Before a drone even takes off, it's often more sensible to discuss a simpler point: what does this site currently look like from the sky, and how has it changed? Free satellite imagery is actually beneficial for this — pre-flight reconnaissance, some historical land-use context before writing a report, or even just panning over a wide area to see if you want to spend flight hours on it.
Though, do set the expectations early. Anything you can actually get on a regular basis likely only goes to ~10-meter resolution (at best) with "free" satellite imagery. Good for context and change detection. This is well short of what a drone orthomosaic delivers and understanding where the line sits is something you should be arming yourself with to know how to actually use these sources.
The Direct Data Portals
They are dedicated websites for browsing and downloading raw satellite data.
Landsat archive, the real strength of USGS EarthExplorer is depth and not sharpness. The archive dates back to 1972, making it the ideal instrument for monitoring how a coast line, jungle or site has evolved over decades—rather than days. Resolution varies from 15-30m depending on the band which is coarse by modern sensor standards, but more than acceptable for looking at long-term change. In addition to Landsat images, EarthExplorer also contains MODIS/Aster thermal data and free declassified Cold War era CORONA spy satellite imagery that is available for download after creating a USGS account.
Operated by the European Space Agency, Copernicus Browser / Copernicus Data Space Ecosystem tends to be the "best in class" free optical/radar imagery available today. Sentinel-2 offers 10-meter resolution at a revisit time of approximately five days, and Sentinel-1 provides radar imagery capable of penetrating clouds — really handy where optical satellites rarely get a "good" look. The interface is a little difficult to use, but the data quality for free is pretty unbeatable.
NASA Worldview and NASA Earthdata Search cover the other end of the tradeoff: nearly real-time, but lower resolution. Worldview uses MODIS and VIIRS to provide near-daily, full globe coverage, with applications in broad area environmental monitoring like fire tracking or weather context and not for site-level detail.
Is Any Free Imagery High-Resolution?
Only sometimes but only if you: The Maxar Open Data Program (actually) publishes in truly high resolution — the sort with identifiable single buildings and vehicles — except for large disaster events, not as standing global coverage. Unlike disaster response, the free imagery realistically caps off around ~10-meter resolution of Sentinel-2. Anything more precise than that, on-demand for a location of your choice, is commercial.
Free Imagery Accessed Directly in your GIS
Instead of downloading the files and importing them separately, many of these sources — from Copernicus Data Space Ecosystem itself — can be extracted directly within your software you are using.
The OpenLayers plugin in QGIS provides basemap imagery for use as reference layers, adding Google and Bing basemaps, while World Imagery from ESRI could be added as an ArcGIS REST service through the tool "Add ArcGIS Map Server Layer." In the case of real satellite data as opposed to basemap tiles, both Google Earth Engine and SentinelHub plugins introduce their own catalogs directly into the QGIS UI, while a dedicated Maxar Open Data plugin allows catastrophe-event imagery — cloud-optimized GeoTIFFs included — to be browsed and downloaded all within QGIS.
Recently, The team at Google and Woolpert Digital Innovations jointly developed a Python toolbox built to directly bring Earth Engine's data catalog and cloud compute into ArcGIS's Model Builder workflows so that ArcGIS Pro users can more easily access it.
Google Earth Engine proper is still the deepest catalog of the pack — a massive archive with actual cloud computing force behind it — but if you don't like some scripting — either through its JavaScript code editor or python API — there is definitely a real learning curve.
Viewers vs. Actual Downloadable Data
Important distinguished: Google Earth, Bing Maps and tools like our Zoom Earth are great for fast taking a look at a pinpointed location — but they are viewers, not data. Unlike you would do with EarthExplorer or the Copernicus Browser, you cannot extract a raw, analyzable file from Google Earth. If all you want is a quick visual check, then a viewer will do. If you want a goal that you'll actually process, measure, layer into a GIS project, get one of the download-enableable portals instead.
Why would a drone survey company even need to use satellite imagery?
But that does not replace the need for all that drone data — it is a separate tool for a different step in the work process. Satellite imagery can also be handy for a rapid check of vegetative cover, access roads and any recent land changes that may affect mission planning prior to committing flight time to site. In particular, historical satellite imagery can give a far more representative record of a site's condition years, if not decades, before the time of a modern survey (one drone flight alone unfortunately cannot provide sufficient context). And for long-distance work, it is now a sensible means of zoning real estate before determining where drone protection truly makes sense.
When it fails, is when accuracy actually matters. A 10-meter pixel doesn't even begin to cut it as engineering-grade measurements, and certainly not close to what's required for topographic survey work or any deliverable involving real dimensions. And itis that gap—between satellite imagery returning context, and a drone flight returning a measurable model of the ground itself—that is precisely why the discipline of drone imaging and processing even exists.
Which Source Fits Which Job
For decades of historical context, you'll want to start with Archive and Landsat (the latter is on EarthExplorer). If you seek the sharpest regularly available free imagery, youre currently best off with Copernicus/Sentinel-2. NASA Worldview has you covered there too — if near-daily and less sharp detail is all you need! For users which are already operating inside QGIS or ArcGIS, ingesting these sources as a plugin or connector shortcuts a round journey through a browser. And if a project ever requires some real precision —anything that you would need in order for a client to be able to make a decision on grading, drainage or GIS analysis downstream—it is where the accurate geospatial information part of free satellite imagery pushes the baton up the chain to solutions of drone-based data collection (not down).
