System integrators have come to rely on the centimeter-level positioning accuracy made possible with real-time kinematic (RTK) commercial GPS receivers. Many authorized defense customers rely on access to the Precise Positioning Service (PPS) for single-point positioning. The OEM625S will combine a commercial dual-frequency NovAtel GNSS receiver with an L-3 IEC XFACTOR SAASM in a single card solution, reducing overall size and power requirements for end customer applications.

The OEM625S will maintain NovAtel’s OEMV-2 form factor, ensuring a successful drop-in replacement and backward compatibility for existing customers. Integrators can continue to use their existing user interface, which will be enhanced with OEM625S logs and commands for SAASM functionality.

NovAtel’s well-established, comprehensive set of software commands facilitates system integration, NovAtel said. The SAASM position is provided via a dedicated communication port, as well as through NovAtel’s software command protocol, allowing for maximum flexibility.

“For the past 17 years NovAtel’s customers have enjoyed great success in integrating our OEM family of high-precision receivers into a wide array of defense applications,” stated Graham Purves, executive vice president of NovAtel. “Adding the L-3 XFACTOR SAASM to our receiver card will allow defense customers to continue to use our products in the most demanding military environments.”

Ric Pozo, general manager of L-3 IEC’s Navigation Systems business unit, commented, “We are pleased to collaborate with NovAtel and provide the warfighter this highly flexible and capable GPS SAASM product. Our combined teams are looking forward to bringing this one-of-a-kind solution to market.”

NovAtel will accept orders for the OEM625S from authorized customers starting in the third quarter of 2012.

A new hardware assessment tool automates testing and mission replay, managing military GPS receiver input and output data, with an operational implementation and with a better control of initialization conditions, especially direct P(Y) acquisition. The test bench drives a GPS/Galileo simulator, a digital jammer, and software programs for visibility computation based on terrain modeling, and for multipath generation on 3D renderings.

Comprehensive assessment of military GPS receivers becomes more complex as they are integrated into advanced systems. To limit testing on systems under live conditions, laboratory evaluations with real elements are essential.

A new hybrid test bench called Statistical INERtial Gnss HYbrid in Simulation (SINERGHYS) is designed for governmental use to validate the integration of GPS/Galileo receivers within the navigation system for different platforms. As system-level requirements become more stringent, this bench has been designed to assess the behavior of the complete system in an operational context.

This new assessment hardware-in-the-loop tool is designed to automate testing and to replay missions with an operational implementation and with a better control of initialization conditions, especially direct P(Y) acquisition. This test bench drives many simulation tools: a GPS/Galileo simulator, a digital miniaturized jammer, and different softwares such as one enabling the computation of visibility depending on the terrain modeling, or one dedicated to the generation of multipaths on surfaces of realistic 3D scenes.

Figure 1. Depiction of SINERGHYS.

Figure 2. Focus on the bench.

A Common Bench. Since 2000, with the arrival of the new cryptographic generation (the selective availability anti-spoofing module, or SAASM), the French government defence procurement agency (DGA) GPS laboratory decided to buy off-the-shelf GPS SAASM receivers that cover different form factors and applications. To test performance, it was necessary to acquire a test bench suitable for each GPS receiver. Testing procedures became more and more complex, and most of the manufacturer-provided benches could not perform every test required, such as direct P(Y) acquisition. To improve French expertise concerning GPS receivers, the DGA GPS laboratory decided to develop a common, generic test bench taking into account the integration constraints of each receiver. The perimeter of the hybrid test bench consists of a PC and a generic GPS test bench.

Figures 3 and 4 show examples of military GPS receivers integrated into the bench.

Figure 3. MPE-S (Ground-based application, Rockwell Collins).

Figure 4. 1000S (Avionics,Thales).

Figure 5. Embedded jammer.

Figure 6. Jamming environment for a fighter aircraft. (Click to enlarge.)

Bench management is centralized, so test conditions are generic, and all simulation parameters are fully controlled. This enables users to display a unique view of the complete information and to be able to replay specific scenarios.

The bench manages military GPS receivers’ input and output data as described in the respective receivers’ interface control document (ICD) or interface specification: this enables, for example, the initialization of GPS receivers by sending precise time to facilitate direct P(Y) acquisition. This new bench is compatible with many GPS receivers with different form factors and applications.

Several receivers can be tested at the same time with the same software, so that the behavior of the GPS receivers can be compared in real time. Data from the different receivers can be observed on the same window of the graphic user interface (GUI). Specific data from ICDs can be displayed on the GUI. The user can visualize three different windows: the first is related to integrity, the second to alarms, and the third to cryptography. All the data output by the receivers can be recorded and replayed.

To facilitate and enhance trials on GPS receivers, the bench can use a Monte Carlo method, enabling sequentially and automatically chained scenarios, up to 10,000 test sequences, primarily for characterization of time-to-first-fix (TTFF).

Inertial navigation system (INS)/GPS hybridization in real time can be simulated via processing based on a Kalman filter of the information delivered by simulated INS and GPS. Loose and tight coupling can be selected through the GUI as well as filter parameters. The Kalman filter design is independent from the receiver and from the type of trajectory simulated. The user can decide whether the GPS receiver does receive aiding either from the simulated INS, or from the optimal navigation (output of Kalman filter).

Interfaces

The bench can interface with various external means and drive some tools and materials involved in the functioning of the bench.

With GPS Simulator. In the interface with the simulator, an intuitive GUI facilitates scenario preparation. When ready, SINERGHYS begins to drive the GPS simulator in remote-control mode. Any type of trajectory can be simulated with its operational environment modeled. The simulator outputs an RF signal to the receiver, and representative aiding, if required, by ethernet protocol to SINERGHYS.

With Jammer. Two types of interference signal generators can be used with the bench. Any available waveform can be generated. The bandwidth can go up to 20 Mhz for one generator and up to 80 Mhz for the other.

SINERGHYS is also compatible with a specific jammer called Embedded Jammer, designed to test vulnerability of GNSS systems (Figure 5).

The GPS receiver under test tracks the real GPS satellites combined with the simulated jamming signal. Thanks to the position and attitudes provided by the aircraft and to a modelized antenna diagram, the jammer computes in real time representative jamming that would be generated by real jammers.

