PROCEEDINGS OF SYMPOL-2013

Validation Methods Implemented to Ascertain the Positional Data Uncertainty of an Indigenously Developed

Drifter Buoy-Pradyu R.Srinivasan1, Shijo Zacharia2, T.Thamarai3, and Tata Sudhakar4

1National Institute of Ocean Technology, Chennai-100, India. rsrini@niot.res.in

2National Institute of Ocean Technology, Chennai-100, India. zacharia@niot.res.in 3National Institute of Ocean Technology, Chennai-100, India. thamarai@niot.res.in

4National Institute of Ocean Technology, Chennai-100, India. tata@niot.res.in

Abstract---A systematic approach in implementing a suitable validation method which evaluates a newly developed product is highly necessary to ensure its continuous reliability and consistency of its quality[1]performances. More specifically any sensor or instrument developed for the real time data collection and measurement of ocean parameters need to be prejudged and evaluated for its data product quality before its being suggested to use for the practical applications particularly for study and modeling of any physical phenomenon. NIOT has successfully implemented a high accuracy smart sensor GPS receiver module in its drifter buoy which acquires positional (Latitude & Longitude) information and transmits using INSAT modems. NIOT named this drifter buoy indigenization as Pradyu. Performed land based triangulation test method and deployment & comparison of drifters using the imported drifter from Marlin-Yug, Ukraine is carried out in Bay of Bengal. The above validation methods[2]carried out confirms the drifter’s ability to follow and track the surface mixed layer current[5] with a positional accuracy of ±10m. In this paper, the recent outcome of different methods implemented for validating the drifter is presented. Index Terms---Validation methods, Smart GPS receiver, Positional data accuracy, INSAT modem, land and Sea Test Comparisons.

1. INTRODUCTION

As part of the Ministry of Earth Sciences (MoES) Indian drifter buoy programme, drifters with Argos transmitters were deployed Indian waters for SST and surface current observations. The reliability of the drifter buoy data primarily depends on sensors, battery, hull, data acquisition system and

communication systems. Present day drifters deployed is equipped with Argos satellite based devices used for data transmission. Argos satellites are polar orbit satellite and pass tropical latitude four to six times in a day which limits the number of data sets one can get from an observation system. Also the information received in delayed mode and the data is remotely logged in CLS server at France with

R.Srinivasan et. al,: Validation Methods Implemented to Ascertain the Positional Data Uncertainty of Indigenously Developed Drifter Buoy.

on latency period of four to six to its actual measured time. Besides the best possible positional accuracy of the Argos drifters which use Argos-2 transmitters is expected to be around ± 350m to 1000m on clear sky conditions and is pertinent to vary with weather conditions. In order to improve drifter buoy positional data and in-turn to arrive surface current mapping NIOT built drifter with high accuracy smart GPS receiver module which acquires positional data with an accuracy of ±10m. The validation test methods carried out with imported drifter indicates and confirms the practical positional data accuracy of drifter-Pradyu within ±10m. Drifter buoys once deployed at sea will drift and track the ocean surface current according to the water body movements. The averaged data is transmitted in real-time to data reception center using INSAT telemetry modems. The accurate positional data obtained using drifter is used for mapping the surface current of the drifter path. These data collected is vital for the prediction of long term and short term climate changes and weather forecast study[4]and modeling applications.

2. Smart GPS receiver for positional data collection

The PRADYU drifter buoy is the system with a

smart GPS[8] receiver module, SST sensor and INSAT communication device. The main objective of the smart GPS receiver interface[10] is to collect the drifter’s positional[7] details at any given location in pre-determined hourly basis. A tiny patch antenna with printed circuit board type receiver electronics is incorporated with Pradyu data acquisition systems. The embedded system has built-in real time clock (RTC) which works on external 32.768 kHz crystal. The satellite modem, GPS and sensor capsules are connected to the universal asynchronous receiver/transmitter (UART) blocks. The embedded system works in sleep mode except for the programmed measurement period. Data acquisition system is programmed in such a way to acquire GPS time and update the system clock once in a day. GPS is turned on every hour to collect the drifter’s positional information and the same is transmitted to shore station using INSAT transmitters. The measurement of surface current is achieved by collecting the drifter positional data and drifter track mapping is done which depicts the surface mixed layer current velocities. Thus the system development is used to measure ocean mixed layer surface current with improved accuracy. The specification of smart

GPS incorporated with drifter is indicated in the Table below.

