A satellite is above the horizon. Does that mean UE can connect to it? Not necessarily. The satellite must illuminate the UE with a usable beam, and the network must provide service through that beam.

There is also a time dimension. A stationary UE can enter and leave a moving beam. A steered beam can stay over the UE while the satellite moves, but only for a limited interval. These changes affect measurement timing, cell mobility and when service needs to transfer to another satellite.

This page separates satellite visibility, beam coverage and scheduled service. It uses the NR NTN coverage definitions in TS 38.300 and simple geometry for the worked examples. For the payload and gateway arrangement, see NTN Architecture.

• What Do We Mean by Coverage ?
• Earth-fixed, Quasi-Earth-fixed and Earth-moving
• Elevation, Slant Range and Visibility
• Worked Example: A 600 km LEO Satellite
• How Long is the Satellite Visible ?
• How Long Does One Beam Cover UE ?
• Visibility Time and Service Time
• When Coverage is Not Continuous
• What Changes for UE Mobility ?
• What to Check in a Coverage Example
• References

What Do We Mean by Coverage ?

Start with the geometric limit. Earth can block the line of sight, and a minimum elevation requirement can further reduce the area where a satellite is usable. That gives a visibility region, not an automatic cell footprint.

A satellite can form several beams inside that region. Each beam has an antenna pattern, a pointing direction and an operational coverage boundary. The boundary depends on the criterion being used, such as antenna gain or usable link quality.

Then the network chooses which areas to serve and when. A visible satellite with a beam pointing elsewhere does not provide coverage to this UE. A suitable beam also needs the relevant radio resources and network path.

Keep these three questions separate when reading a coverage map. Each row adds a condition that the previous row cannot establish.

The beam footprint and the NR cell are also different concepts. A physical beam change does not necessarily change the cell identity. Check the beam-to-cell mapping before treating every footprint transition as handover.

Earth-fixed, Quasi-Earth-fixed and Earth-moving

These names describe how the coverage area behaves relative to Earth. They do not describe whether the payload is transparent or regenerative. Beam pointing and payload processing are separate design choices.

TS 38.300 distinguishes the following service-link coverage patterns. Pay attention to whether the geographical area stays fixed, and for how long.

The comparison below uses two observation times. In the quasi-Earth-fixed case, steering changes the pointing direction to hold the area during this interval. The actual footprint need not keep an identical shape as the look angle changes.

Figure 1. Satellite motion and footprint motion are different: steering can hold a geographical area temporarily.

A satellite moving across the sky does not automatically imply an Earth-moving cell. Steering can hold the footprint temporarily. Conversely, a fixed UE does not imply a fixed relationship to its serving beam.

The definitions are in TS 38.300, clause 16.14.1. The term quasi-Earth-fixed is useful because it makes the limited serving period explicit.

Elevation, Slant Range and Visibility

Elevation is the angle from UE's local horizontal plane up to the satellite. A satellite directly overhead has 90 degrees elevation. At the ideal geometric horizon it has zero degrees elevation.

Slant range is the straight-line distance from UE to satellite. It equals satellite altitude only for the overhead case in the spherical-Earth model. At lower elevation, the path is longer.

The central angle psi separates UE from the sub-satellite point as viewed from Earth's centre. The sub-satellite point is where the radial line from the centre to the satellite intersects the surface. Its surface distance from UE is R x psi when psi is expressed in radians.

Figure 2. Elevation is measured at UE; slant range exceeds altitude except for an overhead satellite.

Use a spherical Earth of radius R and a satellite at altitude h. Let r = R + h. The following relationships come from the triangle formed by Earth's centre, UE and satellite; they neglect terrain, refraction and Earth flattening.

d = sqrt((R + h)² - R² cos²(e)) - R sin(e)

psi = acos((R / (R + h)) cos(e)) - e

Surface distance to the sub-satellite point = R x psi

Here d is slant range and e is elevation. Use consistent angle units in the trigonometric functions, and radians when multiplying psi by R. A minimum elevation e_min sets the maximum permitted central angle in this model.

That angle defines an elevation-limited visibility cap around the sub-satellite point. It does not specify the antenna beamwidth. A narrow spot beam can cover only a small part of that cap.

Worked Example: A 600 km LEO Satellite

Take R = 6371 km and h = 600 km. The values below are calculated from the spherical geometry, not copied deployment requirements. The zero-degree row is an ideal horizon limit, not a recommendation for operation.

For each elevation, the surface-distance column gives the distance between UE and the sub-satellite point. When that elevation is used as the minimum limit, it becomes the radius of the corresponding visibility cap.

The diagram below puts all five rows into the same geometry. Keep UE in one place and move the satellite along the 600 km altitude circle. Follow each color from the straight radio path to its projected point on Earth, then read the matching values in the key. These are five possible positions, not five satellites required at the same time.

Figure 3. Five elevation cases in one view: slant range is a straight path; surface distance follows Earth; psi is measured at Earth's centre.

At 10 degrees elevation, the slant range is more than three times the altitude. At 30 degrees, the allowed surface radius is much smaller. A larger elevation limit therefore trades a smaller visibility region for a shorter path and a higher look angle.

This geometric calculation does not establish received power or throughput. Antenna gain, losses, interference and receiver performance still matter. Low elevation also makes local obstructions more relevant, even if the ideal spherical model predicts line of sight.

How Long is the Satellite Visible ?

Now turn the angular region into a time interval. Consider a circular orbit passing directly over a stationary UE on a non-rotating spherical Earth. This deliberately simple model isolates altitude and minimum elevation.

