Monday, 11 January 2016
For over three years, the Soumi National Polar-orbiting Partnership (S-NPP) satellite, the first spacecraft of NASA and NOAA's planned Joint Polar Satellite System (JPSS), has been broadcasting the majority of its state-of-the-art science data directly to the ground in real time. This High Rate Data (HRD) is critical to users in remote locations who have limited access to satellite data through other means, or who require minimal latency for their forecasting work. The HRD antenna is limited to transmitting at 15 megabits per second (Mbps), which restricts the total data content of the HRD stream. In December of 2014, at the behest of the science community, we modified the operational flight software of S-NPP's Cross-track Infrared Sounder (CrIS) to measure higher resolution data in the short-wave and mid-wave infrared bands. To yield the bandwidth margin necessary to accommodate this "full spectrum" mode, the HRD content was reconfigured to remove several data sources, including Moderate Resolution Band 7 (M7) of the Visible Infrared Imaging Radiometer Suite (VIIRS) and one of the nine fields of view (FOV 4) from CrIS (these data sources are still recorded and downlinked later as Stored Mission Data (SMD)). The VIIRS instrument is the primary driver of data rate behavior on S-NPP. While all other S-NPP data sources transmit at a deterministic data rate, VIIRS's data rate varies with the Earth scene's entropy. Over particularly entropic scenes, despite omitting M7 and FOV 4, VIIRS can still drive the HRD data rate close to the limit of 15 Mbps. From an on-orbit test of CrIS's full spectrum mode in 2013, we know that exceeding this rate on S-NPP causes both HRD and SMD to be permanently lost. Thus, in order to further mitigate the risk of a data loss event, NOAA and NASA mission management, with consent of the science community, have agreed to remove a second CrIS FOV (FOV 6) from the HRD stream before the end of 2015. Under this configuration, loss of S-NPP data due to high entropy scenes will be exceptionally unlikely. We will present the current configuration of HRD content along with a detailed data rate budget broken out by each contributor, as well as provide justifications for omitting certain science data from HRD. For VIIRS, we will quantify its data rate limits and show that its highest rates occur specifically over scenes with high densities of small diameter clouds, and that the incidence of such scenes varies seasonally. For future JPSS satellites, data loss when exceeding the HRD antenna's 15 Mbps limit will not be reflected in SMD, which may permit more liberal management of the HRD budget's margin. We will explore possible configurations or seasonal management of HRD content which would accept a small risk of HRD data loss in return for delivering more science data to the direct broadcast community.
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