber (PIF). The mean PIB is around�23 dBm, while

the mean PIF is below the detector threshold. Moving up

one data set, where the remote AO is OFF and local AO is

ON, the PIB statistics appears only slightly improved, on

the order of�20 dBm. The PIF curve goes up significantly.

The conclusion is that the local AO provides improved

coupling into the fiber. In other words, it focuses incomingradiance better but does not reduce scintillation/fading

significantly. When the remote is ON and the local is OFF,

the PIB statistics is significantly improved but the PIF is

little changed. The top data segment shows the PIB and PIF

statistics with both remote and local adaptive systems ON.

Fig. 12 shows a similar data set taken later that

afternoon, 4:00 pm. The turbulence was less, but the same

trends discussed above are apparent.We have been able to model these experiments and get

reasonably good agreement by using the bottom, uncom-

pensated data sets to estimate the strength of turbulence and

then use a turbulence profile that zeros the strength of

turbulence at some distance from the transmitter or the

receiver or both, to best emulate the measurements. Results

for the measurements are shown in Table 2. This table

shows reasonable agreement between modeled and mea-sured data with correction distances from the receiver,

transmitter, or both in the range of 4 to 7 km, as compared

with our rule of thumb Rayleigh range estimate of 4.5 km.

On August 23, another set of experiments over this

10 km range was conducted to compare tip/tilt-only com-

pensation with full compensation from both receiver and

transmitter and no compensation from either. Figs. 13–16

show results from four different times during that day.

In the daytime figures, we see that full AO compen-sation gives a mean PIB around �4 dBm and a mean PIF

around �16 dBm. In Fig. 13, we see that PIB difference

between full AO and tip/tilt only is �4.3 dB, and for PIF

�9 dB, where the turbulence is not bad, i.e., D=r0 ¼ 2:1.

These differences are even smaller in Fig. 16, 6:00 pm

data, where D=r0 ¼ 1:8. In Fig. 14, we have higher

turbulence conditions D=r0 ¼ 3:2, and the PIB difference

between full AO and T/T then is �2.7 dB and, for PIF,�8 dB. These results are essentially the same for Fig. 15. It

is clear that full AO really focuses the energy better than

tip/tilt or no AO by its pronounced system Strehl ratio,

which demonstrates what full AO brings to the table.

Again, we modeled these experiments using the

rightmost uncompensated data to estimate the strength

of turbulence and then used a turbulence profile, which

zeroed the strength of turbulence at some distance fromboth the transmitter and the receiver. Results for mea-

surements in Figs. 13–16 are shown in Table 3. The

modeled results are a good match to the data, with receiver

and transmitter correction ranges of 4.0–4.5 km, with the

exception of the lowest turbulence case, 6:00 pm, which

has a correction range of only 2.6 km. We think there may

have been larger fluctuation in levels of turbulence in that

particular case. The rule of thumb use of Rayleigh rangeseems useful in these cases as well. Future work will

separate higher order aberrations and attempt to develop a

more precise heuristic estimate.

In all cases, we seem to be left with scintillation/fading

with PIB and PIF means on the order of �4 and �16 dBm,

respectively, with good spread. The conclusion from this is

that when one has ranges in excess of the Rayleigh range, we

cannot completely compensate for all the atmosphericturbulence. The question now is how much extra

scintillation/fading will remain from longer ranges, with

limited Rayleigh ranges, and how should we compensate for

Fig. 11. July 23 10:00 am PIF and PIB are shown for compensation

at both transmitter and receiver, transmitter only, receiver only,

and no compensation at either end.

Fig. 12. July 23 4:00 pm PIF and PIB are shown for compensation

at both transmitter and receiver, transmitter only, receiver only,

and without compensation at either end.



this residual signal loss. Let us begin by expecting higher

atmospheric turbulence losses when transmitting over longer

ranges and discuss one approach to compensate for theselosses, an optical modem composed of an OAGC and FEC.

