How to achieve maximum data consistency and meet the needs of data users?
The options available to the Terra project for using its remaining fuel expanded when the altitude-lowering criteria for leaving the Earth Sciences Constellation (ESC) was reduced. In 2015, the A-Train Mission Operation Working Group (MOWG) approved a new exit plan that reduced the constellation exit from a 19-km lowering to a 4-km lowering. The change provides Terra with fuel availability for additional orbital maneuvers for maintaining its tight (10:29-10:31 am) equator crossing time window. The Terra science team seeks recommendations on how to responsibly use Terra’s remaining maneuvering fuel while achieving maximum data consistency and meeting the needs of Terra’s diverse group of data users.
The original EOS program developers recognized the importance of the data record length when they planned for three AM platforms with six-year lifetimes to provide a consistent 18-year data record. Terra achieves that original 18 year goal in 2017 based on its December 1999 launch date. However, the shift in the platform’s crossing time from 10:45 to 10:30 AM in the years 2000 to 2002 has led to many of the Terra climate-variable studies basically omitting the 2000-2002 timeframe because of the uncertainties related to shift in crossing time. One noted result has been a shift in cloud parameters, which is still not known whether it is caused by the shift in the platform’s crossing time or by a significant climate event. In terms of data consistency and length, Terra still needs until 2020 to reach the originally planned consistent 18-year record.
The standard way to estimate the significance of a trend in a data series is to compare its recent observations to the long-term historical baseline. For example, the global-mean surface temperature (GMST) is the most used indicator of global climate change, because it is directly related to the global energy budget and to the climate impacts and risks on society. Therefore there is a large interest in the time evolution of GMST, both to the scientific community and general public, to assess its variability and its agreement with global climate model (GCM) projections. Terra’s on-board instruments provide unique, high-quality global observations capable of answering these questions with a high degree of certainty, and enhanced certainty with increasing length of time series of observations. A panel of experts well versed in Terra’s instruments and data products came to the conclusion that there is significant scientific value to be gained by extending the mission at the current 10:30 AM crossing time for as long as possible. The additional time of observations at 10:30 AM will be invaluable for climate trend analyses, in particular those related to MODIS-derived sea and land surface temperature (S/L-ST) measurements and MISR-derived cloud measurements. There is an unequivocal agreement that small changes in the crossing time have the potential to significantly alter this diurnally-dependent record and weakens the statistical significance of the analysis.
In order to provide a data set of the best quality for the scientific community, the Terra Project Science Office, including the five instrument teams, recommended that the current crossing time be held until the fall of 2020. Doing so, would sustain an equatorial crossing time within a two-minute window for an unprecedented 18 years, conforming to the original plan of the AM platform, while still allowing a safe exit from the Earth Science Constellation (ESC). This recommendation is based on optimization of the length of the mission while keeping the risk to the morning constellation low. The unique science possible from a near-constant crossing time for 18 years is viewed to offset the added risks involved from the additional time on orbit as compared to an earlier re-entry.
An earlier re-entry, to a certain extent, decreases the probability that Terra will be impacted by space debris, although all orbiting satellites are at risk of being hit by debris and rely on collision avoidance maneuvers to minimize this risk. Terra will maintain fuel reserve to perform these maneuvers after lowering or exiting the constellation to minimize risk of collision. The results of the 4-km exit plan and the debris risk analysis led to a strong concurrence within the science community to propose maximizing Terra’s mission life while minimizing changes in orbital altitude and crossing time. Such an approach extends the continuity of the operational data and capitalizes on the uniqueness of the Terra data record and its use as a climate observing platform.
The Project is currently searching for possible science benefits from the orbit lowering and whether there are any science applications that would benefit from either a shift in crossing time or an orbit lowering. The project plans to evaluate any comments or feedbacks through a discussion panel in December 2017 and present the results of this panel in January 2018.
The Terra Science Team welcomes any feedback or suggestions as to whether to maintain the current orbit and the long-term data continuity, or lower the orbit, and therefore, shift the crossing time. Please send any feedback or suggestions by contacting us.