Constellation shape

A 66-satellite near-polar shell, the Iridium pattern

What a 66-satellite near-polar shell at 780 km delivers as idealised geometry: the classic pattern for global voice and messaging, including the poles.

66 satellites in 6 planes of 11, at 780 km and 86.4 degrees. The oldest still-flying answer to global coverage, and the one shape that genuinely serves the poles. Near-polar inclination buys latitude at the cost of thinning the tropics.

At latitude 60 with a 10 degree elevation mask, this shape gives a worst-case outage of 20 s, and about 1436 service minutes a day. The service band reaches the poles.

Across latitudes it is uneven, as most shells are. At the equator the worst outage is 1.7 h; at 70 degrees it is continuous service. Move the latitude control below to walk the whole range.

Scenario

SatellitesWalker 66/6/1

Orbital planes

Altitude780 km

Inclination86.4°

Min elevation10°

Service latitude60° N/S

Latitude only. Results are worst-cased around the whole latitude circle, so this is the band a city sits on rather than the city itself.

Walker delta 66/6/1 · 48 h window · 20 s step · worst of 8 longitudes at 60°

Worst outage

20 s

longest wait, worst longitude

Average wait

20 s

mean gap between windows

Windows / day

distinct service windows

Service / day

1436 min

of 1440 min

Coverage

99.7%

time in view

Service timeline at your latitude

48 h · worst-case longitude at 60°

Longest gap 20 s. Filled blocks mark at least one satellite above 10° elevation.

Outage vs constellation size

Worst outageAverage wait

At 60° latitude, min elevation 10°, 780 km / 86.4°. Log scale. Labels mark the worst outage.

Worst outage vs latitude

66 sats · 780 km · 86.4° · min elevation 10°

Latitude sweep is sampled at 60 s steps across 4 longitudes, so it is coarser than the headline figures.

Numbers

Constellation	Planes	Worst outage	Avg wait	Windows/day	Service min/day	Coverage
3 sats	3 × 1	1.8 h	66.3 min	19.3	156	10.8%
12 sats	4 × 3	28.7 min	23.0 min	46.0	386	26.8%
22 sats	11 × 2	24.3 min	20.5 min	32.4	780	54.2%
48 sats	8 × 6	40 s	21 s	15.8	1435	99.6%
66 sats	6 × 11	20 s	20 s	11.7	1436	99.7%
90 sats	10 × 9	continuous	0	1	1440	100.0%
200 sats	20 × 10	continuous	0	1	1440	100.0%

Same sampling as the headline tiles: 8 longitudes, 20 s steps (40 s above 400 satellites). Every row except your own uses the automatic plane rule.

Model and assumptions

Geometry: spherical Earth (R = 6371 km), circular orbits, Walker-delta constellation with evenly spaced planes and phasing F = 1, no J2 drift or drag. Service means at least one satellite above the minimum elevation angle.
Sampling: headline numbers and the table simulate 48 h (96 h for fleets of 12 or fewer) at 20 s steps, worst-cased across 8 longitudes at your latitude. The latitude chart uses 60 s steps and 4 longitudes.
Planes: "Auto" spreads satellites across the divisor of N nearest above the square root of N, a revisit-friendly default. Real constellations may choose otherwise: a single-plane test block clusters its passes.
Fidelity: planning-grade, for sizing intuition and commercial conversations. Contractual coverage commitments need full-fidelity tooling (STK, GMAT) with real ephemerides, beam patterns and link budgets. This tool models geometry only, not capacity or link margin. Full method and validation anchors.

Why near-polar

Inclination sets the highest latitude an orbit reaches, and the coverage circle around each satellite adds a little more. At 86.4 degrees this shell reaches everywhere: the worst outage at latitude 80 is continuous service, where a 53 degree shell of the same size and altitude gives nothing at all. That is the whole argument for near-polar geometry, and it is not a small one if your customers are ships, aircraft, or anything north of Scandinavia.

What it costs

Orbits converge at the poles and spread out at the equator, so a near-polar shell spends proportionally less time over the latitudes where most people live. The same 66 satellites give 1.7 h at the equator. A constellation is a choice about which customers to serve well, and this one chooses reach over density.

The mask decides the fleet

This page uses a 10 degree mask, which suits a handset with a stub antenna and a user willing to stand outside. Raise it to 25 degrees, the geometry a modern direct-to-device service needs, and the same 66 satellites give 15.7 min. Nothing about the shell changed. The requirement did.