Why Surveyors Plan a Mission Before Going on Field
Pre-field planning is the difference between a clean day of work and a day spent re-doing measurements because of a satellite black-out you didn't anticipate. Every professional GNSS surveyor, whether running RTK, static post-processed, or PPP-style workflows, opens a mission planner before loading the truck. It tells them, for the specific site and day they care about, how many satellites are above the horizon, how the geometry behaves, and which two hours of the day will give them the lowest PDOP.
The new GNSS Mission Planner on SurveyingPedia does exactly that — for free, in any browser, without an account, and with all four major constellations enabled by default: GPS, GLONASS, Galileo, BeiDou. This article explains what the planner shows and how to read it like a working surveyor.
What Sky Plots and DOP Charts Actually Mean
A sky plot is a polar projection of the sky as seen from a point on Earth. North is up, east is right, and the center of the diagram is the zenith — directly overhead. Each satellite is a dot. The closer to the edge, the closer to the horizon; the closer to the center, the higher overhead.
When evaluating a project site, look at three things:
* Spread. Are satellites clustered together, or are they spread across the whole hemisphere? Even spread = good geometry = low DOP. * Mask. Is the dashed red circle eating into your usable sky? Urban canyons, dense forest and steep terrain force a larger mask, often 15° or even 20°. * Time evolution. A sky plot is just one moment. The constellations change over the next hour. The companion charts under the sky plot show satellites visible and PDOP over time.
PDOP, HDOP, GDOP — What's What
* PDOP (Position DOP) — overall 3D position dilution. Below 3 is good for most field surveying. * HDOP (Horizontal DOP) — horizontal coordinate accuracy. Critical for cadastral and topographic surveys. * VDOP (Vertical DOP) — vertical accuracy. Critical for leveling-style work where elevation matters more than xy. * GDOP (Geometric DOP) — combines position + clock accuracy. Surveyors mostly look at PDOP, but engineers running long-baseline static work check GDOP.
A working rule of thumb: if PDOP is at or below 2 for the entire two-hour window you plan to work in, you'll likely hit centimeter-level accuracy on RTK with good baseline length. If it spikes above 4, you'll see your fix solution shake and you may have to wait it out.
Why Multi-Constellation Matters
Modern receivers track every GNSS they can — typically all four major ones, plus regional systems like QZSS over Japan. With ~30 GPS, ~24 GLONASS, ~28 Galileo, and ~45 BeiDou satellites operational at any time, the typical mid-latitude site sees 40+ satellites above a 10° mask. The geometry gets so good that PDOP drops below 1.0, which was unimaginable when GPS was the only game in town.
The planner lets you toggle constellations on and off. This is useful when:
* You're using an older receiver that only supports GPS + GLONASS — turn off Galileo and BeiDou to see what your kit actually receives. * You're investigating why a particular constellation is failing on site — turn off the others to isolate it. * You want to see how multi-constellation matters: compare PDOP with GPS only versus all four. The difference is dramatic.
How to Use the Planner: A Workflow
The planner uses the same workflow professional surveyors run every morning:
1. Set your location. Click "Use my location" for the field site, or punch in the lat/lng of the project. The presets cover major cities for quick what-if scenarios. 2. Pick the day. The default is today, but you can plan tomorrow's session or even next week's static observation. 3. Set the window length. 12 hours is the default; shorten it if you're planning a specific morning or afternoon. 4. Set the mask. 10° is the global default; raise it to 15° for forested or urban work, or 20° for steep mountain valleys. 5. Read the recommendation. The green "Best 2h window" banner at the top tells you when the geometry is sharpest. That's when you want your rover collecting data. 6. Hover the time chart. The sky plot updates to show what the sky looked like at that hovered time. Useful for understanding satellite passes. 7. Share the plan. The "Copy permalink" button gives you a URL with all your settings baked in. Send it to the field crew before they leave.
Limitations to Be Aware Of
The planner uses Two-Line Element sets (TLE) from CelesTrak, refreshed every 6 hours. TLE propagation via SGP4 is accurate enough for field planning (sub-arcminute on satellite positions), but it is not precise enough for orbit determination work — for that, use IGS final products or your receiver's own broadcast ephemerides.
The planner also does not model multipath or local obstructions. A 15° mask is a global heuristic; the real obstructions around your station — a building wall to the south, a high tree to the east — will eat into the sky differently. The planner gives you the satellite geometry; reading the actual site survey for obstructions is still on you.
Finally, satellite health flags are not currently filtered in. Approximately 1–2 satellites at any given time are marked unhealthy or in maintenance and shouldn't be counted in your PDOP calculation. The error introduced is small — typically a few percent on the DOP — but worth knowing about for precision work.
Why This Tool Is Free
Most GNSS planners are either tied to a manufacturer (Trimble Planning, Leica MissionPlanner) or are pay-per-use enterprise tools. We believe pre-field planning is foundational professional practice, and the data behind it — orbit elements, propagation math — is fully public. There is no good reason for a working surveyor to be locked out of basic planning.
So we built one. It runs in the browser, it's free, there's no account, and you can share permalinks. We pay for the server out of the same budget that runs the rest of SurveyingPedia. If you find it useful, link to it from your own surveying notes or blog — that's how we keep the lights on, indirectly: by getting found by other surveyors who need the same tool.
Related Tools on SurveyingPedia
* Deformation Monitoring Dashboard — real-time earthquake feed + CORS station network * CORS Station Directory — 2,700+ continuously operating reference stations worldwide * Geodetic Benchmark Map — 634,000+ control points * Coordinate Converter — transformations across 25+ reference systems * Embeddable Map Widgets — drop our live maps into your blog or university page
Open the planner now and bookmark it. It will save you a re-do.