GNSS Constellations and the Three GPS Segments: 18-Geom-B2 Satellite Navigation, May 2015, Q1

Question 1: GNSS Constellations and the Three GPS Segments (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part 1 — Operational and emerging GNSS (5 marks)

Global Navigation Satellite Systems fall into global constellations (a full complement of satellites, nominally 24 or more in Medium Earth Orbit, giving worldwide coverage) and regional systems plus space-based augmentation systems (SBAS) that serve a limited area. At the time of this examination two global systems are fully operational, two more are in active deployment, and several regional systems are being built.

Fully operational (global). GPS (NAVSTAR), operated by the United States, has been at Full Operational Capability since 1995 with a nominal 24-satellite (currently ~31-satellite) constellation. GLONASS, operated by the Russian Federation, was restored to a full 24-satellite constellation in 2011. These two are, on their own, globally complete and continuously usable.

Under development (global). Galileo (European Union, civil-controlled) has completed In-Orbit Validation and is launching toward its 30-satellite constellation; BeiDou/Compass (China) is operational regionally over Asia-Pacific and is expanding to a global BeiDou-3 constellation. Regional / augmentation. QZSS (Japan, quasi-zenith orbits) and IRNSS/NavIC (India) are regional; the SBAS — WAAS (North America), EGNOS (Europe), MSAS (Japan), GAGAN (India) — broadcast wide-area differential corrections and integrity from geostationary satellites. For Canadian users GPS+GLONASS is the operational multi-constellation backbone, with WAAS augmentation available nationwide.

Part 2 — The three GPS segments (10 marks)

GPS is organized into three functional segments — space, control and user — each with a distinct task, that together deliver a positioning, velocity and timing (PVT) service.

The three GPS segments and the information flow between them: the control segment tracks the satellites and uploads corrected ephemerides and clock parameters; the space segment broadcasts the ranging signals; the user segment measures pseudoranges and computes PVT.

Space segment. The satellites themselves — nominally 24 (currently ~31) in six orbital planes at about 20 200 km altitude with a ~12-hour period. Their task is to generate and continuously broadcast the ranging signals: the L-band carriers modulated with the C/A and P(Y) ranging codes and the navigation message (satellite ephemeris, clock corrections, almanac and health). Each satellite carries highly stable atomic (rubidium/cesium) clocks that provide the precise time reference on which ranging depends.

Control segment. The ground infrastructure — the Master Control Station (at Schriever AFB), globally distributed monitor stations, and ground antennas. Its task is to monitor and manage the constellation: the monitor stations continuously track every satellite and measure its signal; the Master Control Station uses these observations to compute precise orbit (ephemeris) and clock corrections, predict them forward, and upload the refreshed navigation message to each satellite through the ground antennas. It also commands manoeuvres, manages satellite health and maintains system time (GPS Time).

User segment. The receivers — from survey-grade instruments to navigation and timing devices. Their task is to receive and exploit the broadcast signals: acquire and track the satellites, measure pseudoranges (and carrier phase and Doppler), decode the navigation message, and solve for the user’s position, velocity and time. The user segment is passive — it only listens — which is what allows an unlimited number of simultaneous users.

Concept & Theory

Topic. GNSS landscape and the space/control/user architecture of GPS.

Key relations. “Fully operational” = complete nominal constellation (24+ MEO satellites) with continuous global coverage; a solution needs ≥4 satellites (four unknowns: X, Y, Z and the receiver clock). The three segments map onto make the signal (space), keep it correct (control), use it (user).

Why this works. Ranging accuracy rests on knowing each satellite’s position and clock precisely; the control segment supplies exactly that through the uploaded navigation message, the space segment radiates it, and the user segment turns four or more ranges into a position fix. Separating the functions lets one control segment serve an unlimited, passive user population.

Common pitfall. Confusing augmentation systems (SBAS such as WAAS/EGNOS) with independent constellations — SBAS does not position on its own; it corrects and monitors the core GNSS. Also, constellation status changes yearly, so any “current status” answer must be dated.

Source. Hofmann-Wellenhof et al., GNSS, ch. 1, 9; Kaplan & Hegarty, Understanding GPS/GNSS, ch. 2.

Question 1 of 9: GNSS Constellations and the Three GPS Segments

Question 1: GNSS Constellations and the Three GPS Segments (15 marks)

Part 1 — Operational and emerging GNSS (5 marks)

Part 2 — The three GPS segments (10 marks)