Joseph A. Ishac, Dennis C. Iannicca, and Kurt A. Shalkhauser

Glenn Research Center, Cleveland, Ohio

Brian A. Kachmar

Vantage Partners, LLC, Brook Park, Ohio

Control and Non-Payload Communications (CNPC)

Prototype Radio—Generation 2 Flight Test Report

NASA/TM—2014-218391

November 2014
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Joseph A. Ishac, Dennis C. Iannicca, and Kurt A. Shalkhauser

Glenn Research Center, Cleveland, Ohio

Brian A. Kachmar

Vantage Partners, LLC, Brook Park, Ohio

Control and Non-Payload Communications (CNPC)

Prototype Radio—Generation 2 Flight Test Report

NASA/TM—2014-218391

November 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The National Aeronautics and Space Administration thanks the faculty and staff of the Avionics Engineering Center at the

Ohio University Russ College of Engineering for their generous logistical and institutional support, enabling us to successfully

install and operate the Albany, Ohio ground station described in this report.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 

This report is a formal draft or working 

paper, intended to solicit comments and 

ideas from a technical peer group.

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
Summary 

NASA Glenn Research Center conducted a series of flight tests for the purpose of 

evaluating air-to-ground communications links for future unmanned aircraft systems 

(UAS). The primary objective of the test effort was to evaluate the transition of the 

aircraft communications from one ground station to the next, and to monitor data flow 

during the “hand-off” event. To facilitate the testing, ground stations were installed at 

locations in Cleveland, Ohio and Albany, Ohio that each provides line-of-sight radio 

communications with an overflying aircraft. This report describes results from the flight 

tests including flight parameters, received signal strength measurements, data latency 

times, and performance observations for the air-to-ground channel. 

NASA/TM—2014-218391 1

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Introduction 

NASA is currently working with the Federal Aviation Administration (FAA), RTCA Inc. (a 

Federal Advisory Committee), and other organizations to develop technologies and 

procedures to allow Government (public) and commercial (civil) unmanned aircraft (UA) 

to operate safely in the National Airspace System. An essential element of this new 

national capability is the radio communications channel linking each UA to a network of 

fixed ground stations (GSs). Data communication necessary for flight is referred to as 

control and non-payload communication (CNPC) and is exchanged over the UA and GS 

channel to ensure safe, reliable, and effective UA flight operation. 

In Phase I, NASA Glenn Research Center (GRC) performed flight testing using Generation 

1 prototype CNPC radios to characterize the performance of the air-to-ground (AG) 

channel operating in the 960-977 MHz frequency range (L-band). [1] Results indicated 

that a significant separation distance could be achieved between a single aircraft and a 

single ground station while maintaining a reliable data connection. Channel bandwidth, 

transmitter output power level, and the impact of terrain obstructions on line-of-sight 

(LOS) communications were studied under the Phase I flight testing campaign. 

This report presents results on a Phase II system utilizing two spatially-separated ground 

stations communicating with one overflying aircraft. The principle objective of this test 

campaign was the evaluation and characterization of an in-flight “hand-off” event where 

the aircraft communication path transfers from one ground station to another with 

minimal interruption to the CNPC data flow. This transfer also required development of 

an automated system of ground terminals with the associated routing protocols to 

provide network mobility for a seamless hand-off. Other objectives of the Phase II test 

campaign were to demonstrate remote-controlled operation of multiple ground stations 

in a network environment. Data acquired from this test campaign will be used in 

subsequent UA flight test experiments of increasing complexity. 

NASA/TM—2014-218391 2
The Phase II flight tests utilize “Generation 2” software-defined radios in both the 

aircraft and ground systems to transmit and receive the CNPC signals. The radios were 

developed under cooperative agreement NNC11AA01A between the NASA Glenn 

Research Center and Rockwell Collins, Inc., of Cedar Rapids, Iowa. Descriptions of the 

radio modulation, signal waveforms, and operating modes are presented in [1]. The 

radios are prototypes only and were designed to address the initial “seed” requirements 

from RTCA, Inc., Special Committee 203 (SC–203)—a Federal advisory committee 

developing standards for UA performance requirements. For this series of flight tests, 

the Generation 2 prototype radio remains similar to the Generation 1 model, especially 

in terms of radio frequency (RF) operations. Changes made to the radio include 

modifications to the status and control interface, radio-to-radio link health reporting 

functions, and other “housekeeping” adjustments, resulting in only small changes to the 

over-the-air message overhead content. 

This report presents results from the networking hand-off tests. The data presented are 

intended only as an early gauge of the general capabilities of the CNPC links. On-going 

iteration of system and radio software and firmware are underway to optimize CNPC 

data throughput rates. Those improvements, along with detailed discussion and test 

data from data header compression and data security flight tests, will be presented in 

subsequent reports. 

Test System Description 

The major elements of the Phase II CNPC radio test system are displayed in Figure 1. 

The system consists of two identical GSs, one installed at the GRC Lewis Field campus in 

Cleveland, Ohio and one installed at the Ohio University Gordon K. Bush Airport in 

Albany, Ohio (airport designation: KUNI). Each GS has a network connection to the 

Internet through hardwired connections for remote control, monitoring, programming, 

and test operations. Both GSs have automatic fail-over switching allowing them to use a 

3G/4G cellular wireless connection if the Internet service at the GS site is interrupted. 

Using this arrangement, both GSs are networked to the GRC UAS laboratory where the 

test flights and data collection are centrally managed. 

NASA/TM—2014-218391 3
Figure 1. - Test system overview 

Ground Stations 

The ground stations used in the Phase II test campaign were designed to meet several 

primary requirements. First, the GSs had to be sufficiently compact, where each one 

could be transported by a small crew and installed into a potentially small or confined 

space. With ground stations possibly being installed at various locations around the 

country, surviving the shipping process and deployment into harsh environments is 

necessary. Thus, the ground stations are packaged in a ruggedized enclosure to protect 

the GS electronics during shipment and operation. The second requirement was for 

remote controllability. With the ground stations separated by 133 nautical miles (nmi), 

the ability to control stations remotely from one location would eliminate the need for 

deployed personnel. The third requirement was for expandability. Currently each 

prototype radio can only act as a single receiver. While the current tests utilize only one 

L-Band radio per ground station, future builds must support simultaneous operations of 

additional radios in order to support multiple aircraft. 

NASA/TM—2014-218391 4
Figure 2 illustrates the final arrangement of components in the ground station. To keep 

the GS physical size manageable but still provide for expandability, the smallest readilyavailable components were chosen. To provide uninterrupted electrical power to the 

GS electronic equipment, a 120V mains power supply was identified that would require 

only two “rack units” (2U) of height. A 28-volt direct current power supply is used to 

drive the radios and occupies only 1U in the equipment enclosure. A 1U computer was 

installed with sufficient processing power to satisfy the control and data-gathering 

needs of the GS. An Ethernet switch, firewall/gateway, and 3G/4G cellular modem were 

consolidated onto a single shelf occupying 1U of rack space. To accommodate all of the 

current Phase II GS electronics and to provide space for additional future equipment, a 

14U tall equipment rack was required. 

The radios are installed onto the rack by using a radio tray. Each tray supplies power, 

cooling, and connectivity for up to two radios. Currently, each tray contains one L-Band 

and one C-Band radio. The 28Vdc power supply provides up to 25.7 amperes to easily 

drive up to four CNPC radios. A second, identical power supply will be added to the GS 

for later phases of flight testing when more than four radios will be in operation. An 

eight-port Ethernet switch isolates the radio network, allowing the master radio to 

communicate with any slave radios should they be present. 

A ruggedized polyethylene transport case was selected to enclose the GS equipment. 

The case included a built-in rack-mount rail system on shock-absorbing mounts, 

waterproof covers, lifting and tie-down hardware, and wheels. The internal rack frame 

accommodates 14U of equipment height at the standard 19” equipment width. 

Figure 2. - Ground station component arrangement

NASA/TM—2014-218391 5
Figure 3 shows the GS equipment in the transport case at a typical installation site. All 

antenna, power, and network cabling exiting the case to external locations is visible at 

the lower right. Waterproof covers are shown in stowed positions on the sides of the 

case, allowing front and rear ventilation for the GS equipment. Only one CNPC radio 

tray is installed in the case at this time. 

Figure 4 shows electronic and electrical connections within each ground station. A 

majority of the equipment is electronically interconnected with RJ-45 Ethernet cables 

for control operations and data collection. 

