PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "Received from David Mitchell,
1999-05-13;
Mark Sharlow (revised), 1999-12-08;
Mark Sharlow (revised), 2000-03-02;
Mark Sharlow (revised), 2000-11-03;
Mark Sharlow (minor revisions),
2000-11-20;
Mark Sharlow (revised to reflect
resubmissions by team),
2001-06-27;"
RECORD_TYPE = STREAM
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = "MGS"
INSTRUMENT_ID = "ER"
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = "ELECTRON REFLECTOMETER"
INSTRUMENT_TYPE = "ELECTROSTATIC ANALYZER"
INSTRUMENT_DESC = "
=========================================================================
Instrument Overview
===================
The Electron Reflectometer system consists of a ''symmetric
quadrisphere'' imaging electrostatic analyzer, followed by a
microchannel plate (MCP) detector system with a resistive imaging
anode. The analyzer has a narrow energy pass band (about 25%), which
can be set by the inner hemisphere potential. This potential is
generated by a programmable high voltage supply, which is swept
through its range to measure electrons from 10eV to 20KeV. Electrons
are imaged onto the MCP, which multiplies individual electrons by a
factor of about a million. This cloud of electrons then hits a
resistive anode. The relative signal level on each end of the anode
is measured by the Pulse Position Analyzer (PPA) to determine the
location on the anode that the electron cloud landed. This location
translates into the direction in the field-of-view plane the incident
electron was coming from. The output of the PPA is an 8 bit digital
value proportional to the incident direction, from 0 to 360 deg.
This information is run through a Pitch Angle Mapper (PAM) which
sorts the events into 16 pitch angle bins, which are then counted in
a bank of counters. The PAM is programmed by the main electronics
package to convert the PPA output to pitch angle bins based on the
measured magnetic field vector provided by the magnetometer sensors.
The counters are read out to the main electronics package over a
serial interface 32 times a second, synchronized to the telemetry
clock (RTI). The analyzer control voltages and PAM are programmed by
the main electronics package via another serial interface.
=========================================================================
Platform Mounting Descriptions
==============================
The ER is mounted on the instrument deck. The ER's symmetry axis
(Z, which is orthogonal to its FOV) is orthogonal to the spacecraft
Z axis. The projection of the symmetry axis onto the spacecraft XY
plane is 10 degrees from the -Y axis and 80 degrees from the +X
axis.
========================================================================
Principal Investigator
======================
The Principal Investigator for the MAG/ER experiment is Mario
Acuna. The Lead Investigator for the Electron Reflectometer is
Robert P. Lin.
For more information on the Electron Reflectometer see
[ACUNAETAL1992].
=========================================================================
Scientific Objectives
=====================
See the MISSION_OBJECTIVES_SUMMARY item in the file MISSION.CAT.
(This file is in the CATALOG directory on this disk.)
=========================================================================
Operational Considerations
==========================
Parts of the spacecraft are within the instrument's FOV. During pre-
mapping, the stowed high gain antenna (HGA) blocked ~70 degrees.
Once the HGA was deployed, only small amounts of blockage remained,
which are caused by attitude control thrusters and the -Y solar array
gimbal and yoke assembly. One effect this has on the measurements is
to block ambient electrons from the directions of the obstacles. This
is most clearly seen at high energies (❯ 100 eV), which are only
slightly deflected by the spacecraft floating potential. In
addition, when these obstacles are illuminated by the sun, they emit
photoelectrons up to ~50 eV, which can enter the ER aperture and
elevate the counting rate at low energies. The detailed signature of
this effect depends on the illumination pattern as the spacecraft
rotates, which is a function of the angles between Earth, Mars, and
the Sun. These angles vary over the course of the mission.
Photoelectron contamination has not been removed from the data;
however, the presence of contamination is readily identified in the
low energy channels (❮ 50 eV) by a sharp (nearly discontinuous)
increase in counting rate which appears once per spacecraft spin
(~100 minutes during pre-mapping, ~120 minutes during mapping). The
contamination disappears as abruptly as it appears.
