Effectiveness of Redundant Communications Systems 

in Maintaining Operational Control of Small 

Unmanned Aircraft 

Jaewoo Jung Sreeja Nag Hemil C. Modi 

Aviation Systems Division Bay Area Environmental Research Institute Science and Technology Corp. 

NASA Ames Research Center NASA Ames Research Center NASA Ames Research Center 

Moffett Field, USA Moffett Field, USA Moffett Field, USA 

jaewoo.jung@nasa.gov sreeja.nag@nasa.gov hemil.c.modi-1@nasa.gov 

Abstract—As a part of NASA’s Unmanned Aircraft System 

(UAS) Traffic Management (UTM) research, a test was performed 

to evaluate the effectiveness of the redundant Command and 

Control (C2) communications system for maintaining operational 

control of small UAS in the airspace over a rural area. In the test, 

operators set up a primary and a secondary UAS C2 

communications system, sent a maneuver command to an 

Unmanned Aircraft (UA) with and without a functioning primary 

system, then verified the execution of the sent command to confirm 

the operator control. Operators reported that the tested 

redundancy configurations were effective in maintaining 

operational control in the test airspace over rural locations. Since 

the next phase of UTM research focuses on operations in an urban 

area where an increased level of Radio Frequency (RF) activities 

occur compared to a rural area, four recommendations are 

provided to sustain the effectiveness of redundancy in urban 

operations. First, the operator should not include C2 systems that 

use the industrial, scientific, and medical (ISM) radio bands in 

redundancy configurations. Second, the operator should verify the 

RF characteristics of the intended operation area and examine the 

area’s radio noise floor. Third, the operator should monitor the 

availability, quality, and reliability of communications services 

used by a redundant system. Fourth, the small UAS community 

should adopt a standard set of contingency steps to handle the loss 

of C2 communications so that such events are managed in a 

consistent manner across the airspace. The insights from the test 

will be used to accommodate the FAA’s UAS integration effort. 

Keywords—operations, control, communications, redundancy 

I. INTRODUCTION 

NASA has been researching prototype technologies for an 

Unmanned Aircraft System (UAS) Traffic Management (UTM) 

system to facilitate safe and efficient civilian low-altitude 

airspace and UAS operations [1,2]. The UTM research 

accommodates the FAA’s UAS integration effort, and the UTM 

system is designed to provide services at low altitudes where the 

FAA’s Air Traffic Management (ATM) services are not 

available, therefore complementing the ATM system [3]. The 

FAA expects that the UTM capabilities will be incrementally 

implemented to address this gap over the next several years [4]. 

To effectively transfer NASA’s UTM findings to the FAA, 

NASA and the FAA formed a Research Transition Team [5]. 

The team is divided into four subgroups, Concepts and Use 

Cases; Data Exchange and Information Architecture; Sense and 

Avoid; and Communications and Navigation (C&N). Of the 

four subgroups, this paper discusses activities within the C&N 

subgroup whose objective is to explore communications and 

navigation solutions to ensure that Unmanned Aircraft (UA) are 

under the operational control of the remote pilot and remain 

within a defined area. In particular, the paper describes a test that 

is designed to evaluate the effectiveness of redundant Command 

and Control (C2) communications systems in maintaining 

operational control of small UAS (sUAS) in the airspace over a 

rural area and presents a summary of the test reports. Since the 

next phase of UTM research focuses on sUAS operations in the 

airspace over an urban area, where a higher level of radio 

activities occur over broad Radio Frequency (RF) spectrum 

compared to a rural area [6], the paper also provides 

recommendations about configuring effective redundant C2 

communications systems for urban operations. 

II. TEST DESCRIPTION 

In 2018, NASA contracted six FAA-designated UAS test 

sites to evaluate the UTM capabilities developed to date. In this 

evaluation, four of the six sites, Alaska, Nevada, New York, 

and North Dakota, performed a test to evaluate the effectiveness 

of the redundant C2 communications system, described in this 

paper, in maintaining operational control of sUAS in the 

airspace over several rural locations. 

