Radar systems consist of powerful transmitters that emit high-powered pulsed broadband signals and sensitive receivers to capture the returning low-powered echoes. Characterizing radio frequency (RF) emissions from radar transmitters presents particular challenges due to the nature of radar signals. Many federally operated radar stations emit peak effective isotropic radiated power (EIRP) levels that are among the highest of all radio transmitter systems. Even theoretically perfect radar transmitters cannot confine their electromagnetic emissions entirely to their assigned operating frequencies, or for that matter even to allocated radar spectrum bands, due to the inherent pulse modulation characteristics of these systems. While measuring radar emissions requires equipment that is capable of fast-response wideband measurements at high power levels, analyzing interference to radar reception requires a measurement system that can capture signals at very low power levels and distinguish them clearly from background noise.
The Institute’s Radio Spectrum Measurement Science (RSMS) program has developed special expertise and custom-built capabilities particularly suited to measurements of high-dynamic-range radar emissions, general measurements involving radar technologies, and electromagnetic compatibility (EMC) involving radar systems. In addition to measuring RF emissions, the RSMS program has comprehensive resources and tools to synthesize radar signals and inject them into all types of radio systems to examine receiver performance in the presence of interference.
Radar Emissions Measurements
All U.S. radar systems must meet emission limits imposed by the NTIA Radar Spectrum Engineering Criteria (RSEC) as described in the NTIA “Redbook” “to ensure an acceptable degree of electromagnetic compatibility among radar systems, and between such systems and those of other radio services sharing the frequency spectrum.” Many foreign customers who purchase radar system from U.S. industry also require that the radars comply with the RSEC. The Institute has developed highly specialized capabilities for performing RSEC compliance measurements on radar transmitters using RSMS resources. Procedures for RSEC measurements are provided in available literature, but building measurement systems that can perform these measurements can be prohibitively difficult and expensive for individual organizations, as compared to working directly with ITS to perform the measurement. ITS performs RSEC measurements for OSM, other agencies, and private sector companies. RSEC measurements for other agencies and private industry are performed under IA and CRADA funding, respectively. ITS performed the following RSEC compliance measurements during FY 2013:
- AN/TPQ-53: ITS performed emission measurements on this new radar at the Yuma Proving Grounds in Arizona using the RSMS mobile laboratory. The work was performed for an industry sponsor via a CRADA. The AN/TPQ-53 radar detects and tracks artillery rounds, short-range rockets and mortar rounds in flight. Live fire tests were conducted while the ITS measurements were in progress. The measurement results were analyzed by ITS engineers and provided to the sponsor to attain spectrum certification for the radar under the NTIA RSEC. These measurements and the resulting analysis helped to move this important new system forward to deployment with U.S. and allied forces worldwide.
- AN/APY-10: ITS performed emission measurements on this new radar at the ITS Table Mountain field site using a portable RSMS system. The work was performed for an industry sponsor via a CRADA. The AN/APY-10 airborne radar is currently being deployed in P-8 maritime surveillance aircraft with the U.S. Navy and foreign customers. ITS engineers analyzed the data and provided results to the sponsor. These measurements and the resulting analysis showed RSEC compliance, helping to move this new system forward to deployment with U.S. forces and foreign customers worldwide.
- AN/SPY-1(B/D): ITS performed emission measurements on this radar at the Navy’s Wallops Island Test Range in Virginia using the RSMS mobile laboratory. The work was performed for OSM and the Naval Research Laboratory. This AN/SPY-1 variant is deployed on a variety of naval vessels. These measurements and the resulting analysis are being used in spectrum sharing studies for the U.S. Administration, domestically and internationally.
- AN/SPN-43: ITS performed emission measurements on this radar at the Naval Electronic Systems Engineering Activity (NESEA) test range in Maryland using the RSMS mobile laboratory. The work was performed for OSM and the Naval Research Laboratory. The AN/SPN-43 is the U.S. Navy’s air marshalling radar, deployed on a variety of aircraft carriers. These measurements and the resulting analysis are being used in 3.6 GHz spectrum sharing studies for the U.S. Administration, domestically and internationally.
- ASR-11: ITS performed emission measurements on the FAA’s Airport Surveillance Radar, model 11 (ASR-11) units in Colorado using the RSMS mobile laboratory. The work was performed for OSM and in support of an EMC study conducted jointly with the National Weather Service (NWS). The ASR-11 is the latest radar deployed by the FAA in and around smaller airports for air traffic control. These measurements and the resulting analysis have been used in 2.7–2.9 GHz spectrum repacking studies for the U.S. Administration, domestically and internationally.
Electromagnetic Compatibility Studies
In support of domestic and international initiatives to more efficiently use spectrum below 6 GHz, ITS undertook several studies with OSM on effects of radar interference on radio receivers. ITS synthesized radar interference and injected it into victim receivers at field sites in Virginia and Oklahoma. Quantifying the effects of radar interference on other radio systems, under controlled conditions, is critically important to moving spectrum sharing forward in a number of radio frequency bands.
- 3.6 GHz LTE Spectrum Sharing Study: In support of spectrum sharing initiatives between incumbent radar systems and proposed new terrestrial broadband LTE networks at 3.6 GHz, ITS injected a wide variety of radar waveforms into LTE base station receivers and measured interference effects on those receivers under controlled conditions. The radar waveform parameters spanned the range of all existing and future planned Federal radar systems at 3.6 GHz. Interference power levels were varied while LTE receiver performance was recorded. The results of this extensive study were published in NTIA Technical Report TR-13-499 and were used as part of NTIA’s response to an FCC Notice of Proposed Rulemaking (NPRM).
- 2.7–2.9 GHz Band Repacking Study: In support of domestic and international studies for more efficient spectrum use, ITS, in collaboration with the NWS, investigated radar-to-radar spectrum sharing criteria in the 2.7–2.9 GHz band. ITS engineers used a portable interference test bed to inject synthesized radar signal into an NWS NEXRAD weather radar in Oklahoma. The NEXRAD is used by the NWS to monitor local weather conditions and generate severe-weather warnings. Criteria need to be established for minimum separations in both distance and frequency from one NEXRAD to the next and between NEXRADs and FAA ASRs that utilize the same band. Both NEXRAD and ASR signals were injected into an engineering test-and-development NEXRAD receiver under controlled conditions and the effects of the interference were recorded by NWS. NWS personnel are evaluating those data to develop radar-to-radar sharing criteria in the band. The results are being incorporated into studies for repacking of radars in the 2.7–2.9 GHz band.