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by Ed Gough

Why Altimetry?
Any sailor can tell you that the surface of the sea is not flat. Ocean waves induced by winds and currents cause it to undulate with waves that range from less than a foot to massive hurricane swells the size of office buildings. In the background, there are also other undulations that are much less noticeable. Even when the ocean is perfectly calm, with no waves whatsoever, these underlying differences in height form gentle hills, ridges, and valleys similar to those on land.

However, these differences in altitude at sea are much smaller than on land, and the areas they cover are much larger. The ocean's "hills" and "valleys" differ in height by only a few meters, at most, over the course of many miles, which is why, even on windless days with a glass-smooth sea, the most discerning observer cannot perceive their gentle slopes with the naked eye.

The constant variation of sea surface topography—also called sea surface height, or altimetry—may seem like an esoteric scientific concern of interest only to oceanographers. The differences in surface height are much too small to have any direct effect on most day-to-day ship operations. For example, a "hill" of ocean water 50 nautical miles across and only six inches high has no effect on navigation either above or below the surface.

Nevertheless, the accurate, consistent, and repeated measurement of the ocean's surface plays a vital role in the U.S. Navy's undersea warfare effort. It does so because even small altitude differences greatly influence the direction and strength of sound energy as it moves through the water beneath the varying ocean terrain. Through complex physical processes, the water under areas of higher altitude tends to be moving downward, forcing the thermocline deeper in those areas. In areas of lower altitude, the reverse happens; with the thermocline being pulled upward toward the surface.

By applying known relationships between the height of the sea surface and the movement of the water below, it is possible to calculate the structure of the subsurface water column and thus its acoustic properties. By accurately measuring the ocean surface, we can calculate how acoustic energy will propagate through the water column and thus how the sonar systems of submarines and surface ships will perform against target vessels, regardless of whether the targets are nearby or far away.

The Challenge of Timeliness
But there's a catch. Just like analogous high and low pressure systems in the atmosphere, the ocean's "highs" and "lows"—its hills and valleys—do not just stay in one place, they constantly move around and change in size and shape depending on factors such as the water depth, wind, temperature and current.

For example, strong and swift western boundary currents like the Gulf Stream in the Atlantic and the Kuroshio Current in the Pacific constantly shed warm and cold core eddies that spin off from the main current. These eddies can produce fast-moving ocean features that can disrupt or focus sound energy and impact acoustic performance at scales that are tactically significant for naval operations.

Therefore, unlike terrain maps, which generally do not become outdated even after years without a new survey, mapping the constantly changing topography of the ocean surface requires remeasurement on the order of days to ensure that the information remains up to date and accurate. Revisiting mapped areas frequently and providing near-global broad ocean coverage are both key to successful ocean mapping.

The only sensors that can meet both the temporal and the spatial requirements for ocean mapping are radar altimeters operating from satellites. A radar altimeter is simple in concept, working in much the same way as any other radar. From the satellite, it directs a pulse of radio-frequency energy to the target—in this case, a known location on the ocean surface beneath the satellite's flight path. Since the position of the radar and the velocity of the energy pulse are also known, the system can automatically calculate the height of the surface from the amount of the time it takes for the energy pulse to reach it and be reflected back up to the satellite.

Ozone and water vapor in the atmosphere can complicate this computation somewhat, but dealing with atmospheric complications is relatively simple. The real challenge is the overall process of mapping huge areas of the sea surface and relating the resulting information to sonar performance.

Current Altimetry Satellites
The U.S. Navy's own altimetry satellite, the GEOSAT Follow-On (GFO), was recently decommissioned and taken out of service after operating many years beyond its design life. Two other satellites now provide the U.S. Navy with all of its sea surface altimetry data. One of these is JASON-1, which is operated by a consortium of the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the French space agency. The other is the Envisat satellite, operated by the European Space Agency.

The loss of GFO raises concerns because it was the only altimetry satellite designed specifically to meet the Navy's requirement to capture features that impact undersea warfare operations and provide a complete picture of the ocean dynamics. JASON-1 and Envisat were both designed to monitor long-term climate change and are therefore in orbits less suitable to properly capture ocean features on the time and space scales that the Navy requires. Moreover, both are also operating past their designed life. The Navy is making good use of them, but it will continue to feel the loss of its primary ocean measuring system until a replacement can be launched in 2013.

