Chronometer for space pioneers

In the 18th century, the British Empire actively expanded its borders, trade, and influence. The main tool was the fleet - no wonder this small island country is called the Lady of the Seas. And in the early 18th century, the improvement of navigation required the creation of a more accurate method for determining the position of ships. The Parliament of Great Britain has appointed an unprecedented reward for solving this problem. The decision was to create ultra-precise (for those times) "sea" hours . But the story goes in a spiral: today, the tasks of mastering the Solar System require the use of the most accurate navigation clock — atomic — on spacecraft.

Time is one of the most important parameters for course laying and navigation. Knowing our speed and the elapsed time from the beginning of the movement, we can calculate how far we have moved, how much is left to pass / drive / fly from point A to point B. And the more accurate the measuring instruments, including the clock, the more accurately we can paving the course, the less chance of error. This is vital in situations where distances between points of the route are extremely large, and travelers' resources are extremely limited and do not allow to wander in space in search of a destination. For example, when traveling from Earth to Mars.

The most accurate clock created by mankind is the atomic clock. They are based on the idea of ​​counting units of time using a certain number of periods of oscillation of atoms of various substances. For example, cesium, strontium, rubidium, hydrogen, calcium, iodine and other chemical elements. Today, atomic clocks are used mainly in satellite navigation systems and for controlling spacecraft. At the same time, many countries are working to improve the accuracy of atomic clocks, their compactness and resistance to external influences. For example, in the autumn of 2016, a prototype of optical atomic clocks based on thulium atoms, which today are among the most accurate in the world, was presented at the Russian Institute of Physics.
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But the more boldly we think about the development of the Solar System, the more technical difficulties it confronts designers with engineers. One of them is to increase the accuracy of navigation in outer space. The cost of a course-laying error in this case is VERY great even if the use of unmanned probes, not to mention manned flights to Mars and even further, to the satellites of Jupiter and Saturn.
Today, for satellite navigation, the transit time of a radio signal between the spacecraft and the control center is measured. Knowing the speed of propagation of radio waves, you can determine the distance traveled by the device, and its relative speed.

Deep space

Several years ago, NASA began to develop compact atomic clocks for long-distance space missions - Deep Space Atomic Clock (DSAC). Hours represent the module in the form of a parallelepiped with sizes of 29 x 27 x 23 cm. Weight - 16 kg. Power consumption - 44 watts. The DSAC uses mercury atoms, so the watch is very resistant to external magnetic fields and temperature extremes. The accuracy level of the clock is less than 1 microsecond in 10 years.



In March 2017, it is planned to launch the module in a test flight on the Orbital Test Bed. During the year, the device will determine the height of its orbit with high accuracy.

What for?

But why do we need new atomic clocks, and even for installation on spacecraft, when, even when controlled from the Earth, a tremendous accuracy of measurements of flight parameters is achieved?

Take a practical example: determining the trajectory of a satellite orbiting Mars. The distance from it to the Earth is on average 225 million km (minimum - 55.76 million, maximum - 401 million km). With this distance, the radio signal goes back and forth for about 25 minutes. And the key point here is the accuracy of time measurement. Today, atomic clocks used in ground-based flight control systems make it possible to calculate the distance to a spacecraft with an accuracy of less than a meter, and its speed relative to the MCC - with an accuracy of less than a millimeter per second. After a two-day period of data accumulation, the trajectory of the spacecraft around Mars can be determined. And, if necessary, correct it.

According to NASA's Jet Propulsion Laboratory , which is developing DSAC, during two days of observing a satellite in Mars orbit, the accumulated time measurement error is a few picoseconds, which gives the total measurement error of the distance to the vehicle in fractions of a meter, and speed - about 1 μm / s. The collected statistics are used by complex trajectory calculation algorithms with an error within 10 meters.

If we place an atomic clock on a spacecraft, then to calculate the flight parameters, you do not need to send a command from Earth every time so that the device responds by measuring the time it takes for the signal to travel back and forth. It is enough that the onboard navigation system itself periodically sent signals, or the signals can be sent unilaterally from the Earth, and all calculations of flight parameters will be carried out on board. That is, we halve the calculated time intervals, and hence the magnitude of the error.

In addition, according to the same Jet Propulsion Laboratory, it will be possible to switch to higher frequencies, which will increase the tracking accuracy by an order of magnitude, reducing the error value by the same amount.

In addition, with an increase in the number of spacecraft that need to be controlled from the Earth, the problem of limited resources of antenna complexes will inevitably arise. And if it will be possible to send a signal only in one direction, without waiting for a response, then at the existing capacities it will be possible to effectively manage twice as many devices.



If the device is designed for one-way transmission of the measuring signal, it will be possible to save on the size of the antennas, because they will not need to be very accurately aimed at the ground to send a response. Moreover, it will be possible not to spend the valuable time of research probes on the sessions of sending measuring signals, devoting it to scientific measurements. And the accumulation of navigation data on board will allow them to be used in real time for maneuvering and plotting. This is especially important in cases where reaction time becomes critical. For example, when approaching the planet of the robotic apparatus. Or, when driving on a rough terrain, the robot rover / moon rover / titan rover / euro walk.



Even in the case of manned flights, astronauts will be very helpful to have on hand all the data on their trajectory, so that you can quickly build a further course in difficult conditions.

As for the Russian cosmonautics, then our work also does not stand still. In particular, in 2018, it is planned to launch the first GLONASS satellite with a hydrogen atomic clock , which demonstrated an accuracy of 1.8 microseconds in 10 years (0.5 picoseconds in 12 hours).