This jammer works in two modes: localized mode (coordinates, jammer power, and waveform) and power profile mode. It was initially designed to be used inside an aircraft but can be used for laboratory testing as well.

The simulated environment is defined in the configuration software: waveform, emitter, scenario definitions (bands, number of emitters), and antenna diagram.

Four GNSS bands can be selected: GPS L1 and L2 (40 MHz) and Galileo E6 (40 MHz) and E5 (90 MHz). The embedded jammer can generate up to 14 simultaneous jammers per band, each with different waveforms. Therefore, up to 56 simultaneous jammers can be simulated.

The center frequency of the jamming signals can be chosen anywhere in the bandwidth. Modulation examples: continuous wave, broadband noise, binary phase shift keying), binary offset carrier (x,y), and so on.

Figure 7. Modulation examples.

External software interfaces fall under three categories.

Warfare. Electronic warfare software, which provides jamming coverage, performs a precise assessment of propagation (reflection and diffraction) of the interfering signals (depending on terrain modeling). Interference levels are transmitted to SINERGHYS during pre-processing.

Figure 8. Warfare GUI.

Satellite Tool Kit (STK). This software is designed to provide sophisticated modeling and visualization capabilities and  performs functions critical to all mission types, including propagation of vehicles, and determination of visibility areas and times. STK generates paths for space and ground-based objects, such as satellites, ships, aircraft, and land vehicles. STK also provides animation capabilities and a two-dimensional map background for visualizing the path of these vehicles. Within SINERGHYS, STK is used for real-time visualization.

Figure 9. STK GUI.

Ergospace. This software is designed to generate multipaths, enabling the modeling of reflected paths of different satellite signals on surfaces of realistic 3D scenes. Pre-processed multipaths are sent to SINERGHYS and generated by the GPS simulator. The software is also used for real-time visualization.

Figure 10. Ergospace GUI.

Figure 11. Example of the window showing the general state of the GPS receiver (c/n, svid, gram receiver and channel states, code and frequency tracked).

Operational Mission Characterization

The bench can evaluate and characterize receiver performance in most possible representative conditions.

Management of GPS Inputs/Outputs. Both black and red keys can be loaded inside the GPS receivers in both DS101 and DS102 protocols. This loading can be performed manually through key loaders such as KYK13 or DTD/ANCYZ10, but also through the host application with hexadecimal keys.

The bench can send commands to GPS receivers such as non-volatile memory erasure command, INS, precise time source, precise time and time interval (PTTI) activation commands, or choices between “mixed mode” and “all Y,” between “L1 primary” and “L2 primary,” and so on. Depending on user requirements, the bench can provide time, position, speed, almanac, ephemeris, or specific navigation sub-frames.

To test the jamming resistance of GPS receivers, it is essential to be able to provide INS aiding. SINERGHYS uses perfect or degraded aiding and adapts the format or the frequency for the considered GPS receiver.

Direct P(Y) acquisition functionality is an important case that needs to be evaluated. The GPS receiver needs a precise time to perform direct P(Y) acquisition. The time accuracy, from a few nanoseconds to several milliseconds, has a strong impact on the GPS behavior. A special delay box applied to the pulse-per-second signal of the GPS simulator in accordance with PTTI message (that is, time figure of merit), enables such a simulated accuracy.

A standard IS 153-like interface was developed to display GPS data on a convenient GUI in order to have a common software to visualize output data from the GPS receivers. The user can also visualize some specific data from GPS ICDs concerning integrity, alarms, and cryptography.

All receiver output data are recorded for later analysis.

Table 1. Example of Direct P(Y) acquisitions in accordance with time uncertainty (with times to get “GRAM state 5” and “protected status”).

Monte Carlo Trials

The bench enables sequentially and automatically chaining scenarios (up to 10 000 test sequences) to perform statistics on acquisition times. Indeed, it is primarily used for the characterization of TTFF. GPS signal acquisition is dependent on many different parameters, as described in Figure 12. To properly characterize receiver acquisition times requires a large number of tests. The comparison with GPS Receiver Applications Module requirements can be easily performed.

Figure 12. Setup parameters to study GPS signal acquisition.

Figure 13. Example of a random selection for the position error.

One Monte Carlo trial consists of a repetition of unitary test: powering the receiver, then sending to the GPS receiver random errors of position, speed, time, levels of jamming, and finally stopping the test sequence on trigger. At the end of Monte Carlo trials, statistical computing enables accurate analysis and expertises.

The random selections are optimized to reduce the number of cases. The bench can replay a particular case: as the seeds are deterministic, a special case of Monte Carlo method can be selected and replayed.

Real-Time INS/GPS Data Fusion

The information delivered by INS and GPS are processed by a Kalman filter. The INS trajectory is provided by the simulator or by an external file.

Two types of coupling are considered: loose coupling with position and velocity information, and tight coupling with pseudoranges and delta ranges to estimate errors. In both cases, the GPS receiver receives aiding from either the simulated INS or the optimal navigation (Kalman filter output).

Figure 14. Example of an optimal navigation along a specified trajectory in a jamming environment.

Figure 15. Position and velocity errors and navigation corridor.

The purpose of the Kalman filter is to estimate the navigation errors (position, velocity, and attitudes) and sensor errors (INS, GPS).

The filter design is original because it is independent from the receiver under test and from the type of application (hardiness privileged with reference to jamming). It is also able to estimate the time offset between position and velocity measurement on any GPS receiver under test.

Conclusion

SINERGHYS combines several resources into a single test bench. A complex mode can simulate an operational implementation with different interfaces and by chaining test sequences: receiver initialization, management of the switching of antenna patterns during a simulation, masking of GPS signals, management of jamming, INS/GPS data fusion, and so on. In this mode, missions can be replayed in a realistic environment. This bench is a complementary resource for flight trials and digital models because it can characterize the initialization phases with a good control of initial conditions. SINERGHYS enables users to know, as precisely as possible, the capabilities and limitations of a specific global navigation chain.

