The Defense Department could achieve a more jam-resistant Global Positioning System capability quicker and cheaper by improving receivers or integrating signals from the commercial Iridium network rather than focusing just on launching new satellites with stronger signals, says the Congressional Budget Office.

GPS is relatively easy to jam; theoretically, a jammer with just one watt of power broadcasting on the correct frequencies could prevent a military receiver within a 40 mile radius from picking up a signal.

In a report released Oct. 28, the CBO says DoD plans to spend $22 billion through 2025 on fielding advanced GPS satellites, a ground control system and new receivers able to pick up the M-band GPS signal. The CBO says that amount could go down by between $1 and $3 billion, were the department to stick to purchasing GPS IIIA satellites and skip out on even-stronger signal GPS IIIB and GPS IIIC satellites, the latter of which also promises the ability to selectively focus GPS signals into concentrations 30 times stronger than those of IIIA satellites.

Instead, to increase jamming resistance, the Army and Marine Corps could upgrade current GPS receivers by fitting them with better antennas and by adding inertial navigation systems. That would save $2 billion dollars, the CBO says. Alternatively, they could use the Iridium network to relay data to modified military GPS receivers, also making the signals more robust to terrestrial users. That would save $3 billon, the CBO says.

The military could also go for a combination of the first two options, which would save only $1 billion but be the most robust of the three, the CBO report also notes. Any three of the CBO scenarios assume that DoD would still purchase 40 third-generation GPS satellites (just not IIIB or IIIC birds), that it would still continue to develop a ground control system for third-generation satellites (and IIR-M and IIF orbiters) and that it would also buy M signal receivers in the same numbers. Those irreducible (under the report's assumptions) costs add up to $17.9 billion.

Upgrading existing GPS receivers rather than continuing to operate with existing receivers until the newer M signal devices can be bought--the CBO estimates only about half of military users would have such a receiver by 2026--would get jamming-resistant devices into the hands of military users 8 years earlier, the report says. Roughly half of military users under any of the three scenarios could receive upgrade devices by 2018, the CBO estimates.

There are disadvantages to the cost-saving options, however. All of them would require extra hardware adding bulk and power requirements to devices. Also, the first option would result in less-than-expected increases in precision; under it, accuracy would go down to a radius of about 3 feet, rather than the 6 inches that might be possible with implementation of the DoD's full plan. The second option, which incorporates Iridium satellite signals, would increase accuracy to within about 8 inches--but CBO analysts also note that the government can't guarantee the existence of Iridium Communications.

The option that combines the first two would have the greatest performance, but also be the most difficult for personnel operating on foot, the report notes.

For more:
- download the CBO report, "The Global Positioning System for Military Users: Current Modernization Plans and Alternatives" (.pdf)

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