GPS Satellites Lecture Notes PDF

1/29/2016 1 The GPS Satellites Block II/IIA/IIR/IIR-M satellite Availability of at least 24 operational GPS satellites, 95% of the time. 31 operational GPS satellites....

1/29/2016 1 The GPS Satellites Block II/IIA/IIR/IIR-M satellite Availability of at least 24 operational GPS satellites, 95% of the time. 31 operational GPS satellites. 2 1  1/29/2016 GPS and Trilateration ⚫ GPS and trilateration rely on measurement of distances to fix position ⚫ Range is the GPS term for distance ⚫ Trilateration measure distance from controls on ground ⚫ GPS ranging measure distance from satellites orbiting at nominal altitude of 20,183 km above earth. 3 A Passive System ⚫ GPS signals are broadcast in the microwave part of the electromagnetic spectrum ⚫ It is passive system because only the satellites transmit and the users only receive them ⚫ There is no limit to the number of users without danger of overburdening the system One-way ranging 4 2  1/29/2016 5 Time ⚫ Distance is a function of the speed of light (c), signal frequency (f), and elapsed time (t) ⚫ The GPS signals do not return to the satellite ⚫ A clock in the satellite marks the time it departs from the satellite and a clock in the receiver can mark the moment it arrives ⚫ The range depends on the time it takes a GPS signal to make a trip from the satellite to the receiver. ⚫ Therefore the satellite must tell the receiver the exact time it left the satellite 6 3  1/29/2016 Control ⚫ In GPS, the control points are the satellites themselves but it is somewhat complicated because the satellite is always moving. ⚫ The GPS signal must communicate to its receiver: 1) what time it is on the satellite; 2) the instantaneous position of a moving satellite; 3) some information about atmospheric correction; and 4) satellite identification and where to find the other satellites. 7 The Navigation Code ⚫ In order to communicate the needed information by the receiver, codes are used and carried by 2 carrier waves. ⚫ A carrier has at least one of characteristics such as phase, amplitude, or frequency that may be changed (modulated) to carry information. ⚫ The 2 carrier waves are radio waves that are part of the L-band (390 MHz – 1550 MHz) ⚫ L1-band at 1575.42 MHz is a bit higher than the strict L-band frequency 8 4  1/29/2016 Modulation types 9 Wavelength ⚫ A wavelength with a duration of 1 second or 1 cycle per second has a frequency of 1 hertz (Hz) ⚫ The lowest sound human ears can detect has a frequency of about 25 Hz and the highest is about 15,000 Hz or 15 kilohertz (KHz) ⚫ Most modulated carriers used in in EDMs and GPS have frequencies that are measured in million cycles per second or megahertz (MHz) ⚫ The 2 fundamental frequencies of GPS are: L1 at 1575.42 MHz (λ=19.0 cm) and L2 at 1227.60 MHz (λ=24.4 cm) 10 5  1/29/2016 Codes ⚫ GPS codes are binary, strings of zeroes and one, the language of computers ⚫ 3 basic GPS codes: 1) Navigation code, 2) Precise code (P code) and 3) coarse acquisition code Navigation Code ⚫ Navigation code has a frequency of 50 Hz and is modulated into both L1 and L2 carriers. ⚫ Each of the 5 subframes of the navigation message begins with the same two words: the telemetry word (TLM) and the handover word (HOW). ⚫ Unlike nearly everything else in the NAV message, these two words are generated by the satellite itself. ⚫ These data contain the time since last restart of GPS time on the previous Sunday 0:00 o’clock. ⚫ GPS time restarts each Sunday at midnight (0:00 o’clock). 11 Navigation Message The entire Navigation message contains 37,500 bits and at a rate of 50 bits- per-second takes 12½ minutes to broadcast and to receive from a cold boot with a GPS receiver. 12 6  1/29/2016 Navigation message ⚫ The TLM indicates the status of uploading the control segment if it is in process or not. This allows your receiver to know that. ⚫ The HOW provides the receiver information on the time of the GPS week (TOW) and the number of the subframe, among other things. ⚫ The HOW’s Z count (an internally derived 1.5 second epoch) tells the receiver exactly where the satellite stands in the generation of positioning codes. ⚫ The HOW actually helps the receiver go from tracking the C/A code to tracking the P(Y) code, the primary GPS positioning codes. It is used by military receivers. ⚫ Information in the Navigation message deteriorates with time. ⚫ For example, every 2 hours the data in subframes 1, 2 and 3, the ephemeris and clocks parameters, are updated. ⚫ The data in subframes 4 and 5, the almanacs, are renewed every 6 days. ⚫ These updates are provided by the government uploading facilities. 13 ⚫ The Control Segment 14 7  1/29/2016 GPS Time ⚫ Subframe 1 contains the time-sensitive information. ⚫ GPS Time is the time standard of the GPS system. It is also known as GPS System Time (GPST) ⚫ Subframe 4 contains information needed by the receiver to correlate its clock with that of the satellite clock. ⚫ The rate of GPS time is kept within 1 microsecond of the rate of the worldwide time scale, which is called Coordinated Universal Time or UTC. ⚫ The rate of UTC, determined 65 timing laboratories and hundreds of atomic clocks around the world, is more stable than the rotation of the earth itself. ⚫ This causes a discrepancy between the UTC and the earth’s actual motion. ⚫ This discrepancy is kept to within 0.9 seconds by the periodic introduction of leap seconds in UTC. 15 GPS Time ⚫ Since GPS is not earthbound, leap seconds are not used in GPS time. ⚫ GPS time was identical to UTC on midnight January 5, 1980. ⚫ Since then many leap seconds have been added to UTC but none have been added to GPS time. ⚫ Even though their rates are virtually identical, the numbers expressing a particular instant in GPS time are different by some seconds from the numbers expressing the same instant in UTC. ⚫ For example, GPS time was 16 seconds ahead of UTC on July 1, 2012. 16 8  1/29/2016 Satellite clocks ⚫ Each GPS satellite carries its own onboard clocks in the form of a very stable and accurate atomic clocks regulated by the vibration of frequencies of the atoms of 2 elements: 2 clocks by cesium and 2 by rubidium. ⚫ Block IIR satellites are all rubidium ⚫ The clocks in any one satellite are completely independent from those in the other, hence they are allowed to drift up to 1 millisecond from the strictly controlled GPS time. ⚫ These individual drifts are carefully monitored by the Control Segment and is eventually uploaded into subframe 1 of where it is known as the broadcast clock correction. 17 Satellite clocks ⚫ The broadcast clock correction is only part of the solution to the problem of correlating the receiver’s clock to the satellite’s clock. ⚫ The receiver will need to rely on other aspects of the GPS signal for a complete time correlation. ⚫ The drift of each satellite’s clock is not constant, nor the broadcast clock correction be updated regularly. ⚫ Therefore, one of the 10 words in subframe 1 provides information about the Age of Data Clock or AODC. 18 9  1/29/2016 The Broadcast Ephemeris ⚫ Subframes 2 and 3 of the contain information about the position of the satellite, with respect to time. This is called the satellite’s ephemeris. ⚫ The satellite’s ephemeris is given in a right ascension, RA, system of coordinates. ⚫ There are 6 orbital elements: a, e, Ω, ω, i and v. ⚫ This information is needed by the user’s computer to calculate the ECEF, WGS84 coordinates of the satellite at any moment. ⚫ The broadcast ephemeris is far from perfect. It is expressed in parameters that appear Keplerian. ⚫ But they are the results of least-squares curve fitting analysis of the satellite’s actual orbit. ⚫ The broadcast ephemeris also deteriorates with time. ⚫ As a result one of the important portion of the Navigation message is called IODE or Issue of Data Ephemeris. 19 Ephemeris and clock data parameters decoded from subframes 1, 2 and 3 ⚫ EPHEMERIS FOR SATELLITE 2 : ⚫ PRN number for data.................. 2 ⚫ Issue of ephemeris data.............. 224 ⚫ Semi-Major Axis (meters)............. 2.65603E+07 ⚫ C(ic) (rad).......................... 1.88127E-07 ⚫ C(is) (rad).......................... -1.00583E-07 ⚫ C(rc) (meters)....................... 321.656 ⚫ C(rs) (meters)....................... 87.6875 ⚫ C(uc) (rad).......................... 4.36418E-06 ⚫ C(us) (rad).......................... 2.70829E-06 ⚫ Mean motion difference (rad/sec)..... 5.04521E-09 ⚫ Eccentricity (dimensionless)......... 0.0139305 ⚫ Rate of inclination angle (rad/sec).. 4.11089E-10 ⚫ Inclination angle @ ref. time (rad).. 0.950462 ⚫ Mean Anomaly at reference time (rad). -2.62555 ⚫ Corrected Mean Motion (rad/sec)...... 