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High-Speed Internet Access via Stratospheric HALO Aircraft

Nicholas J. COLELLA <ncolella@angelcorp.com>
Angel Technologies Corporation

James N. MARTIN <james.martin@ieee.org>
Raytheon Systems Company


Angel Technologies Corporation (http://www.broadband.com) and its partners (Deskin Research Group, Endgate Corporation, Raytheon Systems Company, and Wyman Gordon Corporation) have committed significant resources to date to pioneering broadband wireless millimeter wavelength (MMW) services from piloted high-altitude, long-operation (HALO) aircraft. Scaled Composites in Mojave, California, a subsidiary of Wyman Gordon, is flight testing the HALO/Proteus "proof of concept" airplane at full scale, and its sister company, Scaled Technology Works in Montrose, Colorado, will type-certify the airplane through the Federal Aviation Administration and will be the series producer of the airplane. Angel and Raytheon demonstrated a symmetric 51.8 Mbps link from a rooftop, tracking antenna to a general aviation airplane in flight at a slant range of about 25 miles, through which the data services highlighted in this paper were delivered.

The HALO airplane will be the central node of a wireless broadband communications network with a star topology, the HALO Network, whose initial capacity will be on the scale of 10 Gbps, with a growth potential beyond 100 Gbps. The packet-switched network will be designed to offer bit rates to each subscriber in the multimegabit-per-second range. A variety of spectrum bands licensed by the Federal Communications Commission for commercial wireless services could provide the needed MMW-carrier bandwidth. An attractive choice for the subscriber links is the LMDS (local multipoint distribution system) band. The signal footprint of the network, the Cone of Commerce, will have a diameter on the scale of 100 kilometers.

The airplane's fuselage can house switching circuitry and fast digital network functions. A MMW antenna array and its related components will be located in a pod suspended below the aircraft fuselage. The antenna array will produce many beams -- typically, more than 100. Broadband channels to subscribers in adjacent beams will be separated in frequency. For the case of aircraft-fixed beams, the beams will traverse over a user location, while the airplane maintains stationary overhead, and the virtual path will be changed to accomplish the beam-to-beam handoff. The aircraft will fly above commercial airline traffic, at altitudes higher than 51,000 feet. For each city to be served, a fleet of three aircraft will be operated in shifts to achieve around-the-clock service. Flight operational tactics will be steadily evolved to achieve high availability of the node in the stratosphere.

Raytheon and Angel recently conducted a demonstration of the first commercial wireless broadband link from ground to a moving aircraft, a 50-mile round trip connection of 52 Mbps (OC-1 rate). The following services were demonstrated over this wireless link: T1 access, ISDN (integrated services digital network) access, Web browsing, high-resolution videoconferencing, large file transfers, and Ethernet local-area network bridging.


1. Background

High-Altitude Long Operation (HALO) aircraft present a new layer in the hierarchy of wireless communications -- a 10-mile tall tower in the stratosphere above rain showers and below meteor showers (i.e., high above terrestrial towers and well below satellite constellations). Our talk will present the architecture and explain the concept of operations of the HALO Network. It will describe key characteristics of the HALO aircraft, the network equipment onboard, and the user terminals. Earlier papers1,2 introduced the HALO Network. The paper by Djuknic3 highlighted the unique advantages of stratospheric platforms for providing wireless communications services and is a good reference for the engineering community.

2. Introduction

Angel Technologies Corporation and its partners are highly encouraged by technological and manufacturing advances in the aviation, millimeter wave wireless, data communications, computer networking, and multimedia communications fields. We believe we have an opportunity to deploy a novel broadband communications infrastructure. Our work suggests the HALO Network will be able to offer wireless broadband communications services to a "super metropolitan area," an area encompassing a typical large city and its surrounding communities. The aircraft will carry the "hub" of the network from which we will serve tens to hundreds of thousands of subscribers on the ground. Each subscriber will be able to communicate at multi-megabit per second bit rates through a simple-to-install user terminal. The HALO Network will be evolved at a pace with the emergence globally of key technologies from the data communications, millimeter wave RF, and network equipment fields. The HALO Network will be a template that Angel will evolve and replicate to grow a global business.

Much of the technology needed already exists. The engineering development effort is thus focusing on adapting and integrating components and subsystems from competitive markets. Proven technology, components, and subsystems will be used as pervasively as possible. Adaptation has been given priority over innovation and basic development.