C. Optical ModemFig. 17 shows the basic architecture for the Johns

Hopkins University-Applied Physics Laboratory (JHU-APL)

optical modem. In this system, the OAGC translates the

time variant optical input (IðtÞ) into constant optical

amplitude output with a variable optical signal-to-noise

ratio OSNRðtÞ. The modem normalizes, decodes, andformats the received signal as well as regenerates, encodes,

and modulates the transmit signal.

The JHU-APL OAGC system consists of multiple stages

using a feed-forward design to control the gain on a

submillisecond time scale to get the scintillated signal

stream up to nominal average power level without saturating

Table 2 Modeled and Measured Results, July 23

Fig. 13. PIF, PIB, and system Strehl for data taken at 1:00 pm:

D=r0 ¼ 2:1.


D=r0 ¼ 3:2.


D=r0 ¼ 1:8.


D=r0 ¼ 4:2.



or damaging the optical detector. The gain of the first-generation system is between 40–45 dB, depending on

configuration. It also has field proven WDM capability.

The OAGC reduces the dynamic nature of the FSO

link, where > 40 dB swings can occur in submillisecond

time scales in heavy atmospheric turbulence. Issues

addressed by the JHU-APL equipment to ensure that the

ORCA system is reliable are:

• correction of amplitude swings from detector noisefloor to damage threshold in submillisecond time

frames;

• reduction of AM to PM conversion in receiver

electronics;

• reduction of induced jitter, which can lead to bit

errors and make clock/data recovery more difficult;

• reduction of fast baseline drift that can be ac-

coupled into the receiver, leading to power pen-alties exceeding 10 dB.

Given the entire discussion above, how does the OAGC

work in practice? Fig. 18 illustrates the effects on the BER

with the use of the OAGC. The figures show eye diagrams

for a 10 Gbps test link (received optical power varied via a10 kHz sine wave �2 to �16 dBm) taken directly out of a

10 Gbps PIN/TIA/Limiting amplifier combination. The

power range was chosen to be well within the error-free

range of the PIN/TIA/limiter (þ2 to�20 dBm). The figure

clearly shows that the resulting eye diagram of the FSO

link cleans up nicely with the OAGC on, reducing the raw

Table 3 Modeled and Measured Results, August 27

Fig. 17. JHU-APL optical modem.

Fig. 18. Eye diagrams for FSO link with and without the OAGC.

The optical power was varied between�2 and�16 dBm; the waveform

was a 10 kHz sinusoid. The uncorrected case shows intersymbol

interference, leading to an increased bit error rate.



BER from 2.1� 10�6 to G 10�12 (Berror free[). The major

penalty in this case is timing jitter caused by uncorrected

amplitude modulation, with the penalty due to eye closure

also having an impact. As noted above, the OAGC sets theresidual optical signal from the adaptive optics receiver

terminal to the expected average power. Its job is to close

the link at the desired BER without saturating or damaging

the detector. It is clear that in this case, the result link is

Berror free[ in the sense discussed previously.

Fig. 17 showed the basic structure for the modem,

where the FEC electronics are located. Specifically, the

modem uses COTS Reed–Solomon [255, 239] FEC chip-sets for optical links, which are designed to operate in a

high received power, variable OSNR environment. Low

optical received power leads to decision errors by the

limiting amplifier, which cannot be corrected by the FEC.

The result is that the OAGC system maintains a constant,

high input power to the optical detector, but the OSNR

varies. Lab tests have proven that COTS FEC chips can

provide the full designed 8 dB of gain, even when thepower into the OAGC varies over four orders of mag-

nitude. The results of using the COTS FEC with the first-

generation OAGC is shown in Fig. 19 from laboratory tests

using simulated data. The detector in both cases was a

InGaAs PIN/TIA combination. Let us now estimate the

effective gain from the optical modem.

