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.
The GRACE-FO spacecraft will be launched into a near-circular polar orbit with an inclination of 89 degrees and an orbital period of approximately 90 minutes.
The time of launch is determined by the launch requirements of Iridium.
The launch date is based on the readiness of the payloads, the Falcon 9 launch vehicle and the Western Test Range at Vandenberg Air Force Base. Launch is currently scheduled for no earlier than May 19, 2018, at 1:04:24 p.m. PDT (4:04:24 p.m. EDT). The launch window on subsequent days falls earlier by approximately 5 minutes, 35 seconds each day.
A SpaceX Falcon 9 launching
with an Iridium payload
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.
Artist's concept of GRACE-FO separating from the launch vehicle
Image credit: NASA/JPL-Caltech
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.
In the absence of major unexpected events, LEOP will be completed within the first five days after launch. This phase provides frequent opportunities for monitoring the satellites’ status so that controllers can intervene from the ground if required.
Throughout the mission lifetime, telemetry and telecommanding activities will be carried out by DLR/German Space Operations Center (GSOC) at its mission control center in Oberpfaffenhofen, communicating with the satellites via ground stations in Weilheim and Neustrelitz, Germany, and, during the first days of the mission, at the NASA Near Earth Network ground stations at McMurdo, Antarctica; Poker Flat, Alaska; Svalbard, Norway; and Wallops Island, Virginia for uplink. The GFZ Satellite Receiving Station at Ny-Ålesund on Spitsbergen will be the primary downlink station, with backup provided by the uplink stations.
After the satellites are simultaneously released from the Falcon 9’s second stage, the leading satellite will move away from the trailing satellite at a relative speed of about 1.6 feet (0.5 meters) per second. Separation from the launch vehicle causes systems on board the satellites to activate. Less than a minute later, a boom that holds each satellite’s radio frequency S-band antenna is deployed, and the low-rate radio transmitter is activated. The S-band transmitters will continue to transmit until turned off by ground command. During LEOP, the satellites must be capable of survival and attitude recovery with little or no real-time ground interaction.
The separation from the launch vehicle will leave the two satellites in somewhat different orbits. The goal of the maneuvers in LEOP will be to establish the operational formation with the two satellites separated by 137 miles (220 kilometers) sometime between two and a half and four days after separation.
During LEOP, between two and three days after separation from the launch vehicle, the leading spacecraft (which is in the lower orbit) will perform a maneuver to increase the size of its orbit to match that of the other spacecraft and stop the spacecraft from drifting apart. The stability of the separation orbit will be verified six days after launch.
Following their first pass over McMurdo, Antarctica, where separation will be verified, the two satellites will come within radio range of Svalbard, Norway, and then Poker Flat, Alaska. These two ground stations will be used to receive telemetry and to relay commands issued by the German Space Operations Center. On later orbits, the performance of both satellites will be verified, and commands will be issued as needed.
The orbits of the two satellites will evolve naturally for the remainder of the mission. Due to differences in drag forces, the separation between the satellites will vary between 106 and 193 miles (170 and 270 kilometers). Station-keeping maneuvers will be carried out as necessary, approximately every five months, to keep the two satellites at their desired separation.
To insure uniform exposure and aging of the K-band microwave antennas on each satellite, once or twice during the mission, the leading and trailing satellites may exchange positions. The altitudes of the two satellites will decay (gradually get lower) in tandem, from approximately 305 miles (490 kilometers) at the beginning of the mission to 186 miles (300 kilometers) toward the end of the mission; however, the exact altitude decay over time will depend on solar activity and corresponding radiation pressure. At various intervals in the mission, it will also be necessary to carry out certain science instrument calibration maneuvers.
Up to 3.7 gigabytes of instrument, ancillary and spacecraft data will be downlinked to Earth via the S-band antenna from each satellite every day. The DLR German Remote Sensing Data Center will serve as the Raw Data Center at Neustrelitz.
After the orbit and basic satellite operations are well established during LEOP, there will be an In-orbit Checkout (IOC) phase of approximately 85 days, during which the science instruments are powered up and the instruments and satellite systems are evaluated, and calibrations and alignments are carried out. The mission transitions from LEOP to the IOC phase when the LEOP conditions are fulfilled and confirmed by the project.
A Post-launch Assessment Review will take place near the end of this phase to verify that the operations system is ready to proceed into the validation and operational part of the Science Phase.
Following the In-Orbit Checkout phase, the mission will enter its Science phase, in which science data will be routinely gathered and processed. This phase will continue until the end of the mission and will include brief interruptions for orbit maneuvers and instrument re-calibrations. The science phase begins with a 120-day gravity model validation sub-phase focused on providing an end-to-end characterization of the science instrument and data systems before making the first science delivery. During this time, measurements for three monthly gravity science products will be acquired.
The orbits and ground tracks of the GRACE-FO satellites are not actively controlled during the In-Orbit Checkout and Science phases. The orbits are freely evolving, except when interrupted by station-keeping maneuvers or a satellite exchange maneuver. To maintain the separation between the GRACE-FO satellites to 137 miles (220 kilometers) plus or minus 31 miles (50 kilometers), station-keeping maneuvers will be carried out.
During the Science phase, the science and satellite housekeeping data are routinely collected by GRACE-FO’s Mission Operations System and analyzed by the Science Data System team. DLR ground stations at Weilheim and Neustrelitz will be used for tracking, telemetry and telecommand activities for the duration of the Science phase. The GFZ ground station at Ny-Ålesund will be used as the primary downlink station. Each satellite transmitter is powered for a maximum of 19 minutes every orbit to support data transmission.
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.
