Tag: Platform

Printed Wire Assemblies Back Online

To return function to 16 printed wire assemblies, including the two most recent, Terra’s Flight Operations Team rebooted Terra’s solid state recorder, on September 22, 2021. The successful reboot of Terra’s solid state recorder returned Terra to a level that allows full science acquisitions from all five instruments (ASTER, CERES, MISR, MODIS, and MOPITT). 

The recorder reboot took nearly 12 hours to complete. The board-by-board power up sequencing resulted in all 16 printed wire assemblies of the 16 offline assemblies returning to operational status.  Terra now has all 58 assemblies in service, and Terra’s data storage capacity is at the same level it had at launch. There are currently no known hardware limitations to the operation of Terra or any of its sensors and there is currently no official passivation (mission end) date. 

For over two decades Terra has collected valuable data about Earth’s systems. In February 2020 Terra stopped doing platform maneuvers that would control its equator crossing time in order to maintain fuel for collision avoidance maneuvers. The remaining fuel will also be used to lower Terra’s orbit as part of its eventual passivation process.  Now with the printed wire assemblies being returned to full function, Terra will be able to collect full data acquisitions from all five instruments throughout this process.

Terra will also continue collecting data after an orbit lowering maneuver planned for Fall 2022. The maneuver upholds agreements to ensure Terra is a safe distance from the other missions in the 705-km Earth Observing Satellite Constellation when fuel to maintain Terra’s orbit has been depleted.

“The science community views the changes in crossing time and orbit altitude as continuation of normal data collection for those products not affected by the orbital changes as well as an opportunity to do novel science with those that are affected,” according to Kurtis Thome, Terra Project Scientist.  The Terra Project fully expects the Terra platform and all five instruments to operate past 2026 allowing them to maintain their status as leaders in Earth science data production.  Terra’s long-term data record will continue to contribute to the Earth Science Division’s key science questions:

  • How is the global Earth system changing?
  • What causes these changes in the Earth system?
  • How will the Earth system change in the future?
  • How can Earth system science provide societal benefit?

Terra, the flagship Earth Observing Satellite, continues to be a leader in Earth science data, contributing to scientific research and applications worldwide, now with data capacity levels restored by a successful solid state recorder reboot.

Two printed wire assemblies (PWA) on the Terra satellite stopped working, resulting in lost data from the Multi-angle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) on June 28th and June 29th, 2021.  Radiation from the Sun and deep space likely caused an overcurrent issue that stopped the printed wire assemblies from working.

“We have to manually send the update that says, don’t record to that PWA anymore, but until we do that it keeps trying to save data to the bad PWAs,” says Jason Hendrickson, Terra Flight Systems Manager.  Terra’s flight operations Team quickly worked to initiate commands to mark the printed wire assemblies as bad so that they were no longer used. During the time it took to initiate these commands, both MISR and MODIS lost data. However, repercussions from these non-functioning printed wire assemblies have longer lasting impacts ­– reduced storage capacity of MODIS and MISR data on Terra’s solid-state recorder.

Prior to these printed wire assemblies malfunctioning, fifteen other printed wire assembly stoppages had occurred, each time reducing Terra’s solid-state recorder data storage capacity. However, in 2001, after the first printed wire assembly stopped working, rebooting the solid-state recorder corrected the issue, restoring function. Now with only 43 of 59 printed wire assemblies functioning, data storage capacity has decreased to a critical point for MODIS and MISR.

In a working group meeting held in 2019, this scenario was discussed. All instrument teams made choices about what to do if any more printed wire assemblies stopped working. The options discussed included: downlinking data more frequently, reducing the number of science observations by MISR and MODIS, or rebooting the solid-state recorder in hopes that the printed wire assemblies begin to work again. Weighing the risks and benefits of these courses of action is not taken lightly.

“It takes less time to fill up the storage once we mark the printed wire assembly bad, but the same amount of data is being produced by the instruments as always,” says Dimitrios Mantziaras, Terra Mission Director. To keep all the data that is being produced by the instruments, the data would need to be downlinked more frequently. While this may sound easy, data downlink is limited by the number of passes Terra makes past Tracking and Data Relay Satellites. Without more of these satellites in Terra’s line of sight, data cannot be downlinked more regularly.

Another option is to reduce MODIS and MISR science observations.

“The impact on MODIS data is considered significant,” says Michael King, MODIS Instrument Lead. The MODIS science team is faced with choosing to record data from fewer bands or to accept gaps in their data.

MODIS typically operates with 50% of the orbit recording of daytime bands (all 36 bands), whereas the other 50% of the orbit records only 17 thermal bands (night mode).  The Flight Operations Team can reprogram the percentage split, extending the night mode data collection and decreasing the daytime data collection. This reduces the total amount of data that is collected between data dumps. However, daytime bands are essential for many MODIS data products including: ocean color and ocean optical property products, many atmospheric products, and many land products. Additionally, the daytime bands are important in an early part of data processing that identifies clear sky from clouds.  “The loss of the ability to record these bands and derive the corresponding products will affect time series of Earth observations over the course of the Terra mission, leaving gaps in data in the polar regions,” says King.

Similarly, MISR is faced with reducing about 6 minutes of data per orbit. To mitigate data loss, the MISR science team strategized trimming ascending node data collected near the poles (most, but not all, dayside data are acquired on the descending node when Terra is flying from north to south). “Near the summer and winter solstices the scientific impact is minor, but during fall and spring the required trimming includes either loss of descending node data at high latitudes in both hemispheres or choosing between Arctic or Antarctic coverage,” says David Diner, MISR Principal Investigator. To minimize impacts to the science community that relies on MISR data, the Arctic region — where MISR’s data significantly contributes to measuring sea ice minima, albedo decrease, and support of field campaigns — will be prioritized. “Unfortunately, this will curtail the long-term record of sea ice albedo, anthropogenic aerosol influences, and ice sheet breakup in southern polar regions, which have been the subject of published investigations by the science community,” says Diner.

While these options for data reduction come at costs, the teams are considering rebooting the solid-state recorder, which was done once in 2001. “There is no identifiable risk to doing the reset,” says Kurt Thome, Terra Project Scientist. The successful restart of the solid-state recorder in 2001 brought the first inoperative printed wire assembly back online. If a restart works, then MISR and/or MODIS could continue to collect data as was done prior to the recent malfunctions, potentially even returning Terra’s data storage capacity to the amount it had at launch.

As Terra continues to drift toward it’s expected constellation exit in fall of 2022, the Terra science and Flight Operations Teams continue to preserve Terra’s more than two-decades of data with as much continuity as possible, adding to Terra’s historic long-term data record of Earth observations.

Two printed wire assemblies on the Terra satellite failed, resulting in lost data from the Multi-angle Imaging SpectroRadiometer (MISR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) on June 28th and June 29th. The Terra Flight Operations Team, quickly worked to identify the location of the failures and initiate commands to prevent the solid state recorder from trying to use those locations. Data for both instruments is now continuing to be collected. However, data that is collected may be at a reduced quantity, but not decreased quality.

-Kurt Thome, Terra Project Scientist

On February 27, 2020, the Terra flight operations team conducted Terra’s last inclination adjust maneuver.   The inclination adjust maneuver is one of three basic maneuvers requiring fuel that the Terra Flight Operations Team uses to maintain Terra’s orbit and the safety of the platform.  Inclination adjust maneuvers are used to control the platform’s mean local time (MLT) of its equator crossing.  The other two are drag makeup maneuvers (DMUs) to control the orbital altitude (apogee and perigee) and risk mitigation maneuvers (RMM), or debris avoidance maneuver (DAMs), that are used to protect the platform from collisions with known on-orbit debris or other spacecraft.

The importance of orbit maintenance is that changes in orbit can affect the quality of data from a sensor.  Earlier crossing times for a morning platform like Terra mean lower solar elevations (leading to more prevalent shadows), decreased cloud probabilities, lower surface temperatures, and lower reflected energy (leading to lower signals).  A decrease in orbit altitude alters the spatial coverage of the sensor including possible gaps in spatial sampling, decreased spatial coverage, but better spatial resolution. 

The flight operations team works to maintain both crossing time and platform altitude to provide the highest quality data.  The trade is that fuel is often a life-limiting factor for long-life platforms.  The amount of maneuvering fuel is a trade between platform weight at launch, expected platform life, and safe disposal of the platform at end of life. Fuel use is dominated by inclination maneuvers since they require significantly more fuel than drag makeup maneuvers.

Fortunately, Terra benefited from a near perfect launch and accurate orbit insertion by the launch vehicle leading to almost a full tank of fuel for maneuvering.  Inclination maneuvers have been done on average four times per year since launch to maintain Terra’s crossing time of 10:30 am (though it was 10:45 am for the first two years on orbit).  IAMs continued until the remaining fuel was just enough for a Constellation Exit and perigee-lowering burns.  The Constellation Exit lowers the platform’s orbit by 6 km to remove it from the 705-km orbit altitude of the Earth Sciences Constellation.  The perigee-lowering burns take place at the time of Terra’s passivation and all remaining fuel is used to lower Terra to begin its slow re-entry to an uncontrolled de-orbit.

After Terra’s IAM it began to drift to an earlier crossing time.  Drag makeup maneuvers are being continued to maintain Terra’s orbit altitude of 705 km.  Risk mitigation maneuvers will also continue to avoid orbital debris.  When Terra’s crossing time reaches a 10:15 AM MLT (currently predicted for October 2022), the flight operations team will have Terra exit the Earth Sciences Constellation and lower Terra to an altitude of 694 km by performing two retrograde maneuvers. Risk mitigation maneuvers continue, but no further drag make up maneuvers will take place and Terra will slowly decrease in altitude as well as continue drifting to an earlier MLT crossing.  It is predicted to reach a crossing time of 9:00 am in December 2025 at which point the perigee-lowering burns will take place to lower Terra by an additional 20 km.

Terra’s instruments will continue to be operational during this entire process.  The impacts of the changing crossing time are expected to be minimal and will impact the data from each of Terra’s five instruments in a slightly different fashion.  To learn more about the effects please visit the individual instrument websites. 






*Information regarding impacts to instruments is still pending.

Key Dates of Drift

  • Exceed 10:30 MLT (+/- 1 min) – April 2021*
  • Exceed 10:15 MLT – September 2022*
  • Constellation Exit – September 2022*
  • Exceed 9:00 MLT– December 2025*

Passivation is expected to take place due to signal to noise limits from lower solar zenith angles, limited battery power from changing solar angle on the solar panels, or programmatic funding limits.

*Dates are subject to change.