GPS - Selective Availability to be Turned Off?

GPS - Selective Availability to be Turned Off?

Post by Ian Stirlin » Tue, 25 Feb 1997 04:00:00



: >This question may be answered best by Scott4ASG (I believe he wrote
: >the article), but I'll throw it out to everyone and see what happens.

: >I got my April '97 copy of American Survival Guide in the mail
<snip>
: >future President Clinton has directed the Defense Department to delete
: >the Selective Availability (SA) feature of the GPS system..."

: Like in the next 5 years.  When it was announced, everyone was jumping
: up and down with praise, but the actual EO just directs the DoD to find
: a way within the next 5 years to turn off SA unless it's needed.

: DGPS will still be needed for real accuracy (like instrument approaches
: to airports).

IIRC, technology has advanced substantially since GPS was introduced.
It's now possible to track carrier edges on the incoming signal, to get
accuracy to around 250ps (assuming SA is off)
now, 250ps = around 10cm or so.
Accuracy well within a meter should be easily possible.

Another thought just occured to me.
Take several hundred GPS recievers round the world. (well, however many)
All fixed in position barring continental drift/... Can you use this
to derive the real positions of the satellites?
Assuming all the recievers are on the net.
You'd also need a real-time link to this database, pager, mobile/satelite
phone?

: >Cheers,
: >Mark Stoddard (no relation)

--
Ian Stirling.                     Currently designing a new PDA, see homepage.
Homepage:                         http://www.mauve.demon.co.uk/
Money is a powerfull aphrodisiac, but flowers work almost as well.
Robert A Heinlein

 
 
 

GPS - Selective Availability to be Turned Off?

Post by RayP4795 » Tue, 25 Feb 1997 04:00:00


Quote:>: >I got my April '97 copy of American Survival Guide in the mail

<snip>
: >future President Clinton has directed the Defense Department to delete
: >the Selective Availability (SA) feature of the GPS system..."

: Like in the next 5 years.  When it was announced, everyone was jumping
: up and down with praise, but the actual EO just directs the DoD to find
: a way within the next 5 years to turn off SA unless it's needed.

: DGPS will still be needed for real accuracy (like instrument approaches
: to airports).<

The really stupid part about all of this is that a differental GPS system
gets rid of the SA anyway. Yet another example of the Government spending
your money for no reason.

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Jon Van Alle » Tue, 25 Feb 1997 04:00:00

[BIG snip]

Quote:>I got my April '97 copy of American Survival Guide in the mail
<snip>
>future President Clinton has directed the Defense Department to delete
>the Selective Availability (SA) feature of the GPS system..."

> Like in the next 5 years.  When it was announced, everyone was jumping
> up and down with praise, but the actual EO just directs the DoD to find
> a way within the next 5 years to turn off SA unless it's needed.

> DGPS will still be needed for real accuracy (like instrument approaches
> to airports).

>IIRC, technology has advanced substantially since GPS was introduced.
>It's now possible to track carrier edges on the incoming signal, to get
>accuracy to around 250ps (assuming SA is off)
>now, 250ps = around 10cm or so.
>Accuracy well within a meter should be easily possible.

SA can be turned on and off at will by the Defense Department. It was
enabled during the Gulf War, for instance and is enabled on occasion. The
US Military and select NATO allies use the enhanced mode, SA is for the
rest of us peons if and when the need arises.

Differential GPS uses LORAN type signals to enhance a GPS receiver's
accuracy to within one meter, or so it's claimed. This system is in place
along part (or all?) of the US West Coast, don't know about anywhere else
yet, but it's being planned and possible installed and operational by now.

FYI, the USAF shut down the SatNav system as of midnight 31 December,
1996. The sats are now just space junk.

Jon Van Allen

Jon Van Allen, KF7YN

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Mark Thom » Wed, 26 Feb 1997 04:00:00


Quote:>SA can be turned on and off at will by the Defense Department. It was
>enabled during the Gulf War, for instance and is enabled on occasion. The
>US Military and select NATO allies use the enhanced mode, SA is for the
>rest of us peons if and when the need arises.

Actually, you have it backwards.  SA is normally enabled, dithering the
signals giving reduced accuracy.  When it is DISABLED (or turned off),
the improved accuracy results.

Quote:

>Differential GPS uses LORAN type signals to enhance a GPS receiver's
>accuracy to within one meter, or so it's claimed. This system is in place
>along part (or all?) of the US West Coast, don't know about anywhere else
>yet, but it's being planned and possible installed and operational by now.

This is absolutely incorrect.  DGPS is based on a GPS receiver located at
a precisely surveyed location.  It broadcasts information on the satellites
it is receiving, the amount of difference between its actual location and
the location derived from those satellites.  Properly equipped GPS receivers
in the area can compare the satellites they are using for their fix, and
when they match the DGPS receiver, they can use the adjustments broadcast
by the DGPS receiver to adjust their own calculated locations.  

That is why DGPS is VERY regional; you must be close enough to be using
the same satellites.

