The GPS system with Dasy and miniLAP

Introduction
The GPS (Global Positioning System) has now entered in everyday lives, the satellite navigation systems for cars, and more recently also for bikes are becoming more and more and more widespread, also advanced mobile phones now have GPS receiver. The GPS system is also growing in importance in the world of “only track” motorbikes, radically changing the approach to data acquisition and timing systems. Sadly still a lot of confusion and misconceptions surround the use of this technique.
Accuracy
The first question that spontaneously arises talking about these systems is: what is their accuracy? When the GPS is used for detection of lap times the accuracy is in the order of 1/100s for classic infrared stopwatch. To obtain a higher accuracy it is necessary to pass to transponders with track  drowned antenna
This with 1Hz GPS?
On this question there is much confusion, the calculation of time is not done "simply" looking at the first point beyond the target, the precision would be unacceptable! The software analyzes the two points before and after the goal, it records speeds of both and thanks to an algorithm that takes into account the physical characteristics of the "bike system" it shall calculate the exact position of the crossing. Given the nature of a motorcycle (its maximum acceleration for example) and putting the finish line in a straight portion of the track, it is found that 1 Hz is more than enough and so you are allowed to use even consolidated and hyper sensitive GPS receivers.
Speaking of a receiver, which should be its characteristics?
GPS receivers rely on their inside "engine", product by some large brands including SirfStar, MTK, uBlox. What is important is that they are very sensitive, for example, we use a SirfStar III-159dBm for our miniLAP and a MTK II -165dBm for our Dasy6 system (the higher this number the better). Nowadays this is for example one of the highest possible values and this warrants that you have a signal still valid even in quite prohibitive conditions. Another matter on which there is still much confusion is the number of channels. This parameter indicates how many satellites simultaneously can hook up the receiver. It is to be remembered that satellites constellation has 12 as a maximum number of viewable satellites (you can add few support satellites), therefore 14-15 channels would then be more than enough. For reasons most of the image than of real needs, it has been developed 30-50-60-channels receivers. Our MiniLAP system uses a 20 channel receiver to get the same performance (measured) than those obtained by 66 channels system used in our Dasy6.
Analysis of trajectories
EVERY commercial GPS systems, based on frequency L1 has an accuracy in the tens of meters up, regardless the number of channels, sensitivity, frame rate or used algorithms. How can then use these systems for analysis of the trajectories?
The absolute accuracy of a GPS antenna is about 10m, while much higher is its relative precision. This means that GPS can locate the position in the world of an antenna with an error of about ten meters, but then it is able to follow antenna movements with extreme precision and to give very accurate tracks. For this reason, the GPS tracks in themselves are accurate but it is not unlikely that in the medium term, comparing the same track (eg. one of a racetrack) obtained in different days, we find it again very accurate, but shifted by few meters. The antennas used in GPS navigators are just apparently much more accurate, but this is due to the fact that they can exploit additional information contained in their databases, for example they can put the antenna in the middle of the road you are following, although the actual signal received fluctuate every second of several meters.
It is important to stress that it is absolutely not important to the overlap the paths with a real circuit, which would be highly inaccurate due to the unavoidable errors of the GPS, but it is extremely useful the analysis of the SHAPE of trajectories and the comparison with speeds graphs. With this kind of analysis one can understand where he is losing time, simply by placing some splits.
Comparison between trajectories and speeds
One of the most useful features of GPS is the ability to compare the graphs of the trajectory, but also speed of different sessions belonging to the same person or different people. By tese means one can easily understand why he is unable to obtain the results obtained in the past or how to improve facing with graphs of a pilot who gets better results. To facilitate this operation i2m provides a database of tracks that is constantly updated with data from users who want to share their experiences.
Important precautions for best results
When a device is switched on, the GPS receiver receives and stores, as well as location coordinates of the satellite, other information: such as ephemeris (namely the precise location of satellites and data to determine its future position), the corrections to the clock board, the data on earth's atmosphere that are used to calculate the signal propagation speed and the status of satellites. If a GPS receiver is turned on shortly after a previous use or just momentarily loose the signal, it has in its memory all these data, including ephemeris, and thus the time for the first calculation of position and speed is just a few seconds. It's called "hot start". If the shutdown lasts rather longer, we can talk instead of "warm start" because the ephemeris (no longer valid) are no longer stored in the receiver and then you need a longer technical times to download them from the satellite. If the receiver does not have any data stored, which happens after long inactivity, the first position signal is received not less than after one minute, after you've downloaded all the satellites and GPS system codes: it is the so-called "cold start". It 'important to note that these times are extremely fast if the system is stationary during these early stages but may become longer if the receiver is in motion. Is therefore highly recommended to turn the receiver on at least 5-10 minutes before starting the race if the system had remained shut down for more than an hour so as to give time, even in the worst case, to the receiver to search for satellites and to offer the best performance.