[LEAPSECS] GPS certified for navigation?

[Jonathan E. Hardis]

On Sep 23, 2010, at 9:55 PM, Steve Allen wrote:


> On Thu 2010-09-23T20:45:45 -0400, Jonathan E. Hardis hath writ:

>> As the link makes clear, WAAS is funded by the FAA "for aircraft,"  

>> not

>> "for surveying."

>

> As the folks who are trying to abandon leap seconds seem to think,

> just because something was designed for one thing doesn't mean it

> can't be used for something else.


Now, now...


> There are now handheld GPS units which have WAAS but are not certified

> for flight.  Instead they have the ability to display coordinates not

> in Lat/Long, but in State Plane Coordinates.  These are being used

> for surveying, and not for precision geodesy of the sort needed for

> crustal deformation caused by tectonics.


You're blurring two separate issues here.

A GPS receiver can display in different coordinate systems, regardless  
of whether or not it is WAAS-enabled.  For example, the Military Grid  
Reference System (MGRS) and the U.S. National Grid (USNG) are  
alternatives to Lat/Long that some GPS receivers use.

As for WAAS, it is broadcast (from geosynchronous satellites) on the  
same frequency ("L1") that GPS uses.  So, from a receiver standpoint,  
the signal is picked up by the antenna and front-end that's there  
anyway.  That's where the cost would be -- decoding the WAAS signal on  
the same silicon ASIC that's decoding the GPS signals is very  
inexpensive to do.  So, WAAS capability is quite common in consumer  
(non-aviation) gear -- it's in the chip sets.

There are some similarities between WAAS and the Real-Time Networks  
(RTNs) that surveyors use.  The main difference is that WAAS alone  
does not give the accuracy that surveyors need.  In both cases, there  
is an ensemble of monitoring stations at known, fixed locations from  
which correction functions for GPS-computed positions can be devised.   
For WAAS, the monitoring stations cover most of North America, as do  
the correction functions.  For RTNs, the area covered is typically  
much smaller, so the correction functions have greater resolution in  
the location of interest.  For WAAS, the correction functions are  
transmitted to the receiver on the L1 frequency.  For RTNs, some other  
data channel must be used, e.g., cellular packet data, but this  
provides the advantage of greater bandwidth.

     - Jonathan