For a duration of ~4 minutes every half-spin of the spacecraft,
sunlight can directly enter the ER aperture and scatter inside the
instrument, creating secondary electrons. A tiny fraction of these
photons and secondary electrons can scatter down to the anode and
create a ''pulse'' of spurious counts. This sunlight pulse appears
at all energies, but is most noticeable from 10 to 80 eV and above
1 keV. Sunlight pulses have not been removed from the data.
The instrument's energy scale is referenced to spacecraft ground.
In sunlight, spacecraft ground floats a few volts positive relative
to the plasma in which the spacecraft is immersed. Electrons are
accelerated by the spacecraft potential before they can enter the
ER aperture, thus all energies are shifted upward by a few eV. In
addition to shifting the electron energy, the trajectories of low
energy electrons can be significantly bent by electric fields
around the spacecraft. Thus, the energy scale and imaging
characteristics are relatively poor at the lowest energies (10-30
eV), becoming much more accurate at higher energies.
=========================================================================
Calibration
===========
The ER may be put into one of 5 automated calibration modes by
command. These modes (1, 2, 3, 4, and 6) are designed to measure
specific instrument performance characteristics. Each mode
consists of a sequence of steps, each of which lasts one packet
collection interval. A calibration mode sequence can be programmed
to repeat from 1 to 255 times. After the selected number of
iterations has been performed, the system is returned to its
original state, with a few minor exceptions. These exceptions
include:
1. The test pulser is turned off at the end of Cal mode 1,
independent of its original state.
2. The ER_PAM_FIX table and ER_PAM_OFFSET value are modified in
all Cal modes.
In all modes except mode 6, the telemetry format is identical to
the normal format, but the instrument is operated differently, as
described below, except that Case Current telemetry data is usually
garbled. In mode 6, a block of each packet is used for the ADC
measurements, and the rest of the packet is scrapped.
In all Calibration modes, the PAM_FIXED mode is used (see below),
and the PAM_FIX table and PAM_OFFSET value are modified. The
nominal values for the table are described below, but all can be
reprogrammed.
(1) ER Calibration Mode 1 (Test Pulser)
This mode is used to stimulate the instrument when the analyzer is
not functional (high voltages off). It can also be used to
calibrate the PPA anode. If the instrument high voltage is turned
on, real particle counts will be mixed with the test pulser counts.
The PAM is loaded with a fixed map which maps the region near the
pulser input with highest resolution (1 PPA bin per counter), with
the rest of the anode going into counter number 15 (the last pitch
angle bin). Next the test pulser is turned on. Its amplitude is
increased linearly from zero to full scale synchronous with the
normal one second analyzer voltage sweep, so that what is normally
energy step in the telemetry is now test pulser amplitude. This is
repeated for one packet duration, and then the cycle is repeated
for the next packet with the next test pulser. The whole sequence
lasts four ER packet intervals, as shown in the table below:
Packet Counter Test Pulser PAM_OFFSET
0 A (30 deg) 13
1 B (180 deg) 120
2 C (330 deg) 227
3 B (180 deg) 120
PAM_OFFSET is set to center the nominal test pulser location on the
high resolution part of the PAM. The values of PAM_OFFSET and the
sequence of test pulsers are programmable.
(2) ER Calibration Mode 2 (MCP Bias)
This mode is used to determine the optimum setting of the MCP bias
voltage. The instrument continues to operate in the normal mode,
except that the MCP bias voltage is modified once per packet, over
a cycle of 8 ER packets. Also, the PAM_FIXED mode is used, with
the PAM table set to 16 equal 22.5 deg fixed image plane bins.
This allows a look at MCP efficiency as a function of location on
the MCP. If there is one particular point on the MCP that should
be monitored, the PAM_FIX table generated for Cal mode 2 can be
modified in ER_CAL_PAM table - see discussion above.
