GPS World

Category: Transportation

  • NDGPS Gasping

    RITA and the Coast Guard have a tough job ahead. Between them, the Department of Transportation’s (DOT’s) Research and Innovative Technology Administration and the multi-mission maritime service are trying to save a national differential GPS (NDGPS) program that faces termination next year.

    History. The U.S. Coast Guard (USCG) implemented the concept in the mid-1990s. Their requirement was for marine navigation, and the system now provides service for coastal coverage of the continental United States, the Great Lakes, Puerto Rico, portions of Alaska, Hawaii, and a greater part of the Mississippi River Basin. In a testament to NDGPS’s success, many countries around the world have duplicated the concept.

    Since the DGPS signal is broadcast in a 360-degree radius, inland users close enough to the USCG broadcasting station can receive and use the corrections. All of this happened before SA (Selective Availability) was turned off, so the accuracy improvement was staggering; from 100 meters down to 1-3 meters. Once inland users tasted the sweetness of the USCG DGPS system, a groundswell of support arose for expanding the system inland. The NDGPS system was born.

    Manufacturers began to integrate “Coast Guard” DGPS receivers into their products. Companies like CSI and Starlink offered after-market DGPS receivers to enable virtually any GPS user to receive the free DGPS signal, whether it was a $200 consumer GPS unit or a $10,000 submeter mapping receiver. Since then, tens of thousands of “Coast Guard” DGPS receivers have been sold around the world.

    Between the USCG, the Army Corp. of Engineers (USACE), and the DOT, 86 stations now blast out DGPS corrections free of charge that cover more than 90 percent of the U.S. landbase.

    Trouble. The program ran smoothly through the first half of this decade. Each year, a few new stations were added to expand coverage. The HA (High Accuracy)-NDGPS concept to provide decimeter-level positioning was proven to work. However, the rug flew out from under NDGPS last year when DOT’s Federal Railroad Administration (FRA) announced it would no longer sponsor NDGPS. NDGPS supporters had long hung their hats on Positive Train Control (PTC) as the killer application for NDGPS, as it would save the railroad industry billions per year and justify the cost of installing and maintaining the NDGPS. The DOT says the PTC doesn’t need NDGPS any longer.

    Two significant developments have reduced the need for NDGPS since the program began. The first is that autonomous (standalone) GPS accuracy is very good these days, on the order of a few meters. The second is the maturation of the Federal Aviation Administration’s Wide Area Augmentation System (WAAS).

    RITA to the Rescue. Last summer, with no FY07 budget for NDGPS, the scrambling began. The FRA washed its hands of NDGPS so the DOT transferred the program to a little-known agency called Research and Innovative Technology Administration (RITA). At the eleventh hour, RITA scraped up $5 million for NDGPS for FY07 — enough to operate and maintain the system until October 2008. $400,000 of that is allocated for “needs assessment.” In other words, they need to understand who is still using NDGPS and determine if the usage justifies future funding.

    The USCG (39 sites), USACE (9 sites), and DOT (38 sites) fund the 86 stations. USCG has said it would take over 12 of the DOT-funded sites if DOT decides not to fund the program. So the debate only involves about a third of the U.S. land mass. The USCG and USACE sites are not in jeopardy, as their requirements are considered safety-of-life for maritime navigation.

    Is there is a significant enough user base in the areas above to justify the roughly $7 million a year it takes to operate and maintain them? If there are only 1,000 users in those regions who use it regularly, that’s $7,000 per year, per user. That scenario doesn’t make sense. And it doesn’t even consider the cost of complete system build-out. Even at 10,000 users (a very unrealistic number), that’s still a cost of $700 per year, per user to the taxpayer.

    Who still uses NDGPS, anyway? That’s the magic question, and the DOT doesn’t know the answer. Presumably, determining that is part of the needs assessment, to be

    finalized in September 2007. I’ve heard speculators talk about agriculture being a big NDGPS user. While this might have been true five years ago, WAAS now dominates the ag market. Even CSI (now Hemisphere GPS), the largest producer of after-market “Coast Guard” beacon receivers and the leading GPS supplier to the ag market, has shifted its focus from NDGPS receivers to high-performance WAAS receivers.

