Imagine a fully autonomous navigation aid that doesn’t require a signal from either a ground transmitter or satellite constellation. It would virtually eliminate any worry of jamming or other type of signal interruption. What’s more, it would be a low-cost solution that would simply swap out the radar altimeter for a comparably priced unit that delivers both precise navigation and radar altimeter data.
The U.S. Navy is keenly interested in such a capability. The service fears that it may be placing all its eggs in one basket with GPS navigation, which has a weak signal that can be either deliberately or unintentionally interrupted. In a Wall Street Journal interview in September 2002, Rep. Joseph Pitts (R-Pa.) referred to the vulnerability of GPS and the availability of GPS jammers, which were being sold for $39.95 at the Paris Air Show.
The Navy initially is looking at new, non-GPS guidance for its premier cruise missile, the Tomahawk. (More than 800 Tomahawks were launched during Operation Iraqi Freedom, and the U.S. military has fired off more than 1,000 missiles since the Gulf War in 1991.) But the Navy is interested in the capability for manned aircraft, too.
The technology that promises GPS precision without using GPS is called Precision Terrain Aided Navigation, or PTAN. It has been under development at Honeywell Defense and Space Electronic Systems, in Minneapolis, since 1997, as an internal investment research and development (IR&D) program. In 2000 the Navy awarded a contract to Honeywell to establish a PTAN demonstration program that would be managed by the service’s Tomahawk program office.
PTAN is a logical extension of Honeywell’s radar altimeter business. The company has delivered more than 100,000 radar altimeters over a 40-year period. Its APN-209 radar altimeter has become standard on U.S. Army helicopters, and its APN-194 has the same status on U.S. Navy aircraft. The U.S. Air Force, too, has acquired many Honeywell radar altimeters. In fact, Honeywell is developing the radar altimeter for the F-35 Joint Strike Fighter. And there are even Honeywell radar altimeters on Mars, installed in the Mars landing craft for the Exploration Rover and Pathfinder.
With PTAN, Honeywell has combined a precision radar sensor with a digital elevation database and a correlation algorithm. The system also includes three antennas, instead of the normal two antennas for conventional radar altimeters. The extra antenna, positioned between the other two, is required to receive the extra data needed for enhanced precision.
The company took advantage of new technologies, which it developed along with the Johns Hopkins University Applied Physics Laboratory. The two PTAN team members developed a faster and smaller radar altimeter and a digital signal processor. They also developed new algorithms and expanded computer memory capacity to accommodate detailed terrain databases.
“The basic design is capable of expanding to gigabytes’ worth of data, if we decided to build a card with such memory,” says Tom Jicha, senior program manager at Honeywell. These new technologies are packaged in a single box that provides GPS-quality updates.
PTAN, Honeywell believes, could serve as a non-corruptible backup to a GPS receiver or to an inertial navigation system (INS) in place of GPS. Integrated with an INS, the system can provide positioning updates (latitude, longitude and altitude) at intervals of less than a second. Honeywell calls this capability the “continuous upgrade of the platform navigation solution.”
PTAN also could provide other benefits, including:
Improved reliability through enhanced signal processing;
Improved life cycle costs, as compared to existing radar altimeters;
Built-in test technology (some taken from the vehicles that NASA placed on Mars); and
The capability to provide accurate navigation in all weather and in a battlefield environment.
In another backup role, PTAN could be integrated with an enhanced ground proximity warning system (EGPWS), which now relies solely on GPS for aircraft positioning. Jicha says Honeywell is “working on” such an integrated system now.
For the military, PTAN works like stealth, with low probability of intercept, detection and exploitation—advantages that can benefit missiles and manned aircraft alike.
How It Works
Key to PTAN’s success is its precision radar altimeter, also called an “interferometric synthetic aperture radar sensor.” Rather than use a normal radar altimeter, PTAN employs an interferometer that provides significant aperture reduction. The interferometer measures very small distances on either side of a radar beam by splitting the beam into two parts directed over separate paths. Each beam is received by a separate antenna and then reunited. By calculating the difference in path lengths, the system can determine not only that there is a high point, but whether the high point is to the right or left of the beam’s footprint. Comparatively speaking, the interferometer returns plot high points of the terrain below the aircraft like a fine meandering pencil line, compared with the broad-brush returns from a normal radar altimeter. With such precise data, the interferometer not only measures the precise range from the aircraft to the terrain but also provides a range vector. “So it not only knows magnitude, but also direction,” says Jicha.
Terrain Data Selection
The radar altimeter’s precise data is compared to the digital terrain elevation data (DTED), which can come from various sources, including NASA’s Shuttle Radar Topography Mission (SRTM) terrain data and the “edited” SRTM databases produced by the National Imagery and Mapping Agency (NIMA). For the Navy’s demonstration program, Honeywell has been using DTED maps supplied by Johns Hopkins, which used various sources for the raw terrain data.
Honeywell’s goal with PTAN has been to achieve high enough resolution from interferometric technology to make comparisons with a DTED Level 4 map. The system therefore can compare against a map cell, or a representation of a patch of terrain that is as small as 10 feet (3 meters) and deliver navigation accuracy to within 10 feet. However, the PTAN system can be adjusted to accommodate any DTED level, from 1 to 4, depending on the amount of memory the customer chooses to include. The higher the DTED level, the greater the memory required to achieve smaller cell size and more navigation precision.
A correlation algorithm compares the radar altimeter data with the DTED. As with the terrain databases, PTAN can accommodate various options in correlation algorithms. The original terrain contour-matching (TERCOM) algorithm, developed in 1958, has been modified and used in the Tomahawk missile since 1972. However, PTAN also can use the Sandia inertial terrain-aided navigation (SITAN) algorithm, created by Sandia Labs, or some other correlation algorithm.