To address this question, a baseline detection method is

established. Let us propose a 10 Gbps PIN/TIA photo-receiver approach, with BER �10�12 and an input power

of �20.5 dBmVthis is the type of detector measured in

Figs. 18 and 19. Assuming no variable optical attenuator is

used to normalize the input power (additional excess loss

of �2 to 3 dB), the gain of the first generation OAGC over

the PIN alone is 10 dB. Given that the OAGC enables the

full gain of the FEC to be accessed, an additional 8 dB of

effective gain can be considered, leading to a minimum18 dB gain over non-preamplified optical receivers. These

numbers do not take into account the power penalty that

the uncorrected AM to PM conversions would have on the

system. There are assumptions contained in this number,

but it provides the magnitude of what to expect undernormal operations. In situations where there is little var-

iation in the received power, the OAGC would still yield

the same sensitivity improvement, but the need to dy-

namically control the amplifier gain in submillisecond

time scales would be reduced.

D. FSO Performance Using the JHU-APLOptical Modem

The optical modem/OAGC system discussed above was

evaluated as to the performance data of the OAGC and the

BER of the 10 Gb/s link.

The OAGC was developed to reduce the variations in the

received optical power coupled into a single-mode optical

fiber by the adaptive optics terminal. Fig. 20 shows the

Fig. 19. BER as a function of PIF for the first-generation OAGC and a Reed–Solomon [255,239] FEC.

Fig. 20. (Red) Power in the fiber from the FSO terminal and

(white) power out of the optical AGC.



variation in the power in the optical link (red trace) and theoutput of the optical AGC (white trace). This is an indication

of the power variation both over the short time frame of each

data frame (1 s) and as the test day progressed. The

degradation of the link that occurred after sunrise is clearly

apparent. The average received power dropped nearly 20 dB,

and the power distribution increased dramatically. This

would increase the amount of errors seen in the communica-

tions link over time. This impact of the link variation is shownin Fig. 21, which shows the percentage of bits sent error-

free on the bidirectional 10 Gb/s channel on August 27,

2007, for two different receiver sensitivities.

In summary, the optical modem described has been

extensively tested in both laboratory and field environ-

mentsVthe prototype system has been operated 250 hours

in the field. The ORCA system will use the system with minor

changes in the modem design to reflect a higher data ratethan was demonstrated with the IRON-T2 hybrid channel.

E. Challenges for a Reliable RF LinkThe challenge for the RF system is to obtain a reliable

RF communications channel in difficult conditions and

environments to compliment the performance character-

istics of the FSO channel. The IRON-T2 experiments

revealed severe RF channel conditions, where fading of upto 20 dB was observed in the high-altitude (10 000 ft)

long-range (150 km) Hawaiian littoral/maritime environ-

ment. The fading was due to the combined effects of

multipath, radio-holes, tropospheric ducting, rain, and/or

clouds. This situation was shown in Fig. 4. Fig. 22 shows a

sample set of data showing the fading effects.

Airborne systems operating in similar environments

must be designed to reduce the impacts of this RF channel,such as using adaptive equalizers to minimize the multipath

signal-to-interference noise ratio. This is even more

challenging with SWaP constrained systems where limited

transmit power and/or aperture size further limits com-

munications performance. Like the FSO links, means must

be developed to compensate for the above effects. Adaptive

equalization techniques such as decision feedback equal-

izers or minimum squared error equalizers provide thebasis for significant system enhancements in the multipath

environment, in addition to utilizing more powerful FEC

codes as well as other techniques to reduce RF losses.

F. Networking ChallengesOne of the chief challenges is the integration of the

hybrid FSO/RF links into a high-capacity reliable backbone

network. Although not addressed by this paper, it isimportant to understand these challenges. The airborne

nodes and ground nodes will form a mobile ad hoc network

(MANET). This in itself should not be challenging. How-

ever, addressing robustness to airborne link outages that

can range from milliseconds (scintillations) to seconds

(obscurations) to tens of seconds (aircraft turns) will be

challenges. Such robustness needs to address both quality

of service (QoS) constraints for the applications and theimpact to network protocols such as transmission control

protocol.