  Mark

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Jon Van Alle » Wed, 26 Feb 1997 04:00:00



>>SA can be turned on and off at will by the Defense Department. It was
>>enabled during the Gulf War, for instance and is enabled on occasion. The
>>US Military and select NATO allies use the enhanced mode, SA is for the
>>rest of us peons if and when the need arises.

>Actually, you have it backwards.  SA is normally enabled, dithering the
>signals giving reduced accuracy.  When it is DISABLED (or turned off),
>the improved accuracy results.

Yep, I had it backwards, but you get the drift. So noted.

Quote:>>Differential GPS uses LORAN type signals to enhance a GPS receiver's
>>accuracy to within one meter, or so it's claimed. This system is in >>place along part (or all?) of the US West Coast, don't know =

about >>anywhere else yet, but it's being planned and possible installed and >>operational by now.

Quote:

>This is absolutely incorrect.  DGPS is based on a GPS receiver located at
>a precisely surveyed location.  It broadcasts information on the satellites
>it is receiving, the amount of difference between its actual location and
>the location derived from those satellites.  Properly equipped GPS receivers
>in the area can compare the satellites they are using for their fix, and
>when they match the DGPS receiver, they can use the adjustments broadcast
>by the DGPS receiver to adjust their own calculated locations.  

>That is why DGPS is VERY regional; you must be close enough to be using
>the same satellites.

>  Mark

What I said is NOT incorrect! I did not elaborate on how it works, only
that the GPS works in conjunction with a radio signal to pinpoint it's
location more accuately. I use this system, I know how it works.

Jon

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Gregory K Doerschl » Wed, 26 Feb 1997 04:00:00



>This is absolutely incorrect.  DGPS is based on a GPS receiver located at
>a precisely surveyed location.  It broadcasts information on the satellites

                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Quote:>it is receiving, the amount of difference between its actual location and
^^^^^^^^^^^^^^^^
>the location derived from those satellites.  Properly equipped GPS receivers
>in the area can compare the satellites they are using for their fix, and
>when they match the DGPS receiver, they can use the adjustments broadcast

                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Quote:>by the DGPS receiver to adjust their own calculated locations.  

^^^^^^^^^^^^^^^^^^^^^

Since when is a receiver capable of broadcasting?  That's what transmitters
are for.

                                                       Greg

 
 
 

GPS - Selective Availability to be Turned Off?

Post by RayP4795 » Wed, 26 Feb 1997 04:00:00

Quote:>Differential GPS uses LORAN type signals to enhance a GPS receiver's

accuracy to within one meter, or so it's claimed. This system is in place
along part (or all?) of the US West Coast, don't know about anywhere else
yet, but it's being planned and possible installed and operational by
now.<

I seem to recall that there is at least one off shore Coast Guard station
off the east coast also.

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Richard A. De Cast » Wed, 26 Feb 1997 04:00:00




>>This is absolutely incorrect.  DGPS is based on a GPS receiver located at
>>a precisely surveyed location.  It broadcasts information on the satellites
>                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>it is receiving, the amount of difference between its actual location and
>^^^^^^^^^^^^^^^^
>>the location derived from those satellites.  Properly equipped GPS receivers
>>in the area can compare the satellites they are using for their fix, and
>>when they match the DGPS receiver, they can use the adjustments broadcast
>                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>by the DGPS receiver to adjust their own calculated locations.  
>^^^^^^^^^^^^^^^^^^^^^
>Since when is a receiver capable of broadcasting?  That's what transmitters
>are for.

The DGPS site is a transmitter that overlays the corrected signal onto the
GPS receiver.  

Quote:>                                                       Greg

--
==============================================================================

Rick N6RCX NREMT SAR Tech  Do Not try this yourself - it could get ugly......
Richard A. De Castro -     To those who have defended it, Freedom has a flavor

                    = = DON'T TREAD ON ME = =
=============================================================================
 
 
 

GPS - Selective Availability to be Turned Off?

Post by David Sieb » Wed, 26 Feb 1997 04:00:00


 on 25 Feb 1997 12:54:53 GMT,


>>Differential GPS uses LORAN type signals to enhance a GPS receiver's
>accuracy to within one meter, or so it's claimed. This system is in place
>along part (or all?) of the US West Coast, don't know about anywhere else
>yet, but it's being planned and possible installed and operational by
>now.<

>I seem to recall that there is at least one off shore Coast Guard station
>off the east coast also.

As can be seen by the previous posts, the start of this thread had some
 "inaccuracies".  The topic of Selective Availability  (and DGPS) is well
  covered on the newsgroup: sci.geo.satellite-nav.

Besides US coast guard stations, commercial service providers transmit
differential corrections within the US. Providers (private/governmental)
are also available within other countries.

CHECK OUT sci.geo.satellite-nav!!!!!!!!!

    David  Sieber
    Lockheed Martin  -- Portsmouth Hampshire UK

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Mark Thom » Thu, 27 Feb 1997 04:00:00


Quote:>>>Differential GPS uses LORAN type signals to enhance a GPS receiver's
>>>accuracy to within one meter, or so it's claimed. This system is in >>place along part (or all?) of the US West Coast, don't know =
>about >>anywhere else yet, but it's being planned and possible installed and >>operational by now.

