urn:nasa:pds:context:instrument:mag.near 1.0 1.7.0.0 Product_Context 2016-10-01 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.near::1.0 instrument_to_instrument_host Acuna, M., C.T. Russell, L.J. Zanetti, and B.J. Anderson, The NEAR Magnetic Field Investigation: Science Objectives at Asteroid Eros 433 and Experimental Approach, Journal of Geophysical Research, Vol. 102(E10), pp. 23751-23760, 1997. reference.ACUNAETAL1997 Cheng, A.F., A. Santo, K. Heeres, J. Landshof, R. Farquhar, et al., Near-Earth Asteroid Rendezvous: Mission Overview, Journal of Geophysical Research, Vol. 102, pp. 23695-23708, 1997. reference.CHENGETAL1997 Kivelson, M., L. Bargatze, K. Khurana, D. Southwood, R. Walker, and P. Coleman, Magnetic Field Signatures Near Galileo's Closest Approach to Gaspra, Science, Vol. 261, pp. 331-334, 1993. reference.KIVELSONETAL1993 Kivelson, M.G., Z. Wang, S. Joy, K.K. Khurana, C. Polanskey, D.J. Southwood, and R.J. Walker, Solar wind interaction with small bodies: 2. What can Galileo's detection of magnetic rotations tell us about Gaspra and Ida, Advances in Space Research, Vol. 16, Issue 4, pp. 59-68, 1995. reference.KIVELSONETAL1995 Lohr, D.A., L.J. Zanetti, B.J. Anderson, T.A. Potemra, J.R. Hayes, R.E. Gold, R.M. Henshaw, F.F. Mobley, D.B. Holland, M.H. Acuna, and J.L. Sheifele, Near Magnetic Field Investigation, Instrumentation, Spacecraft Magnetics and Data Access, Science Reviews, Vol. 82, pp. 255-281, 1997. reference.LOHRETAL1997 Wang, Z., Kivelson, M.G., S. Joy, K.K. Khurana, C. Polanskey, D.J. Southwood, R.J. Walker, Solar wind interaction with small bodies. 2: What can Galileo's detection of magnetic rotations tell us about Gaspra and Ida, Advances in Space Research (ISSN 0273-1177), vol. 16, no. 4, p. (4)59-(4)68, 1995. reference.WANGETAL1995 MAGNETOMETER Magnetometer not applicable not applicable Instrument Overview =================== The Near Earth Asteroid Rendezvous (NEAR) mission is the first in the Discovery Program of low cost planetary and space science missions. Its objective is to characterize the S-type near-Earth asteroid 433 Eros to determine its composition and structure to help understand the origin of Eros in particular and near-Earth asteroids in general. After achieving rendezvous, NEAR will orbit the asteroid for one year conducting extensive observations using infrared and visible spectroscopic imaging, X-ray and gamma-ray fluorescence spectroscopy, laser ranging and in-situ magnetic field measurements [CHENGETAL1997]. The magnetic field experiment (MAG) will be used to characterize the magnetization intensity and geometry of 433 Eros as described in detail by [ACUNAETAL1997]. This paper describes the in-flight calibration activities for MAG including analyses of spacecraft magnetic fields and the post-reception processing steps developed to remove these effects to yield valid data for scientific study. Because Eros is immersed in the magnetized plasma of the solar wind, the performance requirements of the MAG instrument are related to the orbit distances and the solar wind perturbation signatures associated with the asteroid. During the one year rendezvous, the NEAR spacecraft will execute a variety of orbits at Eros to optimize the imaging, composition and ranging measurements. The planned orbit radii will range from 400 km down to 35 km, as low as 15 km from the asteroid surface. For NEAR to directly sample Eros' magnetic field, the field would have to stand off the solar wind dynamic pressure which will average ~2.5 nPa at perihelion (1.13 AU) and ~1 nPa at aphelion (1.78 AU), corresponding to magnetic field strengths of 80 nT and 50 nT at perihelion and aphelion, respectively. The goal for MAG is to achieve 10% accuracy in a direct detection of Eros' magnetic field which is therefore provided by an accuracy of 5 nT. This is a factor of more than 50 less demanding than for space probes designed for solar wind studies. Achieving better than 5 nT accuracy is important for several reasons. Signatures of the solar wind interaction due to whistler waves should be present near the asteroid [WANGETAL1995] but are expected to be only a few nT in magnitude. Such indirect observations were used to infer the magnetic character of Gaspra [KIVELSONETAL1993] and Ida [KIVELSONETAL1995]. In addition, if Eros presents a small obstacle, < 100 km in size, to the solar wind, one expects kinetic plasma interactions to play a prominent role in the interaction because solar wind protons have gyroradii of ~50 km. Thus, the solar wind-Eros interaction may not be well described by magnetohydrodynamics (MHD) and detecting magnetic signatures of a kinetic interaction is highly desirable to evaluate the nature of the interaction. Signatures of a kinetic effects are likely to be a few nT or smaller. Reasonable efforts within cost and schedule constraints were therefore made to achieve noise levels below 5 nT. It turned out that the net dynamic magnetic field of the spacecraft field is roughly 10 times larger than this (the static field is roughly 190 nT) so that a thorough characterization of spacecraft magnetic fields was required. A detailed