As it continues to refine PTAN, Honeywell is coming up with its own, “customized correlation algorithm, which takes from a number of the existing algorithms,” says Jicha. “But the point is that you can use any of these algorithms to get the results with PTAN. We want to emphasize the system’s versatility.”
Terrain-aided navigation truly is versatile, but it does have one requirement: it needs terrain below the flying vehicle. PTAN provides altitude data but no navigational assistance when the vehicle is flying over water, where no terrain correlations can be made.
Honeywell first flew its prototype PTAN system in September 2001, in the company’s Sabreliner, out of Phoenix (Ariz.) Deer Valley airport. “The demonstration was very much a success in terms of accuracy,” says Jicha. Honeywell used the very precise positioning data from a differential GPS “truthing” system to compare with the PTAN positioning data. Johns Hopkins evaluated the flight test data. “We were achieving GPS-or-better performance,” Jicha adds.
PTAN flight testing was interrupted by the 9-11 terrorist attack in New York and Washington, D.C., but was continued in April 2002 and in April 2003 over increasingly diverse terrain in Arizona and at White Plains, N.M. “In our first demonstration, we flew from 500 to 6,000 feet,” says Jicha. “Since then, we have flown the system up to 27,500 feet.”
However, so far the flight tests haven’t been in real time, according to Jicha. “We actually developed a complex system that captured the radar data from the flight test, and the results were post-processed.” In July Honeywell plans to begin a real-time bench demonstration of the system, and in October it intends to conduct its first real-time flight demo. The flight tests and other development work are parts of the engineering, manufacturing and development (EMD) phase in a program managed by the Tomahawk program office. The Navy has submitted a request for 2006 program objective memorandum (POM) funding to integrate PTAN into a Tomahawk missile, which would be part of the EMD phase.
Meanwhile, Honeywell has delivered a PTAN system to Raytheon, in Tucson, Ariz., for installation in a “hardware-in-the-loop” laboratory. In this facility, engineers will be able simulate PTAN performance in a Tomahawk missile.
UWB Precision Positioning
The search for a GPS alternative also includes a small company in Germantown, Md. The U.S. Army Space and Missile Defense Command, in Huntsville, Ala., has contracted Multispectral Solutions Inc. (MSSI) to develop an ultra wideband (UWB) precision positioning system. In mid-November 2003 MSSI received a Small Business Innovation Research contract worth $730,000 to proceed with Phase 2 of the project. The company will evaluate the system’s accuracy and the use of a new, higher-powered C band transmitter.
The UWB positioning system, called the Precise and Accurate Positioning Device (PAPD), would be as accurate as PTAN and GPS but would provide “localized navigation” in GPS-denied areas, according to Lester Foster, senior member, technical staff, and program manager at MSSI. PAPD is particularly effective in “bad clutter situations,” says Foster. It is effective in all weather conditions and in dust and sand, which makes it applicable for helicopter operations in, say, the Iraqi desert.
The PAPD system architecture comprises four elements:
Geolocation pseudolites positioned to transmit their geolocation data to receivers;
Geolocation receivers located within the range of the pseudolites to compute location based on the transmitted data;
A pseudolite support network that synchronizes, calibrates and coordinates a pseudolite’s operation; and
PAPD command and control, to manage the network of pseudolites.
Much like GPS, the UWB system determines positioning based on relative time difference of arrival (TDOA) of radio frequency (RF) signals. In other words, it measures the time delay of a transmitted signal to an onboard receiver, or sensor, to calculate range from the transmitter. However, instead of satellites, PAPD uses pseudolites that can be positioned on the ground, atop a building or other structure, or even on aircraft. What is critical is that all pseudolites in the constellation are precisely positioned and that their internal control processors maintain clock synchronization. On the ground, GPS may be employed to secure a pseudolite’s exact position. Airborne pseudolites may be accurately positioned using GPS, ground beacons, or electro-optical or infrared cameras that capture terrain images and correlate them with geo-registered maps.
Regardless of the pseudolite constellation’s structure, the key to accurate positioning is, like GPS, an optimum geometry of the transmitted signals. With GPS, measurements collected simultaneously from four satellites are processed to determine the three dimensions of position, velocity and time. The number of PAPD pseudolites required for precise position can vary, according to Foster, depending on their locations and the geometry they create.
Position and time data from the pseudolites is transmitted over UWB nanosecond-pulse waveforms that are spectrally confined. This allows multiple RF systems to occupy the same platform without interference. The short pulse widths spread energy over a large bandwidth, making detection by intercept and surveillance receivers difficult, according to the final report of PAPD Phase 1. This also makes the UWB signal less likely to interfere with other RF systems.
MSSI’s pseudolites have been transmitting over the C band portion of radio spectrum, at about 6.0 to 6.4 GHz, says Foster. However, with multispectral UWB, PAPD has multichannel definition in the radio spectrum, and thus data can be transmitted using any channel, according to Foster. Changing channels would “require only a simple swap-out of the system’s front-end circuitry,” he adds. “In the future, we could even have frequency-hopping geolocation.”
During the current, second phase of PAPD’s development, MSSI plans to evaluate a new C band transmitter. It will include pulse amplification, designed to boost the transmitter’s radiation power from 35 milliwatts to 5 watts. The transmitter’s source power would remain at about 2 watts, says Foster.
As for the airborne system, the pilot would receive the navigational information in the same format as with GPS or other navigational aids.