The GEOSAT Follow-On program is the Navy's initiative to develop an operational series of radar altimeter satellites to maintain continuous ocean observation from the GEOSAT Exact Repeat Orbit.
The METOC Systems Office of the Space and Naval Warfare Systems Command has overall responsibility for executing the procurement of the Navy's environmental sensor satellite, GFO. Competitive procurement in 1992 resulted in the selection of Ball Aerospace Corporation as the Prime contractor for the spacecraft. Subcontractors to Ball include: E-Systems Corporation (payload integration and fabrication of the altimeter), AIL Systems Inc. (manufacture of the microwave radiometer), Rockwell International (manufacture of the GPS receiver), and Lockheed Missile and Space Company (launch vehicle manufacture and operations).
Navy requirements for geodetic and oceanographic information have been a driving force through the history of satellite radar altimetry. The Navy's collection of geodetic information was initially acquired by ship surveys which were slow, expensive, and, incomplete. Now, space-borne altimeters provide a more efficient method of collecting the necessary information to support its environmental predictions and enhance its warfighting capability.
Plans for Navy/NOAA/NASA use of the data were formulated and discussed at GFO User Meetings and Program Meetings. Navy users stressed the shift in Navy tactical focus from blue-water antisubmarine warfare to an integrated ocean monitoring and prediction system that supports Navy needs in blue water, on the slope, and on the continental shelf (coastal waters). Deep ocean basins make up 95% of the ocean area; and, coastal models of the ocean are most viable when coupled to open ocean models with an integrated system. Coastal oceanography and assimilation of altimeter data into an integrated ocean monitoring system will be a dramatic application of GFO altimetry.
Navy applications of GFO include use of altimeter data in coastal oceanography, in mapping mesoscale fronts and eddies, and, in using basin-scale data for generating eddy-resolving global ocean models. The length and time scales of these processes are too large for conventional in-the-water oceanographic instrumentation configurations to measure. Satellite altimetry is the only known method by which oceanographers can precisely measure sea surface topography. The shape of the sea surface is the only physical variable directly measurable from space that is directly and simply connected to the large-scale movement of water and the total mass and volume of the ocean.
The GFO Mission supports Navy, NOAA, NASA, and University ocean science and ocean monitoring. It is believed that ocean circulation may be a major cause of decadal climate change. New scientific evidence indicates that dramatic, even catastrophic climate changes can occur over the space of only a few years. Recent ice core samples support several models where shifts of as much as 10 degrees Celcius occur in as short a time as 3-5 years. By comparison, the "little ice age" of 750 years ago resulted from a climate change of only 2 degrees Celsius.
Habitable temperatures in the North Atlantic region is due primarily to flow of warmer surface water. As water flows north is becomes more dense and saline through evaporation. Part of the flow sinks and flows south as deep water currents. The remainder flows east toward the United Kingdom before turning south as a surface current. Rising global temperatures could cause high volumes of meltwater from ice caps to alter the density and salinity in North Atlantic waters. This might dramatically change the deep and surface circulation of the water leading to drastic climate changes in North America and Europe. NOAA has established the prediction of decadal climate as a major strategic goal.
The measurement performance of the satellite altimeter has been extensively validated. During the Synoptic Ocean Prediction (SYNOP) experiment, an aircraft underflight of a GEOSAT pass dropped Air Expendable Bathythermographs (AXBT's). The rms difference in absolute topography between the altimeter and the AXBT's of 6.8 cm when processing the altimeter data with a "synthetic geoid" to provide a precise surface of no motion at mesoscale wavelengths.
The GFO Spacecraft and Mission Design
The satellite includes all the capabilities necessary for the precise measurement of both mesoscale and basin-scale oceanography. The spacecraft adds a water vapor radiometer and GPS receiver to the basic GEOSAT measurement capability. GFO was launched on February 10th, 1998 aboard an LLVI booster from Vandenberg Air Force Base in California. The launch was originally expected in mid-1996 near the minimum of the solar cycle. During the 10 year mission life, the satellite is in the GEOSAT Exact Repeat Mission (ERM) orbit (800 km altitude, 108 deg inclination, 0.001 eccentricity, and, 100 min period). This 17-day Exact Repeat Orbit (ERO) retraces the ERM ground track to +/-1 km. As with the GEOSAT ERM, the data will be available for ocean science through NOAA/NOS and NOAA/NESDIS.
The 300-kg spacecraft is approximately 3-m long and supports the following payloads:
Radar Altimeter - single frequency (13.5 GHz) with 3.5-cm height precision.
Water Vapor Radiometer - dual frequency (22 and 37 GHz) nadir-looking with a path correction accuracy of 1.9 cm rms.
GPS Receivers - redundant receivers allow precision orbit determination with an rms accuracy in the radial component of 10 cm on wavelengths less than 40,000 km.
Doppler Beacon - GEOSAT performance-stable oscillators and doppler beacons will allow operational orbits to be determined with 1.8-cm rms radial orbit error for mesoscale oceanography (after tilt and bias removal along a 3000-km arc-filter length.
The payload features complete redundancy, light weight (47 kg total), and low power consumption (121 W total).
All payload data are provided on an encrypted, continuously operating tactical downlink to AN/SMQ-11-equipped Navy ships and facilities. Sea surface topography derived from the altimeter data are used for tactical environmental aids and for boundary conditions for shipboard ocean model. Also, payload data are dumped approximately every 12 hours and sent to the NAVOCEANO Altimeter Data Fusion Center (ADFC) at Stennis Space Center, Mississippi for processing.
The GFO Ground Segment includes two NAVSOC remote tracking sites at Prospect Harbor, Maine, and Pt. Mugu, California. Satellite payload and engineering data will be relayed to the Satellite Operations Center (SOC) at NAVSOC Headquarters with payload data sent directly to the Payload Operations Center (POC) at NAVOCEANO. The SOC provides all system and satellite operations with remote commanding via the remote sites.