GOES 12 high resolution magnetometer data

GOES 12 magnetic field data at 0.512 s time resolution. The vector magnetic field is given in the spacecraft Earth-referenced coordinate system: Hp, He, Hn, and Ht, where Hp is perpendicular to the satellite orbital plane, or parallel to spin axis of the Earth in the case of a 0° inclination orbit, He is perpendicular to Hp and directed earthward, Hn is perpendicular to both Hp and He and directed eastward, and Ht is the total field.

This data product also provides GOES 12 satellite positions and velocities derived from SSCWEB GEI ephemeris, interpolated to 1 min time resolution.

The magnetometer data are downlinked from the spacecraft in real-time to the NOAA Space Weather Prediction Center (SWPC) in Boulder, Colorado. The data from the raw Command and Data Acquisition (CDA) telemetry stream are processed by pre-processors at SWPC and from there 1 min averages are distributed to SWPC Space Weather Operations, as well a archived at the NOAA National Geophysical Data Center (NGDC) and the NASA Coordinated Data Analysis Web (http://cdaweb.gsfc.nasa.gov/). The raw high resolution data are corrected and processed at the NOAA SWPC, and later archived at NGDC.

The major issue for processing the high resolution data is correction for the magnetic signature of spacecraft torquer currents as briefly discussed in the magnetometer instrument descriptions. In normal operation, the magnetometer samples its three axes every 0.512 s, synchronized with the telemetry frames. The magnetometer has an anti-aliasing low-pass filter on each axis. As part of the attitude control on the spacecraft, there are two magnetic torquers. At the magnetometers, these torquers can generate magnetic fields larger than the ambient fields to be measured. Therefore signals from these torquer currents need to be removed. The torquer currents are measured at the same rate as the magnetometer rate, and the currents are used to correct the magnetometer measurements.

However, the torquer currents were not low-pass filtered. As a result, large instantaneous steps in the torquer currents show up in the magnetometer data as a delayed, slowly rising wave with some ringing, just what one would expect from the magnetometer low-pass filters. The so-called "Farthing coefficients" simulate, from a short past history of the torquer measurements, the effects of the magnetometer filters. The torquer current changes are asynchronous with the telemetry frames and the timing between these events must be measured to a eighth of a telemetry frame interval for good performance. The transient responses in changes in torquer currents are reduced, but not eliminated for large changes in torquer currents.

There exists one opportunity at the beginning of each mission to perform a spacecraft rotation maneuver to determine magnetometer offsets. These offsets are used through the duration of the lifetime of each satellite.

Approximate coefficients are:

* 0.8 mA per torquer current count
* 4 counts per torquer current command step (3.2 mA per command step)
* 410 mA full scale current
* 32 magnetometer counts per nT (approximate)
* 8 torquer step counts can be up to approximately 0.5 nT (depends on axis, which torquer, which satellite, etc.)

GOES 12 磁场数据，时间分辨率为 0.512 秒。矢量磁场以航天器地球参照坐标系给出：(H_p, H_e, H_n, ext{及} H_t)，其中 (H_p) 垂直于卫星轨道平面，或在 0° 倾斜轨道的情况下与地球自转轴平行；(H_e) 垂直于 (H_p) 并指向地球；(H_n) 垂直于 (H_p) 和 (H_e) 并指向东方；(H_t) 为总场。

本数据产品还提供了由 SSCWEB GEI 星历推导出的 GOES 12 卫星位置和速度，时间分辨率为 1 分钟。

磁力计数据实时从航天器传输至科罗拉多州博尔德的 NOAA 空间天气预报中心（SWPC）。原始的指令与数据获取（CDA）遥测流数据在 SWPC 的预处理程序中进行处理，随后 1 分钟的平均值被分发至 SWPC 空间天气预报操作，并存档于 NOAA 国家地球物理数据中心（NGDC）以及 NASA 协调数据分析网络（http://cdaweb.gsfc.nasa.gov/）。原始高分辨率数据在 NOAA SWPC 进行校正和处理，随后存档于 NGDC。

处理高分辨率数据的主要问题是对航天器扭矩电流磁场特征进行校正，如磁力计仪器描述中简要讨论。在正常操作中，磁力计每 0.512 秒对其三个轴进行采样，与遥测帧同步。磁力计在每个轴上都有抗混叠低通滤波器。作为航天器姿态控制的一部分，存在两个磁扭矩器。在磁力计上，这些扭矩器可以产生比待测环境磁场更大的磁场。因此，需要移除这些扭矩电流的信号。扭矩电流以与磁力计相同的速率进行测量，并用于校正磁力计测量。

然而，扭矩电流未经过低通滤波。因此，扭矩电流中的大瞬时阶跃在磁力计数据中表现为延迟的、缓慢上升的波，并伴随一些振荡，这正是磁力计低通滤波器所预期的。所谓的“Farthing 系数”从扭矩测量短历史记录中模拟磁力计滤波器的影响。扭矩电流的变化与遥测帧不同步，并且这些事件之间的时间间隔必须测量到遥测帧间隔的八分之一，以确保良好的性能。扭矩电流变化的瞬态响应减少，但并未消除对于扭矩电流的大变化。

在每次任务的开始阶段，存在一个机会对航天器进行旋转机动以确定磁力计偏移。这些偏移在整个卫星寿命期间被使用。

近似系数如下：
* 每个扭矩电流计数 0.8 毫安
* 每个扭矩电流命令步长 4 个计数（每个命令步长 3.2 毫安）
* 饱和电流 410 毫安
* 每 nT 32 个磁力计计数（近似值）
* 8 个扭矩步长计数可达约 0.5 nT（取决于轴、扭矩器、卫星等）