Figure 3. - Transportable ground station installed at Ohio University Airport

Figure 4. - Ground station internal connections

NASA/TM—2014-218391 6
The ground station connects to the Internet through either a hard wired host-site 

network or a 3G/4G cellular network. The firewall/gateway is responsible for managing 

the networks provided by the 3G/4G connection and host network. In situations where 

both exist, the gateway will utilize the cellular network as a backup. Should the host 

network experience and outage or other disruption, the gateway will automatically 

switch to the cellular connection. The cellular modem is selected to provide data 

access using one of several cellular service providers, depending on service available at 

the specific GS location.

The uninterruptable power supply (UPS) delivers 120-volt power to three load circuits, 

two of which are independently controllable over the network. The Load 1 circuit feeds 

the power supply for the radios and a xenon safety strobe light. The light provides local 

visual indication that a radio transmitter is enabled and that personnel should maintain 

a safe distance from the antennas. The Load 2 circuit is connected to the Ethernet 

switch and the computer, which allows for power cycling (yielding a reboot of the 

computer) if necessary. The Load 3 circuit, which is not switchable, provides continuous 

power to the cellular modem and the firewall. In the event of a prime power (mains) 

loss at the GS installation site, the 1350 watt UPS has battery capacity to provide backup power to the ground station for approximately thirty (30) minutes. During this time, 

operators would secure all test data and perform a controlled shut-down of the network 

and computer. 

The Global Positioning Satellite (GPS) antenna feeds a power divider which provides a 

GPS signal to the CNPC radio(s) and cellular modem. The CNPC radio is used as a time 

server for the GS computer. This allows the GS to be synchronized with itself as well as 

with the other GSs and test equipment at the various test locations. This 

synchronization is necessary for measuring data flow latency throughout the GS 

network. 

The computer has an Intelligent Platform Management Interface (IPMI) that permits 

remote monitoring and control of the computer without the use of the operating 

system. In the event of a computer failure, the GS systems can be powered down, 

rebooted, and have errors logged via IPMI. 

Ground Station Site Selection 

Demonstration of in-flight CNPC hand-off events required installation of a minimum of 

two ground stations separated by approximately 138 nmi (153 miles). This separation 

NASA/TM—2014-218391 7
corresponds to twice the recommended nominal GS coverage radius of 69 nmi [3]. This 

arrangement would allow each GS radio to be operated at its full transmit power level 

for communications throughout its individual coverage area, and would additionally 

create a flight region where LOS CNPC links could be established with both ground 

stations simultaneously. 

Site selection for each GS involved multiple considerations to ensure that test 

requirements were met. Frequency authorizations for the area including most of Ohio, 

West Virginia, and western Pennsylvania have been obtained allowing some flexibility in 

site selection. The ability to operate the GSs by remote control also provided an 

increased number of placement options. Locations at or near airport property were 

desired (but not essential) in order to represent actual in-service operation. Flight 

planning also influenced GS site locations. Since test flights were anticipated to require 

non-standard and repeated flight maneuvers near the GS, isolated locations with low air 

traffic were preferred. This consideration holds true for the corridor between the two 

GSs, where straight and level flight was important for data collection. Terrain features 

were considered as well in order to avoid obstruction of the LOS signal path between 

the GS and aircraft. Once a site is selected, the GS equipment rack must be placed in a 

strategic location. The fundamental requirements included a 120V, 15-amp electrical 

power source, an Internet connection, a secure, indoor location near an exterior wall, 

and an antenna mounting location within 60 feet of the equipment rack. 

GS1 was installed into an existing building in the western area of NASA’s Glenn Research 

Center, adjacent to the Cleveland Hopkins International Airport (KCLE). Electrical power 

and Internet connections were readily available, as was access to an adjacent 50-foot 

tower for antenna installation. The close proximity of the GS installation site to the GRC 

UAS control center, approximately 0.25 miles, allowed for convenient GS servicing, if 

needed. 

GS2 was installed into an existing aircraft hangar at the Ohio University Gordon K. Bush 

Airport in Albany, Ohio. An indoor mezzanine provided the necessary electrical 

connections and allowed easy access to the roof. Ohio University and the Avionics 

Engineering Center have a strong experience base in air-ground testing, so their facilities 

and staff were well-equipped to support the NASA GS installation. The Ohio University 

met all criteria for the GS equipment installation, including placement of the equipment 

in a well-secured area. 

NASA/TM—2014-218391 8
Figure 5 shows the relative p

separated by a distance of 13

radius of a candidate CNPC c

of 187° relative to the GRC G

in Figure 5 repeatedly during

Four omnidirectional antenn

the GS. Each installation incl

transmit/receive antenna, on

one broadband cellular trans

available products with stand

placement of the transportab

order to minimize the length

The antennas at the OU GS w

aircraft hangar, achieving an 

The antennas at the GRC GS 

an installed elevation of 42 fe

Figure 5. - Locations of ground

Figure 6. - Antenna installation at Ohi

in Albany, Ohio.

position of the GSs within the state of Ohio. The

33 nmi, which represents approximately two tim

coverage area [3]. The OU GS lies on a true vect

GS. The NASA S-3B aircraft traversed the flight p

g the test campaign. 

nas are installed on masts at each location and c

luded one L-Band transmit/receive antenna, on

ne Global Positioning System (GPS) receive ante

smit/receive antenna. The antennas are all com

dard mounts and connectors. Each GS location 

ble electronics case to be within 60 feet of the a

h of the cable runs and amount of signal attenua

were mounted on an elevated platform above th

installed elevation of 47 feet above ground lev

were mounted to an existing 50-foot, open-fram

eet above ground level (figure 7). 

d stations within the State of Ohio. Separation distance 133 n

io University Airport Figure 7. - Anten

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NASA/TM—2014-218391 9
Aircraft 

The Lockheed model S–3B Viking aircraft owned by Glenn Research Center (registration 

number N601NA) was used for all Phase II flight tests. This airborne platform is the 

same one used in channel sounding measurements [2] and in the CNPC Phase I flight 

tests [1]. The aircraft offers significant electrical power, pressurized area for the radio 

and test equipment, significant flight duration (>4 hrs), room for two experimenters, 

and provides excellent flight stability and maneuverability. 

The aircraft equipment consisted of a Rockwell Collins radio, spectrum analyzer, GPS 

time server, two computers, and networking equipment. These components were 

mounted in two 19-inch wide equipment racks in the rear of the aircraft. An aircraftrated, low-profile, omnidirectional antenna was used for the CNPC radio. The antenna 

was connected to the radio by a custom-length, shielded coaxial cable with low insertion 

loss, allowing the radio system to be operated without the need for additional signal 

amplification. 

The L-band antenna was mounted on the bottom surface of the aircraft. The CNPC 

antenna was physically separated from all other antennas by a distance of several 

wavelengths to reduce antenna-to-antenna coupling. When in level flight with the 

aircraft landing gear in stowed position, the flattened contour of the bottom of the 

aircraft provided a clear LOS path to the ground station with little or no airframe 

obstruction. Figure 8 shows the location of the L-Band CNPC antenna and underside 

geometry of the S-3B test aircraft. 

Figure 8. - S-3B Viking test aircraft showing L-band antenna 

mounting location. 

Image © 2008 – TarmacPhotos.com [9], used with permission

NASA/TM—2014-218391 10
Network Description 

Overview 

The prototype CNPC network environment developed for the second generation radio 

was designed using Mobile IPv6 [4] with Network Mobility (NEMO) [5]. The architecture 

is comprised of three major components: the home network, the ground station, and 

the mobile node or aircraft (Figure 9). 

Figure 9. - Overview of network architecture used for phase II flight tests. 

A home agent (HA) is placed in the agency's “home” network and is used as a central 

communications point for contacting the mobile node. The home agent serves to 

maintain a binding database of a mobile node's current foreign network addresses, or 

care-of address. The bindings map the home address, a known address in the home 

network, to the shifting care-of address in the foreign network in order to transparently 

reroute traffic from external users. 

Each ground station (GS) serves as a broker providing a UAV with its foreign network 

IPv6 address. This is done by means of an IPv6 access router (AR) which communicates 

with the aircraft through the prototype radio. Router advertisements or other dynamic 

host configuration methods can then be used to provide the UAV with a network 

address. The ground station does not need to be owned by the same agency as the 

aircraft and will have address space associated with the terrestrial service provider used 

at that location. 

The aircraft communicates to the ground station using the same prototype radio. A 

mobile IPv6 router attached to the radio allows the NEMO network to communicate 

NASA/TM—2014-218391 11
with the home network seamlessly. This allows the various components behind the 

mobile router to be addressed directly by using their “home” address. Users would not 

need to know the addressing schemes used at the various ground stations. 

Internetworking 

In a production environment, communication between endpoints would utilize IPv6 

end-to-end, without the need for any encapsulation of traffic within legacy IPv4 packets 

for transmission across the Internet. However, the GRC perimeter network is currently 

unable to route IPv6 traffic natively.1

 In order to emulate a native IPv6 environment, 

“6in4” (IP Protocol 41) tunnels were created between the home agent and the ground 

station computer system to allow for utilization of the readily available IPv4 backhaul 

communications networks. While there are several alternative ways to configure the 

test architecture to transmit IPv6 packets across an IPv4 network, the 6in4 approach 

reduced configuration complexity and eliminated the need for devices between the 

home agent and ground station computer to be IPv6-compatible. 

The home network and the ground stations are each protected by a dedicated firewall 

system that serves as both an IPv4 network firewall as well as a VPN gateway to secure 

the transmission of data across the Internet. Both ground stations were designed to 

support a primary wired WAN connection along with a backup WAN connection 

provided by a 4G LTE router. Each GS utilizes an IPv4 IPSec VPN connection to 

communicate with the home network and carry the 6in4 traffic between the home 

agent and the ground station computer. 

Aboard the S-3B test aircraft are two research computers running Ubuntu Linux with 

multiple Ethernet interfaces connected into a switch. One system is configured as the 

mobile router and the other is a workstation statically configured with an address on the 

home network. The workstation routes all IPv6 traffic to the mobile router as a default 

gateway. By configuring the second research computer to a known home network 

address, users are able to communicate directly with the system regardless of what 

foreign network the aircraft is currently connected to. The aircraft computer will retain 

the same home network address as the mobile router’s care-of address moves from 

foreign network to foreign network. 

Addressing Scheme 

The Home Network and Ground Stations were designed to support a dual-stack 

IPv4/IPv6 environment in order to support direct communications with legacy devices 

1

 NASA GRC is currently working on implementing IPv6 in response to industry and federal mandate [6] 

NASA/TM—2014-218391 12
such as UPS battery backup network interfaces or IPMI management interfaces on the 

ground station computers that may not support IPv6. In addition, an IPv4 addressing 

scheme was needed to serve as an endpoint for the 6in4 tunnels between the Home 

Agent and the Ground Station computer. 

The ground station computers were configured to use the radvd daemon in order to 

support IPv6 neighbor discovery across the RF link by multicasting periodic Router 

Advertisement (RA) packets every 10 seconds.[7] The remote aircraft computer, upon 

receipt of one of these RA packets, would be able to self-configure its IPv6 Care-Of 

address and begin communicating with the ground station, and thus the home agent, at 

the network layer. 

Network Mobility Concept 

The Mobile IPv6 with NEMO design allows the aircraft to maintain seamless networklayer communications between the pilot’s ground control station and the onboard 

avionics systems as the unmanned aircraft transitions between ground stations. This 

design may result in a few dropped packets, but established sessions should continue on 

without needing to reconnect. This method eliminates the costly procedure of tearing 

down and re-establishing active sessions. 

In a fully implemented communications system, hand-offs would be performed 

automatically by software based on link conditions and other factors. For this phase of 

testing, all hand-offs were conducted manually onboard the aircraft by the flight 

researcher, as the exact conditions for automating the hand-offs are being explored. 

The process of performing hand-offs between ground stations was to first establish 

network communications with the new ground station, wait until an IPv6 address was 

acquired via the IPv6 neighbor discovery process, and finally terminate the network link 

to the original ground station. The mip6d process aboard the aircraft would sense that 

the active link was no longer available and use the newly established link to send a 

binding update to the home agent. 

Flight Testing 

The flight test campaign consisted of eight separate flights occurring over the period of 

April 11 through May 28, 2014. Each flight test consisted of multiple, preplanned 

aircraft maneuvers and flight path segments. The overall sequence of testing was to 

first examine the CNPC radio communications range and link performance using only 

the GRC-based ground station with an omnidirectional antenna. This data was used for 

NASA/TM—2014-218391 13
comparison to models and previous flight test data. Second, the radio communications 

range measurements were made with only the OU-based ground station. Once the RF 

link performance and slant range capabilities of the individual GSs were well 

understood, the GS’s were then simultaneously operated and the combined radio 

environment was evaluated. From this information a “hand-off region" was established 

where CNPC signals could be simultaneously communicated between both ground 

stations and the aircraft, and where hand-off testing could be conducted. 

Characterizing the digital data flow during the hand-off event was one of the primary 

objectives of the Phase II flight tests. The latency of the data packets through the CNPC 

radio link and the magnitude of any interruptions or loss of data during the hand-off 

event were of significant interest. As a result, the Phase II flight test plan included 

multiple periods of time where the aircraft would be flying in and through the hand-off 

region. Flight and ground data were sent to Rockwell Collins for a more detailed 

examination so that changes could be made before the Generation 3 radios are 

delivered. A summary of the Phase II flight tests is presented in Figure 10. 

Test 

Number Test Date Ground Terminal Name (location) 

Radio 

Configuration Objectives Comments 

1 4/11/2014 GS1 (GRC) 1 Characterize L-band performance of 

GS1 

Flight patterns selected to examine 

omnidirectional antenna performance. 

2 4/18/2014 GS1 (GRC) 2, 3 Characterize L-band and C-Band 

performance of GS1 

Flight patterns selected to examine 

omnidirectional antenna performance. 

3 4/21/2014 GS2 (KUNI) 5, 6 Characterize L-band performance of 

GS2 

Flight patterns similar to GS 1, selected to 

examine omnidirectional antenna 

performance at GS 2. 

4 4/22/2014 GS1 (GRC) 

GS2 (KUNI) 6 

Determine hand-off region, examine 

data latency and hand-off 

performance 

Flight patterns directly between ground 

stations, through hand-off region. 

5 4/23/2014 GS1 (GRC) 

GS2 (KUNI) 1, 4 

Examine data latency and hand-off 

performance, investigate interfering 

signal, monitor other ops modes 

Flight patterns directly between ground 

stations, through hand-off region. 

6 4/30/2014 GS1 (GRC) 

GS2 (KUNI) 1, 11, 12 Test firmware updates to improve 

latency. UDP and TCP tests 

Flight patterns directly between ground 

stations, through hand-off region. 

7 5/28/2014 GS1 (GRC) 

GS2 (KUNI) 1, 11 

Demonstrate multiple hand-offs and 

record full-system latency 

performance, perform pre-test of 

"sensed traffic" downlink, 

characterize C-band performance of 

GS2 

Flight patterns directly between ground 

stations, through hand-off region. 

8 5/29/2014 GS1 (GRC) 

GS2 (KUNI) 6 

Integrated test of "sensed traffic" 

downlink into LVC, characterize Cband performance of GS1 and GS2 

Flight patterns between ground stations, 

through hand-off region with moderate inflight re-direction for demonstration of 

avoidance software. 

Figure 10. - Flight test summary 

NASA/TM—2014-218391 14
A customized radio configuration was required for each flight test, depending on which 

ground stations were used or type of test. Each configuration explicitly arranged the 

radio’s CNPC waveform parameters, including uplink and downlink frequencies, modes 

of operation (command and control (C2) data, weather data, video data), and frame 

timing. For example, test flight #1 utilized only GS 1 and the aircraft radios. The radios 

operated both the uplink and downlink modes at a center frequency of 968 MHz and 

interleaved narrowband (75 kHz) and wideband (875 kHz) streams at a 10 Hz rate. A 

complete listing of the radio configurations utilized in these tests is included in Appendix 

A. A more complete description of radio waveform and formatting is presented in [1]. 

Aircraft Flight Path 

Flight plans were individually tailored to address the type of radio test being performed. 

Flights to characterize an individual ground station, such as that for test number one at 

Cleveland, Ohio, included a circular orbit at an approximately constant altitude and 

radial distance from the GS (Figure 11). This maneuver allowed a general assessment of 

antenna performance for a typical ground terminal installation, and identified the 

presence of any antenna pattern nulls or shadowing by ground obstructions. Once the 

circular orbit was completed, the pilot adjusted aircraft altitude and aligned for an over 

flight of the GS on an outbound vector heading of 195° (relative to magnetic north.) The 

aircraft then traveled directly towards GS2 along the vector at constant altitude and 

airspeed until the CNPC radio received signal strength dropped below sensitivity levels 

and the communications link was broken. The aircraft then reversed course and flew 

inbound along the same vector (magnetic compass heading 15°). 

An orbital flight path and inbound/outbound flights were also performed for flight test 

three at GS2 in Albany, Ohio (Figure 12). Small, “racetrack” patterns were executed 

when the CNPC signals fell below the sensitivity level, and were used to verify the slant 

range distance where signal drop-out occurred. Narrowband waveform modes 

generally provided greater range than did the wideband modes, resulting in two distinct 

drop-out orbiting areas. The orbit nearer to the transmitting GS (labeled as “DLvideo” in 

Figure 12) confirmed the signal range for the downlinked video stream. The orbit 

farther from the transmitting GS (labeled as “DLC2”) confirmed the signal range for the 

downlinked command and control data stream. 