The HALO aircraft will be operated in shifts from regional airports. While on the ground, the network equipment aboard the aircraft will be assessed, maintained and upgraded on a routine basis to ensure optimal performance. Our operating plan specifies regular equipment upgrades in order to leverage technology advances for yielding lower cost and weight and for providing increased performance.

The HALO/Proteus airplane has been specially designed to carry the hub of the HALO Network. In the stratosphere, the airplane can carry a weight of approximately one ton. The airplane is essentially an equipment bus from which commercial wireless services will be offered. A fleet of three aircraft will be cycled in shifts to achieve continuous service. Each shift on station will have an average duration of approximately eight hours.

The HALO/Proteus airplane will maintain station at an altitude above 51 Kft in a volume of airspace resembling a distorted torus with a typical diameter of eight nautical miles. The look angle, defined to be the angle subtended between the local horizon and the airplane with the user terminal at the vertex, will be greater than a minimum value of 20 degrees. (The minimum look angle (MLA) for a given user terminal along the perimeter of the service footprint is defined to occur whenever the airplane achieves the longest slant range from that terminal while flying within the designated airspace.) Under these assumptions, the signal footprint will cover an area of approximately 2,000 to 3,000 square miles, large enough to encompass a typical city and its neighboring communities. Such a high value for the MLA was chosen to ensure a line-of-sight connection to nearly every rooftop in the signal footprint and to ensure high availability during heavy rainfall for most of the major cities in North America, especially for broadband data rates propagated in the K/Ka bands (above 20 GHz).

By selecting MMW frequencies, a broadband network of high capacity can be realized, since carrier frequency bandwidths on the scale from 100 MHz to 1,000 MHz have been licensed and may be made available through partnerships. Small antenna apertures on the scale of one foot will provide narrow beamwidths, and thus the user terminals can be compact yet offer high gain. Also, a multi-aperture antenna array can fit in an airborne pod with dimensions practical to an aerodynamicist.

The airborne antenna array can be configured to project a pattern of many cells numbering from 100 to more than 1,000. Each cell on the ground will cover an area of a few square miles to several tens of square miles. A variety of spectrum re-use plans can be selected to cover the signal footprint with patterns of cells. For example, each cell can use one of four frequency sub-bands, and a fifth sub-band can be used for gateways (connections to the public network or to provide wideband links to dedicated users). By reusing the spectral bandwidth, a total network capacity in the range of 10 Gbps to 100 Gbps appears feasible. In this talk, we will explain how this scale of capacity is practical.

The HALO Network provides an alternative to satellite and terrestrial broadband communications networks. Unlike a satellite node, the airborne node can confine all of its spectrum use to a geographic area of the scale of a typical dense population center, which in turn reduces frequency coordination problems and permits sharing of spectrum with terrestrial networks. Enough power is available from the aircraft power bus to the communications network equipment onboard to allow broadband data services with small user terminals, even during storms with high rainfall rates.

3. The HALO Network concept

3.1. Overview

Many types of organizations -- schools, hospitals, doctors' offices, and small to medium-size businesses -- around the world will benefit from the low pricing of broadband services provided by the HALO Network. Standard broadband protocols such as ATM and SONET will be adopted to interface the HALO Network as seamlessly as possible. The gateway to the HALO Network will provide access to the Public Switched Telephone Network (PSTN) and to the Internet backbone for such services as the World Wide Web and electronic commerce. The gateway will provide to information content providers a network-wide access to a large population of subscribers.

3.2. Desirable features

Some desirable features of the HALO Network include the following:

3.3. Service attributes

Various classes of service can be provided to subscribers sharing the bandwidth of a given beam, for example, 1 to 10 Mbps peak data rates to small businesses, and 10 to 25 Mbps peak data rates to business users with larger bandwidth appetites. Because each link can be serviced according to "bandwidth on demand," the bandwidth available in a beam can be shared between sessions concurrently active within that beam. While the average data rate may be low for a given user, the instantaneous rate can be grown to a specified upper bound according to demand. A dedicated beam service can also be provided to those subscribers requiring 25-155 Mbps.

3.4. Network access

Various methods for providing access to the users on the ground are feasible. In one approach, each spot beam from the payload antenna serves a single "cell" on the ground in a frequency-division multiplex fashion with 5-to-1 frequency reuse, four for subscriber units and the fifth for gateways to the public network and to high-rate subscribers. Other reuse factors such as 7:1 and 9:1 are possible. Various network access approaches are being explored. The talk will summarize a few representative examples.

