GOME-2 is a medium-resolution double UV-VIS spectrometer, fed by a scan mirror which enables across-track scanning in nadir, as well as sideways viewing for polar coverage and instrument characterisation measurements using the moon. The scan mirror directs light into a telescope, designed to match the field of view of the instrument to the dimensions of the entrance slit. This scan mirror can also be directed towards internal calibration sources or towards a diffuser plate for calibration measurements using the sun (see Figure 4.1).
GOME-2 comprises four main optical channels which focus the spectrum onto linear silicon photodiode detector arrays of 1024 pixels each, and two Polarisation Measurement Devices (PMDs) containing the same type of arrays for measurement of linearly polarised intensity in two perpendicular directions.
The four main channel detectors are actively cooled in a closed loop configuration to -38°C to maximise sensitivity and minimise noise. In contrast the two PMD detectors are cooled in an open loop configuration to an operating temperature of around 0°.
The PMDs are required because GOME-2 is a polarisation sensitive instrument and therefore the intensity calibration must take account of the polarisation state of the incoming light. This is achieved using information from the PMDs.
Figure 4.1: Artists impression of the GOME-2 optical layout (courtesy of ESA)
|1 - Disperser||10 - Beam splitter||19 - Channel # 2|
|2 - Calibration Slit||11 - Channel # 3||20 - Grating # 1|
|3 - Detector||12 - Channel # 4||21 - Grating # 2|
|4 - Double Brewster Prism||13 - 590 -790 nm||22 - Calibration lamp|
|5 - Telescope mirror||14 - 401 - 600 nm||23 - Calibration Unit|
|6 - Predisperser Prism||15 - 240 - 315 nm||24 - Sun diffuser|
|7 - Channel Separator||16 - 311 - 403 nm||25 - Telescope mirrors|
|8 - Grating # 3||17 - Electronics box||26 - Scan mirror|
|9 - Grating # 4||18 - Channel # 1|
Light is collimated by an off-axis parabolic mirror, behind the entrance slit, onto the double Brewster and pre-disperser prisms which generate the s- and p- polarised beams. These beams are subsequently dispersed onto detectors contained within the Polarisation Unit (PU).
Figure 4.2: The GOME-2 Polarisation Unit (PU) detailed optics (courtesy of ESA)
Light passing through the pre-disperser prism is also directed onto the main spectrometer. A second off-axis parabolic mirror focuses the dispersed beam onto the channel separator prism. This is a quartz prism, the first surface of which is partially coated with a transmission coating for channel 1 and a reflective coating for channel 2. The light for channels 3 and 4 passes the prism edge and a dichroic filter separates it into the two channels.
The four main channels provide continuous spectral coverage of the wavelengths between 240 and 790 nm with a spectral resolution (FWHM) between 0.25 nm and 0.5 nm. Compared to the main channels, the PMD measurements are performed at lower spectral resolution, but at higher spatial resolution. The two PMD channels are designed such that maximum similarity in their optical properties is ensured. The wavelength-dependent dispersion of the prisms causes a much higher spectral resolution in the ultraviolet than in the red part of the spectrum.
In order to calculate the transmission of the atmosphere, which contains the relevant information on trace gas concentration, the solar radiation incident on the atmosphere must be known. For this measurement a solar viewing port is located on the flight-direction side of the instrument. When this port is opened, sunlight is directed via a ~40° incidence mirror to a diffuser plate. Light scattered from this plate, or in general, light from other calibration sources such as the Spectral Light Source (SLS or HCL) for wavelength calibration, and the White Light Source (WLS) for etalon (and, optionally, pixel-to-pixel gain) calibration are directed to the scan mirror using auxiliary optics. Diffuser reflectivity can be monitored internally using light from the SLS. All internal calibration sources with their optics are assembled in a subsystem called the 'Calibration Unit' (CU). The only exception are light emitting diodes (LEDs) which are located in front of the detectors to monitor the pixel-to-pixel gain. For more information on the GOME-2 instrument see [SCD1].
