1. Recognizing First and Second Order Features
Investigation of First Order Line Settings for MARACOOS High Frequency Radars
Erick Rivera1, Hugh Roarty1, John Kerfoot1, Josh Kohut1, Mike Smith1, Ethan Handel1, Colin Evans1, Scott Glenn1, Belinda Lipa2, Bruce Nyden2, Hector Aguilar2, Bill Rector2
1Rutgers University Institute of Marine and Coastal Sciences, 2 Codar Ocean Sensors, Inc.
Introduction
An important requirement of the SeaSonde data processing software is the correct delineation of the first order Bragg region. Effective first order delineation can be achieved using a combination of CODAR recommendations and careful examination of the local conditions influencing the relative positions of first and second order sea echo. Using cross spectra maps and power plots (Range Slices), appropriate boundaries can be set, resulting in the best overall quality data. The following is a discussion of steps, examples and recommendations to help achieve the proper First Order Line (FOL) settings.
The MARACOOS region consists of fourteen long range systems that measure sea surface currents along the Mid-Atlantic coast. In this poster we have identified four domains that show different characteristic in first order region. The settings were set according to visual inspections in Range cell 3, 6 and 11 and by changing time with each cross spectra file
Determining First Order Line Settings
SITE
Signal Above Noise (Factor)
Max Velocity (cm/s)
Peak Dropp off (dB)
Peak Drop off
(Factor)
NAUS
6 dB (3.98)
150 21
125.89
NANT
MVCO
BLCK
MRCH 20
100.0
HOOK
LOVE
BRIG
ASSA
7 dB (5.01)
21.8
151.36
CEDR
LISL
DUCK
DOWN
HATY
250 26
398.11
Type of Site
5 MHz
13 MHz
25 MHz
Decide not to look for Null (Nsec = 0)
Decide to Look for Null (Nsec = 1)
From Peak Power down by Peak Drop off
20 – 25 dB
From Peak Power down to Peak Drop off
From peak Power down to Peak Null ~6 dB
Sets the boundaries
Finds the Null
Identify the type of frequency that radar will be operating at
Decide to look or not to look for null to set the boundaries surrounding First Order Lines
Recognizing First and Second Order Features
Findings
It is critical to understand factors affecting flow in the area sensed by each individual radar when determing first order line settings
The settings of FOL for each site differs along the Mid-Atlantic from north to south. We identified four different current regimes. For large networks, we found it helpful to analyze each site independently to get high quality radials
Careful inspection of both power plots and cross spectra maps significantly increase the accuracy of Bragg peak delineation
Accurate Bragg peak delineation is achieve on 5 MHz systems without looking for second order features (nulls)
Dominant Features:
Fast currents
Strong tidal influence
Figure shows the boundaries between First Order (shaded dark) and Second Order (shaded light). Our primary objective is to identify the boundaries surrounding the first order region (dark blue) for each spectra. These boundaries are based on the interaction of the radar with the ocean waves.
Dominant Features:
Coastal shelf environment
Some tidal influence
5 mHz system showing Second Order features
X (PEAK POWER)
X / FNULL x
13 mHz system showing Second Order features
Dominant Features:
Coastal environment
Fresh water discharge from two estuaries
Since the lower frequency systems have greater separation between the first and second order peaks (above), settings for a 5 MHz system DO NOT look for nulls. The boundaries are identified with the Peak Drop off (FDOWN) from the peak power.
Since the higher frequency systems typically have less separation between the first and second order regions, settings for a 13 & 25 MHz system DO look for nulls. The algorithm looks for nulls from the Peak Power down by the Peak Null (FNULL) to constrain the boundaries of the first order region. If no nulls are found, it will go down by Peak Drop off (FDOWN)
When looking for nulls it is IMPORTANT, that Peak Null (FNULL) < Peak Drop off (FDOWN)
Dominant Features:
Shelf/Coastal environment
Strong Western Boundary Current