Scalability of the network for air nodes, ground gateways,

and customer premise equipment (ground nodes) will

present challenges. The capability of the network to support,

with QoS, both internal and external network traffic is a key

attribute. Although the number of nodes will be limited, the

network will need to support a large number of InternetProtocol (IP)-addressable communications nodes and net-

works that may be connected via gateway nodes to the

airborne backbone.

A separate challenge for the network is the dynamic

range of the data rates that will be presented to the mobile

network routers. Aggregate data rates of 8 Gbps or higher

are possible. This will require additional levels of hardware

and software robustness.

V. THE ORCA HYBRIDFSO/RF PROTOTYPE

The advances in adaptive optics, optical modems, and

optical automatic gain control have been significant. High-

speed air-to-air and ground-to-air RF communication

systems have been matured under the development ofthe CDL. Network technology for MANETs and for high-

speed routers appears to be sufficient to support airborneFig. 21. Link performance on August 27, 2007.

Fig. 22. Example of IRON-T2 RF data.



backbone networks. The IRON-T2 experiments have

shown that an integrated hybrid communications system

is feasible, but the reliability of such a system still needs to

be addressed.

As noted earlier in this paper, DARPA is proceeding

with the next level of development through the initiationof the ORCA program. The intent of the ORCA program is

to design, build, and test a secure, IP, hybrid electrooptical

and radio-frequency backbone network for tactical reach-

back and data dissemination applications, as well as to

provide a demonstration of technologies for hybrid FSO/

RF networking between ground sites. In particular, the

ORCA objective is an actual prototype demonstration of a

tactical network containing ground-based on-the-move/at-the-halt and airborne nodes (see Fig. 23).

Proposed usable data rates for the ORCA system

include a nominal node-to-node uncorrected 274 Mbps

data rate for the RF portion of the hybrid link and an

uncorrected > 5 Gbps data rate (though higher is accept-

able) for the FSO portion of the hybrid link.

The tactical networks should operate as Bstub-

networks[ to the higher capacity demonstration ORCAnetwork. The ORCA network may operate as a Bstub-

network[ to the high-speed segments of the GIG as well as

the transformational communications architecture. There-

fore, an ORCA network must be able to distinguish inter-

and intranetwork traffic flows and compensate through

QoS traffic prioritization.

The current ORCA team, under Northrop Grumman

Corporation (NGC) management, is composed of L-3

Communications, AOptix, and JHU-APL, with the last

three being the IRON-T2 contractors. The NGC team is

continuing to upgrade the IRON T-2 technologies for

eventual integration into the NGC aircraft pod for field

testing. Phase 1 will result in brassboards that will be field

tested in Maryland and/or California to validate that thelink will close under harsh ORCA environmental condi-

tions and that the technologies are ready for integration

into the pod.

VI. CONCLUSION

The use of hybrid FSO/RF links to provide robust, high-

throughput communications fixed infrastructure links andtheir associated networks has been thoroughly investigated

for both commercial and military applications. The

extension of this paradigm to mobile, long-range networks

has long been a desire by the military communications

community for multigigabit mobile backbone networks.

The FSO communications subsystem has historically been

the primary limitation. The challenge has been address-

ing the compensation of propagation effects and dynamicrange of the received optical signal. This paper discussed

the various technologies required to compensate for the

effects referenced above, as well as those incurred by a RF

link. We outlined the effects FSO and RF links experience

and how the new ORCA program plans to overcome these

degradations. Results from field experiments conducted,

including those from the AFRL IRON-T2 program, were

presented and discussed. h

Fig. 23. ORCA network architecture.



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ABOUT T HE AUTHO RS

Larry B. Stotts (Fellow, IEEE) received the

B.A. degree in applied physics and information

sciences and the Ph.D. degree in electrical engi-

neering (communications systems) from the

University of California at San Diego.

