Why now is the time for solutions like L5-direct™ Assured-PNT from oneNav to anchor a more robust system.
We at oneNav have been working for more than five years to develop the first commercially viable L5-direct GNSS receiver, which is now implemented in both FPGA and ASIC silicon. Unlike current
L1/L5 GNSS receivers that tend to support L5 as minimally as possible, oneNav fully implements L5-direct search and track capabilities to ensure maximum resilience to jamming and spoofing.
The receiver has been validated for jamming and spoofing immunity at defense service proving grounds on land vehicles, aircraft and sea vessels, and has been tested for accuracy and sensitivity in dense urban settings using wrist worn smartwatch antennas. Table 1 summarizes these key benefits.
The availability of L5-direct coincides with significant developments that make now the time for it to anchor a new, more robust Assured-PNT system. For example, when the Next Generation Operational Control System (OCX) is completed in 2025, GPS will officially join Galileo and BeiDou as an operational L5 constellation allowing both military and commercial segments to use L5 signals. With 73 space vehicles (SVs), the L5-band now has nearly three times more satellites than any single constellation at L1, making it the dominant unified-signal structure in GNSS.
Then there’s the growing concern for the impact an electronic warfare (EW) attack would have on critical infrastructure and the defense sector. This was recognized in the March 6 Federal Communications Commission (FCC) Notice of Inquiry (NOI) for Promoting the Development of Positioning, Navigation and Timing (PNT) technologies and solutions. Because all receivers available on the market today rely on the L1 C/A signal developed in the 1970s, they are vulnerable to potential jamming and spoofing attacks that could easily disable all commercial receivers. A more sophisticated attack on L1 and L2 signals could also disrupt M-Code, a military-grade encrypted GPS signal, indirectly impacting military operations.
The FCC inquiry prompted responses from more than 100 companies, each proposing an innovative solution. While the government can now consider complementary sources for PNT, resilient GNSS remains at the center of each solution. Direct acquisition of the L5-band signals delivers the significant improvement in resilience modern navigation systems need. oneNav is the only company that offers direct L5 acquisition, and until that functionality and the resilience it offers is available in commercial and non-commercial receivers, retirement won’t be in my plans.
L5 Offers Enhanced Resiliency
Our response to the FCC explains the resilience L5-direct provides. Live demos we’ve presented show a difference in jamming acquisition resiliency of up to 10 dB between L5 and L1 and 15 dB in tracking. We’ve also tested L5-direct in simulated EW environments. In February, L5-direct was evaluated in-flight while strapped to a UAV. In March, it was tested in a maritime environment on Chesapeake Bay, operating together with a tactical grade IMU and high stability reference clock. In both cases, the L5-direct capability to acquire at and switch between the E5a and E5b sidebands along with industry leading tracking KPIs gave us a superb anti-jamming and anti-spoofing advantage.
Jamming and Spoofing at L5 Requires More Power
In the maritime in-flight testing cases, the simulated jamming targeted a bandwidth of 2 MHz for the L1 C/A signal and 30 MHz for an M-Code BOC(10,5) signal. The latter is broadcast on both L1 and L2 frequencies, providing M-Code with additional resilience against interference. The L5 signal with 50 MHz BW and increased transmit power has jamming resilience like M-Code. Compared to L1, jamming and spoofing at L5 requires more than 20x transmission power because of the wider bandwidth, higher satellite transmit power and multiple signals. Jamming devices targeting L5 will be easier to detect and eliminate.
The acquisition and tracking resilience in oneNav receivers will further improve by 3dB when the U.S. National Space-Based Positioning, Navigation, and Advisory Board (PNTAB) helps to make sharing navigation data symbols available on the national level. The data would be available for free in real time, so connected receivers can remove the data symbol phase rotations and exploit the phase coherence of the data and pilot channels on all L5 signals.
Spoofing at L5 Requires Much More Sophisticated Transmissions
Spoofing has become a major threat both in and outside of conflict zones. An increasing number of flights make the news as spoofing disrupts takeoffs and landings. To test the performance of L5-direct against leading smartphones and smartwatches, oneNav set off to Haifa in Israel, an area heavily spoofed by the Israel Defense Forces (IDF). All competitor devices tested in Haifa were spoofed 125 km away from the Beirut Airport while L5-direct remained accurate to within one meter throughout the test drive. Smart devices are so often spoofed that shops in northern Israel have started selling paper maps again.
