1 A base station in orbit

1.1 Why the words had to be pinned down first

3GPP writes rules for radios, and a rule needs a subject. Before anybody could say what a satellite has to transmit, somebody had to say what a satellite is.

TS 38.108 is the document that did it. It sets the smallest radio behaviour a satellite base station has to manage, for the 5G radio and for the narrowband internet-of-things radio ^[7], and it opens with the page of definitions the rest of this subject leans on.

1.2 The whole idea has one name

The short name is NTN, which TS 38.108 spells out as Non-Terrestrial Network ^[8]. The definition is wider than the name suggests.

Two words in there are worth slowing down for. Segments — the whole network need not fly, only a piece of it. Airborne — the definition also covers a vehicle in the air rather than in orbit.

1.3 What a satellite is, in the document's own words

A payload is the radio equipment the vehicle carries, and which of the two kinds it carries is the biggest thing to know about any satellite here.

1.4 The base station of this world

The base station is called a satellite access node, shortened to SAN ^[9]: the node that terminates the 5G radio protocols towards a satellite-capable phone and connects to the core network.

In plain words: UE is the phone, gNB is the 5G base station, 5GC is the 5G core network, NG is the line between the two, user plane is your data and control plane is the network's housekeeping.

1.5 Two links, never one

A phone talking through a satellite is on two radio hops, each with its own name. The service link is the radio link between the satellite and the phone ^[10]. The feeder link is the wireless link between the satellite and the ground station ^[11].

The ground station itself is the satellite-gateway — an earth station on the surface of the Earth with enough transmit power and enough receive sensitivity to reach the satellite ^[12].

1.6 Two kinds of payload, and the whole design follows from it

A transparent payload only moves the signal along.

UL and DL are the two directions, up from the phone and down to it, and RF is the radio signal. Nothing in that definition understands anything; the intelligence stays on the ground.

A regenerative payload does understand.

Demodulating and decoding means reading the bits back out. A payload that does that is running part of the base station in orbit rather than relaying for one.

1.7 Where in the sky

GEO stands for Geostationary Earth Orbiting and LEO for Low Earth Orbiting ^{[13] [14]}. The first has a precise definition worth reading once.

35,786 km above the equator, going round once in the time the Earth turns once, and so standing still over one spot. A low earth orbiting satellite does not stand still, and the definition of a satellite names both orbits side by side.

1.8 No mast in between

TR 22.887 defines satellite access as direct connectivity between the phone and the satellite ^[15] — no mast, no tower, nothing on the ground in the middle of the path.

1.9 Where this meets the rest of the course

Why anyone wanted a phone to talk to orbit is why the work was proposed, in the proposals' own words. Who is building it, and where the weight sits is who does it, and What exists on paper, and where to look yourself names the documents these definitions were read out of.

1.10 What people get wrong