He is Deputy Director for the Strategic Tech-

nology Office, Defense Advanced Research Agency

(DARPA). He support the Director in guiding and

directing a team of program managers developing

communications, networking, information opera-

tions and battle command technologies for network-centric operations

(warfare and enterprise) and generalized C4ISR. Prior to joining DARPA,

he was Director for Technology in the Office of the Assistant Secretary of

the Army for Acquisition, Logistics and Technology from 1999 to 2002.

From 1996 to 1999, he was Chief Scientific and Technical Advisor and the

Integrated Product Team Leader for Aircraft, Avionics and Navigation

Systems in the Office for Communications, Navigation and Surveillance

Systems Department, Federal Aviation Administration. From 1987 to

1996, he was a Program Manager and then Assistant Director for Special

Projects, Tactical Technology Office, DARPA. From 1971 to 1987, he was

the Associate for Image Processing with the Naval Ocean Systems Center

(NOSC). He has published 85 journal articles, conference papers, and

technical reports. He is a coauthor of Optical Channels: Fiber, Atmo-

sphere, Water and Clouds (New York: Plenum, 1988) and the BOcean

Optical Propagation[ entry in the SPIE Encyclopedia of Optical Engineering.

He has received seven U.S. patents.

Dr. Stotts is a Fellow of the Society of Photographic and Instrumen-

tation Engineers (SPIE) and a member of numerous professional

societies. He received a DARPA Technical Achievement Award for his

management of the Future Combat Systems Communications Program

in 2006. He received a National Partnership in Reinventing Government

as part of the Maritime Differential Global Positioning System (GPS)

Service Team and the Nationwide GPS Service Team in 1999. He

received the Secretary of Defense Medal for Meritorious Civilian Service

in 1991 and 1996. He received the Technical Cooperation Program

Technical Achievement Award in 1991; the NOSC Technical Director’s

Award in 1986; and the DARPA Outstanding Technical Achievement

Award in 1985.



Larry C. Andrews (Senior Member, IEEE) re-

ceived the doctoral degree in theoretical me-

chanics from Michigan State University, East

Lansing, in 1970.

He is a Professor of mathematics at the

University of Central Florida, Orlando, and an

Associate Member of the College of Optics/CREOL.

He is an Associate Member of the Florida Space

Institute. Previously, he held a Faculty position at

Tri-State University and was a Staff Mathematician

with the Magnavox Company, antisubmarine warfare operation. He has

been an active researcher in optical wave propagation through random

media for more than 25 years and is the author or coauthor of ten

textbooks on the topics of differential equations, boundary value

problems, special functions, integral transforms, wave propagation

through random media, and mathematical techniques for engineers.

Along with wave propagation through random media, his research

interests include special functions, random variables, atmospheric

turbulence, and signal processing.

Paul C. Cherry (Member, IEEE) received the

B.S.E.E. degree from the University of Colorado,

Boulder, in 1986 and the M.S.E.E. and Ph.D. degrees

from the University of Utah, Salt Lake City, in 1992

and 1996, respectively.

His doctoral research was in electromagnetics.

From 1986 to 1996, he was an RF/Microwave

Engineer with Unisys and Loral (now L-3 Commu-

nications, CS-W), Salt Lake City. From 1996 to

2003, he was with Thomson Consumer Electron-

ics, Indianapolis, IN, first in their Satellite Receivers Group and then in

their Advanced Communications Group. He returned to L-3 Communica-

tions in 2003, where he worked in the Advanced Communications Group

and is currently a Systems Engineer focusing on high-rate RF networked

and hybrid RF/FSO communications systems and technologies.

James J. Foshee (Member, IEEE) received the master’s degree in

systems engineering and in business administration from Wright State

University, Dayton, OH.

He is a Development Engineer with the Connectivity Branch,

Information Directorate, Air Force Avionics Laboratory. He is involved

in the development of wireless data links (RF, optical, and combined RF/

optical) for the transfer of high-capacity high-value data. His past

experience includes the development of airborne/ground satellite

communications terminals, communications propagation research,

and communications technologies flight testing. He has authored/

coauthored/presented papers in a range of related technology areas.

