Contents:
Overview
How to Check the Station's Current Data
Averages
Programming Information
Suggested Projects for Students
Acknowledgements
The WALTA Prototype Station, located at Mercer Island High School, is at present (6/2002) simply a computer which records time and weather information. This setup will allow us to make preliminary studies of how best to design the fully equipped stations for future installations.
The MIHS station consists of a PC connected to a data box which operates two temperature sensors and a Global Positioning System (GPS) receiver. GPS is a satellite system operated by the US government, which allows anyone to determine their geographic location and the exact time with great precision. A central computer at the Particle Astrophysics laboratory at UW polls the MIHS station at regular intervals for information from the sensors. Web pages hosted at the central computer can generate time-series plots of data received from the station or provide stored files of archived data. But anyone, anywhere in the world, can check the station's current data at any time, simply by visiting the station's World-Wide Web address, or URL.
| Item | Name | Format | Units |
|---|---|---|---|
| 1 | Station ID (MIHS=3) | Integer | none |
| 2 | Location Name | Text | none |
| 3 | Computer Clock Time | Integer | seconds since Jan 1, 1970 |
| 4 | GPS Time (Pacific time) | Integer | seconds since Jan 1, 1970 |
| 5 | GPS Latitude | Decimal | degrees and fraction |
| 6 | GPS Longitude | Decimal | degrees and fraction |
| 7 | GPS Altitude | Decimal | meters above sea level |
| 8 | Internal Temperature | Decimal | degrees Celsius |
| 9 | External Temperature | Decimal | degrees Celsius |
Each time you visit the station's website, you will get a new reading.
The station computer's simple internal clock will drift relative to GPS time, which is regulated by atomic clocks on the ground and in the satellites, and is accurate to approximately 0.0000001 second (100 nanoseconds). In other words, if someone anywhere else on earth were to take a GPS time reading at exactly the same instant you do, you would be guaranteed that the two time readings were within 100 nanoseconds of one another. The fact that GPS receivers can be purchased for about $100 is a remarkable feat of mass-market technology. GPS time accuracy is a key feature that makes the WALTA project possible: stations at different schools can easily be synchronized to great accuracy, without expensive equipment.
Both internal clock and GPS time values are reported in the peculiar way that computers like, as the number of seconds since the beginning of the computer era (taken to be January 1, 1970). Many kinds of software, including spreadsheet programs like Microsoft Excel, can convert these values into date and time of day.
The GPS receiver in the WALTA station also interprets satellite data to determine the geographic location of its antenna, which is mounted on the MIHS radio station tower, with an accuracy of about 30 meters. Finally, we installed a temperature sensor outdoors, near the antenna, and indoors, in the station's electronics box. These temperature sensors give readings with precision about 0.1 degree Celsius, although their absolute accuracy is only about 1 degree.
In addition to the basic data described above, you can also visit http://168.212.144.251/walta/vissat.csv, which will show you the following data about the GPS satellites currently in use for getting time and location information. These data are supplied as one line per satellite, with each line containing the data items described below, separated by commas.
| Item | Name | Type | Units |
|---|---|---|---|
| 1 | GPS satellite ID number | Integer | none |
| 2 | Azimuth (0 deg = N) | Integer | degrees |
| 3 | Elevation | Integer | degrees |
| 4 | Health | Text | none |
| 5 | Tracking Status | Text | none |
See Document-Type-Definition at the beginning of the XML.
One line of information. Each parameter is a name-value pair, enclosed
within braces, and separated by white space. For N pieces of information,
the line looks like-
{name1 value1} {name2 value2} ... {nameN valueN}
The names of the parameters currently available are id, cpu_time, gps_time, latitude, longitude, altitude, t0, t1, gps_visible, gps_tracked. The parameters and their order are subject to change.
The variation from fix to fix is due to a combination of factors. One factor is the presence of fundamental uncertainties in making the necessary measurements. For example, our knowledge of the speed of radio waves in the upper atmosphere is limited, and this uncertainty affects the precision of time and location fixes. Another is the arrangement of satellites in the sky, which is termed the "geometric dilution of precision". If all the satellites in view at a particular time happen to be clustered together in the same region of sky, precision is poorer than it would be if they were more spread out. Another factor is the US military's deliberate altering of the information, known as "selective availability", but this source of inaccuracy was turned off by Presidential order in May, 2000.
What is the best way to average our GPS position measurements together? How do you estimate the accuracy of an average reading?
Contact person: R. J. Wilkes
