Airborne
Emergency
Communications
Node Technical
Approaches
National Conference on Emergency Communications
December 12-13, 2005
Dr. Chris Hawkins
Chief Technologist of Unmanned Systems
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Northrop Grumman Corporation
High-Level Requirements for Airborne Relays
 Support communications systems of all responders
–
–
–
–
Police, fire, medical, power, water, roads, National Guard, Coast Guard, etc.
Voice, text, and data communications for all appropriate communication systems
Possibly include commercial cellular
Interface to public telephone networks and internet
 Provide regional coverage with large numbers of relayed
communication channels
– Regional areas spanning 200 miles or more
 Will require far more comms channels than a single ground based repeater or base
station
– Potentially hundreds to thousands of simultaneously active relayed channels
– Support channel frequency reuse over the region by dealing with inherent radio
frequency interference problems
 Support any geographic region on short notice
– Self-deploying in all weather conditions is best, with sufficient speed to overcome
–
–
headwinds
Ability to fly through national airspace on short notice
Best to not be dependent on a high data rate line-of-sight or satellite link to a
ground station
 Gateways, bridges, routers, and some data servers would need to be located on-board
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System Solution Range
 This briefing will focus on high capacity regional coverage approaches
– Lower capacity solutions are subsets of this, and the technical
issues are common to all
High Capacity,
Airborne Node
Regional Coverage
Low Capacity, Local Coverage
Airborne Node
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Satellite
Low Capacity,
Airborne Node
Regional Coverage
Multiple Low Capacity, Local Coverage
Airborne Node
Airborne Node
Typical Signal Characteristics
 Frequency Division Multiplexed (FDM)
– A comms channel is a frequency of a specified bandwidth and modulation
– The frequency is dedicated or dynamically allocated (trunked)
– Base method for Project 25
– Well suited for long range relay
 Time Division Multiple Access (TDMA)
– Proprietary systems and in consideration for Phase 2 of Project 25
– Often not well suited for long range relay to due timing issues
 Simplex: transmit and receive on same frequency
– Transmit or receive (not both) at a given time
– Direct handset to handset. No repeater involved
 Half-duplex: transmit on one frequency, receive on another
– Transmit or receive (not both) at a given time
– Signal retransmitted by a repeater or base station
– Typical land mobile radio used by public safety
 Full-duplex: transmit on one frequency, receive on another
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– Transmit and receive at the same time
– Signal retransmitted by a repeater or base station
– Typical of cell phones
Typical Signal Characteristics (2)
 Trunked systems
– The communications channel is assigned dynamically to maximize
spectrum usage efficiency. The user’s radio requests a comms
channel via a known control channel when needed. All radios in
the call group are sent the comms channel when it is assigned.
– Common in Land Mobile Radio Systems, and included in Project 25
 Modern Commercial Cellular
– CDMA
 Well suited for long range operation – 185 km theoretical maximum
– GSM and iDEN
 Both are TDMA signal types – problems for long range operation due
to timing issues
 GSM is limited to 35 km maximum at normal rates, and 70 km at half
rate
 Military Comms
– Of the types that would be used for disaster response, well suited
to long range operation
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Background: Land Mobile Radio Repeaters
G3
G1
Repeater,
(Relay, Base
Station)
 High power repeater on a tall
tower allows mobile units to
communicate better
–
–
Mobiles don’t need much power
High tower gets over many
obstacles
F1: Mobile Transmit Frequency
F2: Mobile Receive Frequency
Gn: Group of transmit and receive
frequencies assigned to a
repeater
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G1
G6
G4
G2
G2
G7
G5
G3
 Cellular pattern of frequency group
assignments on a map (idealized)
 Same frequency groups are separated by
enough distance that the signal loss is
sufficiently great that no interference
occurs.
 Cells are connected by landline or
microwave backhaul links
 Allows greater total capacity than high
power mobile radios
Airborne Repeater Frequency Reuse Challenge
 Airborne repeater sees many
cells at once
– Signal loss is much higher near
G3
G1
G1
G4
G2
G2
G7
G5
6
–
G6
G3
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the ground than well above the
ground, thus limiting range of
ground based radios and
repeaters, but the range for an
airborne repeater can be very
long
Radio path to airborne repeater
also has fewer obstacles
(buildings, mountains, etc.)
Airborne repeater must either
 use a group of frequencies that is
not repeated for a very long range
(limiting the number of channels
for the region – low capacity)
Or
 Create cells on the ground using
directional antennas or antenna
arrays (higher capacity)
Frequency Reuse using Beams
Regional
Coverage
Area
G3
G4
G3
G1
G2
G1
Frequency Group Reuse in Different Beams
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Smaller (More) Beams = greater frequency reuse
= higher regional capacity
G2
Antennas
Antenna Type
Characteristics
Omnidirectional
Blade



Reasonably uniform response in azimuth
Comparatively low antenna gain
Simple to mount
Panel (or sector)

Can size to select vertical beamwidth separately from horizontal
beamwidth
Good gain response




Yagi


Long and narrow – points towards the signals
High gain and narrow beamwidths possible

Useful for mounting locations where wide or tall antennas are not
appropriate, but length is available
Example: 30 degree beamwidth at 800 MHz band (35 x 7 inches)

Parabolic Dish
(circular or
rectangular)