Cell Coverage by Frequency Division Multiplexing Using Spot Beams

3.5. Network services

The HALO node can provide a multitude of connectivity options. It can be used to connect physically separated Local Area Networks (LANs) within a corporate intranet through frame relay adaptation or directly through LAN bridges and routers; or it can provide videoconference links through standard T1 interface hardware. The HALO Network may use standard SONET and ATM protocols and equipment to minimize the cost of the equipment and to take advantage of the wide availability of such components.

4. HALO Network architecture

At the apex of a wireless Cone of Commerce, the payload of the HALO aircraft serves as the hub of a star topology network for switching data packets between any two user terminals within the service footprint. A single hop with only two links is required, each link connecting the payload to a subscriber. The links are wireless, broadband, and line of sight. Single link delays range from ~60 msec under the airplane to ~200 msec at the edge of the signal footprint. Information created outside the service footprint is delivered to a subscriber's terminal through terminals operated by businesses, Internet Service Providers (ISPs), or content providers within that region, and through the HALO Gateway (HG) directly connected to distant metropolitan areas via leased trunks. Again, only two air links are required for terminal-to-terminal communications via the node in the stratosphere. The HG is a portal serving the entire network. It allows system-wide access to content providers or advertisers, and it allows any subscriber to extend their communications beyond the HALO Network service area by connecting them to dedicated long-distance lines such as inter-metro optical fiber.

High rainfall rates can reduce the effective data throughput of the link serving a given subscriber. Angel plans to ensure the maximum data rate more than 99.7% of the time and to provide an acceptable minimum data rate more than 99.9% of the time. Angel also plans to limit outages to small areas (due to the interception of the signal path by very dense rain columns) to less than 0.1% of the time. The plans are to locate the HG close to the HALO orbit center to reduce the slant range from its high-gain antenna to the aircraft and correspondingly its signal path length through heavy rainfall. The link margin requirements have been assessed and are thought to be achievable due to having high power available for the airborne equipment.

5. HALO aircraft

The HALO aircraft is being flight-tested in Mojave, California. The first flight was accomplished there in July 1998 and the flight envelope is being steadily expanded. The aircraft has been specially designed for the HALO Network and it can carry a large pod suspended from the underbelly of its fuselage. The HALO aircraft will fly above the metropolitan center within a volume of airspace, a distorted torus with a diameter of eight nautical miles or less. The pod containing the antenna array interfaces to the fuselage via a pylon through which power and coolant flow. If encountering a persistent wind at altitude, the aircraft will vary its roll angle as it attempts to maintain its station. Various antenna concepts allow the signal footprint to be maintained on the ground as the airplane rolls. The talk will describe alternative schemes for aircraft-fixed beams and ground-fixed beams.

6. Communications pod

The HALO Network will use an array of narrow beam antennas on the HALO aircraft to form multiple cells on the ground. Each cell covers a small area, e.g., several to several tens of square miles. The wide bandwidths and narrow beamwidths of each beam or cell are achieved by using MMW carrier frequencies. Small aperture antennas with high gains can be used at opposite ends of the subscriber link, corresponding to the user terminal and the airborne antenna. A description of the network equipment was given in a prior technical paper.1 Updated details will be shared in this talk.

7. Subscriber units (user terminals)

The user terminal entails three major sub-groups of hardware: the radio frequency unit (RU), which contains the MMW Antenna and MMW Transceiver, the Network Interface Unit (NIU), and the application terminals such as PCs, telephones, video servers, video terminals, etc. The RU consists of a small dual-feed antenna and MMW transmitter and receiver mounted to the antenna. An antenna tracking unit uses a pilot tone transmitted from the HALO aircraft to point its antenna at the airplane. The antenna tracks the airplane with a mount possessing low-rate two-axis gimbals. Other schemes for performing the auto tracking function are feasible and appear to be competitive in cost. The high-gain antenna is protected beneath a radome from wind loading and the weather.

The MMW transmitter accepts an L-band intermediate frequency (IF) input signal from the network interface unit (NIU), translates it to MMW frequencies, amplifies the signal using a power amplifier to a transmit-power level of 100 - 500 mW, and feeds the antenna. The MMW receiver couples the received signal from the antenna to a Low Noise Amplifier (LNA), down converts the signal to an L-band IF, and provides subsequent amplification and processing before outputting the signal to the NIU. The MMW transceiver will process a single channel at any one time, perhaps as narrow as 40 MHz. The particular channel and frequency are determined by the NIU.

