What is GPS and how does it work

GPS (Global Positioning System) was invented as a precision guidance system for ICBMs (Inter-Continental Ballistic Missiles). Before GPS, missiles were steered by inertial navigation systems (spinning gyroscopes). That would get them to the right city, but not necessarily anywhere near the right block within that city. With GPS, you could get to within about 10 meters of the target that was marked out on the map. That meant that you could take out an army barracks or an airforce hangar building instead of the whole city. This in turn meant you might fight a war without killing several million civilians. That was a great improvement.

Of course, we wanted to give this technical advantage to our own missiles without giving our enemy's missiles a free ride on the system. So the technology was classified, and the receiver modules that were sold to the public for civilian use were crippled in 2 different ways:
1) They were blocked from providing any position data at all at altitudes above 10,000 feet (3,000 meter)
2) The readout precision was "smudged" so that it had a random variation of about 50 meters. To get a "good" position, you had to average over a 24-hour period.

The better signal was encrypted so that only military receivers could get it in "normal" times, and the civilian signal could be completely disabled (globally or in a specific international region) when the military commanded it to be.

This all changed in the early 1990'es, when the civilian signal was allowed to get the full precision during the first Gulf war, so that the US Army could give a cheap civilian receiver to every vehicle in the US Army. Where a US Army dropped into a foreign desert used to be severely handicapped by the unfamiliar terrain, they now had better map data than the Iraqi army and could easily outmaneuver Saddam's army. A couple of years later, Bill Clinton quietly made the improvement permanent, and we have all seen the benefits of that.

Having learned about the GPS system before it was ever launched, the Soviet military quickly decided that they needed a system like that, too. Their clone was called GLONASS but like everything else produced by that system it did not work nearly as well as their literature indicated that it did. In fact, they had trouble keeping the satellites operating for more than a year at a time, so the system never achieved worldwide full service. They are now finally scheduled to reach full service this year.

Since then, both China (Compass) and Europe (Galileo) have designed similar systems and started to launch satellites. It will be a few years still before all 4 systems are each capable of independent world wide service. But the commercial builders of GPS receivers are building devices that will make use of any additional satellites they can see. At least one company, JAVAD (headquartered in Moscow, but chaired by an Indian), has promised that their newest series of high-precision receivers will be able to compute a fix from four arbitrary satellites, even one from each of the four constellations. That is VERY impressive.

Most of us have used a GPS receiver, but few have any idea how they work. Most people seem to think that the receiver transmit a signal that is picked up my the satellites. That is exactly backwards. While there are minor differences between the systems, they are essentially similar, and the following description is more or less true for all of them, although the one I know best is the American GPS system.

The full GPS constellation is 24 satellites (plus a few spares) in medium high orbits at different inclinations betweenn equatorial orbits and polar orbits, designed so that most points on earth will have between 8 and 12 satellites above the horizon at all times. For a given service (such as the civilan "public" signal) all satellites in the constellation transmit on THE SAME frequency, using a direct-sequence spread spectrum signal with a long spreading code - a different code for each satellite. Similar to CDMA mobile telephone encoding, this allows a receiver that knows the proper code for a specific sender to extract that one signal out of the resulting mess. And a receiver can have a single front end (tuned to the shared radio frequency) for all the satellites and apply a separate back end loaded with a separate code for each channel that it needs to decode. Each of the satellites then transmits a complex data stream at a low data bit rate which contains a description of the orbit for each of the satellites as well as a very precise time signal - in the nanosecond range of precision. All of these data streams are precisely synchronized to an absolute time reference. Each satellite actually contains an atomic clock (and two spares!)

When decoding these data streams, the hand held receivers can triangulate to find their position. Since they know the precise position of each satellite (from knowing the orbit of each and the precise current time) and they can measure the relative delay between the multiple satellites that they can receive, they can compute their own position in 3 dimensions from 4 satellite data streams. With more satellites, they can improve on the solution by solving for multiple different combinations of 4 and then averaging.

Even with the best equipment and without the deliberate introduction of errors, this tends to only get to about 10 feet (3 meters) of precision for a variety of reasons. One of these is atmospheric disturbances, especially in the ionosphere, which can affect areas up to a few hundred kilometers at a time. But the error induced by this effect (and several others) tends to be similar for all receivers within a particular geographic area. This means that you can install a stationary receiver, learn its exact location by averaging over several weeks of time, and then compute its error vector by comparing its reading with the known location in real-time. You can then broadcast this correction factor to other receivers on the neighborhood, for example using a local ethernet network or a cellphone network. (Some geostationary satellites broadcast such a corrective data stream in a way that lets some hand-held receivers apply the correction in real time. This is called the WAAS (Wide Area Augmentation Service). While it is not as precise as a local differential reading, it may still get you from 20 feet to 5 feet. A local differential correction obtained from a station a mile or so away may get you in the centimeter range.

