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foxy04
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Hi Guys

 

please can you tell me if you need to be outside when marking your position for the first time?

 

i know this may sound a stupid question but everytime i try to mark my position on the gps in the house, it says no postion!!!!

 

any other help greatly appricated

 

foxy

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hi edel

you do need to see the sky to get a lock on the sats

if will take a little bit of time (15 secs) to get a lock then you should be ok

may also have some probs under heavy leaf cover or in a city with tall buildings

 

works ok underwater however

 

for more info have a look here <Manual>

or email me

 

J

Edited by *bingoboy*
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you do need to see the sky to get a lock on the sats

if will take a little bit of time (15 secs) to get a lock then you should be ok

may also have some probs under heavy leaf cover or in a city with tall buildings

 

works ok underwater however

Some GPSRs are waterproof, but none can receive a GPS signal underwater.

 

Radio waves of the frequency at which the NavStar satellites transmit the navigational message to our GPSrs cannot penetrate more than an inch or two (at most) through water.

 

The way military submarines use GPS to navigate involves taking a quick fix from a GPS antenna sent up to a few inches above water level. They do this in two ways. The first is by means of an antenna on an extending mast which is deployed from periscope depth. The other is to float a stealthy catamaran-shaped antenna platform which is towed along the surface by the submarine at depth. The float has several antennae and other sensors, including a GPS antenna. The bearing and distance from the float to the sub's datum point, called 'layback', is calculated from the speed through the water of the boat and the amount of wire paid out and the depth of the boat and the known velocity of the ocean currents.

 

The primary navigation system of a military submarine is an inertial reference system (INS) which is comprised of a system of gyros and accelerometers which continuously monitor the movement of the submarine in all six axes. The INS does not measure position directly, only changes in position. You therefore have to tell it where it is in space to start its navigation mission. Although remarkably accurate, it does have a certain amount of unmeasurable error with time and so needs to have its drift corrected with an updated actual position. Without correction, the accuracy of the computed position would drift at a rate of several tens of metres per hour which would accumulate huge errors over the duration of a multi-month dive which the modern nuke subs are capable of. Such position updates are nowadays usually made by taking a GPS fix.

 

The missiles too have their own INS systems and they too have to be told their exact position before launch, so the sub's pre-launch sequence involves fixing its own position by GPS and then calculating the missile tube's position. The resultant fix is passed to the missile via the datalink bus between the navigation room, the missile control centre and the missile's tube. In flight, the Trident missile's own INS is continuously updated by GPS if a signal is available. Else it updates its INS midflight by an electro-optical gizmo which measures its position and orientation by taking realtime astronav fixes. The cruise missiles also have an INS and they too can use GPS signals for midflight course corrections,but they also have an automatic map-reading system which continually compares its realtime radar scans of the terrain with a digital map of the ground which it has in its memory.

 

The accuracy of these systems is of the order of 3 to 7 metres, which is not bad for the 1970s technology which they use, but is not as good as the WAAS-assisted accuracy of better than 3 metres which we can achieve in geocaching!

 

Civil submarines, such as the Remotely Operated Vehicles which are extensively used in the offshore construction industry, are navigated by acoustic nav systems which make range and bearing measurements which are slaved to the surface position of the mother ship's datum point. That surface position is nowadays usually mostly derived from DGPS which is accurate to about half a metre when working well.

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<snipped: lots of top secret interesting facts about GPS and the military and stuff>

The man's an encylopaedia! Blimey, makes for an interesting read B) . Thanks for that :P

From recent posts, I'm starting to wonder what The Forester has done in his other former / current lives:

 

He knows about:

Military Field Medicine

Submarine and Missile Inertial Navigation Systems

Wildlife (and probably what you can eat in the jungle to survive)

Lots of other stuff which I don't have time to filter through the forums to remind me about

 

So I surmise that he must be some sort of Top Secret Super Agent. :P

 

So, The Forester, any chance of getting us all access to the GPS P-code? Is it much more accurate than the C/A-code?

