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Satellite imagery specifications describe three different things: how much ground detail a pixel samples, how soon a satellite can observe a place again, and how much ground or how many places a mission covers. They are related, but none alone tells you whether an image will answer your question. The useful choice depends on the feature you need to see, how quickly it changes, and the area you need to monitor.
What does spatial resolution mean?
Spatial resolution, often expressed as ground sample distance (GSD), describes the ground footprint represented by a pixel or detector sample. A stated GSD of one metre means each pixel corresponds to a nominal one-metre ground sampling interval under the specified viewing geometry; it does not guarantee that every one-metre object will be recognizable.
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NASA’s Applied Remote Sensing Training Program describes spatial resolution in terms of the ground surface represented by a pixel, while the USGS glossary describes the area an imaging system can distinguish. In practice, visible detail also depends on the spectral band, contrast, atmosphere, viewing angle, motion, processing, and product quality. The cited definitions do not establish a universal minimum object size that can be identified from a given pixel size.
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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Check whether a number refers to a native measurement or to the pixel spacing of a resampled or displayed product. USGS defines resampling as modifying image geometry; creating a finer output grid does not, by itself, add captured ground detail.
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“Resolution” can also mean spectral, radiometric, or temporal resolution. For questions about vegetation, water, or heat, the bands and measurement type may matter as much as spatial detail. The USGS glossary distinguishes these resolution dimensions: USGS: What does resolution mean in satellite imagery?
What does revisit time mean?
Revisit time is the elapsed time before a system can observe the same point or target area again. It is a user-centered measure of another observation opportunity, not simply the time until an orbital ground track repeats exactly. NASA ARSET and the European Space Agency explain that practical revisit depends on factors including orbit, swath, overlap, latitude, and sensor pointing or acquisition capability.
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A specification may describe a nominal constellation capability, an actual opportunity to acquire an image, or the interval between observations in an archive. Those are not interchangeable. Even when another pass or pointing opportunity exists, clouds, illumination, viewing angle, tasking priority, and processing delay can affect whether a usable image is collected and available when needed. There is no single universal formula that converts nominal revisit into usable observations.
For example, NASA ARSET’s 2023 training material gives a 16-day interval for an individual Landsat Operational Land Imager (OLI) in its comparison with MODIS. NASA’s Landsat 9 mission page describes eight-day temporal coverage when Landsat 9 operates in coordination with Landsat 8. That eight days refers to the paired Landsat coverage, not a repeat interval for Landsat 9 alone. The Landsat 9 page also states a 185 km swath.
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What does coverage mean?
Coverage is ambiguous unless the specification says which kind it means. It may describe the width of ground recorded in one pass, the overall area represented in a dataset or mission archive, or the frequency with which imagery is acquired.
- Swath: the width of ground observed on a pass. Wider swaths can collect data over more ground at once and may create more opportunities to image a target, depending on orbit, latitude, overlap, and acquisition mode.
- Spatial extent: the total surface area covered by a dataset or mission archive. The USGS glossary defines spatial extent as the overall surface area covered by a dataset.
- Temporal coverage: how often imagery is collected or available for an area. Clarify whether this means a nominal revisit, actual acquisitions, or archive frequency.
When a specification gives a swath width, note how it is defined. The Joint Agency Commercial Imagery Evaluation (JACIE) guidance advises specifying whether swath width is measured at nadir—the point directly beneath the sensor—or using a different geometry. JACIE also advises reporting GSD at nadir and, for very wide-swath instruments, at the swath edges, where sampling can differ. Planned performance should be replaced with actual on-orbit performance when that information is available.
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Why detail, revisit, and coverage involve trade-offs
Finer spatial detail commonly comes with a narrower swath. That can mean more images, passes, or time are needed to cover a large region. A broad-swath instrument may collect a wide area frequently but provide coarser spatial detail. The right balance depends on whether the job is to identify small features, track a rapidly changing event, or survey a large area consistently.
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The European Space Agency illustrates the trade-off with examples of WorldView-2 at 0.46 m spatial resolution and a 16.4 km swath, compared with a Sentinel-2 example at 5 m spatial resolution and a 290 km swath. The ESA guide’s 5 m figure is an example in that comparison, not a general statement of current Sentinel-2 product-band GSD; check the band and product specification before comparing it with another sensor.
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NASA ARSET’s 2023 training material contrasts MODIS, which it describes as providing global coverage every 1–2 days, with OLI’s narrower swath and 16-day individual interval. These figures illustrate broad, frequent coverage versus finer land observation at longer intervals; they do not mean either instrument is best for every monitoring task.
How to compare satellite imagery specifications
- Define the detail you need. Identify the smallest feature or change relevant to the decision, then compare the product’s stated GSD at nadir and, where applicable, at the swath edge. Do not treat pixel size as a guarantee of object recognition.
- Set the required observation frequency. Decide how soon you need another observation, and check whether the quoted interval is for one satellite, a constellation, potential acquisitions, or an archive.
- Check location and viewing conditions. Consider target latitude and whether off-nadir viewing opportunities count. The practical revisit can differ from an exact ground-track repeat.
- Match swath to the area. Compare the swath and the total area you must cover. A narrow swath may require more acquisitions for a large region.
- Check the measurement, not just the pixel size. Confirm the spectral bands or thermal measurement needed for the subject you are monitoring.
- Look for actual performance and product details. Review on-orbit geometry, calibration, processing, and product quality alongside planned specifications. For commercial tasking, confirm coverage, acquisition availability, product resolution, processing, and terms for the specific target.
JACIE’s guidance on reporting GSD and swath conventions is available at USGS JACIE. For an example of commercial high-resolution capability, NASA’s Planet overview reports SkySat imagery at 50 cm/pixel and possible multiple revisits per day. Its page also gives a latitude coverage limit; these figures do not establish availability or guaranteed tasking for a particular location. See NASA Science: Planet and verify current acquisition options and product terms with the provider.
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