Applications

The primary scientific objective of GOMOS was the global monitoring of the stratospheric and mesospheric vertical ozone distribution with high accuracy, high vertical resolution and full global coverage. The expected high accuracy of the ozone profile measurements allowed ozone trends to be studied over the lifetime of Envisat in the stratosphere and mesosphere.

GOMOS ozone trend observations addressed:

• The degree of Arctic (polar) ozone loss under different meteorological conditions section
• Increased data for determining height-resolved trend analysis section
• Monitoring, which will indicate the beginning of expected 'recovery' in ozone section

The primary scientific mission objective pointed to the use of an instrument applying the occultation technique, as this was self-calibrating, protecting the measurements from long-term instrument drifts. Stars as light sources had been chosen in order to provide sufficient global coverage of the measurements, which could not be achieved with, for example, a solar occultation instrument.

The occultation technique had one significant advantage, which was that an absolute estimate of the molecule density was obtained from the ratio of two measurements taken with the same instrument within a few seconds. This made the method inherently self-calibrating, since even if the spectral sensitivity of the instrument was changing with time, the ratio will be measured correctly. This protection against long-term drift was ideal for the study of atmospheric trends.

GOMOS measured in the UV and visible wavelength regions where ozone shows strong absorption. The measured wavelength range was 250 nm to 675 nm, covering the ozone Huggins and Chappuis band. The UV wavelengths range was particularly well suited for mesospheric and upper stratospheric measurements. The visible band was well suited for the probing the stratosphere, including the lower stratosphere, where the UV signal was no longer strong enough anymore because of the UV absorption by ozone. Roughly, the UV yielded the best results for the mesosphere and stratosphere above 35 km and the visible band for the stratosphere below 35 km.

A highly desirable follow-on programme with GOMOS or COALA-type instruments provided long-term trend observation.

Secondary Scientific Objectives

NO2 and NO3 play important roles in the nitrogen chemistry relevant to ozone.

Observations of NO2 and NO3 help to address:

• The degree of denoxification/denitrification in Arctic winter and availability of NO2 to deactivate ClO
• The abundance of lower stratosphere NO2, which varies with the aerosol loading
• Future NOx observations relevant to the ozone recovery. As NOx dominates ozone loss on a global scale, NOx observations are important to interpret ozone changes as halogens decrease.

GOMOS therefore observed NO2 and NO3 (enhanced), in the UV-visible band. OClO and BrO are key species in the polar ozone depletion processes. They can be detected with GOMOS if their concentration is enhanced. Measuring the aerosol extinction is an additional secondary objective. Aerosols are important for a number of reasons, including the provision of surfaces for heterogeneous reactions, which can strongly affect the partitioning of chlorine and nitrogen compounds.

Stratospheric water vapour is fundamental to the budget of many trace gases in the stratosphere. It was therefore important to determine its three-dimensional distribution and long-term trends. Water vapour was measured in a near-infrared channel at 926-952 nm.

A further secondary objective was to study stratospheric dynamics. This required measuring the atmospheric temperature and density profiles. Temperature and air density was derived from a near-infrared channel (756-773 nm). Complementary to this, a high-resolution temperature profile could be retrieved from atmospheric scintillation, which was be measured using two fast photometers sensitive in the blue and red, respectively.

The measurement of temperature was also needed for supporting the primary mission objective, ozone monitoring. The ozone cross-sections in the Huggins band (310-350 nm) were strongly dependent on the atmospheric temperature. Therefore, a long-term variation of UV absorption might be due to a temperature variation and could be wrongly attributed to an ozone variation. Temperature measurements were possible with the A-band of O2 at 760 nm. O2 is a perfectly mixed gas and the air can be assumed to be in hydrostatic equilibrium. Therefore, its scale height was directly connected to the atmospheric temperature. The O2 measurements also allow to relate all measurements of ozone density (and other species) to the air density to yield the mixing ratio [O3]/[air], which is a quantity most readily used in models. Furthermore, temperature and air density are essential parameters for atmospheric dynamics, including mixing of gases.

Long-term, Campaign and Permanent Objectives

The scientific objectives were divided into three groups. The first group consisted of long-term objectives. These objectives were enacted according to the overall GOMOS mission plan during the whole mission lifetime in a more or less regular manner.

The most important of these objectives was the monitoring of stratospheric ozone. The second group consists of campaign type objectives. These had more limited scope than those of the long-term objectives. The activation of a campaign objective was based on an agreement among the partners of the GOMOS mission planning.

