GEOMAGNETIC DATA ACQUISITION SYSTEM DEVELOPED FOR THE PLASMON PROJECT
L. Merényi (1), B. Heilig (1), L. Szabados (1)
(1)
Geological and Geophysical Institute of Hungary, H-1143 Stefánia út 14, Budapest, Hungary,
[email protected]
SUMMARY PLASMON is a global collaboration of 11 institutions, funded by the EU (FP7-SPACE), addressing near real-time monitoring of the state of the plasmasphere based on ground VLF and ULF wave observations. A geomagnetic data acquisition (DAQ) system has been developed in Geological and Geophysical Institute of Hungary to fulfill the requirements of the ULF pulsation stations of the PLASMON project. Hardware elements of the DAQ include fluxgate magnetometer, A/D converter, additional data acquisition electronics, embedded computers, GPS unit and power supply unit. The DAQ software incorporates a DOS program used as a real-time interface towards the A/D converter, and a Linux system with a graphical data acquisition program and networking scripts. In this paper we present the hardware and software elements, and some test results related to the estimated noise of the A/D converter and settling time of the magnetometer. 1. INTRODUCTION A geomagnetic data acquisition (DAQ) system has been developed to fulfill the requirements for ULF pulsation stations of the PLASMON project. In this system care has been taken to achieve high timing accuracy and to get high resolution, low noise magnetic data. Stable and robust operation, small size and very low power consumption were also critical design issues for a reliable system running at unmanned stations powered by solar PV panels. Linux was selected as the operating system for the main acquisition computer of the DAQ. Linux is a trusted and stable system that runs efficiently on low-power computers having limited computational power, memory and disk space. Moreover, Linux allows easy implementation of the networking functions required for automated quasi real-time data transfer and remote access. Four identical DAQ systems have been built and operated successfully in Tihany Observatory, Hungary and at other PLASMON sites.
2. HARDWARE AND SOFTWARE ELEMENTS Figure 1 presents the schematics of the DAQ system. The magnetometer noise level should be low, below 10 pT at 1 Hz, moreover, the amplitude response of the magnetometer at the analogue output should be flat from DC to several Hz. Long term base-line stability and low temperature sensitivity – that are generally important factors for geomagnetic observatories – are less important for this application. Lawson Labs M250 type 23 bit A/D converter was selected for fast and low noise A/D conversion. Three input channels from the six 23 bit multiplexed channels are used to sample the fluxgate analogue output signals. The remaining three spare input channels can be used for additional recordings, such as temperature monitoring. The Lawson Labs M250 converter communicates with the host computer through the parallel (LPT) port. A PC/104 industrial computer running a DOS program (in a FreeDOS system) is interfaced between the A/D converter and the main acquisition computer. The task of this DOS computer is to realize the parallel port communication in real-time and to buffer the raw digitized data before it is retrieved and processed by the main computer. For high accuracy timing, a micro-controller based device with a GPS module is used (“GPS electronics” in Figure 1). A Trimble LassenIQ type GPS module provides timing information in NMEA format messages; moreover, it outputs accurate Pulse-Per-Second (PPS) signals. The PPS output is wired to the M250 A/D converter trigger input to restart the A/D sampling every second. Between two PPS events, the sampling is based on the M250’s internal clock. The GPS electronics has a buffer to temporarily store GPS NMEA messages, PPS events and headers of data records from the A/D converter, coming through the DOS interface PC. All these data and events are time 54
stamped in the buffer by the electronics’ internal clock. The data acquisition program on the main data acquisition computer periodically retrieves the contents of the GPS electronics buffer and the digitized raw data from the DOS program running on the PC/104 computer. GPS time stamps are then calculated for each digitized raw datum. Both the PC/104 DOS computer and the main Linux computer are compact size XCore86 based computer systems, with onboard hardware watchdog function. Xcore86 (also called Vortex86MX) is an x86 compatible system-on-chip, with very low power consumption.
