Why Astro?

In a recent article the discussion centred on our over-reliance on GPS for navigation at sea and the need for back-up systems. The conclusion drawn was that we already have a back-up system, one that has been tried and tested over hundreds of years and that is astro navigation or celestial navigation as it is also known. Was this the correct conclusion though? In this article, we set out to explore other alternatives to GPS and to examine the pros and cons of astro navigation.

(Note. The terms astro navigation and celestial navigation are synonymous but for simplicity’s sake, we shall stick to astro navigation for the rest of this article).

What are the risks to the GPS?

Spoofing – misdirecting GPS navigation receiver so that it thinks it is somewhere it isn’t.

Jamming – the intentional emission of radio frequency signals to interfere with the operation of GPS receivers by saturating them with noise or false information.

Hacking – breaking into GPS software to discover a receiver’s location or to corrupt it..

Malicious viruses causing GPS to malfunction.

Magnetic storms can put power grids out of action, blank out communications systems and the GPS.

Electro-magnetic interference – can disrupt radio signals causing distorted GPS readings.

Damage to aerials and equipment can leave a vessel without access to the GPS

What are the alternatives?

Sebastion Anthony suggests creating a ground-based system which would involve blanketing the Earth with hundreds or thousands of radio transmitters at an immense cost. Surely though, that would be a waste of money and time; any system that is based on radio signals would be susceptible to the risks of spoofing, jamming and hacking in the same way that the GPS is.

There has also been talk of re-commissioning some of the electronic navigation systems that were in use before the advent of the GPS such as Omega, CONSOL, DECCA, and LORAN but once again, we are back to the problem of re-introducing radio based systems that are susceptible to the same risks as GPS.

George H Kaplan of the US Naval Observatory talks of using the Stellar Reference Frame as an alternative to GPS but this system also relies on electro-magnetic signals to communicate with satellites and so it is susceptible to exactly the same risks as the GPS.

Kaplan also talks of employing inertial navigation systems which are used in guided missiles, spacecraft, submarines and other naval ships and aircraft; however, he points out that these are simply sophisticated dead-reckoning systems that need to be aligned to a reference point, usually provided by GPS. So, we come back to the problem of reliance on GPS. However, he does suggest that where radar plots and weapon control systems in naval ships and aircraft need some sort of electronic input of position, inertial navigation systems may fit the bill during short periods of GPS malfunction.

However, none of this matters much to ‘yachties’ and small merchant ships unless small and cheap versions of such equipment becomes available to them.

The Only Real Alternative It seems that the only real alternative to GPS is Astro Navigation and that is probably why the US Navy has recently re-introduced it in its training programmes while the Royal Navy continues to keep it in the curriculum for specialist navigating officers.

Advantages of Astro Navigation:

It has global coverage.
Does not require expensive equipment.
Does not require a ground based support infrastructure.
Does not emit electro-magnetic signals that can be detected by an enemy.
Cannot be jammed, spoofed or hacked..
Is not susceptible to disruption by solar storms or other electro-magnetic disturbances.

Disadvantages of Astro Navigation:

Can be hampered by cloud cover except in aircraft.
Inherent Errors in data and calculations. U.S. Navy and Royal Navy navigators are taught that the accuracy of astro navigation is ±1 minute of arc or 1 nautical mile. For details of inherent errors in astro navigation click here.
Even with a highly skilled navigator it can take several minutes to obtain a celestial fix whereas a GPS fix is more or less instantaneous.

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