THEMIS Error
Budget
Tech Memo, 20Jan03, E.R.Taylor
REVISIONS:
27 October 2003 – Draft
07 November 2003 – Revised per telecon, 06 Nov 03 (Paul Turin, Rob Eppler, Vassilis
Angelopoulos, Richard LeBoeuf, Ellen Taylor, Tom Ajluni, Kevin
Brenneman)
11 November 2003 – Added clarifications on timing
20 January 2003 – Clarified 0.1 nT stability is required for inner probes from perigee to apogee
(over 12 hours) only
PURPOSE: The purpose of this Tech Memo is to document and illustrate the flow-down of THEMIS Error Budget
Requirements. The intent is to explain revisions to the lower level subsystem requirements in THM-SYS-001
THEMIS Mission Requirements Document Rev B, released prior to the THEMIS Mission PDR. This memo only
addresses the stability, absolute knowledge and absolute time requirements as related to science objectives, not those
derived from maneuvers or other probe bus considerations such as thermal. For an explanation and flow-down of
those other requirements, please see SAI-SPEC-TBD THEMIS Attitude Control Error Budget.
REFERENCE DOCUMENTS:
THM-SYS-001 Mission Requirements Document, Rev B
SAI-SPEC-TBD Attitude Control Error Budget
FGM Stability Requirement 0.2nT, Vassilis Angelopoulos, e-mail 10/24/2003
Boom Angles, Vassilis Angelopoulos, e-mail 08/15/2003
POINTING STABILITY REQUIREMENT:
For pointing stability, the driving Level 1 Science requirement is:
S-8: Determine the cross-current-sheet current change near the current disruption region (+-2Re of meridian, +-2Re
of measured current disruption region) at substorm onset from a pair of Z-separated probes using the planar current
sheet approximation with relative (inter-probe) resolution and inter-orbit (~12hrs) stability of 0.2nT.
This requirement can be broken into the following segments:
1.
2.
Inter-orbit stability over 12 hours shall be <0.2nT (relates to the measurements of the FGM sensor only)
Inter-probe resolution shall be <0.2deg (relates to knowledge only)
a. ACS knowledge (FGM internal) shall be <0.1deg (Bx and By)
b. Mechanical/thermal stability shall be <0.1deg
This shows the Level 1 requirement met in the following way:
Over 12 hours the FGM sensor may drift up to 0.2nT. Dynamic determination on the ground can compute ACS
knowledge to within 0.1deg every hour. 0.1 degrees translates into 0.1nT knowledge in a 60nT field. This method
allows knowledge in Bx and By to be determined to within 0.1nT every hour but does can not allow knowledge of
the Bz offset. The Bz offset can build up to 0.2nT at the end of 12 hours and invalidate the original inter-probe
calibration made at the beginning of the interval. However, at a magnetic inclination of 30 deg (max, CSR Figure E18), the Bx-contribution due to Bz offset is sin(30)=1/2 of 0.2nT, or 0.1nT over 12hrs. Therefore, the total interprobe knowledge of Bx is 0.1nT from dynamic determination and 0.1nT from the Bz offset contribution on Bx, or
0.2nT total.
So, taking each segment separately:
1.
To meet the inter-orbit stability requirement of <0.2nT over the time of interest (~12 hours), the FGM
sensor must meet the following Instrument requirement:
IN.FGM-3a: The relative stability of the FGM shall be less than 0.2nT over 12hrs
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2.
To meet the inter-probe resolution requirement of <0.2deg, the requirement is split into two components:
(a) ACS knowledge (FGM internal); and (b) mechanical/thermal stability. For (a), the FGM-to-spin axis
angle can be dynamically determined to <0.1 degrees at 1-hour intervals by science data processing on the
ground, assuming the FGM is also relatively stable over this period. Therefore, the following two
requirements must be met:
GS-SOC-13: ACS knowledge (FGM-to-spin axis) shall be estimated to within 0.1 degree every hour using
ground-based processing; and
IN.FGM-3b: The relative stability of the FGM shall be less than 0.1nT over 1hr
For (b), mechanical/thermal stability is captured in the following requirement:
IN.BOOM-2: Magnetometer Boom stability shall be better than 0.1 degree
More explicitly, this requirement applies from the FGM sensor to Probe Z-axis. Therefore, the 0.1 degree
should split between two components: (i) contribution from the boom (sensor mounting and boom
stability); and (ii) contribution from the bus (boom mounting and thermal stability of deck). Stability of the
probe geometric axis to principle axis is considered negligible, allowing the FGM-to-Ecliptic to be known
within the required 0.2 degrees including sensor instability as well.