Predicting Undersea "Weather"
The Naval Oceanographic Office (NAVOCEANO) receives all of the Navy's—and most of the world's—real-time ocean data to feed its operational ocean models. Over 100 times more data comes from altimetry satellites than from all other sources of ocean data combined.

The data first enters NAVOCEANO through the Oceanographic Data Division. The job there is to receive the data, apply all necessary corrections and calibrations, and process it through a series of quality checks to ensure the values are correct and the collection system is working properly. The goal at this point is to ensure that the data is accurately depicting the current state of the ocean surface before it is passed to the next group, the ocean modelers, for further processing.

Collecting the data is just the first step in the process. The data represents the state of the ocean at some time in the near past, like yesterday or this morning. This is very useful information for building what are called historical climatologies—data bases that store data collected repeatedly in a given area for years. However, that is not NAVOCEANO's end game. Rather than merely collecting and storing sea surface data for later analysis, NAVOCEANO's goal is to provide information that will help submariners and other operators make successful decisions in the demanding real-time world of acoustic-driven operations.

Consequently, NAVOCEANO's final products are exactly analogous to local weather forecasts. The National Weather Service receives data from satellites and weather stations all over the country and uses it to forecast tomorrow's temperature and other atmospheric conditions in specific localities. When the weatherman refers to the average high and low temperatures for any given day, he is using output from a historical climatology. However, the actual highs and lows usually differ significantly from these climatological averages, so the goal of the Weather Service is to provide accurate forecasts for specific locations at specific times.

The U.S. Navy cannot rely on historical averages alone for conducting real-world operations. Actual conditions usually differ greatly from averages based on historical data, and even a small change in underwater conditions can be very important, because it can make a huge difference in acoustic propagation. Near-term measurements of past conditions are therefore absolutely essential for predicting acoustic performance.

graphic caption follows
Example of ocean model output, with colors representing the velocity of ocean currents: (left) black arrows show the direction of surface currents in the Atlantic from Cuba to Cape Cod; (center) surface current speeds along the Virginia – Maryland – Delaware coast (a closer look at a portion of the image to the left.); (right) surface currents at the entrance to Chesapeake Bay. (a closer look at a potion of the center image.)

How Forecasting Works
Even near-term measurements are just data, however, until NAVOCEANO turns them into information by inputting this data into numerical ocean forecast models that can predict the state of the ocean in a future place and time where operations will occur. The real value of oceanographic data is its ability to reveal the shape of the ocean, which is much like the atmosphere, only denser, with high and low pressure systems that can reveal the location of currents and eddies, their potential velocity, and the water temperature. All of these are determiners of acoustic detection ranges.

Before the Navy had access to satellite altimetry data, submariners and other operators had to assume that the thermal structure of the water at a distant location they were observing with sonar was the same as the thermal structure of the water at their own position. Oceanographers knew this wasn't the case, but they had no way to accurately estimate the critical properties of the water at any distance from a location where they could collect current data. Attempting to estimate conditions as little as a mile away from that specific location was merely guessing—educated guessing, perhaps, but still just guessing.

That all changed with satellite altimetry. According to Dr. Frank Bub, NAVOCEANO model and prediction system technical lead, nothing else provides as effective and complete a picture of the ocean as satellite altimetry—not buoys, and not ocean gliders. With altimetry, the location of each data point is known within centimeters, satellite passes come at regular intervals, and the data points are for exactly the same location pass after pass. Greg Jacobs, the model developer at the Naval Research Laboratory at Stennis Space Center, added that the model would not look like the real world without continuous data, since ocean features such as eddies, fronts, and currents cannot be predicted without a near constant stream of input.

From Data to Predictions
The Modeling Department at NAVOCEANO, with about 30 employees, models all of the world's oceans from the deep ocean to near-coastal areas continuously, 24 hours a day, 365 days a year. The modelers run three ocean models each day—a three-dimensional circulation model run at both regional and global scales; a two-dimensional circulation model for near-coastal areas; and wave models run on every scale from global to the surf zone. The department also does special requests, which it prioritizes according to the operational load and mission priority.

Each day, the Modeling Department produces for the Fleet about 15,000 graphics that illustrate results of the model runs for various places in the world. "The Naval Oceanographic Office is the only organization in the world that provides fully dynamic global ocean forecasts out to 72 hours in the future," Bub noted.