Assuming conditions are stable over the test cycle time, the data
from the different MCP voltage settings can be compared on the
ground, and an optimum voltage can be selected and commanded up.
The pattern of MCP voltages used is programmable using the
ER_CAL2_MCPOFFSET command; the default values are +3, +6, 3, 0, -3,
-6, -3, 0. The values from this table are added to the current MCP
DAC setting. The last entry in this table should be 0; the MCP
will be left at the voltage level of the final entry. Note that
the default table has steps of about 48 volts.
(3) ER Calibration Mode 3 (Background)
Calibration mode 3 is used to measure the instrument background
counting rate from noise and cosmic rays. The instrument sweep is
stopped at about 10eV with the deflector attenuator set on. The
PAM_FIX table and MCP bias are set and sequenced identically to Cal
mode 2, so that the background can be measured as a function of MCP
bias voltage. Unfortunately, unlike the case of Mars Observer, one
cannot completely shut off incoming electrons, so this mode is of
marginal usefulness.
(4) ER Calibration Mode 4 (Deflector Attenuator)
This mode is used to inter-calibrate the instrument sensitivity
with and without the deflector attenuator on. This is done by
disabling normal sweeps, and fixing the analyzer voltage at the
place where the grid attenuator is normally turned on. For the
first packet, the attenuator is off, and for the second packet it
is turned on (the cycle lasts 2 ER packet intervals). The PAM_FIX
table is loaded for 16 equally spaced 22.5 deg bins.
(5) ER Calibration Mode 6 (Voltage Calibration)
This mode is used to measure the various analyzer voltages which
normally vary during a packet via the analog housekeeping ADC. The
voltages are swept over their range slowly, and the ER analog
housekeeping ADC read-out time slot is dedicated to the measurement
(giving 4 samples per second). The voltage sweeps are synchronized
to the ADC read-out times. The data from the ADC is loaded directly
into the ER telemetry packet right after the header, taking 48 16
bit words (48 12-bit ADC samples per packet, with the 4 MSB of each
word set to zero). Normal telemetry data is lost, and the rest of
the packet will be garbled. The mode consists of 2 different
cycles, each looking at different voltages, and each taking 1
packet interval to complete.
(5a) ER Calibration Mode 6, Cycle 1 (Analyzer and Deflector
Attenuator Voltages)
This cycle measures 32 samples of the analyzer voltage, using every
4th value of the normal analyzer sweep table. Note that the gain
of the housekeeping channel switches when the gain of the
programming DAC for the analyzer is changed, and by the same
amount, so to correctly interpret the measurements, the setting of
the gain bit must be known for each measurement. The MSB of each
sample contains the gain bit of the DAC for this purpose. The
remaining 16 values are measurements of the attenuator grid made
while the analyzer and grid voltages are programmed in their normal
pattern over the second half of the sweep table, again using every
4th entry of the table.
(5b) ER Calibration Mode 6, Cycle 2 (Case Voltage)
The second packet of ER Cal mode 6 contain measurements of Vcase
(the case voltage), Icase (the case current), and Icase*80 (the
high gain Icase measurement), while the Case Voltage DAC is ramped
over its full range in 16 steps (starting at 0, 16 DAC steps each
sample). The data is ordered as 16 Vcase samples, followed by 16
Icase samples, and finally 16 Icase*80 samples.
=========================================================================
Operational Modes
=================
(1) Energy sweep and attenuator
Data is accumulated over one or more energy sweeps. Each energy
sweep takes 1 second, and is divided into 128 equal steps
(synchronized to the RTI). The analyzer high voltage is swept from
high to low energy during the sweep in an approximately exponential
decay. Data is collected for 16 pitch angle bins 30 times each
sweep, dividing phase space into 16 pitch angles by 30 energy bands
over the energy range of the instrument. Note that data is not
collected during the first 4 energy steps when the analyzer high
voltage is re-charged, or during the 4 steps that the attenuator
grid voltage is charged up.