0.000145859 ⚫ Computed Mean Motion (rad/sec)....... 0.000145854 ⚫ Argument of perigee (rad)............ -2.56865 ⚫ Rate of right ascension (rad/sec).... -8.43857E-09 ⚫ Right ascension @ ref time (rad)..... 1.75048 ⚫ Sqrt (1 - e^2)....................... 0.999903 ⚫ Sqr root semi-major axis, (m^1/2).... 5153.67 ⚫ Reference time ephemeris (sec)....... 240704 20 10  1/29/2016 Atmospheric Corrections ⚫ Subframe 4 addresses atmospheric correction. ⚫ As with subframe 1, the data only offer a partial solution to the problem. ⚫ The Control Segment monitoring stations find the apparent delay of a GPS signal caused by its trip through the ionosphere through an analysis of the propagation rates of L1 and L2 frequencies. ⚫ A single-frequency receiver depends on the ionospheric correction in subframe 4 to help remove part of the error introduced by the atmosphere. 21 The atmosphere correction 22 11  1/29/2016 GPS Satellite Ionospheric Model Parameter Decoded from Subframe 4 Ionospheric parameters: Alpha : 1.397E-08 Alpha : 2.235E-08 Alpha : -1.192E-07 Alpha : -1.192E-07 Beta : 1.044E+05 Beta : 9.83E+04 Beta : -1.966E+05 Beta : -3.932E+05 23 Antispoofing ⚫ Subframe 4 also contains a flag that tells the receiver when a security system known as antispoofing or AS has been activated by the Control Stations. ⚫ Since December 1993, the P code on all Block satellites have been encrypted to become the more secure Y- code. ⚫ Block IIR-M and II-F - both will utilize the new M- code in 2020 ⚫ Subframe 4 also holds information about satellites 25- 32. 24 12  1/29/2016 The Almanac ⚫ The almanac contains the ephemerides of the satellites. ⚫ Subframe 4 contains the almanac data for satellites with pseudorandom noise (PRN) numbers from 25 through 32. ⚫ Subframe 5 contains almanac data for satellites with PRN numbers from 1 through 24. ⚫ The Control Segment generates and uploads a new almanac every day to each satellite. ⚫ Once, the receiver finds its first satellite, it can look at the ephemerides to figure the position of more satellites to track. ⚫ But it can only collect the entire ephemeris once the receiver acquire the satellite’s signal and look there for subframe 2 and 3. 25 Time To First Fix (TTFF) ⚫ The time to first fix (TTFF) is longest at a cold start, less at warm and least at hot. ⚫ Cold start, also known as a factory start. A receiver has no previous almanac or ephemeris data in its memory. It downloads the almanac full information in 12.5 minutes. ⚫ If a receiver has been in operation recently and has some left over almanac and position data in its non-volatile memory from its last observations it can begin its search with what is known as a warm start. A warm start is also known as a normal start. It can be as short as 30 seconds. ⚫ A receiver that has a current almanac, a current ephemeris, time and position can have a hot start. A hot start can take from 1/2 to 20 seconds. 26 13  1/29/2016 Satellite Health ⚫ Subframe 1 contains information about the health of the satellites. ⚫ Subframes 4 and 5 also include health data of all the satellites, data that is periodically uploaded by the Control Segment. ⚫ These subframes inform users of any satellite malfunctions before they try to use a particular signal. ⚫ They may tell the receiver that all signals from the satellite are good and reliable or that the receiver should not currently use the satellite because there may be tracking problems or other difficulties. ⚫ They may even tell the receiver that the satellite will be out of commission in the future, perhaps it will be undergoing a scheduled orbit correction. ⚫ GPS satellites are vulnerable to a wide variety of breakdowns, particularly clock breakdown. ⚫ This is the reason why GPS satellites carry as many as four clocks. 27 The P and C/A Codes ⚫ Like the Navigation Message, the P and C/A code are impressed on the L1 and L2 carrier waves by modulation ⚫ Unlike the Navigation Message they are not vehicles for broadcasting information that has been uploaded by the Control Segment. ⚫ They carry raw data from which GPS receivers derive their time and distance. 28 14  1/29/2016 PRN ⚫ The P and C/A code are complicated; in fact they appear to be nothing but noise at first. ⚫ They are known as pseudorandom noise, or PRN codes. ⚫ But these codes have been carefully designed and capable of repetition and replication. 