By Mr. Paul C. Manz
Global Position System (GPS) signals are extensively used by a multitude of Army and Joint military products and applications. GPS is highly accurate, affordable, and pervasive. Most typical GPS-based systems automatically listen for GPS signals, use these signals to determine the exact location of the GPS satellites in the sky, and then (when it sees at least four GPS satellites) use this information to precisely determine the system’s geo-location coordinates (i.e. X, Y, Z) on Earth. Because these GPS signals contain data from their extremely accurate on-board satellite clocks, they can also be used to synchronize time across multiple systems. Thus, GPS is a simple, yet effective, tool which enables many military position, navigation, and timing (PNT) related capabilities used to maintain combat overmatch against the enemy. These GPS-enabled capabilities include indirect fires which support the Maneuver Commander in performing essential tactical operations such as “Movement to Contact.”
Knowing where your weapon system is located and where the target is located are two of the five critical requirements for accurate predicted indirect fires. Additionally, many indirect fire Precision Guided Munitions (PGMs) use GPS to deliver lethality exactly where it is required to quickly defeat enemy targets with minimal collateral damage, even when the enemy target is very far away. Unlike most typical GPS-based systems, some indirect artillery and mortar fire PGMs must “hot start” or pre-load the locations where GPS satellites are in the sky (i.e. GPS ephemeris data) in order to rapidly start looking for and navigating off these GPS satellites within a few seconds after exiting the weapon system.
Why is “hot start” GPS data important?
Similar to pulling your car out of the garage after a two week vacation and turning on your vehicle’s navigation system (i.e. “cold start”), it can take up to two minutes to acquire and start navigating off of GPS if you don’t pre-load this GPS data. Since the time of flight for many such PGMs can be under one minute, this means the PGM may never navigate and can become an unguided “lawn dart.”
Where does “hot start” GPS data normally come from?
Usually a handheld Defense Advanced GPS Receiver (DAGR), or other GPS device co-located with the weapon system shooting the PGM, transfers the GPS Satellite information it sees in the sky to the PGM using a specialized Fuze Setter device. Unfortunately, if a weapon system and its co-located DAGR are located in a vertically-challenged terrain environment (ex. at the bottom of a deep valley in Afghanistan or in an “urban canyon” location), the required visibility of at least four GPS satellites in the sky may be terrain-masked during certain times of the day (see Figure 1). This terrain-masking effectively prohibits GPS-based PGMs from being fired (i.e. making the weapon system not “precision capable”) since not enough “hot start” GPS satellite data can be preloaded to rapidly acquire, track, and navigate off of GPS.
PEO Ammunition, Joint Center Picatinny Arsenal, and its other Army research, development, and acquisition (RDA) partners have designed, developed, and successfully tested an innovative system-of-systems solution called Network Assisted GPS that provides complete “hot start” GPS satellite data for PGMs—even in the presence of almost full terrain masking! Network Assisted GPS takes advantage of multiple, sunk-cost, acquisition Programs of Record across multiple PEOs and deployed across multiple Services. Leveraging these deployed capabilities and combining them with a modest amount of new software “glue”, Network Assisted GPS is a reasonable cost, non-traditional program that will dramatically increase the availability of indirect artillery and mortar fire PGMs in vertically-challenged terrain environments.
How does it work?
The US Air Force (USAF) GPS Operations Center (GPSOC) publishes the exact location of the GPS satellites orbiting around the Earth several times each hour on a classified network. Joint Battle Command – Platform (JBC-P) is managed by PEO C3T and similarly has centralized Network Operations Centers (NOCs) at a few key sanctuary locations around the globe. These JBC-P NOCs are always connected to the same classified network as the GPSOC and are also always connected via Satellite Communications (SATCOM) to JBC-P systems on the ground. These terrestrial JBC-P systems are found in most vehicles as well as Tactical Operations Centers (TOCs). The Advanced Field Artillery Tactical System (AFATDS) is co-located with JBC-P in these TOCs. AFATDS is the command and control system that generates fire missions which tell who, what, where, when, and how to shoot enemy targets. AFATDS is connected via tactical terrestrial communications to all targeting systems and indirect fire weapon systems in the area of combat operations.