Are you impressed yet? I sure am, and I work with wireless engineering every day.

Another article soon will talk about novel applications for GPS and other GNSS (Global Navigation Satellite Systems).

Do we learn anything from listening to the news ?

My local NPR (National Public Radio) affiliate (KCLU) runs an hour of BBC World Service from 8 PM to 9 PM every evening. That works out to be the 3AM hour in London, which is probably a sleepy time there, but would be morning drive time around the Persian Gulf.
Tonight, on my way home after a business dinner, I heard the news anchorman say: "To give you an idea of the magnitude of Ireland's financial problems, let me share this number: The total bank debt of Ireland is equal to one and a half time the value of Ireland's entire economy."

The more I thought about this, the less I understood. What did he say ?

The easiest part of this is "the value of Ireland's economy". He is almost certainly talking about the annual gross domestic product figure for Ireland. Although "value" to me would more closely be associated with "worth" (assets) we are so used to comparing everything to the annual GDP that one needs a good reason to choose any other yardstick, so as long as he is not clearly describing a different yardstick that is probably what he meant; he just wanted a more colloquial phrase for it.

But what is "the total bank debt"? I can think of several possibilities:

a) The total debt owed to the banks in Ireland.
As far as the banks in Ireland are concerned, that would be an asset.
If this includes the total amount of mortgage debt owed on all the
"real estate" in Ireland, that does not seem excessive.
b) The total debt owed *BY* the banks of Ireland.
Without comparing it to the assets of Ireland, we don't know if the banks
are in good shape or in bad shape.
c) The *net* debt owed by the banks of Ireland.
This sort of implies that the banks are all grossly insolvent. Indeed,
if the sum of the negative balance sheet of all the banks is equal to
the annual GDP, then the banking system is in very bad shape.
That matches the headline. But is is even conceivable that the banks
would be allowed to continue doing business (by the local banking
regulators as well as by the foreign banks that they do business with)
if they were that grossly insolvent ? I sure would not want to leave any
transaction unsettled overnight with a bank that deep in the hole.

I would not be surprised at hearing this kind of garbage on an American commercial radio station, but I used to think that the BBC was better than this.

GPS, Personal Navigation Devices etc

By now, most people are familiar with the abbreviation GPS - Global Positioning System - and most people seem to associate it with at device with a small display screen that tells you where to drive your car to. Most people I know thinks that this is new and growing and must be a great business to get into.

Those people would be surprised to learn that the business of selling Personal Navigation Devices is already in decline: European sales declined by 18.6% in the 4th quarter of 2010 (presumably compared to the year before).

http://news.thewherebusiness.com/report/6364

The thing is that enough people can use Google Maps on their internet enabled cellphones so they don't need to buy a special navigation device.

I have an old GPS receiver without a map display, that I bought before internet cellphones were available; I have used it for a bit of geocaching, and hooked it up to a laptop with a map application loaded from a CD-ROM. But these days, I just use the Maps on my iPhone. I still think the navigation system in my daughter's car is cool, and I will almost certainly get one when I need a new car. The car devices speak, while my iPhone Maps require reading, thus distracting me when I am driving.

Meanwhile, the positioning provided by these device is getting remarkably accurate. That will be the subject of another post in the near future.

The Plight of the Homeless

It is winter in California, and we have rainy days almost every week. Occasionally, there's water in our rivers ....

A local news story on my NPR station this morning told of how police and social workers in Ventura, the next city down the coast from here, had spent yesterday clearing out homeless encampments from the bed of the Santa Clara River, I did not catch the total numer of people that they rounded up, but they said they had removed 17 tent camps and 14 tons of trash.

While camping in a riverbed in the raining season is certainly not a very good housing situation, I thought for a long time afterward, wondering how many of the people will actually be helped into a better situation as a result of this action. I expect that most of these are people with multiple problems that put them in this miserable place; pick a few from this list:
- military veterans with PTSD
- learning disabilities
- mental illness
- illiteracy
- brain damage from years of substance abuse

In an ideal world, we would have a "boarding school" where we could house those who are willing to be treated for some of these problems, with the goal of getting back to a functioning state and in the best cases, employability.

But in a state with a twenty billion dollar budget deficit, such things just do not happen.