A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Sems Aktug
Date: 2013 Mar 27, 00:23 -0700
Do We Still Need to Use Sextants?
Piri Reis University
The title of this paper may give you the impression that article intends to discuss the requirements of the celestial navigation again, but it does not. It discusses the possibility of ending the sextant era and starting a new era in celestial navigation.
For almost than 350 years sextants have been helping the seafarers to find their ways in the open sea. It has been one of the most important navigation equipment for them. The sextant is used to measure altitude of the celestial bodies. Once the altitude of the celestial body is obtained by the sextant, with a known altitude, position of the observer can be calculated by using spherical trigonometry. At the beginning this was a painful and tedious process of calculations by trigonometric functions, but the speeds of the ships at that time were such slow to bare it. The methods changed in time, tables surfaced, calculators shortened the time, then the computers and software took the load and made it easy.
On the other end the observation process is still same as it was 350 years ago. To observe a star, you have to wait for twilight period, check the instrument error, select the proper stars and identify them. After that if the horizon is clear, “shooting” begins by sextant, not only once, even not depending on your experience, more than one time, you have to record the altitudes and time. Those readings should be corrected for different errors to make it reliable data for calculation.
What can be done to shorten this procedure and improve the reliability of the celestial navigation fix positions? There are different error sources, like interpolations, assumptions, rounding etc. in celestial navigation but errors done during the altitude measurement with sextant is the main error source. Sextants principle is same for ages. While seeing the horizon through a transparent horizon mirror, with help of another mirror (index mirror) the image of the star reflected on to the horizon mirror. The star is lowered to the horizon mirror by a micrometer which is linked to the index mirror. The observer should be sure the sextant is held perpendicular to the horizon and adjust the micrometer drum to see the star and the horizon at same level. The measured altitude of the star is always two times the angle between the two mirrors and is displayed directly on the micrometer.
If we go back to WWII there are still some lessons to be learned. One of them is the German Gyro (Kreisel) sextant which is used on board aircrafts. This sextant used an air-driven gyroscope as an artificial horizon so no need to wait for the twilight or clear horizon. The initial navigation system of B-52 aircrafts had a star tracing system supported by a gyroscope. Also this type of sextants was found to be quite useful at sea, since gyroscopes do not respond to lateral acceleration the way the bubble sextants do. But the problem here is how can we implant a gyro in a sextant or do we still have to use sextants?
By using the newer technologies a solution can be implied. Micro electro mechanical systems (MEMS) gyroscopes or gyro chips may create a revolution for sextants. In a routine daily life we are using those chips in our smart phones, video game controllers or in navigation systems. Gyroscope MEMS are an inertial sensing integrated circuit that measures the angle and rate of rotation in an object or system. This technology has been improved in the last 10 years and now, 3-axis gyroscope, 3-axis accelerometer and 3-axis compass are integrated in the same chip can be found on the shelf. With the advent of MEMS, gyroscopes and other inertial measurement devices can now be produced cheaply and in very small packages in the micro domain. So it can be fitted to a binocular or monocular.
This device is a binocular and will use the artificial horizon as the reference which is produced by gyro MEMS and the name is “altcular”. During the observation, officer of the watch has to choose any star and when the star is in the centre of the “altcular”s crosshair, he will shoot it by pushing a button and the altitude and not necessary but also azimuth of the body will be transmitted to the system via Bluetooth. This system can be used at any time if someone can see the sky because there is no necessity to see the horizon. Without waiting twilight, all night long stars, planets and moon and during the day sun can be observed. Another advantage of the “altcular” is that it will minimize the measurement errors. OOW can shoot one star may be 10 times in a very short time and the system will decide the correct measurements and will find the correct altitude. Also the system can identify the body automatically. With a known time and position of the ship and altitude and azimuth of the body which is transmitted by the altcular, software will identify the body and will inform the OOW about the identification. If identification done automatically, there will no need to choose 57 stars listed in nautical almanac and one can use any star as she/he wishes.
If the sextant is not hold perpendicular to the horizon, measured altitude will be wrong. To compensate this holding error, altculors’ gyro mems will produce a signal to show the corrected holding way to the OOW.
The time of the sight is very important, because actual positions of celestial bodies will be calculated according to this time. When the sight data transmitted to the processor, first line of the program should record the altitude with time. Time input can be gathered from the inner clock of the ECDIS. ECDIS inner clock gets the time from satellite navigation system and when transmitted, it corrects the errors. So the inner clock precession should be high.
ECDIS are either using satellite position finding system, or Radar fix method. At open sea radar is useless and DR cannot be considered as a position finding method so only satellites can be used. ECDIS with a new software patch which covers the necessary steps of celestial navigation calculations will manage the process and the result as a fix position will be displayed on screen of ECDIS. This type of approach will improve the reliability of ECDIS also.
Piri Reis University
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