Maarg Manthan · Topic 12.13

Deep Space Atomic Clock

Science & Technology › Space Technology and ISRO · Topic 12.13

The Deep Space Atomic Clock is a small, very precise clock built by NASA's Jet Propulsion Laboratory that uses trapped mercury ions to keep time. It was launched in June 2019 on a SpaceX Falcon Heavy as part of a technology demonstration and worked in Earth orbit until September 2021. It was designed to be about 50 times more stable than the clocks flown on navigation satellites such as GPS, with an error of less than a billionth of a second over a day. The purpose is to let spacecraft on deep space journeys work out their own position without waiting for tracking signals to go to Earth and back, which can take many minutes or hours. Such clocks are also vital to navigation systems such as NavIC.

5 min read · UPSC Prelims · Science & Technology · Space Technology and ISRO

At a Glance

Built by NASA JPL
Launched June 2019
Ion used Mercury
Stability About 50 times better than GPS clocks
Aim Autonomous deep space navigation
Exam link UPSC Prelims, Science & Technology: Space Technology and ISRO

Where Does It Fit?

This topic follows the account of remote sensing. It looks at timekeeping technology, a base for navigation and for spacecraft operations far from Earth, and it links to the atomic clocks used in NavIC. Previous topic: Remote Sensing Applications and Ground Segment Services. Next topic: Space Weaponry: Anti-Satellite (A-SAT) Technology.

How Does the Clock Work and Why Is It Useful?

Aspect	Details
Atomic clock principle	Atoms absorb microwave energy at an exact frequency. Locking a microwave source to that frequency gives a very steady time signal
Mercury ion trap	Charged mercury atoms are held in a trap by electric fields and cooled, and their transition frequency about 40.5 gigahertz keeps the clock steady
Size	About the size of a four-slice toaster, a big reduction compared with earlier ion clocks
Launch	STP-2 mission on a Falcon Heavy, 25 June 2019, carried on the Odyssey spacecraft
Demonstration	Ran for more than two years in low Earth orbit, ending in September 2021, and showed stability over 10 times better than other space clocks tested
Stability	Gain or loss of less than about 4 nanoseconds over more than 20 days in tests
Next step	A follow up clock, DSAC-2, was selected for a Venus mission

Normal deep space tracking: a signal is sent from Earth to the spacecraft, which returns it, and the round trip time gives distance. Calculations are done on Earth and commands sent back
Delay: for Mars, signals take 4 to 24 minutes one way, so the spacecraft must wait for instructions
One-way tracking: with an onboard atomic clock the spacecraft can use a one-way signal from Earth, and calculate its own position
Autonomous navigation: the craft decides its course quickly, which helps in landing, orbit insertion and in crewed missions
More tracking: one Earth antenna can track several spacecraft at once, since two-way tracking is not needed
Better science: precise timing improves radio science measurements such as gravity fields
Safety of crews: faster navigation solutions are valuable for human missions to Mars
Reduced load: fewer demands on the Deep Space Network of antennas

What Else Should You Remember?

Atomic clock types: caesium and rubidium clocks are common, hydrogen masers are very stable, and optical lattice clocks are the best in laboratories
Definition of the second: since 1967 the second is defined by 9,192,631,770 oscillations of a caesium-133 atom
GPS clocks: each GPS satellite carries several rubidium and caesium clocks, and a time error of one microsecond gives a position error of about 300 metres
NavIC clocks: NavIC satellites carry rubidium atomic clocks, and failures of early clocks reduced the system, as in Navigation Satellite Systems: GPS, Galileo, GLONASS, NavIC (IRNSS), GAGAN
Indigenous clocks: ISRO’s Space Applications Centre developed an indigenous rubidium atomic clock for the NVS satellites
One nation, one time: India’s national time scale is kept by the National Physical Laboratory, which operates caesium clocks and distributes Indian Standard Time
Relativity: clocks in space run at a different rate from clocks on Earth, which must be corrected for
Other uses: financial transactions, telecom networks, power grids and scientific experiments need precise time
Navigational power: a clock is a key part of the accuracy of any satellite navigation system
Quantum clocks: optical clocks can be 100 times more precise and may redefine the second in the coming years
Mercury toxicity: the mercury quantity in the clock is tiny and sealed
Limit: the demonstration was a short mission and further tests are planned

Why Does It Matter for the Exam?