For the test, NASA provided the instruction shown in Table 

1 to each Test Site Operator (TSO). The instructions were 

designed to show that sUAS equipped with redundant C2 

communication systems could execute a maneuver command 

via either C2 system, maintaining operational control when one 

system is compromised. Given the variety of technologies and 

services that can be used for sUAS communications, the test did 

not specify a specific redundancy system and the TSOs used 

redundancy configurations of their own choosing. Table 2 

shows a summary of the redundancy configurations used in the 

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test. Note some similarities and differences between the 

redundancy setups. For example, most redundancy 

configurations involved two communication systems. The 

range of communication systems included radio, LTE cellular, 

Wi-Fi, and satellite. Most redundant systems identified a 

primary communication system with automatic switching to a 

secondary system when the primary fails. One automatically 

switched to the system providing the best data quality. One was 

switched manually by the operator. One merged the data from 

both systems. 

The TSOs were required to conduct the test using at least 

three different flight plans that were significantly different in 

terms of takeoff location, takeoff time, altitude profile, and 

lateral flight profile. After the test, each TSO debriefed and 

reported on the test processes and results. 

III. SUMMARY OF TEST REPORTS 

A. Alaska 

The Alaska flight test was conducted on April 25, 2018. Two 

quadcopters were flown in a box pattern in Fairbanks, at the 

locations shown in Figure 1 [7]. During the flights of a 

quadcopter with two identical radio modems, the C2 

communications link established by one radio was manually 

disconnected by the operator to force a switchover to the other 

radio. During the flights of another quadcopter with an LTE 

cellular modem and a radio modem, the LTE C2 

communications link failed without intervention from the 

operator and caused a switchover to the radio. The Alaska TSO 

reported that in both cases having a secondary C2 

communications system improved reliability in maintaining the 

operator control. 

Table 1. Test Instruction 

STEP INSTRUCTION 

1 Equip sUAS with more than one C2 

communications system 

2 Prepare UA to fly within a defined area 

3 Once UA is airborne, send a maneuver command 

4 Confirm execution of the sent command 

Take one C2 system off-line (e.g., disconnect, 

turn off signal, etc.) 

5 

Use redundancy to send another maneuver 

command 

6 

7 Confirm execution of the second sent command 

B. Nevada 

The Nevada flight test was conducted on March 9, 2018. A 

fixed-wing UA equipped with two different radio modems was 

flown in three planned patterns in North Valleys. Figure 2 

shows the planned flight locations [8]. During a flight, the 

operator turned off the C2 communications signal of one radio 

to force a transition to the other radio. The Nevada TSO 

reported that the operator was able to maintain control with this 

transition throughout the test. 

C. New York 

The New York flight test was conducted on April 18-19, 

2018. An octocopter UA equipped with a Wi-Fi modem and an 

Table 2. Redundant sUAS C2 Communications System Configurations Used in the Test 

TEST SITE/ 

UA TYPE COMM. SYSTEM #1 COMM. SYSTEM #2 NOTE 

Radio modem operating Radio modem operating Each radio connected to a separate 

ALASKA/ between 902 to 928 MHz between 902 to 928 MHz Ground Control System (GCS), #2 link 

QUADCOPTER echoing #1 link with automatic 

switching to #2 when #1 fails 

ALASKA/ Long Term Evolution (LTE) Radio modem operating #1 used as primary, automatic switching 

QUADCOPTER cellular modem between 902 to 928 MHz to #2 when #1 fails 

NEVADA/ Radio modem operating Radio modem operating Operator switches between the two 

FIXED-WING between 2200 to 2500 MHz between 2405 to 1470 MHz systems 

NEW YORK/ Wi-Fi modem with the center LTE cellular modem #1 used as primary, automatic switching 

OCTOCOPTER frequency at 5875 MHz, 20 to #2 when #1 fails 

MHz channel width 

NORTH Radio modem with the center Radio modem with the center Both radios transmit all data, 

DAKOTA/ frequency at 757.5MHz, frequency at 907.5MHz, automatically merging to a single 

HELICOPTER running a 90 KHz waveform running a 90 KHz waveform receiving stream with duplicate data 

filtered out. 

