| INSTRUMENT_DESC |
Instrument Overview
===================
The MIRO instrument will provide both very sensitive continuum
capability for temperature determination and extremely high-
resolution spectroscopy for observation of molecular species.
The MIRO instrument consists of two heterodyne radiometers, one
at millimeter wavelengths (1.3 mm) and one at submillimeter
wavelengths (0.5 mm). The millimeter and the sub-millmeter
radiometers have continuum bandwidths of 0.5 GHz and 1.0 GHz
respectively in addition, the submillimeter receiver has a total
spectroscopic bandwidth of 180 MHz and a spectral resolution of
44 kHz. In the spectroscopic mode, 4096 channels, each having a
bandwidth of 44 kHz, are observed simultaneously.
Instrument Summary
==================
The performance parameters that govern the MIRO instrument
design include system sensitivity, spatial resolution,
radiometric accuracy (both absolute and relative), beam pattern
and pointing accuracy, together with the mass, power, volume
envelope, and environmental conditions available within the
spacecraft. The MIRO instrument performance characteristics
are summarised in Table below.
Equipment Property MM-Wave SubMM-Wave
Telescope Primary Diameter 30 cm 30 cm
Primary F/D 1 1
Sidelobes -30 dB -30 dB
Spatial Resolution 24 arcmin 8 arcmin
Footprint (at 2 km) ~15 m ~5 m
Spectral Freq Band 188.5-191.5GHz 547.5-580.5GHz
Performance 1st IF Bandwidth 550MHz 11GHz
1st IF Freq Rng 1-1.5GHz 5.5-16.5GHz
Spectral Resolution n/a 44 kHz
Spectral Rng per line n/a nominally 20 MHz
Accuracy n/a 10 kHz
Spectrometer Center Freq/Bandwidth n/a 1350/180 MHz
Number of channels n/a 4096
Radiometric DSB Receiver Noise 800 K 3800 K
Performance Temperature
SSB Spectroscopic Sens
(300 KHz, 2 min)
relative n/a 2 Krms
absolute n/a 3 Krms
Continuum Sens (1 sec):
relative 1 Krms 1 Krms
absolute 3 Krms 3 Krms
Data Rates Instantaneous Rate
Continuum Mode <1 kbps
Spectroscopic Mode 2 kbps
On-board Storage 0.2 Gb (1)
(1) One day's data volume, Mode 3, 100 Duty cycle, see Vol 6., Table
6.3.1-1)
Abbreviations used in above:
Freq = Frequency
Rng = Range
Sens = Sensitivity
MIRO Data Modes
===============
The MIRO instrument has 6 major modes of operation and data-
taking that reflect operational combinations of its two continuum
radiometers and the spectrometer, engineering mode, millimeter
continuum mode, submillimeter continuum mode, dual continuum
mode, CTS/submillimeter continuum mode, and CTS/dual continuum
mode.
In addition, a special mode has been designed for planetary and
asteroid flybys. A number of data compression options are
obtained in each mode by varying the data-taking rate (integration
time per sample) and/or spectral resolution of the radiometers
and spectrometer. The parameters for each mode are summarized
below. For more details, see the MIRO User Manual (AD4 of the
EAICD, RO-MIR-IF-0001_14), Volume 6.1.
1. Engineering Mode
In engineering mode the MIRO software is collecting
engineering data from 56 internal sensors. The sampling of
these sensors is at a 5 Hz rate. All engineering measurements
are 12-bit A/D converted values. The engineering mode
telemetry is sent to the spacecraft in the form of a
housekeeping telemetry packet. One engineering telemetry
packet is typically generated every 11 seconds.
2. Millimeter Continuum Mode
In millimeter continuum mode continuum data are collected from
the millimeter radiometer at a 20 Hz. rate. All continuum data
consist of 16-bit values. The millimeter continuum data are
nominally packetized into science telemetry packets every 10
seconds. A 'summing value' parameter can cause the MIRO
software to sum either 1, 2, 5, 10 or 20 separate continuum
values prior to putting them into the telemetry packet. This
feature can reduce the data rate to as little as one
millimeter continuum packet every 200 seconds.
3. Submillimeter Continuum Mode
Submillimeter continuum mode is identical to the millimeter
continuum mode in data collection and packing, except that a
different set of electronics is powered on. Millimeter and
submillimeter continuum data are contained in separate science
telemetry packets, identified by a field in the source data
header.
4. Dual Continuum Mode
In dual continuum mode the millimeter and submillimeter
continuum are collected simultaneously. When running in dual
continuumvmode, the summing value parameter mentioned earlier
is applied to both sets of data, causing equal amounts of
millimeter and submillimeter data to be generated.
5. CTS / Submillimeter Continuum Mode
This mode adds the collection of chirp transform spectrometer
(CTS) data. The CTS is programmed by the MIRO software to run
for an initial sub-integration period of approximately 5
seconds. An internal LO frequency generator is then switched
and another 5 second period is observed. These pairs of
observations are repeated with the respective results being
summed over time. Selectable integration periods are 30, 60,
90 and 120 seconds. The data from the two LO frequencies are
then subtracted from each other to provide a single
4096-element difference spectrum. The 4096 data values can
be further reduced by application of a smoothing function
whereby data from several channels are combined and weighted
to produce fewer final channels. Smoothing window sizes are 1,
5, 7 and 9 channels. A mask is applied to the CTS data and
only 12 bits of each resulting measurement are returned. CTS
data collection and the LO frequency switching is
coordinated with the collection of continuum data. Exactly 100
continuum samples are taken during each CTS scan. Upon
receipt of the data on the ground it is known at which LO
frequency all of the continuum measurements were made at. If
the CTS has just been powered on, an internal calibration of
the CTS is performed. This consists of loading the 4 CTS sum
of square tables with a linear ramping pattern. A 10,000 cycle
integration is then performed and the resulting data read out.
The data are then averaged to yield the mid-point of the
table. The resulting mid-point values for each table are
downlinked in telemetry packets for monitoring over time.
6. CTS / Dual Continuum Mode
This is the same as the CTS / SMM continuum mode except that
the millimeter data are also collected.
7. Asteroid Mode
This special data-taking mode has been implemented for the
asteroid and planetary encounters to enable MIRO to follow the
rapid Doppler shift of spectral lines that may be visible. The
primary characteristic of this mode is that LO frequency
switching is turned off. The LO is set to either +5 MHz or - 5
MHz from the nominal frequency prior to the encounter. At the
specified encounter time, the LO frequency is switched _ 5 Mhz
(opposite from the first setting) from the nominal frequency.
Continuum data are collected at 20 Hz. Each set of CTS data
consists of a single 5-second integration with all 32 bits
returned for each 4096 channels.
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