Performance comparison of RDI ADCPs: broadband versus narrowband

Abstract

The acoustic Doppler current profiler (ADCP) manufactured by RD Instruments of San Diego CA can potentially greatly enhance the temporal and spatial resolution of underway current measurements relative to older ADCP technology. Despite this, adoption of broadband ADCPs has been slow in the US national research vessel fleet operated by the University-National Oceanographic Laboratory System (UNOLS). Narrowband ADCPs are now considered standard equipment in UNOLS intermediate and large class vessels. Adoption of the potentially superior newer technology has been delayed by a lack of rigorous intercomparison studies between the narrowband and broadband ADCP, plus some anecdotal evidence of decreased performance when substituting a broadband for a narrowband (such as significantly decreased profiling range). A series of intercomparison experiments were performed aboard the Research Vessel Seward Johnson during month-long cruises in May 1996 and May 1997 in the vicinity of Barbados, West Indies. The instruments involved were narrowband 150 kHz and broadband 150 kHz vessel mounted ADCPs. The two ADCPs were set up in as similar a manner as possible to provide the most valid intercomparison. In order to avoid any possible interference between ADCP systems experiments were run in time-interleaved fashion. Results showed that the broadband ADCP produced data quality superior to the narrowband under similar conditions. Profiling range in broadband water profiling mode 7 was equal to that of the narrowband. In broadband modes 4 and 1, profiling ranges averaged 78% and 75% of narrowband ranges respectively. Ensemble to ensemble variation of speed at 57 meters depth averaged 13.26 cm/sec for the narrowband versus 6.16 cm/sec for broadband mode 7 and 4.80 cm/sec for broadband mode 4. Broadband water profiling mode 7 provides profiling ranges equal to a narrowband of the same frequency with increased data accuracy, while water profiling mode 4 provides even greater data accuracy with only a loss of 20-25% in range.