NASA/TM—2014-218391 15
The air corridor between Cleveland, OH and Albany, OH was generally clear of air traffic, 

minimizing the need for en route flight path adjustments. The underlying terrain 

features and elevations along this path had been reviewed and modeled to determine 

necessary flight altitudes. Modeling and test flights confirmed that aircraft altitude of 

approximately 13 000 feet above mean sea level (MSL) would provide unobstructed LOS 

between the GS antennas and the aircraft over the entire flight path span. Weather and 

air traffic issues had only minor effects on the flights. This flight path shown in figure 13 

was used repeatedly during the Phase II campaign. 

Figure 11. - Aircraft flight track for characterization of ground 

station 1. Flight track created using GPS Visualizer [8], Map 

data from the National Atlas [10]

Figure 13: Ground Station 1 - to - Ground Station 2 flight track. 

Flight track created using GPS Visualizer [8], Map data from the 

National Atlas [10]

Figure 12. - Aircraft flight track for characterization of ground 

station 2. Flight track created using GPS Visualizer [8], Map 

data from the National Atlas [10]

NASA/TM—2014-218391 16
Flight Test Results

April 11, 2014 

The objective of the first flight test was to examine the general radiofrequency 

characteristics of the CNPC link between the S-3B and GRC ground terminal GS1. Flying 

the flight path shown in Figure 11, investigators could observe the general capabilities of 

the CNPC radio pair operating with L-band omnidirectional antennas. Figure 14 displays 

data for the entire flight period, from aircraft take-off through landing, with time 

annotated along the horizontal axis. Received signal strengths at the aircraft radio and 

at the GS1 radio are plotted along the vertical axis. Data traces for narrowband (75 

kHz), wideband (875 kHz), uplink, and downlink combinations are shown to illustrate the 

similarity in channel performance. Taking off from Cleveland Hopkins International 

Airport (airport designation: KCLE), the aircraft ascended above the local ground 

obstructions (buildings) allowing the received signal strengths to reach their maximum 

level at the “spikes” indicated at 15:54 UTC. The signal strength curves quickly dropped 

to the -85 to -100 dBm range where it remained for approximately 30 minutes during 

the 25 nmi circular orbit. 

The next four traces in figure 14 present data on the percentage of frame loss averaged 

over one-second at the ground and aircraft receivers. Individual traces for the UL1, 

UL20, DLC2, and DLvideo channels are shown. Where the CNPC communications path 

was transferring all data without error, the data are presented as 0-percent loss and no 

trace is visible on the grid. Where errors occurred in the radio link, the lost frame data 

created a visible trace ranging from 0 to 100 percent, with the latter representing a total 

loss of the radio link and clearly indicated by the heavy colored sections in the figure. 

With the exception of a blockage period near time 16:06, the observed CNPC link 

streams encountered minimal frame losses during the 25 nmi circular orbit, the 

outbound flight towards GS2, and the inbound flight towards GS1. Similar to previous 

flight tests, the UL20 and DLvideo channels were lost near 70 nmi slant range and the 

UL1 and DLC2 channels were lost at a slightly greater distance. 

The peaks, trends, and other perturbations displayed in the curves in Figure 14 can be 

explained by aircraft position, aircraft attitude, ground obstructions, antenna patterns, 

slant range, and other factors. Similar data traces are analyzed extensively in [1.] For 

this report the goal of Figure 14 is to display the overall similarity of the traces to one 

another, indicating both the uplink/downlink channel reciprocity and the 

narrowband/wideband (UL1/UL20) performance similarity. 

NASA/TM—2014-218391 17
Figure 14. - Flight Test 1 radio performance data and associated aircraft parameters at Glenn Research Center on April 11, 2014

GS, Ground Station; UTC, Coordinated Universal Time 

NASA/TM—2014-218391 18
One notable difference between the four traces in Figure 14 is the presence of multiple, 

closely-spaced “spikes” in the UL1 and UL20 uplink data traces. These are most visible 

in the UL1 trace during the period of 17:03 through 17:29. While the S-3B aircraft was 

flying at a range of approximately 70 nmi, the signal strength received at the aircraft 

radio was nearing its sensitivity limit near -119 dBm. While the underlying signal 

strength trace appears to trend further downward, the amplitudes of the spikes appear 

to be staying relatively constant. The April 11, 2014 flight is the first test where this 

effect has been recorded; previous tests have not displayed this effect. The spikes are 

not visible in the DLC2 or DLvideo “downlink” traces recorded by ground station radio. 

Subsequent investigation revealed that the automatic dependent surveillance-broadcast 

(ADS-B) transponder and other equipment on the S-3B aircraft had been changed prior 

to the Phase II test series. The new ADS-B unit, equipped with an extended squitter 

transmitter at 1090 MHz, was suspected to be a probable cause of the spikes. Although 

the transponder is designed to operate outside of the 960-977 MHz CNPC radio band, it 

is apparent that the transponder transmissions are being received by the CNPC radio on 

the aircraft. 

April 23, 2014

To investigate the possible source of the spikes, a flight test was planned in which the 

ADS-B transponder would be briefly turned off during flight. Radio operating conditions 

were identical to those of the April 11, 2014 test, including radio configurations, flight 

path, and downrange distance. In coordination with local air traffic controllers, the 

transponder was de-powered for approximately five minutes during the outbound pass 

and then reactivated for reversal and the inbound pass. Data for the test is presented in 

figure 15, showing the UL1 signal from ground-to-aircraft and the DLC2 signal from 

aircraft-to-ground. 

The airborne ADS-B transponder was in operation from the time of take-off until the S3B was approximately 80 nmi downrange from the GRC ground station. At this time the 

received signal strength at the aircraft was approximately -103 dBm which is above the 

sensitivity limit of the radio and at a level that the spikes would become visible in the 

data trace. From 14:48 the 14:53 the transponder output was deactivated while the 

aircraft continued on a straight and level flight path, extending to approximately 110 

nmi downrange. The data trace in figure 15 clearly shows that the spikes have abated 

throughout the five-minute period, especially when compared to the same airspace and 

flight conditions on the inbound run (15:08 through 15:13) when the ADS-B transponder 

NASA/TM—2014-218391 19
Figure 15. - Flight Test 5 radio performance data and associated aircraft parameters at Glenn Research Center on April 23, 2014

GS, Ground Station; UTC, Coordinated Universal Time; ADS-B, Automatic Dependent Surveillance-Broadcast

NASA/TM—2014-218391 20
was back in operation. The ADS-B deactivation appears to have greater impact on the 

UL1 trace, as it shows greater reduction in number of spikes. The wider bandwidth 

UL20 signal does not experience the same cleansing during the ADS-B off period, 

indicating that some other interferer is still present. Noise spikes were not visible in the 

downlink signals received by GS1. The overlap of the uplink and downlink data traces 

indicates the strong similarity of the characteristics of the channels, and also gives a 

forecast of how the uplink signal should be received once the interfering signal is 

removed. 

The outbound portion of the flight test, beginning at 14:33 UTC, yielded high quality 

data transmission (0-persent losses) both ground-to-aircraft and aircraft-to-ground until 

a few spikes were recorded at the aircraft beginning at approximately 14:46. The ADS-B 

transponder was de-activated at 14:48 which caused the spikes in the UL1 trace to 

disappear. The ADS-B transponder was re-activated at 14:53, at which time the UL1 

spikes returned. The UL1 frame loss trace shows signal deterioration prior to the onset 

of DLC2 frame loss. This indicates that the interfering signal is having negative impact 

on the uplink CNPC channel. 

On the inbound portion of the flight, the impact of the interfering signal is visible in the 

UL1 frame loss trace from 15:08 through 15:13. The multitude of vertical lines indicates 

that the radio is experiencing frame losses. Over the same time period, the DLC2 frame 

losses are much lower, resulting in little visible trace on the grid. 

The ADS-B interference issue described in this section and other co-site effects will be 

examined in future test activities. Factors including antenna placement, RF filtering, and 

radio waveform configuration/synchronization will be investigated in subsequent tests. 

April 30, 2014 

The primary objective of the early flight tests and one reason for characterization of the 

individual ground stations was to establish an environment to support networking and 

GS-to-GS hand-off testing. To perform the networking and hand-off tests, the aircraft 

would need to operate in a region where radio contact could be simultaneously made 

with both ground stations, preferably allowing the radio links to operate near the edge 

of their coverage range. The hand-off region for the Phase II tests was established as a 

sub-segment of the KCLE-KUNI flight path. 

