PASSIVE MICROWAVE REMOTE SENSING AT C-BAND: CONSIDERATIONS ON THE NEW FREQUENCY ALLOCATIONS PROPOSED IN AGENDA ITEM 1.19 OF THE WORLD RADIOCOMMUNICATION CONFERENCE 2027

P. de Matthaeis^1,2,3, B. Backus^4,3

¹NASA Goddard Space Flight Center, ²University of Maryland Baltimore County, ³IEEE Geoscience and Remote Sensing Society, ⁴Johns Hopkins Applied Physics Lab, USA

Introduction

A World Radiocommunication Conference (WRC) is organized by the International Telecommunication Union (ITU) every four years to revise and update the Radio Regulations (RR), which determine the use of the radio frequency spectrum by radio services. The next World Radiocommunication Conference, WRC-27, will be held at the end of 2027 (WRC-27). The WRC-27 agenda contains several topics of relevance to microwave remote sensing, called Earth Exploration Satellite Service (EESS) in ITU terminology. This presentation will address in detail one of these topics, of specific importance for passive microwave remote sensing, or EESS (passive).

Agenda Item 1.19 of WRC-27

Microwave radiometers have been using frequency bands with the range 6425-7250 MHz for many years, however they do so without an EESS allocation, and they are only recognized by RR Footnote 5.458. Their measurements are essential for estimating global sea surface temperature (SST) and used in combination with other frequencies for other applications such as measuring soil moisture, temperature of the ocean surface and sea surface wind through clouds. 6425-7250 MHz is a unique frequency range for SST observations, since it corresponds to the peak sensitivity to this geophysical parameter [1]. Recognizing the importance of these measurements, Agenda Item 1.19 was established for WRC-27 to seek new allocations in the 4200-4400 MHz and 8400-8500 MHz bands to complement the measurements with the range 6425-7125 MHz [2].

Use of the proposed new bands for SST retrieval

Creating new EESS (passive) allocations at 4200-4400 MHz and 8400-8500 MHz bands does not imply that the use of the range 6425-7125 MHz will be discontinued. Existing instruments operating at this band will continue to acquire measurements, however, they will be increasingly affected by Radio Frequency Interference (RFI) from IMT systems, particularly close to the coast.

Although the sensitivity of brightness temperature to SST is not as high in the new proposed frequency bands as at 6425-7125 MHz under the most common ocean conditions, measurements at 4200-4400 MHz and 8400-8500 MHz bands could complement those made in the currently used frequency band. Adding the two new frequency allocations would make the retrieval of SST more resilient against RFI. Since it is very unlikely that all three bands are affected by interference, a comparison between them can help identify interference so that the measurements in the band corrupted by RFI are not used in the estimation of SST. However, other factors such as different spatial resolution and increased attenuation from rain and clouds also need to be considered.

Risk of interference from other systems

At the World Radiocommunication Conference 2023 (WRC-23), frequency bands with the range 6425-7125 MHz were identified for use by the terrestrial component of International Mobile Telecommunications (IMT), with some small differences by ITU Region [3]. IMT deployment following this WRC-23 decision is expected to greatly increase the level of interference received by microwave radiometers operating at 6425-7250 MHz.

A new primary allocation for passive microwave remote sensing in the 4200-4400 MHz and 8400-8500 MHz bands would not shield these sensitive systems from interference caused by existing services. Key sources of this interference include the Aeronautical Radionavigation, Mobile, Fixed, Fixed Satellite and Radiolocation (radar) services. To address this risk, the ITU Radiocommunication Sector (ITU-R) is conducting studies to predict the level of interference that spaceborne microwave radiometers may experience from these operations.

Summary

WRC-27 will consider new primary frequency allocations at 4200-4400 MHz and 8400-8500 MHz to spaceborne passive microwave remote sensing under its Agenda Item 1.19. The presentation will discuss advantages and disadvantages offered by the two new proposed frequency bands. It will also summarize various scenarios of in-band and out-of-band interference that may impact the use of these new band as well the potential RFI into the currently used frequency range by new services.

References

1. D.M. Le Vine and E.P. Dinnat, The Multifrequency Future for Remote Sensing of Sea Surface Salinity from Space, Remote Sensing, 2020; 12(9):1381. https://doi.org/10.3390/rs12091381

2. Results of the first session of the Conference Preparatory Meeting for WRC-27 (CPM27-1), ITU Radiocommunication Bureau (BR) Administrative Circular CA/270, Geneva, 26 January 2024, Resolution 813, p. 668.

3. Resolution 220 (WRC-23) in ITU World Radiocommunication Conference 2023: Final Acts, Geneva, 2024. https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.16-2024-PDF-E.pdf