The sweep voltage is controlled via the ER sweep registers to a 12
bit DAC with a gain switch. The gain switch changes the sweep
voltage by a factor of 16 to increase the accuracy of the voltage
setting at the low end. The software generates a log sweep pattern
on turn-on (which may be over-written by ground command), and
automatically sets the gain bit appropriately.
At a selectable point in the sweep, the deflection attenuator turns
on (default is at step 124, which means it never turns on). This
decreases the sensitivity of the instrument by a factor of 43.5.
This is needed to avoid saturating the instrument at the low energy
end (where there are typically a lot of electrons), while allowing
maximum sensitivity at the high energy (where there are few
electrons). When energized, the deflectors bend electrons from the
normal aperture out of the analyzer field of view, while
simultaneously bending electrons from the lower, attenuated
aperture into the analyzer field of view. The deflection supply
runs at 8 times the analyzer high voltage, but the supply tops out
at about 600 volts, and works only in the low gain range of the
analyzer high voltage. The sweep voltage pauses while the grid
attenuator voltage comes on for one accumulation sector time, and
the data collected from that interval is discarded.
(2) PAM-variable Mode: (onboard pitch angle sorting)
The events are converted into 16 pitch angle bins, corresponding to
the 16 counters, using the PAM table. This table is generated by
the software based on the direction of the magnetic field vector.
The PAM table is updated every 2, 4, or 8 seconds at 1296, 648, and
324bps respectively. A one second average of the magnetic field
samples is computed for this purpose. Offsets are then subtracted
from a programmable table. Next, the vector is rotated into ER
sensor coordinates using a variable rotation array set by the solar
array motion model (see below), followed by a second fixed rotation
matrix. The vector is then normalized and the azimuthal angle
(PHI) and cosine of the elevation angle (COSTH) are computed. PHI
is the angle around the FOV plane, coded in an 8 bit number such
that 256=360 deg, zero degrees being at the anode break point,
increasing clockwise as viewed from the top of the analyzer (the
anode break point is 135 deg clockwise from the RPA aperture).
COSTH is the cosine of the elevation angle out of the FOV plane,
zero degrees being is the image plane. COSTH is always positive,
since the sign is unimportant for PAM table computation. These
quantities are the basis of the PAM table generation, and are
transmitted in the ER packet so that the PAM table can be
reconstructed on the ground for computing the bin weighting.
The PAM table is generated by computing the image plane location of
the pitch angle bin boundaries, and then filling in the rest of the
table, using the relationship:
Bin = PHI +- Cos-1(COSPAi/COSTH)
where:
Bin is the image plane bin corresponding to pitch angle boundary # i
COSPAi is the cosine of the i-th pitch angle boundary
+- is the 'plus-or-minus' sign, normally represented by a + sign
above a - sign, but written here as +- for typographical
reasons.
The COSPAi are programmable.
(3) PAM-fixed Mode.
This computation can be bypassed by going to a 'Fixed' PAM table,
which is independent of the magnetic field direction. This can be
done by command or automatically during calibration cycles. The
'fixed' map is characterized by a set of 16 image plane boundaries
and a rotation of that pattern similar in function to PHI.
Before the PAM table is loaded into the ER, the table is masked to
remove a programmable set of 'bad' regions. These are parts of the
image plane which are noisy or which have objects in the FOV
distorting the trajectories. This table defaults to 'none'.
========================================================================
Measured Parameters
===================
The ER measures particle counts from 1 to 507904. The instrument
integrates for 0.0625 seconds at each energy channel every 2
seconds. The noise level is about 10 counts per second at all
energies."
END_OBJECT = INSTRUMENT_INFORMATION
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "ACUNAETAL1992"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "CUEVAS1989"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "JOHNSON1990"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "SAUNDERSETAL1990"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
END_OBJECT = INSTRUMENT
END
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