A recent upgrade to a commercial cell phone tower near the U.S. National Science Foundation Very Large Array (NSF VLA) briefly disrupted some observations, but operations are back to normal following a cooperative effort between the NSF National Radio Astronomy Observatory (NSF NRAO) and the wireless provider.
In early 2025, the NSF NRAO was notified that a tower on Gray Hill, near the NSF VLA’s east arm, would be upgraded to add new LTE and 5G services transmitting at higher frequencies than previously used in the area. The upgrade, which went into use on January 25, 2026, introduced signals above 1 GHz for the first time at that site, including bands around 2.1 GHz that overlap with one of the NSF VLA’s most heavily used observing bands, known as S-band (2–4 GHz).
Because the NSF VLA’s antennas are extraordinarily sensitive, the new signals were immediately visible in the observatory’s data during an ongoing sky survey. This interference caused subtle “compression” effects in some receivers, especially on antennas closest to the tower. Within weeks, the NSF NRAO temporarily suspended most regular S‑band observing while engineers and scientists analyzed the impact and coordinated tests with the wireless provider.
Over March and April 2026, NSF NRAO staff and the provider carried out two days of carefully planned tests, turning different cell tower sectors on and off and adjusting their power levels while the NSF VLA observed standard reference sources. They also used a dedicated spectrum monitor at the center of the array to independently track the new signals over time and frequency. The tests confirmed that one sector of the tower, pointing toward the heart of the VLA (the “Gamma” sector), had the largest impact on data quality when transmitting at full power in the new 2.1 GHz band. Another sector (the “Alpha” sector), pointing largely away from most antennas, produced much smaller effects even at full power.
Based on these results, the NSF NRAO requested that the provider continue using the new 2.1 GHz service only in the Alpha sector and keep the same band turned off in the two sectors facing the VLA (the Beta and Gamma sectors). The provider agreed, and on April 24, 2026, it shut off the 2.1 GHz transmissions in those two sectors while maintaining service in Alpha. Spectrum monitoring now shows that the 2.1 GHz signal is only weakly detectable at the VLA’s center, and follow‑up checks indicate that most antennas can once again observe across the full S‑band without significant degradation.
During the investigation, the NSF NRAO offered NSF VLA observers an alternative setup that temporarily shifted part of the S‑band coverage away from the tower’s frequencies, allowing many projects to continue at reduced bandwidth. Normal, full‑band S‑band operations resumed on May 6, 2026, after the sector changes took effect and engineering teams verified that standard observing configurations were again usable. The two antennas closest to the Gray Hill tower may still see some effects at the very lowest S‑band frequencies in certain array configurations, and the NSF NRAO will continue to study those residual impacts.
“We face many challenges—and possibilities—sharing the radio spectrum between next‑generation communications technology and cutting‑edge science,” said Chris De Pree, who leads spectrum management for the NSF NRAO, “The VLA sits within the New Mexico Radio Coordination Zone, a region created to encourage voluntary coordination with wireless providers. Spectrum is a limited resource, and the cooperation we saw here is exactly what we need as new wireless technologies roll out across the country.”
About NRAO
The National Radio Astronomy Observatory is a major facility of the U.S. National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