Manufacturers

SINERGHYS was developed by Bertin Technologies and specified by the French Ministry of Defense (MoD)DGA Information Superiority. It drives a Spirent GPS/Galileo simulator, Agilent 4431B and MXG generators, and software programs such as Analytical Graphics, Inc. (AGI) Satellite Tool Kit and Ergospace 3D scenes. The embedded jammer was developed by Ineo Defense in 2010 to MoD-DGA specifications.

Stéphane Gallot works at the French MoD (DGA Information Superiority) as a radionavigation expert. His particular interest is the integration of military GPS receivers including SAASM modules within French platforms.

Pascal Dutot is an architect engineer at the French MoD (DGA Information Superiority). His main activity is to optimize and control GPS integration in the global navigation chain.

Christophe Sajous works at the French MoD (DGA Information Superiority) as a radionavigation expert. He is also responsible for the “navigation per satellites” laboratory within the radionavigation department.

Jules McNeff

By Jules McNeff

An old adage says, “Be careful what you wish for, you might get it.” That is particularly relevant in today’s world of GPS and the positioning, navigation, and timing (PNT) dependencies it has created. In business, it’s all about location, and in military circles, something called real-time situational awareness, driven by the ready availability of PNT from GPS. However, it has been reported (and validated by experience) that U.S. soldiers believe that the GPS equipment they are issued through official channels is too big, too heavy, uses too many batteries, and is old-looking and not sexy like the multi-color, multi-app personal electronics and smart phones they are accustomed to at home.

Furthermore, they reportedly feel encumbered by Department of Defense (DoD) policies that require the use of encrypted military GPS signals when executing combat mission command-and-control or performing combat-related actions such as synchronizing tactical networks, designating targets, and calling for fire support when in contact with an adversary force. They wish they could just use their iPhone, or iPad, or similar smart device with its integral location-based apps and ready communication capabilities, and not have to deal with what many see as obsolescent gear and antiquated policies. Unfortunately, were that wish to really come true across the joint force and mission domain, it could have disastrous and deadly consequences.

This is not intended to be a defense of the DoD requirements and acquisition processes, for there is much that could be improved within both. Adherence to those processes in the procurement of PNT equipment means that it will take longer to develop and produce the equipment than comparable commercial units, and that the equipment will probably be heavier and less user-friendly than commercial products.

However, those processes exist and are rigorously followed, first because they are required by statute, but also for practical reasons of justifying investments of taxpayer resources and ensuring as much as possible that whatever is procured will withstand the rigors of service in its intended military application. For GPS equipment, this includes not only the rigors of the physical environment but also those of the electronic environment, including threats of both unintentional and hostile interference and signal imitation. It is precisely that threat environment that presents the greatest danger to reliance on commercial GPS products in military applications.

The U.S. military and coalition forces have been fortunate from a PNT perspective over the last couple of decades in facing relatively unsophisticated adversaries with either limited access to or limited desire to routinely employ PNT countermeasure technology. Consequently, we have seemingly become complacent to the risks posed by overreliance on commercial-derivative PNT products. This complacency is apparent in the recent reporting from the Army’s forward-leaning Network Integration Evaluation (NIE) program, in which the Army assesses leading-edge commercial technologies and identifies those with great promise in order to fast-track them into operation, bypassing as much as possible the aforementioned DoD requirements and acquisition processes. 

At the same time, the Army gives a wink and a nod to the GPS security policies requiring use of encrypted military GPS signals for combat operations. It is a virtual certainty that if GPS drives the location-based applications in the commercial-derivative technologies evaluated by NIE, those applications are all powered by civilian GPS and not the encrypted military GPS. As noted, civilian GPS is frequently seen by those not thoroughly familiar with PNT technology as the cheap, expedient choice because more secure or integrated PNT sources are too expensive, too heavy, too much bother, and so on. 

It is also apparent, though not confirmed, that during NIE field testing, the opposing force toolkit does not include navigation warfare (NAVWAR) techniques for GPS jamming and spoofing. If it did, and if the test scenarios included active GPS jamming and spoofing, then the commercial location-based apps with civilian GPS as their input would not work or would derive erroneous solutions. In that case, the Army might have to reconsider its rapid deployment decisions for these vitally important devices. Clearly, it is not doing that.

The highly touted Rifleman Radio, advertised by the Army as a success, uses civilian GPS as its source of PNT information. The Army is planning to deploy tens of thousands of these radios for operational use over the next several years. While soldiers may be told or even admonished not to use the position and timing solutions derived from these radios for other than situational awareness — in other words, not to use them for direct combat or combat-support tasks — the likelihood of that policy being followed in the real world is nil. Either of necessity or for convenience, soldiers will use what is made available to them for whatever purposes they deem appropriate. That will be true whether the commercial-derivative PNT solution is in a smartphone or a Rifleman Radio. 

For the near term, that may not be a problem. However, at some point, in a contested environment against a knowledgeable adversary, mission effectiveness will be compromised and soldiers’ lives will be endangered by such devices. Further, proliferation of these devices will constrain our own commanders in their ability to employ offensive NAVWAR techniques that might be necessary to disrupt adversary use of open civilian GPS signals against our forces in the combat theater.

These statements are not mere speculation. The vulnerability of civilian GPS signals to unintentional interference and intentional jamming is well known. Reports of personal privacy devices interfering with reception of civilian GPS signals at Newark Airport provide a recent example (see “Personal Privacy Jammers,” page 28 in this issue). What is less well understood, but even more sinister in a combat environment, is civil GPS susceptibility to spoofing: the intentional creation of false, but believable, signals. 

In a recent interview with Fox News, Todd Humphreys, a well-regarded GPS researcher from the University of Texas, stated, “The civil GPS signal is completely open and vulnerable to a spoofing attack, because they have no authentication and no encryption. It’s almost trivial to mimic those signals to imitate them and fool a GPS receiver into tracking your signals instead of the authentic ones.” In a combat environment, such deception could result in mission failure or loss of life through loss of command-and-control communications in high tempo lethal actions, erroneous target designations, or misdirected fires.

All those who recommend providing soldiers in combat situations with PNT capabilities derived from civilian GPS, whether via smart phone, iPad, or Rifleman Radio, in lieu of or even in addition to their less convenient but more reliable military GPS devices, should reconsider that recommendation in light of the above. 

