Real-time satellite data is a trendy concept and a question that our customers sometimes ask.
But real-time satellite data is not what you think it is.
As with most high-quality feeds, limitations on data refresh come from transfer speeds and processing speeds. In best-case scenarios, it could take only a few minutes between data capture and delivery. However, in most cases, for high resolution imagery, it takes a few days.
The number of Earth observation satellites is limited, which means not all satellites are always within range of a downlink station. Satellites do not capture data at all times.
Each day, satellites capture hundreds of terabytes of data. The captured data is stored on the satellite until it reaches a 'downlink station,' where it will be decoded, sent to the provider, further processed, and sent to its final destination.
There was a time when capturing space and shipping it could take weeks, but today the commercial space industry is booming. As computing costs fall, space data is being pushed to the cloud, and companies such as SkyWatch are automating satellite data access, new entrants are getting into the game. In other words, we can expect that the time between capturing a scene and transferring the high-resolution data to the customer will drop from days to minutes.
Let’s dig into why live, real-time satellite data video feeds, as portrayed by Hollywood, do not currently exist.
How many satellites are watching us right now?
There are two types of Earth observation satellites: geosynchronous equatorial orbits (GEO) and low Earth orbits (LEO).
GEO satellites are geostationary, meaning they orbit at the same speed as the Earth's rotations. This results in them staying in the same spot above Earth all the time. However, to achieve such an orbital path, they must be placed at an altitude of around 35,786 km (22,236 mi). Since they keep a relative position to the Earth’s surface, they can simply transfer down their data as they capture it since they don’t move out of range of the reception dish. Yet this remoteness also implies that these EO satellites cannot take photos with sharp details - cities are usually seen as big areas - since their spatial resolution is limited. Thus, GEO satellites are mostly used for weather observation and are typically thought of as having very low spatial resolution.
Here is an example of GEO satellite data:
Most people expect satellite data to be comparable to what you would get if you zoomed in on Google Maps. Though, ordinarily, aerial photography is the highest 'zoom' displayed. To date, the greatest resolution accessible with commercial optical satellite data is 30 cm per pixel, which equates to a 12 x 12-inch area on the ground under optimal conditions. Here's what very high resolution data looks like:
EO satellites in low Earth orbit take sharper images of the Earth, such as the one above. They cover the globe one strip at a time by travelling from pole to pole. Most polar-orbiting satellites return to the same pole after 90 minutes. The next imaging strip will cover a different part of Earth as the Earth rotates under the satellite.
Satellites can either take a short number of 'zoomed out,' low-resolution images in fewer rotations or a higher number of 'zoomed in,' high-resolution images over a longer rotation period.
A wide-covering satellite like MODIS, pictured above, with a swath of 2300 km (or 1400 mi), can capture the entire globe in 1-2 days at a resolution of 250 m (820 ft). One of the Sentinel-2 satellites, designed to cover 290 km (180 mi) at 10 m (33 ft) resolution, will take 10 days to take a new image of a specific area under the same viewing angle. With both satellites in the Sentinel-2 constellation (S2A and S2B), this revisit time can be reduced to 5 days.
To improve ‘refresh rates,’ satellite operators can launch multiple satellites with the same sensor to follow the same orbit. This technique, called ‘pearling,’ allows operators like Planet to provide SkyWatch, and their other partners, with the ability to obtain medium and high resolution data over the same area daily.
Here is a website that illustrates how Planet can image the entire globe daily, thanks to its large constellation (several hundreds of satellites).
How long between satellite data capture and data delivery?
Data capture and transfer are delayed when satellites are not in range of a downlinking station for a few hours.
The speed of polar-orbiting satellites is 7.8 km/s (4.8 miles/second). While passing over a downlinking station, they are within range for less than 10 minutes, during which time they have to downlink all the data collected since the last pass. Satellite operators must ensure they do not capture more data than they can send in one pass, plus the cost of downlinking/processing data.
Therefore, many satellite operators concentrate only on capturing ‘interesting’ scenes and do not capture data over the Earth at all times unless a client requests them to. It is also common for tasking data prices to be higher than archive data prices because operators need to update their collection decks to accommodate specific clients.
To ensure you receive data for your area of interest, it is crucial that you ‘task’ a satellite to capture your area of interest rather than waiting for archival data to be available. Considering platforms like EarthCache that can automatically place tasking orders for your application is a good idea if you need data collected within a week.
How long does it take to process satellite data?
The data captured by satellites is very different from what can be viewed on the Google Earth engine.
The station receiving the raw satellite data (or ‘level 0’) processes the data, decodes it, and transforms it into usable data, including geotagging, time, and sensor information. As a result of this initial step, it can take from a few minutes to an hour for the data to be transferred back to the data provider.
It is often further processed to turn data into analysis-ready data (ARD). In addition to sharpening the image, atmospheric correction, and smoothing out geographical distortions caused by the angle of the sensor, this step is increasingly being done automatically since manual processing could delay delivery by days.
As a final step, the ARD is stored online and made available. When developers use automated distribution platforms, like EarthCache, this step only takes a few minutes.
Our recent advances in automating the capture and distribution of EO data, including in-flight preprocessing, cloud storage, and automated ordering of archive and future satellite data, will mean the time between scene capture and high resolution data transfer to the customer will soon drop from days to minutes. If you want to automate your access to low, medium, and high resolution satellite data, contact us to request access. If you prefer only to use open (free) data, we recommend the following sources of free satellite data.