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Shallow in depth towards the radio signal direction (mount in
sides of aircraft)
Needs to be tall to obtain narrow vertical beamwidths, or wide for
narrow horizontal beamwidths
Example: 16 degree vertical by 45 degree horizontal beamwidths
at 800 MHz band (Dimensions: 48 x 15 inches)
Can size to select vertical beamwidth separately from horizontal
beamwidth
Generally best gain response for a given width and height.
Deeper than panel antennas in direction of signals.
Needs to be tall to obtain narrow vertical beamwidths, or wide for
narrow horizontal beamwidths.
Awkward to mount many of them on an aircraft.
Antenna Arrays
 VHF Frequency Bands (30 to 300 MHz = 1m to 10m
wavelength)
– Blade antennas mounted over a large area on bottom of aircraft
– Antennas combined with amplitude and phase weighting to create
directional beams
– Needed only if frequency reuse in VHF bands is desired
 UHF
– Antenna arrays would only be used in the low end of UHF (300 to
500 MHz) only if frequency reuse is desired and if the directional
antennas for that band are too large to be mounted on the selected
aircraft.
– Advanced Electronically Steered Arrays (AESA)
 Panel of individual antenna elements that are electronically controlled
and weighted with amplitude and phase offsets to create multiple
steerable beams from a single array
 Very advanced and flexible, but also expensive
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Some Frequency Management Issues
 If beams used, need to steer and/or switch antennas to
keep each frequency group on the same ground cell
location.
 Airborne relay can’t reproduce the same cellular
pattern of the ground stations
– Must coordinate frequency sets with local authorities to avoid
interference
 Desirable to have well separated blocks of uplink
frequencies (mobile to repeater) from blocks of
downlink frequencies (repeater to mobile)
– Same as commercial cellular does
– Allows for an overall smaller, lighter weight equipment package on
the aircraft by using wideband technology
 Fewer analog filters and power combiners
 Allows many more relay channels than individual narrowband
transceivers
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High-Level Architecture Concept
Non-Ethernet Control and Data
Airborne Comms Node
Comms Node Controller
Network Manager
Voice/Data Routing
Data Traffic Bridging
Gateways
Ethernet Switch
Voice Digitization
and Packetization
(VoIP)
Commercial
Cellular
Switch/Controller
Analog Audio
LMRS
Radios/Repeaters
(Project 25 Compliant)
Military
Narrowband
Radios/Repeaters
Commercial
Cellular
Base Stations
Wideband
Data Links
(e.g., Ku SATCOM,
TCDL, video links, etc.)
VHF/UHF Antenna Interfaces
(Diplexers, Filters, Low Noise Amplifiers, Splitters,
RF Switch Matrix, High Power Amplifiers as needed, etc.)
Wideband Link
Antenna Interfaces
VHF/UHF Antenna Group
Wideband Link Antenna Group
Voice and lower-rate data users
(data can include imagery at limited download speed)
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Servers
(Information Databases,
Chat, Imagery, etc.)
High Data Rate Users & Networks
(Command Centers, imagery and video users, public
telephone network interfaces, Internet Interface,
backhaul relays, etc.)
Concept Points
 Recommend using a federated architecture
– Integrate multiple vendors’ radio and controller systems rather than try to build a
–
tightly integrated custom system that “does it all”
Keeps development and upgrade costs lower overall
 Focus on Project 25 standards for public safety land mobile radio
– Support of proprietary systems would be very complicated or cost prohibitive due
–
to the number of types
Rapid add-on proprietary systems could be considered to deal with specific regions
 Use Ethernet with Internet Protocol (IP) as the backbone for the system
– Bridge all comms gear to Ethernet/IP (Voice using Voice over IP)
 On-board controllers should be capable of operating the node without
high-rate data links to the ground
– For example, the commercial cellular system should be able to operate as a standalone network if necessary, and dynamically reconnect to the ground network as
data links are available.
 Encryption units and trusted gateways/filters were not shown in the
high-level block diagram, but can be included
– Caution: encryption can cause communications problems if not setup carefully, and
it must not jeopardize disaster response
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Sharing with the Military
 Only those military radios and comms systems that might be
needed for disaster response operations need to be
accomodated
 However, the military also has needs for airborne
communications nodes, but with additional military comms
equipment
– An experimental comms relay node program is in progress, but with a
limited number of simultaneous channels
 Sharing with the military could offset the cost of the aircraft(s)
and maintenance
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Aircraft Considerations
 High flying (> 50,000 feet)
– Advantages:
(e.g., Global Hawk, Proteus, U2, WB-57, etc.)
 Above the weather allows for constant comms coverage during hurricanes and
thunderstorms
 Above commercial air traffic means no conflict with other aircraft and hence optimal
comms relay flight paths
 Better angle of incidence to the ground radios reduces radio signal blockage by obstacles
and reduces ground signal loss
– Disadvantages:
 Generally more costly than low flying aircraft
 Exposed to more radio interference
 Long endurance (24+ hours) recommended
– Reduces the number of aircraft needed due to less time spent flying to and from
–
airbase
Reduces how often comms interrupts occurs when aircraft are swapped
 Velocity > 250 mph recommended
– Can self deploy from long distances if also long endurance
– No local airbase required (airbase could be 2000 miles away)
– Can combat high velocity headwinds
 Weight and Power: A high capacity system will likely weigh 1000 to 3000
pounds, and consume 5,000 to 15,000 Watts.
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Summary
 High capacity airborne comms relay nodes are possible
– High capacity can be achieved using directional antennas and frequency
–
reuse
Low capacity has the same node architecture and radio interference
issues, but simply to a lesser degree
 Rapid response and all weather operations are achievable
 Communications node should be standards based with a flexible
interconnect backbone
– Project 25 standard for public safety and public works systems
– Commercial cell phone support
 CDMA best suited due to long distance relay issues
– Standard military radios for National Guard, Coast Guard and other
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–
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military support
Commercial and military standard wideband data links
Ethernet/IP backbone using Voice over IP and data bridges