Figure 4.3: The GOME-2 calibration unit (courtesy of ESA)
Every 375 milliseconds, GOME-2 generates one science data packet. A data packet comprises 9369 2-byte words, leading to an average data rate of 8x2x9369/0.375 bit/s = 400 kbit/s. A detailed description of the science data packet format is provided in [RD9]. Briefly, a GOME-2 data packet consists of three basic parts (apart from header information): instrument housekeeping (HK) data (e.g., temperatures, scan mirror angles, lamp currents and voltages), PMD data, and main channel Focal Plane Assembly (FPA) data. The maximum temporal resolution differs between main channel FPA and PMD data. One data packet contains up to two main FPA readouts, corresponding to a 187.5 ms temporal resolution, and up to 16 PMD readouts, corresponding to a 23.4 ms temporal resolution. A detailed description of the options for PMD readout and data transfer is given in Appendix B of [RD7] GOME-2 Level 1 Product Generation Specification.
A basic concept in the operation of the GOME-2 instrument is that of the 'scan'. A scan is defined as a time interval of 6 seconds, consisting of 16 'subsets' of 375 ms each, equivalent to one data packet. The subsets are numbered from 0 to 15. In the Earth scanning mode, a scan consists of one scan cycle: 4.5 s forward scan (subsets 0 to 11) and 1.5 s flyback (subsets 12 to 15). In the static and calibration modes the scan mirror does not move, but the data packet structure is identical to the scanning mode.
In the default measuring mode, the nadir scan, the scan mirror sweeps in 4.5 seconds (12 subsets) from negative to positive viewing angles, followed by a flyback of 1.5 seconds (the last 4 subsets) back to negative viewing angles as shown in Figure 4.4.
Figure 4.4: The GOME-2 scan pattern in the default measuring mode
Solid line: forward scan; dashed line:
Each subset pixel (0-15) corresponds to 375 milliseconds. In one of the four subsets of the flyback (subset 14 is shown as an example) the 'unused' parts of the PMD detectors (i.e. Block A - see Appendix B of [RD7]) are read out.
Note that the figure is not drawn to scale.
The default swath width of the scan is 1920 km which enables global coverage of the Earth's surface within 1.5 days (note that other swath widths are also commandable). The scan mirror speed can be adjusted such that, despite the projection effect, the ground is scanned at constant speed. The along-track dimension of the instantaneous field-of-view (IFOV) is ~40 km which is matched with the spacecraft velocity, such that each scan closely follows the ground coverage of the previous one. The IFOV across-track dimension is ~4 km. For the 1920 km swath, the maximum temporal resolution of 187.5 ms for the main channels (23.4 ms for the PMD channels) corresponds to a maximum ground pixel resolution (across track x along track) of 80 km x 4 0 km (10 km x 40 km for the PMDs) in the forward scan.
The actual integration time used (and thus
the ground pixel size) will depend on the light intensity. The integration
time can be separately set for each channel; in channels 1 and 2 it is even
possible to subdivide each channel into two parts (called 'band 1a', 'band
1b' and 'band 2a', 'band 2b' respectively) having separate integration times.
It is anticipated that a default integration time of 187.5 ms (yielding
two spectrum readouts per data packet) will be used in all channels but
with two exceptions where longer integration times are needed because of
low light intensity:
(i) Band 1a has a default integration time of 1.5 seconds (yielding three spectra per scan and one from the fly-back with the possibility of co-adding spectra to improve signal to noise characteristics).
(ii) The integration time for all channels will be increased for low solar elevations (high solar zenith angles).
This section gives a classification of the GOME-2 observation modes. The observation modes can be assigned to three categories: Earth observation modes, calibration modes, and other modes.
The observation mode is derived in the data processing chain by combining fields from the data packet, such as scan mirror position, subsystem status flags, etc. There is no dedicated field in the data packet indicating the observation mode. Any GOME-2 data packet which does not fit into one of the modes below will be classified as "invalid" by the Level 0 to 1 data processor.
Earth observation (or "earthshine") modes are those modes where the Earth is in the field of view of GOME-2. They are usually employed on the dayside of the Earth (sunlit part of the orbit). The scan mirror can be at a fixed position (static modes), or scanning around a certain position (scanning modes). All internal light sources are switched off and the solar port of the calibration unit is closed.
This is the mode in which GOME-2 will be operated most of the time. The scan mirror performs a nadir swath as described above. The swath width is commandable, its default value is 1920 km. Scanning can be performed either with constant ground speed, resulting in equally sized ground pixels (this is the default), or with constant angular speed ("GOME-1 mode"), resulting in larger ground pixels for the extreme swath positions as compared to the swath centre.
