Univeristy of Athens / Hellenic Center for Marine Research Profiling Drifters

Univeristy of Athens / Hellenic Center for Marine Research
Profiling Drifters

Introduction
Automatic data processing & web-page generation
Reliability statistics
Argos Data Telemetry
GTS publication of float profiles
Contact Information

Introduction

Welcome to the Web Site for the University of Athens and Hellenic Center for Marine Research profiling drifter study.

Automatic data processing, data distribution, & web-pages generation

This web site contains thousands of dynamic objects that must be updated as new data become available. Examples include postscript files, jpeg images, tables, HTML files, hydrographic profile data, and data distribution services. These objects are automatically generated or maintained in quasi-real-time by an extensible system of autonomous software. Consequently, maintaining this web site requires no regular manual intervention.

Reliability Statistics

The reliability of the floats can be measured in terms of the number of profiles expected during a specified time period as compared to the number actually executed. These statistics are presented in the following table (updated daily).

Profiling Drifter Reliability

Profiler Reliability Statistics for the period from Jan 01 00:01 GMT 2000 to Oct 28 09:02 GMT 2013.
Group	Size	Profiles Executed	Profiles Expected	Reliability (%)
APEX260	3	443	1695	26.1
APF8	3	443	1695	26.1
Total	3	443	1695	26.1

Argos Data Telemetry

The data are recovered from the floats via the Argos system. These floats spend 7 to 8 hours transmitting on the surface with one message transmitted every 46 to 54 seconds. This means that each float transmits from 470 to 530 messages for each profile. Most of these transmissions are lost because no satellite happens to be overhead. In fact, telemetry statistics below show that only 7 to 10 percent of these messages are transmitted when a satellite is overhead. Some statistics have wider appeal and we have collected several of them in the table below (updated daily). All of these statistics were computed on a per-profile basis.

Float Id: A unique 3 digit identifier assigned to each float.
Number of Profiles: The number of profiles the float has executed so far.
Incomplete Profiles: The number of profiles that have an incomplete Argos message set. This condition results in a missing segment of data in the profile plots. The accompanying percentage gives the ratio of the Incomplete Profiles to the Number of Profiles.
Satellite Passes: The average number of times that a satellite passed overhead while the float was on the surface.
Messages Expected: This is the average number of messages that should have been received when satellites were overhead. If two or more satellites are overhead simultaneously then each transmission was multiply-counted. The number of messages expected for a given satellite pass is determined from the Argos data stream by counting the number of messages received and adding the number of messages missing. Due to averaging and rounding effects, these entries in the table below may not add up exactly.
Messages Received: This is the average number of messages that were actually received by satellites (without regard to the CRC check). The accompanying percentage gives the average ratio of the messages received to the messages expected.
Messages Missing: This is the average number of messages that were transmitted while a given satellite was overhead but that were not received by the satellite. These are detected in the Argos data stream by gaps in time and message number. The accompanying percentage gives the average ratio of the messages missing to the messages expected.
Good Messages: This is the average number of messages received that passed the CRC check. The accompanying percentage gives the average ratio of the good messages to the messages received.
Bad Messages: This is the average number of messages received that failed the CRC check. The accompanying percentage gives the average ratio of the bad messages to the messages received.
Telemetric Efficiency: This measures the average rate of successful transmission when satellites are overhead. It is the average ratio of the number of good messages to the number of messages expected.

University of Athens / Hellenic Center for Marine Research Profiling Drifter ARGOS Receipt Statistics

As of 09:05 GMT on Apr 22, 2010				Average Per-Profile Statistics for ARGOS Data Telemetry
Float Id	Profiles Executed	Incomplete Profiles		Satellite Passes	Telemetric Efficiency	Messages Expected	Messages Received		Messages Missing		Good Messages		Bad Messages
2014	9	6	66.7%	7	38%	54	37	74%	17	26%	19	51%	17	49%
2041	168	33	19.6%	11	35%	124	74	60%	50	40%	43	57%	32	43%
2058	192	158	82.3%	8	30%	79	42	56%	37	44%	22	52%	20	48%
2620	57	3	5.3%	10	38%	123	80	66%	42	34%	45	56%	35	44%


Ensemble Average:			43.5%	9	35%	95	58	64%	37	36%	32	54%	26	46%
Standard Deviation:			36.8%	2	4%	34	22	8%	14	8%	14	3%	9	3%

Publication of Profiles to the Global Telecommunications System (GTS)

The profiles from these floats are published in quasi-real-time via the GTS system using TESAC/KKXX format. Each float is assigned a unique WMO identifer that can be found on our Present Status web page. The status of each profile with respect to GTS publication is also available on our GTS Status web page.

Quality control of profile data is managed automatically by a QC module that is integrated into the GTS publication software. Each profile must pass a number of quality control criteria in order to be published via GTS. If even a single observation in the profile violates any of the criteria then publication of the entire profile is inhibited. The following criteria are used to inhibit publication of rejected profiles.

A profile that is older than 30 days will be rejected as required for GTS publication.
A profile that does not have a valid location fix will be rejected as required for GTS publication. This might happen if ARGOS reception is poor.
A profile with a gap between adjacent observations that exceeds 300 decibars will be rejected. A profile with the shallowest observation deeper than 300 decibars will also be rejected. This is to maintain reasonable accuracy in the pressure to depth conversion calculation as required for GTS publication. This might happen if ARGOS reception is poor.
A profile with an observation where the pressure is reported to be less than zero or greater than 3000 decibars will be rejected as indicative of a pressure sensor problem or malfunction.
A profile with deepest observation less than 50 decibars is rejected. For example, this might happen if the float drifts into shallow water or becomes entangled in mats of surface-drifting seaweed.
A profile with pressures that are not monotonic will be rejected. This is intended as a general criterion that a properly collected profile should satisfy.
A profile with fewer than 5 observations will be rejected. This is intended as a general criterion that a properly collected profile should satisfy.

This list is exhaustive in the sense that any profiles that do not violate these criteria will be published via GTS. In the absence of established GTS quality control criteria, the approach of caveat emptor has been assumed with respect to the hydrographic data.

Contact Information: Please note that the data and material on this web site are the property of the Univeristy of Athens. Questions concerning the information available here can be addressed to Dr. Sarantis Sofianos, (email: sofianos@oc.phys.uoa.gr).

This web site was developed and is maintained by Dana Swift (email:swift@ocean.washington.edu) of the University of Washington, Seattle, Washington.

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