The DEMETER spacecraft will fit into an approximately 1 m^3 bounding box and have a mass of approximately 150 kg. The instrument itself will be approximately 0.5 m x 0.5 m x 0.25 m, with each BOM (up to 4) having a mass of approximately 2 kg.
The optimal altitude range to achieve limb-to-limb coverage with 4 BOMs is between 750 – 824 km.
If formation flying with JPSS – yes. Otherwise, not necessarily. It will depend on the mission profile.
DEMETER uses a thermopile microarray detector consisting of 3 rows, each 65 “pixels” across. Each row represents a different broadband channel. The absorptive coating of the thermopile is gold black, selected for its stability and heritage on similar space systems.
The DEMETER sensor is a microarray-style thermopile coated in gold black. Each individual thermopile is called a “pixel”. The array is 65 pixels across by 3 rows. Each row measures a different broadband channel, determined by the filter window throughput for the given channel. The sensor relies on a freeform mirror as the optical mechanism for directing incoming radiation into the filter assembly.
DEMETER will measure three spectral channels: shortwave (0.3 – 5 um), longwave (5 – 50 um), and total (0.2 – 50 um). Each BOM will measure all three channels.
DEMETER retains the ability to rotate in elevation and azimuth, but this is done only for internal calibration or targeted scans. Nominal operations will have no rotation or moving parts.
DEMETER will maintain global coverage in 1 deg bins. True footprint coverage will be ~90% per day, gridded into global bins.
To achieve global coverage, we will use 4 BOMs given the optimum altitude range and in a sun-synchronous orbit. DEMETER can fly in any given altitude and orbit, but achieving global coverage requires a sun-synchronous orbit and >750 km altitude.
One pixel will have a footprint of ~7 km at nadir.
Flying in formation with a VIIRS-class imager or carrying a multispectral imager on board would allow DEMETER to produce highly detailed data products through Level 4. DEMETER can still produce useful science data, albeit with limited data products, without a multispectral imager.
DEMETER carries two onboard calibration sources: a blackbody longwave source and an integrating sphere with LED and QTH lamps as a broadband/shortwave source. DEMETER will routinely orient its BOMs to view these sources for a given amount of time.
Radiometric stability, on any platform, is tracked primarily via two methods: onboard calibration and vicarious measurement validation. On DEMETER, each BOM will be routinely calibrated using views to cold, dark space and onboard longwave and shortwave calibration sources. The calibration sources will be well-characterized using SI-traceable sources prior to launch. Additionally, vicarious sources such as the lunar disc, deep convective clouds, and desert targets, for example, provide invariant, diverse scenes that DEMETER can measure to track instrument drift and validate its measurements.
There are 65 pixels per row in each BOM. Each pixel needs to be individually tracked with respect to precision and accuracy, which is being done in custom software using tracking trees. All pixels will be tracked and validated using views to cold, dark space, and Earth targets to track instrument drift. Additionally, both calibration sources are designed to overfill the FOV of a BOM, and the emitting surface of each calibration source will be spacially and spectrally characterized using SI-traceable sources. Because each pixel shares ~70% of its FOV with its neighbor, each pixel can be intercompared across the row pairwise to intercompare each pixel within a BOM.
DEMETER carries onboard calibration sources that are spectrally resolved and broadband and therefore covers the required calibration range. Solar calibration is a nice-to-have.
Lunar scans are not strictly necessary for baseline operations. They can be done for validation and tracking instrument drift.
Yes, DEMETER tracks spectral degradation using its onboard calibration sources and vicarious measurements with invariant targets.
