Growing awareness of the potential vulnerabilities introduced by the military's dependence on GPS data has driven demand for new deployed approaches for detecting threats to GPS and providing position, navigation, and timing (PNT) information to the warfighter in GPS-denied environments.

Growing awareness of the potential vulnerabilities introduced by the military’s dependence on GPS data has driven demand for new deployed approaches for detecting threats to GPS and providing position, navigation, and timing (PNT) information to the warfighter in GPS-denied environments.

The data from GPS isn’t only used to provide location and mapping information; it also delivers critical timing information used for mission synchronization and a myriad of other activities. If the satellites that provide GPS become inoperable, or terrain conditions make that data unavailable, the effects will be felt across many onboard ground vehicle systems. To compensate for the loss of GPS data, and to provide an alternative resource that enables GPS data to be trusted if an adversary attempts to deliver spoofed PNT data, system designers are beginning to offer subsystems that deliver complementary PNT data. The combination of a ground-based GPS receiver application module (GB-GRAM), inertial navigation system (INS), chip scale atomic clock (CSAC), and other complementary PNT services enables the warfighter to help establish Assured PNT truth.

The prevalence of GPS-reliant capabilities on ground vehicles is a key factor behind the U.S. Army’s VICTORY [Vehicular Integration for Command, Control, Communication, Computers, Intelligence, Surveillance, and Reconnaissance/Electronic Warfare (C4ISR/EW) Interoperability] initiative. One of the results sought by proliferating VICTORY-compliant architectures across the ground vehicle fleet is the elimination of the forest of GPS antennas that sprout from the exterior of today’s ground vehicles, each devoted to supporting a single, stove-piped subsystem. Because VICTORY enables network-based data sharing between a platform’s electronic subsystems, it can reduce the number of GPS antennas on a vehicle down to one.

The “bolt-on” approach

In the effort to provide Assured PNT (A-PNT) services in GPS-denied environments, two competing approaches have recently emerged. In one approach, the vehicle’s Selective Availability Anti-Spoofing Module Defense Advanced GPS Receive (SAASM DAGR) unit, which provides the vehicle’s serial-based ICD-GPS-153 compliant services, is housed in a standalone appliance that is bracketed to the interior wall of the platform. With this scenario, adding complementary PNT services, such as the CSAC or the VICTORY network, can require the bolting on of expansion mezzanine componentry to the appliance.

Also, in this approach, to provide the INS capability needed to ensure A-PNT, requires a separate third-party external unit, further adding to the vehicle’s size, weight, and power (SWaP) burden. What’s more, because the DAGR appliance is exposed in the cabin environment rather than enclosed in an unobtrusive rugged chassis, the numerous cables it requires to communicate with the vehicle’s antenna and GPS-dependent subsystems are also exposed.

Another approach?

An alternative approach for delivering A-PNT takes advantage of the pre-existing space claim of an open-architecture commercial off-the-shelf (COTS) line-replaceable unit (LRU) that is already resident in the vehicle. This approach, in which the A-PNT capability is essentially poured into an existing chassis, delivers compelling advantages when compared to the bolt-on solution. Such an LRU can support A-PNT by tightly integrating the GB-GRAM, INS, CSAC, and other complementary PNT services needed to help establish A-PNT truth within an existing chassis. Compared to the DAGR bolt-on approach, integrating A-PNT into an already resident LRU eliminates the entire bracketed DAGR receiver appliance, obviates the need for a separate stand-alone INS subsystem, and removes all of the DAGR’s exposed cabling.

The existing LRU approach for hosting A-PNT takes up no additional in-vehicle real estate and requires only minimal configuration changes to the vehicle. This open architecture approach also lowers total cost of ownership by enabling cost-effective incremental upgrades, reduces the logistical burden, and provides a COTS roadmap for technology insertion. Use of a COTS-based LRU design eases the deployment of additional state-of-the-technology A-PNT services. For example, it eases upgrading from a GB-GRAM to an M-CODE (Military Code) GB-GRAM.

Curtiss-Wright’s COTS-based Digital Beachhead (DBH) product family is an example of such an approach. (Figure 1.)

Figure 1: The Curtiss-Wright DuraDBH-672 Digital Beachhead, a rugged COTS single-box solution integrating a GbE switch and quad-core vetronics processor for supporting VICTORY services, is an example of a pre-installed LRU that can host complementary PNT services on ground vehicles.