What is the difference between B1I and B1C?

B1I is the legacy BeiDou signal (carried since BDS-2) at 1561.098 MHz using BPSK(2) modulation with 2.046 Mcps chipping rate and 2,046-chip code length. It provides backward compatibility with older BDS-2 receivers. B1C is the new BDS-3 interoperable signal at 1575.42 MHz (same center frequency as GPS L1 and Galileo E1). B1C uses MBOC(6,1,1/11) modulation, which is a multiplexed binary offset carrier combining BOC(1,1) and BOC(6,1) components. This modulation was jointly agreed upon by GPS, Galileo, and BeiDou for maximum interoperability. B1C has a data component (25% power) and pilot component (75% power) for improved tracking sensitivity. The pilot code length is 10,230 chips at 1.023 Mcps (10 ms period), matching GPS L1C pilot. Key advantage of B1C: a multi-GNSS receiver can process GPS L1C, Galileo E1, and BeiDou B1C with a single RF front-end and shared correlator architecture, reducing cost and power consumption.

What are the RF receiver requirements for BeiDou B1?

A BeiDou B1 receiver front-end must handle both B1I (1561.098 MHz) and B1C (1575.42 MHz) or select one: Antenna: RHCP (right-hand circular polarization), gain >3 dBic at 10° elevation, 3 dB beamwidth >140°. For multi-GNSS: cover 1559-1591 MHz (GPS L1 + BeiDou B1I + B1C + Galileo E1 + GLONASS L1). LNA: noise figure -20 dBm (to handle nearby cellular signals). Filter: SAW or BAW, center 1575 MHz, bandwidth 30-40 MHz for B1C, or dual-band for B1I+B1C. Out-of-band rejection >40 dB at 1559 MHz (GLONASS boundary) and >50 dB at GPS L2 (1227 MHz). ADC: 2-4 bit (single-bit for mass market), sampling rate 16.368 MHz (16× 1.023 MHz) or higher for wideband B1C. Sensitivity: -130 dBm acquisition, -160 dBm tracking (with 10 ms coherent integration on B1C pilot). Thermal noise floor: kTB = -174 + 10log(32.736×10⁶) = -98.9 dBm. Processing gain: 10log(10,230 × 1 ms / (1/1.023×10⁶)) = 40.1 dB.

How does BeiDou B1 compare to GPS L1 and Galileo E1?

The three GNSS systems now share the 1575.42 MHz center frequency with compatible modulations, enabling true multi-constellation receivers: Frequency: GPS L1 = 1575.42 MHz, Galileo E1 = 1575.42 MHz, BeiDou B1C = 1575.42 MHz. BeiDou B1I at 1561.098 MHz is the only non-overlapping signal. Modulation: GPS L1C = TMBOC(6,1,4/33), Galileo E1 OS = CBOC(6,1,1/11), BeiDou B1C = MBOC(6,1,1/11). All are variants of MBOC, enabling shared tracking loops with minor firmware differences. Chipping rate: all 1.023 Mcps (C/A code equivalent). Code length: GPS L1 C/A = 1,023 chips (1 ms), GPS L1C = 10,230 chips (10 ms), Galileo E1 = 4,092 chips (4 ms), BeiDou B1C = 10,230 chips (10 ms). Power at earth surface: GPS L1 C/A = -158.5 dBW, Galileo E1 = -157 dBW, BeiDou B1C = -159 dBW (data) / -158.5 dBW (pilot). Constellation size: GPS = 31 SVs, Galileo = 30 SVs, BeiDou = 36 SVs (30 MEO + 3 IGSO + 3 GEO). Combined: 97 satellites visible, enabling 50+ simultaneously visible from any location.

GNSS Navigation

BeiDou B1

/BAY-doh bee-wun/

The primary navigation band of China's BDS-3. Two signals: B1I (legacy, 1561.098 MHz, BPSK(2), 4.092 MHz BW) and B1C (interoperable, 1575.42 MHz, MBOC(6,1,1/11), 32.736 MHz BW). B1C shares center frequency with GPS L1C and Galileo E1 for multi-GNSS receiver compatibility. Constellation: 30 MEO (21,528 km) + 3 IGSO + 3 GEO = 36 SVs. B1C pilot: 10,230 chips at 1.023 Mcps. Min received power: −159 dBW (B1I), −158.5 dBW (B1C pilot).

B1C: 1575.42 MHz

B1I: 1561.098 MHz

SVs: 36

Understanding BeiDou B1

BeiDou's B1 band strategy reflects China's dual objective: maintaining backward compatibility with the older BDS-2 regional constellation (B1I) while achieving full interoperability with GPS and Galileo (B1C). The B1C signal at 1575.42 MHz uses MBOC modulation specifically designed for cross-system compatibility, allowing a single receiver front-end to process navigation signals from all three major GNSS constellations.

The multi-constellation advantage is substantial: with GPS (31 SVs), Galileo (30 SVs), and BeiDou (36 SVs) all transmitting at or near 1575.42 MHz, a receiver can track 50+ satellites simultaneously from a single RF channel. This dramatically improves positioning in challenging environments (urban canyons, dense foliage) where single-constellation receivers may lack sufficient satellite visibility.

B1 Signal Parameters

B1I (Legacy):
Center: 1561.098 MHz
Modulation: BPSK(2), 2.046 Mcps
Code: 2,046 chips, 1 ms period
Bandwidth: 4.092 MHz (null-to-null)
Power: −159 dBW min at earth surface

B1C (Interoperable):
Center: 1575.42 MHz
Modulation: MBOC(6,1,1/11)
Data: BOC(1,1), 25% power
Pilot: QMBOC(6,1,4/33), 75% power
Code: 10,230 chips, 1.023 Mcps, 10 ms
Bandwidth: 32.736 MHz

Processing Gain (B1C pilot):
G_p = 10·log(10,230 × 10 ms × 1.023 MHz)
= 10·log(10,230) = 40.1 dB

Multi-GNSS L1/E1/B1 Comparison

Parameter	GPS L1 C/A	Galileo E1	BeiDou B1C
Center freq	1575.42 MHz	1575.42 MHz	1575.42 MHz
Modulation	BPSK(1)	CBOC(6,1,1/11)	MBOC(6,1,1/11)
Code length	1,023 (1 ms)	4,092 (4 ms)	10,230 (10 ms)
Min power	−158.5 dBW	−157 dBW	−158.5 dBW
Constellation	31 SVs	30 SVs	36 SVs

Common Questions

Frequently Asked Questions

B1I vs. B1C?

B1I: legacy at 1561.098 MHz, BPSK(2), 2,046-chip code, backward compatible with BDS-2. B1C: interoperable at 1575.42 MHz, MBOC, 10,230-chip code, compatible with GPS L1C and Galileo E1 for multi-GNSS receivers.

Receiver requirements?

RHCP antenna, LNA NF <1.5 dB, 30 to 40 MHz SAW/BAW filter at 1575 MHz, 2 to 4-bit ADC at 16.368 MHz+. Sensitivity: −130 dBm acquisition, −160 dBm tracking. Multi-GNSS: single front-end covers GPS+Galileo+BeiDou.

How does B1C compare?

All three share 1575.42 MHz with MBOC variants. Combined: 97 SVs, 50+ simultaneously visible. BeiDou adds 3 GEO + 3 IGSO for enhanced Asia-Pacific coverage. Single RF channel for all three constellations.

Related Terms

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