Chapter 2 - Landsat 7 Spacecraft
The Landsat 7 satellite was successfully launched from Vandenburg Air Force Base on April 15, 1999. The Delta II launch vehicle left the pad at 11:32 PDT and performed flawlessly. The injected spacecraft, depicted in Figure 2.2 and in this 1.8 MB Quicktime movie, is a 5000 pound-class satellite designed for a 705 km, sun synchronous, earth mapping orbit with a 16-day repeat cycle. Its payload is a single nadir-pointing instrument, the Enhanced Thematic Mapper Plus (ETM+). S-Band is used for commanding and housekeeping telemetry operations while X-Band is used for instrument data downlink. A 378 gigabit Solid State Recorder (SSR) can hold 42 minutes of instrument data and 29 hours of housekeeping telemetry concurrently. Power is provided by a single Sun-tracking solar array and two 50 amp-hour Nickel-Hydrogen batteries. Attitude control is provided through four reaction wheels (pitch, yaw, roll, skewed), three 2-channel gyros with celestial drift updating, a static Earth sensor,
1750A processor, and torque rods and magnetometers for momentum unloading. Orbit control and backup momentum unloading is provided through a blow-down monopropellant hydrazine system with a single tank containing 270 pounds of hydrazine, associated plumbing, and 12 one pound-thrust jets. Spacecraft weight is approximately 4632 pounds at launch.
Figure 2.2 The Landsat 7 satellite as viewed from the sun side.
2.2 Platform Characteristics
The satellite provides active, three axis stabilized, attitude control; NASA standard telemetry, tracking and command for narrowband communications; Landsat unique direct, and store and forward wideband data; direct energy power transfer, including a single solar array with multiple panels, with on-board power storage; on-board computing for power management, attitude control, satellite commanding and failsafe protection management; and hydrazine reaction control for momentum compensation and orbit adjust maneuvers.
The satellite attitude control uses precision mode, which is a combination of stellar and inertial guidance sensors to maintain the spacecraft platform within 0.015 degrees of earth pointing. The capability exists for an initial attitude control rate nulling mode, local vertical acquisition mode, yaw gyro backup mode, and an orbit adjust maneuver mode. Narrowband communications include processing of real-time and stored commands, processing and authentication of command messages and transmission of telemetry data which is the collection of housekeeping and satellite processor reports.
Wideband communications for payload data transmission to the ground incorporates four X-band transmitters, switchable to three steerable antennas that have a downlink beam widith of 1.2 degrees. Also, an on-board solid state recorder is used to store imagery of foreign land masses for subsequent transmission to EDC.
The on-board processor performs autonomously executed functions for wideband communications, electrical power management, and satellite control. These include attitude control, redundancy management, antenna steering, battery management, solar array pointing maintenance, thermal profile maintenance, and stored command execution. The Satellite Segment also includes the Aerospace Ground Equipment (AGE) which is composed of all the electrical (EAGE), mechanical (MAGE), instrument and propellant ground equipment and related software used to integrate the satellite to the launch vehicle. Additionally, it consists of support system test functions and protection of the satellite during transport. One particular type of EAGE, the launch site equipment (LSE), is used to verify the readiness of the satellite during pre-launch testing and during launch operations and readiness reviews.
Planning and scheduling takes place in three categories; long-term planning, short-term planning, and daily scheduling.
The prime goal of the Landsat 7 mission is to refresh the global archive. Because the Landsat 7 mission orbit profile operates on a repetitive 16-day cycle, the global archive refresh strategy can and was designed years before the Landsat 7 satellite launch.The long-term plan was generated early, to provide ample time for coordination with the science community, program management, international resources, and project elements. The long term plan takes into account the calibration plan, planned satellite maneuvers, and international imaging needs. The MOC will ingest the long-term plan prior to launch and use the data as the main database from which to plan daily schedules of instrument and SSR activities.Short-Term Planning
The objectives of short-term planning are to schedule communication contacts for telemetry, tracking and commanding services on the LGN, to include special requests into the scheduling process, and to generate daily reports summarizing the disposition of imaging requests and time-ordered scheduled ETM+ imaging events for the latest 48 hour period.Daily Scheduling
On a daily basis, the MOC must generate a nominal 48-hour set of imaging, SSR activities, and X-band downlink services from which the MOC will generate an absolute time stored command load. The MOC will schedule the 48-hour set of activities based on a number of criteria, including global refresh requirements, request priority, SSR or other resource availability, and cloud cover predictions. After the FOT has iterated through the scheduling function and resolved all conflicts, the MOC will pass the 48-hour schedule to the load generation function for final compilation. To assist in the scheduling process, the MOC will receive planning aids from the FDF (Table 2.1), a cloud cover prediction from the National Meteorological Center (NMC), and metadata on acquired or archived imagery from the EDC DAAC.
2.3.2 Orbit Maneuvers
During the Landsat 7 mission, the MOC operators will have to perform two types of orbit maneuvers to ensure the satellite's orbit remains within the Landsat 7 mission parameters. The two types of orbit maneuvers are:
The In-plane maintenance maneuver, also called a drag make-up maneuver, maintains the semi-major axis within an acceptable tolerance of the mission orbit semi-major axis. The semi-major axis is biased high and permitted to decay over time. The bias applied to the orbit varies with the amount of environmental drag which is a function of solar activity.
