Xylem SIT65 SATELLITE INTERNET TELEMETRY - Manual (Page 8)

Global Water Instrumentation
8
Time Stamps and Latency Time
Latency time is the total time between the SIT65 transmitting data and the data
appearing on the hosting web site. The data is held in the radio module until one of
the satellites is within range, and then sent. The information is further held at the
satellite until its data buffers are full before relaying it to one of several ground
stations where it is transferred to the Internet. These two factors contribute most of
the latency time but additional time delays can be added by the transfer of data
through servers. This latency time also applies to control and interrogation
commands being sent to the SIT65. Total delay time is not predictable but areas
with good satellite coverage and an unobstructed view of the sky can expect a total
delay of 2 to 5 minutes. Some international locations may not be as well covered
and could experience longer delays; as can installations in canyons, mountainous
areas and places surrounded by tall buildings where the view of the open sky is
partially obstructed.
The latency time will delay alarm event transmissions sent, and control commands
received, by the field station. This will result in unpredictable delays between alarm
event transmissions and the email and text message notifications sent by the
database. These delays also apply to 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 latency time and does not indicate the actual
time the data was taken. Because the latency time varies from one transmission to
another, the interval between data readings also varies. This variation can be
several minutes. If the SIT65 is programmed to transmit once each hour, a typical
time stamp may vary from 55 minutes to 65 minutes between transmissions, which
does not create a significant error in most cases. However, the variation in time
stamps remains the same regardless of the transmission interval. For systems
transmitting at faster intervals, the variation in time stamps can produce a much
greater percentage of error. This puts a practical limit on how often the SIT65 can
transmit and still produce useful data. It is generally not recommended that the
system be programmed to transmit more than once every 15 minutes.