VLBI - Very Large Baseline Interferometry
| Connecting two dishes over a 800 km distance ... |
...the big one |
...and the small one |
 |
|
Overview about measurement data acquisition
This page intends to provide an overview about the software that collects data from a fast sampling device.
Since the project is an ongoing effort the software represents a development snapshot. It will possibly
be replaced by a next snapshot as soon as new functionality has been added. The current version has reached
a stable stage for writing data to a DataX repository on harddisk.
A description of the sampler hardware can be found on the ERAC website at:
http://eracnet.org/workshop/sampler/sampler.htm
The sampler hardware has been connected to the serial port of a Linux computer.
The connection runs at 38400 Kbit/sec and allows to obtain two blocks of sample data per second
with 1952 8-bit samples in each block.
Online access to DataX repository
All data that are collected by the Smp32 device can be accessed over the Internet.
The specification of the software as well as an implementation example can be found under "Connectivity".
Software components
The software consists of the following components:
smp32.cpp
A deamon that collects data on a Linux computer and writes it into a DataX repository on hard disk.
DataX.h/DataX.cpp
A DataX class which allows for writing huge data volumes in suitable chunks to harddisk.
Types.h/Types.cpp
Every C/C++ program needs some basic types where other components can build on.
The Types component implements for example string classes, a class for date/time handling as
well as a sophisticated benchmark class. The benchmark class is good for determining the runtime
behavior of various components with a resolution down to a single processor cycle. Finally a class
for handling exceptions supports the handling of all exceptional situations.
makefile
It is common to write a command file which compiles all C++ sources and generates the executable just by typing:
$ make
on the command line (The $ is the command prompt). With
$ make go
the resulting executable will be started after it has been built.
Structure of a DataX file
The definition of a DataX file has been layed out in a comprehensive document:
DataX file/stream data structure documentation
The following lines give an example about a possible structure of a DataX file:
EKD@JN58ve.Smp32,1111867200,duration:180
,Dish:cm,90
,Receiver:AR5000
,A/D converter:SMP32
,antenna direction:azimuth:deg:247
,antenna direction:elevation:deg:26
,antenna direction error:deg:3
,position:abs:WGS84:latitude:deg:48.2
,position:abs:WGS84:longitude:deg:11.8
,position:abs:WGS84:elevation:m:530
,position error:m:3000
,sample rate:Hz:2633000
,sample block:1952
,frequency:MHz:10600
,A/D no of bits:8
,analog filter::type:lp
,analog filter::bandwidth:MHz:1.5
,Data
:Time,Samples
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DataX repository
When connecting the hardware to a Linux PC and running the software then a repository will be
generated on harddisk which will look similar to the following screenshot:
