When you hear the term "drift," you may think of a satellite floating aimlessly through space. In reality, drifting a satellite is a highly scripted maneuver. What exactly happens when NOAA GOES-16 starts to drift and why is there a lag time before the satellite becomes fully operational?
Let's start with the basics. On November 30, GOES-16 will begin moving east. Normally, a geosynchronous satellite like GOES-16 remains in a fixed position in the sky relative to Earth's rotation. This won't be the case once the drift process gets underway. Currently positioned at 89.3 degrees west (roughly the longitude of New Orleans, Louisiana), GOES-16 will embark upon an 11-day journey, moving eastward at a rate of 1.41 degrees longitude per day. By December 11, the satellite will reach its new operating position, at 75.2 degrees west (roughly the longitude of Philadelphia, Pennsylvania).
During its relocation, GOES-16's five main instruments (ABI, GLM, SUVI, SEISS, and EXIS) will be turned off and placed in safe or diagnostic mode. These are the high-tech sensors that allow us to do things like map lightning flashes to track thunderstorms or monitor solar flares from space. When GOES-16 begins drifting, all of this data collection will be disabled. Features such as the Search and Rescue Satellite-Aided Tracking (SARSAT) system will also go off-line.
After drifting for 11 days, GOES-16 will reach its new permanent vantage point - 22,300 miles above Earth - on December 11. However, the satellite will remain off-line for a short while.. The delay occurs because the satellite's instruments first have to be calibrated, a process that can take anywhere from three to nine days. During the calibration period, all of GOES-16's instruments return to their normal imaging modes and the transmitters aboard the spacecraft get turned back on. NOAA's Office of Satellite and Product Operations (OSPO) will monitor the instruments to make sure they are working properly. If everything checks out, the instruments can start collecting and transmitting a plethora of weather data and imagery back to Earth. NOAA's mission operations team expects GOES-16 to be back online and fully operational by December 20th.
You may be wondering, how exactly does a satellite physically get moved from point A to point B thousands of miles above Earth? NOAA's operations team can plan all of these maneuvers using navigation software. For a satellite to change its orbital position, it follows a series of commands uploaded by the operations team to the spacecraft's memory. The mission operations center validates and rehearses these maneuver sequences on the ground using a satellite simulator.
Normally, satellites maintain the same distance from Earth while operational (i.e. transmitting data). However, during the drift period, GOES-16's altitude will actually be lowered slightly (by about 60 miles). This maneuver helps nudge the satellite to begin moving into its new orbital position. After GOES-16 finishes drifting, NOAA's operations team raises the satellite back to its normal operating position. NOAA's missions operations division uses this lowering and raising process any time its geosynchronous satellites need to change orbital positions.
The New GOES East: Replacing GOES-13
If you're an avid follower of satellite news, you may have heard that GOES-16 will officially become NOAA's new GOES East satellite, replacing GOES-13 as the primary satellite that keeps watch over the eastern U.S. and the western Atlantic Basin. What will happen to GOES-13 when GOES-16 replaces it? For three weeks, the two satellites will practically be neighbors in the sky - GOES-16 at 75.2 degrees west longitude, and GOES-13 at 75.0 degrees west. The old satellite, GOES-13, will continue providing data until January 2, 2018. Thereafter, the satellite will stop transmitting data and begin its own three-week drift period. GOES-13's final resting place, 22,300 miles above Earth, will be at 60 degrees west longitude.
But before then, several things have to take place to ensure a seamless transition of data-transmitting power. On December 7 - four days before GOES-16 has finished drifting - GOES-13 will begin relaying its data to another geosynchronous satellite, GOES-14. Watching Earth's skies at 105 degrees west longitude, GOES-14 will act as an intermediary, assimilating the data from GOES-13 during its final few weeks in operation. Once GOES-16 becomes operational, there will be a period of overlap, with both the new and old GOES East satellites transmitting data for about ten days.
Given all the critical weather data NOAA's satellites provide, any gaps in data transmission can mean a make-or-break difference in the accuracy of forecasting a significant weather event. NOAA's Office of Satellite Operations follows special rules to ensure that there are no data interruptions during the GOES East transition process. Any time the National Weather Service declares a critical weather day (for example, during a major storm, wildfire or other event of national importance), OSPO prohibits any special operations activities that could impede the flow of satellite data back to Earth.
If all goes according to schedule, GOES-16 will officially begin its new service life by December 20. Once fully operational, the satellite will provide forecasters with near-real-time data and imagery on developing weather events in North America, from its new perch high above Earth’s equator, 22,300 miles away.