Once a SAR image has been collected by a satellite, it still has a long way to go – literally. The data must travel all the way back down to Earth, and then on again to a SAR processing unit, before it can be put into the hands of tactical decision-makers.
This part of the journey can introduce significant latency into the process. Ground-station availability, satellite geometry and throughput speeds all play a role in how quickly customers receive their imagery.
We touched on the downlinking basics in Beyond the Echo - The Ability to Act, our first blog on latency. In this post, we’ll look at steps ICEYE is taking to really move the needle in data transmission. This includes:
Expanding ground-station coverage to bring satellites within reach of an antenna sooner
Optimizing scheduling to make the most effective use of available ground-station capacity
Adapting downlink parameters to maximize the amount of data transferred during each pass
Streaming data directly from the ground station to processing to remove unnecessary delays
Moving toward sovereign delivery, giving customers greater control over how and where their data reaches the ground
In simple terms, SAR satellite communications involve two steps. Uplinking is when data is sent from a ground station (on Earth) to a satellite with instructions on what imagery to capture. Downlinking is when image data is sent from the satellite back down to Earth.
When you task a satellite to capture an image, a ground station first needs to establish a contact to uplink the command. The satellite receives the command, captures and pre-processes the data, and stores the resulting data onboard until it can be downlinked to a ground station. From there, the data can be transferred for further processing and delivery.
A single image therefore typically requires two separate ground-station contacts: one to send the task to the satellite and another to bring the resulting data back to Earth. However, achieving contact is not always straightforward. Ground stations serve multiple satellite missions on a first-come, first-served basis, and each antenna handles one satellite communication at a time.
Throughout the process, there are several key factors that impact the speed at which data can be transferred.
Ground-station coverage: How far does a satellite have to travel before it is within range of a ground station?
Antenna availability: Can the uplink or downlink happen immediately or is the antenna being used by other satellites?
Downlink speed: How much data can be transferred between satellite and ground station in a single pass?
Transmission speed: How quickly is the data transferred from the ground station to a location where it can be processed?
Let’s look at each of those questions in turn and see what steps we have been taking at ICEYE to reduce latency.
The more ground stations you have, the sooner a satellite can get within range of an antenna and downlink its data. At ICEYE, we use a vast global network of ground stations, providing coverage across more than half of our planet’s surface.
Our network is deliberately concentrated where it matters most: the regions where there is relevant human activity. This dense coverage helps reduce latency by shortening the distance satellites need to travel before they make ground-station contact. In well-covered regions such as Europe, for example, most satellites will already be within range of a ground station when they are ready to downlink.
However extensive the ground-station network, each antenna can only communicate with one satellite at a time. This makes antenna availability a scarce resource that has to be actively optimized in the planning and scheduling process. It’s not just about deciding what to image. It’s also about deciding when and where to uplink the command and downlink the resulting data.
The larger the constellation, the more complex the challenge becomes. Multiple satellites are now likely to be within range of the same ground station simultaneously. Yet only some of them will be able to downlink due to limited antenna availability. The scheduler has to decide which satellite gets which antenna and time slot, balancing the need to move data to the ground as quickly as possible with other competing factors such as scheduling constraints, satellite requirements and ground-station costs.
By improving the ability of the scheduling system to optimize, we can therefore reduce the average time required to uplink or downlink data.
Once ground-station contact has been secured, the next challenge is to downlink the data as quickly and efficiently as possible. Crucially, the amount of data that can be downlinked varies from pass to pass.
This is due to two key factors:
1. Downlink window duration
The amount of time available for downlink depends on the geometry of each satellite’s pass. For a typical Low Earth Orbit (LEO) satellite, a pass can last approximately from 1 minute to 10 minutes. A direct overhead pass maximizes this window, while a low-elevation pass might only offer one or two minutes.
2. Downlink throughput speed
Signal strength changes continuously during a pass, affecting how quickly data can be transferred. While factors such as antenna characteristics and pass geometry also play a role, distance from the ground station is the key factor here: as the satellite gets closer, the signal generally becomes stronger, allowing higher throughput.
Greater signal strength does not necessarily mean higher throughput because the speed at which data is transmitted depends on the signal configuration being used – and, traditionally, only one signal configuration scheme was used per pass. This meant choosing between a more robust configuration that could operate across the full pass but at a lower data rate, or a more aggressive one that offered higher throughput but could only be used while the signal was strong.
We adjust configurations during the pass and switch to higher throughput configurations when signal strength allows. By optimizing data transmission across the entire pass, we not only reduce downlink times – we also increase the likelihood that the image fits within a single downlink. This, in turn, has a huge impact on latency.
What is the most efficient way to get downlinked data from the antenna site to the SAR processing unit where it is turned into usable insights?
The legacy standard based on file transfer required waiting for the whole file to be downlinked before the data transfer could start, adding significant latency. To eliminate this delay, we have migrated to payload streaming, where satellite data starts moving to our cloud or local processing nodes the moment the first bits arrive.
The impact this has had on latency is significant, with raw data now available instantly. For example, if a downlink lasts five minutes, streaming essentially saves those five minutes of waiting time.
From satellite-based technologies and smarter system architectures to optimized data pipelines and faster processing, the improvements we are making across the SAR data delivery chain mean that latency is now measured in minutes rather than hours.
The next step is not just to get customers their data faster but to give them greater control over how and where it is delivered. This includes providing customers with their own ground stations, which they can either operate themselves or ask us to host on their behalf.
This matters because the more control customers have over ground-station availability, and over the physical and technical path their critical data takes, the easier it is to guarantee reliable, low-latency access to guide decision-making, even in challenging, high-stakes situations.