    The forest products industry comprises some big users of GPS, but they’ve been post-processing for years, and some have even stopped doing that because autonomous GPS is sometimes good enough.

    What’s left is a fragmented group of utilities, federal/state/local government, engineering, surveyors, universities. and various -ologists. Honestly, as much traveling, conference attending, speaking engagements, and training as I do, I can’t recall the last person who told me they use NDGPS. That’s the fundamental problem.

  • GMV Buys Controlling Interest in Masisconvi

    Spanish technology conglomerate GMV has purchased a 66 percent controlling interest in Masisconvi S.A., another Spanish company that specializes in design and manufacture of electronic fare-collection systems.

    This purchase will allow GMV to incorporate Masisconvi’s wide range of electronic fare-collection systems into its own range of products in the passenger transport telematics area, where it has traditionally concentrated on GPS-based fleet management systems. This means that GMV can now offer fleet operating companies a complete, across-the-board coverage of all their possible needs in the field of information systems and communications, the company says.

    There is a growing trend of merging fleet management systems and fare-collection systems, according to GMV. Its takeover of Masisconvi will thus enable it to give a better service to its long-standing customers and develop lower-cost systems with improved service features to break into emerging markets, the company says.

    Masisconvi already has a strong foothold in some of these emerging markets, such as South America and North Africa, so its integration into GMV will not only boost its growth prospects but also improve the joint market position of both companies in the transport telematics market, according to GMV.

  • EGNOS Performs Well in Flight Trials

    The European Geostationary Navigation Overlay Service (EGNOS) recently passed flight trials in Limoges, France with flying colors, according to the European Space Agency (ESA).

    EGNOS, a venture between the ESA, the European Commission and Eurocontrol, is the first step in Europe’s satellite navigation plans, paving the way for Galileo. EGNOS supplements GPS data, offering more accurate vertical positioning data to pilots, similar to systems already in operation in the United States. The system can provide a precision of better than two meters, according to the ESA.

    In the most recent EGNOS flight trials, a French civil aviation authority test plane was specially equipped to make tests using EGNOS at an airfield in Limoges, France. It made a number of approaches and landings using the new procedures, in each case aligning itself with the runway’s axis and then following a descent path to touchdown.

    Inside the plane, which is normally used for calibration of airport systems in France, the method of analyzing the quality of the EGNOS signals was done by comparing the landing phases guided by satellite with landings using traditional means, such as the plane’s Instrument Landing System (ILS).

    The results of Limoges trials demonstrate again that EGNOS signals allow approaches and landings that meet the safety standards that govern international air traffic, the ESA says.

    One of the main advantages of EGNOS is that it is available everywhere without the need for ground infrastructure and it provides vertical guidance procedures for every runway, the ESA says. Furthermore, the cockpit data display is the same as that of ILS, so there are no familiarization problems for the pilots and no additional training costs.

    Currently in pre-operational service, EGNOS will be certified in 2008 for safety-of-life applications such as air traffic control. It will be comptible and interoperable with similar systems elswhere in the world, according to the ESA.

  • AmbientNAV Releases Marine LCD Monitors

    AmbientNAV has launched a line of marine LCDs featuring its SeaEye technology. The SeaEye electronic design reduces internal operation heat and the effects of ambient temperature, while the display technology minimizes solar thermal gain, eliminating blooming and black splotches associated with the breakdown of LCD crystals due to heat; this also eliminates the need for an internal fan. Designed for operation in rugged marine conditions, these monitors incorporate advancements in optical bonding, backlighting circuitry, optical enhancement films, and anti-glare/anti-reflective treatments to the external glass surface.

    SeaEye monitors employ a fully enclosed aluminum and stainless steel case design as well as bonded glass, both of which serve to eliminate water egress, internal condensation, screen fogging, and corrosion. SeaEye monitors also incorporate an easily accessible rotary analog control knob that allows for adjustment of the backlight from full brightness to off (less than 1 nit) for safe night operation. Unlike other monitors that dim to red at night, SeaEye monitors dim to black, minimizing confusion and loss of information when using navigational chart images with red coloration.

    “Our SeaEye technology represents a quantum leap in not only image clarity in all lighting conditions, but in overall performance and product life,” said Peter Meagher, president, AmbientNAV.