To accomplish (i) and (ii), hole tolerances, mounting templates, mounting edges, etc. will be implemented
as indicated in the FGM ICDs.
Adding up all components of Segment 2, 0.2deg knowledge can be computer every hour, and consequently
over a 12-hour period. However, the dynamic determination of FGM-to-spin axis angle of <0.1 degree
over 1-hour interval is only possible at the P3/4/5 apogee assuming stable offsets and booms within that
period (easier at perigee). Therefore, eclipse periods and post-shadow intervals of temperature stabilization
(period of rapid temperature variations) should be excluded.
ABSOLUTE KNOWLEDGE REQUIREMENT:
For absolute knowledge, the driving Level 1 Science requirement is:
S-6: Track between probes the earthward ion flows (400km/s) from the reconnection site and the tailward moving
rarefaction wave in the magnetic field, and ion plasma pressure (motion at 1600km/s) with sufficient precision
(dV/V=10% or V within 50km/s whichever is larger, dB/B=10%, or B within 1nT whichever is larger, dP/P=10%,
or P within 0.1nPa whichever is larger) to ascertain macroscale coupling between current disruption and
reconnection site during >10 substorm onsets (>188hrs of four-probes aligned within dY of +-2Re).
This requires absolute FGM orientation on one probe to 1deg (translated from 1nT in 10nT field, near the current
sheet where flows and current disruption measurements are made). This requirement can be broken up into the
following components, where the addition of all components is less than the 1-degree total:
1.
Probe spin axis inertial orientation shall be < 1 degree
a. ACS knowledge (FGM-to-spin axis) shall be <0.1 deg (apogee and perigee)
b. ACS inertial knowledge (Spin-to-Probe Z axis) shall be <0.5 deg (around 6 hours at perigee)
i. Deployment of the boom shall be <1 deg.
c. Magnetometer drift shall be negligible (drift is 0.2nT/300nT, or <0.03 deg, at perigee)
d. Contribution from probe magnetics shall be <0.1 deg (equivalent to DC stability <0.1nT at 2m)
e. Boom stability shall be <0.1 deg
f. Bus stability (Probe Z axis-to-Izz principle axis) shall be negligible
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g.
Time tag accuracy shall be <0.1 deg
Again, taking each segment separately:
1.
The direct flow-down of the Level 1 requirement is levied on ground system processing (GS-SOC-12 was
PB-42 in MRD Rev A. See change log.):
GS-SOC-12: The probe spin axis inertial orientation shall be estimated in ground-based processing to
within 1 degree, 3 sigma
To meet this requirement, the following components are considered:
a.
As with the stability requirement, the FGM-to-spin axis angle can be dynamically determined to <0.1
degrees at 1-hour intervals by science data processing on the ground, assuming the FGM is also stable
over this period. Therefore, the following two requirements are levied:
GS-SOC-13: ACS knowledge (FGM-to-spin axis) shall be estimated to within 0.1 degree using
ground-based processing; and
IN.FGM-3b: The relative stability of the FGM shall be less than 0.1nT over 1hr
b.
Perigee modeling of FGM data and spin-to-Probe Z axis gives inertial knowledge to <0.5deg from 6
hours around perigee, given an initial FGM deployment knowledge of 1deg. Therefore, the following
two requirements are levied:
GS-SOC-14: ACS knowledge (spin-to-Probe Z axis) shall be estimated to within 0.5 degree using
ground-based processing
IN.BOOM-1: Magnetometer Boom deployment shall be repeatable to 1 degree, 3 sigma.
The 1 deg absolute knowledge budget relates to the need of the FGM, SCM and EFI instruments to
inter-calibrate and have an apriori knowledge of where they will end up. Knowing that there is little
spin ripple along SCM Bz, allows Bz to be selected as the waveform data quantity of choice.
Therefore, the 1 deg repeatability of X/Y direction for MAG booms is more, not less critical, since
that's the angle that is more difficult for FGM to determine. I.e, 1-deg repeatability is required in both
spin-plane and Z directions.
Note: 0.5 degree inertial knowledge is guaranteed from 6 hours around perigee only. However, note
that the other components of the budget are
c.
Magnetometer drift can be considered negligible at perigee, given that the drift shall be less than 0.2nT
in a 300nT field, or <0.03 deg. Thus, only following requirement is levied:
IN.FGM-3a: The relative stability of the FGM shall be less than 0.2nT over 12hrs
d.
