Earth Exploration-Satellite Service - Active Spaceborne Remote Sensing and Operations

1. Earth Exploration-Satellite Service - Active Spaceborne Remote Sensing and Operations Bryan HUNEYCUTT (NASA/JPL) WMO, Geneva, Switzerland 7 October 2002

2. Active Sensor Types • SYNTHETIC APERTURE RADARS - Sensors looking to one side of nadir track, collecting phase and time history of coherent radar echo from which typically can be produced a radar image or topographical map of the Earth surface • ALTIMETERS - Sensors looking at nadir, measuring the precise time between a transmit event and receive event to extract the precise altitude of ocean surface • SCATTEROMETERS - Sensors looking at various aspects to the sides of the nadir track, using the measurement of the return echo power variation with aspect angle to determine wind direction and speed on Earth ocean surface • PRECIPITATION RADARS - Sensors scanning perpendicular to nadir track, measuring the radar echo from rainfall to determine the rainfall rate over Earth surface, usually concentrating on the tropics • CLOUD PROFILE RADARS - Sensors looking at nadir, measuring the radar echo return from clouds, to determine cloud reflectivity profile over Earth surface

3. ACTIVE SENSOR AND SERVICE DEFINITIONS Active Sensor: a measuring instrument in the Earth exploration-satellite service or in the space research service by means of which information is obtained by transmission and reception of radio waves (RR) Earth Exploration-Satellite Service: a radiocommunication service between earth stations and one or more space stations, which may include links between space stations, in which: -      information relating to the characteristics of the Earth and its natural phenomena including data relating to the state of the environment is obtained from active sensors or passive sensors on earth satellite; -      similar information is collected from airborne or earth-based platforms; -      such information may be distributed to earth stations within the system concerned Space Research Service: a radiocommunications service in which spacecraft or other objects in space are used for scientific or technological research purposes

4. Active Sensors Applications by Sensor Type

5. Active Sensor Characteristics

6. Active Sensor Examples SAR-Radar Image Honolulu,HI Altimeter-Sea Level Scatterometer-Wind Speeds Precipitation Radar-Rain Rates Cloud Radar-Cloud Reflectivity Profile

7. Synthetic Aperture Radars (SARs) • Provide radar images of the Earth’s surface • RF center frequency depends on the Earth’s surface interaction with the EM field • RF bandwidth affects the resolution of the image pixels • Allowable image pixel quality degradation determines allowable interference level SAR Illumination Swath Chirp Spectrum

8. Altimeters • Provide altitude of the Earth’s ocean surface • RF center frequency depends on the ocean surface interaction with the EM field • Dual frequency operation allows ionospheric delay compensation • TOPEX/POSEIDON uses frequencies around 13.6 GHz and 5.3 GHz • Allowable height accuracy degradation determines the allowable interference level Illustration of Altimeter Return

9. Scatterometers • Provide the wind direction and speed over the Earth’s ocean surface • RF center frequency depends on the ocean surface interaction with the EM field and its variation over aspect angle • Narrow RF signal bandwidth provides the needed measurement cell resolution • Allowable wind speedaccuracydegradation determines the allowable interference level Variation of Backscatter with Aspect Angle

10. Scatterometers SEAWINDS scanning pencil beam illuminates scans at two different look angles from nadir, and scans 360 degrees about nadir in azimuth NSCAT illuminates the Earth’s surface at several different fixed aspect angles

11. Precipitation Radars • Provide precipitation rate over the Earth’s surface, typically concentrating on rainfall in the tropics • RF center frequency depends on the precipitation interaction with the EM field • Narrow RF signal bandwidth provides the needed measurement cell resolution • Tropical Rainfall Measurement Mission (TRMM) uses only 0.6 MHz RF bandwidth • Allowable minimum precipitation reflectivity degradation determines the allowable interference level

12. Cloud Profile Radars • Provide three dimension profile of cloud reflectivity over the Earth’s surface • RF center frequency depends on the ocean surface interaction with the EM field and its variation over aspect angle • Antennas with very low sidelobes so as to isolate the cloud return from the higher surface return illuminated by the sidelobes • Narrow RF signal bandwidth provides the needed measurement cell resolution • Allowable reflectivity accuracy degradation determines the allowable interference level

13. Active Sensor Interference And Performance Criteria

14. Typical Interference Levels at Earth’s Surface

15. Compatibility Studies

16. Compatibility Studies

17. WRC’03 Issues/Concerns

18. 420-470 MHZ BAND CURRENT STUDIES The WRC-2003 resolution 727 resolves to consider provision of up to 6 MHz of frequency spectrum to the EESS (active) in the band 420-470 MHz. Several studies from the spaceborne active sensor community has analyzed the interference from spaceborne SARs in the band 420-470 MHz into Earth stations, the radio amateur service, fixed service, and ISM equipment, concluding that although there may be occasional interference to the various other services, that the interference will be short in time and will have a very long interval of six months or longer and thus the affected service will not be rendered incapable of operating effectively; current studies include sharing analyses with terrestrial radars, military radiolocation systems, and amateur systems. Another study by the the spaceborne active sensor community analyzes the interference levels of a lower power, lower sidelobe spaceborne SARs into the amateur and amateur satellite services and airborne radiolocation radars, offering that the SAR parameters can be chosen to reduce the interference level to acceptable levels. Mitigation techniques include lowering the antenna sidelobes in both azimuth and range, and lowering the average power with reduced backscatter sensitivity. Limiting operations to geographical regions, delineated by boxes of latitude, longitude may also be an option.

19. 5460-5570 MHZ BAND CURRENT STUDIES The WRC-2003 resolution 736 resolves to consider additional primary allocation for the EESS (active) and SRS (active) in the band 5460-5570 MHz. For the 5460-5570 MHz band, the previous studies in the 5250-5460 MHz band showing compatibility between spaceborne active sensors and the radiolocation/ radionavigation services, have been extended to the band 5460-5570 MHz. For interference mitigation techniques as provided in the Rec. ITU-R SA.1280, the selection of active spaceborne sensor emission characteristics can help to mitigate the interference potential to terrestrial radars. One consideration for this 5460-5570 MHz band is to use it for the wideband SARs with high time- bandwidth products, which tend to give high on-tune rejection factors and thereby lower the interference levels into terrestrial radars. In addition, several studies from the spaceborne active sensor community have analyzed the compatibility between spaceborne SARs in the band 5470-5570 MHz and wireless access systems including RLANs in the mobile service. The wider SAR bandwidths also give higher kTB system noise temperatures, allowing greater acceptable interference levels from other services, and further improves the sharing situation.