Size Matters: Why Smaller Telescopes Limit OGS Network Performance

Section : Aerospace and Defense

Publication date: 29 June 2026

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Optical ground stations (OGS) are often compared through the cost, size, and deployability of each individual site. From this point of view, a smaller telescope can look attractive. Yet the reality of deploying and operating a network of OGS across many sites changes the equation. Aperture diameter can determine the number of ground stations required across an operational network, directly affecting how many passes can be used and how much data can be transferred.

A smaller aperture may reduce the cost of one station, but it can increase the cost, complexity, and inefficiency of the whole network. In this article, we explain how aperture size shapes communication opportunities, network throughput, and the number of ground stations required to support reliable space-to-ground optical connectivity.

What Happens When You Reduce Aperture Size?

A smaller telescope collects less light from the satellite transmitter. This decreases the received signal power, forcing the link to require either more power transmitted from the satellite terminal (already in a power constrained environment) or lower data rates (reducing throughput). This is especially important when the satellite is low on the horizon, where the beam crosses much more distance and correspondingly accumulates more signal loss and beam distortion. A ground station that is compact but only works reliably during the best part of a satellite pass does not provide the same network value as a station that can close the link over a wider range of elevation angles.

Figure 1: There is much more distance and atmosphere between a satellite at a fixed altitude and an OGS when the satellite is near the horizon, rather than directly overhead. This directly translates to more free space path loss and stronger atmospheric turbulence penalties for an optical link between the two.

Most Satellite Passes are Not Overhead Passes.

Figure 3 Distribution Of Leo Sso — Figure 2: Distribution of LEO SSO passes available to an OGS, over a 30 day period, binned by maximum elevation angle during the pass. There are more than 30 thousand pass opportunities, and the median peak elevation angle is 15 degrees over the local horizon.

An OGS system must be designed to reliably close the link budget across a wide range of elevation angles to effectively add operational capabilities at the network scale. We model the distribution of pass opportunities over 30 days between a low Earth orbit (LEO) constellation comprised of 15 lasercom-capable satellites in Sun Synchronous orbits (SSO) at 550 km altitude and a single OGS in central Europe. Categorizing the distribution by the maximum elevation angle of each pass reveals the risk of having too small of an aperture.

Less than a third of LEO satellite pass opportunities will exceed 30 degrees elevation angle over the local horizon relative to a fixed OGS location. Being unable to reliably close the optical link budget at low elevation represents a massive reduction in viable pass opportunities for an OGS network.

Smaller Apertures Mean Greater Sensitivity To Atmospheric Turbulence.

As the optical link budget gets stressed closer to the horizon, the light also passes through substantially more atmosphere, meaning the scintillation due to turbulence becomes worst when the link is most vulnerable. Scintillation is the rapid time varying signal loss measured at the lasercom receiver due to localized random changes in temperature, pressure, and humidity in the air between the satellite and OGS.

This beam distortion takes a well-defined round beam emitted by the satellite and randomly scatters its energy across the receiver aperture, as shown in the simulation aperture in Figure 4. Small apertures average over fewer of these turbulence cells than larger apertures and therefore experience reduced aperture averaging. As a result, at low elevation angles, received power fluctuations become more severe and link margin must accommodate deeper dynamic fades, while also collecting less total light over time.

Figure 3: Snapshot of received light intensity at the OGS telescope aperture simulated for moderate turbulence conditions. Dashed lines indicate diameters of 10, 30, 50, 70, and 80 cm apertures.

Figure 4: Simulations of total light collected by various diameter telescopes during moderate turbulence (r0 = 8 cm), representative of typical conditions at 20 degrees elevation over the horizon. Transmission is normalized relative to the mean collection of the 80 cm aperture trace. The degradation in both average aperture collection and dynamic scintillation is clearly visible as the diameter reduces.

The Aperture Determines How Much of the Sky is Operationally Useful.

The consequence of reducing aperture size is simple: the usable part of the pass becomes shorter or vanishes entirely. And because the received signal is weaker, the system may also need to operate at a lower data rate. Less time and lower throughput both reduce the amount of data delivered during the pass.

The following figures are built from this logic: for each pass, we estimate when the link has enough margin to work, then calculate the usable duration, the achievable data rate, and the total data volume. To account for the delay associated with link acquisition, we begin transmission after the first 15 second period over which a link was possible.

Figure 5: The data rate vs time for a single lasercom pass between a LEO satellite and OGS. The solid curves represent the average data rate for a rolling average window of 1 second using different telescope apertures. Translucent dots mark localized fluctuations in data rate, sampled over a finer time resolution. Each curve is annotated with how many GB were transmitted over the pass.

It is assumed that the 80 cm case has 2 dB margin for the maximum optical data rate of 10 Gbps at 20 degrees elevation angle over the horizon. Following the Shannon-Hartley theorem, we allow for the data rate to vary proportionally to the link budget between the maximum 10 Gbps and a minimum rate of 1 Gbps to maximize the total capacity for the pass for all aperture sizes. The total data transmitted per pass is equal to the area under each curve.

In short, reducing aperture size directly results in:

Less collected light, so less link margin.
Larger minimum elevation angle, so a shorter communication window.
Shorter time and at a lower data rate, so less data transferred per pass.

How Much Useful Connectivity Does Each OGS Actually Provide to the Network?

The link margin impacts end-to-end network performance along two key metrics, service continuity and service throughput.

For service continuity, the network must keep high-capacity optical path available between the ground and the satellite constellation. This is where aperture size has a direct network-level impact. Enabling links at low elevation is key for 2 reasons.

Higher ground-station coverage: restricting optical links to high elevation angles reduces the sky coverage of each OGS: some satellite passes become shorter, while others are not usable at all. Larger apertures enable lower-elevation links, increasing the fraction of time during which a valid optical feeder link is available from each site.

Map Reseau — Figure 6: Map of a proposed 10 OGS network, with overlays illustrating the possible satellite contact windows for an 80 cm and 30 cm aperture size OGS in each location.

Fewer handovers and less overhead: shorter communication windows mean more frequent transitions between satellites. Each transition requires acquisition, tracking, and link setup procedures during which no user data is transferred. As a result, a larger fraction of the network capacity is consumed by overhead rather than useful traffic.

Figure 7: Total daily capacity for a network comprised of different numbers of OGS with 80 cm and 30 cm aperture sizes.

For service throughput, we consider the amount of payload data that can be downloaded every day. The impact of aperture size becomes particularly visible at network scale. In the example below, an OGS network built around 80 cm telescopes delivers nearly 3x more data per day than a network using 30 cm apertures with the same number of sites.

Conclusion: Compactness Is Not The Same As Operational Efficiency

The appeal of smaller optical ground stations is easy to understand. A compact telescope can look simpler to deploy, easier to replicate, and cheaper at the level of an individual site.

But optical communications do not operate at the level of one ideal pass or one isolated ground station. They operate as networks.

At network scale, aperture determines how much of each satellite pass is actually usable, how much data can be transferred, how often links need to be reacquired, and how many ground stations are required to reach a given level of availability and throughput.

A smaller aperture may reduce the apparent cost of a single site. But if it shortens communication windows, limits low-elevation operation, reduces data return, and requires more ground infrastructure to compensate, then the system has not become simpler. The complexity has simply moved from the station to the network.

For operational optical communications, the question is not how small an OGS can be made. The question is how much useful connectivity each OGS can reliably deliver.

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