    All SeaEye-enabled monitors feature custom-designed, fully isolated power supplies with a voltage input range of 9-36 VDC, and powder-coated, anodized aluminum enclosures with a NMEA 4x rating. The thin bezel design (3/8-inch lip) reduces the footprint of the monitors. Monitors can be mounted with either a quick clamp mount for rigid flush mounting, or the VESA and RAM mount points located on the back of the monitor.

    The new AmbientNAV SeaEye product lineup comprises two 15-inch and two 17-inch LCD monitors, available in both daylight-viewable models (MDDL) and enhanced sunlight-viewable models (MDSL), with multi-port inputs including, RGB, DVI-D, S-video and composite video. The 15-inch monitors offer a contrast ratio greater that 500:1, 160-degree-plus viewing angles, support for display resolutions ranging from 640×480 to 1280×1024 (1024×768 native). The 17-inch models offer a contrast ratio greater that 700:1, 170-degree+ vertical and horizontal viewing angles, and support for display resolutions ranging from 640×480 to 1280×1024 (1280×1024 native). All monitors have an operating temperature range of 0-60 degrees Celsius, and an amperage draw of 1.5 amps (daylight models) and 2.5 amps (sunlight models) at 12 volts.

  • NDGPS Abandoned?

    The Federal Railroad Administration has relinquished responsibility for national differential GPS (NDGPS), saying that it no longer supports NDGPS for positive train control. DOT’s Research and Innovative Technology Administration (RITA) now has sponsorship of NDGPS, and will hold a meeting on October 4 to assess.

    The outlook for NDGPS appears bleak. “Without funding, the system faces tough choices.” Options reportedly under consideration include moth-balling the system for $2 million or shutting it down for $10 million. NDGPS supporters claim that a large mass of users of NDGPS have not yet been heard.

  • Troubleshoot Before Takeoff

    Modeling and simulation, two separate but related activities conducted prior to flight tests of high-performance military navigation systems, can reduce costs, shorten timelines, and remove some uncontrollable variables from the process, to deliver more accurate, verifiable results.

    Flight tests on an outdoor range provide engineers with an abundance of data to improve performance of weapon navigation systems. Whether the guidance package is hosted on a fighter jet, a cruise missile, an artillery shell, or a hypersonic kill vehicle, the navigation performance of a weapon during flight trials closely reflects its suitability for combat. The roar of jet engines, the percussion of a gun barrel, live video feeds, or a radio link to the cockpit can make live flight tests exciting — and sometimes unpredictable.

    While these activities are critical for weapon system qualification and validation, live tests nonetheless have significant limitations. Lengthy flight trials can be quite expensive. The rising price of aircraft fuel combined with test-range costs, travel expenses and data analysis can easily exceed millions of dollars for extended operational test and evaluation (OT&E). Also, test results can vary widely between trials due to a number of uncontrolled variables such as flight dynamics, RF interference, and atmospheric effects. Finally, the tests themselves often involve safety and technical risks and usually require careful planning and coordination, often months in advance.

    For these and other reasons, military navigation system flight tests are generally limited in number and are preceded by extensive modeling and simulation in the lab, where a methodical and disciplined test approach can produce substantial savings and more detailed insight. Successful completion of these activities provides system developers and integrators with the confidence required to proceed to the next stage of testing. This phased approach is useful for a broad range of weapon systems including GPS-guided ground-, sea- and air-launched munitions; high-performance aircraft; and long-range interceptors.

    Model, Then Simulate. Evaluating weapon navigation systems in the lab prior to outdoor testing usually consists of two sequential activities. First, performance of the GPS/inertial navigation system (INS) throughout the flight trajectory is modeled in software using representative data for the satellite constellation, RF signal environment, body masking, antenna pattern, receiver processing algorithms, and other parameters.

    Second, the GPS/INS flight hardware is exercised with controlled inputs to collect data relating to the operation of its code- and carrier-tracking loops, its susceptibility to interference, and the performance of the navigation filter under simulated highly dynamic conditions. Changes in vehicle velocity along its three axes are represented by inertial delta-V (ΔV) terms that are delivered to the INS navigation filter, while changes in attitude are delivered as delta-q (Δθ) terms. For normal flight operations, accelerometer and gyro sensors within the INS measure and report these forces, but in test mode the inertial simulator PC generates these terms based on translational and rotational motion described within the simulated trajectory. Computing these terms involves mathematical translation between the sensor frame and the vehicle body frame, and includes all errors due to sensor bias, drift, and misalignment. The inertial simulator PC delivers these terms to the INS via appropriate electrical interface.