Figure 16 presents radio performance data for the complete Phase II flight test system, 

including both ground stations and the S-3B aircraft. Received signal strength traces 

NASA/TM—2014-218391 21
show the UL1 and DLC2 channels to and from both GSs as the aircraft flies along the 

KCLE - KUNI flight path. Reading the figure from left to right from time 14:30 represents 

the S-3B aircraft traveling northbound from overhead of GS2 at the KUNI airport. As 

expected, the received signal strength recorded at GS2 and on the aircraft decrease as 

the aircraft moves northward away from GS2 and the slant range between aircraft and 

ground station increases. In the same period, the signals between the aircraft and GS1 

begin to increase in strength as that slant range distance decreases. Near time 14:40 on 

the plot, the signals from the two ground terminals are nearly equal at approximately -

105 dBm. Further progression to the right on the plot shows the received signals 

between the aircraft and now-distant GS2 becoming lower in power until the CNPC link 

can no longer communicate without data errors. No data errors are measured from the 

aircraft-GS1 CNPC links as the aircraft progresses northward and overflies GS1. 

The region of the flight path during which both ground stations were communicating 

with the aircraft without significant data errors is considered appropriate for the 

networking and hand-off testing. The first “ideal” test region occurs between times 

14:35 and 14:44 in Figure 16, as the aircraft was traveling northbound from GS2 towards 

GS1. This region is readily observed as the period when the signal strength curves from 

the two GSs cross, and when the frame error percentages are zero or very low. Two 

other test periods were achieved when the aircraft repeated southbound and 

northbound runs on the KCLE-KUNI flight path; nominally 15:03 to 15:19 and 15:38 to 

15:47. Test periods at 14,000 feet altitude lasted approximately 9 minutes during 

northbound flights and approximately 16 minutes during southbound flights. The 

difference in duration of the test periods was influenced primarily by differences in 

ground speed caused by winds aloft. Groundspeed during each of the flight segments 

was nominally 349 knots (northbound #1), 260 knots (southbound), and 344 knots 

(northbound #2). Network testing periods were often extended beyond the ideal 

periods in order to measure radio performance in the presence of frame errors. 

The ADS-B extended squitter transponder was in operation during this flight test. 

May 28, 2014 

The May 28 flight provided the same test environment and flight conditions as in April 

30, 2014 flight, offering constant speed and level flight along the GS1-GS2 route. The 

hand-off testing was conducted during the period when the CNPC communications path 

was established with both GSs, ranging from 16:28 to 16:42. The radio performance 

data for the hand-off test period is displayed in Figure 17. 

NASA/TM—2014-218391 22
Figure 16. - Flight Test 6 radio performance data and associated aircraft parameters at Glenn Research Center and Ohio University Airport on April 30, 2014 

GS, Ground Station; UTC, Coordinated Universal Time

NASA/TM—2014-218391 23
Figure 17. - Flight Test 7 radio performance data and associated aircraft parameters at Glenn Research Center and Ohio University Airport on May 28, 2014 

GS, Ground Station; UTC, Coordinated Universal Time

NASA/TM—2014-218391 24
Networking Tests and Results 

Using the network configuration and architecture described earlier, tests were 

constructed to ascertain actual performance metrics of basic network statistics while in 

flight and with the CNPC radio system in operation. In addition, tests were performed to 

evaluate the ability to do mobile hand-offs while in range of both ground stations. Since 

these tests were done in flight and outside of the controlled laboratory setting, the 

results are subject to variations in the environment and operational conditions. Thus, it 

is important to reference the appropriate flight plan when discussing these results. 

Times discussed in the networking test results will directly correlate to figures found 

earlier in this document. 

Delay and Jitter 

One of the basic goals in these networking tests was to measure the time delay and 

delay variation (jitter) through the prototype radios while in a networked configuration. 

In order to obtain these values Internet Control Message Protocol (ICMP) echo requests, 

or “pings”, were sent directly from each GS to the aircraft. The aircraft-based test 

electronics would then reply to each request, sending the response to the appropriate 

GS. The total time between the initial request and its corresponding response, as 

measured at the GS, is the round trip time (RTT). Since the pings are generated directly 

at the GS, this test helped to isolate the delay through the radio as no other network 

queues or nodes were involved in the path. Figure 18 shows the components involved in 

this test. Measurements were made by capturing the ICMP traffic on the ground station 

interface to the radio. 

Figure 18. – Diagram showing round-trip path of ping message through CNPC flight test system.

NASA/TM—2014-218391 25
The data for this test was collected during a segment of flight on April 30, 2014. During 

this time the aircraft was returning from GS2 in Albany, Ohio and heading towards GS1 in 

Cleveland. The UL1/C2 waveforms were used for these tests and were configured for a 

transmission rate of 10 Hertz. Testing consisted of sending ten ICMP echo requests 

every second from each ground station. Ten requests were sent every second in order to 

pair up with the 10 Hertz rate of the waveform. The payload size of each request was 20 

bytes. With packet overheads of 8 bytes for ICMP and 40 bytes for IPv6, the total IP 

datagram size was 68 bytes, which is less than the allotted 103 bytes of user data 

allowed for each transmission event. However, the goal of these tests was not to 

saturate the link with data. Rather, it was to observe the latency and variations in delay 

if a user utilized a message frequency closely matching that of the underlying waveform. 

Network mobility was not used for these tests. The echo requests were sent directly to 

the appropriate care-of address of the aircraft. This allowed for testing to measure the 

delays over just the radio portion of the link. Round trip times were calculated by 

processing a time-stamped packet trace at the ground station interface. Each request is 

tracked and when the corresponding response is seen in the trace the RTT is calculated 

by taking the difference in the two times. The time of each RTT sample is recorded with 

the time of the request. If a response was not received at the ground station, then the 

ping is marked as lost. Lost pings are not incorporated into the calculation of RTT 

metrics and simply reduce the number of samples in the time period the loss occurs. 

The test had duration of roughly 23 minutes in which thousands of pings were 

transmitted. In order to better visualize the results, data was grouped into ten second 

ranges in order to obtain roughly one hundred samples per data point. This approach 

provided a good statistical basis for each data point while still allowing the metrics to be 

visualized over time and compared to the RF channel if necessary. For example, a 

marker at time 14:40:00 contains data from 14:39:55.00 to 14:40:04.99. The exact 

number of data points in each sample is shown in Figures 19 and 20, along with the 

number of losses encountered. Results falling just short of the expected 100 samples 

per data point will be discussed later in this section. The other major dips in the number 

of samples, such as the entry at 14:39:50, are due to briefly stopping and restarting the 

pings during the anticipated time a hand-off would normally occur. 

NASA/TM—2014-218391 26
Figure 19. -Number of Total Samples/Losses at the Glenn Research Center ground station.

Figure 20. - -Number of Total Samples/Losses at the Ohio University Airport ground station.

NASA/TM—2014-218391 27
Figures 21 and 22 show the calculated round trip times at both ground stations by 

displaying a box-and-whisker plot of the samples for each data point. The darker “box” 

portion of each data set represents the first (Q1) and third (Q3) quartiles of the sample 

set. It is sometimes helpful to think of this as the “middle 50%” or “middle half.” The 

line within the box is the statistical median of the data. The “whiskers” of the plot 

extend to the farthest data sample that is not an outlier. For these tests, an outlier is 

defined as any point more than 1.5 times the interquartile range (Q3-Q1) from the end of 

the “box”. Each outlier is shown as a dot on the plot. 

A few immediate conclusions can be drawn from figures 21 and 22. First, there are a 

large number of outliers which are often grouped together in clusters. It will be shown 

that the cause of these outliers is due to periodic IP traffic that is concurrently 

transmitted with the ping data. Second, no notable correlation is observed between RTT 

and time at a macroscopic level. Thus, as the radiofrequency link degrades or improves, 

there does not seem to be an impact on RTT. So, while figures 19 and 20 show an 

increase in lost pings as the link sufficiently degrades, the RTT of the pings that do 

succeed remain consistently with a range of 200 to 250 milliseconds.

With no apparent dependence on the RF link quality, the performance differences 

between ground stations were explored. To do so, all RTT samples were collected at 

each location and a cumulative distribution function (CDF) was generated. Figure 23 

shows the CDF for each ground station. From the plot, it is shown that the distribution 

of ping travel times at each ground station is nearly identical. It is also confirmed that a 

majority of the data falls within 200 and 250 milliseconds, with 95% of all data having a 

RTT less than 282 ms. The outliers from earlier are visible producing the long tail at the 

top of the plot. Furthermore, there is a notable “plateau” between 190 and 200 ms, 

indicating that few samples fall into that range and warrant further analysis. 

