The hook
GNSS isn't one system — it's a constellation of constellations: GPS (US), GLONASS (Russia), Galileo (EU), BeiDou (China). A modern receiver listens to dozens of satellites at once and trilaterates position from precise timing of microwave signals. Survey-grade work uses three flavors: static, RTK, and PPP.
Memorize these
Concepts that show up on the exam
GNSS
Global Navigation Satellite System — umbrella for all positioning constellations. GPS is one GNSS; the term GNSS implies multi-constellation.
Static GNSS
Receiver collects data on a known or unknown point for hours, then post-processes against base station data. Highest precision (sub-cm); used for control networks.
RTK (Real-Time Kinematic)
Base station broadcasts corrections to a rover; the rover computes cm-level position in real time. Standard for topo, layout, boundary work in open sky.
Network RTK / VRS
Instead of your own base, your rover gets corrections from a state CORS network. A "Virtual Reference Station" is computed from multiple network sites near your rover. No base station to set up.
PPP (Precise Point Positioning)
Standalone receiver uses precise satellite orbits/clocks (downloaded from IGS or similar) to achieve cm-level accuracy without a base. Slower convergence (15-30 min).
OPUS
Online Positioning User Service from NGS. Upload 2+ hours of static GPS data; receive a published NAD83 coordinate within minutes. The free workhorse for U.S. control work.
CORS
Continuously Operating Reference Station. NGS and state networks operate ~3,000+ CORS sites in the U.S., publishing data freely. Core infrastructure for RTK and post-processing.
Multipath
Satellite signal bounces off a building, tree, or vehicle before reaching the antenna. Adds cm-level noise. Why open sky matters.
| Method | Time on point | Accuracy (horiz) | Best for |
|---|---|---|---|
| Autonomous (no corrections) | instant | 3-5 m | navigation only |
| DGPS (code corrections) | instant | 0.5-1 m | GIS data collection |
| RTK (single base) | seconds | 1-2 cm | topo, boundary, layout (open sky) |
| Network RTK (VRS) | seconds | 1-3 cm | same as RTK, no base setup |
| Static (post-processed) | hours | <1 cm | control networks, high-order |
| PPP | 15-30 min | 2-5 cm | remote sites, no base/network available |
Don't fall for these
What trips people up
Working under canopy
Trees, bridges, and tall buildings block satellites and cause multipath. Cycle-slip errors creep in. RTK accuracy degrades fast — what shows as "fixed" may not actually be fixed. Always cross-check.
PDOP / GDOP without checking
Geometric Dilution Of Precision tells you how the satellite geometry amplifies measurement noise. PDOP > 6 means your "1 cm" rover is actually maybe 3-4 cm. Mission planning catches this before you drive out.
Wrong datum out of OPUS
OPUS solutions come back in a SPECIFIC NAD83 realization (currently NAD83(2011) epoch 2010.00). Mixing this with NAD83(86) coordinates without transformation introduces 1-2 m errors.
Antenna height mistakes
RTK rovers measure to the antenna phase center, but you record height to the bottom of the rod. Confusing slant height with vertical, or forgetting the antenna model offset, biases vertical by cm-to-dm.
Test yourself
How well did it stick?
A quick 5-question check on GNSS / GPS Surveys. See where you stand and what to review.