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In my previous blog I explored how reliant we are on satellite navigation systems, how vulnerable we might be if they should fail and looked at some ways that our systems could be made more resilient. In this blog I’ll provide an overview of how it’s possible to conceive a System of Systems Approach (SOSA) capable of offering a PNT solution in a congested and hostile environment. One that could be found in a defence application space. In particular, I’ll focus on complementary PNT techniques for encrypted receivers that offer enhanced resilience.

In the defence arena, we are operating in challenging times with a number of conflicts taking place in the Middle East and Europe and increasing tensions in other regions including Asia Pacific. PNT information is critical to a variety of defence applications to ensure that deployed resources can manoeuvre, co-ordinate and strike effectively. However, defence operatives must assume that in any operational environment adversaries will contest or deny access to the Electromagnetic Spectrum (EMS) - and therefore critical PNT information - through offensive Navigation Warfare (NAVWAR) actions. To mitigate this, military PNT systems must offer a suitable level of resilience through their design and adopt a system of systems approach which provides back up and references to any primary PNT source.

An example of the challenges around PNT in defence was highlighted in an article by “Defense One” in April 2024. It indicated that in the Ukraine conflict, a new Ground-Launched Small Diameter Bomb (GLSDB) had failed to hit a number of targets due to Russian electromagnetic warfare. The weapon uses GPS to navigate to its targets and also has a built in inertial measurement unit that navigates to targets through the use of accelerometers and gyroscopes. The article went on to indicate that since the start of the conflict there have been a number of reported issues with a variety of guided munitions including Excalibur, Guided Multiple Launch Rocket System (GMLRS) and Joint Direct Attack Munitions (JDAMS) which all use Global Positioning System (GPS) for PNT. With the increasing effectiveness and use of electromagnetic warfare (EW) in this conflict, there is an increasing desire to enhance the effectiveness of these guided munitions that can cost between $40,000 and $160,000 a round.

Before we consider how defence equipment designers can enhance the resilience of the PNT solution they adopt for an application, it is worth considering the challenges they are up against in a contested environment. Typically, there are two main challenges to GPS navigation which are (signal) spoofing and jamming. Spoofing can come in two guises which are:

• Meaconing
  The re-transmission of authentic GPS signals to a target receiver. If successful the receiver will report its position from the re-transmitted data rather than its true position
• Code / carrier attack
  The attacker replicates a GPS signal using an RF signal generator. Their aim is to align the replica signals to authentic signals being received by the target receiver before increasing power and taking control of the receiver’s tracking loops

GPS jamming is blocking out a particular satellite signal frequency band and this form of attack has become particularly prevalent in both Eastern Europe and the Middle East. It is achieved by overpowering the relatively weak space-based GPS signals with local stronger signals. Commonly used interferers include wideband Gaussian noise, wideband phase/frequency modulation, wideband spread spectrum and narrowband swept pulse. These attacks are typically relatively easy to detect, however for GPS receivers focused on 1 or 2 frequencies, they can be difficult to counter.

For defence applications there are a variety of PNT solutions that can be deployed, however a large proportion have encrypted GPS receivers as their standard. Encrypted GPS (M-Code or legacy P(Y)-Code) is a US Department of Defence (DOD) classified code that uses the L1 (1575.42 MHz) and L2 (1227.60 MHz) frequency bands. It is designed to offer a variety of operational benefits which include:

• Jamming resistance
  M-Code signals can be delivered to specific regions using spot beam transmissions from the ~23 in-orbit GPS block satellites. The high gain directional antenna on satellites aims the M-Code signal at a specific region of the earth which offers gains of some 20dB over full earth coverage beams. This high power signal is more resistant to jamming due to higher Carrier to Noise (C/No) ratio.
• Blue Force Electronic Attack Compatibility
  The unique wider modulation used in the M-Code signal allows for selective jamming of the commercial GPS L1 C/A signal that may be used by adversaries, while still maintaining the M-Code signal for military applications
• Anti-spoofing
  M-Code signals are encrypted and they use a Public Key Encryption (PKE) and a private M-Code key which is closely guarded at a classified level. M-Code receivers are specifically designed to detect and reject spoofed signals that fail cryptographic verification of the private key.

While M-Code receivers offer excellent resilience to spoofing, they can still be jammed with wide band jammers and so PNT designers for military applications need to consider how they can enhance anti-jamming and alternative navigation (Alt-Nav) solutions. Improved anti-jamming can often be achieved through the use of Controlled Radiation Pattern Antennas (CRPA) that use phased array techniques to reject interference signals or boost satellite signals. Alt-Nav solutions to GPS M-Code can come in a variety of guises which are indicated in the table below

Table 1 – Solutions that can complement M-Code receivers

Alternative techniques	Example of alternative	Common feature
Multiple Global Navigation Satellite System (GNSS)	Galileo GLONASS BeiDou QZSS NAVIC	Global or regional in nature with range of satellites Offer different frequency bands to L1 and L2.
Wireless PNT signals	eLoran LEO-provided PNT Space based augmentation Pseudolites	RF signals from transmitters of known location. User computers time and location from pseudorange Maintain user anonymity
Signals of opportunity	WiFi / WLAN LTE / 4 G Cellular 5G Cellular DVB-T DAB LEO Comms Constellations MF DGNSS	Existing RF signals not primarily intended for navigation User computers location from triangulation and ranging User may or may not get time from signal
User equipment based	Holdover clock Chip scale atomic clock Inertial measurement unit LIDAR	No new signals Relies only on modification to user equipment Typically sensors and technologies May maintain user anonymity
PNT resilience technologies	Nulling antennas Direction finders Jamming or signal degradation detection systems	Enables continued use of GPS signals and / or existing user equipment Maintains nominal PNT performance of benign environment

Source: Homeland Security Operational Analysis

In the increasingly challenging NAVWAR environment in which defence equipment and personnel operate, it is imperative to have a well-designed PNT solution that considers a layered and system of systems approach. A simple layered approach might consist of:

• Select the right GNSS signals to use and use as many as possible
• Use encrypted GNSS and ensure receiver is keyed
• Use an Anti-jam antenna
• Integrate additional positioning sensors like inertial measurement units

Through appropriate design and back up, defence PNT designers can ensure their equipment and munitions stay fully operational irrespective of the environment in which they are operating.