US agencies tangle on possible C-band interference

January 18, 2022  - By
Photo: guvendemir/E+/Getty Images

Radio altimeters are critical in aircraft landing systems. (Getty image). (Photo: guvendemir/E+/Getty Images)

As most GNSS industry insiders already know, the Federal Communications Commission (FCC) has licensed adjacent GNSS L1 protection frequencies to Ligado Networks (formerly Lightsquared) for its nationwide 4G-LTE network.

Many objections emerged as expected this second time around from government agencies, industries and U.S. forces — yet the roll-out is still underway, pending actual interference occurring. This all in an attempt to find communications bandwidth for many emerging commercial radio applications.

Now, as 5G C-Band 3.7–3.98 GHz wireless phone networks begin their FCC approved roll-out, the Federal Aviation Administration (FAA) has apparently lodged an unanticipated objection on the grounds that cross-interference could compromise aircraft radar altimeter and wireless communications that operate at 4.2 to 4.4 GHz in the C-band.

While 5G wireless has already been operating in many parts of the world without reports of interference with aircraft systems, the FAA appears to be taking a more conservative approach to how aviation in the United States should co-exist with the new 5G phone wireless system. The FAA has proposed imposing an exclusion zone around airports for 5G wireless networks — which apparently have already been operating with reduced power in these areas — until cooperative operation has been proven.

Now along comes a new C-band wireless network (SkyLink) aimed at providing high-integrity unmanned aircraft systems (UAS) command and control (C2). The SkyLink company uAvionix has also developed a C-band Control & Non-Payload Communications (CNPC) radio for UAS applications.

Together with Thales, uAvionics recently tested its radio with its SkyLink radio network. The network has been qualified in accordance with the RTCA DO-377 standard for a network management system that monitors network and radio link health, and the radio has been developed to the draft FAA Technical Standard Order (TSO) C-213A to support critical UAS operations.

The network uses new DO-362A-compliant SkyLink C-band radios, integrates certifiable aviation-grade hardware and software, uses frequency agility, and provides critical fault monitoring and control capability. The objective is to obviate the loss of the C2 link with the vehicle, and thereby enable beyond-visual-line-of-sight (BVLOS) operations without an FAA waiver.


It’s unclear whether the emergence of the C-band network — approved by both the FAA and FCC — will play a role in the current phone network interoperability issue. However, uAvionix reports that several sites in the United States and offshore are either rolling out C-band SkyLink networks or evaluating doing so.

  • North Dakota already has an ISM-band SkyLink network at its UAS test site that will shortly transition to C-band.
  • The Choctaw Nation in Oklahoma under an FAA program seeks to enable BVLOS operations through a C-band C2 network.
  • New Mexico State University will use a Skylink C2 network around Las Cruces airport for small UAS (sUAS) operations and testing to overcome anticipated interference from nearby Air Force and Space Force operations.
  • The Tillamook UAS test range in Oregon has already installed the first ground site of a SkyLink network.
  • The University of Alaska at the Fairbanks UAS test site will use uAvionics radios for testing large, heavy UAS operations.
  • In Canada near the Jonesburg airport, a Skylink C2 network will support the safety case for BVLOS pipeline inspection operations for the oil industry.

While many of these new networks are not yet fully online, the use of frequency hopping, safety-monitored C-band, and certifiable transmissions for UAS command and control appears to be moving forward rapidly. Because the FAA is supporting this testing phase, it seems inevitable that large-scale C-band network rollout for UAS C2 will happen eventually.

5G phone networks, wireless UAS command and control, and aircraft safety systems essential for landing will need to find a way to co-exist and provide reliable, sustained service to their respective customer bases. Look for much more to develop in this ongoing tussle between industry groups and agencies who appear to have little in common, other than grudgingly sharing a crowded radio spectrum.

Tony Murfin
GNSS Aerospace

About the Author: Tony Murfin

Tony Murfin is managing consultant for GNSS Aerospace LLC, Florida. Murfin provides business development consulting services to companies involved in GNSS products and markets, and writes for GPS World as the OEM Professional contributing editor. Previously, Murfin worked for NovAtel Inc. in Calgary, Canada, as vice president of Business Development; for CMC Electronics in Montreal, Canada, as business development manager, product manager, software manger and software engineer; for CAE in Montreal as simulation software engineer; and for BAe in Warton, UK, as senior avionics engineer. Murfin has a B.Sc. from the University of Manchester Institute of Science and Technology in the UK, and is a UK Chartered Engineer (CEng MIET).