DC stability due to the probe field shall be <0.1nT at 2m drives the following magnetic cleanliness
requirements:
M-31a: DC magnetic noise radiated by the Probe and other instruments shall not exceed 5nT at 2
meters from the Probe (location of FGM sensor).
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M-31b: DC magnetic field generated by the Probe subsystems and other instruments shall be stable to
<0.1nT over 12hrs at 2m from the Probe (location of FGM sensor) over the operational temperature
range of these subsystems and instruments.
Assuming a 0.1nT stability is achieved, the absolute 5nT offset from the Probe can be accommodated
if this offset is measured (see THM-SYS-002 Magnetics Contamination Control Plan).
e.
Boom stability shall be <0.1deg is met with the following requirement, distributed between a
contribution from the boom and contribution from the bus as discussed above in the stability section:
IN.BOOM-2: Magnetometer Boom stability shall be better than 0.1 degree
f.
Bus stability (Probe Z axis-to-Izz principle axis) is considered negligible.
g.
For absolute knowledge, the FGM data must be time tagged to an accuracy of <0.1 deg. Assuming a 3
second spin period, 0.1 degree is equivalent to 0.83ms, driving the following requirement:
IN.DPU-32: The IDPU shall time-tag DFB and FGM data to <0.5 ms
Note that this same requirement (<0.5ms) holds for ESA and SST data, driven by the need to
accurately spin sector ESA data (to within 0.1 degree) and synchronize the data with SST:
IN.DPU-31: The IDPU shall time-tag ESA and SST Spin Reference Pulse (SRP) to <0.5 ms
ABSOLUTE TIME REQUIREMENT:
Finally, to achieve the Level 1 science objective of onset and evolution of substorm instability to within 30 seconds;
the onset time of the auroral breakup, current disruption and reconnection should all be known to within 10 seconds.
With a 3 second cadence due to the spin period on the 3 probes in the current disruption region, error on the absolute
time must be less than 1 second:
PB.CDH-44: The C&DH subsystem shall maintain Coordinated Universal Time (UTC) with an
accuracy of +/- 0.5 sec.
This requirement drives the accuracy of the C&DH oscillator. Oscillator drift then drives how often the probes must
be contacted to update the on-board time.
Note that actual Level 1 requirements are set at t_res = 30 seconds for auroral breakup, current disruption and
reconnection, allowing up to a 10 second cadence due to the spin period but not affecting the overall requirement on
absolute timing.
----------------------------------------------------------------------------------------------------------------------------- --------------NOTE: In Revision A of the MRD, PB-43 and PB-44 were broken down as follows:
PB-43: Angle between FGM instrument Z-axis and the ecliptic plane normal shall not vary more than 0.16
(TBR) degrees (3 sigma) over any 12-hour period during normal science observation (required outside 5RE).
PB.ACS-7: The angle between the probe Izz principle axis of inertia and the probe Z-axis shall not vary
more than 0.1 degrees (3 sigma) over any 12-hour period during normal science observation.
PB.ACS-14: The angle between the probe spin axis (momentum vector) and the observation reference shall
not vary more than 0.1 degrees (3 sigma) over any 12-hour period during normal science observation
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PB-44: Angles between FGM instrument Z axis and the ecliptic plane normal shall not vary more than 1 (TBR)
degrees (3 sigma) over any 3 day period during normal science observation.
PB.ACS-8: The angle between the probe Izz principle axis of inertia and the probe Z-axis shall not vary
more than 0.1 degrees (3 sigma) over any 12-hour period during normal science observation.
PB.ACS-20: The angle between the probe spin axis (momentum vector) and the observation reference shall
not vary more than 0.6 degrees (3 sigma) over any 3 day period, during normal science observation
Where ACS contributions were RSS’d to meet PB requirements (i.e. sqrt(0.1^2+0.1^2) < 0.16deg). However, in
subsequent analysis, ACS contribution was considered negligible compared to the stability of the Magnetometer
Boom:
IN.BOOM-2: Magnetometer Boom stability shall be better than 0.1 degree (FGM to inertial)
The flow-down described in this memo assumes that nothing will perturb the spin axis in short time scales and
therefore, all of the above ACS requirements are considered negligible. Therefore, in Revision C of the MRD, PB 43, PB -44, PB.ACS-7, PB.ACS-8, PB.ACS-14 and PB.ACS-20 are deleted and the top-level requirement of 0.2
degrees is apportioned between the FGM Sensor itself and the Boom.
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