Why continuous coverage is not a threshold you cross once

There is no single number, and any tool that gives you one is hiding the assumptions. Continuous coverage means no gap at all, at the worst longitude on your latitude circle, for the whole simulated period. Whether a fleet reaches it depends on the elevation mask, the altitude, the customer latitude and how the planes are arranged, at least as much as on the satellite count.

For the configuration loaded below (550 km, 53°, min elevation 25°, latitude 50°), the shell first closes at 120 satellites. Change the mask to 10° and it closes at 60. That difference, from one input, is larger than most of the arguments people have about fleet size.

The ladder

At 550 km, 53° inclination, minimum elevation 25°, latitude 50°. Planes are chosen by the automatic rule.

Read down the worst outage column and the shape of the problem appears. Gaps fall quickly at first, then the improvement slows, then the shell closes. At 90 satellites the worst wait is 3.0 min, which is already a usable messaging service and nowhere near continuous. The last few minutes of outage are the expensive ones.

Continuity is not a threshold you cross once

Notice the 150 satellite row. It sits above a fleet size that was already continuous, and it still shows a 40 s seam. That is not a bug in the arithmetic, it is the geometry being honest. Coverage continuity depends on how the planes interleave, and the automatic plane rule picks a different arrangement for each count. A well arranged shell can beat a larger badly arranged one.

The practical reading: treat continuity as a property of a specific design, altitude, plane count, phasing and mask together, not as a satellite count you can quote on its own. If someone tells you the magic number is 66, or 120, or 648, ask which mask and which latitude.

The levers, in order of strength

Elevation mask

The strongest lever by a distance. Dropping from 25° to 10° moves closure in this ladder from 120 satellites to 60. The mask is set by the user terminal, so the hardware decision drives the fleet size decision.

Altitude

Higher orbits see more ground. The same 90 satellite fleet raised from 550 km to 800 km goes from 3.0 min of worst outage to continuous service at this latitude. Altitude costs you link budget and adds debris exposure, so this is a real trade rather than free coverage.

Latitude

A 53° shell serves latitudes near its own inclination best and thins out both above and below. The same fleet that gives 3.0 min at 50° gives 7.0 min at 30°. Where your customers are matters as much as how many satellites you fly.

Plane count and phasing

Two fleets of identical size can differ by an order of magnitude in worst outage. The extreme case is a single plane, which is what an early buildout usually looks like.

Using this for a plan

Set the mask your terminal actually needs, set the latitude your first market sits at, then walk the satellite count up until the worst outage crosses the number your service can tolerate. That crossing point, not the point where gaps reach zero, is usually the commercial answer. Voice needs continuity. Messaging does not. An asset tracker reporting four times a day needs almost nothing.

Other scenarios

• Direct-to-phone coverage at 5, 25, 45 and 90 satellites
  What a direct-to-device constellation can promise at each stage of a buildout, from a first block of five satellites to a 90-satellite shell.
• IoT store-and-forward with a polar constellation
  Revisit time for a sun-synchronous IoT constellation with a 10 degree mask, where the product is message latency rather than continuous service.
• Why single-plane deployments have long outages
  The same satellite count can mean a 15 hour outage or a 25 minute one. Deployment order and plane spread decide which.
• Coverage vs latitude, and why 53 degrees stops at 61
  Inclination plus the coverage half-angle sets a hard latitude ceiling. Above it, no fleet size produces a single second of service.