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Kit Guide

GPS is now not just for Yacht Navigation - Part 1

By Rooster Sailing 30th September 2020
Team Rooster has a proven record of sourcing, adapting and using the latest technology on the sailing racetrack to deliver the best techniques to the dinghy racer such as the Rooster Boat Whisperer DVD Set. Developments in high definition video capture have made it possible to put Steve’s concepts across. Rooster Sailing now can offer a low cost, simple and easy to use system, to analyse your own sailing performance with data processed from the Global Positioning System (GPS). PDF of Full Article Here Part 1 The Global Positioning System has become an everyday tool for most people. For example almost all new vehicles sold today have as standard a GPS receiver and associated mapping display built into the dashboard. The GPS system started out as a global navigation system for the military, with some limited use for the civilian population. It was also very expensive, even for a basic GPS receiver. But today, the tables have turned – there are more GPS receivers used by civilians than by the military! Some History of GPS The first GPS satellite was launched in 1974, and the GPS satellite constellation was declared fully operational in 1994. It was not until May 2000, President Clinton having realised the benefit of GPS to the civilian community, ordered the 100m accuracy degradation (accuracy denial by the military) to be removed. With the removal of this degradation, the doors opened up for civilian users to use GPS for vehicle tracking, fleet management, the emergency services and of course sailing. What is GPS? The US Global Positioning System (GPS) is a worldwide radio-navigation system formed from a minimum constellation of 24 satellites and their ground stations. GPS uses these "man-made stars" as reference points to calculate positions accurate to a matter of metres. In fact, with advanced forms of GPS you can make measurements to better than a centimetre with the right equipment! GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical to build. This makes the technology accessible to virtually everyone including us the dinghy sailor. In many published training, tuning, tactic and rules books on dinghy sailing you see numerous diagrams showing the plan position of dinghies in relation to marks, start lines, and other boats. But as we all know, in a typical dinghy race, all those aids are gone. Now, with low cost, small, light GPS receivers and the associated dinghy specific analysing software, we can record, and in fine detail study our ‘sailing tracks’ and analyse just what actually happed out on the race course. Missed that wind shift or bend? Did you get overlap at the leeward mark? Now see it on your PC screen…. GPS is not just a positioning tool, but associated with GPS measurement is time, and that is used to a few nanoseconds as part of the GPS calculation process. GPS satellites carry highly accurate atomic clocks. And in order for the system to work, our GPS receivers here on the ground synchronize themselves to these clocks. That means that every GPS receiver is in essence, a very accuracy clock. So how does a GPS receiver work? Well you need a GPS receiver for a start. The GPS receiver is a very accurate clock as mentioned, and can track and process the transmission from the orbiting satellites. So to get a position with our GPS receiver, there are a series of steps that happen ‘behind the scenes’ inside the receiver in a very short space of time. They are:
  1. "Triangulation" is the basis of using GPS data from the satellites. I am using the word "triangulation" very loosely here because it's a word most people can understand, but GPS purists would not call it "triangulation" because no angles are involved. It's really "Trilateration."
  2. Trilateration is a method of determining the relative positions of objects using the geometry of triangles.
  3. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
  4. To measure travel time, GPS needs very accurate timing which it achieves with some tricks.
  5. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.
  6. Finally, you must correct for any delays the signal experiences as it travels through the atmosphere.
So we need to measure the distance from at least 3 satellites to obtain a position with our receiver on earth. Measuring time is the tricky bit. First, the times are going to be awfully short. If a satellite were right overhead the travel time would be something like 0.06 seconds. So we're going to need some really precise clocks. So this is how it is done: Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite. To make the measurement we assume that both the satellite and our receiver are generating special a ‘GPS code’ at exactly the same time. By comparing how late the actual satellite GPS ‘code’ appears compared to our receiver's generated ‘code’, we determine how long it took to reach us. Multiply that travel time by the speed of light and you have a distance!! How accurate is GPS? Turn on any GPS receiver and it will give you an approximate horizontal position of 10-15 metres. It does depend on how much positional averaging the manufactures GPS unit does, and if you moving or not. Accuracy is also affected by a number of other factors, including the satellite positions, noise in the radio signal, atmospheric conditions, and barriers to the GPS signals, such as sails and masts. However having a receiver that is accurate to approximately 10 metres is not an issue with the race sailing application, as the 10m accuracy stated above is the absolute position, and not a relative position accuracy. All of the sailing boats within the same race area will have the same GPS errors in there calculated horizontal positions. (The largest error is mainly attributed to the ionosphere & troposphere delays as mentioned above). This can be seen by taking two GPS receivers on a single boat (with the same GPS logging configuration) and the ‘base line’ distances between them will remain reasonably constant. But can I get a better than a 10m accuracy with my handheld GPS receiver? Yes, it is possible thanks to the European Space Agency, but you must have a GPS receiver that supports the EGNOS service (WAAS is the system used in North America). What is the European Geostationary Navigation Overlay Service (EGNOS)? EGNOS is the first pan-European satellite navigation system. It augments the US GPS satellite navigation system and makes it suitable for safety critical applications such as flying aircraft or navigating ships through narrow channels. Since October 2009, three geostationary satellites and a network of ground stations have been transmitting a signal containing information on the reliability and accuracy of the positioning signals sent out by GPS. This improves the user’s position to within approximately 1.5 metres (horizontal position) depending on the users GPS receiver. You need a Qstarz GPS receiver in your Rooster buoyancy aid pocket.This GPS receiver is a pocket-sized rocket, with a huge battery life up to 40hrs -- and a replaceable low cost Nokia mobile phone battery. The included accessories are a battery, cigarette adapter for charging and a mini USB port cable for downloading the sailing tracks. (This cable can also be used to charge the unit via a PC USB port). The PC software to interface seamless with the Qstarz GPS hardware and create the ‘sailing tracks’ and further analyse also needs be purchased. This application software is discussed in PART 2 The GPS unit uses a MTK II GPS chipset which is very sensitive, meaning that it will track and process GPS signals from the protective pocket of your buoyancy aid or spray top. Qstarz GPS Receiver Other features of the Qstarz GPS receiver are:
  • Small size: 73 x 47 x 20mm
  • Fast Position Fix: Cold start 35s, Warm start 33s, Hot start 1s
  • Internal GPS antenna.
  • 8mb of Internal memory, that will store up to 400,000 records.
  • 3 hrs charging time.
  • Bluetooth and USB I/O Interfaces.
  • Simple to use, just switch to ‘LOG’
  • LEDS on the front panel: Battery status LED (Red/Green), Bluetooth status LED (Blue) GPS status LED (Orange)
  • Differential EGNOS support.
  • Warranty: 12 month manufacturers warranty
A ‘One Button’ press. If you really wish to press a button, the front face of the the Qstarz has just one (red) button. This is used to record a point position. This can be used to log features such as racing marks, obstructions, navigation marks etc. Called a POI (point of interest) button, and its designed to record your point of interest immediately. What about ‘boat heading’ with a single GPS receiver? If I use a single low cost receiver can a boats true heading or bearing can be calculated? Yes. This is simply done as the GPS calculates a given position, then 1 second later it knows the new position. Draw a line between them and that is your true heading.....Then it repeats every second or so and updates according. Further reading to follow… In a follow-up paper PART 2, that all important sailing specific application that works with the GPS data is discussed. This unique software for the dinghy sailor is the platform to analyse those starts, beats, mark rounding, download legs, tactics and wind changes. TrackTacker Software This new development for the dinghy sailor is an important milestone, on a par to when you bolted the first compass on the deck of a dinghy....

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