Detection of Biological Scatter Using Spaceborne Precipitation Radar: A Feasibility Study

Historically it has proven very difficult to obtain reliable data pertaining to the mass movements of birds and bats during migration. For those cases when the animal of interest can be captured, banding or tracking devices can be used; however, this requires that the animal be recaptured as well. There are records of banded birds being recaptured long distances from the initial marking location, but this method does not provide any information on the trajectory an animal follows between the times when it is tagged and recaptured. Geolocators are small devices that rely on the amount of detected daylight as a means of estimating the position of an animal in time and as such, do provide trajectories; however, the spatial resolution can be rather poor. Another means of tracking animal movements is miniaturized GPS devices. These provide accurate time and location data, but the mass of the sensor package precludes the tracking of smaller animals. As an alternative to using tagging and tracking devices, one can also rely on remote sensors such as radar to monitor the movements of airborne migrating animals. Using radar, large numbers of individuals can be observed and there are no restrictions based on size, provided the radar has sufficient sensitivity. Of course, radar has limitations as well. It can be difficult if not impossible to determine the species of animal being detected or in some cases, even if a detected radar echo can be associated with biological scatter or not. Another drawback is the limited spatial coverage provided by single radars. This can be mitigated in part by tapping into existing radar networks such as those used to monitor air traffic or weather. In many aspects, weather radars have been shown to be well suited for the study of birds, bats, and insects and there is a growing interest in the use of weather radars to study migration.

Unfortunately, even networked radars due not provide adequate spatial coverage to fully explore weather patterns and long distance migration. Large extents of the atmosphere over the Earth's landmasses and the vast majority of its oceans go unobserved by radar. For the case of weather monitoring, the need for greater spatial coverage has led to the development of spaceborne radar carried on satellites. In the same way that land-based weather radar networks are capable of producing valuable biological information, spaceborne radar could also be used to observe the movements of flying animals. Admittedly the observations would only be available along narrow tracks with limited spatial and temporal coverage. However, these observations could still provide researchers with the needed information to better understand i) migration routes used by birds and bats; ii) how the animals are affected by abiotic drivers such as weather; iii) some measure of their abundance in sparsely sampled regions; and iv) a means for validating radar network data.

In this presentation we focus on measurements from the Precipitation Radar (PR) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite and the CloudSat Profiling Radar (CPR). The TRMM PR operates at 13.8 GHz has a nadir footprint with a resolution of about 5 km. It has a vertical resolution of 250 m and minimum detectability of about 18 dBZ. The CPR operates at 94 GHz, has a nadir footprint of with a resolution of about 1.4 km, and a minimum detectability of about -28 dBZ. Both should be able to produce useful data down to a height of about 500 m above ground level. The sensitivity of the PR should allow for the detection of large flocks of migrating birds. In the case of the CPR, the sensitivity should be adequate to detect much smaller collections of birds and bats. Here we discuss the feasibility of using spaceborne radar to observe and study flying animals during migration and methods of validating these observations. We will also consider the potential of using the precipitation radars to be used within the Global Precipitation Measurement (GPM) project.