Global Position Reporting
Linux, amateur radio and the APRS protocol are only part of the system we have discussed so far. The GPS is truly what makes the system practical. Although the details of the GPS are extremely complex, the basic idea is relatively simple. Triangulation is used to pinpoint a receiver.
For example, assume you and your friend both have accurate synchronized clocks. At some unknown distance from you, she yells, “It is now 6:00 and 0.000 seconds.” When you hear her, your clock shows the time as 6:00 and 0.333 seconds. You can now compute your distance from her as 100 meters by multiplying the speed of sound (300 meters per second) by the elapsed time (0.333 seconds). With this single test point, you are able to compute your distance from the source. Specifically, you are located in any direction 100 meters from your friend. This scenario is shown in Figure 4A by the multiple dots located on the circumference of the circle.
With a second friend, we can further clarify our position. In Figure 4B, a friend at point Y, also with an accurate clock, takes another measurement. Again you make the computation and find the distance from this friend. You now have your position narrowed to two points shown by the two dots where the circles intersect. Using a third friend at point Z and another measurement, you are able to pinpoint your exact location.
The GPS works on a similar principle; however, the speed of light replaces the speed of sound in the experiment, and your friends are replaced by satellites. In the above explanation we have conveniently assumed that the world is flat to provide a clearer understanding of the concept. When the concept is extended to three dimensions, a single measurement does not produce a circle as shown in Figure 4A, but a sphere. A second measurement does not limit our position to two unique locations as shown in Figure 4B, but a circle that is the intersection of two spheres. Last, a third measurement does not yield a unique location as shown in 4C, but two points which are the intersection of three spheres. Thus, with three measurements, we have two possible locations. The good news is that one of the points can be eliminated since it corresponds to a position above the Earth's atmosphere. So unless you are an astronaut, your unique location on earth has been found with only three measurements.
We made a second convenient assumption, specifically that all parties in the experiment had accurate clocks. Although the GPS satellites have accurate atomic clocks, the receiver on the ground does not have such a luxury, nor would it be practical. Fortunately, by adding a fourth satellite, the person on the ground does not require an atomic clock. This is a simple algebraic problem in four unknowns: latitude, longitude, altitude and time. With four satellites we can solve for all four unknowns and provide an accurate and unique position for the listener on Earth. The experienced GPS user may argue that he has obtained accurate positions with only three satellites. This is true; it is done by letting the GPS receiver assume the altitude is 0 (sea level). Therefore, if we are willing to give up knowing our altitude, which is valid in many applications, the GPS can indeed provide an accurate position using only three satellites, since we have three unknowns and three equations.
The above explanation is in many ways an oversimplification. In real life, numerous variables affect the accuracy of the system. For example, radio frequency transmissions are affected by objects such as buildings and trees. These structures cause reflections, referred to as multi-path. Signals from the satellites are reflected off nearby structures, causing delays which ultimately affect the accuracy of the measurement. Radio frequencies are also affected by rain, sleet, snow, humidity and even the temperature of the air, since the speed of the transmission is affected as well as the attenuation of the signal. All of these variables result in loss of accuracy. However, these inaccuracies are small compared with the deliberate error called Selective Availability (SA).
To understand SA, we must understand that GPS applications fall into two service categories: the Standard Positioning Service (SPS) for civilians, and the Precise Positioning Service (PPS) for military and authorized personnel. PPS GPS receivers remove the adverse affects of SA and are therefore far more accurate. SPS GPS receivers provide less accuracy than the GPS is capable of, and each is generally limited to an accuracy of 100 meters. However, there are ways of overcoming the limitations of SPS receivers by using Differential GPS (DGPS). For those interested in DGPS, the web is an excellent source of further information.
Practical Task Scheduling Deployment
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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|The Firebird Project's Firebird Relational Database||Jul 29, 2016|
|Stunnel Security for Oracle||Jul 28, 2016|
|SUSE LLC's SUSE Manager||Jul 21, 2016|
|My +1 Sword of Productivity||Jul 20, 2016|
|Non-Linux FOSS: Caffeine!||Jul 19, 2016|
|Murat Yener and Onur Dundar's Expert Android Studio (Wrox)||Jul 18, 2016|
- Stunnel Security for Oracle
- The Firebird Project's Firebird Relational Database
- SUSE LLC's SUSE Manager
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- Managing Linux Using Puppet
- My +1 Sword of Productivity
- Non-Linux FOSS: Caffeine!
- Google's SwiftShader Released
- SuperTuxKart 0.9.2 Released
- Doing for User Space What We Did for Kernel Space
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide