Controlled Reception Pattern Antennas (CRPAs) have been removed from the United States government’s International Traffic in Arms Regulations (ITAR) control, official documents have revealed.
Having been removed from ITAR strictures, CRPAs will now be classified under less restrictive Export Administrative Regulations (EARs). Whereas ITAR is administered by the US Department of State, EARs are the responsibility of the Department of Commerce. The news is a step forward in helping safeguard users against Global Navigation Satellite System (GNSS) Position, Navigation and Timing (PNT) signal spoofing and jamming. GNSS jamming refers to electronic attack tactics to deny a PNT signal to a GNSS receiver. GNSS spoofing relates to the signal’s manipulation to convey false information. Deliberate disruption to GNSS PNT signals is a growing problem. On 20th January Nkom, Norway’s telecommunications regulator, revealed it had detected incidents of GNSS spoofing in the country’s airspace.
Low power signals
CRPAs are an innovative technology. An informative article by everythingrf.com explains that CRPAs are adaptive beam steering antennas. This means the antenna can adjust the direction from which it receives signals. Known as creating ‘nulls’ this process lets the antennas ignore dubious signals coming from a specific direction. Suppose an unusually powerful PNT-like signal is detected by a GNSS receiver coming from a bearing of 45 degrees. PNT signals are notoriously weak by the time they reach Earth, usually with an amplification of circa -125 decibels/dB. An unusually powerful signal should trigger suspicion.
Decibels measure signal amplification. In RF engineering, the closer a signal is to zero decibels, the stronger it is and hence the easier it can be for a radio antenna to receive. GNSS PNT signals use radio waves usually on frequencies of between 1.1 gigahertz/GHz and 1.6GHz. One GNSS PNT jamming technique involves transmitting much stronger, but fake, PNT signals towards a targeted GNSS receiver. The strength of these false PNT signals can wash out the real signal, preventing the receiver from obtaining the latter. False PNT signals can also be modulated with fake information, primarily false timing signals. Navigation is a function of measuring speed and direction over distance. An accurate timing signal derived from atomic clocks equipping GNSS satellites and transmitted as part of the PNT transmission is essential. Transmitting fake PNT signals into a GNSS receiver can cause the system to develop and display navigation errors.
In our above example, we have assumed that the GNSS receiver is obtaining false PNT signals from a bearing of 45 degrees relative to the receiver’s position. The unusually high power level of the false PNT signal is eliciting suspicion from the GNSS receiver’s processor. The system takes remedial action and blocks out a ‘slice’ of coverage between 40 degrees and 50 degrees allowing the receiver to ignore the fake signal. The CRPA simply no longer monitors that azimuth for PNT signals. The GNSS receiver can still obtain PNT signals, but not from the direction of where the fake transmissions are coming from. The asset of the CRPA is it performs this blanking electronically, making it highly responsive to the appearance of a fake signal. US government documents say such antennas can response thus in less than one second. Moreover, other directions from where additional fake signals maybe coming from can be blanked out.
Share and share alike
In the words of the official US government documents lifting the ITAR restrictions on CRPAs, “certain anti-jam antennas no longer provide a critical military advantage.” Some CRPAs will remain under ITAR restrictions as the US Department of Defence “seeks to control only the most sensitive and effective anti-jam antennas in (the) USML (US Munitions List).” Furthermore, “in removing CRPAs for PNT (from ITAR restrictions), the Department intends to facilitate civil global navigation system resiliency.”
As illustrated by recent incidents involving GNSS jamming, the danger of such occurrences is not limited to military GNSS users. Civilians are also at risk and air travel can be particularly affected. Air Traffic Control (ATC) secondary surveillance radar protocols like the Federal Aviation Administration’s Automatic Dependent System-Broadcast (ADS-B) depend on GNSS PNT signals. These signals help an aircraft determine its position when out of ATC primary surveillance radar range. Protocols like ADS-B let the aircraft’s position be shared with air traffic controllers via its transponder following SSR interrogation.
Unavailable or false PNT signals do not prevent an aircraft from navigation safely. Other techniques like radio navigation and dead reckoning are provide redundancy and prevent single points of failure. However, false PNT signals may hinder ATC efficiency with the risk of attendant flight delays. Some airports also require GNSS PNT signals for approach and landing procedures. This was the case for Tartu Airport in southern Estonia. In April 2024 Finnair was forced to suspend flights between Tartu and Helsinki as Russian GNSS jamming was affecting the PNT signals pilots relied upon to land at the airfield. The airport has since modified its landing and approach procedures so they no longer depend on GNSS transmissions.
Sharing the availability of CRPAs will help improve navigation safety by outflanking GNSS PNT attacks. Moreover, US companies who provide formally ITAR-controlled CRPAs can now do so under a much less restrictive regime.
by Dr. Thomas Withington