He developed, demonstrated, and patented a bit timing extraction (clock)

technique for use in a noisy satellite communications link disturbed by

ionospheric scintillation. He worked with the Defense Advanced Projects

Agency in the initial development of the Tactical Common Data Link

family of terminals and in the initial development of an airborne

communications node, and was involved in the initial development of the

payload communications package during the Predator UAV develop-

ment. He is presently involved in developing data links for the Angel Fire

program (small surveillance aircraft presently deployed in Iraq),

developing the communications payload and ground terminal for the

TACSAT-3 satellite system (a LEO satellite with a sensor package),

accomplishing research in optical and communications technologies

(Lasercomm and RF data link terminal technologies and optical and RF

devices), and providing support to the Missile Defense Agency in their

SBIR/STTR programs to support insertion into BMDS systems.

Mr. Foshee is a registered Professional Engineer in the State of Ohio.

Paul J. Kolodzy (Senior Member, IEEE) received

the B.S. degree in chemical engineering from

Purdue University, West Lafayette, IN. He re-

ceived the M.S. degree in chemical engineering

and the Ph.D. degree from Case Western Reserve

University, Cleveland, OH.

He is a Communications Technology Consultant

in advanced wireless and networking technology.

He has 20 years of experience in technology

development for advanced communications, net-

working, electronic warfare, and spectrum policy. He was Director of the

Center for Wireless Network Security (WiNSeC), Stevens Institute of

Technology; Senior Spectrum Policy Advisor with the Federal Commu-

nications Commission and Director of Spectrum Policy Task Force;

Program Manager with the Defense Advanced Projects Agency; Director

of Signal Processing and Strategic Initiatives with Sanders, a Lockheed

Martin Company; and with MIT Lincoln Laboratory.

William K. McIntire received the engineering

degree from South Dakota School of Mines and

Technology, Rapid City, in 1978.

He spent two years in the Peace Corps prior to

beginning his engineering career. He is currently

with L-3 Communications. He has 20 years of

experience with wide-band microwave digital data

links used for air-to-air, air-to-surface, and air-to-

satellite communications. His past work includes

airborne and surface terminals for DSCS, INTELSAT,

TDRS, and MILSTAR satellites. Currently, he is leading a design team for a

waveform and supporting modem that enables high-data-rate communica-

tions using a radar system as the aperture.

Malcolm Northcott received the B.A. degree in

physics from Christ’s College, Cambridge, U.K., in

1984 and the Ph.D. degree in optics from Imperial

College London, U.K., in 1988.

He is Founder and Vice President of Optics

Software. He is a pioneer in the development of

software for adaptive optics (AO) systems, He has

18 years of experience in adaptive optics based

product development, adaptive optics for astron-

omy, software development, and optical system

modeling. He played a key role in the development of curvature-sensing

AO technology and participated in the construction of the world’s first

curvature-sensing AO system for the Canada-France-Hawaii telescope in

Hawaii. Along with designing the computer control system, including the

architecture of the custom electronics, he wrote all of the software

necessary to operate the system. He contributed to the construction of

AO systems for the NASA IRTF and Mess Solar observatories. He has

continued to work to improve the Gemini North telescope, also based in

Hawaii. Prior to founding AOptix, he was an Astronomer at the University

of Hawaii and Imperial College London. He was with the Institute for

Astronom, University of Hawaii, from 1989 to 1999, where he conducted

research on adaptive optics. He has received eight U.S. patents.



Ronald L. Phillips (Senior Member, IEEE) has

received four degrees in the areas of electrical

engineering and mathematics. He received the

doctoral degree in electrical engineering from

Arizona State University, Tempe, in 1971.

He was Founding Director of the Florida Space

Institute, Kennedy Space Center, University of

Central Florida (UCF), Orlando, started in 1996.