 

Thanks also from me for many informative reads.

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The primary navigation system of a military submarine is an inertial reference system (INS) which is comprised of a system of gyros and accelerometers which continuously monitor the movement of the submarine in all six axes.......

I had an uncle who was a clockmaker and an avid collector of anything with 'cogs' in. I remember him showing me one box-of-tricks that he was particularly proud of. He told me that it had been fitted to a Jeep used by the LRDG in North Africa during the war. It was purely mechanical and somehow kept rack of exactly how far and in which direction the Jeep had travelled since the device had been zero'd. That way, the crew always knew how far away and in which direction their base was. Pretty dadgum clever, if you ask me.

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He told me that it had been fitted to a Jeep used by the LRDG in North Africa during the war. It was purely mechanical and somehow kept rack of exactly how far and in which direction the Jeep had travelled since the device had been zero'd. That way, the crew always knew how far away and in which direction their base was.

If you take a look at The Time Lord's avatar you can see an example of the type of sun compass that the LRDG (and subsequently the Special Air Service) used. The LRDG no longer exists, but each patrol of the SAS usually has one member who has been trained to a very advanced level in land navigation, including astronav.

 

A sun compass may seem pretty useless in our rotten climate, but the Vikings had a brilliant type of sun compass which uses a gnomon to enable the thing to display direction and time of day if you know your Latitude. Measuring Latitude is trivially simple. Longitude has always been the tricky bit in navigation -- at least until Harrison perfected his chronometer. He grasped the most important concept in navigation which is that position is invariably related to time. Everything in navigation is in some way related to time. Even the definition of the length of a metre is actually defined by reference to the length of time that is a second. The clever thing about a gnomonic sun compass is that it is able to tell the time of day as well as direction.

 

The contraption which you describe is an advanced form of log. The earliest form, which gives us the name of a record of events, was quite literally a log onto which was kept a rolling record of headings and duration of legs of a voyage. The other use of a log was a speed log, which consistsed quite simply of a log which was attached to a knotted rope and chucked overboard. The number of knots which passed through the navigator's fingers in a certain time interval was the ship's speed -- in knots.

 

Navigation has always been about time. In fact GPS itself consists of little more than a bunch of speaking clocks in orbit.

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any chance of getting us all access to the GPS P-code?  Is it much more accurate than the C/A-code?

No to both questions.

 

The military receivers used to be a lot more accurate than the civil ones back in the bad old days of SA, but since the advent of WAAS we now have better accuracy from our WAAS-enabled C/A code only GPSrs than the military do with their P-code receivers.

 

Back in the 1980s when some of us were actively lobbying the White House and the Pentagon to try to persuade the Reagan and Bush administrations to abandon the planned Selective Availability, we pointed out to them (and to the subsequent Clinton administration, more successfully) that SA was ultimately doomed because it could so easily be circumvented by Differential correction services. We correctly predicted that in the early 1990s a plethora of mom 'n pop outfits would burgeon with very cheap DGPS services which would be more accurate than the ±7m military receivers.

 

Obeying the Law of Unintended Consequences, the military shot themselves in the foot by restricting their Precise codes to themselves because it led directly to the civil sector building a better mousetrap.

 

I know of at least one unit of the British Army which actually issues civil GPSrs to its troopers instead of the military ones because a WAAS-assisted C/A-derived fix is so much more accurate than an unaugmented P-code one. We can get 1 to 2 metre accuracy with our civil receivers, so there's no advantage in getting hold of one of the P-code enabled receivers. Many of the mass market GPSrs meet and even exceed the milspec for waterproofness and vibration resistance etc and are as cheap as chips when compared to what the military pay for the military receivers.

 

The scheme to restrict civil use of NavStar was ill considered right from the start. They just didn't think it through.

