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GOES I-M INR Compensations
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In order to present information about INR compensations it
is helpful to have insight onto main parts of the INR system. A diagram
below represents the basic
building blocks of the INR system.
1. OATS – Orbit & Attitude Tracking System
2. SPS – Sensor Processing System
3. GIMTACS – GOES IM Telemetry & Command System
4. RPM – Replacement Product Monitor
The GOES I-M INR system generates compensation coefficients (by OATS) that are used
in real time by the onboard computer of the GOES spacecraft (S/C) in order to
compensate for the bus and instruments orbit and attitude deviations. These coefficients
are converted into two (E/W, N/S) time-varying signals that transmitted to the
instrument servos.
There are three INR compensations routinely uploaded to the S/C and corresponding three
groups of coefficients: IMC, SCC and SMC. In addition, the Mirror Motion Compensation
(MMC) is used in order to compensate for the instrument induced (Imager, Sounder)
spacecraft attitude deviations caused by the scan and slew motions of an instrument
itself. The IMC, SCC, SMC coefficients are uploaded to the S/C's onboard computer on
regular basis, whereas the MMC parameters are loaded to the onboard computer memory at
the initialization phase. Thus total INR compensation consists of four models: MMC, IMC,
SMC and SCC.
IMC - Image Motion Compensation
One of the main INR compensations is IMC. The IMC corrects instrument pointing errors
caused by a long-term spacecraft orbital perturbations and thermal distortion of the
optical axis. The OATS generates IMC coefficients on daily basis.
The GOES spacecraft can be operated in two modes: “IMC On” and “IMC Off”. If selected
mode is “IMC On” the IMC coefficients are uploaded by GIMTACS to the on-board memory
of the S/C for direct application to the instrument servo system. However, when
operating in “IMC Off” mode no IMC coefficients get uploaded to the spacecraft and
thus Earth locating need to be performed on the ground by the user's software.
SCC - Single Chord Compensation
Under nominal conditions 2 Earth Sensor (ES) chords are used to Earth-Center the
GOES S/C.
However,there are seasons when sun and moon "intrude" one of the ES chords. This intrusion
causes S/C attitude errors. To avoid these errors one of the chords is "inhibited". The
transition from "Dual Chord" to "Single Chord " and back to "Dual" causes roll errors.
These S/C attitude roll errors are compensated by Single Chord Compensation - SCC.
The OATS generates the Single Chord Compensation coefficients on weekly basis and sends it
to the GIMTACS. When ES intrusions are predicted (per OATS) the scheduling incorporates the
upload of SCC coefficients to the onboard computer. These coefficients are applied when one
of the controlling ES chords is inhibited.
Note about Bevel.
The Bevel is ES entering sunlight phenomenon that cause the S/C pointing errors. These errors
are handle by Single Chord Operation. Since it is sun declination dependent event there are
four distinct periods of Bevel ops throughout the year (see figure below).
SMC - Spacecraft Motion Compensation
There are two types of Spacecraft Motion Compensations: “Batwing” and “Boomsnap”.
The “Batwing” effect is a spacecraft control system response to the sunlight entering the
Earth Sensor (ES) and resulting in roll and pitch attitude errors (note: the “Batwing”
effect only applies to GOES – 8). The “Boomsnap” effect is a spacecraft disturbance
resulting from the projection of the magnetometer boom shadow on the solar sail causing
roll attitude errors.
Since both effects are seasonal (i.e. sun declination dependant) the SMC coefficients are
generated and uploaded to the onboard memory on daily basis when in season
(see figure below).
IN SUMMARY, the total INR compensation that is applied by the on-board firmware is a
summation of all INR compensation models and can be expressed as:
True Instrument Compensation = S/C attitude compensation (MMC,SMC,SCC) + Instrument
Attitude compensation (IMC)
INR Compensation = IMC + MMC + SMC + SCC
The plot below represents sun light driven INR events and the time they occur.
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