There is no argument to the statement that the DoD owes the warfighter more modern, integrated, compact, battery-efficient PNT devices incorporating military GPS. Those will come through the acquisition process, though not as fast as we all would like. In reality, a proliferation of civil PNT devices in military operations will likely delay further the availability of more suitable integrated military equipment. 

In the meantime, we should not be misled because of our experience in today’s war. Instead, we must plan for future actions in anti-access/area denial situations against knowledgeable adversaries. We cannot afford to undermine the warfighters’ cause in advance by advocating reliance on vulnerable and exploitable commercial GPS equipment that can get them killed.

Jules McNeff is vice president for strategy and programs for Overlook Systems Technologies. He served 20 years in the U.S. Air Force, and then was responsible for Defense Department management and oversight of the GPS program. He is a charter member of GPS World’s Editorial Advisory Board.

By Andrei M. Shkel, Defense Advanced Research Projects Agency (DARPA)

The aggregated DARPA Microtechnology for Positioning, Navigation, and Timing (micro-PNT) program is pursuing a new wave of innovation focused on bringing to life revolutionary ideas and fabrication technologies on micro/nano/pico/femto/atto scales, packaging, ultra-low-power electronics, innovative algorithms, never-before-explored architectures, and exploitation of new integration paradigms.

After about two decades of harmonic investment in developments, potential users of so-called small technology for positioning, navigation, and timing (PNT) applications increasingly ask, “Are we there yet?” Clearly, some significant advances have been made, and we see a footprint of the technology in an ever-growing consumer electronics market full of interactive products enabled by inertial and timing microtechnologies. These products include accelerometers for gaming applications, gyros for auto safety, resonators for clocks, and more.

The question remains, however: Is the technology really on the level of what we consider to be precision navigation and timing, that is, is it capable of achieving an accuracy level of at least 10 meters in position and 1 nanosecond in time throughout the entire duration of missions that may range from minutes to hours to days? In reality, small technology remains several orders of magnitude short with respect to long-term stability, dynamic range, and accuracy compared to conventional technology, which is already known to perform adequately for many military applications.

Why does making inertial instruments and clocks small necessarily lead to degradation in performance?

We don’t yet have a complete answer to this question, and we are still working hard to disprove the contention that high-performance inertial micro-instrument is a contradiction in terms. We can make things small, but we cannot yet make them sufficiently precise and uniform; the accuracy of lithography-based manufacturing is on the order of 10^–2–10^–3 (the ratio of the average defect to the smallest feature size), while the accuracy of conventional manufacturing utilizing precision machining is two to three orders of magnitude higher, on the order of 10^–5. We know we can deposit materials layer-by-layer with high precision, but we cannot make micro-devices truly 3D, as is readily achievable using conventional machining. We consistently have an excellent case for low-cost and bulk fabrication, but we cannot seriously challenge so-called boutique processes when it comes to achieving precision, structural complexity, and long-term stability.

We need new knowledge regarding the dimensional stability of materials. We also need a better understanding of material scaling, surface effects, energy-loss mechanisms, and the consequences of fabrication imperfections on the performance of micro-instruments.

PNT applications demand both unusual new fabrication technologies and new materials with special properties. To achieve the required phenomenal accuracy for precision navigation and timing, we need a new wave of innovation in design and refinement of many existing transducers. Future breakthroughs in microtechnology for PNT will likely rely on yet-to-be-exploited physics, new materials, highly specialized fabrication technologies and batch assembly techniques, selective wafer-level trimming and polishing, a combination of passive and active calibration techniques strategically implemented right on-chip, and introduction of innovative test technologies.

Need for Advanced Capabilities

PNT technology usage has doubled every five years since 1960, mostly due to GPS and the miniaturization of electromechanical components. Future PNT usage is expected to double every two years as a result of telecommunication, automobile navigation, robotics, and other commercial markets inserting micro-electromechanical systems (MEMS) technologies. The modern PNT paradigm is based on the assumption that space-based GPS is accessible most of the time to provide position, velocity, and timing information, enabling every user to operate on the same reference system and timing standard.

The size of an apple seed: The micro-PNT objective (conceptual illustration), a single-chip timing and inertial measurement unit, 8 mm³. (Click to enlarge.)

Today’s military systems increasingly rely on GPS, creating a potential vulnerability for U.S. and allied war-fighters should GPS be degraded or denied. When GPS is inaccessible, critical information with respect to position, orientation, and timing can only be gathered through self-contained onboard instruments: a local clock and two triads of inertial sensors (three accelerometers for position and three gyroscopes for orientation). The ideal solution would be a self-sufficient instrument not relying on any external information. Precision microscale clocks and inertial sensors are required to address the paradigm of self-contained PNT.

Clocks. Position and time have a relationship important to a broad spectrum of military applications, including communication systems that feature efficient spectrum utilization, resistance to jamming, high-speed signal acquisition, and an increase in the period of autonomous operation. Other important applications include surveillance, navigation, missile guidance, secure communications, identification friend-or-foe, and electronic warfare.

The emerging applications require new compact time-distribution systems technologies capable of achieving signal phase (time) common synchronization of better than 10^–9 seconds relative to the Coordinated Universal Time (UTC) standard; intersystem synchronization of less than 10^–8 seconds relative to battle group; and less than 10^–9 seconds for interoperability, surveillance, and high-speed communications. Solid-state and atomic oscillators are the key components enabling time and frequency distribution for communication, navigation, and command and control systems.

To support emerging applications, we are interested in clocks with

• signal phase (time) communication synchronization less (better) than 28 nanoseconds (ns) within 5 minutes (real time), UTC;
• intersystem synchronization less (better) than 28 ns relative to other system nodes within 5 minutes (real time); and
• local navigation/communication systems capable of time transfer less (better) than 28 ns, UTC.

The operational frequency mismatch (δf=f), where f is a nominal frequency and δf is a frequency deviation from the nominal, is a measure of oscillator quality and subsequently the quality of the frequency distribution system. Different applications can tolerate different levels of frequency mismatches. For example, for low-accuracy aircraft/land mobile platforms, the requirement for frequency mismatch is 10^–12, while for intermediate land reference sites the requirement is an order of magnitude smaller, 10^–13. For large time-division multiple-access (TDMA) systems, the tolerable frequency mismatch is on the order of 10^–11.