Table. Specification of sensors

Sensor Current @12V (mA)

Range Accuracy

Smart GPS 41 positional ±10m

INSAT Transmitter

42 402.68 MHz

Every Hour

Garmin handheld reference GPS

60

positional

±3m

3. Validation methods implemented

The following field validation tests were carried out to ensure the GPS positional accuracy of drifter buoy systems and the field performance of the drifter buoy. Its water following characteristics[5] is found to be comparable and matching to an imported drifter with improved accuracy.

Triangulation test method Arabian Sea test Bay of Bengal-Comparison test

3.1 Land based positional data test

Land based triangulation test was carried out to ensure the positional accuracy of indigenously developed drifter. Imported drifter from Marlin Yug was also taken into account for comparison purposes. Drifter Buoy System has been mainly used for the ocean water circulation motion study, thus the accurate measurement of surface velocity important.

Figure 1. Triangulation test results

PROCEEDINGS OF SYMPOL-2013

In this test, we adapted land based 24x7 test at Marina beach which has plain surface for a larger length and width grid. Tests were performed by the triangulation method with leg distance of 200m each. All 3 points were fixed and marked with permanent poll. Also in our test we have taken in to consideration one more imported drifter which works with Argos satellite. Initially both the drifters were placed at point-P1 and both the systems were operated to transmit 6 data sets. The same procedure was repeated for all other points as marked in the Figure 1.

Figure 2. Pradyu and Imported drifter (Land

Test) Figure 2. depicts the land based tests conducted at Marina beach, Chennai. The data sets received from both the drifters were analyzed for the positional information and observed that the indigenously developed drifter provides positional details with an accuracy of ±10m. Six sets of data were acquired for arriving the conclusion. 3.2 Arabian Sea trial

One day off-Cochin field tests were carried out to validate the positional accuracy. Pradyu and Imported drifter deployed at Off-Cochin to confirm the error boundaries of Drifter observations. It was observed that the positional error found to be within 110m to 150m.

3.3 Bay of Bengal Comparison test with

imported drifter The first prototype of drifter buoy with SST was

tested in lab for continuous performance evaluation over 120 days at NIOT and twice it was field tested at off Chennai coast for short duration of maximum 9 hour data transmission. Finally the first drifter buoy with SST design was deployed an equatorial region of Bay of Bengal on 21/4/2012 using the vessel Ocean Research Vessel Sagar Manjusha at 100

00.035’N, 880 30.165’E nearly 120km of off-Panama, Srilankan coast as shown in the Figure 3.

Figure 3. Drifter deployed in the field

Similarly the second prototype system was built during November 2012 and deployed at off-Tamilnadu coast in Bay of Bengal using NIO vessel Sindhu Sankalp on 8/03/2013 with cluster[3] of two drifters including imported one from Marlin Yug, Ukraine.

Figure 4. Drifter rate comparison

Figure 4. Illustrates the drift rate of both drifters

taken for comparison and evaluation. The red and blue column represents drift rate of Pradyu and imported drifter respectively. Drifter buoy working satisfactorily since deployment and continuously transmitting SST data along with positional details on hourly basis. SST observation and process technique implemented[6] in drifter SST found to be matching to the WOCE/NOAA requirements. Mixed layer[5] current following characteristic of indigenously developed drifter found to be more precise than imported drifter. Hourly based drift rate is illustrated in the Figure 4. and the Pradyu performance found to

R.Srinivasan et. al,: Validation Methods Implemented to Ascertain the Positional Data Uncertainty of Indigenously Developed Drifter Buoy.

be well comparable on the current following characteristics nature of the system which is the primary requirement on any drifters. Drift rate of Pradyu exactly matches to the design criteria of 10-30cm drift when the wind speed is 10m. Drifters attains little speedy when it is near and around the shallow water region than deep sea. It is observed that the deployed system even provides information on eddy current circulations which occurs due to the reversal of surface current in response to strong winds at deep sea surface.

4. Conclusions

The indigenously developed reliable drifter buoy

built with a smart GPS receiver[7] module and INSAT telemetry system has been proved in the field during the last two years. A continuous quality analysis of data received is vital for the accurate prediction of long term and short term climatic changes. This type of newly developed GPS Lagrangian drifters can be effectively used for ocean water circulation experiments and climate change applications. ACKNOWLEDGMENT

Authors gratefully thank all the members of National Institute of Ocean Technology and heartfelt thanks to director, NIOT for his vital and continuous encouragement and support extended during this developmental work. The development and deployment are fully funded by MoES (Ministry of Earth Sciences, Govt. of India)

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