Due to the volume of data, analysis was based on aggregate ten second groups. In order 

to understand some of the observations, it was necessary to look closer at smaller 

portions of the data. Figure 24 shows individual RTT measurements as a function of 

time for a single minute of measurements. The figure shows a clear “saw-tooth” pattern 

in the behavior of the RTT delay. Furthermore, there are distinct spikes at various 

intervals throughout the data. The spikes correlate directly with ICMP6 router 

advertisement (RA) being sent during the test. The time of each RA is shown as a 

vertical dashed line on the figure. These advertisements are 128 bytes long and delay 

the pings following it as the radio tries to handle the data spike. Thus, the outliers in the 

data can be attributed to the periodic transmission of RA packets across the link.

NASA/TM—2014-218391 28
 

Figure 21. - -Round trip times to the Glenn Research Center ground station.

Figure 22. - Round trip times to the Ohio University Airport ground station.

NASA/TM—2014-218391 29
Explanation of the trace’s saw-tooth behavior required analysis of the inter-arrival time 

of each ICMP echo request. Figure 25 shows a CDF of the delay between each ICMP 

echo request. The figure shows that although the test had initiated ten ping requests 

every second, a majority of pings actually took one to eight milliseconds longer than the 

expected 100 ms. This observation explains why there were less than 100 samples over 

ten seconds. While this extra delay may seem insignificant, it is suspected to be the 

cause of the cyclical RTT pattern.

Within the CNPC radio, data is queued and made ready for transmission before a new 

transmit window starts, otherwise it must wait until the start of the next window. The 

time spent in the queue, TQ, impacts the overall round trip time metric. In the 

Figure 23. - Cumulative Distribution Function of Round Trip Time Samples.

Figure 24. - RTT as a Function of Time at Ground Station 1.

NASA/TM—2014-218391 30
configuration for this test, each transmit window was 100 ms long. Figure 26, shows the 

impact a few extra milliseconds can do the value of TQ. Since the time between pings 

(period) is slightly longer than the transmission window, the value of TQ will slowly 

decrease causing the RTT to also decrease. Eventually this leads to a ping request that 

arrives just prior to the transmission window. The resulting RTT is the “ideal frame” 

shown in the figure and has the lowest value. However, the very next request will “just 

miss” the next transmission event, leading to the worst possible value of TQ and thus the 

longest delay. 

In Figure 24, as the RTT decreases, miniature spikes in the RTT can be observed. The 

erratic behavior is near 190 ms and is the reason behind the plateau observed in Figure 

23. It is suspected that this effect is caused by an implementation detail within the 

radio. Specifically, a ping cannot truly arrive just prior to the transmission time. Rather, 

there is a processing delay that may vary and negatively impact some requests as TQ is 

reduced towards zero. 

Test data shows that the delay through the radio link is consistent over different RF 

characteristics. For the 10 Hertz update rate configuration, the median round trip delay 

through the CNPC link was 225 ms. Excluding outliers caused by other traffic, 99% of the 

RTT varied from 171 to 267 ms. The delay variation or jitter is due to messages being 

held in the radio's queue for different durations based on the messages' arrival time at 

the radio. This range of 96 ms is consistent with the 100 ms between transmission 

events in the waveform configuration that was applied at the time of the test. Thus, the 

amount of jitter is dependent on the configuration of the radio. 

Figure 25. - Cumulative Distribution Function of Time between Ping Requests.

NASA/TM—2014-218391 31
TCP and UDP Testing 

On April 30, 2014, after the completion of the time delay and jitter testing, a series of 

investigative tests were conducted. Network traffic was transmitted between the 

ground station and the aircraft using both Transmission Control Protocol (TCP) and User 

Datagram Protocol (UDP) transport formats. The tests were performed to provide an 

initial assessment of the throughput capacity of the radio pairs. The tools used in these 

tests were based on open-source utilities and were adequate for the initial assessment. 

However, additional software will need to be developed to complement those tools and 

extract additional metrics. The resulting testing suite will be utilized for upcoming NASA 

tests, which will measure network throughput capacity in the flight environment and 

study the effects of header compression and data security. 

 

Hand-off Response Time and Loss 

On May 28, 2014, a series of network hand-off tests were performed in order to 

characterize the performance of network mobility on the current radio waveform. In 

these tests, the aircraft flew to an area that was within range of both ground stations. 

The test began by first establishing a link to ground station 2. This active link would be 

considered the primary connection until an operator on the plane initiated a manual 

hand-off. Each manual hand-off consisted of first establishing a connection to the 

inactive ground station and then manually disabling the primary connection. The mobile 

router would detect that the primary link had failed and hand-off to the newly created 

connection. This procedure was repeated a total of 25 times as the aircraft attempted to 

remain between ground sites. 

Figure 26. - Impact of Radio Queuing Delay for 10 Hz rate

NASA/TM—2014-218391 32
Figure 27 shows the three steps of the hand-off procedure. Even though the process 

first establishes the new link, the hand-off is “break-before-make” as the rebinding is 

done after the active link is destroyed. This will lead to some data loss and is considered 

the worst case for a hand-off when another link is ready and available. Even as handoffs become more automated, such conditions may arise if an impending loss of link 

event happens very suddenly, such as severe shadowing. The figure also shows how 

these ping tests differ from those performed at the ground stations. During this test, 

ICMP echo requests are sent continuously from the HA to the aircraft. The requests are 

never paused and are dynamically re-routed through the correct path as the hand-offs 

occur. The frequency of ping requests matches that of the earlier test at ten requests 

every second. However, the size of each ping packet has increased due to the extra 

headers required for mobility. The end result is a ping packet whose total size is 93 

bytes, slightly larger than the 68 bytes used in the previous test. 

Figure 28 shows the first four minutes of this experiment and the resulting six hand-offs 

that were performed in that time. In these plots, the round trip time of the pings, as 

observed by the home agent, are plotted against time. The plots also show when the 

primary connection was terminated at the aircraft. These termination points are 

indicated by the double dotted dashed lines, and the labels above these lines indicate 

which ground station was acting as the primary link. Binding updated used by the 

mobile node to establish the new link to the home agent are shown with a single dotted 

dashed line. Pings that do not get a response at the home agent are considered lost. 

These losses are plotted as boxes on the figure, with the width of the box indicating the 

duration of the loss and the height of the box indicating how many consecutive pings 

were lost in that duration. The number of lost pings is plotted against a secondary y-axis 

in order to visualize both metrics on a single figure. 

Figure 27. - Testing Points and Break-Before-Make Hand-off Process: 

(1) New Link is Established, (2) Old Link is Destroyed, (3) Binding Update is Issued.

NASA/TM—2014-218391 33
(a) 

(b) 

(c) 

Figure 28. – Hand-off Timing and Signal Losses 

NASA/TM—2014-218391 34
Just as in the earlier ping tests, there are large plateaus in the round trip time caused by 

router advertisements being sent from the ground station. Also, the same overall 

sawtooth pattern, where the RTT slowly decreases, can be observed in all three plots. 

However, unlike the earlier tests, there are additional spikes along the sawtooth pattern, 

which are extremely consistent throughout the experiment and occur every second. The 

source of these spikes was traced back to a radio message that carries RF health 

information and is transmitted across the link every second. The radios transmit this 

health information in order to establish a database of towers and aircraft between 

radios. Since the queuing priority of these messages is higher than that of any user data, 

under loaded conditions it will induce extra delay into any user data. These spikes were 

not seen in the earlier experiment as the size of the ping packets was smaller. This 

allowed for both the health message and the ping packet to fit into a single transmission 

event. However, the size of ping packets increased to 93 bytes for these tests. Thus, 

when contention occurs with the health data, only a fraction of the ping packet can be 

transmitted. The remaining portion is delayed a transmission window adding 100 

milliseconds to the round trip time. 

Similar to the earlier ping tests, the observed round trip times fall into the same range. 

For example, the same 171 to 267ms sawtooth is observed for RTT values in the GS1 

segments. This is due to their being very little latency between GS1 and the home agent 

as they are connected over a local area network. As expected, the RTT values at GS2 are 

50 to 60 ms higher than those at GS1. This is directly attributed to the extra delay of 

traversing the terrestrial network between Cleveland and Albany, Ohio. 

Figure 28 (a) and (c) show the first two minutes and fourth minute of the experiment, 

respectively. In this time, five “typical” hand-offs occur. In total, 23 of the 25 hand-offs 

follow the behavior shown in the two subfigures. The plots show that after the link is 

terminated, there is an amount of time before the mobile router reacts to the loss and 

initiates the first binding update to the home agent via the standby connection to the 

other ground station. In the 25 hand-off requests, the average time taken by the mobile 

router to initiate this first binding update was 1.054 seconds, with a standard deviation 

of 0.042 seconds. It is important to note that no special steps were taken to tune this 

response time for these tests. 