Small size, weight, and power (SWaP) are critical metrics for portable time and frequency distribution systems. The target performance characteristic for low-power clocks and oscillators is long-term stability (aging), which need to be less than 10^–11/month, with less than 1 W power consumption. It is desirable that the oscillators have small SWaP and preserve the level of long-term stability while surviving an inertial environment with accelerations on the level 10,000 g, where g is the gravity constant.

For comparison, the one-way satellite transmission from a GPS satellite in common view at two sites allows one to do accurate time transfer to within 10 ns, with a potential to achieve accurate time transfer of the order of 1 ns. Achieving an accuracy of time transfer on the level of 1 ns is loosely defined as precision timing.

Inertial Navigation Systems. The navigation-grade performance provided by inertial sensors is defined as an INS that accumulates an uncertainty in location not greater than one nautical mile (nmi), or 1.852 km, after one hour of navigation. The error in position is historically defined by the circular error probable (CEP) of 50 percent. The ability to achieve a CEP of 1 nmi in one hour (or 1 nmi/hour) does not translate to a unique performance requirement for a gyroscope and/or an accelerometer. Rather, it presents a trade-off in the overall inertial measurement unit (IMU) error budget. The trades can be generated within a family of gyroscope errors, such as gyro angle random walk (ARW) versus bias drift, or similarly within a family of accelerometer errors. For example, an IMU with gyroscope bias drift of 0.01º/hour combined with an accelerometer bias drift of 25 μg would guarantee a CEP of less than 1 nmi/hour, if no other errors are present. To generate the trade-off space for component performance, one efficient approach is to first generate the parameter space at the linear error covariance level, taking into account the bias drift of components, and subsequently perform  more extensive modeling in a bounded trade-off space by a nonlinear Monte Carlo simulation.

The ability to navigate and keep precise timing has been an important factor in defining the military and economic power of nations for at least a millennium. For almost a century, the development of high-performance inertial instruments has been an extensive area of research. It is anticipated that the following level of performance will soon be achieved, significantly reducing navigation errors and enhancing military capabilities, within the next 5 to 10 years:

• < 0.1 nmi/hour CEP for aircraft, vehicle, or spacecraft for attitude, guidance, and control;
• < 1.0 nmi in 30 hours for ships;
• < 0.4 nmi/hour CEP for missiles.

It is critical that future-generation INS systems be capable of operating through shock levels greater than 1,000 g.

Similar to clocks, the reduction of SWaP and cost (SWaP+C), while not compromising in performance, are the critical metrics for future development of IMUs. The current performance of state-of-the-art MEMS-based IMUs is on the level of tactical grade, with CEP approaching 100 nmi/hour. There is a great potential for achieving performance improvements that will subsequently enable platforms for personal navigation, precision navigation of small unmanned aerial vehicles (UAVs), unmanned underwater vehicles (UUVs), and GPS-free navigators for missiles. It is expected that the performance levels of chip-scale inertial instruments and clocks, shown in Table 1, could be achieved within the next 5 to 10 years, thus significantly enhancing military capabilities. The conservative estimations are projected by the Department of Defense’s Science and Technology List for Positioning Navigation and Timing. The aggressive estimates presume successful completion of the micro-PNT program described here.

Table 1. Projected performance of chip-scale inertial sensors and clocks by 2015.

The military has access to a currently specified accuracy of 21 meters (95 percent probability) from the GPS Precise Positioning Service (PPS). Accuracy can be improved after calibration for some of the GPS errors, for example, by utilizing optimal estimation techniques correlating GPS and INS signals. A CEP of less than 10 meters has been routinely achieved, with a potential to achieve accurate positioning on the order of 1 meter CEP.

Navigation, guidance, and automatic control are not the only military applications that could benefit from improvements in inertial sensors. Azimuth or north-pointing determination systems include celestial devices, magnetic compasses, and inertial sensors. Utilization of gyroscopes to precisely determine orientation has a number of benefits attributed to their immunity to magnetic fields, speed of acquisition, and potentially small SWaP+C. For this purpose, a variety of inertial equipment is being explored, including IMUs, attitude-heading reference systems (AHRS), and gyro-compasses. Providing an azimuth or north-pointing accuracy of less (better) than 0.5 arc minute multiplied by secant latitude has the potential to significantly enhance military capabilities for many targeting applications, especially for anticipated mobile platforms.

Initial prototype illustrating objective of the micro-PNT program.

Current Research

This section provides an overview of the ongoing efforts funded by DARPA (Defense Advanced Research Projects Agency) under the micro-PNT program.

Clocks. The potential payoff of the precision-clock technology developed by the program will enable ultra-miniaturized and ultra-low power absolute time and frequency references for applications such as nano/pico satellite systems, UUVs, UAVs, wristwatch-size high-security UHF communicators, and jam-resistant GPS receivers.

There are currently two efforts within the micro-PNT program involving the development of clocks: Chip-Scale Atomic Clock (CSAC) and Integrated Micro Primary Atomic Clock Technology (IMPACT).

The goal of the CSAC effort is to create ultra-miniaturized, low-power, atomic time and frequency reference units that will achieve, relative to present approaches: more than 200× reduction in size (from 230 cm³ to <1 cm³); more than 300× reduction in power consumption (from 10 W to less than 30 mW); and matching performance (1 × 10^–11 accuracy and 1 ns/day stability). This work, funded by DARPA since 2002, has been supporting 11 teams. The program is currently in its final phase and supports two performers, Symmetricom and Teledyne Scientific. Symmetricom has already demonstrated pilot units that are 1 cm³ in volume, consume on the order of 100 mW of power, and perform on the level of better than 30 × 10^–11 short-term 1 sec instability (Allan Deviation) and 5 × 10^–11/day (1.4 × 10^–10/month) long-term frequency drift.

The IMPACT program seeks to improve the stability and accuracy of microscale atomic clocks by as much as two orders of magnitude. Atomic-clock performance is affected by buffer gases (nitrogen or argon), which are necessarily present in either rubidium- or cesium-based atomic clocks. Buffer gas atoms interact with alkali atoms and effectively shift the resonant frequency of atoms. Emerging atomic-clock technologies based on laser-cooled atoms and trapped ions could overcome the limitations of CSAC.

