Range, Not Precision: Block-Floating-Point Half-Precision FFT and SAR Imaging on Apple Silicon

Abstract

Half precision (FP16) promises to double FFT throughput on GPUs, but the prevailing view is that its 10-bit mantissa makes it unsuitable for radar-grade signal processing. We show this framing is wrong on Apple Silicon: the binding constraint for FFT and Synthetic Aperture Radar (SAR) is not mantissa \emph{precision} but the 5-bit exponent's \emph{dynamic range}. We first measure that an FP16 FFT is mantissa-limited at 56--61~dB signal-to-quantization-noise ratio (SQNR) -- comfortably radar-usable -- yet a na\"ive FP16 SAR pipeline produces \emph{only} \texttt{NaN}, because the conjugate--FFT--conjugate inverse transform grows magnitudes by a factor of $N$, and the matched-filter product ($\sim\!5\times10^6$ at $N\!=\!4096$) overflows FP16's 65{,}504 ceiling. We resolve this with a fixed-shift \emph{block-floating-point} (BFP) schedule: a single $1/N$ scale applied before each inverse transform bounds every intermediate below 4096. A cascade follows: range-compression output becomes $O(1)$ instead of $O(N)$, which in turn keeps the downstream azimuth-FFT output FP16-loadable instead of overflowing at $O(N^2)$. The result is the first quality-preserving FP16 SAR pipeline: peak/integrated sidelobe ratios, target SNR, and resolution match the FP32 reference to within $0.1$~dB at $42$~dB end-to-end SQNR, while a radix-8 FP16 FFT reaches 306~GFLOPS -- $2.2\times$ over the 139~GFLOPS FP32 baseline -- on a fanless Apple~M1. Finally, we measure that FP8 (E4M3/E5M2) collapses to 14--20~dB SQNR, making FP16 \emph{today's} precision floor for FFT-based radar -- one that future precision-recovery methods may yet lower -- and showing that the lever for low precision here is range management, not mantissa bits.