Principles, Applications, and Future of Satellite Positioning (GNSS)

5G and Beidou are national strategic assets. Beidou, as a satellite positioning system, has become a world leader and is now embedded in many aspects of daily life. This article briefly introduces the principles and applications of satellite positioning to help readers better understand Beidou and GNSS.

Principle of Satellite Positioning

GNSS (Global Navigation Satellite System) provides positioning capability. The principle is that satellites broadcast time signals; a receiver measures the signal transmission time by comparing the satellite‑sent time with its local clock, then multiplies by the speed of light to obtain the satellite‑to‑receiver distance.

With signals from multiple satellites, a set of equations can be formed to solve for four unknowns: the receiver’s three‑dimensional coordinates (x, y, z) and the clock bias between the receiver and the GNSS system.

Each satellite’s ephemeris (orbital parameters) can be computed to obtain its precise position.

Ephemeris data can be obtained in two ways: direct broadcast from the satellite (slow, requires ~30 s to download) or via the internet (A‑GNSS, SUPL protocol). A‑GNSS allows devices to acquire ephemeris within seconds and is supported by most modern smartphones.

Satellite broadcasts include satellite ID, current timestamp, and ephemeris data. The timestamp is used to compute pseudorange: (local time – signal transmission time) × speed of light. Because the receiver’s clock is not synchronized with the satellite, this distance is called a pseudorange.

The carrier frequency (~1.5 GHz) has a wavelength of about 20 cm, giving the signal relatively good penetration (can pass thin walls), though indoor reception remains challenging.

Velocity can also be derived from Doppler shift, similar to police radar.

The Doppler‑based velocity equation relates frequency shift, wavelength, satellite and receiver velocities, and the projection direction.

Beyond positioning and velocity, GNSS also provides precise time (global time‑sync), which is more accurate than most device clocks.

Development History of Satellite Positioning

The first system was the U.S. Transit in the 1960s, followed by GPS in the 1970s (now 24 satellites). Other constellations include China’s Beidou, Europe’s Galileo, Russia’s GLONASS, and regional systems from Japan and India.

All systems use MEO satellites (~20 000 km altitude), with some GEO and IGSO satellites. Signals are CDMA‑encoded, allowing multiple satellites to share the same frequency. Data rates are low (e.g., GPS L1 ≈ 50 B/s) to maintain lock under low SNR.

Beidou’s advantages:

Better coverage in the Asia‑Pacific region (6 additional satellites).

GEO satellites can provide short‑message communication (free but requires dedicated antenna).

Architecture of a GNSS Receiver

The main modules are:

Antenna – captures weak satellite signals; larger antennas improve gain and multipath rejection. Types include ceramic patch (car‑top), professional choke‑ring antennas, and tiny smartphone antennas.

RF Front‑End – down‑converts, amplifies, and filters the raw signal.

Baseband Processing – decodes each satellite’s message; many parallel channels are needed for multi‑constellation, multi‑frequency operation.

PVT Solver – extracts timestamps and ephemeris from the decoded messages, then computes Position, Velocity, and Time.

Sources of Positioning Error and Accuracy Improvement

Errors arise from ionospheric and tropospheric delays, multipath reflections, satellite clock bias, and ephemeris inaccuracies.

Improvement methods include:

Dual‑frequency GNSS : Combines measurements at two frequencies to cancel ionospheric delay, achieving sub‑5 m accuracy.

Ground/Space‑Based Augmentation (SBAS, internet‑based corrections) to correct satellite clock and ephemeris errors.

RTK (Real‑time Kinematic) : Uses a nearby reference station to eliminate common errors, reaching centimeter‑level accuracy.

PPP (Precise Point Positioning) : Models all error sources precisely; convergence takes ~30 min.

Combined Positioning : Fuses GNSS with inertial navigation (e.g., Kalman filter) for higher update rates and robustness.

GNSS on Mobile Phones

Early GNSS receivers were specialized (e.g., Trimble, u‑blox). Modern smartphones integrate GNSS chips, leading to billions of devices and massive location data.

Differences between phones and professional receivers:

Small antennas limit signal capture and multipath rejection.

Limited channel count (mostly single‑frequency, few dual‑frequency).

Power and CPU constraints restrict algorithm complexity.

iOS vs Android

iOS hides raw GNSS data; only the final position is exposed. Android provides APIs for NMEA strings, GnssStatus, GnssMeasurement, and raw navigation messages, enabling developers to access satellite IDs, signal strength, DOP values, and raw pseudorange/carrier‑phase data.

Apps such as “Android GPS Test” can display real‑time satellite status.

Future Outlook

With the growth of mobile users and IoT, GNSS will continue to evolve. Low‑Earth‑Orbit (LEO) positioning constellations (e.g., Starlink) promise stronger signals, more data bandwidth, reduced ionospheric error, and faster PPP convergence.

On the device side, high‑precision positioning (RTK) is becoming mainstream; Huawei P40 already supports RTK with ~0.5 m accuracy, and Qualcomm is preparing RTK‑capable chips.

Application scenarios expanding rapidly include traditional surveying, precision agriculture, lane‑level navigation and autonomous driving, bike‑sharing precise parking, and UAV navigation.

References

Beidou official site: http://www.beidou.gov.cn/

GPS official site: https://www.gps.gov/

Galileo official site: https://www.gsa.europa.eu/