The goal of IMPACT is to create miniaturized, low-power, integrated micro primary atomic clock technology that will achieve significant reduction in size relative to conventional clocks, but slightly larger than CSAC (volume less than 5 cm³ in final package, excluding battery); significant reduction in power relative to conventional clocks, but slightly greater than CSAC (50 mW); and two orders of magnitude increase in performance relative to CSAC (frequency accuracy 1 × 10^–13, Allan deviation at one-hour integration time, and stability characterized by 5 ns/day time loss). The work, funded by DARPA since 2008, currently involves four teams: Honeywell, Symmetricom, Sandia National Laboratories, and OE Waves.

The overall approach is based on sampling of atomic transitions at extremely low temperatures, requiring vacuum on the level of 10^–9 Torr and the ability to trap atoms in a small volume. The technology has been previously demonstrated on a large scale, but transferring the technology to small scale is far from trivial, requiring major innovations. The effort has already demonstrated magneto-optical trapping in a 16 cm³ atomic cell, and chip-scale clocks implemented using cold atoms performing on the level, quality factor × signal/noise ratio ∼ 2.6 × 10¹⁰, time loss after 1 ms equal to 10^–4 ns; after 1 second, 6 × 10^–3 ns; after 1 hour, less than 10 ns; and after 24 hours, on the order of 100 ns. Frequency retrace was demonstrated at the end of the phase on the level of 10^–11.

Inertial Sensors and Systems. There are currently three efforts within the micro-PNT program involving the development of inertial sensors and systems: Navigation-Grade Integrated Micro Gyroscopes (NGIMG), Micro Inertial Navigation Technology (MINT), and Information Tethered Micro Automated Rotary Stages (IT-MARS).

The NGIMG effort seeks to develop tiny, low-power, rotation-rate sensors capable of achieving performance commensurate with requirements for GPS-denied navigation of small platforms, including individual soldiers, unmanned (micro) air vehicles, unmanned underwater vehicles, and even tiny (for example, insect-sized) robots. By harnessing the advantages of microscale miniaturization, the NGIMG effort is expected to yield tiny (if not chip-scale) gyroscopes with navigation-grade performance characteristics: overall size less than 1 cm³ (no power source), power consumption less than 5 mW, ARW less than 0.001°/√hour, bias drift less than 0.01°/hour, scale factor stability on the order of 50 parts per million (ppm), full-scale range greater than 500°/sec, and bandwidth on the order of 300 Hz.

The NGIMG effort has been funded by DARPA since 2005, and work is currently being conducted by three teams: Northrop Grumman, Boeing, and Archangel Systems. The work has demonstrated several experimental prototypes (some, but not all, independently verified by the government) performing on the level of ARW 0.01°/√hour,  and bias drift 0.05°/hour.

The MINT effort seeks to develop microscale low-power navigation sensors that allow long-term (hours to days) precision navigation in GPS-denied environments. The goal is to create high-precision, navigation-aiding sensors that directly measure intermediate inertial variables, such as velocity and distance, to mitigate the error growth encountered by integrating signals from accelerometers and gyroscopes alone. In addition to aiding sensors such as velocity sensors, the combination of microscale inertial sensors will be integrated to a form-factor of one or two integrated circuits. Such an integrated sensor suite will be incorporated into the sole of a shoe for accurate and precise velocity sensing using zero-velocity events during walking.

The final goal of MINT is to achieve an overall package and form-factor for a velocity sensor (excluding IMU) of less than 1 cm³, power consumption for the velocity sensor of less than 5 mW, 1-meter position accuracy after 36 hours of walking, and 10 µmeter/second velocity sensing bias per step. The effort has been funded by DARPA since 2008 and involves work by four teams: Carnegie Mellon University, Analog Devices, Northrop Grumman, and Case Western Reserve University/University of Utah. To date, the work has demonstrated positioning error on the order of 4 meters after 30 minutes of walking.

The goal of the IT-MARS program is to implement and demonstrate a MEMS-fabricated rotary stage providing a rotational degree of freedom to planar MEMS structures and sensors, thus enabling free rotation of micro-structures and micro-sensors relative to the package, with coupled power and signal transfer from the rotating platform to the package. The IT-MARS effort may enable highly accurate calibration of inertial sensors and serve as a micro-platform for carouseling of inertial sensors that further enable on-chip calibration and gyro compassing. The ultimate program goal is to achieve an overall volume of no more than 1 cm3, power consumption for actuation on the order of 10 mW, angle position absolute accuracy to within 1 milli-degree, maximum wobble of 10 micro-radians, a rotation rate of 360°/second, and reliability (run time of rotor) greater than 104 hours.

This effort, which has been funded by DARPA since 2009, supports three teams: UCLA, UC-Berkeley, and the Boyce Thompson Institute. The work has already demonstrated free rotated platforms, and future efforts will focus on manufacturability and precision control of the stage-rotation and reduction of wobbling.

Figure 1. Range of missions versus speed of platforms for common Department of Defense (DoD) needs. The duration of missions ranges from seconds to minutes to hours. The majority of missile missions are fast, but less than 180 seconds (3 minutes) in duration. All missions with duration greater than 10 seconds typically use GPS. The Micro-PNT program seeks to develop a single-chip solution enabling self-contained inertial navigation (without the need for external signals such as GPS) for diverse DoD scenarios. (Click to enlarge).

New Initiatives

In January 2010, DARPA launched a coordinated effort focused on the development of microtechnology specifically addressing the challenges associated with miniaturization of high-precision clocks and inertial instruments. The new program, Microtechnology for Positioning, Navigation, and Timing (micro-PNT), aggregated the existing efforts (CSAC, IMPACT, NGIMG, MINT, and IT-MARS) and initiated four complementary new developments:

• Microscale Rate Integrating Gyroscopes (MRIG),
• Chip-Scale Timing and Inertial Measurement Unit (TIMU),
• Primary and Secondary Calibration on Active  Layer (PASCAL),
• Platform for Acquisition, Logging, and Analysis of Devices for Inertial Navigation & Timing (PALADIN&T).

