GPS satellites orbit 12,500 miles from the Earth – resulting in very low signal strength

Can GPS be Trusted? Part 3

In my previous posts, I discussed the shortcomings and benefits of utilizing GPS as a primary Position Navigation and Timing (PNT) source. I also examined methods that provide Assured PNT (or A-PNT). These include hardening the GPS signal against jamming, while at the same time jamming the enemy’s receivers, utilizing encryption to provide spoofing immunity, and complementing GPS with other forms of PNT equipment.

This final post will focus on how complementing PNT systems can be combined together in a military vehicle and how this can be efficaciously integrated with other military ground vehicle systems.

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GPS satellites orbit 12,500 miles from the Earth – resulting in very low signal strength

Can GPS be Trusted? Part 2

In my previous blog post, Can GPS be Trusted? Part 1, I explained why commercial GPS position, navigation and timing (PNT) cannot be trusted for critical systems. This blog explains how a trusted PNT system can still use a GPS as the primary PNT source.

Adding Trust with A-PNT

Assured Position, Navigation and Timing (also known as A-PNT) is an Army initiative that attempts to provide soldiers and systems with a reliable and accurate source of position, navigation and time even during compromised, denied or spoofed GPS transmissions.

A-PNT is a set of goals that are driven by the Army’s Direct Reporting Program Manager PNT (PM PNT) and the PNT System of Systems Architecture (SoSA). They derive from technologies developed by the Army’s Communications Electronics Research, Development and Engineering Center (CERDEC) and generally follow COTS (Commercial Off-The-Shelf) technological improvements.

A-PNT’s goals can be summarized by the following:

  • Harden – resilience to denial, both from enemy and friendly jamming
  • Trust – immunity to spoofing
  • Complement – redundancy to failure by complementing GPS with multiple non-GPS PNT sensor

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GPS satellites orbit 12,500 miles from the Earth – resulting in very low signal strength

Can GPS be Trusted? Part 1

GPS is Everywhere

Most of us don’t think of GPS on a daily basis even though the technology has quickly become a quiet necessity in our lives. A vast majority of us walk around with an active GPS receiver in our pocket. The modern cellphone has been equipped with a tiny GPS receiver ever since the FCC mandated its use for location by rescue workers and 911 calls. We mostly take its presence for granted even when it is accessed by our favorite apps. Our GPS location allows us to navigate, browse the local big chain store inventory, tag our location on photos, get local news, and find our parking spot.

Similarly, the accurate navigation from GPS is also critical for efficient operation of commercial and military vehicles, aircraft, ships, UAVs, missiles, and smart bombs.

GPS Alone Cannot be Trusted!

We generally have pleasant experiences with GPS in our modern phones. With the right software, it gets you from A to B without any issues. However, occasional GPS dropouts do occur, leading to a “loss of GPS” or similar message from our navigation software, for instance, when driving in a tunnel. The fact that dropouts occur should not be surprising when you consider the technology.

GPS satellites orbit at an altitude of about 12,500 miles and each satellite has the radio power equivalent to a conventional light bulb. That results in a minuscule signal strength at the GPS receiver, which makes it very susceptible to radio noise, attenuation, and reflection from tall buildings. It also makes it very easy for attackers to jam with simple, low cost radio transmitters.

Back in 2013, Newark airport was inadvertently hit by a GPS denial attack when an employee, wanting to hide his company vehicle movements from his boss, was driving in the area with a GPS jammer that could be obtained for as little as $100.

The GPS signals are also at risk from legitimate terrestrial transmitters. In the US, the FCC is responsible for keeping the weak signal satellite bands separate from terrestrial ones. But even so, in 2011 a 4G LTE wireless company called LightSquared requested the terrestrial use of spectrum so close to the GPS band that it put the whole network at risk.

Worst still, commercial GPS receivers are susceptible to GPS spoofing attacks. Attackers can connect a strong transmitter to a GPS simulator programmed to fool a target GPS receiver into thinking that it is somewhere it isn’t. An infamous headline of such a spoofing attack was the capture of a US RQ-170 Sentinel UAV by the Iranian government in 2011. This was allegedly done by sending the UAV strong false GPS signals to make it believe it was flying over a friendly airfield instead of hostile territory.
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