*Geomagnetic Measurements Workshop: *DIflux: Absolute Measurement of the Geomagnetic Field orientation in Space prepared for the XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 by Jean RASSON Royal Meteorological Institute of Belgium 1
Magnetic Observations Basics The Diflux Concept Selected DIflux Topics 2
Magnetic Observations Basics • • Geomagnetic field Magnetic conditions in Observatory environment Instrumentation: Absolutes, Variometers The baseline concept XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 3
Geomagnetic field Components and coordinate systems The Earth Geomagnetic Field is a vector field’ Frame of reference: Geographic North and direction of local gravity Sets of components used: XYZ, HDZ, FDI Components recorded: XYZ, HNHEZ, FHEV XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 4
Magnetic conditions in Observatory environment The Magnetic Observatory is constructed so that: • It is not situated on a local magnetic anomaly • All buildings intended for magnetic measurements are non-magnetic XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 5
Magnetic conditions in Observatory environment • Only natural magnetic field is present • Low magnetic spatial gradient Consequently: 1. Magnetic field is highly homogeneous: magnetic field lines are parallel 2. Magnetic field differences between pillars are small (ΔF ~ 1 n. T) 3. Magnetic field changes are supposed to be identical inside the Observatory space Additionally: • Stable pillars • Availability of targets with known azimuths Note: Over time, these conditions may degrade. Check them by performing absolutes on more than one pillar! XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 6
Instrumentation: Absolutes • Absolute instruments measure one or more components of the Geomagnetic Field in absolute units or the direction of the field with respect to geographic North and direction of gravity • DIFlux Theodolite for measurement of the Declination angle D and Inclination angle I • Proton Precession Magnetometer measures Total Field, F in n. T. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 7
Instrumentation: Variometers A variometer measures the variation of the components of the magnetic field. The variometer measures the field from an unknown level called the baseline. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 8
The Magnetic Observatory Baseline Concept Fact: Absolute component instruments (Diflux) cannot yet measure continuously. (Total field instrument can) How to obtain a continuous Geomagnetic recording with absolute quality? 1. Record the variation of the components about a value close to its mean: the baseline. This is done by the variometer 2. Measure from time to time (*) the value of the baseline with an absolute instrument * ideally the baseline is a constant. Practically it changes slowly and barely over time XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 9
The Magnetic Observatory Baseline Concept Exemple Take the measurements made at the same time: • Absolute declination measurement D ------- • Variometer measurement δD ------------ defines the baseline D 0 : D = D 0 + δD Note 1: This equation can be written for any component Note 2: Variometer measurements d. D and d. I in n. T are converted to angular units δD and δI by: XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 10
The DIflux concept for measuring the Geomagnetic Field Orientation • Introduction • Theodolite basics • Fluxgate basics • Diflux measurement protocol • Levelling of (Zeiss) Theodolite • Geographic North: the reference in horizontal plane • Precautions required for obtaining high accuracy XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 11
The DIflux Concept for measuring the Geomagnetic Field Orientation • DIflux is an instrument able to measure the value of the geomagnetic declination D and inclination I. • The instrument consists of a non-magnetic theodolite and a fluxgate sensor mounted on the telescope, so that optical and magnetic axes are parallel. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 12
The DIflux Concept Non-magnetic theodolites (are hard to find) XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 13
The DIflux Concept • Fluxgate basics : A fluxgate sensor is very sensitive to its orientation with respect to magnetic field direction. The highest sensitivity is when sensor magnetic axis and magnetic field are almost ┴ (orthogonal = perpendicular = 2 axis form a 90° angle). XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 14
The DIflux Concept • Determining a plane P ┴ to the geomagnetic vector is the same as measuring its direction (our task) • The fluxgate sensor will give null reading when ┴ to geomagnetic vector • So we can find the plane P ┴ to geomagnetic vector: two alignements of the sensor giving a null reading will define it Here are the 2 usual alignements inside P: 1. We require the sensor to be horizontal : Declination measurement --------------------------- 2. We require the sensor to be in the geomagnetic meridian : Inclination measurement (top drawing) Note : this gives a total of 4 (D) + 4 (I) different positions XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 15
Use of Angle Units • 005° 07’ 35’’ Degrees, minute and seconds: – Most theodolite circles are divided in DMS. • Grades: – Some Zeiss theodolites circles are divided in grades • Radian: – Computation using software (Excel) and compilers • • • 1 sec of arc = 1/200000 rad or 1 mm seen from 200 m distance 1 sec of arc movement of magnetic field direction ~ 0. 2 - 0. 3 n. T 1 sec of arc change of declination depends on value of H. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 16