The third group consisted of a few permanent objectives which had to be considered when a new mission plan was being prepared. In particular, whenever possible, certain stars (e.g. Sirius) were occulted and predefined locations (e.g. selected validation sites) were prioritised. Therefore, these objectives had an overruling priority.

In more detail, the three groups include:

Long-term Objectives

• Stratospheric ozone monitoring (15-50 km)
• Stratospheric chemistry: upper stratosphere (35-50 km)
• Stratospheric chemistry: lower and middle stratosphere (15-35 km)
• Stratospheric dynamics: large scale dynamics, polar vortex, planetary waves
• Stratospheric dynamics: small scale processes
• Mesospheric ozone monitoring
• Mesospheric noctilucent clouds variation (80-87 km)

Campaign Type Objectives

Geophysical validation of GOMOS products:

• Validation of gas concentration retrieval for particular species
• Co-located measurements e.g. overpass flights over validating ground based instrument or co-incident regions withvalidating satellite instrument
• Validation of diurnal condition. (Depending on the validating instrument.)

Inter-comparison with MIPAS and SCIAMACHY:

• At specified times
• Comparison of ozone and aerosols profiles. (High priority)
• Comparison of NO2, air, O2 and temperature profiles. (Lower priority)
• Comparison of H2O, OClO and BrO profiles (Lowest priority) 
• Interesting diurnal condition: depending on the validating instrument (MIPAS day and night, SCIAMACHY nadir and limb modes day, SCIAMACHY occultation mode twilight) 
• Co-located simultaneous measurements with uniform global coverage
• Campaigns: Campaign-specified latitude regions and priorities for gases
• Special events, for example, volcanic eruption: ozone and aerosol profiles with highest accuracy around the eruption
• Stellar spectra, with increased occultation starting altitude

Permanent Objectives

• Fixed region measurements
• Fixed stars measurements
• Long lasting occultations

The permanent objectives had overruling priority over the other mission objectives. But it must be noted that the permanent and other mission objectives could be fulfilled - or nearly so - at the same time, because of the large number of occultations per orbit. More generally, almost all mission objectives were to some extent fulfilled in any mission plan. The degree of fulfilment, of course, varies. The best results were obtained for the mission objective, which was the target of the optimisation.

Design

The Science Data Electronics (SDE)

The science data electronics (SDE) was the CCD sensor control and signal conditioning unit. It controlled a total of eight CCD's (two for the UVVIS spectrometer, two for the NIR spectrometer, two for the fast photometers, and two for the nominal and for the redundant star tracker). This unit had a high degree of programmability.

During the star occultation measurement, three bands of the spectrometer CCD's were read out. They contained the star, the upper, and the lower background spectrum. The typically seven lines of these bands were binned out in the output register of the CCD before reading. The position, the width, and the separation of these bands were all programmable. This gave a high degree of flexibility in optimising the detection performance to specific observation objectives. During specific monitoring modes, the CCDs were read out in unbinned mode, and the integration times were programmable between 0.25 and 10 seconds.

The sequencing of the star tracker CCD was adapted dynamically to the different phases of star detection centering and tracking as follows. In the initial phase of star detection, the SDE detected the coordinates of the most-illuminated pixel of the CCD using a programmable integration between 5 ms and 50 ms, depending on the star magnitude. As soon as the star had been detected, a 10 × 10 pixel window centred around the star was read out with programmable integration times between 5 ms and 10 ms. This window follows the star as the tracking system centres the star in the field of view. As soon as the star was centred, the read out window "collapsed" to 8 × 8 pixels and the read out frequency was set to 100 Hz with 5 ms or 10 ms integration times.

The analogue signal conditioning consisted of low noise, programmable gain channels. The analogue to digital conversion was done with 12 bit ADC's.

The GOMOS Telescope

The GOMOS telescope had a rectangular aperture of 30 cm × 20 cm. It had to operate in a 0.6° field of view and had to have very good transmission in the 250 nm to 950 nm range. The small allowable volume (intermirror distance < 250 mm) coupled with high-quality imaging and stability requirement, and coupled with the high stiffness requirement (first eigenmode above 180 Hz) were very challenging. A Cassagrain design based on aspheric primary and secondary mirrors, and based on a CFRP structure had been chosen. The main telescope requirements, together with the resulting technological and design choices, are summarised in the table below.

Sensor Modes

Mission Operations

GOMOS Resources