 

The whole P-code/SA thing began to fall apart at the beginning of Operation Desert Shield. They has just switched SA on when they began to deploy hundreds of thousands of troops to the deserts of Saudi Arabia. Most US troops aren't trained particulatly well in fieldcraft skills such as navigation, so within days there were dozens of trucks and other vehicles getting lost in the desert . Some even strayed across the unmarked border into Iraq-occupied Kuwait and suffered all kinds of personal indignities at the hands of their captors. The Pentagon ordered its suppliers to ramp up production of GPSrs to the max, but with almost half a million troops being deployed to the region there was an insatiable demand for GPSRs.

 

They turned to the civil sector and bought up every GPSr they could get their hands on. Every yacht chandler and sporting goods store was visited and emptied of its GPS units, such was the desperation to get receivers into the cabs of every wheeled and tracked vehicle in "theater". They soon discovered that they were suffering from the fix quality degradation of SA, so the order was sent from the highest level of the Department of Defense to switch SA off.

 

After the war was over, which only took about four days, they switched SA back on again. Whoops! The Iraqis, mistakenly believing that the American attack would come from the seaward side, had heavily mined the waterways leading to Kuwait. All the allies who had any mine-hinting assets contributed to the huge effort to clear safe passsages through the minefields, including the French. France had only recently rejoined NATO and wasn't yet on Uncle Sam's list of "good guys" who were granted the privilege of accessing the P-codes. Oops!! This meant that when the French cleared a 100 metre wide strip using a nav system which had errors of up to 50 metres, they couldn't certify that patch of seabed as being mine-free. Not much use at all! The DoD decided to swith SA off again until the mine clearance program had been completed.

 

The whole P-code/SA thing had been shown to be the farce that it is and the rot had set in. It was only a matter of time until the top Generals conceded that SA should be abandoned.

 

One day SA suddenly and mysteriously went off. It was a Sunday, I seem to recall. It was only off for a day, but it set our tongues wagging in the chattering classes of the navigation community. It turned out that what had happened was that a homosexual A-10 pilot had had a tiff with his USAF squadron boyfriend and flew his jet symbolically into the side of a mountain which bore his name. It was an extremely remote part of the Rockies and it took the Air Force an embarassing long time to figure out where he had gone. The local mountain rescue team was a civilian one, with civilian GPSrs. They were having great difficulty in navigating through dense fog in the very steep mountain terrain and they asked the USAF if they could borrow a couple of military receivers. This request was nixed by the senior officer in charge of NavStar, which was based in the nearby Cheyenne Mountain, but instead he granted the alternative request that SA be removed while a position fix was obtained on the wreckage of the crashed jet.

 

That farce was pretty much the final straw. Bill Clinton signed an Executive Order which required the military to switch off SA indefinitely. One consequence of the removal of SA was the establishment of the game which we all know as Geocaching.

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Interesting that the british army issues civilian GPSrs. I understood that the US military could effectively 'switch off' the C/A code to any particular area if required. (Presumably to stop the opposition from using it during a conflict) - This would leave the british without GPS navigation.

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Interesting that the british army issues civilian GPSrs. I understood that the US military could effectively 'switch off' the C/A code to any particular area if required. (Presumably to stop the opposition from using it during a conflict) - This would leave the british without GPS navigation.

I'm not sure about the last Gulf War, but I do know that during the campaign to liberate Kuwait, the US ran out of military GPSr units and had to go and buy civilian units over the counter, then switch off selective availability so that the US forces could use GPS.

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Had to reply - the MOD/Military issue off the shelf 'civilian' GPSr to there personnel as it is the cheapest option and probably the best. During my time in the forces of HM Government, various Garmin products were issued, most notably GPS12/12XL and GPS V delux. Garmin also do a range of E-Trex just for the boys in Iraq and very good they are too.

I have also used dedicated military Gps that have an accuracy to 1Meter but I don't think that Rockwell will sell them on the civvy market.

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