21.4 INMARSAT Satellite Communications

21.4 Satellite Communications, INMARSAT & GPS

Quick Answer: LEO (Iridium ~485 mi, ~85 min orbits, 1616–1626 MHz) = moving birds, handheld global voice/data. COSPAS-SARSAT listens on 406 MHz and often fixes position by Doppler. INMARSAT = GEO ~22,177 mi maritime satcom (ocean regions AOR-E/W, POR, IOR). GPS = ~24 MEO sats, L1 1575.4 / L2 1227.6 MHz, 4 sats for full position + time; DGPS adds corrections.

Topic 3-P (Satellite) closes the Element 3 systems block with the space segment GROL techs meet on ships, aircraft, and distress beacons. Keep LEO vs GEO vs GPS MEO altitudes straight—the pool loves swapping the mile numbers.

Orbit classes you must separate

GEO sits near 35,786 km / ~22,300 mi so orbital period matches Earth’s rotation—one satellite hangs over a fixed longitude. LEO periods are about 90 minutes (Iridium pool: approximately 85 minutes). GPS is neither GEO nor low-handheld LEO—it is a medium-altitude navigation constellation.

Low Earth Orbit systems (3-P-095)

Iridium (pool facts)

Iridium-class LEO systems trade tiny handheld antennas and true polar coverage against the need for many satellites and frequent handoffs. Spot-beam footprints are small compared with a GEO ocean region—that is why the constellation is large.

COSPAS-SARSAT — satellite distress detection

The main function of the COSPAS-SARSAT satellite system is to monitor 406 MHz for distress calls from EPIRBs (and related 406 MHz beacons such as ELTs/PLBs). Do not put COSPAS-SARSAT on INMARSAT L-band voice channels or claim it primarily monitors 121.5 MHz voice (legacy 121.5 satellite processing has been phased down; Element 3 still centers 406 MHz).

How position is determined (pool): by measuring the Doppler shift of the 406 MHz signal taken at several different points in the satellite’s orbit. A LEO bird hears a rising then falling frequency as it passes the beacon; the Doppler curve encodes the beacon’s location for Local User Terminals and the SAR distribution network. (Many modern beacons also encode GNSS position in the message—but the classic Element 3 answer emphasizes Doppler.)

INMARSAT communications (3-P-096 & 097)

Geostationary maritime backbone

Orbital altitude of INMARSAT satellites: about 22,177 miles (pool figure). They are geostationary over the equator and provide coverage to vessels in nearly all of the world’s navigable waters—not polar-orbit “true global” in the LEO sense, and high latitudes near the poles can be marginal because elevation angles go to zero.

Ocean region description (pool):

• AOR-E at 15.5° W
• AOR-W at 54° W
• POR at 178° E
• IOR at 64.5° E

What can make INMARSAT-B communications impossible?

Pool answer: all of these—

1. A mast or obstruction blocking the path when the ship steers a certain course.
2. A satellite signal at low elevation / below the horizon.
3. Travel beyond the effective radius of that satellite’s coverage.

Directional SES antennas must stay pointed; shading and geometry are operational failure modes, not just “radio broken.”

INMARSAT-C vs -B antennas and services

INMARSAT-C is the GMDSS workhorse for ships that need distress and safety messaging without a large stabilized dish: the omnidirectional antenna is simpler to install and keep working in heavy weather. Messages are stored and forwarded through the network—think enhanced telex/email-style delivery, not a phone call.

INMARSAT-B (legacy digital GEO mobile) used CODECs to digitize the voice signal for satellite voice. Compare antennas correctly on exam day: C = small omni; B = larger directional—never reverse them.

INMARSAT-B services (pool): voice at 16 kbps, fax at 14.4 kbps, and high-speed data at 64/54 kbps-class rates.

GPS — Global Positioning System (3-P-098)

Constellation

Trilateration and the fourth satellite

Each GPS satellite broadcasts precise time and ephemeris (orbital data). A receiver measures pseudorange to each satellite from signal travel time (speed of light × delay, with clock and propagation errors).

• Three perfect ranges would fix a 3-D position if the receiver clock were perfect.
• Real receiver clocks are imperfect, so a fourth satellite is required to solve for three position coordinates + clock bias—“complete position and time.”

That is trilateration (ranges), not compass bearings. L1 (1575.4 MHz) is the primary civil frequency you will see on every chartplotter and phone GNSS front-end; L2 (1227.6 MHz) supports dual-frequency ionospheric correction and authorized services.

DGPS

DGPS (Differential GPS) is a system that provides additional correction factors to improve position accuracy. A reference station at a known location measures GPS errors and broadcasts corrections to nearby users (historically MF marine beacons, also satellite-based augmentation in broader GNSS practice). It is not merely “digital ground position system” wordplay and not an altitude-only aircraft product in the Element 3 definition.

GMDSS satellite distress path overview

For the GROL tech supporting compulsory vessels, satellite distress is a layered system:

1. Alert: Ship INMARSAT SES distress function and/or 406 MHz EPIRB via COSPAS-SARSAT.
2. Locate: COSPAS-SARSAT Doppler (and/or GNSS-coded position); INMARSAT path delivers identity/position through land earth stations to RCC.
3. Local home: Rescuers use SART on X-band radar and VHF Ch-16 / portable survival radios near the scene.
4. Navigate: Bridge GPS/DGPS feeds DSC, ECDIS, and INMARSAT terminals with position.

If the GEO satellite is shadowed or the ship is outside an ocean region, LEO beacon detection and terrestrial/MF-HF GMDSS paths remain critical—another reason Element 3 tests both INMARSAT GEO facts and COSPAS-SARSAT LEO Doppler facts.

Service notes for GROL maintainers

1. INMARSAT directional antennas: verify clear sky view, stabilization, and cable/waveguide integrity; mast shadow on certain headings is a real “RF works on some courses only” complaint.
2. INMARSAT-C omni: check antenna height/clearance and GNSS input used for distress position injection.
3. GPS antennas: need sky view; multipath off superstructure degrades accuracy—mount away from radar scanners and INMARSAT dishes when possible.
4. EPIRB registration and 406 MHz coding: wrong MMSI/hex ID wastes the satellite path you just studied.
5. Do not confuse GPS L1 with Iridium 1616–1626 MHz or INMARSAT L-band service frequencies.

Exam-day checklist (3-P 095–098)

1. Iridium: 485 mi, 1616–1626 MHz, digital voice/data 2.4 kbps, period ~85 min.
2. COSPAS-SARSAT: monitor 406 MHz EPIRBs; position via Doppler of the 406 MHz signal.
3. INMARSAT altitude ~22,177 mi; regions AOR-E 15.5°W, AOR-W 54°W, POR 178°E, IOR 64.5°E; coverage of nearly all navigable waters.
4. C = store-and-forward, omni antenna; B = directional, CODEC voice, higher-rate services; HSD 64/54 on B, M4, Fleet.
5. GPS: 12,554 mi, 24 sats, six planes ~55°, 1575.4 & 1227.6 MHz, need 4 sats; DGPS = correction factors for accuracy.
6. GMDSS uses INMARSAT + COSPAS-SARSAT + local SART/VHF—know which segment each device belongs to.

Master LEO vs GEO mileages and the C omni / B dish antenna contrast and Topic 3-P becomes high-percentage points on Element 3. Safety Topic 3-Q (RF exposure and electrical safety) is the final technical cluster after this chapter.