The NIU interfaces to the RU via a coax pair that transmits the L-band TX and RX signals between the NIU and the RU. The NIU comprises an L-band tuner and down converter; a high-speed demodulator; a high-speed modulator; multiplexers and demultiplexers; and data, telephony, and video interface electronics. Each user terminal can provide access to data at rates up to 51.84 Mbps each way. In some applications, some of this bandwidth may be used to incorporate spread spectrum coding to improve performance against interference (if so, the user information rate would be reduced).

The NIU equipment can be identical to that already developed for LMDS and other broadband services. This reduces the cost of the HALO Network services to the consumer since there would be minimal cost to adapt the LMDS equipment to this application and we could take advantage of the high volume expected in the other services. Also, the HALO RU can be very close in functionality to the RU in the other services (like LMDS) since the primary difference is the need for a tracking function for the antenna. The electronics for the RF data signal would be identical if the same frequency band is utilized.

8. Flight demonstration results

Raytheon and Angel recently conducted a demonstration of the first commercial wireless broadband link from ground to a moving aircraft, a 50-mile round trip connection of 52 Mbps (OC-1 rate). The following services were demonstrated over this wireless link: T1 access, ISDN access, web browsing, high-resolution videoconferencing, large file transfers, and Ethernet LAN bridging.

Subsequent flight demonstrations are planned to occur on the HALO aircraft in 1999. Higher data rates may be employed and multiple ground terminals will be used. On-board switching will allow any terminal to communicate with any other terminal.


The HALO Network will provide wireless broadband communications services. The feasibility of this network is reasonably assured due to a convergence of technological advancements. The key enabling technologies at hand include GaAs (Gallium Arsenide) RF modules operating at MMW frequencies, ATM/SONET technology, digital signal processing of wideband signals, video compression, ultra-dense memory modules, lightweight aircraft technology including composite airframes and small fanjets capable of operating reliably at low Mach and low Reynolds numbers. These technologies are available, to a great extent, from vendors targeting commercial markets. The HALO Network is predicated on the successful integration of these technologies to offer communications services of high quality and utility to small and medium-sized businesses at reasonable prices. The regulatory climates of the FAA and the FCC are favorable. While a variety of broadband access modalities are promising for the U.S. market, the HALO Network may be a winner for "green field" deployment, especially in regions where the existing infrastructure is not amenable to an upgrade or retrofit.


The author acknowledges key contributions from Leland Langston of Raytheon Systems Company, George Chadwick of Deskin Research Group, and Doug Lockie of Endgate Corporation. Burt Rutan and his team at Scaled Composites are inspiring innovators of world-class stature.


  1. J. Martin and N. Colella, "Broadband Wireless Services from High Altitude Long Operation (HALO) Aircraft," Proc. of the SPIE International Symposium on Voice, Video, and Data Communications: Broadband Engineering for Multimedia Markets, 1997.
  2. N. Colella and J. Martin, "The Cone of Commerce," Proc. of the SPIE International Symposium on Voice, Video, and Data Communications: Broadband Engineering for Multimedia Markets, 1997.
  3. G. Djuknic, J. Freidenfelds, et al., "Establishing Wireless Communications Services via High-Altitude Aeronautical Platforms: A Concept Whose Time Has Come?" IEEE Communications Magazine, September 1997.


Dr. Nicholas J. Colella is the Chief Technology Officer of Angel Technologies Corporation. In prior years, he held senior technical positions at Lawrence Livermore National Laboratory. He invented the RAPTOR/TALON theater ballistic missile defense concept and served as DOD's executing agent for pioneering low-cost, high-altitude, long-endurance unmanned aircraft; high-mass fraction kinetic kill interceptors; and electro-optics and communications systems. He co-created Brilliant Pebbles, led LLNL's spacecraft design and survivability projects, and developed one-steradian wide field of view (WFOV) cameras employing spherically concentric refractive optics for tracking satellites and space objects. He is a founding partner of a multi-chip module company and the National Robotics Engineering Consortium at Carnegie Mellon.

James Martin is the lead systems engineer for the HALO Network equipment under development at Raytheon Systems Company for Angel Technologies. At AT&T Bell Labs, he developed cellular wireless telecommunications equipment and underwater fiber optic transmission systems. Mr. Martin has recently published a "Systems Engineering Guidebook" with the CRC Press. His specialty is systems engineering management, systems architecting, and the total systems engineering process.

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