Re: GPS Megadeath

From: Stuf4 (tdadamemd-spamblock-_at_excite.com)
Date: 06/19/04 

Date: 18 Jun 2004 19:57:51 -0700

>From Derek Lyons:
> tdadamemd-spamblock-@excite.com (Stuf4) wrote:
> >You could likewise argue that Air Force strategic bombers had no need
> >for GPS because they had INS supplemented with TACAN and RADAR.
>
> The behavior of the USAF across it's history supports that argument,
> they were a latecomer to GPS, not an early adopter. The bombers
> depended on radar and visual sightings once they entered the bombing
> run. One suspects their involvement was more attributeable to
> politics (increasing the accuracy of manned bombers increases their
> viability, and being involved in a program that was in space but not
> USAF controlled) as any desire for accuracy.

Perhaps I was not clear enough the first time. Derek, GPS was funded
because:

INS was neither accurate nor reliable.

INS may be accurate after being fixed to a known location, but INS
*does not measure position*. It does not even measure velocity. It
measures acceleration. And that means that the slightest error gets
integrated into a larger error in velocity, which gets integrated into
an EVEN LARGER error in position. Over short periods of time, INS
accelerometers were well known for running the position away with a
skewed platform.

Bomber missions take several hours and there are long stretches of
time between fixes where INS's often behave badly. It took diligence
of a highly skilled navigator to keep the INS "corralled", so to
speak, by regularly pumping good fixes into the system.

...and *even then* it was known to go inaccurate. Aside from errors
within the INS itself, even highly skilled navigators would make
errors in identifying radar targets to fix off of (there were lots of
other sources of error as well).

The optimal solution was to measure position directly, instead of
integrating all those errors.

The Air Force was in on the *ground floor* for the DNSS (renamed GPS),
so I don't know why anyone would say that they were latecomers. The
Navy had a 2-D system that worked at extremely low velocity. Jets
can't stop like subs can. And jets need 3-D position. It *was* the
Air Force who created such a system.

The most significant GPS contribution from the Navy was the atomic
clocks. The basics of the signal theory came from the Air Force. As
derived from MOSAIC, it was the technology that was designed for
positioning ICBM launches that gave us the system we use today. Air
Force technology.

 
> >Notice that even today the space shuttle is involved with a GPS
> >upgrade. For every flight to date it had multiple INS with multiple
> >TACAN. If this was deemed reliable and accurate enough then the GPS
> >conversion would be a complete waste of time and money.
> >
> >Now talk to the astronauts and see how badly the want GPS.
>
> They want it because it's available. There was certainly no such
> clamor during the design phase.

While the shuttle is capable of doing automatic navigation fixes from
TACAN stations, it still suffers the accuracy problems inherent in INS
systems that don't get precise position updates.

Part of astronaut pilot training is practicing landings with *bad
nav*.

This is why they want GPS. They don't like the idea of their nav
system taking them down to a spot that is so far off the runway that
they crash. The primary issue isn't convenience. It isn't technology
fashion trends.

It's a matter of living, versus not.

The reason why there was no "clamor during the design phase" was
because there was no GPS when shuttle was designed. It was just a
theory.

~ CT

Relevant Pages

  • Re: GPS System Could Begin to Fail Within a Year
    ... I'll also paste the US Air Force response mentioned at ... GPS at risk of failure in 2010 and beyond. ... launching replacement satellites, among other things. ... Considered by the GAO to be "essential to national security" the GPS ...
    (uk.tech.digital-tv)
  • Early Orbit Checkout Extended for IIR-20(M)
    ... The following is a press release from Los Angeles Air Force Base. ... from the GPS IIR-20spacecraft launched on March 24, ... bring the satellite into compliance with the GPS Performance ... problems are degrading the accuracy of signals from the latest GPS ...
    (sci.geo.satellite-nav)
  • OCS upgrade this month
    ... Air Force Anticipates Invisible GPS Ground Control Transition ...
    (sci.geo.satellite-nav)
  • Multiple Problems Delay Next GPS Satellite Launch -- Block IIR-M1
    ... Multiple Problems Delay Next GPS Satellite Launch ... But issues arose involving internal components, causing officials to put the brakes on launch plans. ... their subcontractors, to understand root cause of the issues and extensively review the 'reach-back' potential to," the Air Force Space and Missile Systems Center told Spaceflight Now. ...
    (sci.geo.satellite-nav)
  • Re: GPS Megadeath
    ... >You could likewise argue that Air Force strategic bombers had no need ... >for GPS because they had INS supplemented with TACAN and RADAR. ...
    (sci.space.shuttle)
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