gps gardens

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How to Map Your Garden Using GPS without Getting Lost: 

How to Map Your Garden Using GPS without Getting Lost Learn all about GPS (Global Positioning System): what it is and why it is so useful for mapping. We’ll discuss equipment (Will that $39.99 unit be sufficient for your needs?) and mapping software. You’ll learn how to download a base map for your garden. Participants will go out on the campus and collect data on tree locations using a handheld GPS unit and a laser rangefinder, bring the data back to the lab, and learn how to actually place the trees on the maps.


GPS “The Next Utility”


Schedule 9:00 – 10:00 Introductions What is GPS and How it Works Prep for Field work (mission planning) 10:15 – 11:00 Data Collection 11:00 – 12:00 Post processing and mapping


Introductions Who you are and Where you are from Are you currently using GPS Current Mapping Efforts Techniques Tools (Software & Equipment)


GPS: Global Positioning System US System: NAVSTAR (NAVigation System with Timing And Ranging) Managed by: US Dept. of Defense The Russian Federation system: GLONASS (GLObal Navigation Satellite System) European Union: Galileo (2008) Who What Where When Why How


NAVSTAR 24 - 30 orbiting satellites (solar powered, radio transmitting) 4 satellites in each of 6 orbiting planes 3 of the 24 are considered “spares” Orbiting speed: 3.87 km/s Orbiting angle: 55o angle from equator Size: 9m width with solar panels extended Control stations are positioned near the equator, around the globe. Who What Where When Why How


A GPS system is comprised of several components: GPS satellites The satellite ground control system A GPS antenna and receiver Data loggers and/or computers Software for processing Who What Where When Why How

Many Uses: 

Many Uses Location – positioning things in space Navigation – point (a) to point (b) Tracking – monitoring movements Mapping – creating maps based on positions Timing – precision global timing


GPS helps expose indecent snowplow driver By David R. Guarino Tuesday, January 11, 2005 When a snowplow driver allegedly flashed a Dunkin' Donuts worker early yesterday, police figured they'd take the usual path toward tracking him down: witness statements, descriptions, surveillance video…. Gorilla Positioning System Tracking mountain gorillas in the volcanic jungles of central Africa isn't easy. If you avoid malaria, there's still the snipers. A newly forged partnership hopes to lessen the risks by allowing scientists to study the endangered beasts from desktops rather than treetops. ….


Origin of problem: shipping in the 1800’s Predecessor of GPS technology: WWII radar and later, shipping radio transmitters. 1978: First GPS satellite launch 1983: GPS revealed (kept secret until now) 1994: All 24 satellites operational 1996: Investment to date - $12 billion 1999: “Washington, DC -- Vice President Gore announced today a $400 million new initiative in the President's balanced budget that will modernize the Global Positioning System (GPS) and will add two new civil signals to future GPS satellites, significantly enhancing the service provided to civil, commercial, and scientific users worldwide.” 2000: SA Turned Off Who What Where When Why How


13,000 km 20,200 km 12,600 mi Zone of space junk 35,420 km Geosynchronous orbit (communications satellites) GPS satellite orbit Who What Where When Why How


Originally develop by the military, for the military. Now, civilian uses have far exceeded military uses, but the DoD maintained strict control… leading to a long political battle. (GPS was used extensively in Desert Storm.) Who What Where When Why How


The Fundamental Principal: Speed * Time = Distance Radio waves are electromagnetic radiation, and travel at a constant speed: 299,792,458 meters/sec (186,000 mi / sec) Thus, if we can measure how long it takes for a signal to reach us, we know the distance to the satellite. Who What Where When Why How


= GPS satellite (sphere) x x = known distance from satellite If we know our distance from one satellite, we know we lie somewhere on a theoretical sphere, with a radius equal to that distance. Who What Where When Why How


(spheres) If we know our distance from two satellites, we know we lie somewhere on a theoretical circle, that is the intersection of the two spheres. Who What Where When Why How


If we know our distance from three satellites, we know we lie on one of two points, one of which is impossible. Who What Where When Why How


Movietime Movie Clip – GPS Principles

Mission Planning: 

Mission Planning Location Time of Field Collection


In theory, only three satellites are needed to acquire an accurate position fix. In practice, we need four satellites because of error that arises from a variety of sources. The core of a good understanding if GPS is understanding these error sources. Who What Where When Why How


Sources of error: Distance of error: Satellite clock errors < 1m Ephemeris errors (satellite position) < 1m Receiver errors (fraction arithmetic) < 2m Ionosphere (charged particles) < 2m Troposphere (the dense part) < 2m Multi-path errors Variable Selective Availability (when active) (< 33m) Satellite Geometry (PDOP) * 4 - 6 PDOP = Position Dilution of Precision Who What Where When Why How

