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Posted: 11/19/2001 7:28:45 PM EST
[Last Edit: 11/19/2001 7:42:54 PM EST by 300ydClean]
An AN/PSN-11 has Selective Availability and Antispoofing capability. What does that mean? What is the difference between Universal Transverse Mercator and Irish Transverse Mercator? Is Transverse Mercator preferrable to MGRS or vice versa? Check it out:[url]http://www.hqmc.usmc.mil/factfile.nsf/7e931335d515626a8525628100676e0c/7c93296961c94c378525627a006b173a?OpenDocument­[/url] Thanks for any help Chuck
Link Posted: 11/19/2001 7:53:24 PM EST
Selective Availability refers to the built in error (random 15 to 100 meter error) transmitted due to the two tier design - military and civilian. Used to be (and could happen again anytime Uncle Mil sees fit) the P codes were only used by Mil receivers on other than the L1 freq, and civvie recevers could only get the CA codes. Thus the militarys were accurate to 1m best case, whereas civilian units were 100m best case horizontally, so that our "enemies" can't use the system against us. Not sure about antispoofing. UTM and other systems are merely systems of grid coordinates, like lat/long. UTM splits the earth into 60 zones 6 degrees wide. UTM is only good between North 84 and South 80 degrees, and is good for the states because most USGS maps use these coordinates. Other systems are used elsewhere. There is a ton of stuff on GPS, too much to list here. Do a web search and read for days.
Link Posted: 11/19/2001 8:00:09 PM EST
I figured that Irish Transverse Mercator required one to be tipsy on Rye, or it used the Blarney Stone as a zero reference, or some such nonsense. Thanks for the input Wreck. Are maps readily available in MGRS? Chuck
Link Posted: 11/19/2001 8:40:09 PM EST
Selective Availability is no longer. We all recieve the same signal now (as of last year). My GPS (Garmin GPS III Plus) can get down to a few feet Accy.
Link Posted: 11/19/2001 10:07:58 PM EST
selecive avalibility is gone for now or at least thats whats being said. My magellan GPS is extreamly accurate like down to a few feet and i use it but in a real bad situation (economic or whatever) I woludnt trust it because i assume the gov can again activate selective avalibility or turn it the sattilite off to civilian users altogether which isnt real convienent if you have all of your course data to your special place loaded into it and you actually cant get there on your own
Link Posted: 11/19/2001 11:28:16 PM EST
To answer another one of your questions, the MGRS uses the UTM projection as a reference. Of course, this does not work once you get to certain latitudes north and south, after which the MGRS uses something else. I have never heard of ITM. Incidentally, the particular dervey data used by the MGRS is WGS-84. Adam
Link Posted: 11/20/2001 6:47:19 AM EST
UTM and ITMG are both coordinate systems, like the standard 6 digit grid used for military navigation. In that case it is merely 6 digits in what is in reality a 15 digit grid system, which contains the grid zone designator (first 3 digits, 2 numbers and a letter), 100,000 square identifier (next 2 digits, 2 letters) and 10 digits (all numbers, 10 digits gives a 1 meter accuracy). The PLGR (AN/PSN-11) can display 7 different coordinate systems and convert coordinates input in one to another within limits. Both the ITMB and the OBL are based are only applicable in the British Isle and areas surveyed by them. It is based on the Airy Spheroid of 1830 are is being replaced with WGS-84 MGRS is based on the UTM/UPS coordinate system, the only difference being the UTM coordinates is that the 15 digits are all numbers. UTM is used within previous mentioned limits, the UPS system is used in polar regions Don’t get Datum and Coordinate systems confused though. The coordinate system defines locations within a datum. The datum defines the starting point for the measurements that are used in the surveying of the earth within a spheroid. In most cases you can switch between coordinate systems while using the same Datum. There is 2 types of MGRS, the new and old. The New normally is used with the WGS 84 datum and the Old with NADs 27 and other non-US datums. Spoofing is a form of electronic warfare, in which a false signal is transmitted in order to cause confusion or in the case of GPS receivers a decrease in accuracy or an inability to solve a position solution. In normal conditions, the U.S. Government guarantees that these errors do not exceed 100 meters horizontal, 140 meters vertical, and a time accuracy of 340 nanoseconds(ns) 95 percent of the time. A position derived from a CA code however can be much worse in accuracy than 1 KM depending on various conditions. With SA turned off a non-survey GPS receiver can obtain a 12.1 m average error just using the CA code. The P code or precision code is used by the military to increase accuracy. It is transmitted in a coded signal called the P(y) code, and is referred to as the Y code. The use of the Y code is one of the anti spoof capabilities of the PLGR. The exact methods used by US equipment cannot be discussed though. That info falls under the current definition of OPSEC.
Link Posted: 11/20/2001 7:02:25 AM EST
STLRN has it right. I have to disagree with RipMeyer and Oregon Shooter: selective availability or SA is a switchable random error applied to CA codes and HAS NOT GONE AWAY, it has only been switched off. Commercial GPS units only receive CA codes. The P codes are for Military units only and are OPSEC. The SA can be turned back on at any time Uncle Mil sees fit, corrupting the CA codes, and your hand-held purty little Garmin GPS is now innacurate to 100m horizontally again. Selective means just that - the error can be selectively applied whenever they need to screw up our civvie units. Say if there was a SHTF situation the govt could turn it back on and our civvie GPSs are screwed.
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