The benefits of maxL5 over minL5

 

by Paul McBurney CO-FOUNDER AND CTO, ONENAV

Current state of the art multifrequency GNSS receivers operate by receiving L1 first and then L5. L5-first is a viable answer to the call for more resilience in GNSS as is being discussed in government and technical circles to protect vital national infrastructure. It is suggested as part of “Toughening Category 4: Signal Alternatives” to protect, toughen and augment (PTA) the current GNSS systems described by Brad Parkinson’s article in the March 2022 issue of GPS World.

The need arises from attacks directed by bad actors on a large scale, such as electronic warfare, and on a more humane scale, by bad actors such as self-jammers and spoofers. On top of that, normal interference can cause desensitization and denial of service on GNSS receivers from myriad terrestrial and satellite communications.

The PTA plan presents the Denial Radius Reduction Ratio (DRRR) figure of merit and shows that a J/S increase of 15 dB produces a DRRR of 0.18. Whereas a receiver without this additional 15 dB of J/S could be denied fixing out to 1 km from a given transmitter, a receiver with an additional 15 dB J/S would be denied out to only 180 m from the same transmitter.

The improvement in terms of area is proportional to radius squared. The article identifies that the J/S capability is different among GNSS signals and the best performance is obtained with L5, mainly because it has the highest chipping rate. L1C has a code length of 10,230 chips, the same as L5, but it is spread over 10 msec and has the same chipping rate as L1 C/A.

There are currently 72 L5 signals between GPS, Galileo, BeiDou and QZSS transmitting the same physical layer features of 10.23 MHz chipping rate, 1 kHz overlay codes and higher transmit power compared to nearly all L1 signals with a 1.023 MHz chipping rate and lower transmit power. The combination of these features at L5 is close to achieving this 15 dB performance level over L1.

One might conclude that the current start of the art of a receiver with both frequencies (aka, a hybrid L1+L5) has this resilience. However, the market does not currently offer the ability to directly acquire L5 signals overall use cases of GNSS assistance without first acquiring signals at L1. This means they can only achieve this resilience when the interference is encountered after acquiring and fixing at L1. As soon as the L1 is lost and the position and time uncertainty grow beyond the receiver’s capacity to autonomously search for L5 signals, the receiver is denied service at the interference level tolerable at L1. If you cut the receiver into L1 and L5 pieces, only the L1 side is capable of fixing autonomously. As noted by Dennis Akos et al. (“Testing COTS GNSS Receivers Using Only a Subset of Supported Signals,” ION JNC 2023), “support for several signals/frequencies provides integrity and robustness.” Specifically, “under jamming scenarios, signal diversity can allow a receiver to still generate an accurate position solution.”

Current receivers are not able to acquire L5 for reasons related to history, cost and power consumption. Historically, the promise of L5 accuracy was so attractive that it was added to legacy chipsets based on L1 even when it was only partially deployed. It was impractical at that time to require L5 acquisition when there were fewer L5 satellites than at L1. Cost and power are related to the fact that L1 receivers’ acquisition methods are sized to acquire the L1, E1, B1 and G1 signals. Memory and compute capacities, including the digital clock speed, are sized for slower chipping rates and hence shorter code lengths. At this performance level, conventional time domain correlation is adequate. Some receivers deploy frequency domain methods at L1 and achieve a lower cost and power than time domain methods with similar capacity. However, the L5 acquisition complexity with time domain correlation is 100 times more than L1 as its complexity increases with N2 , meaning the cost and power to acquire L5 is out of reach. While using a time domain acquisition engine to acquire L5 may be possible for strong signals when the code and frequency search space is constrained for those signals, directly acquiring L5 with conventional methods would have serious shortcomings in many use cases.

Interestingly, the signal designers across all GNSS systems have cleverly designed the L5 signals so they can be easily acquired after acquiring their counterparts on L1. The L5 primary and secondary code is predictable based on learning the L1 primary code and navigation data bit phase. E5a and B2a primary and secondary codes can be predicted by learning the well-designed E1/B1 primary and secondary code phases that have the same total period: the combination of the 4 msec code lengths synchronous with 25 bits of secondary code are in phase with the E5a 100 msec overlay code. After an L1 fix with fine time, L5 can similarly be directly acquired easily with limited searching.

These signal characteristics help the L1-first receiver. However, they don’t add the resilience suggested by the PTA. These approaches enable an efficient minimum L5 (“minL5”) implementation. What is needed for resilience and L5-first is a maximum L5 capability (“maxL5”).

Today, the L1 spectrum is increasingly crowded with non-terrestrial networks (NTN) and cellular ones. There is also more jamming and spoofing at L1 than at L5 due to the maturity of L1-based products, the workhorses for the past 30 years. If you want to jam a minL5 receiver, you only have to jam L1. Consumers will face more denials of service, but now there is a real big brother to L1 — L5 in the protected aviation band — that can have its back in a dark alley.

Furthermore, L5 is now the dominant unified-signal frequency band in GNSS. With 72 satellites currently, L5 now has nearly three times more satellites than any single constellation has at L1, with a common physical layer: 10 MHz chipping in 1msec with 1 msec overlay codes. This is the first common physical layer across all systems in the history of GNSS. This means it is possible to implement a single DSP to efficiently acquire L5- band signals across all systems. These common signal attributes provide the largest dynamic range for acquisition among GNSS signals at any frequency, and in normal signal environments virtually eliminate cross-correlation, a nuisance that reduces reliability and is more severe with shorter codes.

A cost-and power-effective L5- first acquisition processor requires advanced frequency domain correlation (AFDC). Fortunately, L5 full code acquisition complexity using frequency domain correlation methods is one-quarter of L1 full code acquisition complexity using time domain correlation methods. It is solved by equating the frequency and time domain complexity 3NLog2N + N = 0.21M2 when N=2*10230 and M=2*1023 to achieve ½ chip search spacing. To acquire many L5 satellites in parallel with high sensitivity, there is a need to do many acquisitions in one millisecond, and to employ long non-coherent integration times. The AFDC needs to be completed in as few clock cycles as possible to allow re-use of each millisecond. The AFDC also needs to be able to concurrently capture multiple signal components to take advantage of the additional transmission power of L5 satellites: up to four components on Galileo E5 and BeiDou B2 combined.

Two architectures can be envisioned with L5 first. In an L1 + maxL5 hybrid, acquisition could be enabled in parallel at both frequencies or in series directed by interference detection across both bands. The band with the fastest acquisition could be used to assist the lagging band. This device could be cut in half, (assuming the antenna wires are not cut!) to become two independently operating receivers. Alternatively, a maxL5 standalone would offer the lowest cost with excellent resilience and performance. “Decoupling the L5 from the L1 by using an independent L5 embodiment enables single-frequency receivers to utilize the most modern and secure civilian signals, or alternatively allows realization of the full performance benefits of a multi-frequency receiver,” notes Professor Todd Humphreys, of the University of Texas at Austin.

Of the four “toughening categories” suggested by Parkinson, L5 first is likely the most cost- and power-effective solution to improve J/S by nearly 15 dB. The third method he suggests requires a steerable multiple-element antenna. While effective in improving J/S, this method will require advances in lowcost antenna design and low-power parallel processing. The other two methods, state-3 code tracking and the use of inertial sensors, are available and can be implemented in software. Thus, solutions are available to improve the customer experience towards accurate and ubiquitous navigation.

Read the article in GPS World