Can GPS navigate us to covert underground nuclear test sites?

Posted by () on Oct 06 2011
VERTIC Blog >> Arms Control and Disarmament

Isadora Blachman-Biatch, London

An op-ed recently published in the Bulletin of the Atomic Scientists suggests that there might be a new way to detect underground nuclear test explosions. The authors, a group of scientists from Ohio State University, have been working on turning a troublesome vulnerability of a common system into something useful. Their findings suggest that data from Global Positioning Systems (GPS), commonly used in navigation, could be used to augment existing detection techniques by detecting airborne shockwaves created by underground test explosions. The location of such explosions can then be narrowed down by comparing data on these shockwaves collected from nearby GPS receivers. This seems like a fascinating potential use of existing technology. However, there is an important issue which could prevent it from becoming a reality.

Searching for nuclear tests from space
At least 27 GPS satellites orbit the earth at any given time, 24 of which are operational while the remaining three serve as back-ups. These satellites communicate with GPS receivers on the ground by broadcasting high-frequency, low-power radio waves. GPS receivers can translate these signals into distances based on the time it takes for them to arrive. The receivers then determine their own location by performing similar calculations on signals received from three or more other satellites. This method is known as ‘trilateration’.

With such extensive and constant coverage of the earth’s surface, GPS satellites are particularly effective for global monitoring. Many of these satellites already contribute to the Comprehensive Nuclear-Test-Ban Treaty’s (CTBT) monitoring scheme. For example, GPS satellites operated by the US Air Force already include nuclear explosion detection as one of their core missions. As part of this mission, nuclear explosion detection monitors were added to these and other satellites in 1975, and are now present on 33 satellites. These include optical detectors, such as bhangmeters, neutron detectors, x-ray detectors and gamma ray sensors. Additional systems may be added as technology improves. New detectors may include enhanced optical sensors, autonomous EMP sensors, combined gamma ray and neutron detectors, infrared sensors and x-ray sensors with on-board data processing.

Unfortunately, developing and deploying these extra detectors is likely to be a protracted and expensive process. There is very little space available for new satellites within the orbital radii required by GPS satellites. And attaching new equipment to satellites already in orbit is extremely challenging, particularly with the current lack of available space vehicles. This is why the new proposal sounds promising; it takes advantage of particular aspects of the primary positioning systems already on these satellites.

Navigating to underground test sites by GPS
As GPS radio waves travel through the atmosphere, they encounter the layer of the atmosphere known as the ionosphere. The majority of atoms within this layer, which lies between 90km and 350km above the earth’s surface, have either gained or lost an electron. This imbalance causes these atoms to carry an electric charge. The resulting distribution of electrical charges in the ionosphere can reflect or disrupt radio signals, including those from GPS satellites. It is this effect that allows lower-frequency radio signals to be ‘bounced’ off the bottom of the ionosphere to locations over a transmitter’s natural horizon.

Underground nuclear explosions, as mentioned in an earlier post, generate powerful vibrations. These vibrations are not confined within the ground and are transmitted into the air through the earth’s surface. When the explosion pushes out against the ground, it creates a shockwave in the air which can travel all the way up into the ionosphere. When these shockwaves reach the ionosphere they can compress it, causing spikes in charge density. Significant spikes in the charge density, measured in terms of total electron content (TEC) are referred to as travelling ionospheric disturbances (TIDs). These disturbances cause delays in the receipt of GPS signals, which result in measureable errors in receiver positioning calculations.

By detecting and measuring these errors, the authors argue that they can calculate the length of transmission delay and the change in TEC which caused it. They can then calculate the velocity of the disturbance from a given satellite using the TEC. With the help of some interesting geometry, it is then possible to determine the origin point of the disturbance, or, in this case, the location of an underground nuclear test explosion. However, underground nuclear explosions are not the only cause of GPS signal delays. Geomagnetic storms, tsunamis and tropic storms can also produce similar effects. These disturbances have to be monitored in order to discern a natural change in TEC from one caused by an underground nuclear disturbance.

Can GPS be added to the CTBT monitoring arsenal?
Although the authors concede that this technique is not yet accurate enough to constitute a fully independent monitoring system, it could augment the existing methods used to verify the CTBT. The Comprehensive Nuclear-Test-Ban Treaty Organization’s (CTBTO) international monitoring system (IMS) currently consists of seismic, hydroacoustic, radionuclide and infrasound stations. Although this network of monitoring stations is already highly capable of detecting nuclear explosions, the proposed system could be an inexpensive and quick way of verifying the results from the IMS.

However, there is an issue which might diminish the appeal of this technique. For this system to be feasible, the CTBTO will have to gain access to sufficient sources of GPS receiver data. Their access might depend on a receiver host country’s CTBT status. Gaining access to data from non-signatories may be not be easy. And although state parties to the CTBT may be willing to share GPS receiver data, they are in no way obliged to.

On the other hand, the CTBTO may not need to obtain national receiver data. If the CTBTO was worried about particular regions, it could build its own receivers. Additionally, test experiments to prove this method acquired almost half of their GPS receiver data from an international system of satellites and receivers. The International Global Navigation Satellite System Service, or IGS, combines both US and Russian-operated GPS to produce the largest GPS network available on earth. Even if some countries are unwilling to share local data, the volume and distribution of these worldwide receivers might negate any lack of access. However, it is important to note that the IGS is federation of world-wide agencies, not an internationally-owned resource. Introducing a role in monitoring a treaty, which has not been ratified by one of the key members of this federation, could be politically complex.

With this in mind, the authors suggest that by improving the verification capabilities of the CTBTO, this new technique could encourage more states to ratify the CTBT. Indeed, adapting GPS to help detect underground nuclear test explosions is a relatively cheap and simple way of adding to the IMS. It remains to be seen however if the CTBTO can gain sufficient access to receiver information. And more fundamentally, it remains to be seen if an improved verification system is enough to convince sceptics that the CTBT should be signed and ratified. Despite this, the proposed system should not be rejected too hastily. With further interest, investment and development, the technique may eventually play an important role in the IMS.

Last changed: Oct 07 2011 at 5:07 PM

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