Facts: who built it, ion, year
Concept: one-way versus two-way tracking
Link: atomic clocks in GPS and NavIC
Current affairs link: Indian atomic clocks and time scale
Common errors: saying that it uses caesium or that it is a GPS satellite

As an analytical point, small stable clocks make spacecraft more independent and navigation systems more accurate. India’s work on indigenous atomic clocks for NavIC is part of the same global effort.

Why Is This Topic Important?

Facts Mercury ion clock
Purpose Autonomy
India NavIC clocks

What Came Next?

The next topic covers anti-satellite weapon technology. Next topic: Space Weaponry: Anti-Satellite (A-SAT) Technology.

Key People and Terms

Atomic clock Clock using atom transitions
Ion trap Holds charged atoms
Nanosecond One billionth second
DSN Deep Space Network
One-way tracking Signal in one direction
Maser Microwave amplifier clock

Exam Corner

Points to Remember

DSAC launched: June 2019.
Uses mercury ions.
About 50 times more stable than GPS clocks (NASA claim).
Second defined by caesium-133.
One microsecond error = about 300 m.

Do Not Confuse With

DSAC and GPS clock Mercury ion demo versus rubidium and caesium clocks
One-way and two-way tracking Spacecraft computes versus Earth computes
Atomic clock and quartz clock Atom transition versus crystal vibration

Memory Hook

Mercury ions in a trap keep time for the deep space map.

Mains Angle

Questions ask about the purpose and link to navigation. These points help.

Purpose Autonomous navigation
Value Precise timing
India NavIC clocks

A Question You May Face

An original practice question, not a past paper question.

“Explain the importance of precise atomic clocks in space navigation with reference to the Deep Space Atomic Clock and NavIC.”

How to Answer

Principle Atomic transitions.
DSAC Autonomous craft.
NavIC Clock reliability.
Way Indigenous clocks.

GS Relevance

Prelims: purpose, who built it, ion type, difference from GPS clocks. GS Paper 3: science and technology, space, navigation.

Frequently Asked Questions

What is the Deep Space Atomic Clock?

The Deep Space Atomic Clock is a compact mercury ion clock built by NASA's Jet Propulsion Laboratory and launched in June 2019 for a technology demonstration. It is far more stable than the clocks used on navigation satellites, and its purpose is to let spacecraft on deep space journeys compute their position on board without waiting for instructions from Earth.

Why does a deep space spacecraft need an atomic clock?

A spacecraft far from Earth normally sends a signal back, and calculations of its position are done on Earth, with delays of many minutes. An accurate onboard atomic clock lets the craft use a one-way signal from Earth and compute its own position and path, so it can navigate and manoeuvre faster, which is important for landings and crewed missions.

How accurate are atomic clocks in navigation satellites?

Atomic clocks on navigation satellites keep time to billionths of a second. This matters because radio waves travel about 300 metres in one microsecond, so a timing error of one microsecond causes a position error of about 300 metres. GPS satellites carry rubidium and caesium clocks, and NavIC satellites carry rubidium clocks.

How is the second defined?

Since 1967 the second has been defined as the duration of 9,192,631,770 oscillations of the radiation of the caesium-133 atom. Caesium clocks are the basis of international atomic time, and India keeps its national time with caesium clocks at the National Physical Laboratory in New Delhi.

Why are indigenous atomic clocks important for India?

Indigenous atomic clocks reduce India's dependence on imported clocks for the NavIC navigation system and other satellites. Failure of early imported clocks had reduced NavIC's working satellites, so ISRO developed its own rubidium clock, which has been flown on newer NVS satellites to improve reliability.

PYQ Practice — Statement Analysis

1 The Deep Space Atomic Clock uses mercury ions.

True

Held in a trap.

2 The second is defined using the radiation of a rubidium atom.

False

It is defined using caesium-133.

3 A timing error of one microsecond causes a positioning error of about 300 metres.