Wednesday, July 16, 2014

of trilateration and probably not theft

GPS receivers also have to correct for things like light traveling slower through plasma in the ionosphere. Receivers on the ground actually solve for their positions in 4 dimensions (they need to solve for time!). GPS is so cool.


baddox 5 hours ago | link

Any idea if large-scale weather effects (like the jet stream) can cause noticeable accuracy problems?


kubiiii 4 hours ago | link

It does, in fact GPS positioning error, derived from fixed GPS stations, is often used as an input for weather forecast models like WRF. The error is closely binded to the atmospheric water content. Using the different azimuths from the GPS constellation you can even build up a 3d representation of the water content on different layers of the atmosphere.


mnw21cam 1 hour ago | link

You also have satellites with GPS receivers, so that they can measure the effects when the GPS signal skims the atmosphere. This allows humidity and temperature to be measured with very high accuracy, with a very good vertical resolution.


We received trade execution reports from an active trader who wanted to know why his large orders almost never completely filled, even when the amount of stock advertised exceeded the number of shares wanted. For example, if 25,000 shares were at the best offer, and he sent in a limit order at the best offer price for 20,000 shares, the trade would, more likely than not, come back partially filled. In some cases, more than half of the amount of stock advertised (quoted) would disappear immediately before his order arrived at the exchange. This was the case, even in deeply liquid stocks such as Ford Motor Co (symbol F, market cap: $70 Billion, NYSE DMM is Barclays). The trader sent us his trade execution reports, and we matched up his trades with our detailed consolidated quote and trade data to discover that the mechanism described in Michael Lewis's "Flash Boys" was alive and well on Wall Street.

Let's take a look at what we found from analyzing 5 large trades executed at different times over a 4 minute period in Ford Motor Co. Before each of these trades, the activity in the stock was whisper quiet. Here's a chart showing millisecond by millisecond trade and quote counts in Ford leading up to one of these 5 trades:

from Lizard's Ghost

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