Ambulance, Binary Stars and Saving Lives at Sea

Doppler Effect

There are physical phenomena we experience intuitively, without ever thinking about the fact that they form the foundation of some of the most complex engineering systems in existence. 

One of these phenomena is the Doppler effect. It connects the sound of a siren on a city street, the motion of stars in deep space, and the operation of the world’s global maritime and aviation search-and-rescue system.

An Approaching Ambulance

Almost everyone has noticed it: as an ambulance approaches, the sound of its siren seems higher-pitched and sharper. The moment it passes and moves away, the tone suddenly drops and becomes noticeably lower. What matters here is that the siren itself does not change frequency. It emits sound at a constant frequency. What changes is how that sound is perceived by the listener, because the source of the sound is moving relative to the observer.

Physically, this happens as follows:

• when the source approaches, sound waves are compressed • the distance between wave crests decreases • the observed frequency increases

• when the source moves away, the waves are stretched • the observed frequency decreases

This is the Doppler effect: a change in the observed frequency of a wave caused by relative motion between the source and the receiver. Crucially, the Doppler effect applies to all types of waves: sound waves, radio waves, electromagnetic radiation, including visible light. This universality is what made it a fundamental tool in astronomy, radio physics, and navigation.

The Doppler Effect and Binary Stars

In 1842, Austrian physicist Christian Doppler proposed that changes in the color of starlight could be linked to the motion of stars relative to an observer. He considered binary star systems - pairs of stars orbiting a common center of mass.

In such systems one star periodically moves toward the observer, then moves away and the spectral lines of its light shift: toward the blue when approaching; toward the red when receding. This became the foundation of spectroscopic astronomy, enabling scientists to measure stellar velocities, confirm the existence of binary systems and later, measure the motion of galaxies and the expansion of the universe.

The same physical effect that alters the pitch of an ambulance siren also shifts the light of distant stars.

Rescue Before the Age of GPS

Somewhat unexpectedly, the Doppler effect also became a cornerstone of one of the most important - and least visible - engineering systems of the 20th century: COSPAS–SARSAT.  Its mission is uncompromisingly simple - to detect a distress signal anywhere on Earth and deliver it to rescue services as quickly as possible. The key point is this: the system was created long before GPS, GLONASS or any other global navigation satellite systems existed, yet it was already capable of determining the location of an emergency.

Before Satellite Navigation

Early emergency beacons were highly specialized devices tied to specific modes of transport: ELTs (Emergency Locator Transmitters) for aviation, in use since the 1960s, EPIRBs (Emergency Position-Indicating Radio Beacons) for maritime use, becoming widespread in the 1970s.

They transmitted on 406 MHz frequency, sending: a unique identifier, the type of object (aircraft or vessel), a distress signal. What they did not transmit were coordinates. Compact navigation receivers simply did not exist at the time. Personal Locator Beacons (PLBs) appeared much later, in the era of miniaturized GNSS receivers, and were designed from the outset for a “406 MHz + GPS” architecture.

This raises an obvious question: how do you locate a distress beacon that transmits no position data?

Doppler Navigation from Orbit

Once again, the answer lies in the Doppler effect.  COSPAS–SARSAT satellites in low Earth orbit (LEOSAR) move at approximately 7–8 km/s relative to Earth. As such a satellite passes over an emergency beacon the received frequency is higher while approaching, and lower while moving away.  By analyzing the shape of the Doppler frequency curve, the rate of frequency change, the precise orbital parameters of the satellite,  ground stations could compute a line of possible locations for the signal source. With repeated satellite passes, this line narrowed to two points - and eventually to a single, correct position.

Typical performance in the late 1970s and early 1980s: accuracy 5–10 km, sometimes better under favorable conditions; location time - from several minutes up to about an hour.

For its technical era, this was a genuine engineering breakthrough - personal computers were rare, GPS was still an experimental military system, the internet did not exist - yet a global satellite-based rescue network was already operational.

The Cold War and an Unlikely Partnership

The origins of COSPAS–SARSAT lie in the 1970s, at the height of the Cold War. The rapid growth of civil aviation, long-distance shipping and flights over remote regions demanded a fundamentally new approach to search and rescue. Traditional radio methods had limited range, depended on nearby ships or coastal stations, were often ineffective over oceans or in polar regions.

Paradoxically, geopolitical rivalry led to a rare moment of cooperation. The Soviet Union developed the COSPAS system, The United States, Canada and France developed SARSAT. Instead of competing, the parties chose an unprecedented path: to merge efforts and create a single global search-and-rescue system, open to all nations.

Key milestones: 1979 - first orbital experiments; 1981 - successful detection of emergency beacons; 1982 - system declared operational; 1985 - international agreement on joint operation.

From the beginning, COSPAS–SARSAT was designed as a planetary system, not a regional service.

GPS Changes the Architecture

With the advent of global navigation satellite systems: Navistar, GLONASS, Galileo, BeiDou, it became possible to integrate positioning receivers directly into emergency beacons.

Modern 406 MHz beacons now transmit: a unique identifier, precise latitude and longitude, time of position fix, additional data about the vessel or individual. Location accuracy improved from kilometers to tens of meters, and response times dropped to minutes.

Three Orbital Layers, One System

Today, COSPAS–SARSAT operates using three satellite architectures.

LEOSAR (Low Earth Orbit) - the historical foundation still uses the Doppler effect, works without beacon coordinates, provides polar coverage;

GEOSAR (Geostationary Orbit) - near-instantaneous detection, no Doppler processing, no polar coverage, requires GPS coordinates in the beacon signal;

MEOSAR (Medium Earth Orbit) - the modern standard, using navigation satellites from GPS, GLONASS, Galileo and BeiDou. MEOSAR provides: simultaneous reception by dozens of satellites, high redundancy, coordinate confirmation, position determination even if GPS fails.

Doppler out of date?

No - it has simply changed its role. In LEOSAR, it remains the primary positioning method. In MEOSAR, it serves as a backup, as a validation tool,  as a positioning method when GPS data is unavailable.

Classical radio physics has not vanished - it has become part of a more complex hybrid system.

Conclusion

COSPAS–SARSAT is an example of a system where fundamental physics, satellite navigation, international cooperation work together toward a single goal: saving lives.

 

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