The Sensor Phalanx, only buildable on a moon, is one of the most useful buildings in the game. Using the Phalanx you can scan a planet for 5,000 deuterium. Fleets in flight towards the planet will be shown, together with their composition of ships and time of arrival (note that you don't see resources that fleets are carrying). This allows a player to send their fleet on an attack that is timed to arrive a few seconds after the target fleet gets back, leaving not enough time for it to be saved again. Note that moons cannot be scanned with a Sensor Phalanx due to their lack of an atmosphere.
Sensor Phalanxes are also useful when attacking another player. When attacking another player, the person being attacked can see the attacking fleet coming in. Often, the person being attacked will "ninja" the attacker: move a sizable fleet to the target so that it arrives seconds before the attacking fleet, destroying it, and turning the attacker's fleet into debris. A Sensor Phalanx can be used by the attacker to see if the target is moving a fleet in an attempt to "ninja" the attacking fleet, and the attacker can then decide to call off the attack if need be.
Before moving your fleet to a new area, it is important to be aware of nearby moons. You can find out the ranges of other phalanxes by probing their moons. It should be noted that when another player sensor phalanxes you, there is no in-game message or any message at all notifying you. The only way you could tell that you have been sensor phalanxed is if you see an attacking fleet coming at you within seconds of your own fleet's return. Sensor phalanxes give information on the fleet's origin, size, where it is heading, how long it will take to get there, and when it will get back. If you only see a fleet that has already completed its mission, you won't be able to see the composition of the fleet. In this sense, it is possible to avoid sensor phalanxes by stationing your fleet on a moon, and then fleetsaving from the moon (e.g., to a debris field). As stated above, moons cannot be scanned; therefore, if a player has at least two moons, a fleetsave from one moon to another moon is the safest possible fleetsave and cannot be detected in any way.
Level Table:
Level - Cost - Range
Metal Crystal Deuterium
1- 20,000- 40,000- 20,000 - 1
2- 40,000- 80,000- 40,000 - 3
3- 80,000- 160,000- 80,000 - 8
4- 160,000- 320,000- 160,000 - 15
5- 320,000- 640,000- 320,000 - 24
6- 640,000- 1,280,000- 640,000 - 35
7- 1,280,000- 2,560,000- 1,280,000 - 48
8- 2,560,000- 5,120,000- 2,560,000 - 63
9- 5,120,000- 10,240,000- 5,120,000 - 80
10- 10,240,000- 20,480,000- 10,240,000 - 99
What a Sensor Phalanx Can See:
If you scan the target planet of the fleet
Attacking fleets (ACS too)
Trading fleets
Fleets on deployment mission
Spying fleets
Fleets with the mission hold (ACS)
If you scan the home planet of the fleet
Fleets coming home from any mission (except Recalled deployment)
Fleets on a hold or expedition mission
What a Sensor Phalanx Cannot See:
If you scan the target planet:
Recalled deployment fleets
Fleets heading to the planet's DF
Fleets heading to the planet's moon
Harvesting fleets launched from or returning to a moon
In short, you will see when a fleet is arriving at the target planet (other than a Recalled deployment), whether that fleet is deployed to the target, attacking the target, or some mission from the target that will return the fleet to the target. The key is that a fleet will be making a stop at the target planet, whether it is the target of a mission or the origin of a round trip mission.
(Note : With level 1 sensor phalanx, only the solar system containing the moon with the sensor on it can be scanned.)
Practical Upward Limits:
You can scan every planet in an entire galaxy with a sensor Phalanx of level 16. However, your moon will need to be in the range of X:244:X - X:255:X. A Sensor Phalanx of level 23 will be able to scan the entire galaxy no matter where your moon is located.
Unfortunately, a level 16 Sensor Phalanx would need 655 million metal and deuterium, 1.31 billion crystal, and would take about 5 years and 7 months to build with a level 15 Robotic Factory. Building a level 23 Sensor Phalanx would need 83.8 billion metal and deuterium, 167.7 billion crystal and would take slightly more than 547 years to build with a level 20 Robot Factory.
In addition to this, one should keep in mind the cumulative costs and times of building a level 23 Sensor Phalanx with a level 20 Robot Factory. They are as follows:
Metal: 167.772 billion
Deuterium: 167.772 billion
Crystal: 335.544 billion
Time: 1094 years, 4 months and 27 days
The practical upward limit to the Sensor Phalanx is level 8. It takes 2.56 million metal and deuterium, 5.12 million crystal and can be built by a level 10 Robot Factory in just under 2 weeks (11 days).
A level 8 Sensor Phalanx would allow you to scan 63 systems to either side as well as the home system, giving you 127 systems to scan. A Level 48 Sensor Phalanx if positioned at 4:230:x would allow you to phalanx anyone in the universe, although the cost for level 48 alone would be 2,814,749,767,106,560,000 Metal/ 5,629,499,534,213,120,000 Crystal/ 2,814,749,767,106,560,000 Deuterium and with level 20 Robotics factory it would take you 18,361,055,232 years to build it (which is more time than our universe has been in existance (which is about 14,000,000,000 years)). You'd need level 5,000,000,000 robotics factory to make it within a person's lifetime or 77 years to complete and the robotics factory would cost a number too large
Subject: Sensor Phalanx 