Use Earth's gravitational parameter mu_E = 398600.4418 km³/s². For orbital radius r, angular speed omega and orbital period T follow the circular two-body relationships.

omega = sqrt(mu_E / r³); T = 2 pi / omega

Overhead-pass visibility duration = 2 psi_max / omega

For h = 600 km, the orbital period is approximately 96.54 minutes. That is the time for one orbit, not the time this satellite is visible to one UE.

The visibility intervals below use the central-angle limit associated with each minimum elevation. Earth rotation, off-track passes and orbital perturbations are excluded.

A pass that misses UE's zenith generally spends less time above the same elevation limit in this model. Some passes never reach that limit. A real prediction needs UE coordinates and satellite ephemeris, evaluated in consistent time and coordinate frames.

Do not use the 96.54-minute orbital period as the revisit time for a particular ground location. Earth rotation changes the ground track, and a constellation can provide other satellites between successive passes of this one.

How Long Does One Beam Cover UE ?

Satellite visibility can last several minutes while one moving spot beam covers UE for only a few tens of seconds. The beam dwell time depends on the footprint's motion and the path UE takes through it.

For a simple circular orbit over a non-rotating Earth, orbital speed is sqrt(mu_E / r). The sub-satellite point moves along the surface at R x omega. These speeds differ because they describe motion on circles of different radii.

At 600 km altitude, orbital speed is about 7.56 km/s and sub-satellite ground speed is about 6.91 km/s. For a footprint assumed to move locally at that ground speed, a 200 km central crossing takes approximately 28.94 seconds.

Central-crossing dwell time = footprint diameter / relative footprint speed

200 km / 6.91 km/s = approximately 28.94 s

This example assumes a stationary UE, constant local footprint speed and a circular usable boundary. It is not a general speed formula for every off-nadir or steered beam.

If UE crosses the footprint away from its centre, the chord is shorter. For footprint radius a and closest approach b in a local flat-plane approximation, the path length is 2 sqrt(a² - b²).

With a = 100 km and b = 80 km, the chord is 120 km and the dwell time is about 17.36 seconds. UE motion changes the relative speed and direction as well.

For a quasi-Earth-fixed beam, diameter divided by orbital ground speed is the wrong model. The network steers the footprint to hold the area. Steering limits, geometry and the planned beam schedule determine when that serving interval ends.

Visibility Time and Service Time

A network can begin service after a satellite becomes geometrically visible and stop before it leaves the visibility region. Beam allocation, usable radio conditions and feeder-path availability can shorten the serving interval.

The following hypothetical timeline assumes one planned interval for each of two satellites. Satellite A is visible for 510 seconds, but its scheduled service lasts only 420 seconds. Satellite B starts service before A stops, giving a 30-second overlap.

Figure 4. A service schedule can end before geometric visibility ends; the transfer uses the available service overlap.

The overlap makes a transfer possible in this example; it does not prove that the transfer succeeds. UE still needs a suitable target configuration, valid assistance and the applicable access or re-synchronization procedure.

If B started serving at 490 seconds instead of 450 seconds, the planned service gap would be 10 seconds after A stops at 480 seconds. That gap could occur while A and B are both geometrically visible. Visibility alone does not establish continuous service.

NR SIB19 can provide t-Service, indicating when a cell will stop serving its current area. It is service information, not a satellite-rise or satellite-set calculation. The field applies to the service-link and feeder-link switching cases specified in TS 38.331.

For satellite switch with re-synchronization, t-ServiceStart can identify the target satellite's service start. Its use and time reference follow the RRC procedure. A service timestamp does not replace the target configuration or independently command handover.

When Coverage is Not Continuous

A source service interval can end before the next usable service begins. UE then faces a coverage gap rather than an immediate transfer to another available cell. Predicting that gap can help UE avoid unnecessary searches and help the network manage reachability.

The procedure depends on the radio access and core network. IoT NTN provides satellite assistance for coverage-gap prediction and idle-mode handling. NR satellite access has core-network unavailability procedures in Release 18. These should not be treated as one common RRC mechanism.

For what UE and network do before, during and after the gap, see Discontinuous Coverage. That page also explains the relationship to paging and power saving, with separate NR and IoT NTN procedures and a worked timing example.

• Visibility does not guarantee continuous service. The next usable service interval can start after the current one ends.
• Prediction supports preparation. UE must still acquire a suitable cell when service returns.

What Changes for UE Mobility ?

The coverage pattern determines what information is useful before service changes. A moving footprint can carry its boundary past UE. A held footprint can reach the end of its scheduled serving period while UE remains near its centre.

Radio measurements tell us about the current link. Time and location assistance can help anticipate a coverage transition. The relationship to the NTN mobility mechanisms is outlined below.

D1 conditional handover uses configured fixed reference locations. D2 uses moving reference locations with the associated ephemeris and epoch information. T1 uses a configured execution window. These conditions are part of a configured procedure, not automatic consequences of a particular orbit type.

For the detailed sequences and conditions, see NTN Handover. Changes in slant range and radial velocity also affect Timing Advance and Frequency Compensation.

What to Check in a Coverage Example

Before comparing two coverage results, check their assumptions. A map based on geometric visibility and a map based on received-power thresholds can show different areas without either calculation being wrong.

Record the inputs below with the result. They explain why a calculated visibility interval may differ from the interval seen in a modem log.

The calculations here explain coverage geometry and time intervals. A full link budget and the detailed procedures for discontinuous coverage or power saving are separate topics.

References

The coverage terminology and NR procedure connections use the versions below. The geometry, dwell calculations and service timeline are explanatory models with their assumptions stated in the relevant sections.

• 3GPP TS 38.300 V18.9.0 : Clauses 16.14.1, 16.14.3 and 16.14.4; NTN coverage patterns, mobility and switching.
• 3GPP TS 38.331 V18.8.0 : SIB19 and its service-time fields, conditional events and satellite switch with re-synchronization.