SEOUL, Sept. 19 (Yonhap) -- Lawmakers from the ruling and opposition parties alike on Monday grilled the defense ministry on the military's preparedness against North Korea's jamming of satellite signals.

During the annual parliamentary audit, lawmakers on the standing committee on national defense said North Korea disrupted Global Positioning System (GPS) signals in South Korea in August and December last year, and again in March of this year.

Park Sang-cheon, a representative of the main opposition Democratic Party, said about 30 commercial GPS receivers used in the South's military are vulnerable to North Korean jamming attacks.

"In particular, the Army's C4I (command and control, communication, computer, intelligence) structure, which relies on GPS receivers to get information on positioning, may be affected," Park added.

The military has previously said it would try to replace commercial GPS receivers with military models, but Park countered that military models are no less vulnerable to GPS jamming.

Earlier this month, the defense ministry said in a report that the North has been developing a new GPS jammer with a range of more than 100 kilometers, among other devices for electronic warfare.

The report also said the North imported about 20 communications and radar jamming devices from the old Soviet Union.

Chung Mee-kyung of the ruling Grand National Party (GNP) said the military has lamented a lack of funds to acquire military GPS receivers, and demanded to know how the defense ministry planned to secure funding and whether it was developing technology to counter North Korea's GPS jamming.

Defense Minister Kim Kwan-jin said "a few anti-jamming programs" were secretly under development.

"Taking a long-term view, I think it's right for us to build our own, independent GPS," Kim said. "But to do so, we need about six military satellites. The development for such satellites is still in an early stage, and it may cost a lot of money. It is difficult to develop independent GPS now but we have done our research on that option."

The ministry has said the North operates a regiment-sized electronic warfare unit near Pyongyang, and some battalion-sized units closer to the front line.

The ministry also acknowledged South Korea has not countered North Korea's past electronic attacks with an electronic offensive of its own.

Earlier this year, the Korea Communications Commission (KCC) complained that the North was jamming GPS signals in the South, but the South's attempt to lodge a formal complaint was rejected. The International Telecommunication Union (ITU) at the U.N. also urged Pyongyang to stop disrupting signals in South Korea.