The forecasts that the Modeling Department produces after processing the altimetry data predict oceanographic conditions in the battlespace environment, but they do not yet show how the conditions in the forecasts will impact the Fleet's sonar systems. That is the job of NAVOCEANO's Acoustics Department. The acousticians take the data fields produced by the modelers and use them to make predictions that commanders can leverage to better understand how the environment impacts their mission.

Temperature and salinity affect the propagation of sound waves. Ocean currents shape and move water masses of different temperature and salinity, and therefore density, and these water masses directly affect the propagation of sound waves through the ocean. Analysts in the Acoustics Department observe the ocean properties that the predictive models show for a specific area and determine how those properties will affect sound waves.

The Acoustics Department runs acoustic propagation and performance models that combine the information on ocean conditions with sonar system design parameters to compute acoustic energy propagation for various sonars against different targets at different positions and depths. Predictions from the model runs are often condensed into a series of graphics, called "performance surfaces," that provide operational commanders with an "acoustic map" of the battlespace informing them how their sensors will perform.

Ensuring Accurate Information for the Fleet
The final step in the processing chain is the Naval Oceanography Anti-Submarine Warfare Center (NOAC), which works directly with the Fleet in undersea warfare. NOAC's uniformed Navy personnel use the results of the Acoustic Department's acoustic analysis to brief operational commanders directly on potential sonar performance in their operational area.

Lt. Cmdr. Tim Campo, a former NOAC operations officer, said that his people have to be absolutely certain about the information that they are delivering. Any weak link in the chain — be it in the collection of satellite altimetry data, the fusion of that data, the running of ocean forecast models, or the prediction of acoustic system performance — adds to the uncertainty in the forecast acoustic performance products and reduces the accuracy of the acoustic performance briefs.

"We are about taking uncertainty out of the operation," he said.

NAVOCEANO's systematic effort to improve the quality of its forecasts now enables operational commanders to employ their forces, at least partially, on the basis of the NAVOCEANO "sonar performance surfaces."

"We tell the Fleet operators how their sonar will perform in a specific area, Campo said. "Based on that information, operators place their assets and search for submarines."

The Battlespace-on-Demand Doctrine
All Navy meteorology and oceanography support—in particular the support for undersea warfare described above—is accomplished in accordance with the Battlespace-on-Demand (BonD) doctrine, a 'value chain' approach to provide the Fleet with relevant and actionable information on the physical battlespace environment and how it impacts operations and fielded systems. The BonD doctrine calls for three "tiers."

Tier 1 is called the environment layer. This is where data from oceanographic sensors like a satellite altimeter is fed into numerical ocean models and formed into "nowcast" and forecast fields of data parameters like temperature, water density and sound speed that most influence sound propagation and thus acoustic sensor performance.

Tier 2, the performance layer, is where environmental data computed and delivered from Tier 1 is ingested into acoustic propagation and performance models to determine how a specific sonar system will operate against targets in those waters.

Tier 3 is the decision layer, where Tier 2 sonar performance is fused with other information about the tactical battlespace to create operational products on which operational commanders can base decisions.

Each BonD tier is completely reliant on the one below it. The data collection itself, in this case, the sea height measurements that come from satellite altimeters, is the implied 'Tier 0,' the foundation on which all higher tiers and products rest. Without the satellites that constantly measure the ocean surface, those who are charged with defending America's interests at sea would lack critical operational knowledge about the performance of their sonar systems.

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Example of ocean model output: (top) sea surface temperatures off the Atlantic coast from Florida to Nova Scotia; (center) ocean current velocities in the same area; (bottom) sea surface temperatures and currents in the Gulf of Mexico and western Caribbean Sea.

The Foundation of It All
So the foundation of the entire process remains the continuous, real-time satellite measurement of something as esoteric as sea surface topography. The resulting data points are the basic building blocks for the modern ocean models that ultimately keep the Navy informed about how well—or even how poorly—its sonar systems will perform in any given place at any given time.

The systematic collection of altimetry data by satellites is the indispensible first step toward an accurate understanding of current conditions in the environment beneath the ocean's surface. As such, it is absolutely essential for ensuring that U.S. Navy warfighters have the information they need to make effective operational decisions in the immensely complicated world of undersea warfare.

Ed Gough is deputy commander and technical director of the Naval Meteorology and Oceanography Command.

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