Field Guide · protocol

Also known as: Globalstar

Globalstar is a low-earth-orbit satellite constellation offering satellite-telephone voice, low-rate data, and one-way asset tracking. It uses a bent-pipe CDMA air interface: the satellites carry no onboard processing and simply relay signals between a user handset and a ground gateway, with an L-band uplink near 1.6 GHz and an S-band downlink near 2.4 GHz.1

LEO satellite (~1414 km) — bent pipe handset (1.6 GHz)gateway (2.4 GHz)
Globalstar satellites are bent pipes: each relays the uplink straight down to a gateway, which does the switching on the ground.

Overview

Globalstar was conceived in the 1990s as a lower-cost rival to Iridium, trading Iridium’s complex satellite-to-satellite crosslinks for simple bent-pipe repeaters and a dense network of ground gateways. The constellation of roughly 48 satellites flies in inclined orbits near 1414 km. Because a call must see both a satellite and a gateway at once, coverage is strong over populated land and coastal waters but absent over mid-ocean and the poles.1

Technical characteristics

Property Value
Orbit LEO, ~1414 km, ~52° inclination
Satellites ~48 (plus spares)
Access Qualcomm CDMA spread spectrum
Uplink L-band, ~1.61–1.62 GHz
Downlink S-band, ~2.48–2.50 GHz
Modulation QPSK, direct-sequence spread
Services Voice, packet data, SPOT/simplex tracking

The CDMA waveform is borrowed from terrestrial IS-95 cellular: each user’s data is spread by a pseudo-random code across a wide channel, so many users share the same frequency and are separated by their codes rather than by time or channel.2

History

Service launched commercially around 2000. A first-generation constellation suffered amplifier degradation in the late 2000s that hurt two-way voice, largely resolved by a second-generation replacement fleet. Alongside phones, Globalstar built a large business in one-way simplex data: the consumer SPOT messengers and industrial asset trackers transmit short bursts up to the satellites for relay to a gateway. In 2022 Apple contracted Globalstar’s spectrum and network for the iPhone’s Emergency SOS via satellite.1

Deployment

Globalstar serves maritime, expedition, and remote-industrial users needing voice and tracking beyond cellular coverage, and its simplex link carries huge volumes of low-cost location beacons. The S-band simplex downlink has attracted amateur SDR experimenters, but the CDMA spreading, the proprietary framing, and the 2.4 GHz band make it a far harder target than the VHF data of Orbcomm or the L-band voice of Iridium.

Decoding it with GopherTrunk

GopherTrunk does not decode Globalstar. It is a terrestrial VHF/UHF land-mobile trunking scanner, and Globalstar’s spread-spectrum CDMA physical layer, S-band frequencies, and satellite framing share nothing with the C4FM/QPSK land-mobile protocols GopherTrunk implements. It is listed here as the CDMA, bent-pipe member of the LEO family alongside Iridium and Orbcomm.

Sources

  1. Globalstar — Wikipedia, for the constellation design, bent-pipe architecture, L/S-band plan, and simplex data services. ↩ ↩2 ↩3

  2. Code-division multiple access — Wikipedia, for the spread-spectrum access method Globalstar borrows from IS-95. ↩

See also