Global Positioning System in Science Education PTEE 2000 - Workshop

A Gps receiver can offer modern didactic opportunities for teaching different fields of physics, starting from working examples of space localisation and time scale comparisons. Our workshop showed how to implement the measurement set-up, and how to drive the active participation of the students in developing up-to-date experiments. In particular we gave new evidences of the difference between precision and accuracy (localisation), and of how to deal with different noise measurement spectra due to short and long term instabilities (timing). 1. Measurement configurations We put into operation three different measurement systems, one for timing and two for space localisation. We transported to Budapest from the Department of Physics of the University of Bologna the followings: one universal counter, three GPS receivers, two active antennas, a power splitter, three different quartz oscillators (as built by our students), the software, interconnecting cables and four power supplies. The Technical University of Budapest made available two personal computers and the antenna cables, from the roof down to the workshop room. We report here the set-up configurations: Timing (one system) • Motorola VP Oncore GPS receiver PTEE2000; 14-17 June 2000 Budapest, Hungary 2 Motorola active antenna HP 53131A universal counter n.3 quartz oscillators TAC32Plus (software) Personal computer with two serial ports (one for the GPS receiver and one for the counter). Space localisation (two systems) • Motorola VP Oncore GPS receiver Motorola active antenna TAC32Plus (software) Personal computer with one serial port (for the GPS receiver) • Garmin e-map (hand held GPS receiver) with pre-loaded map. Fig.1 The timing configuration PTEE2000; 14-17 June 2000 Budapest, Hungary 3 A proper installation of the antenna is mandatory for a good operation of the GPS receivers: this means to have an open view of the sky, with moderate obscuring by the nearby buildings. We thanks Nicholas, of the Technical University of Budapest, for having arranged a suitable support for the antenna, at the top of a very steep roof. 2. Workshop activities We were ready to show the basic commands to load and operate the software TAC32Plus (see notes); then how to configure the GPS and the counter interfaces; how to choose the proper operation mode among “position”, “timing”, and “DGPS” (differential mode); how to synchronise the personal computer clock with UTC (Universal Co-ordinated Time). We have also shown which kind of information is made available in real time: what satellites are visible and with what signal/noise they are received; how the satellites cover the sky in a quasi polar diagram, with explanations of the northern “dead angle”; we explained also the instantaneous difference between the local time scale and UTC (PC-U) and the meaning of sideral time, as needed to point at a star in the sky. The following step was a description of how TAC32Plus manages the serial connection which the HP counter: the values of frequency and of time interval can be read in real time both on the counter display and on the pc monitor which runs the software. TAC32Plus also provides simple statistical analysis of the data samples (mean, standard deviation, minimum and maximum). With the counter set in “frequency mode”, we have compared the typical rms noise of the 1 PPS pulse made available by the GPS receiver, and the one generated locally by a quartz oscillator built by our students. The data samples lasted only a few minutes during the workshop, but the measurements can be extended over longer times, if one wants to show the difference between the instabilities of two signals, over different time scales. Finally, with the counter set in the “time interval mode”, we have been able to evaluate the “time jitter” between the “ticks” of the two signals and then to recognise the intrinsic limits of the VP Oncore module (1). As a localisation application, it has been easy to show that, since the removal of the selective availability (S/A) at the beginning of May this year, the repeatability of the positioning solutions is within 3 meters, as measured with the fixed GPS system. PTEE2000; 14-17 June 2000 Budapest, Hungary 4 Finally we have also given practical demonstrations of the use of the portable unit (e-map by Garmin), both during the workshop, in the garden of the University, and through all the four days of the Conference. We have been able to verify both the excellent repeatability of the solutions and the rather large difference between the latitude and longitude co-ordinates measured by the receiver and the available map of the region. As an example, when we were half way on a bridge over the Danube, the coordinates measured by the GPS receiver pointed on the map to a land spot, almost 70 meters away. This is a clear example of the effect of different reference frames of coordinates: while GPS gives results in the so called WGS84, (a global frame with world wide coverage), each nation has its own geographical co-ordinate system, mainly for historical and military reasons (2). We can report here another dramatic example. We have compared the two full tracks recorded while we were onboard of the shuttle from the airport to downtown and back, before and after the conference. The tracks resulted to be absolutely indistinguishable one from the other, at the highest display resolution, apart when the traffic lanes were not adjacent. This again verifies that, with the S/A off, the repeatability of the GPS solutions, even over few days, is within the width of a traffic lane or very few meters.