From: Eric L. Strobel (fyzycyst_at_comcast.net)
Date: Mon Aug 11 2003 - 10:27:08 PDT
on 08/11/03 12:24 PM, Roman Pixell at roman_at_pixell.net wrote:
>> Actually, wasn't SA (selective availability) turned off a few years = ago
>> (meaning that you can now get the meter-type accuracy)?
>>
> its not that, eric.
>
> SA is about scrambling the L2 signal in order to decrease accuracy for
> the L2 frequency. the military L1 frequency gives you a precision from
> a whole different ballpark. not having access to L1 is not the same as
> SA.
Actually, the "military" signal is the L2 (higher) frequency. Both
frequencies carry the P-code (precision navigation) signal and L1 also
carries the C/A (coarse acquisition) signal. SA was when the US military
deliberately 'fiddled with' the satellite ephemeris and clock info being fed
by the constellation to the civilian (single frequency) receivers such that
the resulting accuracy was on the order of 100 meters. Without such
measures accuracies on the order of 10 meters are possible for single
frequency receivers, and even better with receivers that use one or the
other of the possible augmentation schemes.
The second frequency (in terms of accuracy, leaving out discussions of
anti-spoofing, etc.) provides the capability to compensate for much (but not
all) of the inaccuracy introduced by the signal passing through the
ionospheric plasma environment. Single frequency receivers must depend upon
a crude algorithmic ionospheric correction that only eliminates about 1/2
the ionospheric error. It is substantially (but not, IIRC, entirely) this
ionospheric correction that gets the multiple frequency military rigs the
meter or so accuracy.
For any purpose you're likely to need, 10 meters is quite likely to be good
enough, especially if you're only interested in *relative* positions between
nearby points.
- Eric.
-- Eric Strobel (fyzycyst_at_NOSPAM^mailaps.org) ===================================================================== There is never a single right solution. There are always multiple wrong ones, though. ===================================================================== -- This is the NewtonTalk list - http://www.newtontalk.net/ for all inquiries List FAQ/Etiquette/Terms: http://www.newtontalk.net/faq.html Official Newton FAQ: http://www.chuma.org/newton/faq/
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