    The fighter aircraft flight discussed here is based on a flight model overlaid on a map of the Fort Worth, Texas, area (FIGURE 1).

    Figure 1 Simulated tactical fighter aircraft flight trajectory over north Texas
    Figure 1 Simulated tactical fighter aircraft flight trajectory over north Texas

    Mod/Sim Process

     

    Before discussing the simulation results, it’s important to understand modeling and data analysis in its proper context. Disciplined performance analysis is like the scientific method — it begins before the test is initiated and concludes by validating preliminary assumptions using results of the experiment.

    Analysis of military navigation system performance begins with software modeling as shown in FIGURE 2, first panel. In this example, software tools predicted, among other things, SV carrier-to-noise values; azimuth and elevation angles; constellation dilution of precision (DOP) values; and GPS receiver acquisition/tracking performance. If required, software may also predict jammer-to-signal (J/S) ratios at the weapon GPS antenna.

    Figure 2 Navigation system performance analysis
    Figure 2 Navigation system performance analysis

    The next step (second panel in Figure 2) enhances software models with hardware-in-the-loop (HWIL) simulations, replacing predictions with actual performance data. Conclusions from RF and computer simulation stage will often aid significantly in field-test set-up. Benefits of simulation include isolating design flaws, validating algorithm performance, and exercising the GPS/INS system with realistic and stressful inputs. These activities may reduce the number of flight trials required, which in turn reduces the potential for program cost and schedule slips.

    Modeling Results

     

    FIGURES 3 AND 4 show predicted results from the commercially available Navigation Tool Kit (NavTK) regarding the navigation performance of the unaided GPS receiver within the fighter aircraft during its highly dynamic flight. STK generated the 6-degree-of-freedom (6-DOF) trajectory used in this test. This trajectory is based on the dynamic limits of the aircraft; representative roll/pitch/yaw rates; typical climbing; cruising and descent speeds; takeoff and landing distances, and so on. This data was subsequently passed to NavTK. Figure 3 shows number of satellites tracked throughout the flight while Figure 4 predicts East-North-Up errors given actual GPS performance (clock data, ephemeris data, and system performance) for July 1, 2005. This detail is possible because NavTK can accept archived GPS constellation data from the GPS Operations Center (GPSOC) to fully represent the effects of geometry, satellite health, ephemeris errors, and clock variations for individual SV signals.

    rder=”0″ align=”left” class=”content_image” src=”/files/gpsworld/nodes/2006/6535/i4_t.jpg” alt=”Figure 3 Number of signals tracked” />
    Figure 3 Number of signals tracked

    Figure 3 highlights flight portions when loss of lock is expected due to platform orientation and accelerations. When the aircraft initiates a steep turn, the accelerations cause cycle slips that result in loss of lock. These results are useful for isolating discrete periods of time during which performance of the INS-only solution can be scrutinized during hardware simulation. At these times the INS navigation filter relies exclusively on ΔV and Δθ terms from the inertial simulator to compute a fix.

    Figure 4 Predicted navigation errors
    Figure 4 Predicted navigation errors

    When PRN 25 disappears below the horizon approximately 25 minutes into the flight, we see a noticeable shift in predicted accuracy. This suggests that DOP values may worsen noticeably at this point, or that clock/ephemeris errors for this satellite had a noticeable effect on the GPS-only solution. These results depend on inputs provided for the receiver models, constellation behavior and tracking loop algorithms, and provide a start point for pre-HWIL evaluation. Again, results from this modeling exercise highlight a unique moment in time in which the performance of the GPS/INS navigation filter can be tested with a hardware simulator.

    Software modeling tools can also provide valuable inputs to the simulation process with trajectory modeling. STK can create flight simulations for military applications including missiles, rockets, spacecraft, and airborne platforms. In this example, software created a 6-DOF fighter trajectory which was stored as a file on a SimGEN PC.