>>This is absolutely incorrect.  DGPS is based on a GPS receiver located at
>>a precisely surveyed location.  It broadcasts information on the satellites
>>it is receiving, the amount of difference between its actual location and
>>the location derived from those satellites.  Properly equipped GPS receivers
>>in the area can compare the satellites they are using for their fix, and
>>when they match the DGPS receiver, they can use the adjustments broadcast
>>by the DGPS receiver to adjust their own calculated locations.  

>>That is why DGPS is VERY regional; you must be close enough to be using
>>the same satellites.

>>  Mark

>What I said is NOT incorrect! I did not elaborate on how it works, only
>that the GPS works in conjunction with a radio signal to pinpoint it's
>location more accuately. I use this system, I know how it works.

Well, "LORAN-type" signals ain't even close, and it takes SPECIAL GPS
receivers to accept DGPS.  I don't want any neophytes out there to be
confused about what they're buying because of your explanation.

  Mark

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Mark Thom » Thu, 27 Feb 1997 04:00:00




>>This is absolutely incorrect.  DGPS is based on a GPS receiver located at
>>a precisely surveyed location.  It broadcasts information on the satellites
>                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>it is receiving, the amount of difference between its actual location and
>^^^^^^^^^^^^^^^^
>>the location derived from those satellites.  Properly equipped GPS receivers
>>in the area can compare the satellites they are using for their fix, and
>>when they match the DGPS receiver, they can use the adjustments broadcast
>                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>by the DGPS receiver to adjust their own calculated locations.  
>^^^^^^^^^^^^^^^^^^^^^

>Since when is a receiver capable of broadcasting?  That's what transmitters
>are for.

The DGPS units that are precisely located also contain a transmitter
to transmit correction values according to the RTCM SC-104 standard.
How would you prefer I refer to a unit that contains a receiver for
one purpose, and a transmitter for another?

  Mark

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Gregory K Doerschl » Thu, 27 Feb 1997 04:00:00



>The DGPS units that are precisely located also contain a transmitter
>to transmit correction values according to the RTCM SC-104 standard.
>How would you prefer I refer to a unit that contains a receiver for
>one purpose, and a transmitter for another?

A facility which transmits GPS error-correcting data, obtained by comparing
the readings of a precisely located GPS receiver with its known position,
to suitably equipped DGPS receivers.
 
 
 

GPS - Selective Availability to be Turned Off?

Post by Jon Van Alle » Thu, 27 Feb 1997 04:00:00


Quote:

>Well, "LORAN-type" signals ain't even close, and it takes SPECIAL GPS
>receivers to accept DGPS.  I don't want any neophytes out there to be
>confused about what they're buying because of your explanation.

>  Mark

My post wasn't about a techical discussion, and I think you missed the
point of the general gist of the discussion. I already told you I use
DGPS, so why bother to say "it takes SPECIAL GPS receivers to accept
DGPS". Duh...what a concept....

Besides, the article I read about DGPS in one of the hobby rags in fact
likened the differential signals to LORAN, their words, not mine.

Relax, I doubt any neophytes will buy because of either of our
explanations.

Jon

Besides,

 
 
 

GPS - Selective Availability to be Turned Off?

Post by Peter Gottlie » Fri, 28 Feb 1997 04:00:00



Quote:> Well, "LORAN-type" signals ain't even close, and it takes SPECIAL GPS
> receivers to accept DGPS.  

Do you know what frequency the DGPS correction broadcasts use?

It wouldn't happen to be between the AM BCB and 160 meters, would it?

 
 
 

GPS - Selective Availability to be Turned Off?

Post by JT McBri » Fri, 28 Feb 1997 04:00:00

SA was turned OFF during the Gulf War because the US military didn't have
enough Mil GPS receivers (which can decode the p signal and get SA accuracy
even when SA is enabled). Saddam either hadn't bought enough civilian GPS
units to matter, or the US military wasn't worried about his troops trying
to get up and move [the coalition could nail them then].

The gov't bought enough civilian GPS units during the war to kick-start
mass production. We're all reaping the benefits now.

You guys are BOTH right. Differential GPS does involve a local (usable
within about 100 miles) carefully surveyed site, with a GPS receiver
that calculates the difference between each satellite's signal and the
established true position (and time). And it does involve (at least at
present) a LORAN-band [actually a little higher freq. I think] radio
beacon that transmits that difference signal to boats, planes, and
surveyors in the area. The Coast Guard is operating sites along most
of the nation's coastline. If you want to know where coverage is available,
there's a Web page at: http://www.navcen.uscg.mil/dgps/dgpssts.html
to tell you.

Survey crews can also set up a temporary DGPS base station. They subscribe
to a service to get a signal allocation, survey the site and set up the
equipment, then they can gather field points in minutes, instead of hours.

Jim
--
Happiness is (according to Aristotle): "The exercise of vital powers
along lines of excellence in a life affording them scope."
--

"There is already a ban on ALL CRIME. It is not working." - Thomas Sowell