🚀 go-pugleaf

In article <1992Nov3.084856.25305@netcom.com>, hage@netcom.com (Carl Hage) writes:
> henry@zoo.toronto.edu (Henry Spencer) writes:
> : The noise being injected by DoD (the buzzphrase is selective availability)
> : is currently of a relatively simple type that can indeed be removed by
> : long-term averaging.
> 
> I have a few of questions. Suppose we end up with a president who places
> a higher priority on commerce rather than restricting the military buildup
> of countries like Iraq, and accurate navigation information is declassified.
> 
>   1. Can the "noise" be turned off, or do we need new satellites?
>      (With DoD involved, this isn't a stupid question.)

Selective Availability *can* be turned off. I guess you'll now have to go
ask Bill Clinton to do it.

>   2. I thought selective availability mean't that noise was injected only
>      during a military operation, e.g. the Gulf War. However, it wasn't
>      enabled then due to a shortage of military units and many commercial
>      receivers were used. Why do I read that commercial units still don't
>      have the accuracy that military units have, i.e. how is full precision
>      information transmitted?

Commercial units use the C/A code & military units use P or Y code. The P/Y
code rate is 10 times the C/A code rate, so, to a first approximation, the
military receivers can give an order of magnitude better positional accuracy
than C/A receivers without SA. When SA is on, there is part of the data
message encrypted & this part gives information about what distortions have
been applied to the signal.

>   3. Would special or extra hardware be required to receive full precision
>      information over existing receivers?

Yes, you need a military reciver, & they are classified I believe.

>   4. Is differential GPS used just to overcome the selective availability
>      noise, or are there other sources of error?

No, it can give additional precision, if the location of the DGPS station
is known accurately enough. You are now using the system to generate
the vector from you to the DGPS station, and a lot of the common-mode
errors involved in computing the absolute position of you and of the
DGPS station cancel out.

>   5. What is the precision of commercial vs military GPS with/without
>      SA enabled?

C/A receiver manufacturers seem to quote about 25m CEP with S/A switched off.
With it on the DoD quotes not more than 100m 2d RMS error for most of the
time. The altitude accuracy is generally rather worse than the long/lat
accuracy because of the geometry of the system (you can't see satellites
below you because the earth gets in the way!).
As I indicated above, military receivers are probably getting on
for 10 times better than that, though I expect the exact performance available
is classified information.

John C Sager					Mail:	B67 G18, BT Labs
Email:	jcs@zoo.bt.co.uk				Martlesham Heath
Tel:	+44 473 642623					IPSWICH  IP5 7RE
Fax:	+44 473 637614					England