    GPS/INS Simulation

     

    HWIL GPS/INS simulation of many military flight trajectories consists of generating a composite GPS RF signal synchronized with inertial ΔV and Δθ terms. Precise synchronization is achieved via a 1 PPS interface between the RF signal generator and the inertial simulator PC. Latency of the IMU motion data is also specified in microseconds. Together, these RF and inertial inputs are delivered to the navigation system under test.

    FIGURE 5 illustrates the GPS/INS test equipment in more detail. Platform dynamic motion (in this case from STK), GPS constellation data, signal parameters, and GPS/INS error sources are defined within the simulation application (SimGEN). Signal control commands are issued continually from the simulator PC to the RF signal generator via IEEE-488 (GPIB). The RF signal is typically connected to the GPS receiver antenna port via coaxial cable. INS ΔV and Δθ terms are calculated to the vehicle trajectory on a separate PC (SimINERTIAL) and are delivered to the GPS/INS device in test mode via the appropriate interface. The SimINERTIAL PC and signal generator are synchronized via a 1 PPS signal. Barometric altimeter aiding and instrumentation are achieved via MIL-STD-1553B.

    Figure 5 GPS/INS simulation block diagram
    Figure 5 GPS/INS simulation block diagram

    Realistic simulation requires many unique components of the navigation solution to be carefully specified in advance. These include but are not limited to:

    • 1. Trajectory
    • 2. Antenna pattern
    • 3. Constellation geometry
    • 4. Codes and frequencies
    • 5. G-sensitivity
    • 6. Gyro and accelerometer errors
    • 7. Atmospheric effects
    • 8. Multipath
    • 9. Interference

     

    Trajectory. As mentioned earlier, a flight trajectory is usually expressed as a series of time-stamped translational and rotational motion terms. Rotational motion is typically described by Euler angles or quaternions. The GPS/INS simulator uses this trajectory as the basis for calculating SV pseudoranges, Doppler values, and inertial delta-V and delta-q terms. This flight trajectory can be delivered to the simulator remotely via a high-bandwidth interface such as IEEE-488, SCRAMnet or TCP/IP, or, as in this example, may reside as a file on the simulator PC hard drive. Flight times may range from a few minutes to several hours or even days.

    For this simulation, the 45-minute flight over Fort Worth includes take-off, landing, and several steeply banked high-g turns. The airspeed for the majority of the flight is approximately 500 mph.

    Antenna Pattern. If the simulator’s RF signal is injected directly into the antenna port of the GPS/INS system under test, the gain and phase characteristics of the receive antenna must be modeled. For best results, the effects of the body on which the antenna is mounted must be included. Gain and phase pattern measurements are typically collected in an anechoic chamber or outdoor range and are generally available to test engineers evaluating GPS/INS system performance.

    Constellation Geometry. A simulated flight can occur at any time or location to test the effects of constellation geometry on the navigation solution. Historical GPS almanac information is available from the GPSOC and can be loaded into the simulation scenario. This example used historical almanac parameters from July 1, 2005.

    Codes and Frequencies. Most U.S. military GPS/INS systems operate at both L1 and L2 frequencies and track the encrypted P(Y) and unencrypted C/A codes. Future military receivers will also track the M-code on L1 and L2. To fully evaluate unique features of the weapon navigation system including direct-Y acquisition, SAASM functions, and resistance to interference, the simulator must support all modes of classified and unclassified operation.

    G-Sensitivity. Acceleration force can alter the performance of a crystal oscillator. Specifically, changes to the local acceleration vector stemming from high-g maneuvers will change the output frequency of the oscillator. Airframe vibrations may cause frequency modulations of the oscillator output that affect the performance of the tracking loops. To a certain extent, the effects of these forces on the code- and carrier-signal offsets can be modeled. For this scenario, the oscillator G-sensitivity is specified as 2 × 10-11 sec/sec/G per axis, typical for a military-grade oven-controlled crystal oscillator (OCXO).