He also was Founding Director, in 1984, of UCF’s

optics center CREOL, which went on to become the

country’s first university stand-alone college in the field of optical

physics, engineering, and photonics. His academic positions at UCF

include Professor in the School of Electrical Engineering and Computer

Science, Professor of the Department of Mathematics, and Professor of

the College of Optics and Photonics. He has held academic positions on

the Faculties of Arizona State University and the University of California,

San Diego. He has been an active researcher in wave propagation

through random media for more than 35 years. He is the coauthor of

three research books on the topic of wave propagation through random

media and applications to laser communications and radar. He is also

coauthor of a text on advanced applied mathematics. In addition to

optical wave propagation, his research interests include optical space

communications, laser radar, imaging through atmospheric turbulence,

and random processes.

Dr. Phillips is a Fellow of OSA, SPIE, and AIAA. His honors and awards

include: the National Science Foundation’s NATO Postdoctoral Fellow

and ASEE’s Outstanding Contributions to Research for all fields of

university engineering research.

H. Alan Pike received the B.A. degree from the

Massachusetts Institute of Technology, Cambridge,

in electrical science and engineering and the

Ph.D. degree in optics (with a specialty in quantum

electronics) from the Institute of Optics, University

of Rochester, Rochester, NY.

He is President of Defense Strategies & Sys-

tems Inc., a small business specializing in system

architecture and system engineering for defense

related applications. He has extensive experience

in designing, developing, and operating advanced technology ground,

air, and space systems. He has published widely on subjects including

aerospace testing, strategic defense, optical discrimination, and devel-

opment and testing of laser weapon components. He is well known in the

field of lasers propagation. He led the team that first demonstrated

adaptive optical compensation for high-power laser distortions produced

by thermal blooming of a Bmegawatt class[ CW laser. His thesis, BOrganic

Dye Lasers,[ incorporated both theoretical and experimental advances

leading to the first tunable single-mode dye laser, now a model for

commercially available tunable lasers.

Brian Stadler received the bachelor’s degree in

aerospace engineering from the University of

Cincinnati, Cincinnati, OH, and the master’s degree

in systems engineering from Wright State Univer-

sity, Dayton, OH.

He is a Research Engineer and Program Man-

ager in the Electro-Optical Combat ID Technology

Branch, Sensors Directorate, Air Force Research

Laboratory (AFRL), Wright-Patterson AFB, OH. He

is currently the Defense Advanced Research

Project Agency (DARPA) Technical Agent for the DARPA Optical RF

Communications Adjunct Program. Previously, he was a Technical Agent

for various DARPA efforts related to free-space optical communications

and hybrid RF and FSO communications, including the Optical RF

Combined Link Experiment (ORCLE) and Terra Hertz Operational Reach-

back (THOR) programs. Prior to working in the Sensors Directorate, he

was with the Air Vehicles Directorate of AFRL, assessing directed energy

integration issues on aircraft and conducted utility studies of both high

power microwave and high energy laser weapons. He also conducted a

wide variety of advanced technology and flight control simulations using

the Air Vehicle Directorate’s high-fidelity real-time man-in-the-loop

facility at Wright-Patterson AFB.

David W. Young (Senior Member, IEEE) received

the B.S. degree in physics and the M.S. and Ph.D.

degrees in electrical engineering from the Univer-

sity of Connecticut, Storrs.

He is currently a member of Senior Principal

Staff at the Applied Physics Laboratory, The Johns

Hopkins University, Laurel, MD. His current work is

on high-bandwidth free-space optical communica-

tions systems development, focusing on systems

and technologies for enabling robust links in high

dynamic range environments. He has designed, built, and field demon-

strated high channel count multigigabit/second wavelength-division

multiplexed (WDM) free-space optical communication equipment. He

was an early developer of wavelength agile dense WDM ultra-long-haul

systems while with Corvis Corporation.



Hybrid Optical RF Airborne Communications

Documents