Network Assisted GPS works by having the JBC-P NOC request the GPS satellite location data continually published by the USAF GPSOC and “pushes” this small amount of GPS data down to each and every terrestrial JBC-P on a periodic basis whenever SATCOM bandwidth is available. When a Call-For-Fire message comes into AFATDS from a targeting system, AFATDS processes this message and then sends another message to the appropriate weapon system to initiate and conduct an indirect fire mission against a specific target. With Network Assisted GPS, AFATDS also “subscribes” to GPS satellite location and related data from JBC-P over the TOC’s Local Area Network (LAN) using the TOC’s Data Dissemination Service (DDS). AFATDS subsequently “pushes” this GPS information to all these same weapon systems on a periodic basis. This information includes ALL the potential GPS satellites a weapon system should be seeing on that side of the Earth (i.e. as if its location was not terrain-masked and shooting from a “world is flat” position). Whenever the indirect fire weapon system receives a precision fire mission from AFATDS, it loads ALL this potential GPS satellite location data provided by Network Assisted GPS (i.e. GPSOC to JBC-P NOC to JBC-P to AFATDS to Weapon) onto the PGM in lieu of the much lesser number of satellites usually seen at a terrain-masked firing position.
When the PGM is subsequently fired, it utilizes this “hot start” data to immediately start acquiring GPS satellites as they become visible in the sky. As the PGM rises in elevation and clears terrain-masking features (ex. flies out of the valley and above the ridgeline), it sees more and more GPS satellites. Once at least four GPS satellites come into view, the PGM starts navigating and is now able to complete its precision engagement on the target even when the weapon position location saw less than this minimum number of GPS satellites in the sky.
One More Thing
The PGM also needs to know about Ionospheric Correction data (automatically calculated by the DAGR when its sees multiple GPS satellites) since the GPS signals are “delayed” when passing through the Earth’s Ionosphere. This “delay” must be reflected in high-precision, accurate, time calculations which are an essential part of using GPS. Network Assisted GPS developed an innovative local-to-the-weapon-system Ionospheric Correction Extrapolation (ICE) software function. ICE can accurately estimate Ionospheric Correction data using only one GPS satellite along with the information passed down from JBC-P through AFATDS to the weapon system. This enables weapon systems to still be considered “precision capable” by AFATDS and shoot GPS-based PGMs when their firing positions are almost completely terrain-masked.
The Government conducted a system-of-systems live fire test of all the aforementioned elements of Network Assisted GPS (see Figure 2). This live fire test was conducted at Yuma Proving Grounds and went a successful 5-out-of-5 precision indirect fire missions using only one real GPS satellite in the sky (i.e. needed for ICE to work) with the balance of “hot start” data provided via Network Assisted GPS.
Since Network Assisted GPS was specifically designed to work automatically in the background when all of its required system-of-systems elements are present, it is totally transparent to the user. Other than general awareness, no special training is required by most warfighters.
Extending the Goodness of Network Assisted GPS
There are many different applications across the Services that can benefit from Network Assisted GPS just as it is built right now. For example, a dismounted warfighter has been under triple canopy jungle for an extended period of time. The warfighter is masked from “hearing” relatively weak GPS satellite signals through the dense tree foliage but is still able to occasionally get stronger communication signals. The warfighter may generally, but not exactly, know where he is but needs more accurate position information to perform the mission. To determine his exact position, the warfighter or one of his platoon mates would usually have to go out into a larger jungle clearing with a reasonable open view of the sky. They would then have to stay
in this open clearing for a relatively extended period of time – one or two minutes – to obtain accurate “cold start” GPS position information. This exposed time in the open increases their potential risk to enemy observation and threat of hostile fire. Leveraging Network Assisted GPS and ICE, the warfighter could rapidly obtain “hot start” GPS position information in a much shorter period of time (i.e. single digit seconds) to determine his exact position.
Network Assisted GPS was designed in a modular fashion and can also be modified/expanded to address other critical PNT-related problems and capabilities. For example, the mechanisms and “building blocks” established in Network Assisted GPS for current P(Y)-Code GPS applications can be leveraged to support new M-Code GPS applications as well as GPS augmentation capabilities such as Pseudolites. Both these new sources of GPS signal must similarly be pre-loaded to support precision indirect fire operations. Network Assisted GPS also provides a known reference source of GPS information that can easily be used to determine if the signals being heard are true.
The Bottom Line
Network Assisted GPS … Coming Soon to a Precision Fire Mission Near You!
Mr. Paul Manz currently serves as chief scientist for PEO Ammunition at Picatinny Arsenal, the Joint Center for Weapons and Ammunition. He is a multiple-certified senior member of the Army Acquisition Corps and certified Lean Six Sigma Black Belt with more than three decades of experience spanning the entire materiel development life cycle from science and technology through production and deployment. He recently won the 2016 Undersecretary of Defense for Acquisition, Technology and Logistics Workforce Individual Achievement Award in Engineering.
This article is a winner in the 2017 Maj. Gen. Harold J. “Harry” Green Awards for Acquisition Writing competition. A special supplement featuring the winning entries is online now, and will accompany the print version of the April – June 2018 issue of Army AL&T magazine. If you wish to be added to the magazine’s mailing list, subscribe online; if you’d like multiple subscriptions, please send an email to firstname.lastname@example.org.