Ping responses resume roughly one round trip time after the binding update is issued 

from the mobile router. This follows expectations as it is the amount of time needed for 

the binding update to reach the home agent, and for the home agent to send an 

acknowledgment confirming the change. Therefore, the total time to recover should be 

roughly 1.054+RTT seconds. Table 1 shows the statistics for the total time to recover for 

all of the 23 “typical” hand-offs. It shows that the average time of 1.353 seconds 

NASA/TM—2014-218391 35
matches the expected result and is consistent with the round trip times seen earlier in 

the delay tests. The table also shows the average number of losses encountered. Since 

pings are sent ten times each second, it is reasonable that the number of packet losses is 

ten times the time needed to recover. 

Figure 28 (b) shows one of two hand-offs that encountered losses during the start of the 

event. Looking back at the RF data (Figure 17), there is a loss event just after the 

16:30:00 time marker that primarily impacts the uplink portion of the link. This implies 

that data sent to the aircraft was dropped, while data to the ground was not impacted. 

In this hand-off, the time needed to recover is much larger and longer and is due to the 

quality of the underlying RF channel. Figure 29 provides the actual packet trace as 

observed from the aircraft during this hand-off event. The trace spans 3.75 seconds, 

combines traffic from both ground stations, and highlights binding updates and 

acknowledgments in bold text. It begins with pings through GS2, identified by the use of 

the care of address, S3-COA-GS2. The link to GS2 is disabled 21.652 seconds into the 

trace, and the first binding update, with sequence number 42523, is sent out roughly 

one second later over the link to GS1. The home agent actually receives this update as 

echo requests begin to arrive over the GS1 care of address, S3-COA-GS1. However, the 

aircraft does not reply to these pings since it has not yet received an acknowledgment of 

the change from the home agent, which was lost due to the issues in the uplink RF path. 

During this time, there are 14 pings that arrive and do not get routed properly. 

Eventually, the mobile router retransmits another binding update which is successfully 

acknowledged and the echo replies resume as normal. Thus, the loss event was 

prolonged due to a critical loss of the binding acknowledgment. Figure 28 (b) shows that 

losses continued after the acknowledgment arrived. Those losses correlate with the 

larger of the two loss spikes in the RF plot. 

# Samples N = 23 Median IQR Mean Std. Deviation 

Time to Recover (seconds) 1.268 0.199 1.353 0.439 

Number of Losses (packets) 13.0 2.0 13.7 4.1 

Table 1: Hand-off Times 

NASA/TM—2014-218391 36
Conclusions 

The second phase of the Unmanned Aircraft System in the National Airspace System 

(UAS in the NAS) communications flight test campaign successfully demonstrated inflight transfer (hand-off) of an aircraft radio communication link between two 

independent ground stations. Using remotely-controlled ground stations located at 

Glenn Research Center in Cleveland, Ohio and Ohio University Airport in Albany, Ohio, 

the test flights demonstrated sustained air-to-ground L-Band radio connectivity 

throughout a 133 nautical mile (nmi) flight path. The flight tests also characterized the 

use of hand-offs of the Control and Non-payload Communications (CNPC) data between 

ground stations. Test data was collected on roundtrip times through the 

communications network and on data flow interruptions during hand-offs. 

The Phase II flight tests utilized “Generation 2” software-defined radios developed 

under cooperative agreement NNC11AA01A between the NASA Glenn Research Center 

and Rockwell Collins, Inc., of Cedar Rapids, Iowa. Although the radios are only 

prototypes, they have demonstrated very repeatable and reliable operation in the field. 

Like the performance recorded during previous test series [1], the radios are providing 

connectivity beyond the 69 nmi range initially planned. Omnidirectional antennas were 

used for the L-band communications at both ground terminals and on the aircraft. 

21.551030 IP6 HA > S3-COA-GS2: IP6 HA > S3: ICMP6, echo request, seq 4078 

21.551065 IP6 S3-COA-GS2 > HA: IP6 S3 > HA: ICMP6, echo reply, seq 4078 

21.651806 IP6 HA > S3-COA-GS2: IP6 HA > S3: ICMP6, echo request, seq 4079 

21.651844 IP6 S3-COA-GS2 > HA: IP6 S3 > HA: ICMP6, echo reply, seq 4079 

22.730532 IP6 S3-COA-GS1 > HA: DSTOPT mobility: BU seq#=42523 AH 

23.101219 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4093 

23.201808 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4094 

23.301144 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4095 

23.401074 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4096 

23.501922 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4097 

23.801388 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4099 

24.001369 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4101 

24.101154 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4102 

24.401343 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4105 

24.501082 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4106 

24.601722 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4107 

24.700982 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4108 

24.730623 IP6 S3-COA-GS1 > HA: DSTOPT mobility: BU seq#=42524 AH 

24.801277 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4109 

24.900965 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4110 

25.001046 IP6 HA > S3-COA-GS1: srcrt mobility: BA status=0 seq#=42524

25.101001 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4111 

25.101059 IP6 S3-COA-GS1 > HA: IP6 S3 > HA: ICMP6, echo reply, seq 4111 

25.301948 IP6 HA > S3-COA-GS1: IP6 HA > S3: ICMP6, echo request, seq 4113 

25.301991 IP6 S3-COA-GS1 > HA: IP6 S3 > HA: ICMP6, echo reply, seq 4113 

Figure 29. - Network Trace During a Hand-off with Loss 

(Column 1 time offsets are seconds from Wed May 28 16:30:00 UTC 2014) 

NASA/TM—2014-218391 37
The testing demonstrated the proposed network architecture will allow for smooth 

transitions between ground stations. The proposed architecture was designed to be 

scalable and allows for industry standard security controls to be incorporated in the 

future. Operators do not need to be aware of which network path is currently being 

utilized as the underlying architecture is designed to seamlessly direct messages to the 

correct endpoint as the mobile node traverses the network of ground stations. 

The networking tests showed that the median round trip propagation time between the 

airborne and ground radios was 225 ms, when the CNPC radio was operating in a 10 Hz 

configuration. The observed round trip times had a significant amount of variation, with 

the majority of samples between 171 and 267 ms. This spread of 96 ms was consistent 

with the 10 Hz (100 ms) waveform configuration used for the tests. Thus, the maximum 

targeted 20 Hz (50 ms) configuration should yield the least amount of delay variation. 

Likewise, reducing the waveform configuration to values below 10 Hz would further 

increase the amount of jitter. These values are a result of small data packets that can fit 

into a single transmission event. Larger data payloads would increase the amount of 

time needed to transmit the full payload and decrease the amount of observed jitter. 

Finally, traversal over the terrestrial Internet added an additional 55 ms round-trip for 

the location in Albany, Ohio. This brought the total round trip delay from the home 

network location to the aircraft via the ground station to over 0.25 seconds in the 

average case. 

The use of periodic router advertisements to assign IPv6 addresses, while sufficient in 

the lab environment, proved to add long-lived spikes to the observed round trip times of 

normal data by introducing extraneous traffic into an already severely constrained link. 

Future work will investigate transitioning IPv6 address assignment to using DHCPv6 or 

some other method where the additional bandwidth of periodic router advertisements 

can be suppressed. 

Router advertisements were not the only extraneous traffic encountered in these tests. 

The radio would internally generate and transmit a small amount of “health” 

information every second. These messages would also significantly impact observed 

round trip times in numerous, short-lived spikes. The impact was quite significant and 

the utility and frequency of these messages will be revisited in future updates of the 

radio. 

Network hand-offs are shown to be seamless and transparent to the end user. When 

forcefully breaking a connection, the airborne system waits a set period of time then 

issues a request to use the standby connection. This request is completed in one round 

trip time with a lossless link. The hand-off from one ground station to the next 

NASA/TM—2014-218391 38
nominally required 1.268 s. The set period of time for the mobile IPv6 software 

implementation to detect link failure is currently one second, which comprises the 

majority of the 1.268 s recovery time. 

Several methods are being explored to further reduce this time. Attempting to make 

the one second timer more aggressive can reduce hand-off times in the current breakbefore-make scenario. However, additional analysis would be needed to demonstrate 

that more aggressive timers would not negatively impact performance. Another means 

of reducing this time is by utilizing a make-before-break hand-off. Such a hand-off 

would eliminate the one second timer completely, leaving only the one round trip time 

of roughly 268 ms and thus incur less data loss. Both of these techniques will be 

explored in future testing. 

NASA will continue to investigate the performance of the CNPC radio links in an ongoing flight test program. Upcoming test activities will include but not be limited to: 

further investigation of the 5030- to 5091-MHz frequency range (C-Band) for either 

alternate or simultaneous CNPC operations, measuring user data throughput rates, 

demonstrating data header compression and data security, and evaluating multiaircraft operations. Results from these tests will be presented in future reports.

