Modern Global Navigation Satellite Systems (GNSS) rely on sophisticated signal structures to deliver the precise positioning and timing information used across aviation, telecommunications, autonomous systems, and countless everyday applications. While receivers interpret these signals seamlessly, the underlying modulation techniques represent decades of engineering evolution. This article provides an accessible overview of how GNSS signals are constructed—starting from classic BPSK-DSSS and progressing through BOC, CBOC/TMBOC, and advanced AltBOC waveforms.
Understanding GNSS Signals: Why Modulation Matters
GNSS signals are narrowband carriers carrying navigation data, often enhanced through forward-error correction and pilot channels. To achieve global reliability, these carriers are “spread” across a much wider bandwidth using Direct-Sequence Spread Spectrum (DSSS).
This spreading delivers three key benefits:
- Higher ranging accuracy, through improved correlation and processing gain
- Better multipath resistance, by decorrelating delayed reflections
- Stronger resilience to interference, because narrowband disturbances affect only a small portion of the spread signal
Even though each GNSS satellite uses its own pseudo-random (PRN) code, multiple satellites can coexist within the same RF band without interfering—thanks to these spread-spectrum properties.
1. BPSK-DSSS: The Foundation of GNSS Signals
The simplest GNSS waveform is BPSK-DSSS, exemplified by the legacy GPS L1 C/A signal.
It uses:
- One PRN code
- One navigation data stream
- A chip rate of 1.023 Mcps
- A processing gain of ~43 dB
- A data rate of 50 bps
Modern signals—such as those used on L5—enhance this model by incorporating two orthogonal PRN codes, improving performance and robustness.
2. Binary Offset Carrier (BOC): A Shift Toward Robustness
Many new GNSS signals employ the BOC modulation, which shifts energy away from the carrier center frequency using a ±1 sub-carrier.
Key characteristics:
- Sub-carrier is a square wave (50% duty cycle)
- Two phasing types: BOCₛ (sine-phased) and BOC꜀ (cosine-phased)
- Example: GPS L5 (i and Q channels)
- 10.23 Mcps spreading
- 100 bps data rate
- Processing gain increased to ~50 dB
This increased spreading improves both multipath performance and interference resistance, making BOC the backbone of modern high-precision GNSS signals.
3. Composite & Time-Multiplexed BOC: Greater Spectral Control
Some signals require even more flexible spectral shaping. That’s where CBOC and TMBOC come in.
Composite BOC (CBOC)
Galileo’s E1 CBOC(11/6, 1/1) waveform combines two sub-carrier frequencies—6.138 MHz and 1.023 MHz—across its data (E1B) and pilot (E1C) channels.
Time-Multiplexed BOC (TMBOC)
This method alternates between wide and narrow BOC components over time.
Both approaches allow designers to fine-tune signal shape, improving compatibility across frequency bands while optimizing performance for different user needs.
4. AltBOC: The Advanced Hybrid Approach
The AltBOC (Alternate BOC) waveform represents one of the most sophisticated GNSS modulation techniques, used notably in the Galileo E5 band.
Its design includes:
- Four sub-carriers (two wide at 6.138 MHz, two narrow at 1.023 MHz)
- A constant-envelope composite signal
- Eight carefully weighted chip amplitudes
- Interleaved pilot and data channels
This hybrid design provides:
- Superior timing precision
- Inherent notch filtering (reducing the burden on receiver filtering)
- Outstanding rejection of narrowband interference
- High ranging accuracy without expanding the occupied spectrum
AltBOC is often considered the “gold standard” for modern GNSS waveform engineering.
Conclusion: The Evolution Toward More Resilient GNSS Signals
As GNSS modernization continues, we see a clear progression:
- BPSK-DSSS: simple, foundational
- BOC: improved accuracy and interference robustness
- CBOC / TMBOC: enhanced spectral refinement
- AltBOC: exceptional precision and built-in signal protection
These advanced modulation schemes are carefully documented in official interface control documents and form the backbone of today’s resilient GNSS systems. Their inherent waveform-level protections—such as AltBOC’s strong spectral notch—help ensure reliable, high-precision performance even in challenging RF environments.
A Simple Analogy to Understand GNSS Signal Evolution
- BPSK-DSSS (GPS L1 C/A) → a standard incandescent bulb: simple, even illumination
- BOC (GPS L5) → a bulb with reflectors: more focused, efficient distribution
- AltBOC (Galileo E5) → a multi-spectrum LED streetlamp: multiple beams, built-in filtering, higher clarity
Each step improves brightness, clarity, and resilience—just like GNSS signals.