The overall goal of the new aggregated micro-PNT program is to focus all of these complementary efforts toward achieving one specific overarching goal: self-contained chip-scale inertial navigation (see opening illustration). The reduction of SWaP+C of IMUs and timing units (TUs) is the technological objective. The developments consider a number of operational scenarios, ranging from dismounted-soldier navigation to navigation, guidance, and control (NGC) of UAVs/UUVs and guided missiles. The new micro-PNT initiatives will increase the dynamic range of inertial sensors, addressed by the new MRIG effort; reduce the long-term drift in clocks and inertial sensors, addressed by the PASCAL work; develop ultra-small chips providing position, orientation, and time information, addressed by the TIMU effort; and provide a universal and flexible platform for the testing and evaluation of components developed within the comprehensive micro-PNT program, addressed by the PALADIN&T effort.

The primary goal of MRIG is to create a vibratory gyroscope that can be instrumented to measure the angle of rotation directly, thereby extending the dynamic range and eliminating the need for integrating the angular rate information; MRIG will thus eliminate the accumulation of errors due to numerical/electronic integration.

The final goals are to:

• extend the dynamic range to 15,000°/second;
• achieve drift repeatability on the level of 0.1°/hour (angle dependent) and 0.01°/hour (bias-dependent) under variable –55°C to 85°C thermal conditions;
• achieve ARW of 0.001°/√hour, an operation range of 1,000 g with acceleration sensitivity of 10^–5 degrees/hour/g, vi
  bration sensitivity angle random walk of 0.01°/√hour per g/√Hz, and drift rate of 0.01°/hour per g²/√Hz.

These performance characteristics are thought to be achievable through development of precision 3-D fabrication technologies utilizing high-Q materials; development of wafer-level balancing and trimming techniques that reduce the effects of aniso-inertia (mass misbalance), aniso- compliance (stiffness misbalance), and aniso-damping (damping misbalance); and development of active control and an active calibration architecture.

These performers have been selected for the initial phase of the MRIG effort: Draper Labs, Honeywell, Northrop Grumman, Systron Donner, UC-Irvine, UC-Davis, UCLA, Cornell, University of Michigan, and Yale University.

The TIMU effort will address challenges associated with the development of a miniature (10 mm³), low-power (200 mW), high-performance (CEP on the order of 1 nmi/hour), and self-sufficient navigation system on-a-chip. The smallest state-of-the-art IMUs perform on the level of tactical-grade instruments (CEP on the order of 100 nmi/hour) and are about the size of an apple (more than 104 mm3). This effort intends to develop a technological foundation for a navigation-grade TIMU (CEP less than 1 nmi/hour and time accuracy of 1 nanosecond/minute) with a significant reduction in SWaP, potentially miniaturizing the TIMU to the size of an apple seed (10 mm³).

PASCAL will develop self-calibration technologies intended to eliminate long-term bias drift of inertial sensor and clocks. The grand challenge of this effort is to raise long-term bias stability to the level of 1 ppm.

This level of stability represents a two-orders-of-magnitude improvement compared to state-of-the-art inertial microsensors, currently at 200 ppm. The work will investigate an approach for fabricating sensors on an active layer that may serve as a calibration layer for micro-PNT systems.

The PALADIN&T effort will develop a universal platform for test and evaluation of early prototypes developed in the micro-PNT program. The effort will also simplify the uniform evaluation of pilot prototypes within the program and provide an early field demonstration, advancing the technology readiness level.

Conclusions

Current state-of-the-art microscale clocks and inertial instruments can provide the required level of precision only for missions having a duration of no more than about one minute. The micro-PNT program at DARPA is developing small SWaP+C inertial sensors for a variety of operational scenarios for missions ranging from minutes to hours. Current projects (CSAC, IMPACT, NGIMG, MINT, IT-MARS) mainly focus on navigation, characterized as missions of prolonged durations in relatively benign environments (a few hours of operation on a platform moving at relatively low speed, less than 100 km/hour).

The new initiatives (MRIG, TIMU, PASCAL, and PALADIN&T) target the challenges of missile guidance for precision engagement scenarios, short duration missions in highly dynamic environments (10 seconds to 3 minutes of operation at speeds of 1,000 km/hour and higher). Ongoing efforts and new initiatives explore new physical phenomena, high-quality factor materials, specialized fabrication technologies, and innovative approaches to system integration.

Disclaimer. The views, opinions, and findings in this article are those of the author and should not be interpreted as representing official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. The document GPS0911 [DISTAR case 17952] is approved for public release, distribution unlimited.

Andrei M. Shkel received a Ph.D. in mechanical engineering from the University of Wisconsin-Madison and is a program manager in the Microsystems Technology Office at the Defense Advanced Research Project Agency (DARPA), and on-leave professor of mechanical and aerospace engineering at University of California, Irvine, where he is also the director of the UCI Microsystems Laboratory. He holds 15 U.S. and international patents (12 pending) on micromachined angle-measuring gyroscopes, wide-bandwidth rate gyroscopes, light manipulators and tunable optical filters, and hybrid micromachining processes.

Troops take part in GPS handheld training with Ed Meyer.

Operation Waypoint, a Minnesota-based, non-profit program administered by American Legion Post 621, has broadened its program from a state and regional focus to national in scope with its new website, gpsfortroops.org.

Run by volunteers, the program is committed to increasing the safety of military men and women deploying to the Middle East with the guidance of highly accurate, handheld GPS units and mapping cards for Iraq and Afghanistan. Since its inception, Operation Waypoint has relied heavily on its partnership with GPS device manufacturer Lowrance to provide GPS products and charts to soldiers preparing to serve, as well as generous donations from service and social organizations and numerous individuals to fund the effort.

Operation Waypoint was started in 2005 by retired educator Ed Meyer after a former student, preparing for deployment to Iraq, contacted him to ask what type of GPS unit would be best for his mission. As the military only provides one GPS device per unit, which is usually mounted in a vehicle, Meyer contacted a friend at Lowrance, requested three GPS handheld devices, and trained the company commander and two former students how to use them.

