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the use of global navigation Satellite Systems (gnSS), largely through the uS global Positioning System (gPS), pervades almost every area of modern life, and this is particularly true of the military. there are few, if any, aspects of military operations that are not dependent on Pnt: Position (knowing where you are), navigation (how you are going to get to where you want to be) and timing.

It is unlikely that any modern military mission could be successfully executed without accurate PNT. This is currently supported largely by GPS, which provides supremely accurate position and timing, with the latter linked to atomic clocks monitored by the GPS ground control segment and traceable to Universal Coordinated Time (UTC) at the US Naval Observatory. PNT provided by GPS is fundamental to precision-guided weapons; the provision of synchronised situational awareness; unmanned platforms; in fact almost every area of military activity. GPS has been so successful that it now pervades much of civil society, and commercial activities such as banking, stock

markets, utilities and transport all rely on its capabilities.

However, there is now increasing concern about overreliance on GPS given the fragility of the signal. GPS is vulnerable to jamming, interference and “spoofing” – the attempted deception of the receiver by the broadcasting of incorrect signals. During the Russian exercise “Zapad 2017” GPS signals were reportedly lost by commercial aircraft and other platforms over the eastern Finnmark region of Norway as far as 250 km from the Russian border, with similar large-scale interference experienced over land and sea across northern Finland and Sweden. And earlier in the same year it was reported that GPS anomalies affected the navigation of some 20 vessels in the Black Sea, which was suspected to have been caused by a spoofing attack.

GPS is also not always universally available simply because of physical factors. Being inside buildings, under dense foliage or underground can defeat the signal, and it

has never been usable underwater or in deep space.

There is therefore a need for devices to replicate the integrity and reliability of GPS to provide an alternative Assured PNT (APNT) capability which will provide positional data with the same accuracy as GPS, and timing with the equivalent traceability to that derived from the atomic clock. This must not only address the requirements of GPS users who are temporarily denied its use because of physical circumstances or human interference but also extend the benefits of APNT to those who have never had access to the GPS signal.

Northrop Grumman Mission Systems has a strong history of over 50 years of delivering trusted navigation and positioning systems, rooted in its Litton Industries heritage of the development of lightweight inertial navigation systems (INS) for aircraft in the late 1950s. By 1983 Litton had produced 20,000 aircraft INS. Litton was acquired by Northrop Grumman (NG) in 2001 and continues to be a major force in the aviation

The Future Challenge

Northrop Grumman has a long and distinguished history of supplying trusted navigation and positioning systems for a wide range of US ships and aircraft (US Navy/Lt j.g. James Griffi n)

The Future ChallengeAssured Position, Navigation and Timing:

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or timing or both which cannot all include anti-jam protection. As a result there can be a multiplicity of self-contained GPS receivers on a single platform, leading to what Ebert describes as “an integration and resilience nightmare that needs to be managed”.

The NG solution is to harness as much of this sensor data as possible to contribute to an accurate and robust APNT solution which will be trusted. The core of this philosophy is a software-defined open architecture that can affordably integrate data from diverse sensors, both GPS and non-GPS, in a single hub for distribution throughout the platform.

The sensor fusion framework functions at both the sensor measurement and signal processing levels, providing access to the different signals and enabling multiple levels of checks on their validity. The raw information from all the relevant sensors on board the platform is available to the hub, with each additional source of data increasing resilience, adding accuracy and improving the integrity of the solution.

The centralised framework for GNSS and other RF PNT signal processing which the hub provides also gives it the capacity to extract the particular signals it needs to provide APNT for a specific mission or threat environment, thus offering flexibility in creating the sensor measurements for fusion as well as flexibility in the fusion process itself.

A hub solution reduces vulnerability through its use of a variety of information sources and reduces cost as it utilises existing sensors. The modular open systems architecture allows rapid reconfiguration to allow acceptance of different signals, with standard interfaces developed for individual sensors which can access a signal as early as possible to reduce the amount of processing.

This common philosophy espoused by NG has a wide range of applications which are appropriate to diverse users, but to achieve this it must be scalable and flexible and realised in widely differing hardware.

Integrated SyStemS

A good example of this approach is the USAF’s Embedded GPS/INS-Modernization (EGI-M) programme, for which NG was awarded a contract by the USAF to develop the technology in May 2017. The EGI-M will have a modular, open systems architecture to support the rapid insertion of new capabilities and adaptability based on specific platform requirements and it will incorporate the more robust military-

the ng PhIloSoPhy: the hub

The NG approach moves the burden of trusted APNT away from sole reliance on GPS to the complementary use of inertial devices augmented with additional alternative methods of navigation. INS calculate direction moved over time, with a varying degree of drift, and with the use of high precision oscillators provide continuity of time. Their great advantage is that they cannot be spoofed or jammed.

On an individual platform there may be a number of sensors, each of which is a potential source of complementary information to support navigation. For example, in addition to INS and celestial

systems, vision-aided navigation using EO/IR and LADAR imagery sensors is capable of maintaining a position of near GPS quality both on the ground and in the air through a variety of techniques, including visual odometry, simultaneous localisation and mapping (SLAM) and absolute positioning (map matching).

There can also be several systems each with its own GPS to provide either position

navigation market as well as in stellar-inertial and maritime navigation: NG products are in use in ICBMs and on more than 60 naval surface and subsurface platforms from a number of countries, including the USN’s Los-Angeles-, Sea Wolf- and Virginia-class submarines.

With this background NG is well aware of the challenge set by the need to provide APNT under all circumstances, a challenge accentuated by the needs of a broad user base which ranges from the submarine to the dismounted soldier, with a wide range of missions, requirements and constraints.

