The PNT industry must adapt to a changing landscape and find a way to provision for continual improvement. Why SDR architectures may be the answer.
In Ernest Hemingway’s “The Sun Also Rises” Mike is asked how he went bankrupt. His response: “Two ways, gradually and then suddenly.”
It is increasingly obvious that GPS is falling behind when compared with the European Union’s (EU) Galileo and China’s BeiDou Navigation Satellite System (BDS). There are many opinions as to why, but one theme echoed by many is it’s because GPS was first. It has the least modern signal designs and therefore has the least capabilities.
Requirements to maintain backward compatibility impose heavy burdens on any attempts at modernization and so, we fail to progress, weighed down by a half century of legacy baggage. The opportunity costs are immense. The L1 C/A signal was designed 53 years ago, has numerous deficiencies, and still, a 45-year-old receiver will operate with current broadcast signals (or spoofing variants). I’m of the opinion that software defined radio (SDR) architectures present an opportunity to escape the tyranny of unbounded backwards compatibility—but only if carefully managed.
The SDR Opportunity
At this year’s ION GNSS+ plenary, the European Space Agency’s (ESA) Marco Falcone, when asked about future directions for Galileo, observed that ESA needs to better coordinate with receiver manufacturers. Thinking about it more deeply, when both satellites and receivers are SDR, it presents important opportunities to better manage the retirement of obsolescent or failed signals and, instead, move toward a paradigm of continual and planned improvement cycles. Installed hardware can stay the same because signal upgrades are software upgrades. This is particularly true with civil signals, as the associated receivers are mostly connected, minimally via a USB port. The “PNT solver” in your cell phone is mostly an SDR.
Using the PC industry as a parallel, specific signals might have minimum required SDR capability. Think of minimum A/D conversion rate, minimum number of correlators, minimum required RF memory, etc. And of course, there needs to be requirements for hardware authentication of software loads and machine-readable signal specifications [1] using an industry standard like the trusted platform module (TPM). Similarly, like an OS, generous advance warning as to when support for a particular signal release is ending, such as we are turning it off or we are upgrading the message format, and so forth.
A Different Approach
Looking toward the cellular industry for inspiration, they recognized they had a serious innovation problem more than 30 years ago. Two important outcomes were the genesis of SDR and the standardization of release cycles. Every 2 years, there is a “major release” where specifications for new capabilities are rolled out. The process has been phenomenally successful, leading to several orders of magnitude improvements in data rates. The 3G signal, state of the art in 2001 when first broadcast, was decommissioned in 2022. Staged improvements within each generation pay more attention to backwards compatibility but again, the lifetime of a signal is limited. Each generation is an opportunity to revitalize the enterprise.
At this point, many of you may be thinking this approach may work for cellular but not for safety critical PNT systems requiring onerous certifications. Think of commercial aviation. A 2-year major release cadence is probably too fast, but what about 5 years or 10 years? I think it is doable but requires a considered approach.
Toward this end, signal modifications fall into roughly 3 categories:
1. Backwards Compatible: These are modifications that are fully backwards compatible with receivers designed to use prior signal generations. Examples include new message types for data authentication, code puncturing for range authentication ala. Chimera, emergency warning services ala. Galileo’s Emergency Warning Satellite Service, and so forth.
2. Digital Impact: This is where digital aspects of a signal are redesigned to achieve an important objective but with the caveat that the modulation format is not changed. A BPSK(1) signal remains BPSK(1), but without the burden of backwards compatibility. Using GPS as an exemplar:
• Except for L1C, the forward error correction (FEC) on civil signals is antiquated. L1 C/A doesn’t have FEC at all and the non-interleaved convolutional codes used on L2C and L5 offer greatly inferior performance when compared with 4G and 5G FEC standards (TS36.212 and TS38.212 respectively). Substantial performance gains in scintillation, rapid fading, pulsed interference and in Eb/No are possible. Bit rates can be increased to support enhanced message content.
• The Keplerian curve fit representations developed in the 1970s provides point solutions for satellite position given time. Much better, bit-efficient, long-term orbit representations are now available [2] where medium Earth orbits (MEO) can be accurately conveyed for up to a week with less than 1-meter errors at the end of the week. Propagating the orbit forward in time requires more computations, but when combined with active clock steering on the satellite, using such a representation can have major impacts on receiver power consumption because regular data reading is not required. Occasional snapshots collecting a few milliseconds of RF are fine for many applications, such as asset monitoring, IoT applications and hiking.
• Short code sequences have been good for acquisition but bad for structured interference response. L1 C/A code is efficiently jammed using structured Gold code jamming. Longer codes and/or concatenated secondary sequences offer major improvements and can be handled readily by modern receiver designs.
3. Analog Impact: Changes to a signal’s modulation format or its center frequency have direct impacts on a receiver or transmitter’s interaction with the signal and with the environment. Multipath characteristics shift. Filter effects shift. Antenna effects on observables like carrier phase and pseudorange also shift, especially when using CRPAs. Any changes to a signal’s format that impact analog characteristics need to be considered very carefully but not out of hand rejected. OFDM, CPM and multi-h formats[3]have great merit, but tread cautiously.
Returning to a prior point, what is a failed signal? L2C was first broadcast in 2005 with CNAV messaging starting in 2014. It has seen limited adoption mainly because it is in a less protected frequency band and because the L5 signal is more capable in dual frequency applications. L2C is not a bad design, it just doesn’t provide a distinct capability. Rather than continue with L2C, I would argue for an L2 signal that does something different. For example, a secure ranging signal for civil users to use in spoofing mitigation, authentication and proofs of location.
I would also add there is no substitute for live sky testing. SDR development paradigms have repeatedly shown the power of being able to fix design flaws by changing the signal structure. Operational SDR satellites can broadcast experimental and regional signals as part of their transmit portfolio. Design flaws can be worked out in the laboratory of real life. Without SDR, it can take years to fully understand operational weakness in a signal’s design. With SDR, you can fly it before you buy it.
Future Proofing
GNSS operators must find a way to provision for continual improvement. The ideas presented here are in nascent form, but I do believe they point toward a way to break the tyranny of backwards compatibility. The GPS Block III satellites are expected to be 35 years on orbit. Needs and requirements change much faster, witness the “sudden need” for civil signal authentication. SDR offers a mechanism for future proofing, but only if we employ carefully engineered approaches for managing and fielding innovation. We must adapt, or face technical bankruptcy, first gradually, then suddenly.
References
(1) Sanjeev Gunawardena “A High Performance Easily Configurable Satnav SDR for Advanced Algorithm Development and Rapid Capability Deployment,” ION ITM 2021 https://doi.org/10.33012/2021.17808.
(2) Oliver Montenbruck et.al. 2020, DOI: 10.1002/navi.404 “A long-term broadcast ephemeris model for extended operation of GNSS satellites.”
(3) Logan Scott, “Continuous Phase Modulation for Navigation” ION JNC2017.
Author
Logan Scott has over 45 years of military and civil GPS systems engineering experience. He is a consultant specializing in radio frequency signal processing and waveform design. Logan has been an active advocate for improved civil GPS location assurance for over 20 years and was the first to describe how civil navigation signals could be authenticated using delayed key concepts central to the Chimera signal. For the past 10 years, he has been developing advanced signal concepts, including Chimera for NTS-3, AFRL and the University of Colorado. He is a Fellow of the Institute of Navigation and a Senior Member of IEEE. He received ION’s Weems award in 2022 and the Kepler award in 2025 and is a member of the National PNT Advisory Board. He is the author of “Interference: Origins, Effects, and Mitigation in PNT21” and holds 46 U.S. patents.