Levelling of (Zeiss) Theodolite: finding the reference in Vertical plane • Coarse levelling using circular bubble level • Accurate levelling using two linear bubble levels • For best levelling use automatic vertical index • During normal D and I measurements accurate levelling is not necessary as the build-in vertical compensator can correct for minor tilt of theodolite. • A precise levelling however makes the measurements faster. • During measurements with elevated telescope, where readings of both circles are needed (sun and star observations) extremely careful levelling is needed. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 17
Geographic North: the reference in horizontal plane The sequence for Declination measurement positions is as follows: § Measurement of Azimuth mark sensor up § Measurement of Azimuth mark sensor down § D 1 : Telescope horizontal towards E, sensor up § Test NOW for magnetism of Observer § D 2 : Telescope horizontal towards W, sensor down § D 3 : Telescope horizontal towards E, sensor down § D 4 : Telescope horizontal towards W, sensor up § Measurement of Azimuth mark sensor down XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 18
Diflux measurement protocol XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 19
Diflux measurement protocol XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 20
Diflux measurement protocol: Inclination Note: from the D measurement we obtain the direction of the magnetic meridian. Note: We can orient theodolite around the vertical axis so that the horizontal axis is ┴ to the magnetic meridian: the telescope will then always swing in the magnetic meridian. The sequence for I measurement positions is then as follows: § I 5 : Telescope in magnetic meridian pointing towards N, sensor up § I 6 : Telescope in magnetic meridian pointing towards S, sensor down § I 7 : Telescope in magnetic meridian pointing towards N, sensor down § I 8 : Telescope in magnetic meridian pointing towards S, sensor up XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 21
Diflux measurement protocol I 5 : Telescope in magnetic meridian pointing towards N, sensor up I 6 : Telescope in magnetic meridian pointing towards S, sensor down I 7 : Telescope in magnetic meridian pointing towards N, sensor down I 8 : Telescope in magnetic meridian pointing towards S, sensor up Note: in fact we get two determinations of I: IH = (I 5 + I 8)/2 which is the inclination at the “sensor up” position and IL= (I 6 + I 7)/2 at the “sensor down” position. ΔI = IH – IL is called the “I gradient” We will come back later on ΔI XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 22
Residual Method The residual method utilize the digital output of the Diflux to eliminate errors and improve the accuracy of the measurements. In each of the 4 D and 4 I positions one does not try to adjust the reading to zero but only close to zero (below 10 n. T). On the minute one reads the circles as usual and also the digital output of the DIflux. Using to the digital reading from the Diflux, one can calculate the circle readings at zero-output of the Diflux. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 23
Residual Method, advantages • The observer can step back from the Diflux at the time of measurement and reduce the effect of magnetic contamination from the observer. • During active period and also at high latitude it is difficult to adjust to 0. 0 n. T on the minute. This is not needed using the residual method. • Using difficult to read theodolites like Zeiss 020, one does not have to estimate the value of the fraction of minute of arc. One simply set the circle to a major division line and use the digital reading to calculate the seconds. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 24
Precautions required for obtaining high accuracy • • Obtain highest precision for target readings and horizontality of telescope for D measurements ”Clean” environment – Absolute house is the ”holy” place in the observatory and should be absolutely clean. – Check area around absolute house • ”Clean” observer – Use the Diflux to check for magnetic material. Shoes are often very magnetic. • Keep observer away from theodolites at the time of reading. – Residual method is highly recommended • • • Keep good timing < 1 sec Use instantaneous variometer data for reduction not filtered minute values Keep list af all available data from the absolute observation – Baseline values, Absolute values, Temperatures, DI-constants, Observer ID and more • Continuous quality check of DI-Theodolite, Observer and Variometer XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 25
Measuring the Geomagnetic Field Orientation: Selected DIflux Topics • Fluxgate electronics basics • Analysing DIflux measurements : Azimuth, Site and magnetisation errors • DIflux magnetic hygiene check • Specs of commercial and other available devices • Troubleshooting of the ZEISS 010/15/20 • Optics cleaning XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 26
Selected DIflux Topics: Fluxgate electronics basics XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 27
Selected DIflux Topics: Fluxgate electronics basics XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 28
Selected DIflux Topics: Analysing DIflux measurements : Azimuth, Site and magnetisation errors ) • • The readings of the four D-positions gives: – Mean Declination D = ( A 1 + A 2 + A 3 + A 4 ) /4 – π rad – Magnetisation error S = ( A 1 – A 2 + A 3 - A 4 ) /4 *H rad – Elevation or Site error ε = ( A 1 – A 2 – A 3 + A 4 +- 2π ) /4*H*/Z rad – Azimuth error δ = ( -A 1 – A 2 + A 3 + A 4 ) /4 rad Similar formulas for the four I-positions gives: – The inclination in the two UP positions (IH) – The inclination in the two DOWN positions (IL) – Magnetisation error S – Elevation or site error ε When you plot the collimation errors it is often convenient to convert them to equivalent n. T so that S, ε and δ can be compared: – Magnetisation error S = ( A 1 – A 2 + A 3 - A 4 ) /4 *H n. T – Elevation or Site error Z*ε = ( A 1 – A 2 – A 3 + A 4 +- 2π ) /4*H n. T – Azimuth error H*δ = ( -A 1 – A 2 + A 3 + A 4 ) /4*H n. T Please be careful using these formulas as the sequence of positions 1, 2, 3, 4 may be different that the one your are using. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 29
