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    Re: GPS Maybe Not as Healthy as Thought
    From: Frank Reed
    Date: 2009 May 22, 11:36 -0700

    I've read the GAO report all the way through and also the Air Force replies on 
    the matter. I think this issue has been over-blown by the media. They're 
    quoting the first page summary of the report, and like most GAO reports, it's 
    intentionally dire.
    For marine navigation, the potential reduction in active GPS satellites is not 
    likely to be noticeable. Marine users have the best conditions for GPS signal 
    acquisition. From a boat at sea, a GPS receiver can generally "see" the whole 
    sky and reduced numbers of satellites will rarely matter (unless there is a 
    major program delay, see below, which would not have a practical impact until 
    about 2014).
    The one market that may be adversely affected, sooner rather than later, is 
    "urban canyon" navigation since those users benefit most from the excess 
    satellite capacity which currently exists (far from being in trouble, the US 
    GPS constellation currently has EXCESS capacity). These are the car 
    navigation and mobile phone location services, for example, which are 
    becoming dramatically more popular with each passing year. Since these 
    services are already impaired in environments like this where GPS signals 
    propagate poorly, if at all, this report will probably encourage more rapid 
    adoption of alternative location services like SkyHook (that's the one based 
    on triangulation of home and business Wi-Fi signals). The challenge is to 
    make the hand-off between GPS and other services seamless and invisible to 
    the end-user.
    There are two issues raised in later sections of the GAO report which are 
    serious and more relevant than the supposed increased risk in 2010. The first 
    of these is the potential decline in service that could result if there is a 
    serious delay in the launches of the next series of satellites, the GPS-III 
    satellites. If they're delayed by two years, then there could be a serious 
    disruption in the US GPS signals starting around 2014 and lasting for as long 
    as seven years. Have a look at figures 4 and 5 in the GAO report. Although 
    GAO reports are intentionally pessimistic, I believe that the low probability 
    of having a full GPS constellation around 2017 is realistic. That is, it's 
    realistic IF there is a significant delay in the GPS-III program. But note 
    that the EU's Galileo system as well as the Russian GLONASS systems are 
    expected to be fully operational in this period. So this isn't an issue for 
    global navigation directly; it's an issue for US-pre-eminence in the 
    satellite navigation business, and it's an issue for the US military which 
    would be loath to rely on a non-US position-finding system. Beyond the 
    specifics of the GAO report, the US Air Force claims that they already have 
    procedures lined up which would extend the lives of existing satellites 
    (mainly by powering down other military hardware on the satellites) and so 
    even this supposed gap could potentially be covered.
    The second significant problem outlined in the GAO report applies only to the 
    US military. They haven't funded receivers to take advantage of the new 
    satellites' capabilities. There are details in the report.
    To sum up, the "sky is falling" media reports are misleading. The GPS system 
    is not on the verge of failure in 2010. However, it is on the verge of 
    disappointing, to some extent, the rapidly growing commercial market for its 
    services in urban areas because the current excess capacity is considered 
    temporary, and there are some significant issues for the US military. So 
    unless there is some major collapse in the system unrelated to the issues in 
    the GAO report, there do not appear to be any significant issues for marine 
    navigation with the single exception that marine users may need to upgrade to 
    systems capable of picking up signale from multiple satellite constellations 
    (GPS, GLONASS, Galileo, Beidou/Compass) at a somewhat earlier date (c.2014) 
    if the worst predictions in the GAO report are borne out.
    PS: There are a couple of distinct reports on GPS from the GAO. The longer of them is linked here.

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