A Verification of Magnetometer’s Time Accuracy by using a GPS Pulse Generator

Some magnetic observatories are exchanging one-minute geomagnetic field data in near real in a program of International Real-time Magnetic Observatory Network (INTERMAGNET). On the other hand, one-second data distribution have been a hot issue and requirements of the one-second data, for example resolution, absolute accuracy, noise level have been discussed among relevand people of INTERMAGNET. Time accuracy is one of problems to be solved, but any standard methods to examine time accuracy of magnetometers’ output are not established. We had been entrusted a GPS pulse generator made by geomagnetic laboratory/ natural resources Canada (www.nrcan.gc.ca) by Dr. Jean Rasson who is a core staff of INTERMAGNET. In order to verify a magnetometer’s time accuracy by this pulse generator, we made magnetic pulses correspond to GPS signals into and check its output. We examined output of a fluxgate magnetometer which is introduced in 2010.

•The pulse generator system is consists of a GPS receiver, microcontrollers, and a pulse generator. /

•Outputs are rectangular pulses synchronized with GPS signals.

•Cycle time of the rectangular pulses is available between 2 and 3,276,810 seconds.

Yasuhiro MINAMOTO 1, Takeshi Oowada 1, Takashi Ookawa 2

 1. Kakioka Magnetic Observatory, Japan Meteorological Agency, 595

Kakioka 595, Ishioka-shi, Ibaraki-ken, Japan, minamoto@kakioka-jma.go.jp

2. Matsushiro Seismological Observatory , Japan Meteorological Agency, Nishijo 3511, Matsushiro-machi, Nagano-shi, Nagano-ken, Japan,

Sensor of fluxgate magnetometer

GPS pulse generator’s controller

•coil diameter: 116mm

•length: 63mm

•total number of turns: 44

•inductance: 5.5×10-5 H

(measured by an LCR tester)

Fig.1 external appearance of the GPS pulse generator

Fig.2 The coil was set north of the censor. 　

Table 1 Specifications of the three axis Fluxgate magnetometer FM10

temperature cofficient (sensor) < 0.1nT C/ ﾟ

temperature cofficient (amplifier) < 0.2nT C/ ﾟ

stability < 0.1nT/ day

measuring range ± 625 Tｎ

frequency band DC to 5Hz

output reasolution < 0.01nT/ 5HZ

data collectiong 1Hz,10Hz,100Hz(option)

A new highly sensitive fluxgate magnetometer, FM10 was installed at KAKIOKA. The FM10 have made a trial run and we can test for ascertaining its timing accuracy with any noise in published data.

Furthermore we can examine responses from GPS pulses to 0.1 second value and 0.01 second value, in addition to one-second value.

t t+11000msec= 1sec

00 01 02

X

99

Y

Z

10msec

one-second value of “t”

Fig.6 Timing chart of one-second value sampling for FM10

9900 01 02

GPS1PPS

A/D recording

250Hz

Simultaneous sampling for three components

Variations of geomagnetic field with magnetic signals by the GPS pulse generator.

Fig.4 plot of 0.1sec values of X component with eight seconds period

Fig.5 plot of 0.01 second values of X component : before to after switching

1sec

0.1sec

1sec

references•Rasson, J. L., Testing the time-stamp accuracy of a digital variometer and its data logger, Proceedings of the XIIIth IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition, and Processing, 225-231, (2009).•Jankowski, J. and C. Sucksdorff, Guide for Magnetic Measurements and Observatory Practice, 139-142, International Association of Geomagnetism and Aeronomy, Warsaw (1996)•International Association of Geomagnetism and Aeronomy,10 seconds digital recording for 1 minute means of geomagnetic data, Resolutions of IAGA the 1979 Assembly, No.12 (1979)

Fig.3 plot of 1sec values of X component with eight seconds period

We calculate differences between one-second value FM(Fig.3) and time with 1000 waves by using a method based on the idea of linear least squares parameter estimation which was shown by Rasson(2009). The results show delays of FM’s records are 0.385 second. It is same for each period; 4, 8, 16 seconds. This delay is not caused by error of time signals but due to making procedure for one-second value.

Fig.6 shows timing chart of one-second value sampling for FM10. Analogue/digital recording in FM10 is taken with 250Hz for each component simultaneously. 0.01-second values are calculated from a few values obtained with 250Hz. 0.1-second value at “t” is calculated from ten values of 0.01-second values between t and t+0.1. And 1-second value at “t” is calculated from ten values of 0.1-second values between t and t+1.

IAGA(1979) noted the one-minute mean value for the minute XX calculated for example from ten-second values is the mean of the values of the times between (XX-l)m30s and XXm30s. An hourly mean value is the mean of the values taken at from 00 m to 59m. The accuracy of time has to be at least 1 s for one-minute values, and better for faster sampling(Jankowski and Sucksdorff 1996).Taking into account of these procedures, we adopt the time chart of 1-second value for FM10 in order to reduce dispersions of data. While one-second INTERMAGNET sample refer to the top of the second, i.e. the epoch hh:mm:ss.000(Rasson 2009).

0.05sec

Plot of 0.1-second values (Fig.4) shows the digital switching was performed between 00:08:00 and 00:08:00.2.While, 0.01 second value indicate that the decline of output started about 0.09s, and it takes about 0.05 second (fig.5) for switching.

JMA’s mascot is named Harerun (in the hope of hare, the Japanese word for “fine weather”), and is designed with elements of sun, cloud and rainfall. Harerun holds a green baton in prayer for a disaster-free, peaceful world.

0.00

0.10

0.05

0.15