NORTH Radio modem operating Satellite communication 

DAKOTA/ between 902 to 928 MHz modem operating in 1616 to 

FIXED-WING 1626.5 MHz 

NORTH Two radio modems with Dual LTE mobile modem- Automatic switching to the 

DAKOTA/ center frequency at 915 MHz, router that connects to two communications link with the best 

HEXACOPTER one at low and the other at different cellular services quality 

high transmit power setting 
 

Figure 1. Alaska Test Location 

Figure 2. Nevada Test Location 
Figure 3. New York Test Location 

Figure 4. North Dakota Test Location 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

LTE cellular modem was flown in Rome. Figure 3 shows the 

flight location [9]. The flight was designed to bring the UA 

outside of Wi-Fi range and when this occurred, C2 

communications were automatically transferred to the LTE 

modem. The New York TSO reported that no issues occurred 

during this transfer and the operator maintained control. 

D. North Dakota 

The North Dakota flight test was conducted on April 11-12, 

2018. A helicopter UA equipped with two different radio 

modems, a fixed-wing equipped with a radio modem and a 

satellite communications modem, and a hexacopter equipped 

with a radio modem and an LTE cellular modem were flown in 

Camp Grafton. Figure 4 shows flight locations [10]. The three 

UA were flown in two conditions. In the first condition, the 

operator manually disconnected one C2 communications link 

to force switching. In the second condition, the UA has flown a 

long distance where one C2 link failure occurred without 

intervention from the operator. The North Dakota TSO reported 

that although there was a latency of about 50 seconds in 

switching to the satellite communications modem, this was 

known beforehand and did not impact the operator’s control of 

UA in both test conditions. 

IV. RECOMMENDATIONS FOR CONFIGURING REDUNDANT C2 

COMMUNICATIONS SYSTEM FOR URBAN OPERATIONS 

The next phase of the UTM research focuses on sUAS 

operations in an urban area. Compared to a rural area, one can 

expect an increased level of RF activities across a broad 

spectrum to take place in an urban area due to wireless voice 

and data communications, broadcasting venues, and electronics 

used by a larger, concentrated population [11]. Also, highdensity, fast-tempo sUAS operations carrying out tasks such as 

newsgathering and package deliveries are expected to take 

place in the airspace over an urban area in the future. Therefore, 

the following are recommended to assist sUAS operators for 

configuring an effective redundant C2 communications system 

to maintain operational control during urban operations. 

Recommendation 1: The operator should not use C2 

systems that use the ISM radio bands for redundancy as 

they are not reliable in an urban setting. 

Although the tested redundant C2 communications 

configurations, shown in Table 2, were found effective in 

maintaining operational control of sUAS in the airspace over 

rural locations, all configurations include C2 systems that use 

the industrial, scientific and medical (ISM) radio bands, such as 

902 MHz ~ 928 MHz and 2.4 GHz ~ 2.5 GHz [12]. The ISM 

bands can be used freely within the regulated transmit power 

limit [13] and due to its open nature, C2 communications 

systems using the ISM bands can experience interference from 

applications, such as Wi-Fi routers, at any time in an urban area. 

As another example, radio energy from everyday electronics, 

such as a microwave oven, can be released to the ISM radio 

bands to interfere with the C2 systems using the same bands, 

and this phenomenon is likely more intense in an urban area 

than in a rural area. Therefore, for urban operations, C2 systems 

that use ISM radio bands should not be included in redundancy 

configurations as they will be subjected to difficult-to-predict 

interferences and will not be reliable. 

Recommendation 2: The operator should verify the RF 

characteristics of the intended operation area and examine 

the radio noise floor during operations. 

To configure a redundant C2 communications system that 

is robust to RF interferences, the operator should obtain the RF 

characteristics in the intended operation area, such as locations 

of broadcasting antennae, transmit frequency bands, power, and 

schedule. Thereafter, the operator should configure redundancy 

by including C2 systems using frequencies that are away from 

the known high-powered transmission frequencies, unless the 

high-powered transmission is for the C2 communications. Once 

the redundant C2 communications system is configured, it is 

prudent for the operator to check the noise floor of all C2 

communications frequencies in the operation area. When noise 

floor increases in one radio band, the operator can switch C2 

communications to the other band to proactively prevent loss of 

communications and maintain control. Also, it is not 

recommended to feature two identical systems in the 

redundancy, such as two identical radio modems using the same 

frequency bands. While this arrangement can be effective in 

mitigating a communications system malfunction, RF 

interference affecting one system will affect the other, negating 

redundancy. 