Having 73 active L5 satellites and more on the way means that up to 25 satellites with the same physical layer are visible. L5-direct acquires the L5 signals in the same way for GPS, Galileo, QZSS and BeiDou in two frequency bands (E5a and E5b) and with 2-4 PRN codes transmitted per satellite.
To be effective, spoofing our receivers would require a significant multisystem upgrade that can generate up to 75 PRN codes with fine time alignment to GPS time. The 1 kHz overlay codes in all L5 signals mean an L5 spoofer must align to GPS time 20x more accurately than an L1 spoofer would to capture a receiver that has already obtained a correct GPS time. Additionally, a MEMs oscillator can maintain fine time long enough to allow the L5-direct receiver to complete a mission without allowing spoofers with errant time to be strong enough to overpower the true signals. This means most devices equipped with L5-direct would reach their target.
Dense Urban Accuracy
Eliminating dependence on L1 band signals improves dense urban accuracy. By having a larger bandwidth, signal observables contain additional information that can be used to detect when reflections are present in the combined received signal. By identifying these reflections, they can be corrected, and the signals can be used in the solution rather than eliminated because of measurement uncertainty. The GPS L1 chipping rate is 1.023 MHz, while the modernized signals in the L5 band have a 10x higher chipping rate of 10.23 MHz. This means one side of L1’s correlation peak covers 293 m, while one side of an L5 peak covers only 29.3 m. This yields inherently more precise measurements. In dense urban areas, where multipath reflections are typically tens of meters, the first arriving signal can be detected using L5-band signals, but not with the L1-band.
To compare the performance of L5-direct and L1 receivers in urban canyons, oneNav recorded sample location data in the San Francisco Financial District using a pedestrian wristwatch antenna. The data revealed two reflections after the true signal delayed by 45 and 60 meters. Because the half-chip spacing for the L1 channel is 146.5 meters, constructive combining of the correlation peak in an L1 receiver typically (Figure 3b) produces an artificially high SNR. The resulting multipath error incorrectly shifts the peak to the right. The 14.5-meter L5 half-chip spacing (Figure 3c) is 10x more precise. L5-direct can identify and eliminate dense urban reflections, resulting in more precise location.
Leveraging the Tools We Have
To paraphrase a statement made two decades ago by a famous Secretary of State, we combat threats with the tools we have, not with the tools we might one day have. Because interference is a present-day problem, it calls for immediate solutions using the best available resources. L5 is the most resilient GNSS signal, and L5-direct, through direct acquisition, can help build a more resilient Assured-PNT in civilian and military domains.
The Resilient-GPS (R-GPS) satellite program currently being planned in the U.S. is an effort to improve resilience by adding more satellites to the U.S. fleet. The program would launch new, small satellites with Y-M-CA codes to address space EW and satellite viability. The Y-M-CA receivers will coherently process a specific set of the P(Y), M-Code, and C/A on L1 and L2. Knowing they come from the same satellite, they’re combined to improve overall GPS performance. This new system must include L5 signal transmission capability for maximum resilience.
oneNav is optimistic GPS L5 will become officially operational in 2025, helping the defense and commercial sectors harness the advantages of the modern signals. EW is a real threat, but with resilient GNSS and direct acquisition of the L5 signals, we can be prepared. L5-direct is ready to be implemented in all devices as a standalone solution or an augmentation to existing solutions. With the resilience of L5-direct protecting us against EW, I will happily retire.
PAUL MCBURNEY is the co-founder and CTO of oneNav, his second GNSS startup. He began his focus on GNSS while earning his PhD in electrical engineering and has since been developing location technology and systems at Stanford Telecom, Trimble, eRide (his first GNSS startup as co-founder and CTO), and GopherHush (his location analytics startup). Most recently, he was a GNSS architect at Apple. At Trimble Labs, he co-invented a host-based GPS IP that was successfully licensed into consumer and automotive segments. eRide successfully licensed GPS IP into the Japanese mobile phone and automotive segments and was acquired by Furuno. Paul holds a BS in Engineering and Physics from St. Ambrose University, and a PhD in Electrical and Electronics Engineering from Iowa State University. Paul has more than 40 location related patents.
