How Low Can Satellites Go? Air Force

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How Low Can Satellites Go
How Low Can Satellites Go

Satellites which fly in low Earth orbit (LEO) typically ascend to an altitude of 500 kilometers (310 miles) or higher above earth. VLEO sats like Earth Observant’s proposed “Stingray” imaging satellite fly at 300 km or less. At 250 km where the startup says Stingray will fly, satellites are still basically in Earth’s atmosphere. That comes with some downsides like aerodynamic drag and strong gravitational pull, which are significant enough to make a spacecraft’s orbit decay in less than 5 years, requiring changes in traditional designs.

Earth Observant's conceptual overview of its VELO ″Stingray″

But there are also real advantages. Flying at lower altitude can improve the resolution of optical sensors, radiometric performance (infrared/microwave sensors) and geospatial accuracy. Those sensing benefits can also reduce required payload size (optical, radar or communications) and thus cost.

VLEO Earth-observing satellites could be more competitive, either by flying more capable platforms at the same cost, or by offering the same capabilities at a reduced cost. One could argue that lower costs, allowing for greater numbers, also yield better coverage.

And there’s another issue. Low-Earth orbit is increasingly crowded.

Estimates suggest that by 2025, the number of man-made objects sent into space annually will surpass 1,100. Most will be stationed in LEO. SpaceX offers a prime example: It has launched over 600 LEO satellites for its Starlink broadband internet constellation and plans to launch thousands.

The Starlink team is building 120 smallsats each month and the Federal Communications Commission has approved SpaceX’s scheme to build out the Starlink constellation to 12,000 satellites. The company has applied for rights to add 30,000 more.

Low Flying Fish

Earth Observant’s Stingray is a 400-pound, 8-foot by 8-foot optical imaging satellite with a Space Shuttle-like body, or “bus,” as it’s called in the industry. The shape helps reduce aerodynamic drag at the altitude at which Stingray will fly. It may also aid maneuverability for other purposes though we didn’t discuss it with Earth Observant.

Earth Observant's Stingray satellite with its Space Shuttle-shaped ″bus″ or body.

That’s shorter than average partly due to the marginally lower latency that comes with low altitude transmissions, but also due to Earth Observant’s goal of carrying out some image processing (raw optical image data needs to be processed and formatted for use) onboard Stingray using edge computing methods rather than simply sending raw data to a ground station.

“Our goal is to have [the imagery data] skip the traditional ground station processing stacks,” Smith affirms. “We knew it was important, we didn’t know how important. But after talking to the Air Force and Army it became pretty clear that getting the data faster is their fundamental desire. It’s something they’re looking for all of us out there in space to do.”

Stingray will send imagery via Ka band (26.5 to 40 GHz) which fits into the multi-domain sensor linking TITAN (Tactical Intelligence Targeting Access Node) and AMBS (Airborne Battle Management System) systems that the Army and Air Force are respectively developing. Its potential to cut out the middleman (i.e. ground processing stations), relaying imagery directly to tactical units/assets would surely be of interest to the services and is something that EA’s Smith hopes the Air Force will test Stingray on.

Seeing But Not Being Seen in VLEO

Colonel Eric Felt, director of the Air Force Research Laboratory Space Vehicles Directorate at Kirtland Air Force Base in New Mexico, says he’s an advocate of using all kinds of orbits, not just the traditional ones for imagery and communication

“I see VLEO as a new orbit that has potential for us. I’m really glad that [Earth Observant] is going after this mission area.”

Colonel Felt agrees that the resolution which satellites flying closer to Earth can deliver for smaller payload is desirable. It yields a cost per high-res image advantage that the Air Force is eager to leverage, he says. “More satellites and more capacity is definitely of interest to us.”

Looking at the artwork above, you may note that Earth Observant stresses Stingray is capable of adjusting in-orbit altitude, has low cross-sectional area and is highly maneuverable.

Such attributes could be useful to the Air Force.

“The more you can maneuver, the less predictable your orbit is. That’s good,” Felt says. “A small cross-section is good too. If they can’t find you, they can’t tell that you’re [flying] overhead. If you really get into a shooting war, it’s harder for them to attack you or otherwise defeat you.”

The military appears to be going for agile, low-altitude satellites at low cost. That’s perfect competitive space for fledgling satellite service providers.

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