Compare Earth Observation Satellites: Resolution, Coverage, Revisit
This guide explains how to compare Earth observation satellites by three key specs: spatial resolution, swath coverage, and revisit time. You’ll learn what each spec means, how to read satellite datasheets, and how to trade them off for your application. No prior remote sensing experience is needed, but a basic familiarity with satellite imagery helps.
What You’ll Need
- A list of candidate satellites (e.g., from ESA’s Earth Observation Portal, USGS, or commercial provider websites)
- The official spec sheets or mission pages for each satellite
- A notepad or spreadsheet for comparison
- Your application requirements (e.g., detect 5-meter objects, cover a 200 km region weekly)
Step-by-Step
- List your candidate satellites. Start with the missions you’re considering. Include both public (Landsat, Sentinel-2) and commercial (WorldView, Planet) options if relevant. Write them down in a table with columns for each spec.
- Find the official specification source. Go to the mission’s official page or its operator’s datasheet. Avoid third-party summaries unless they cite the original. Look for a “Technical Specifications� or “Instrument� section.
- Record the spatial resolution. This is the pixel size on the ground. It’s usually given in meters (e.g., 10 m, 30 m, 0.5 m). Note whether it’s panchromatic (black-and-white), multispectral (color), or SAR (radar). Write down the resolution for the band you care about—often the multispectral resolution is coarser than panchromatic.
- Record the swath width. This is the ground strip the satellite images in one pass, typically in kilometers (e.g., 185 km for Landsat, 290 km for Sentinel-2). Write it down.
- Record the revisit time. This is how often the satellite can image the same point on the equator, assuming clear skies. It’s usually given in days (e.g., 16 days for Landsat, 5 days for Sentinel-2). Note that some satellites have a shorter revisit at high latitudes due to orbit convergence.
- Check for off-nadir capability. Many commercial satellites can point sideways to image targets before they pass directly overhead. This reduces revisit time but increases the effective ground resolution (pixels get larger) and can reduce swath width. Note the maximum off-nadir angle and the resolution at that angle.
- Create a comparison table. For each satellite, list: resolution (m), swath (km), revisit (days), and any off-nadir limitations. Use consistent units.
- Evaluate against your needs. For resolution, ask: can I detect the smallest object I care about? A rule of thumb is that you need at least 2-3 pixels across the smallest feature to detect it reliably. For coverage, check if the swath is wide enough to cover your area of interest in one pass, or how many passes are needed. For revisit, consider cloud cover—if the area is cloudy, you may need a shorter revisit to get a clear image.
- Rank your options. Assign a score or simply rank by how well they meet your minimum requirements. If no single satellite meets all, consider combining multiple missions (e.g., use Sentinel-2 for frequent wide-area monitoring and a commercial satellite for detailed spot checks).
Common Mistakes
- Mixing panchromatic and multispectral resolution. A satellite might have 0.5 m panchromatic but 2 m multispectral. If you need color, use the multispectral number.
- Ignoring off-nadir resolution loss. Revisit times are often quoted for nadir (directly below), but if the satellite points sideways to get a quicker revisit, the actual resolution can be significantly coarser.
- Using equatorial revisit for high-latitude areas. Revisit times are shortest at the equator; at higher latitudes, the same satellite may pass more frequently due to orbit convergence.
- Forgetting cloud cover. A 5-day revisit is useless if the area is cloudy 80% of the time. Effective revisit is often longer than the orbital revisit.
- Confusing swath width with coverage. Swath width is the width of a single pass, not the total area covered in a day. Total daily coverage depends on the number of orbits and the swath.
Tips
- Use the ESA Earth Observation Portal or the CEOS database to compare multiple missions quickly.
- For change detection, prioritize consistent revisit and consistent viewing angle over absolute resolution.
- Check if the satellite has a “tasking� mode—some can be pointed to specific areas, which can improve revisit but may cost extra.
- For agricultural or environmental monitoring, consider the spectral bands (e.g., red edge) as well as resolution.
- Look for the “revisit time� in the mission’s “observing capacity� section—it often includes off-nadir pointing.
- If you’re comparing SAR satellites (like Sentinel-1), note that resolution and swath depend on the acquisition mode (e.g., Stripmap vs. Interferometric Wide swath).
Troubleshooting
- Specs seem contradictory. Check if the resolution is given as “ground sampling distance� (GSD) or “ground resolution.� GSD is the pixel spacing, which is slightly different from the actual resolution. Use GSD for comparison.
- Revisit time varies by source. Some sources quote the “temporal resolution� including off-nadir capability; others quote only the orbital repeat cycle. Make sure you’re comparing the same type.
- You can’t find swath width. Look for “swath� or “coverage width� in the instrument description. If not listed, check the mission’s user handbook.
- Resolution seems too good to be true. Verify if it’s the resolution at nadir or at the edge of the swath. Edge resolution is usually coarser.
- You need historical data. Revisit time matters for future acquisitions, but for historical data, check the archive’s actual acquisition dates. Some satellites have gaps in coverage.
Frequently Asked Questions
Q: What is the difference between spatial resolution and pixel size?
A: Pixel size (or ground sampling distance) is the distance between pixel centers. Spatial resolution is the ability to distinguish two close objects. They are related but not identical; resolution is usually slightly worse than pixel size.
Q: How does revisit time affect my ability to monitor a disaster?
A: For disaster response, you want a short revisit (1-3 days) to see changes quickly. Satellites like Sentinel-2 (5 days) or Planet (daily) are better than Landsat (16 days) for this.
Q: Can I get both high resolution and wide swath?
A> Generally, no. There’s a trade-off: high-resolution sensors have narrow swaths (e.g., WorldView-3 has 0.3 m resolution but only 13 km swath). Wide-swath sensors like MODIS have 250 m resolution but 2330 km swath.
Q: What does “off-nadir� mean in satellite specs?
A> Off-nadir is when the satellite points its sensor sideways instead of straight down. This allows imaging areas not directly under the satellite, reducing revisit time, but it increases the look angle and degrades resolution.
Q: How do I compare SAR satellites to optical ones?
A: SAR (radar) satellites have resolution and swath specs similar to optical, but they can image through clouds and at night. Compare them on the same metrics, but note that SAR resolution depends on the radar mode (e.g., Stripmap vs. ScanSAR).
Q: Is revisit time the same as temporal resolution?
A: Often yes, but temporal resolution can also include the effect of off-nadir pointing. Some sources use “revisit� for the orbital repeat and “temporal resolution� for the actual time between images of the same point, which may be shorter.
Related
Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.