model of the spacecraft magnetic contamination was developed that makes extensive use of spacecraft engineering data and describes dynamic as well as static field sources. The spacecraft contamination field is subtracted from the measurements in ground processing. Because a detailed magnetic survey of the NEAR spacecraft was not performed on the ground, the analysis was performed using data obtained during cruise. This paper documents the methods and results of this analysis showing that the ultimate accuracy of the magnetic field data returned from NEAR is 1-2 nT in each axis. The NEAR magnetic fields experiment will therefore return direct measurements Eros' magnetic field, if its magnetic moment is sufficiently large, as well as signatures of the solar wind interaction. Instrument Description ======================== The NEAR magnetometer is described in detail by [LOHRETAL1997] and essential instrument characteristics are summarized in Table 1. The instrument consists of a triaxial fluxgate sensor, drive and sense electronics with separate 20-bit A/D converters for each axis. The instrument includes eight ranges covering full scale sensitivities from +-4 nT to +-65,000 nT. Pre-flight sensor calibration provided absolute gain and cross talk calibration to 0.5% of full scale in each range. Range control can be selected to be either manual or automatic. Automatic range control provides transition to less sensitive ranges when any one axis exceeds 87.5% of full scale for 0.25 seconds and shifts to a higher sensitivity when the field on all three axes falls below 17% of full scale continuously for 1 minute. This ensures that the instrument will follow rapid increases in field strength without toggling between ranges. The internal instrument sampling is fixed and variable output rates are accomplished by a combination of digital filtering and subsampling. The analog voltage outputs are anti-alias filtered with a combination of a single pole analog Butterworth low pass filter, 3dB attenuation at 10.7 Hz, at the input of each A/D and an IIR filter built into the A/D, 3 dB attenuation at 16.7 Hz. The analog signals are sampled continuously by the A/D converters at 20 samples/sec and output to the instrument data processing unit (DPU) via gate arrays. The same DPU serves both MAG and the Near Infrared Spectrometer (NIS) instruments. Subsequent digital filtering, range control, noise rejection, time tagging, formatting and other pre-processing and logic control are performed in the DPU. Intrinsic instrument sensor noise is about 1pT. Time tagging is provided to 0.05 second accuracy and the internal timing latency of the instrument including both analog delays and digital processing is 0.64 seconds. Additional known latencies are introduced by the on-board digital filters. To accommodate the data collection strategies of NEAR, the instrument sample rate is commandable from 0.01 to 20 samples/sec. Digital filtering commensurate with the output rate is applied for rates down to 1 sample/sec. The filtering calculations are performed on the internal 20 bit resolution data. To avoid integer roundoff calculations in the filtering, the data are padded with 5 zeroes and the resulting 25 bit integers are carried forward in the 32-bit processor. The output is rounded to 16-bit resolution providing digitization step sizes from 0.2pT to 2nT. To avoid undesirably long averaging and to ensure that data are not smeared significantly in position relative to Eros, sample rates from 0.5 samples/sec to 0.01 samples/sec are sub-samples of the 0.5 Hz filtered time series. During cruise the instrument is operated in range 3 at the 0.01 samples/second rate and checkouts (Section 3.1 below) consisting of a calibration with the internal bias current, 20 sample/second data collection (5 minutes) and anti-alias filter turn-off are performed approximately weekly as spacecraft operations allow. The sampling rate at rendezvous will be 1 sample/second and checkouts performed weekly as during cruise. Table 1. NEAR magnetometer characteristics. Tri-axial Fluxgate: 15 kHz drive/30 kHz sense 1 pT intrinsic rms noise Linearity: <1 part in 104 Selectable electronics calibration bias current Survival heater -15-C set point. Sensor body mounted on antenna x cm above top deck. Sampling: 20/second internal Separate 20 bit A/D for each axis Output: 16 bit resolution 10 Hz anti-alias filter, 3-pole Butterworth Ranges: eight selectable Range Full scale (nT) Resolution, LSB (nT) 0 +-4 (a) 1 +-16 (a) 2 +-64 0.002 3 +-256 0.008 4 +-1,024 0.031 5 +-4,096 0.125 6 +-16,384 0.500 7 +-65,536 2.000 Software: RTX-2010RH, 32 bit, FORTH language Output sample rates: Sub-sampled 1/2 Hz filter: 0.01,0.02,0.05,0.1,0.2,0.5/s Filtered (fNyquist=1/2 rate): 1, 2, 5, 10/s No digital filtering: 20/s Filtering: Butterworth IIR, 25 bit resolution 0.5, 1.0, 2.5, 5.0 Hz fNyquist Ranging: Automatic: max/min F.S.: 87.5%/17% Manual Other: Noise rejection, range offsets, sample synch. pulse. Resources: Mass, power: 1.5 Kg, 1.7 W (not including DPU) Data rate: 65 bits/sample, 5.6 Mbits/day at 1/s (a) Limited by intrinsic 1 pT instrument noise.