Close Call in Baghdad. Shortly after the soldiers arrived in Iraq, while traveling at night, their 24-vehicle convoy took a wrong into a dangerous Baghdad neighborhood following the lead truck’s Army-issued GPS unit. Realizing the mistake, the convoy commander called Sgt. Gaylen Heacock, one of the soldiers equipped with a Lowrance GPS supplied by Meyer. Heacock’s device determined the correct route and was able to guide the convoy to safety. Upon hearing of how the Lowrance units aided in safety, Meyer worked through the American Legion Auxiliary and Post 621 to broaden the idea into a full not-for-profit program.

“Our goal is to spearhead an even larger movement where communities nationwide can directly support our troops in a very meaningful way,” said Meyer. “I believe that every soldier that feels a GPS would aid them in their mission in the Middle East should have one with them.”

With the enhancement of GPS accuracy and advanced features, today’s GPS units are even better suited to the challenges often seen by the military than when the program began. Operation Waypoint provides soldiers with Lowrance Endura Safari handheld GPS units that contain a precision GPS+WAAS antenna with 42-channel receiver and 3-axis magnetic compass to ensure troops have pinpoint accuracy for proper guidance or calling in air support when needed. The combination of the touchscreen, simple menus, and the ability to control one-handed or with gloves keeps usability fast and seamless, Meyer said. However, the most important benefit is the ability to store up to 2,000 waypoints for areas of safe passage, suspected insurgent buildings, and other items that are marked and identified with any of 193 different icons and then shared between GPS units over time or added to satellite maps.

“The [GPS] unit helped ensure the safety of crews while running convoys through the worst part of Iraq,” said Sgt. Heacock. “It’s helpful in pinpointing casualty evacuation points and points of hostile action.”

To date, Operation Waypoint is responsible for delivering more than 200 handheld devices into the hands of deploying soldiers. The St. Augusta American Legion accepts donations for Operation Waypoint and purchases its Endura Safari handheld GPS units directly from Lowrance. Lowrance also provides permission for the organization to copy and encrypt its Middle East mapping onto locally sourced microSD cards. While more work, this avoids packaging and operational overhead costs that would normally be seen by a manufacturer. Once the GPS and mapping cards are prepared, each participating soldier is personally trained on the GPS device and mapping before he or she takes it overseas.

“Each Lowrance GPS and chart card costs $115 after corporate discounts are factored in,” said Meyer. “Unfortunately, there are still times when we can’t purchase enough units. I have even given my personal GPS away, because I can’t imagine turning down a brave soldier. The challenge, as with most non-profits, is maintaining enough donations to support the program effectively.”

Operation Waypoint seeks to grow nationally by working with other American Legion Posts and organizations with a goal to provide a GPS device to every deployed unit. The Operation Waypoint website was redesigned to build awareness, make it easier for visitors to donate, and encourage other organizations to become partners in the project to provide GPS devices for soldiers in their own communities.

Operation Waypoint GPS handheld recipients.

The first time I ever heard of the Magnavox Research Laboratory in Torrance, California, was in 1966, as a young engineer working at Hughes Aircraft. We were building large (46-foot diameter) ground stations for the Defense Satellite Communications System (DSCS). Magnavox was supplying the secret anti-jam modems used in the terminals.

Because of this, I also learned a little about spread-spectrum pseudo-noise (PN), something quite esoteric at the time and not taught in engineering school. I noticed a widespread respect for Magnavox from my colleagues who referred to the company and its equipment as “Magicbox.”

Within a year I had transferred to the Hughes division responsible for developing satellites. We were working on a study known as 621B, for using satellites for positioning. Our teammate for the study was Magnavox. That team was responsible for the payload signal design, for which the team chose PN as the modulation to provide for multiple access, ranging, data transmission, and anti-jam.

Before long, my boss decided to leave Hughes and go work for Magnavox. He took two of his systems engineers with him. I was one of them.

In1968, the U.S. Air Force could not yet sell the 621B concept as an Advanced Development Program, so instead opted to experiment and prove that PN modulation could be used to accurately measure a half-mile of cable. Hughes bowed out since there wasn’t any satellite procurement in the offing. Magnavox and the other 621B contractor, TRW, each took on the challenge of measuring the cable.

Where Hughes had been 10-deep in Ph.D.s in every discipline, Magnavox was 10-deep in PN experts, which I believe at that time was the world’s majority. Thus it was natural for the Air Force to ask them to continue, and develop a receiver to be used in the next phase of 621B. An inverted range was set up with four PN transmitters, and an aircraft with the receiver and a bottom antenna flew over them. The aircraft’s position was determined using the PN range measurements and the known locations of the transmitters. The data from that receiver, called the MX450, was used to help justify the Department of Defense (DoD) decision to proceed into the Advanced Development Phase of GPS. Some of the people who contributed to this were named in Dr. Brad Parkinson’s recent articles on the origins of GPS. During that time I was working on the next generation of spread-spectrum modems for the DSCS.

Magnavox went on to develop these PN satcom modems for all three services, and thus was a natural choice to develop the first military GPS receivers (known as X and Y sets and the first Manpack), as well as the first C/A receiver, the Z set, and the very first spaceborne receiver called GPSPAC.

As soon as we completed the first Manpack, I approached Col. Paul Weber, the Joint Program Office Army Deputy Program Manger, and asked if he would pose with the Manpack on his back for a brochure we wanted to produce to show to potential Army and Marine Corps users. He agreed, dressed in his combat uniform, and went with our photographer into the wild woods of San Pedro (near the Port of Los Angeles) for the picture shown in the brochure.

Magnavox also developed the military GPS Engineering Models in competition with Rockwell Collins. Magnavox lost the production contract to Rockwell Collins a year after I left to join IEC, now known as L-3 Communications.

Magnavox also pioneered commercial GPS sets for use in the marine and survey markets. Today, you will still find many of the original GPS user equipment developers still at it as consultants and engineers at Raytheon, Navcom, Trimble, IEC, and others. Perhaps our most famous alumni is Dr. Min Kao, the “min” of Garmin.

Len Jaconbson

LEN JACOBSON is a consultant to the GPS industry and has served as an expert witness in many legal proceedings involving GPS. He is the author of the book GNSS Markets and Applications, published in 2007, and is a longstanding member of this magazine’s Editorial Advisory Board.