Dean Ebert, vice president, navigation and positioning systems, Northrop Grumman,

emphasizes that the initial response to a GPS-challenged environment should be “to hang on to GPS for as long as possible”. This may mean increasing the sensitivity of the GPS receiver and its antenna, improving its anti-jam capability or using other onboard sensors to check its integrity as an anti-spoofing precaution. However, this approach can run into issues of size and affordability so there needs to be an alternative.

TOP Two ships in company maintain Assured PNT with a satellite and each other as they approach a GPS denied area BOTTOM Once in the GPS-denied zone they mutually maintain Assured PNT using alternative methods and provide this for a helicopter (Northrop Grumman)

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Integrating the data from existing sensors in this fashion improves accuracy and assurance at little extra cost.

As a possible future example, an F-16 equipped with a Litening targeting pod, the software framework architecture and an APNT distribution sensor fusion hub might utilise all the EO/IR capabilities of the pod to reinforce the PNT solution from the platform’s existing GPS/INS system, particularly under GPS-challenged conditions.

SuPPortIng SolutIonS

The modernisation of GPS satellites to broadcast M-code requires the modernisation of GPS receivers which are currently hard-wired for the legacy signal. To meet this need NG has developed the SERGEANT software defined GNSS transceiver. This utilises COTS processors, allowing rapid upgrades to take advantage of technology developments and outpace threats, and it has advanced anti-jam capabilities. SERGEANT’s software design now allows it not only to access M-code but also any satellite navigation signal, providing significantly increased resilience and flexibility. It has a modular architecture and can be integrated with other systems: it has already been successfully hosted in the proven NG LN-260 INS/GPS which is in service with the USAF.

The flexibility of the hub approach and its scalability for use on a range of platforms is reinforced by improvements NG is making

inertial device, a chip-scale atomic clock (CSAC) and a GPS receiver, and it will also have access to existing data, such as from a vehicle odometer. Other sensors which are already installed on the platform, such as EO/IR devices, could also be integrated into the hub to reinforce the APNT solution.

code (M-code) GPS signal, which has been developed in response to the increased GPS-hostile environment. EGI-M is also designed for compatibility with current systems on legacy aircraft.

The same approach is being taken with the US Army’s Mounted Assured PNT System (MAPS) programme. This will provide APNT data to land vehicle platforms, distributing it from a fusion hub over the standard VICTORY electronic vehicle architecture to any warfighting function application that requires it. It will use anti-jam antennas to improve the anti-jam margin of existing military GPS receivers and will detect and report GNSS interference sources. By integrating timing capabilities and a PNT distribution hub it will replace the need for multiple GPS receivers on a single platform.

MAPS will be deployed in large numbers so the solution needs to be affordable. It must therefore rely on a different balance of inertial and other sensors to complement the GPS data and maintain accuracy when it is absent. While the MAPS sensor suite is not finally decided it is likely to include an

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LEFT A range of sensors can contribute to the provision of Assured PNT, with their data fused using an open architecture, open interfaces and modular software (Northrop Grumman)

Northrop Grumman will leverage its hub philosophy for the US Army MAPS programme, which will provide a low-cost solution for APNT utilising all the available data on a vehicle platform such as this Stryker IFV (US Air Force/Justin Connaher)

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to inertial systems. It is taking advantage of new technology to shrink size and cost with the Precise Robust Inertial Guidance for Munitions Navigation-Grade Inertial Measurement Unit (PRIGM: NGIMU) programme funded by the US Defense Advanced Research Projects Agency (DARPA), for which it was selected in March 2016.

The resultant highly accurate microelectromechanical systems (MEMS)-based IMU, designated the LR500, is only 5in3 in volume and weighs 0.35lbs, a 4000% improvement on some of its predecessors, and it combines the best features of current MEMS gyro technology. Coupled with NG’s proprietary dynamic self-calibration (DSC) this gives a positional drift rate near that of high end airborne inertial navigation systems. A prototype at technical readiness level six will be available in early 2019.

This development is of particular value to weapon systems, where the provision of APNT in a low size, weight and power (SWaP) form factor will make a significant contribution to weapons guidance in a GPS-hostile environment.

There is also a new focus on stellar and celestial navigation as this can provide another non-interferable alternative to GPS. Celestial navigation, which uses the known orbits of resident space objects (RSOs), provides an absolute position fix. Stellar navigation, which uses the stars and is therefore not always available, gives roll, pitch and drift. These can be combined with an inertial system to lower the drift rate: while an inertial system by itself drifts as much as 9 nautical miles in 12 hours, a combined solution drifts by far less. Such a combination of these technologies, which can be used when mission-relevant, provides more resilient APNT.

The NG LN-220 is a Stellar-Inertial system that is the latest in an evolutionary process that has seen systems shrink by 80% in size and weight and over 90% in power consumption. It integrates proven NG advanced inertial sensors with a star tracker and video processor. The result is a device which in a GPS-denied environment suffers from a drift of less than 30m in an hour, which can be corrected by the embedded GPS if the service becomes available.

“In developing these APNT solutions Northrop Grumman brings its world-class experience and innovative approach to inertial instrument technologies and navigation systems. In particular, making inertial navigation units smaller and lighter than ever before and introducing software-defined open architecture-based systems will make a huge difference in GPS-denied and highly contested environments,” said Ebert.

An F-16 with a Litening pod. In future the pod’s capabilities could be used to reinforce the PNT solution via a sensor fusion hub (US Air Force/Senior Airman Jorden Castalan)

Northrop GrummanNavigation and Maritime Systems

21240 Burbank BoulevardWoodland Hills, CA 91367 USA1-866-NGNAVSYS (646-2879)

www.northropgrumman.com

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