The inclination maintenance maneuver involves a yaw slew by approximately +/- 90 degrees. The operators perform the inclination maneuver to keep the satellite descending node within a 30 minute box (09:45 to 10:15 AM). The FDF is responsible for monitoring the satellite orbit to ensure that its orbit remains within mission parameters. The FDF receives tracking data from the SN and Landsat telemetry data from the MOC that are necessary to monitor the orbit and informs the MOC when the orbit has been perturbed sufficiently to require an orbit adjust. The FDF calculates the orbit maneuver command data, including an orbit maneuver time window, and passes the command data and a maneuver plan to the MOC.
The MOC ingests the orbit maneuver command data and generates a set of absolute time commands. The Flight Operations Manager (FOM) reviews the data and the resulting commanding prior to merging the commanding into the absolute time command load for the day of the orbit maneuver. The FOT mission planner will insert an orbit state vector activation command that is timed to execute at the end of the maneuver window.
The FOT will coordinate the orbit maneuver times with the NCC for TDRSS support and with the NOAA SOCC for CDAS support to facilitate the communications support needed to monitor the orbit maneuver in real-time. Additionally, the FOT will schedule TDRS tracking passes to occur immediately after the maneuver.
On the day of the orbit maneuver, the FDF will send the MOC two sets of orbit state vectors: one to execute in a burn scenario and one to execute if the MOC cancels the orbit maneuver. The MOC will generate orbit state vector loads for each set and make them available to the FOT.
Once the FOM makes the final decision to perform the orbit maneuver, the FOT will uplink the absolute time stored command load to the satellite. Sometime after the FOT activates the stored command load and before the orbit maneuver occurs, the FOT will uplink the appropriate orbit state vectors load. The orbit state vectors load will remain in the on-board scratch buffer until after the burn takes place. At that time, the FOT will execute real-time commands to activate the post-burn orbit state vectors by copying the orbit state vectors from the scratch buffer to the active orbit state vectors table and starting ephemeris processing of the new orbit state vectors.
Following the orbit adjust, the MOC conducts TDRS contacts for acquiring tracking data. The FDF calculates the new satellite orbit, using this tracking data, verifies the success of the orbit adjust, and generates new orbit state vectors for routine delivery on the day following the burn. The FDF will evaluate the performance of the satellite components used during the orbit adjust and provide that information to the MOC to update the MOC-resident propulsion model. The MOC will use the MOC-resident propulsion model to calculate the remaining propellant on-board the satellite and analyze the propulsion system.
2.3.3 Tracking and Control
2.3.4 Mission Operations
2.4 Communication Links
Ground sites are planned for Sioux Falls SD (The Landsat Ground Station, or LGS), Poker Flat AK (Alaska Ground Station, or AGS), Wallops VA (WPS), and Svalbard Norway (SGS). SGS and AGS are also known as the EOS Polar Ground Sites (EPGS)
and are managed by Wallops Flight Facility (WFF). All ground sites will be equipped with 11 meter antennas. AGS, SGS, and LGS will be capable of receiving both S-band (housekeeping) at a downlink rate of 4 Kbps and X-band (payload) data simultaneously. Figure 2.3 provides a view of the ground stations' acquisition circles.
Figure 2.3 U.S. Ground Station Acquisition Circles
In addition to the ground sites, the Tracking Data and Relay Satellites (TDRS), operated by the Space Network, NASA code 530 will be utilized. TDRS enables downlink of real-time and stored S-band data and command of the spacecraft. TDRS is also used tracking data collection for FDF generation of Landsat 7 spacecraft ephemeris which is subsequently uploaded for use in precision attitude control. All network lines will be provided by the NASCOM division of NASA.
2.5 Solid State Recorder
The SSR is used to capture wideband data from the ETM+. The SSR accepts two inputs at 75 Mbps. The SSR can hold up to 42 minutes (approximately 100 scenes) of data at 150 Mbps. The SSR records and plays back wideband data in numbered logical blocks which are used by the MOC in commanding the recorder.
The SSR records ETM+ channel access data units (CADU) as two bitstreams, each at a nominal rate of 74.914 Mbps. CADUs are recorded in the same order as received from the ETM+. Partial CADUs may be recorded if the ETM+ collection interval extends beyond the commanded SSR record interval, if the the ETM+ is turned off before the end of the SSR data recording area is achieved, or as a result of a ground command to disable wideband recording.
During playback, the two 75 Mbps bitstreams are read out of memory and sent to the broadband switching unit. A second pair of 75 Mbps bitstreams can also be played back for a total aggregate rate of 300 Mbps. The bitstreams include the CADUs generated by the ETM+. Record intervals, each corresponding to a ETM+ collection interval, which consists of one or more Landsat scenes, may be subdivided for playback if more than one scene is collected. In this case, each resulting subinterval is defined such that data in the vicinity of each subinterval boundary are included (redundantly) with both subintervals. Each subinterval includes all of the CADU data required to process the subinterval as a separate ETM+ collection. As a result, individual subintervals may contain partial CADUs. The SSR contains error detection and correction capability to preserve the integrity of the stored wideband and narrowband data. Reed-Solomon encoding is performed on record while Reed-Solomon decoding is performed on playback to recover data from possible dynamic RAM list errors.
Narrowband data is captured from by the SSR from the Telemetry Data Formatter. The SSR accepts two input rates of 1.216
Kbps or 4.864Kbps and plays back stored telemetry data at 256 Kbps to the S-Band transponder.
The SSR is capable of either recording or playing back wideband data (but not both simultaneously) while simultaneously recording and/or playing back narrowband telemetry data. s-Band telemetry data is stored separately from wideband image data and can be recorded during load shedding.