21.7 Low Earth Orbit Systems: Iridium & COSPAS-SARSAT

21.7 Low Earth Orbit Systems: Iridium & COSPAS-SARSAT

Quick Answer: Iridium: orbit 485 miles, band 1616–1626 MHz, digital voice and data at 2.4 kbps, 48 spot beams per satellite with a 30-mile footprint each. COSPAS-SARSAT: monitors 406 MHz for distress calls from EPIRBs, and fixes position by measuring the Doppler shift of the 406 MHz signal at several different points in its orbit.

Sub-topic 3-P-095 (Low Earth Orbit Systems) covers the two LEO constellations a marine technician meets. It pairs naturally with the geostationary Inmarsat material, because the contrast between the two orbit types explains everything about what each system can do.

Why low earth orbit at all

Two consequences drive everything else. First, a geostationary satellite sits above the equator, so from high latitudes it is at or below the horizon — which is exactly why sea area A4 (the polar regions) is defined as the area outside Inmarsat coverage, and why LEO systems matter there. Second, because a LEO satellite is roughly 46 times closer, path loss is dramatically lower and a handheld terminal becomes practical.

Iridium: the numbers

The pool asks four separate factual items about Iridium, so learn them as a block:

A few notes that make the numbers stick:

• 1616–1626 MHz is in L-band, the same general region as GPS (1227.6 and 1575.4 MHz, section 21.8) and Inmarsat. L-band is used for mobile satellite service because it penetrates weather with minimal rain fade and works with small, non-tracking antennas.
• 2.4 kbps sounds slow, and it is — Iridium was designed around voice and short messaging, not broadband. For a GROL technician the relevance is that a vessel's Iridium terminal is for telephony, position reporting and short data, not for bulk transfer.
• 48 spot beams per satellite is how the constellation reuses its limited spectrum: each satellite paints its coverage as a honeycomb of small beams, and the same frequencies serve non-adjacent beams simultaneously — the same cellular principle used on land, projected from orbit.
• Because the satellites move fast relative to the ground, a call is handed off between beams and between satellites during a conversation, which is why the system needs inter-satellite links.

COSPAS-SARSAT

What is the main function of the COSPAS-SARSAT satellite system? Monitor 406 MHz for distress calls from EPIRBs.

COSPAS-SARSAT is the alerting backbone of GMDSS. It is an international, government-run, non-commercial system whose only job is to detect and locate distress beacons — EPIRBs at sea (section 5.2), ELTs in aircraft, and PLBs carried by individuals.

406 MHz is the frequency to remember. It is dedicated to satellite distress alerting, and a 406 MHz beacon transmits a digitally coded burst carrying a unique identity that ties back to a registration database — vessel name, owner, emergency contacts.

Position by Doppler shift

How does the COSPAS-SARSAT satellite system determine the position of a ship in distress? By measuring the Doppler shift of the 406 MHz signal taken at several different points in its orbit.

This is the elegant part, and worth understanding rather than memorising.

A LEO satellite passing overhead is moving fast relative to the beacon. As it approaches, the received frequency is shifted up; as it recedes, shifted down. The instant of zero Doppler shift — the moment the frequency crosses the beacon's true value — occurs at the satellite's closest approach. The rate at which the shift changes through that crossing indicates how close the pass was.

Take those measurements at several points along the orbital pass, combine them with the precisely known satellite ephemeris, and the beacon's position falls out — typically to within a few kilometres, with no position data required from the beacon at all.

Two practical points follow:

1. Doppler location needs relative motion, so it works with LEO satellites, not geostationary ones. The geostationary part of COSPAS-SARSAT provides instant alerting but no Doppler position.
2. A modern GPS-equipped EPIRB transmits its own position inside the 406 MHz message, giving a fix in minutes instead of the up-to-an-hour wait for a suitable satellite pass. Doppler remains the fallback and remains the pool's answer.

Recall from section 5.2 that COSPAS-SARSAT is the international satellite-based SAR system that detects and locates 406 MHz EPIRB alerts — the same fact framed from the Element 1 side.