    Gyro and Accelerometer Errors. Note that GPS/INS simulation does not test the performance of the sensor elements themselves; this exercise is generally conducted with mechanical spin tables or centrifuges. Rather, military INS manufacturers typically include a test interface that allows simulated motion terms to be directly injected into the navigation system. This allows test engineers to evaluate the performance of the Kalman filter during high-dynamic or stressed conditions. For one manufacturer’s system, this in
    terface is proprietary to the Inertial Sensor/Recorder Simulator (ISRS-2); another’s systems accept these test inputs via RS-422. A non-proprietary inertial interface known as NATO Standard Agreement (STANAG) 4572 also accepts simulated ΔV and Δθ terms via RS-422.

    Embedded gyro and accelerometer sensors are imperfect and are subject to a variety of errors. Stochastic errors may be modeled by random walk or by a Gauss-Markov noise estimator, while deterministic errors such as biasing, axis misalignment, and scale- factor errors should also be specified. For this simulation, a second-order Gauss-Markov model is used as described in STANAG 4572 for gyros and accelerometers with unique values for the natural frequency, damping factor, and power spectral density.

    Atmospheric Effects. After Selective Availability was set to zero in May 2000, the primary contributor to GPS ranging error remains ionospheric delay. Dual-frequency receivers can effectively eliminate this effect by processing both carriers, but range measurements at L1 and L2 are still subject to multipath errors. Furthermore, ionospheric scintillation is a complex phenomenon that can affect the tracking performance of dual-frequency receivers. This simulation specifies the historical broadcast Klobuchar coefficients for July 1, 2005, and modifies the SV code and carrier offsets to approximate the effects of the ionosphere for the time, date, and location of the scenario.

    Tropospheric delay varies by latitude and elevation angle and is a function of temperature, humidity, and atmospheric pressure. For more realistic results, the effects of this ranging error can also be simulated. For this simulated flight over Texas the surface refractivity index is set to 324.8.

    Multipath. While the aerial environment is generally free of the same satellite obscuration effects more common on the ground, signals may still be reflected off wings, stabilizers, and flight control surfaces. These path delays affect the performance of the GPS receiver correlators and may introduce position and velocity errors into the navigation solution. GPS simulation allows the tester to specify reflective effects including fixed-delay multipath, Doppler multipath, and vertical plane multipath on a per-satellite basis.

    Interference. Over the last 10 years a variety of techniques have been developed to mitigate the effects of intentional interference on military navigation systems. Adaptive antenna arrays may steer nulls in the direction of jammers or beams in the direction of satellites. Spectral filtering divides the L1 and L2 bandwidths into discrete bins in which interference can be excised using fast Fourier transform (FFT) techniques. Polarization filters attempt to reject interference from sources that are not right-hand circularly polarized (RCHP). Regardless of the mitigation technique, the performance of antijam GPS systems is generally evaluated in the lab prior to conducting flight tests.

    Simulation allows the tester to specify a variety of interference waveforms including broadband Gaussian noise, continuous wave (CW) and swept-CW, AM, FM, and coherent jammers. The relative signal strength of these sources will vary as a function of the distance between the navigation system under test and the jammer.

    HWIL Simulation Results

     

    Military navigation engineers are usually interested in the following performance characteristics:

    • 1. Accuracy
    • 2. Availability
    • 3. Resistance to interference

     

    FIGURE 6 shows example results that can be obtained using this equipment to test INS devices under these simulated highly dynamic test conditions. The results presented here are for illustrative purposes only and show the type of data obtainable to fully characterize the performance of a specific INS under all possible conditions. In this example, latitude (black), longitude (green), and height (blue) errors are plotted separately. Discontinuities in the calculation of latitude and longitude may be observed when high-g turns are initiated. Errors in the height component may correspond to rapid climbs and descents. RMS error for this entire illustrative mission was slightly higher than 16 meters. Again, these results are based on the specific inputs for this simulation. Navigation engineers would need to determine whether similar inputs represent their own flight environment and whether these results will satisfy their operational objectives. In practice, results will vary as a function of simulated gyro/accelerometer errors, oscillator g-sensitivity, pre-flight calibration time, flight dynamics, barometric aiding, and SV visibility.

    Figure 6a INS simulation results: latitude upper left, longitude upper right, height lower left
    Figure 6a INS simulation results: latitude upper left, longitude upper right, height lower left

    As expected, availability of the navigation solution was 100 percent. In contrast to the GPS-only performance modeled in Figure 3, the INS solution used ΔV and Δθ inertial inputs to compute a solution whenever a GPS fix was temporarily unavailable.

