Solutions GPS and SatNav Pinpoint Accuracy Explained

As the use of SatNav systems increases among the general user population the misconceptions that have arisen on how these systems work are spreading. This article covers one of the author's favourites, the 'fact' or 'common knowledge' that GPS accurately pinpoint you anywhere in the world. This article is aimed at increasing awareness of what is really going on with accuracy.

Well, can anyone define pinpoint accuracy? It must vary as specs for different GPS units state different figures. My new Garmin handheld states less than 10m 95% typical with the proviso 'subject to accuracy degradation to 100m 2DRMS under the US DoD Selective Availability (SA) Program when activated'. I have had it down to saying 3m or even 2m in the higher accuracy differential mode (DGPS). However, whether to believe it or not at these better figures? A survey GPS unit states, Single Point L1 1.8m CEP (most will be explained later).

These figures are very very good, but are they pinpoint?

Let us keep analysing. Perhaps if it is good enough that it doesn't matter, it is pinpoint. If your GPS can get you close enough to use the best navigation system in the world, the Mark I Eyeball, then it has done its job. Depending on light/dark and weather this can range from a few meters to 10 miles or more. Is it pinpoint if it gets you to within 10 miles, where you can see your target, on a clear bright day?

I always thought the definition was, er, the point of a pin, less than 1mm. Only the best GPS left stationary for days will get anywhere near this.

Furthermore many talk of accuracy loosely without understanding. Even I have been loose with the term. There is accuracy, precision or repeatability and resolution, often with cross meanings to different people. Probably that is why many are loose with the term.

When someone says pinpoint I am tempted to ask for a figure and when and how often, resulting in the blank look. This brings in 95%, 2DRMS and CEP. Accuracy is defined as a statistical measure, it is pointless not to. The figure, for example, <10m is maybe the expected under best conditions rather than the average. These best conditions are where in the world there is full coverage and good satellite geometry, their position relative to the receiver. Its all about angle of cut. An example of how often is the 95%, the accepted but not exactly 2 sigma. 95% of recorded positions are expected to be within 10m of the true position. If you wanted 99% that would mean something in the order of maybe 15m. Circular Error Probable (CEP) is 50%.

Now for the anywhere in the world. Due to the orbital inclination you can expect less visible satellites above 55° north and south, less to choose from to obtain the required accuracy position solution. This is not the end for the position is also relative to some reference system and can cause issues if you are combining the use with a map.

GPS works on a reference system that is the best global fit. You may have come across WGS84 Datum. The world has a mathematically horrible shape, all lumpy and bumpy so only a best fit is viable. For some areas this is not a good fit so a different datum is used. Many of these precede GPS. The result is that plotting your WGS84 position on a map of a region of different datum can mean hundreds of meters out. For the car SatNav which is of proprietary manufacture this will be of no consequence as the map would, or should, come already referenced. If navigating with a hand held you need to know the datum for the area. Again the conversion sums are horrible but, no fear, most hand helds have most datums in the software, as long as you know this and where to find them you will be OK.

Essentially that pinpoint is more degraded by plotting on the wrong datum, something not always considered by many.

So there you go. If you arrive safely at your desired destination then the system has done its job. But lets not call it what it isn't.

There are other misconceptions but these are left for a separate article, 'GPS and SatNav. Removing Misconceptions' by the same author.

Will Really Apple Cut iPad 2 Prices for the iPad 3?

Let's hop into a time machine and look back at the scale of rumors about Apple's iDevices in the past year. On our journey through the iPhone 5 rumors, we remember that the summer of 2011 was spent rummaging through the web and forecasting the future of the latest iPhone through each piece of circumstantial evidence to suggest the imminent announcement of the iPhone 5. And while the iPhone 3G and 4 are in sales, the general assumption is that retailers are trying to get rid of old stock while waiting for new Apple products. The same scenario took place when the iPhone 3 was released and the iPhone 3G was in the works.

Back in the present, we seem to be hearing the same news for the iPad generation. We are recently reading reports that the iPad 2 has started to go on sale. News from Apple Insider tells that one of the biggest stores in U.S., Meijer, has reduced in-store prices for 16GB iPad 2 with Wi-fi for $70. The company has announced the weekly sale to run from February 12 to 18 and is offered exclusively in the company's stores. The mere facts that Meijer has slated sales before the end of February 25 may be a sign that Apple will release the iPad 3 on February 29, the leap day of this year, although a lot of other reports are suggesting that the date may actually be March 7. Upon hearing this news, iPad 2 owners are now considering to sell iPad 16GB before the prices are cut. Meanwhile, non-iPad 2 owners are looking to buy their own units to take advantage of the sale.

It is good to note that looking back, the clearance sale for the iPhone 3G and iPhone 4 came in a few months before the announcement of the iPhone 4S. In another published article last August, it was reported that Target had reduced the price of the iPhone 3G from AT&T for the price of only $19.99 and the iPhone 4 for $169.99 for the 16GB version and $249.99 for the 32GB version. The said store had reduced prices for Verizon iPhone 4 in the same way. Best Buy joined at the forefront on August 22. Experts and techies are also looking forward to the same pattern for the iPads and several blogs are now telling people to sell iPad 16GB used units soon before they lose value.

But since history has proven that retailers have a track record of wrong predictions, the price reductions for the iPad 2 are still a little hard to swallow for the moment. And while we have always expected that major retailers have an upper hand once the iPad 3 is released, it seems that the exclusion of Apple's mobile operator partners shows that the retail world remains largely shooting in the dark with regards to Apple product launches.

So dear iPad owner, before you sell iPad 16GB on eBay, think about how much you'll probably be getting from it and how much hassle it'll cause especially when price cuts are just around the corner. Meijer can just gamble an iPad 2 clearance sale, betting that the February 29 or March 7 date is the third major announcement for the iPad and will be followed by a rapid market release of the iPad 3 (it's a big company anyways). Our safe bet is that it will fall sometime between February 29 and March 29.If this bet turns out to be a winner, then it's about time for you to sell iPad 16GB online to eliminate your iPad 2 parts, which will allow you to start fresh with the iPad 3.