Recommendation 3. The operator should monitor the 

availability, quality, and reliability of communications 

services that are used by the redundant system. 

When the redundant part of the C2 system uses an external 

service, such as LTE cellular and satellite communications, 

redundancy is maintained only when the service is reliably 

available in the required quality. Therefore, it will be necessary 

for the operator to know whether the required service is 

available in the intended operation area and have a mechanism 

to keep track of the service quality and reliability. For this 

monitoring to be effective, the operator must know the 

performance of the communications required for the intended 

operation, such as minimum data transfer rate and maximum 

tolerable latency. When the monitoring finds the performance 

of one system below-required, switching to the other system 

should happen within the maximum tolerable latency so that 

operational control is maintained during this switching. 

Therefore, the operator should not include a C2 system that 

requires switching time longer than the maximum latency. For 

example, if switching to satellite communications requires 50 

seconds, as reported by the North Dakota TSO, the operator 

must exclude such a system from the redundancy configuration 

if the 50 seconds is larger than the maximum tolerable latency. 

Recommendation 4. The sUAS community should adopt a 

standard set of contingency steps to manage the loss of C2 

communications in a consistent manner across the airspace. 

Loss of C2 communications is identified when all C2 

communications systems in a redundancy configuration 
 

 

 

 

 

 

 

Figure 5. High-Level Loss of C2 Communications Mitigation Steps 

experience persistent below-required performance. This can 

occur due to broad-spectrum RF interference, hardware 

malfunction, etc. As high-density sUAS operations are 

envisioned to take place in the airspace over an urban area, 

when Loss of C2 communications occurs in one sUAS 

operation it will be important for that sUAS to display 

predictable behavior. This will facilitate other sUAS in the 

vicinity to anticipate an impact on their operation and take 

necessary measures to minimize any interruptions. 

High-level mitigation steps to manage a Loss of C2 

communications situation in a consistent manner are presented 

in Fig. 5 for adoption and further development by the sUAS 

community. To instill a predictable behavior while giving the 

freedom to operators to design a method to regain 

communications as they see fit, a notion of “timeout” is 

introduced in these steps. That is, sUAS should be programmed 

to regain communications when Loss of C2 communications 

occur until the timeout is reached, and upon the timeout initiate 

safe landing process. 

100: Loss of C2 Communications condition identified. 

102: Start counting time in Loss of C2 Communications 

condition, to compare against set timeout value at 106. 

104: Execute a plan to regain C2 communications. Example 

plan: fly higher and loiter for better reception, etc. 

106: Compare the elapsed time in Loss of C2 

Communications condition to the timeout value. 

108: Continue nominal operation. 

110: UA initiates a safe landing process. A safe landing is 

achieved when UA lands without causing a detrimental impact 

on people and property. 

112: The operator prepares a report that contains offnominal situation details. 

The length of the timeout is to be specified per a type of 

operation and its environment. For example, the length of 

timeout for solo operation over a cornfield can be much longer 

than one for high-density operation over a city as Loss of C2 

Communications poses less risk for the former than the latter. 

Discussion about the safe landing process is out of the scope of 

this paper and will be addressed in other UTM documentation. 

V. CONCLUSION 

Flight tests showed that the redundant C2 communications 

systems implemented by four UAS test sites were effective in 

maintaining operational control of sUAS in the airspace over 

several rural locations, as defined by the operators. Based on the 

test experience, four recommendations are made to assist sUAS 

operators for configuring an effective redundant C2 

communications system to maintain control in the airspace over 

an urban area where an increased level of RF activities takes 

place compared to a rural area. The NASA FAA UTM research 

team will use insights from the test to accommodate the FAA’s 

UAS integration effort. 

ACKNOWLEDGMENT 

The authors thank test site operators and their partners for 

conducting safe and successful flight tests. Also, appreciations 

go to the NASA UTM team. 

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J. E., “Unmanned Aircraft System Traffic Management (UTM) Concept 

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[13] Federal Communications Commission, Code of Federal Regulations 

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