[swaghauler]

Originally Posted by rcaf_777

Besides GPS use batteries which will run out, so forget about the signal where are you going to find spare batteries? or recharge the ones you might have?

And finally if GPS is so important to the Airforce and Navy, why is Loran-C and Tactical air navigation system (TACAN) still in use?

I'm not trying to offend you here, but... are you joking (I'm sensing a bit of sarcasm in your reply)?

Loran-C: Loran uses very large radio towers (requiring huge amounts of power) to beam directional signals which can be picked up by receivers to tell bearing (direction to the transmitter). The transceiver is small (about the size of a car stereo) and not very expensive (I still have one on my boat), but the transmitters are HUGE (like 1,000 feet tall for the transoceanic ones). Furthermore, they have a limited range of about 500 Nautical Miles per station. This is why there are "chains" of identical stations (often containing a "master" and one or more "slave" transmitters) for long range navigation. The accuracy of Loran-C is about 100 meters per kilometer over most ranges (it is better if you are traveling shorter ranges because the radio waves aren't as distorted). This means that a ship traveling 5,000km will be off on its navigation by about 50km if they used Loran alone (assuming perfect weather which also degrades the Loran's signal). The average accuracy combining "Dead Reckoning" WITH multiple Loran-C fixes during the voyage is about 1km per 1000km. That ship traveling 5000km will be off 5km at the end of their voyage. While 5km may not seem like much, it assumes very good Dead Reckoning navigation in addition to the Loran-C fixes.

Additionally, if the Soviets are attacking the GPS satellites, there is NO WAY they are going to allow the Loran network to keep transmitting. All it would take to shut down the Loran-C system (besides the loss of the power grid) are a few HARM missiles.

VOR/DME NAVIGATION (VHF Omni-Directional Ranging/Distance Measuring Equipment: This civilian radio ranging and bearing system uses two separate systems and is commonly used by Commercial Aviation for determining bearings (VOR) and range (DME) to an airport. This is a VHF radio beacon system that is easily jammed and only useful at a limited range (about 300km). It provides 2-DIMENSIONAL ranging (ILS is 3-dimensional) to an airport and can produce SIGNIFICANT errors in Slant Ranging of aircraft (slant range is both the distance AND altitude to the airport). Ranging errors can be from 400m to 900m with altitude variations being the more common error. This is why VOR/DME is only authorized for NON-PRECISION Approaches to an airport. It is too inaccurate for a Precision Approach. Attempts to correct this resulted in the TDME (or Terminal DME) where a 0 on range means you are at the head of the runway. The downside to TDME is that it is STILL a 2-D system, you may be at the end of the runway in distance, but there is no "calculation" for altitude (like with ILS).

The system uses both a transmitter (at the airport) and a TRANSCEIVER (in the aircraft). The aircraft's transceiver "pings" the transmitter and the transmitter replies. This means that BOTH the aircraft and the airport are emitting signals that can be easily tracked and triangulated (especially since they are VHF). Once again, a HARM missile would end this system's "life."

TACAN (Tactical Air Navigation Unit): This more accurate "cousin" to VOR/DME uses a similar system but in the MHz range. The system in the 90's was fairly ponderous (800lbs) and used a significant amount power (10kw) given its maximum range of 390 Nautical Miles. Modern systems are much smaller and more economical (100lbs and 400 watts) but both still have the drawback of emitting large amounts of EM energy which can be detected and triangulated. A carrier turning on its TACAN is essentially a "sitting duck" until the aircraft are retrieved. Modern (21st Century) TACAN have an interrogation mode that only turns the system on if "interrogated" by a "friendly transceiver" (using IFF systems) and then stays on just long enough to provide range and bearing info before shutting off again. The modern transceiver has 3 settings: Recieve (no outgoing "ping") gives the range to the TACAN only. Transmit/Recieve ("pinging" the TACAN) gives both range AND bearing to the TACAN. Finally, 21st Century TACAN Transceivers have an Air-To-Air setting which gives Slant Range and Bearing to another transceiver equipped aircraft. The accuracy of TACAN is about 20m per Kilometer which is VERY GOOD. Even though this is good, TACAN is being replaced by JPALS (a binary GPS system with an accuracy of 1 meter).

The Global Positioning System (GPS): The GPS system uses space-based satellites combined with PASSIVE RECEIVERS to perform triangulation to determine your EXACT position on the Earth. The US government, worried about misuse, degraded the civilian receivers so that you could only get an accuracy of 20 meters from your requested location (President Clinton ended this in 2000). Military personnel were getting 3-meter accuracy readings during the Twilight War. Additionally, since the receivers were passive (emitting no signal themselves), the enemy could not detect the user. The receivers are also small, use minimal power, and cheap (unfortunately, the satellites aren't). The accuracy of GPS was an "order of magnitude" better than radio-based navigation.

Things are not all "sunshine and puppies" though. The GPS receivers can be jammed locally and the satellites are subject to EM interference in space. Local weather or overhead cover can interfere with the signal as well, but strategically, GPS is "unjammable" across the globe. The accuracy of GPS was so good, that civilian mariners began to ignore other types of navigation.
All it took was one instant of forgetting about the 20-meter "bias" the government installed, and you had yourself what the Coast Guard called "A GPS-assisted grounding."

Navigation in the Game:

In response to your first statements about general direction finding, I would say "yes and no." People get lost following road maps and street signs all the time. The real test comes when you have to travel to a PRECISE LOCATION which you have NEVER been to before. I like to throw in "Navigational Tests" during gameplay (usually with a time constraint as well) to change things up. The ultimate test; Navigating the Desert or Jungle in the dark (no "terrain association" here boys). So GPS is VERY important to my players. It means the difference between an Easy:Navigation or a Formidable:Navigation test.