Figure 1 –Schematics of the data acquisition system
Figure 2 –Photo of a running system
An AC/DC power supply unit was designed and built to supply all DC power requirements of the hardware elements (+15Vdc, -15Vdc, +12Vdc, +5Vdc). Surge protection with varistors and transient suppressors on the AC line have been added. The measured overall power consumption of the DAQ system is 18.5 W when operated from 230 VAC, and 16-17 W when operated from 12 V battery. Automatic charging of a 12 V lead-acid battery (9- 55Ah) with over-discharge protection is possible. The system can automatically switch to battery operation in case of power outages. A solar cell (PV) driven version has also been built and is under test. The raw sampling rate was chosen as 128 Hz. Time stamped raw samples are filtered in the data acquisiton program by two subsequent digital Gaussian filters. The first filter has a flat response up to 3 Hz (16 Hz data product), while the second filter used to produce the 1 Hz data cuts off at 0.25 Hz. The filters have zero phase shift. The MAGLIN data acquisition program is used in the main data acquisiton computer. MAGLIN is a graphical program designed for continuous, long term data acquisition. It features modular acquisition configuration, one or more different instruments with one or more
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components can be added to the system. MAGLIN can calculate arithmetic averages or filtered values with user selectable averaging windows and user defined filter coefficients. MAGLIN not only saves, but also plots raw data, filtered/averaged data and GPS timing and positional information data in different time scales. This helps the operator to check the system either on the local console or through a remote access connection.
3. FILE STORAGE AND NETWORKING 1 Hz filtered data, and optionally also the 16 Hz filtered data and raw 128 Hz data are saved in compact binary format files using a simple compression algorithm. Then gzip compression is applied to these binary files to further reduce file size. Using the compact binary format and gzip in sequence performs better than applying zip compression to simple binary or text files. Gzip compressed compact binary files containing three-component full-day. 1 Hz magnetic data have file sizes typically between 200 and 260 kilobytes, while zip-compressed IAGA-2002 format or space delimited ASCII format files with the with the same information content are about three times bigger. 1 Hz data files are automatically sent to PLASMON's server every 5 or 10 minutes, using FTP and/or SFTP protocol. The high file-size reduction rate with the double-compression has been found very beneficial when files are transferred from stations having a limited network connection (slow bandwidth and/or limited total monthly amount of data, typical of mobile Internet network services). Files are also archived locally on the main flash disk of the data acquisition computer. Oldest data files are automatically deleted when disk space goes too low. Disk space is sufficient to store at least 1 year of 1 Hz data files. The automated file transfer works for stations having Internet services with either a static or dynamic IP address. However, sometimes it is neccessary to log in to stations for manual system checks and for making configuration changes and remote login is generally not possible for stations with a dynamic IP address. To overcome this limitation we intend to update the Linux system and implement a remote access service supporting connections to computers with a dynamic public IP address.
4. SOME TEST RESULTS The measured time delay of the whole system with a LEMI magnetometer is about 32 ms in the frequency range of interest. The error of this delay estimation is about 1-2 ms, probably due to the timing errors of emitted magnetic signals. According to comparison of different tests, the delay is probably due mainly to the magnetometer. The 32 ms delay corresponds to approximately four samples (31.25 ms) of the raw data, and hence could easily be corrected with a simple shift of the filter. However, since this small delay causes only 2° phase delay in a 200 mHz signal (and even less at lower frequencies) we decided not to correct the data for this delay. The value of the delay will be added to the header information instead. The other reason for not correcting the data is that a known time delay (supposing linear phase delay) can easily be corrected for during data processing if required. Noise on the A/D converter was evaluated by measuring zero voltage on shorted analog inputs. Noise on raw 128 Hz data were examined in windows containing 100 samples. Noise on 1 Hz filtered data (produced by the two subsequent digital Gaussian filters) were also examined, in 30 seconds wide windows. Typical (average) noise on raw 128 Hz data is 8.9 ȝVRMS, and noise on the 1 Hz filtered data is about 1.4 ȝVRMS. Typical (average) peak-to-peak noise is 44.5 ȝV and 4.0 ȝV for raw 128 Hz and filtered 1 Hz data respectively.
5. ACKNOWLEDGEMENT The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no 263218.
6. REFERENCES PLASMON FP7-Space project: A new, ground based data-assimilative model of the Earth’s Pasmasphere – a critical contribution to Radiation Belt modeling for Space Weather purposes. http://plasmon.elte.hu J. Lichtenberger, M. Clilverd, B. Heilig, M. Vellante, J. Manninen, C. Rodger, A. Collier, A. Jørgensen, J. Reda, R. Holzworth, and R. Friedel (2012) First results on plasmasphere in PLASMON project, (submitted to Journal of Space Weather and Space Climate)
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