WASHINGTON — Lawmakers on Tuesday voiced frustration that, after tens of billions of dollars spent on America’s sophisticated homeland defenses, low-tech objects of mysterious origins managed to enter North American airspace.

As the Biden administration gave senators a classified briefing Tuesday on four incursions by unidentified floating objects in recent days, lawmakers were left with many questions unanswered, and several have vowed to go over the North American Aerospace Command’s budget and capabilities with a fine-toothed comb.

“The latest incidents reveal that we have real gaps when it comes to policing our airspace, and we need to have better situational awareness and more of a protocol of how exactly to react,” Sen. Susan Collins of Maine, the top Republican on the Appropriations Committee and its defense spending subpanel, told Defense News on Monday.

“If there are gaps that are caused by a lack of equipment or sensors or better radars, the Department of Defense needs to provide us with that information. The fact that the military was able to act so quickly on these subsequent objects raises very serious questions about why they did not act quickly on the first one,” Collins added.

Since Biden ordered the shoot-down of the suspected Chinese spy balloon that transited the United States earlier this month, NORAD — the U.S.-Canadian organization responsible for warning and defending against missile threats to North America — has been looking more closely for potential airborne threats, including by tweaking radar detection parameters.

After three more unidentified devices were spotted and shot down, with the latest on Sunday over Lake Huron, U.S. officials attributed the spike in part to NORAD sharpening the ability of its radar “gates” to detect small objects at high-altitudes, moving at slow speeds.

“Radars essentially filter out information based on speed. So you can set various gates — we call them velocity gates — that allow us to filter out low-speed clutter,” Gen. Glen VanHerck, who leads NORAD, said in a call with reporters on Sunday.

U.S. airspace detection capabilities haven’t always been attuned to spot these sorts of objects. VanHerck and other officials say at least three Chinese surveillance balloons transited the United States during the Trump administration and weren’t immediately detected.

“The intel community after the fact assessed those threats from additional means of collecting and made us aware of these balloons that were previously approaching North America or transited North America,” VanHerck said, adding that NORAD has a “domain awareness gap that we have to figure out.”

Exiting Tuesday’s briefing, the top Republican on the Senate’s military readiness subcommittee, Sen. Dan Sullivan, stressed that the incident highlights the seriousness of the “domain awareness gap” that VanHerck has been warning lawmakers about for years.

“We have to have systems and capabilities that can pick up a slow-moving balloon, that can pick up a hypersonic missile that’s traveling eight, nine times the speed of sound, that can pick up low-flying cruise missiles,” said Sullivan, of Alaska. “These are all things that I think this episode is revealing are still challenges.”

NORAD has long operated the North Warning System, a line of short- and long-range radars across northern Canada and Alaska, and a network of alert fighter bases for response.

But a Center for Strategic and International Studies report last year faulted NORAD’s employment of Cold War-era technology and decision processes as outmoded. One of its authors, missile defense expert Tom Karako, said that while NORAD has been experimenting with machine learning and artificial intelligence, it needs to be using them more broadly to analyze the gobs of data collected by its radars to spot potential threats.

“You can’t defend against what you can’t see,” Karako told Defense News. “We have to get more and better sensors and get better at using the data coming off those sensors.”

That effort is underway. In June, the Canadian government announced it would invest $4.9 billion over the next six years to create a new Northern Approaches Surveillance system of two over-the-horizon Radars and a network of sensors with classified capabilities, distributed across northern Canada, called Crossbow.

Melissa Dalton, the assistant secretary of defense for homeland defense and hemispheric affairs, told lawmakers at a Feb. 9 hearing that the Defense Department “continues to be acutely aware of the need to enhance persistent surveillance of the airspace and maritime approaches to North America.

“To that end, we are working closely with our Canadian allies to modernize NORAD’s surveillance capabilities, paced to the current and future geopolitical environment,” Dalton said.

“In the near-term, we are taking steps with Canada to augment the existing NORAD warning system, including development of a new system of sensors called Crossbow that will enhance NORADs ability to detect approaching airborne threats.”

At that hearing, the Senate’s top defense appropriator for defense, Sen. Jon Tester, D-Mont., also said he supports more funding in the 2024 budget, due in March, to help the Pentagon stop incursions into U.S. airspace. The suspected Chinese spy balloon was first spotted over Montana, home to several nuclear weapons sites.

“When this budget comes forth [from the White House] I hope this is addressed in some line item and not only addressed with dollars but addressed with actions on how these moneys will be put to work to make sure that this never happens again,” said Tester.

On Tuesday, another member of the appropriations committee, Alaska Republican Sen. Lisa Murkowski, said the Pentagon budget request ought to provide funding to fix any vulnerabilities.

“When we get to the starting point, we’ll have a better understanding of how serious they are about shoring up existing gaps,” Murkowski’s office told Defense News in a statement after the classified briefing on Tuesday. “At this point, nobody knows more than administration officials about what is needed to fill the shortfalls the senior military leadership have identified.”

But Arkansas Sen. Tom Cotton, the top Republican on the Armed Services Committee’s air and land warfare panel, argued that shooting down unidentified objects like the Chinese balloon before they enter U.S. territory is more important than pouring more money into NORAD modernization.

“We should not spend billions – probably tens or even hundreds of billions – of dollars changing the way we protect our own airspace against small, slow-moving objects,” Cotton told reporters after Tuesday’s briefing. “We have the capability in certain settings to detect those.”

White House National Security Council spokesman John Kirby said that President Joe Biden ordered National Security Advisor Jake Sullivan to lead an interagency effort to explore the parameters and protocols in place for dealing with such objects, and to find potential improvements.

None of the three objects shot down over the weekend have been recovered. Kirby said military officials are still working on recovering debris from the recent shoot-downs to better understand what the objects were. He said the devices may not have posed any real national security threat, but officials won’t know for sure until after that work is finished.

“Having investigators look at that debris would certainly be of immense value, in terms of our ability to positively identify what these objects were and what their purpose was,” he told reporters.

However, Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, said separately that the recovery efforts for the object downed over the Arctic would be hampered by “very, very difficult terrain.” The same Pentagon officials who briefed Congress on Tuesday told senators last week that the complicated recovery efforts in the Bering Sea were a key factor in waiting to shoot down the Chinese surveillance balloon over the South Carolina coast after it had already traversed the country.

Amid all the mystery surrounding the incursions into U.S. airspace and NORAD capability gaps, lawmakers and Biden administration officials took pains to make one thing clear: it’s not aliens.

“There is no serious discussion that this is some sort of extraterrestrial contact,” Sen. Thom Tillis, R-N.C., said Tuesday after the classified briefing. “And you’d think if they did, they would use something more sophisticated than a balloon.”

Leo Shane III contributed to this report.

Joe Gould was the senior Pentagon reporter for Defense News, covering the intersection of national security policy, politics and the defense industry. He had previously served as Congress reporter.

Bryant Harris is the Congress reporter for Defense News. He has covered U.S. foreign policy, national security, international affairs and politics in Washington since 2014. He has also written for Foreign Policy, Al-Monitor, Al Jazeera English and IPS News.

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