A Commandable Lightning Detector in Space

FORTE is the only space-based platform to feature instrumentation to simultaneously record the optical and Radio-Frequency (RF) signals produced by ligthning. It was also operated in campaign mode, meaning that its instrumentation could be reconfigured on-orbit to record different aspects of lightning.

The Fast On-Orbit Recording of Transient Events (FORTE) satellite was a DOE-sponsored platform for studying transient optical and RF pulses in the Earth-atmosphere system that are primarily produced by lightning. FORTE was launched in late 1997 into a low Earth orbit at ~800 km with a 70-degree inclination. The spacecraft was notable for being the first satellite built with a lightweight all-composite structure.

FORTE was the only space-based platform to feature coincident optical and RF instrumentation for analyzing the same lightning event. Its Optical Lightning System (OLS) consisted of a high-speed photodiode detector (PDD) and a pixelated Lightning Locating System (LLS). The LLS was based on the LIS / OTD lightning imager design with 10 km pixels and an 80-degree Field of View. The FORTE Radio Frequency System consisted of 3 VHF receivers connected to 2 LPA monopole antennas. In one operating mode, 2 VHF receivers are each connected to one of the antennas, while the third receiver records signals from both antennas in the other mode.

Unlike the NASA / NOAA platforms that have flown lightning imagers, FORTE was a commandable satellite that was reconfigured over its mission to maximize the scientific impact of its recorded events. The RF and optical systems could be commanded to trigger independently, or both systems could be triggered by the optical system. The frequency bands for the RF receivers could be selected between 30 and 300 MHz. Polarization information could also be obtained in the 2-receiver mode if both receivers were set to a common reference signal. FORTE's niche in the space-based lightning community lies in the impressive variety of measurements that it made.