Why can't the First Order scan low altitudes?

Question

In The Force Awakens there are two scenes in which

the Millennium Falcon flies at low altitude in order to remain hidden from the First Order's scanners.

Why can't the First Order scan low altitudes?

Is some reason given in the novelization or elsewhere? Are technical specs given anywhere as Marvel gives for their fictional technology?

Answer 1

The novelization implies they used ground-based radars, and that Han was hoping they weren't using the orbital satellites to scan the ground:

The Falcon went plowing through them as both pilot and copilot struggled to bring the ship up. A moment later it was clear of the ground and shooting skyward—which was an equally undesirable outcome.

“Any higher, they’ll see us!” Han shouted. Of course, if the vicinity of the First Order base was monitored by ground-scanning satellites, they were likely to be seen anyway. They could only hope that the instruments on board any such reconnaissance craft were aimed out toward space and not down at the landscape.

Answer 2

This is similar to the way that real-life radar works, so I'm going to assume the issues here are the same.

The problems with scanning for objects at very low altitude is that most such scanning devices work be sending out a signal, which bounces off distant objects and returns to an antenna/receiver. At low altitudes, there are a lot of things that get in the way of this signal working properly:

• Uneven terrain often means the signal bounces off mountains, etc. and cannot "see" past them
• Lots of natural objects (note that the Falcon is flying through trees at one point) confuse the signal and "hide" objects inside them
• The geometry and configuration of the broadcast and receiving antenna are likely optimized to look for objects coming in from far away -- in space, for example -- which may leave "blind spots" near the ground.

(Modern radar is getting much better at eliminating these blind spots, but the other principles of interference and terrain features still apply.)

Answer 3

Assume the First Order uses radar (say, a more advanced and precise version but still based on the same technology) to scan for intruding ships, which is currently used by airports' air traffic control. The principle behind radar is: radio waves or microwaves are transmitted by an antenna, reflect on everything they encounter and in particular moving objects like spaceships, and are then received by a second antenna. So this technology suffers from various limitations, such as:

• the range is limited by the possibility to have a line-of-sight between the antenna and the object;
• clutter, which designates echoes returned by ground, atmospheric turbulence and other natural objects (like trees).

So, flying at low altitude helps keeping the Millennium Falcon as it interposes a lot of obstacles to the scanners of the First Order.