    Figure 6B Close-up look at latitude (black), longitude (green) and height errors (blue) from lower right panel
    Figure 6B Close-up look at latitude (black), longitude (green) and height errors (blue) from lower right panel

    Intentional RF interference was not created for this HWIL simulation. However, adding this component to the RF signal is possible with additional equipment, and a similar chart could be created to illustrate the effects of jamming on the INS solution.

    In addition to this simple error plot, truth data from the GPS/INS SimINERTIAL simulator were also delivered in WGS-84 ECEF coordinates to STK for graphical performance analysis. Time-stamped latitude, longitude, and height data extracted from the military GPS/INS MIL-STD-1553B interface were also provided to STK. Several key performance parameters were then analyzed including carrier-to-noise levels (C/N0), tracking states, pseudorange error per channel, and instantaneous position/velocity errors. FIGURE 7 and the opening graphic compare truth location at a single moment in time as prescribed by the simulator (gray aircraft) versus the actual GPS/INS computed location (color-coded per figure of merit, or FOM). STK can be used to visualize these outputs in real time. Instantaneous position error is projected on the red vector between the aircraft centers of gravity. Vectors pointing toward each SV tracked may be color-coded to J/S, while line type may be selected based on tracking state. Instantaneous pseudorange error for each channel is also provided. Many other data items can also be displayed graphically or in text.
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    Figure 7 INS simulation results
    Figure 7 INS simulation results

     

    Conclusion

     

    Field tests generally provide the most indisputable evidence of military navigation system performance. When a GPS/INS-guided gravity weapon is released above a test range and impacts the ground near its target, we may physically measure the miss distance. We may also assume a similar weapon released at a future time under similar conditions will achieve approximately the same results. But how can we be sure? One approach would be to perform hundreds of trials, if costs and schedules were of no concern.

    However, this is rarely the case. GPS receiver performance models used with the HWIL SimINERTIAL simulator allow the test engineer to minimize flight tests while achieving the same level of confidence. As we have shown, modeling and simulation activities may be tailored for a specific objective — for example, highly dynamic aircraft motion with a fixed satellite constellation geometry while using inertial sensors with known bias and drift characteristics. This simulation may be valuable in conducting analysis of alternatives for different user equipment mixes (receivers, antennas, inertial sensors, platforms, and so on) or to ensure the system meets requirements for all missions.

    Other modeling and simulation objectives may differ from those described in this article. FIGURE 8 enhances our aircraft flight test model to include intentional jamming, an advanced digital antenna, and future modernized GPS space capabilities. Similar enhancements can be added to the RF simulation.

    Figure 8 Real-time 4D data display
    Figure 8 Real-time 4D data display

    Whatever the application, a variety of commercial products allow navigation engineers to prepare for live flight trials on the test range. GPS receiver performance analysis models can be used to create specific GPS/INS simulations that focus on key performance characteristics and environmental challenges. Graphical analysis software can simplify the task of combing through thousands of data records to isolate specific periods during which performance is most important. With careful planning and a disciplined test approach, military and defense industry engineers can use these tools to ensure their weapon systems will deliver at the most critical times on the battlefield.

    Manufacturers

     

    Analytical Graphics, Inc. developed the Navigation Tool Kit and STK commercial software products used for this simulation. GPS/INS test results were collected using the Spirent GSS7700 simulator with SimINERTIAL.

    CURTIS HAY is director of GPS business development for Spirent Federal Systems. He has master’s degrees in electrical engineering and business administration and served eight years as an officer in the United States Air Force in a variety of GPS-related assignments.

    COLIN FORD is a senior software engineer at Spirent Communications and is the lead architect for the SimINERTIAL navigation test suite.

    GREG GERTEN supports satellite navigation and electronic warfare programs for Analytical Graphics, Inc. He received a master’s degree in electrical engineering from the University of Dayton and has completed graduate courses in GPS at the Air Force Institute of Technology.