Selected DIflux Topics : Analysing DIflux measurements : Azimuth, Site and magnetisation errors • All three instrument errors ε, δ and S are eliminated by averaging the four position readings for D and for I • The size of the three instrument errors should be kept small to make the measurements faster and convenient • However, large errors will not influence the accuracy of the measurements as long as they remain unchanged during the measurement sequence. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 30
Selected DIflux Topics: DIflux magnetic hygiene check 1. The best magnetic check is done by participating in the international IAGA workshops for intercomparing magnetometers 2. Another check can be done by a second Diflux, installed in the I measuring position. DUT should be rotated in front of fluxgate and the extrema subtracted. !! Attention: the fluxgate sensor is magnetic 3. Inspection of the vertical I gradient ΔI gives indication of “non-hygienic” Diflux. (ΔI = IH – IL) Note: magnetic elements on the telescope do not introduce errors as long as they remain constant during the whole measurement protocol. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 31
Selected DIflux Topics Specs of Theodolites Circle • • • Zeiss THEO 010: Zeiss THEO 015 Zeiss THEO 020 3 T 2 KP-NM YOM MG 2 KP (Russia) Ruska theodolite Vertical index +- 1’’ +- 0. 3’’ +- 1’’ +- 6’’ +- 1’’ +-1” +- 1’’ ? ? 30” Bubble levels Sys. errors none eccentricity* none • *eccentricity errors are also removed using 4 positions XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 32
Selected DIflux Topics: Troubleshooting of the ZEISS 010/15/20 ü Caution: do not troubleshoot a Zeiss theo if you are not at ease with fine mechanics ü Caution: always use the correctly sized screwdriver Special tools for repair of Zeiss theos XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 33
Selected DIflux Topics: Troubleshooting of the ZEISS 010/15/20 • • If footscrews are loose you will have random horizontal circle reading errors and problems for levelling Therefore check regularly the footscrew play and adjust if necessary Footscrew play adjustement differs from theo type and series Below it is adjusted by tightening a screw, visible and accessible for a single specific position of the footscrew XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 34
Selected DIflux Topics: Troubleshooting of the ZEISS 010/15/20 • • Footscrew play adjustement differs from theodolite type and series Here it is adjusted by tightening the footscrew, after inserting a pin in the holes, visible and accessible for a single specific position of the footscrew XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 35
Selected DIflux Topics: Troubleshooting of the ZEISS 010/15/20 Horizontal axis clamp adjustment---------- Vertical axis clamp adjustment ↓ XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 36
Selected DIflux Topics: Optics cleaning • Optics cleaning is difficult, so avoid dirtying them in the first place: never touch or contaminate the lenses and optical glass surfaces of theodolite. (!! greases and lubricants) • Valid tool for optics cleaning is cotton held on a brass needle. A fresh cotton tip is OK for easily accessible places. • Golden rule: never reuse a cotton wad for optics cleaning. • Cotton should be used dry (moderate dirt) • Otherwise the correct fluid for optics cleaning is ether. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 37
Special Issues of Repeat Survey Measurements: Precise levelling using Zeiss/UOMZ pendulum • Zeiss 010/015/020 and UOMZ 3 T 2 KP theos have automatic vertical circle index levelling (index is automatically levelled by a pendulum ) • It is possible to use this level for levelling theodolite. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 38
Special Issues of Repeat Survey Measurements: Precise levelling using Zeiss/UOMZ pendulum • • When use pendulum levelling? When very high levelling accuracy is required (elevated target like North Star, Cruz del Sur or Sun at noon) When bubble level is malfunctioning (f. i. direct sunshine heats the bubble and this causes drifts in the level) So, before sunshots or starshots XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 39
Special Issues of Repeat Survey Measurements: Precise levelling using Zeiss/UOMZ pendulum • • Procedure (theo should already be leveled within a few ’): Select mentally two footscrews (1 and 2) and position horizontal telescope along the line they form: call it horizontal direction A Set telescope perfectly horizontal (VC reads 90° 00’ 00”) Rotate 180° around vertical Read VC. (say VC reads 90° 00’ 48”). Levelling error in direction A is 48/2=24” Using Horizontal axis slow motion screw bring VC to 90° 00’ 24” Using footscrew 1 bring VC to 90° 00’ 12” Using footscrew 2 bring VC to 90° 00’ 00”. Theo is now perfectly levelled in direction A XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 40
Special Issues of Repeat Survey Measurements: Precise levelling using Zeiss/UOMZ pendulum • • • Rotate 90° around vertical. This is horizontal direction B Set telescope perfectly horizontal (VC reads 90° 00’ 00”) Rotate 180° around vertical Read VC. (say VC reads 90° 00’ 20”). Levelling error in direction B is 20/2=10” Using Horizontal axis slow motion screw bring VC to 90° 00’ 10” Using footscrew 3 bring VC to 90° 00’ 00”. Theo is now perfectly levelled XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 41
Acknowledgements • Thank you for your attention, • Thanks to CEA and Dr Dongmei Yang for the invitation to this Workshop, • and thanks to our Masters Kring E. Lauridsen and Daniel Gilbert, for teaching us so many things. XIVth IAGA WORKSHOP ON GEOMAGNETIC OBSERVATORY INSTRUMENTS, DATA ACQUISITION AND PROCESSING September 13 - 23, 2010 42
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