User Manual

Global Water Instrumentation
8
Time Stamps and Latency Time
Latency t im e is the t otal time between the SIT 65 trans mitting data and the dat a
appearing on the hos ting web site. The dat a is held in the radi o m odule until one of
the satellites is within range, and then s ent. The information is further held at the
satellite until its data buffer s ar e full before relaying it t o one of several ground
stations wher e it is transferred to the Internet . These t w o factors contribute m os t of
the latency time but additional time delay s c an be added by the transfer of data
through servers. This latency time also applies to control and interrogation
commands being sent to the SIT65. Tot al delay time is not predictable but ar eas
with good satellite coverage and an unobst r uc ted view of the sky can expect a tot al
delay of 2 to 5 m inutes. Some international locations may not be as well covered
and could experience longer delay s ; as can installations in cany ons , mountainous
areas and places sur r ounded by tall buildings wher e the view of the open sky is
partially obstructed.
The latency time will delay alarm ev ent transmissions s ent, and control commands
received, by the field station. This will result in unpredictable delays between alarm
event tr ans m is s ions and the email and text message notifications s ent by the
database. These delays also apply t o the time between control commands being
sent and the digital outputs acting on them.
The time stamp for data readings is applied by the database at the time the data
arrives. This time stamp includes the latenc y time and does not indicat e the actual
time the data was taken. Becaus e the lat enc y time varies from one transmis s ion to
another, the interval between data readings also varies. This variation can be
several minut es . If the SIT65 is program med to transmit once each hour, a t y pic al
time st am p may v ar y fr om 55 minutes to 65 minutes betw een transmissions, whic h
does not create a significant er r or in m ost c as es. Howev er , t he v ar iation in time
stamps r emains the same regardless of the transmiss ion interval. For systems
transmitting at faster interv als , the variat ion in time stamps can produce a much
greater perc entage of error. This puts a practical limit on how of ten the SIT65 can
transmit and still produce useful data. It is generally not r ec om mended that the
system be pr ogr am med to transmit more than once ev er y 15 m inutes.