  • GPS and Aviation Safety

     

    U.S. federal agencies, aircraft and avionics manufacturers, airlines, and research centers are brainstorming ways to prevent a repeat of the tragic events of September 11, in which highjacked aircraft were used as missiles. Under these new circumstances, options previously dismissed out of hand suddenly are attracting renewed attention.

    One recurring proposal is to automate the landing of hijacked aircraft. In this scenario, a “dead-man switch” would allow the pilot to turn over navigational control to an on-board GPS-based autolanding system. The system would broadcast a mayday to air traffic control (ATC), search an on-board database for the nearest suitable airport, alert that airport, receive landing authorization, and land the aircraft there. During these operations, no one on board would be able to regain control of the aircraft. The pilot would be like an employee who, when confronted by a robber, does not have the combination to the company’s safe. No amount of violence on board would allow hijackers to use an aircraft as a missile against a target.

    Technologically feasible. The strong consensus of airline, industry, and academic experts interviewed for this article is that the above scenario is technologically feasible. In fact, the autolanding technique has been amply demonstrated and at least one major avionics manufacturer is actively working on producing an emergency landing system.

    The Federal Aviation Administration (FAA) is working on two GPS-based systems that could enable this sort of antihijacking capability: the Wide Area Augmentation System (WAAS) that will enable aircraft to reach the so-called Category 1 decision point in an approach to an airport, and the Local Area Augmentation System (LAAS) that would enable aircraft to reach the ground in zero visibility, known as a Category 3B landing. The agency plans for many airports to be equipped with LAAS transmitters eventually and will require WAAS/LAAS systems on commercial airliners.

    Although both systems still await final certification, testing, and installation at U.S. airports, commercial airliners and military aircraft have already demonstrated fully automatic instrument approach and landing under Category 3B conditions.

    Features

    Although technologically feasible, operational considerations pose obstacles for implementing an automated emergency landing system. The following scenarios address some of these issues as well as technical features of such a system:

    1. A “multiple key” arrangement could restore manual control with codes from the pilot, the co-pilot, and the ground-based ATC operators. Ground control would con- tribute its code only when absolute sure that the aircraft could not be used to attack a population center.
    2. To protect it from being disabled, the system would require a hardened compartment not accessible from the cabin and an autonomous power source not controlled from the cockpit circuit breaker panel.
    3. Prior to landing, the onboard system would notify ATC, which, in turn, would alert and re-route other aircraft as needed.
    4. If the highjackers jammed the GPS signal, the system would put the plane in a holding pattern until it reacquired a clear signal. By refusing to turn off the jammer, terrorists could force the aircraft to run out of fuel and crash – but could not guide it to a target.
    5. According to an industry source, the system should first put the aircraft in a holding pattern in any case, to give a chase plane time to reach it and visually monitor it. In the very unlikely case that the highjackers were able to regain control of the aircraft and aim it toward a target, the chase plane could challenge the aircraft, order it to land, and shoot it down if it did not comply.
    6. The airport database would need to include data on possible flight path obstructions – terrain or tall buildings – so that the system could select a clear approach path. Avionics systems coming onto the market that are designed to prevent controlled flight into terrain essentially have this capability now.
    7. The autolanding system would require permission from the ground to land on a particular runway. If permission were denied for any reason, the system would search its database for the next-best runway.

    Cockpit philosophy. An airline pilot who is now an aide for the operations chief of a major airline reacted very negatively to the idea of an emergency autolanding system that could not be disengaged by the pilot. Any system that restricts the crew’s options, he said, clashes with a key tenet of “cockpit philosophy”: to keep the pilot in charge and never relinquish control of an aircraft completely to automation.

    An emergency autoland system also conflicts with a basic principle of aeronautical engineering – namely, that an aircraft should have multiple, redundant ways to control it.

    However, in extreme emergencies, the alternative may warrant overriding such concerns, according to Bradford Parkinson, a professor emeritus at Stanford University’s School of Engineering who first proposed fully automated cargo planes years ago. He points out that, although an antihijacking system used routinely would have to be extremely reliable, when the alternative is a 100 percent probability of death for all aboard, “Boy, that sure changes the equation in a flash.”

    Further reading: “Soft Landings: Navy Proves Hands-Off Touchdown,” by Matteo Luccio and Glenn Colby, GPS World, August 2001.