1. LCDR George Wright, USN OC 3570 – Winter 2008 Friday, March 14th 2008
VALIDATION OF HIGH FREQUENCY RADAR USED IN OCEAN SURFACE CURRENT MAPPING VIA IN-SITU DRIFTER BUOYS
LCDR George Wright, USN
OC 3570 – Winter 2008
Friday, March 14th 2008

2. Purpose
Goal:
This study focuses on the validation aspects of High Frequency radar through the use of four drifters placed in-situ from January 2008 in the Central California Coast from Monterey to San Francisco.
Statistical comparisons of radial current velocity data from 8 CODAR (Coastal Ocean Dynamics Application Radar) stations to the radial velocity of each of the drifters were obtained
VMADCP (Vessel Mounted Acoustic Doppler Current Profiler) data from approximately 8 meters depth from the research ship Point Sur was compared with the CODAR data utilizing similar techniques as with the drifter buoys.

3. Scope Review of HF Radar Theory Methodology Results and Analysis:
Drifters vs. COMM HF RADAR
Drifters vs. FORT HF RADAR
Drifters vs. MONT HF RADAR
Drifters vs. PPIN HF RADAR
Summary / Conclusion

4. Review of HF Radar Theory
The primary type of wave that reflects high frequency radar is shown to be surface gravity waves which are characteristic of having approximately 10m wavelengths.

5. Review of HF Radar Theory
The backscatter spectrum that contains the Bragg peaks will show a slight Doppler shift which is attributed to the underlying ocean current as well as the deep water phase speed of the wave.
Since the phase speed of the wave is theoretically known due to the dispersion relation of surface gravity waves, the remaining shift is therefore due to the current only.

6. Review of HF Radar Theory

7. Microstar Drifter and
1 meter drogue

8. Deployment of the Pacific Gyre Microstar drifter
Deployment of the Pacific Gyre Microstar drifter. The surface float contains the telemetry system, antenna, batteries and sensors. Drifter positions are calculated by an onboard Global Positioning System (GPS) receiver.

9. Summary of Instrumentation used for Analysis
Period 2008
Measured depth
Measured Period
Data Interval
HF Radar
22 Jan 17: Jan 23:00 (GMT)
~ 1 m
1 hour
VMADCP
22 Jan 17: Jan 23:55 (GMT)
~ 8 m
5 minutes
Drifters
23 Jan 20: Jan 18:00 (GMT)
1 hour /
10 minutes

10. U’D = UD x cos (α) + VD x sin (α)
EQUATIONS
U’D = UD x cos (α) + VD x sin (α)
RMS=sqrt(mean(Vd-Vr)2)
sy|x2  is the square of the error of a linear regression of xi  on yi
sy2  is just the variance of y

11. HF RADAR STATIONS STUDIED
Radar Station ID
Name
Position
Center Frequency
Beam Pattern
COMM
Commonweal Center
N , W
MHz
Measured
FORT
Fort Funston
N , W
MHz
MONT
Montara Sanitary District
N , W
MHz
PESC
Pescadero
N , W
MHz
BIGC
Big Creek
N , W
Not Operable
SCRZ
Santa Cruz
N , W
MHz
MLML
Moss Landing
N , W
MHz
NPGS
Naval Postgraduate School
N , W
MHz
PPIN
Point Pinos
N , W
MHz
GCYN
Granite Canyon
N , W
MHz
Idealized
PSUR
Point Sur
N , W
4.660 MHz

12. Plot of drifter tracks obtained during cruise: Drifter 899 (magenta), drifter 901 (green), drifter 625 (red), and drifter 860 (blue). Positions obtained in 10 minute intervals via GPS. HF radar station locations used during the comparison are shown as well.

13. SAMPLE HOURLY DATA OUTPUT OF A HF RADAR STATION
COMM DATA 01 FEB Z

14. Visual representation of radial velocity calculations done between drifter and typical HF radar station. Shown here is sample hourly MONT HF radar pattern, data points (black squares) <3km away from drifter position used as measurement points, and hourly averaged drifter positions (red dots). Representative radar radial velocity vectors (red) and drifter radial velocity vectors (blue) shown too. Drifter used for plot is #901.
1630
1620
1610
1600
3KM
1550
1600 avg
1540
1530

16. COMM RESULTS

17. FORT RESULTS

18. MONT RESULTS

19. PPIN RESULTS

20. SUMMARY OF DATA Radar Site Correlation Coefficient Drifter R^2 (%)
VMADCP R^2 (%)
RMS Difference (cm/s)
Slope
Intercept
Number of hourly averaged observations
Total Number of 10 min observations
COMM
-0.04
0.10
5.20
65.66
-0.02 33 39
236
FORT
0.35
12.60
39.40
17.26
0.32 1 68
410
MONT
0.75
81.70
42.50
8.52
1.1
8.4 95
1036
PESC
12.20
5.40
36.52
0.43
-4.4
168
978
SCRZ
0.69
47.20
35.50
9.81
0.65
3.6
123
743
MLML
0.90
56.00
42.00
19.04
0.86
173
581
NPGS
0.62
38.50
5.60
17.61
0.49 -2
117
715
PPIN
47.50
41.50
16.01
0.81
-13.4
186
1115
Average
0.54
36.98
27.14
23.80
0.58
3.36
121
727

21. Previous validation studies have been conducted in the past, specifically with moored current meters and profiler data.
Holbrook and Frisch (1991) and Schott et al. (1996) studied the correlation between HF radar and current meters and found the RMS differences ranged between cm/s.
Paduan and Rosenfeld (1996) compared both ADCP and drifter data to show RMS differences between cm/s.
Chapman et al. (1997) compared shipborne current meter data and HF radar data and showed the upper bound of HF radar accuracy to be around 7-8 cm/s.
Recent studies by (Kohut and Glenn 2003; Emery et al. 2004; and Kaplan et al. 2005) showed RMS differences between 7 and 19 cm/s.

22. FURTHER RESEARCH
More drifters to increase sample size
Multiple approach to data / coexist
drifters/ moored ADCP / glider / etc
Saturate spatial scales to study effect of
radar algorithm averaging on radial vectors.

23. CONCLUSION
This study’s results fall in line with the previous studies mentioned above. With the exception of COMM and PESC data, the RMS differences had a reasonable spread of between 8.52 cm/s to upwards of cm/s.

25. QUESTIONS?