NMT Interferometer

The New Mexico Tech (NMT) interferometer maps lightning at 274 MHz (VHF) in 2 dimensions (azimuth and elevation) with high time resolution ($\sim$2 $\mu$s). The interferometer is able to map lightning by obtaining phase data from multiple pairs of antennas along orthogonal baselines (Rhodes et al., 1994; Richard and Auffray, 1985; Hayenga and Warwick, 1981). Prior to 1995, the interferometer antennas were confined to a horizontal plane and this limited the interferometer's ability to resolve flash structure beyond $\sim$20 km range (Shao, 1994). In 1995, a vertical baseline was added to the interferometer which extended the interferometer's useful range to beyond 50 km. However, the vertical baseline's systematic errors have not yet been adequately determined and this limits its effectiveness in this study, particularly since the sprite-producing flashes were centered at or beyond 80 km range.

In addition to the phase data, log-RF and electric field data are sampled by the interferometer's data acquisition system (see Appendix A.2). Like the interferometer phase data, the log-RF is centered at 274 MHz with a 3 dB bandwidth of 6 MHz. The log-RF was sampled with 8-bit resolution.

The electric field data were acquired from a slow antenna (SA). Additional electric field data were sampled in the form of ``fast antenna'' (FA) data, which are amplified and high-pass-filtered SA data. Both the SA and FA were sampled at 12-bit resolution. The SA electric field data will be used in Section 3.5 to estimate the charge moment at various times after a return stroke for both sprite-producing and non-sprite-producing discharges.

The interferometer data acquisition system acquires data at a 500 kHz rate and can be armed in either an RF-threshold mode or in a FA-trigger mode. In the RF-threshold mode, a 100 $\mu$s data block of phase, log-RF, and FA data is acquired whenever a log-RF sample exceeds a user-specified threshold within a data block. The slow antenna data is sampled continuously at 5 kHz in this mode. In the FA-trigger mode, a user-specified FA threshold is used to trigger the system. A user-specifed pre- and post-trigger length of phase, log-RF, and FA data is stored around the trigger. The SA data is also sampled and stored for the same time interval, albeit with a 10 kHz sample rate.