North polar scanning
The scan mirror performs a swath around the viewing angle +46.696º (default value) in order to cover the North Pole which would not be observable with the normal nadir scanning mode. This mode will typically be used during northern hemisphere spring.
South polar scanning
The scan mirror performs a swath around the viewing angle -46.172º (default value) in order to cover the South Pole which would not be observable with the normal nadir scanning mode. This mode will typically be used during southern hemisphere spring.
The scan mirror performs a swath around another off-nadir position.
The scan mirror is pointing towards nadir. This mode will typically be used during the monthly calibration. It is valuable for validation and long-loop sensor performance monitoring purposes.
The scan mirror is pointing towards an off-nadir position.
In-orbit instrument calibration and characterisation data are acquired in the various calibration modes. They are usually employed during eclipse with the exception of the solar calibration which is performed at sunrise. Both internal (WLS, SLS, LED) and external (sun, moon) light sources can be employed. The various sources are selected by the scan mirror position.
The scan mirror points towards the GOME-2 telescope. All internal light sources are switched off and the solar port is closed. Dark signals are typically measured every orbit during eclipse.
Sun (over diffuser)
The scan mirror points towards the diffuser. All internal light sources are switched off and the solar port is open. Solar spectra are typically acquired once per day at the terminator in the northern hemisphere. The Sun Mean Reference spectrum will be derived from this mode.
White light source (direct)
The scan mirror points towards the WLS output mirror. The WLS is switched on and the solar port is closed. The WLS can be operated at four different currents (360, 380, 400, 420 mA). Etalon (and optionally Pixel-to-Pixel Gain (PPG) calibration) data will be derived from this mode.
Spectral light source (direct)
The scan mirror points towards the SLS output mirror. The SLS is switched on and the solar port is closed. Wavelength calibration coefficients will be derived from this mode.
Spectral light source (over diffuser)
The scan mirror points towards the diffuser. The SLS is switched on and the solar port is closed. Light from the SLS reaches the scan mirror via the diffuser. This mode is employed for in-orbit monitoring of the sun diffuser reflectivity.
The scan mirror points towards the GOME-2 telescope. The LEDs are switched on and the solar port is closed. PPG calibration data will be derived from this mode.
The scan mirror points towards the moon (typical viewing angles are +70º to +85º). As the spacecraft moves along the orbit, the moon passes the GOME-2 slit within a few minutes. This mode can be employed only if geometrical conditions (lunar azimuth, elevation and pass angle) allow it which will typically occur a few times per year.
These modes are either transitory (idle mode) or used in instrument maintenance (dump and test modes). In these modes, data packets are generated; however, they do not contain any useful scientific data. A typical example is during an in- or out-of-plane manoeuvre of the satellite.
This mode is reached during instrument switch-on or switch-off.
In place of PMD and main channel data, memory contents are downlinked. This mode is used for diagnostic purposes.
In place of PMD and main channel data, a fixed test pattern is downlinked. This mode is used for diagnostic purposes.
The GOME-2 instrument may be operated using timelines (GTL) and timeline tables (GTT). Timelines are used primarily to reduce the load on the satellite uplink and additionally to provide on-board autonomy. One GTL is pre-loaded as a series of up to 33 individual instrument commands that are executed without the intervention of the Instrument Control Unit (ICU) or the Payload Module Controller (PMC). GOME-2 can store up to 16 timelines. Twelve default timelines will be loaded prior to launch and represent a library immediately available for use at the start of instrument operations.
GOME-2 operations and science data acquisition are strongly linked to the viewing geometry and the solar zenith angle (SZA), which determine the expected intensity of light received and the corresponding integration times required. There is in principle no restriction on the duration of a timeline or when it can be activated during an orbit. However, in order to simplify the sequencing and generation of timelines all default timelines start with an SZA equal to 90° minus an offset of 580 seconds and will have a duration of one orbit. The sequence of commands as well as their duration will remain constant over the year.
A GTT can be loaded, started and stopped by use of a macro command. The GTT allows 28 timelines and their execution times to be pre-loaded without the intervention of the ground segment or the PMC. There will be no default GTT stored on board and it is currently not planned to use the GTT capability for instrument operations.
For a full description of instrument operation and the default timelines see the [RD4] MetOp GOME-2 Instrument Operation Manual.
GOME-2 timelines are scheduled according to a fixed timeline pattern per orbit repeat cycle. In the schematic shown below (Table 4.1) the day numbers are indicative only. This scheme has the following advantages: