Question 5 of 7: Satellite Positioning — WGS84, ITRF, PPP and RTK Networks

Question 5: Satellite Positioning — WGS84, ITRF, PPP and RTK Networks (20 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.

Given. GPS positioning referenced to WGS84, and the relationships of WGS84 to NAD83(CSRS) and to ITRF, plus the PPP and network-RTK positioning techniques.

Find. (a) WGS84↔NAD83(CSRS) compatibility and level; (b) WGS84↔ITRF compatibility; (c) the PPP concept, field procedure and accuracy; (d) the definition, principle, accuracy and an Ontario example of a network-RTK service.

(a) WGS84 vs NAD83(CSRS). They are not the same datum, WGS84 is geocentric and aligned to the ITRF, while NAD83(CSRS) keeps the NAD83 origin about 2 m from the geocentre, so they are “compatible” only at the metre level (about 1–2 m). Because NAD83 carries its historic ~1–2 m non-geocentric offset while modern WGS84 is aligned to ITRF at the centimetre–decimetre level, coordinates of the same point in the two systems differ by 1–2 m. For any survey requiring better than a metre they must be related by a proper transformation (with epoch), not treated as equal.

(b) WGS84 vs ITRF. Yes — they are compatible at the centimetre to few-decimetre level. Since 1994 the successive realizations of WGS84 (G730, G873, G1150, G1674, G1762…) have been deliberately aligned to the contemporaneous ITRF, so for practical positioning WGS84 and ITRF coordinates of a point agree to a few centimetres. The justification is definitional: WGS84 is maintained by the U.S. NGA specifically to coincide with ITRF, so any residual difference is at the noise level of ordinary GPS work.

(c) GPS-PPP. Precise Point Positioning determines absolute coordinates with a single dual-frequency GNSS receiver — no local base station — by applying precise satellite orbit and clock products (from the IGS or NRCan) and rigorously modelling the ionosphere (via the ionosphere-free combination), troposphere, solid-Earth tides, antenna phase-centre variations and relativity. Field procedure: occupy the point with a geodetic dual-frequency receiver and log carrier-phase and code data (typically for tens of minutes up to a few hours), then submit the RINEX file to a PPP service such as NRCan’s CSRS-PPP, which returns coordinates directly in NAD83(CSRS) or ITRF at a chosen epoch. Accuracy: after the ambiguity/solution converges, a few centimetres in static mode (down to ~1–2 cm horizontally with long occupations); kinematic PPP gives decimetre-level results. The trade-off is a convergence period of typically 20–40 minutes.

(d) GPS-RTK network. A network RTK service is a permanent array of GNSS reference stations (a CORS network) whose data are combined at a central server to model the spatially correlated errors (ionosphere, troposphere, orbits) across the region. Principle: the server generates area corrections — VRS (Virtual Reference Station), FKP or MAC — and streams them to the rover, usually as RTCM messages over the internet via the NTRIP protocol on a cellular link; the rover fixes carrier-phase ambiguities in real time and computes its position instantly. Accuracy: 1–3 cm horizontal, 2–5 cm vertical in real time, maintained over the whole network area (not just near one base). Ontario example: the commercial Can-Net network (operated by Cansel), and similarly SmartNet North America, provide network-RTK coverage across southern Ontario; a user subscribes, then connects a cellular-equipped RTK rover to the service’s NTRIP caster (mountpoint) to receive VRS corrections. Natural Resources Canada’s CACS/CBN continuously operating reference stations underpin the national framework these services tie into.