NASA/TM—2014-218391 39
References 

1. Shalkhauser, K. et al: Control and Non-Payload Communications Generation 1 Prototype Radio

Flight Test Report. NASA/TM —2014-218099, 2014.

2. Matolak, D. and Sun, R.:et al.: AG Channel Measurement and Modeling Results for Over-Sea

Conditions. NASA/CR—2014-216674, 2014.

3. Wilson, W., “Terrestrial L-Band and C-Band Architectures for UAS Control and Non-Payload

Communications”, SC203-CC019, RTCA SC-203, December 2010

4. Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility Support in IPv6", RFC 6275, July 2011.

[Online]. Available: http://www.rfc-editor.org/rfc/rfc6275.txt

5. Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert, "Network Mobility (NEMO) Basic

Support Protocol", RFC 3963, January 2005. [Online]. Available: http://www.rfceditor.org/rfc/rfc3963.txt

6. Kundra, V., "Transition to IPv6", Federal Mandate, September 2010. [Online]. Available:

http://www.whitehouse.gov/sites/default/files/omb/assets/egov_docs/transition-to-ipv6.pdf

7. Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6

(IPv6)", RFC 4861, September 2007. [Online]. Available: http://www.rfceditor.org/rfc/rfc4861.txt

8. Schneider, A., “GPS Visualizer”, Computer Software, http://www.gpsvisualizer.com/

9. Derrickson, W., “Lockheed S-3B N601NA NASA Glenn”, 2008, Cleveland National Air Show,

http://tarmacphotos.com/

10. National Atlas of the United States, March 5, 2003, http://nationalatlas.gov

NASA/TM—2014-218391 40
Appendix A 

 

Band LSID 

/CID 

Mode Channel 

center 

frequency 

Transmit 

Power 

level 

Frame 

Rate 

(Hz) 

Aircraft GS1 (GRC) GS2 (OU) 

 

L 1 UL1 968 MHz 63 10 X X 

Configuration 1 2 DLC2 968 MHz 63 10 X X 

3 UL20 968 MHz 63 10 X X 

4 DLVideo 968 MHz 63 10 X X 

C

 

L

Configuration 2 C 1 UL1 5060 MHz 63 10 X X 

2 DLC2 5060 MHz 63 10 X X 

3 UL20 5060 MHz 63 10 X X 

4 DLVideo 5060 MHz 63 10 X X 

 

L 1 UL1 968 MHz 63 10 X X 

Configuration 3 2 DLC2 968 MHz 63 10 X X 

3 UL20 968 MHz 63 10 X X 

4 DLVideo 968 MHz 63 10 X X 

C 1 UL1 5060 MHz 63 10 X X 

2 DLC2 5060 MHz 63 10 X X 

3 UL20 5060 MHz 63 10 X X 

4 DLVideo 5060 MHz 63 10 X X 

 

L 1 UL1 968 MHz 63 4 X X 

Configuration 4 2 DLWx 968 MHz 63 10 X X 

3 UL4 968 MHz 63 4 X X 

4 DLVideo 968 MHz 63 10 X X 

5 UL8 968 MHz 63 4 X X 

7 UL12 968 MHz 63 4 X X 

9 UL16 968 MHz 63 4 X X 

C 1 UL1 5060 MHz 63 4 X X 

2 DLWx 5060 MHz 63 10 X X 

3 UL4 5060 MHz 63 4 X X 

4 DLVideo 5060 MHz 63 10 X X 

5 UL8 5060 MHz 63 4 X X 

7 UL12 5060 MHz 63 4 X X 

9 UL16 5060 MHz 63 4 X X 

 

L 1 UL1 968 MHz 63 10 X X 

Configuration 5 2 DLC2 968 MHz 63 10 X X 

3 UL20 968 MHz 63 10 X X 

4 DLVideo 968 MHz 63 10 X X 

C 1 UL1 5060 MHz 63 10 X X 

2 DLC2 5060 MHz 63 10 X X 

3 UL20 5060 MHz 63 10 X X 

4 DLVideo 5060 MHz 63 10 X X 

 

NASA/TM—2014-218391 41
L 1 UL1 968 MHz 63 5 X X 

Configuration 6 2 DLC2 968 MHz 63 5 X X 

3 UL20 968 MHz 63 5 X X 

4 DLVideo 968 MHz 63 5 X X 

5 UL1 968 MHz 63 5 X X 

6 DLC2 968 MHz 63 5 X X 

7 UL20 968 MHz 63 5 X X 

8 DLVideo 968 MHz 63 5 X X 

C 1 UL1 5060 MHz 63 5 X X 

2 DLC2 5060 MHz 63 5 X X 

3 UL20 5060 MHz 63 5 X X 

4 DLVideo 5060 MHz 63 5 X X 

5 UL1 5060 MHz 63 5 X X 

6 DLC2 5060 MHz 63 5 X X 

7 UL20 5060 MHz 63 5 X X 

8 DLVideo 5060 MHz 63 5 X X 

 

L

Configuration 7 C 1 UL1 5060 MHz 63 10 X X 

2 DLC2 5060 MHz 63 10 X X 

 

L 1 UL1 968 MHz 63 20 X X 

Configuration 8 2 DLC2 968 MHz 63 20 X X 

C

 

L

Configuration 9 C 1 UL1 5060 MHz 63 10 X X 

2 DLC2 5060 MHz 63 10 X X 

3 UL1 5060 MHz 63 10 X X 

4 DLC2 5060 MHz 63 10 X X 

 

L

Configuration 10 C 1 UL20 5060 MHz 63 10 X X 

2 DLVideo 5060 MHz 63 10 X X 

3 UL20 5060 MHz 63 10 X X 

4 DLVideo 5060 MHz 63 10 X X 

 

L 1 UL1 968 MHz 63 10 X X 

Configuration 11 2 DLC2 968 MHz 63 10 X X 

3 UL1 968 MHz 63 10 X X 

4 DLC2 968 MHz 63 10 X X 

C

 

L 1 UL20 968 MHz 63 10 X X 

Configuration 12 2 DLVideo 968 MHz 63 10 X X 

3 UL20 968 MHz 63 10 X X 

4 DLVideo 968 MHz 63 10 X X 

C

 

L 1 UL20 968 MHz 63 10 X X 

Configuration 13 2 DLC2 968 MHz 63 10 X X 

3 UL20 968 MHz 63 10 X X 

4 DLC2 968 MHz 63 10 X X 

C

NASA/TM—2014-218391 42
Appendix B 

Hardware/Software Inventory 

Home Agent / Network 

• Intel Xeon E3-1230 CPU @ 3.30 GHz (Quad Core)

• 16 GB RAM

• 240 GB SSD (Seagate ST240HM000)

• 2 TB HDD (Seagate ST2000NM0033)

• 4x Intel 82574L Gigabit Ethernet Interfaces

• Ubuntu Linux 12.04.4 LTS Server

• Linux 3.11.10.4-mobility00 kernel (custom compiled to support Mobile IPv6)

• UMIP v1.0 (www.umip.org) (mip6d daemon)

• Radvd 1.8.3 (Router Advertisement Daemon)

• Cisco ASA 5510 Firewall

Ground Stations 

• Intel Xeon E3-1230 CPU @ 3.30 GHz (Quad Core)

• 16 GB RAM

• 240 GB SSD (Seagate ST240HM000)

• 2 TB HDD (Seagate ST2000NM0033)

• 4x Intel 82574L Gigabit Ethernet Interfaces

• Ubuntu Linux 12.04.4 LTS Server

• Linux 3.8.0-38-generic kernel (Ubuntu distributed kernel)

• Radvd 1.8.3 (Router Advertisement Daemon)

• Iperf 3.0.3

• Custom Python and BASH shell scripts to communicate with radio

• Cisco ASA 5505 Firewall

Mobile Router (Onboard Aircraft) 

• Intel Xeon E5-1660 @ 3.30 GHz (Hex Core)

• 16 GB RAM

• RAID-1 2x 256 GB SSD (Crucial M4-CT256M4SSD2)

• RAID-1 2x 2 TB HDD (Toshiba MK2002TSKB)

• 2x Intel I350 Gigabit Ethernet Interfaces

• Ubuntu Linux 12.04.4 LTS Desktop

• Linux 3.11.10.4-mobility00 kernel (custom compiled to support Mobile IPv6)

• UMIP v1.0 (www.umip.org) (mip6d daemon)

• Radvd 1.8.3 (Router Advertisement Daemon)

• Iperf 3.0.3

• Custom Python and BASH shell scripts to communicate with radio

NASA/TM—2014-218391 43