3DM
®
-GX5-35
DCP Manual
2.7
Data Synchronicity
Because the MIP packet allows multiple data fields to be in a single packet, it may be assumed that a
single timestamp field in the packet applies to all the data in the packet. In other words, it may be
assumed that all the data fields in the packet were sampled at the same time.
IMU and Estimation Filter data are generated independently by two systems with different clocks. The
importance of time is different in each system and the data they produce. The IMU data requires
precise microsecond resolution and perfectly regular intervals in its timestamps. The Kalman Filter
resides on a separate processor and must derive its timing information from the two data sources.
The time base difference is one of the factors that necessitate separation of the IMU and Estimation
Filter data into separate packets. Conversely, the common time base of the different data quantities
within one system is what allows grouping multiple data quantities into a single packet with a common
timestamp. In other words, IMU data is always grouped with a timestamp generated from the IMU time
base, and estimation filter data is always grouped with a timestamp from the Estimation Filter time
base,etc.
All data streams (IMU and Estimation Filter) on the 3DM- GX5- 35 output a “GPS Time”- formatted
timestamp. This allows a precise common time base for all data. Due to the differences in clocks on
each device, the period between two consecutive timestamp values may not be constant; this occurs
because periodic corrections are applied to the IMU and Estimation Filter timestamps when the GPS
Time Update Command is applied.
2.8
Communications Bandwidth Management
Because of the large amount and variety of data that is available from the 3DM-GX5-35, it is quite easy
to overdrive the bandwidth of the communications channel. This can result in dropped packets. The
3DM- GX5- 35 does not do analysis of the bandwidth requirements for any given output data
configuration, it will simply drop a packet if its internal serial buffer is being filled faster than it is being
emptied. It is up to the programmer to analyze the size of the data packets requested and the available
bandwidth of the communications channel. Often the best way to determine this is empirically by trying
different settings and watching for dropped packets. Below are some guidelines on how to determine
maximum bandwidth for your application.
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