Ultraviolet Visible Spectrometer (UVS)
AIM-North would use an ultraviolet-visible spectrometer (UVS) to measure reflected sunlight to retrieve O3, NO2, BrO, HCHO, SO2, aerosols and other species for air quality research and operational forecasting. The UVS would be a dispersive spectrometer spanning 290-786 nm with 1240 spectral elements, giving a spectral sampling of ~0.4 nm (before binning 2 spectral samples). Its mass would be ~51 kg with an aperture diameter of 74 mm and would image the above species with pushbroom scanning by acquiring 480 simultaneous 3×3 km2 observations every 1.2-1.3 seconds using one dimension of a 480×1280 element space-qualified focal plane array (FPA), imaging the field of regard every ~60-90 minutes of daylight.
Near Infrared and Shortwave Infrared (NIR & SWIR) Spectrometer
AIM-North would image CO2, CH4 and CO by observing spectra of reflected shortwave infrared (SWIR) and near infrared (NIR) solar radiation. Dispersive and Fourier Transform Spectrometers (FTSs) are being studied for this instrument in Phase 0. The baseline design from mission concept feasibility studies was an Imaging Fourier Transform Spectrometer (IFTS) with 4 NIR-SWIR spectral bands using separate but identical FPAs and the same fore-optics with a large input aperture. The interferometer would use two moving mirrors on a V-shaped pivot arm, rather than a traditional Michelson interferometer design (one fixed and one moving mirror). With the two mirrors moving in opposite directions, only half the displacement is needed to achieve a maximum optical path difference of 4 cm and spectral sampling of 0.25 cm-1 on a ~150 kg instrument. The IFTS pointing and scanning pattern is still being investigated, but under one scenario, it would image approximately 480×480 pixels on each FPA at 3×3 km2, then step to the next position and repeat, imaging the field of regard every ~60-90 minutes of daylight. The IFTS interferometer design has LEO spaceflight heritage in ACE, GOSAT, GOSAT-2 and other missions; however, imaging would be a new application facilitated by the possibility for longer stare times from a HEO or GEO vantage point. The Canadian Space Agency (CSA) continues to invest in raising the technology readiness level of IFTS by supporting research by Canadian industry and academic partners in consultation with Environment and Climate Change Canada (ECCC).
Spectral Bands, Sampling and Species
Solar Induced Fluorescence (SIF) could be observed with both instruments. The UVS would observe SIF over a broad spectral range and low spectral resolution spanning both photosystems I and II, while a few isolated solar Fraunhofer lines for SIF retrieval would be included in the NIR O2 (~760 nm) band, which is primarily intended for surface pressure retrievals (for XCO2, XCH4 and XCO) and aerosol information.
Earlier Instrument Studies and Potential Mission Enhancements
The AIM-North concept evolved from an earlier proposal called the Polar Highly Elliptical Orbit Science (PHEOS) Weather, Climate and Air quality (WCA) instrument suite. PHEOS-WCA was proposed as an enhancement to the Polar Communications and Weather (PCW) mission and completed Phases 0 and A in 2012, led by Professor Jack McConnell of York University. Like AIM-North, PHEOS-WCA also consisted of an IFTS and a UVS but these instruments were constrained to work with very tight mass (< 50 kg) and volume allocations. The primary objective of PHEOS-WCA was to support the PCW weather observations, so the baseline IFTS included wide longwave infrared and mid-wave infrared bands along with only one NIR and one SWIR band. Although an optimal configuration proposed by the science team included both XCO2 and XCH4 capability, the fully compliant configuration would not have measured XCO2.
GHG and air quality observations are the primary objectives of the AIM-North mission in the baseline design. An enhancement to the IFTS above the baseline is also possible by adding bands in the longwave and mid-wave infrared. These bands would enable measurements of temperature, water vapour and atmospheric motion vectors in northern regions (for weather forecasting) along with numerous AQ species and CO2 and CH4 with mid- to upper tropospheric sensitivity during days, nights and all seasons, but would require infrared FPAs and a cryo-cooler adding ~60 kg to the IFTS. Other potential enhancements that could also be explored include: a cloud imager to enable better pointing and real-time cloud data for forecasts; or a small dedicated aerosol instrument for improved air quality health forecasts; as well as a number of hosted payload options to enhance the business case for the mission.
Figure 4. With AIM-North’s imaging approach and the example of a Three-Apogee (TAP) highly elliptical orbit (HEO), each colored region is a Field of Regard (FOR) that would be scanned every ~60-90 minutes during daylight.
Data Quality and Validation
AIM-North accuracy and precision targets are aligned with international GEO missions. Validation is a key component of ensuring high accuracy and precision. Canada currently operates two certified sites for satellite XCO2, XCH4 and XCO validation in the global Total Carbon Column Observing Network (TCCON) that use high spectral resolution solar-viewing ground-based Fourier Transform Spectrometers (FTSs). Canada has an Arctic site at Eureka, Nunavut (80°N, 86.4°W) and boreal site at East Trout Lake, Saskatchewan (54.4°N, 105°W). The FTSs at these sites also measure a number of other species, while Eureka hosts a wide range of other instruments for both science and validation. Northern high latitude TCCON sites in Europe will also contribute to validation. Additional XCO2, XCH4 and XCO validation can be carried out with lower resolution FTSs, for which there are a growing number in Canada and Europe as well as one in Alaska. Air quality validation is becoming more standardized with observations from the Pandonia network that carries out ground-based remote sensing using Pandora/Pandora-2S instrumentation, with a number of high northern latitude sites in Canada and Europe. Existing northern TCCON, Pandonia and other validation sites along with some new sites will be required to assess data quality and ensure that accuracy targets are met.
Spatially and/or temporally averaging AIM-North data can improve precision beyond target values for some applications. Alternatively, sequentially combining multiple images can yield movie-like views of evolving atmospheric composition. No other satellite mission ever formally proposed could offer equivalent data for atmospheric composition or vegetation in northern regions.