Typical Accuracies: 

Typical Accuracies 100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program. 10- 15 meters: Typical GPS position accuracy without SA. 2-5 meters: Typical differential GPS (DGPS) position accuracy. < 3 meters: Typical WAAS position accuracy. Sub-meter: Typical WAAS position accuracy and DGPS


How does satellite geometry influence accuracy? A telemetry example (2D): 90o Shape of area which may contain receiver Receiver Who What Where When Why How Shape of area which may contain receiver


Without correcting for these errors, we can achieve about a 5m accuracy with parallel tracking units (good ones), or 10m with serial tracking units (cheap ones). Several sources of error are very difficult to correct for. Fortunately, the largest error sources can be corrected for using differential processing. With differential processing, we can achieve accuracies of < 1m, and even centimeter accuracy with the right receiver. Who What Where When Why How


Averaging (no correction) + + + + + + + + + + + + + + + + + + 1-2 m Actual location Single GPS location Averaged GPS location Averaging increases accuracy to around 2-4m. The more points you average, the better your accuracy. Who What Where When Why How


Differential GPS 1. Using post-processing Known location data (base station) Remote location data collected simultaneously Data from the known location is used to identify the error. The post-processing removes this error from the remote data. Who What Where When Why How


Differential GPS 2. Real-time differential Known location data (transmitting base station) Remote location data collected simultaneously Data from the known location is sent via radio signals to the remote receiver, which removes the error using real-time processing. No post-processing is needed. We can obtain sub-meter locations in about 5 seconds. Who What Where When Why How


Differential GPS US Coast Guard transmitting station coverage for real-time DGPS in Wisconsin. Who What Where When Why How


Obstacles to GPS signals: GPS signals are high frequency because low frequency signals tend not to travel in a straight line through the atmosphere. The cost is that high frequency signals have very little penetration through matter. They are also easily reflected. The signals pass through: thin plastic, cloth, canvas, etc. They do not pass through: anything metal, or anything containing a high degree of water (flesh, deciduous leaves, very heavy rain, etc). Who What Where When Why How


Obstacles to GPS signals (cont.): Smooth surfaces act as mirrors to GPS signals. “Smooth” to a high frequency radio wave means anything as smooth or smoother than a coarse gravel road. Open water is particularly reflective. Reflectance leads to multipath errors... Who What Where When Why How


Multipath error: Increases the length of time taken for a signal to reach the receiver. Who What Where When Why How


Types of GPS units Low end (recreational grade) Single channel, Track in serial, $100-$400, Accuracy: 10m Mid Range (mapping grade) 8-12 channels, Track in parallel, $2,500 - $10,000 Accuracy: 2-5m, sub-meter real-time, centimeter accuracy with post-processing. High end (survey grade) 12 channels, Track in parallel, $10,000 - $25,000 Accuracy: sub-meter to centimeter real-time, sub centimeter accuracy with post-processing. Who What Where When Why How

Other Considerations: 

Other Considerations WAAS Coordinate System! Compatibility with other users Base Station access See for good review on receivers


1. Why is this equatorial rainforest wildlife biologist sitting on a horse in the middle of a river? 2. What problems is she likely to be having (with her GPS unit)?


The Physics of GPS


How is the signal sent? The signal consists of two parts: the carrier - on all the time the modulation - carries the information Signals are broadcast on 2 frequencies (only one of which is for civilian use). Coarse/Acquisition code: Frequency: 1575.42 MHz (FM radio is around 100 MHz) Wavelength: 20 cm (short, hence difficulty with obstacles). Who What Where When Why How


What is in the signal? 10011101101110001010101010 1023 bits repeated 1000 times/second Called “pseudo-random noise” Contains information about the satellite, the contents of the code, the time the code was sent, etc. Who What Where When Why How


How is time measured? Each satellite keeps accurate time using four atomic clocks ($50,000 each). The receiver has a much less accurate (and cheaper) clock (this is one source of error). The receiver and the satellite generate the same code at the same time. The receiver determines range by matching the satellite code to its’ own code to calculate how long the signal took to reach it, and therefore the distance of the satellite (time x speed = distance). Who What Where When Why How


How does satellite geometry influence accuracy? (cont.) GPS positioning involves 4 dimensions (3D space plus time) The influence of geometry is measured with “Dilution of Precision” (DOP). A DOP value of 1 is perfect geometry. DOP’s > 6 indicate poor geometry and readings are not taken. Who What Where When Why How


DOP North DOP (NDOP) East DOP (EDOP) Vertical DOP (VDOP) Time DOP (TDOP) The 4 dimensions Horizontal DOP (HDOP) consists of NDOP and EDOP Position DOP (PDOP) consists of HDOP and VDOP Geometric DOP (GDOP) consists of PDOP and TDOP Who What Where When Why How

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