The twin GRACE-FO satellites will be launched together aboard a SpaceX Falcon 9 rocket from Space Launch Complex 4E (SLC-4E) at Vandenberg Air Force Base in central California. They will share the launch to Earth orbit with five Iridium NEXT communications satellites as part of a commercial rideshare mission procured by GFZ from Iridium Communications Inc.
LAUNCH EVENTS AND
Image credit: SpaceX
The Falcon 9 will launch GRACE-FO from SLC-4E down an initial flight azimuth of 180.1 degrees from true north (south-southwest). The boost-phase trajectory is designed to place the Falcon 9 upper stage, along with the GRACE-FO and Iridium satellites, directly into an approximately 305 mile (490-kilometer) circular orbit by the time of the first cutoff of the Falcon 9 Second-Stage engine (SECO-1). The nominal altitude of the injection orbit for GRACE-FO was chosen to match that of GRACE.
The Falcon 9’s Merlin first-stage engine start sequence begins approximately three seconds prior to liftoff. After liftoff, the launch vehicle will travel through maximum dynamic pressure (max Q). The nine first-stage engines burn for approximately two minutes and 45 seconds before being commanded to shut down at Main Engine Cutoff (MECO). Separation of the Falcon 9’s first and second stages occurs seconds later, followed by ignition of the second-stage engine for second-engine start 1 (SES1), which burns until reaching the injection orbit. During the second-stage burn, the payload fairing, or launch vehicle nose cone, will separate into two halves, like a clamshell, and fall away.
After separating from the first stage and completing its ascent with the orbit insertion burn, the second stage pitches down 30 degrees to its separation attitude for GRACE-FO and rolls so that one of the GRACE-FO satellites is on the Earth-facing side of the launch stack and the other on the opposite side is facing space.
Approximately 11.5 minutes after liftoff, a separation system on the re-ignitable second stage will deploy the twin GRACE-FO satellites in nearly the same nominal orbit. The separation impulses are within 20 milliseconds of each other and push the two spacecraft in opposite directions, with the only differences being that the separation mechanisms will have pushed the two satellites crossways in opposite directions by 0.8 feet (0.25 meters) to 1 foot (0.30 meters) per second each, resulting in slight relative velocity differences and magnitudes. Thus one of the GRACE-FO satellites will be pushed up into a larger, higher orbit that is slower on average, and the other will be pushed down into a smaller, lower orbit that is faster on average.
Separation occurs over the Pacific Ocean at about 17.5 degrees North latitude, 122.6 degrees West longitude. VAFB will confirm a successful separation using downlinked telemetry data from the upper stage. The first data from the spacecraft are expected to be received through the first pass over NASA’s tracking station at McMurdo, Antarctica. The satellites will be in range of the McMurdo station about 23 minutes after separation and within range of the Alaska Satellite Facility tracking station about 45 minutes later, providing a good chance of acquiring early telemetry data for mission operations.
After separation of the GRACE-FO satellites, the Falcon 9 second stage will coast before re-igniting its engine (SES2) to take the Iridium NEXT satellites to a higher orbit, where they will be deployed, one by one.
The purpose of the Launch and Early Operations Phase (LEOP) is to gain control over the two GRACE-FO satellites and establish nominal formation. The LEOP starts at the time of launch and ends when the following conditions have been met:
Orbital debris are a hazard to all low-Earth orbiting satellites. Even non-operating spacecraft are a concern, as they are passive targets for debris strikes that could spawn more debris to threaten active missions. To limit debris strikes, satellite missions decommission their spacecraft in orbits that are predicted to decay within a specified time. At the end of the Science phase (including any extensions to the mission), the GRACE-FO mission will enter its Decommissioning phase.
Nominally, the orbit at the end of the mission will already be low enough to ensure compliance, so no orbit change will be needed, and the propellant depletion maneuvers that begin the passivation of the satellites will be designed to further reduce the risk of collision during decay of their orbits. The spacecraft with the most remaining propellant will use it to lower its orbit as much as possible. The other spacecraft will perform its depletion maneuver in a manner that separates the orbit planes of the two spacecraft as much as possible.
After propellant depletion, the battery on each spacecraft will be disconnected from the solar array; it will continue to power the spacecraft until discharged by that power draw. Then, fault protection on the spacecraft will be deactivated. Finally, the transmitter of each spacecraft will be turned off, terminating all orbital operations for GRACE-FO.
Final data and science processing will be completed, and data products will be archived.
Per NASA procedural requirements, GRACE-FO’s end-of-mission orbits will assure that the twin spacecraft reenter the atmosphere within 25 years of decommissioning or 30 years of launch, whichever is earliest.
The GRACE-FO mission ground system includes all the assets needed to command and operate the twin satellites in orbit, as well as manage, process and distribute their data.
To communicate with the satellites, the operations center in Oberpfaffenhofen, Germany, sends commands through ground stations in Weilheim or Neustrelitz directly to the GRACE-FO satellites. Once data have been recorded onboard the spacecraft, they are transmitted to the two German stations or to the GFZ station in Ny-Ålesund, Norway. From there, all received telemetry is sent to the Raw Data Center in Neustrelitz, Germany, and to the Physical Oceanography Distributed Active Archive Center (PO.DAAC) at JPL in Pasadena, California, the Information System and Data Center (ISDC) at GFZ in Potsdam and to UT-CSR for monitoring and further analysis. Real-time data analysis takes place at the German Space Operations Center (GSOC), which will respond with new software commands as necessary for optimal operations.
JPL and GFZ will carry out the first level of processing, generating calibrated and processed metric observables. JPL, GFZ and UT-CSR will generate gravity field products from these intermediate products. The validated data products will be distributed to the science community through archives at JPL’s PO.DAAC and GFZ’s ISDC.
Data from the GRACE-FO satellites are returned approximately every 90 minutes. During most of the mission, the ground tracks of the GRACE-FO satellites will trace sufficiently dense patterns over Earth to enable a global gravity field to be generated every 30 days.
Data products will include 30-day estimates of gravity fields as well as daily profiles of air mass, density, pressure, temperature, water vapor and ionospheric electron content.
Once GRACE-FO is fully operational, high-resolution, monthly global models of Earth’s gravity field will be freely available at: https://grace.jpl.nasa.gov/data.