29 P Code ⚫ The P code is generated at a rate of 10.23 million bits per second and is available on both L1 and L2. ⚫ Each satellite repeats its portion of the P code every 7 days, and the entire code is renewed every 37 weeks. ⚫ A GPS receiver must distinguish one satellite’s transmission from another. ⚫ One method to facilitate this is the assignment of one particular week of the 37-week-long P code to each satellite. ⚫ For example SV 14 is so named because it broadcasts the 14th week of the P code. ⚫ The encrypted P code is called the P(Y) code. It is done to prevent spoofing from working. Spoofing is generation of false transmissions masquerading as the Precise Code. ⚫ The P(Y) has been joined by a new military signal called the M-code. 30 15  1/29/2016 C/A Code ⚫ The C/A code is generated at 1.023 million bits per second, 10 time slower than the P code. ⚫ Satellite identification is quite straightforward. Not only does each satellite broadcast a completely unique C/A code on its L1 frequency( and on L1 alone), but also the C/A code is repeated every millisecond. ⚫ The legacy C/A code is broadcast on L1 only. It used to be the only civilian GPS code, but no longer, it has been joined by a new civilian signal known as L2C that is carried on L2. 31 Codes modulated on carrier waves 32 16  1/29/2016 SPS and PPS ⚫ The C/A code is the vehicle for the SPS (Standard Positioning Service). ⚫ The P code provides the same service for PPS (Precise Positioning Service). Is Military GPS More Accurate Than Civilian GPS? The accuracy of the GPS signal in space is actually the same for both the civilian GPS service (SPS) and the military GPS service (PPS). However, SPS broadcasts on one frequency, while PPS uses two. This means military users can perform ionospheric correction, a technique that reduces radio degradation caused by the Earth's atmosphere. With less degradation, PPS provides better accuracy than the basic SPS. http://www.gps.gov/systems/ 33 Latest report on Range, Horizontal and Vertical Accuracy 34 17  1/29/2016 35 36 18  1/29/2016 New GPS Signal and CNAV ⚫ The US government is in the process of fielding three new signals designed for civilian use: L2C, L5, and L1C. ⚫ The legacy civil signal, called L1 C/A or C/A at L1, will continue broadcasting in the future, for a total of four civil GPS signals. ⚫ Users must upgrade their equipment to benefit from the new signals. 37 The Production of a Modulated Carrier Wave ⚫ EDM Ranging Two-way ranging The EDM is both the transmitter and the receiver of the signal ⚫ GPS Ranging One-way ranging Unlike an EDM, a GPS signal cannot be analyzed at its point of origin The elapsed time is measured by 2 clocks: one in the satellite and another in the receiver. Perfect synchronization of clocks is physically impossible. A discrepancy of 1 microsecond can create a range error of 300 meters. 38 19  1/29/2016 Phase angles Phase angles are important to the modulation of the carrier by phase that is the method of attaching the codes to the GPS carriers. 0, 90, 180, 270, and 360 are known as phase angles in a single wavelength. The oscillators in the EDM or in the GPS satellite create very constant wavelengths, because like clocks or oscillators, they're known as frequency standards. They create electromagnetic energy that has a very constant wavelength. Therefore the phase angles occur at definite distances. 39 Oscillators ⚫ Time measurement devices used in GPS are not really clocks. ⚫ They are more correctly called oscillators or frequency standards. ⚫ They don’t produce a steady series of ticks. ⚫ They keep time by chopping a continuous beam of electromagnetic energy at extremely regular intervals. ⚫ The result is a steady series of wavelengths and the foundation of the modulated carrier. 40 20  1/29/2016 EDM Distance Determination 41 42 21  1/29/2016 A Chain of Electromagnetic Energy ⚫ GPS oscillators are sometimes called clocks because the frequency of a modulated carrier, measured in hertz, can indicate the elapsed time between the beginning and end of a wavelength. ⚫ The length is approximately: where λ=the length of each complete wavelength (m) ca=the speed of light corrected for atmospheric effects f=frequency in hertz 43 ⚫ For example, if an EDM transmits a modulated carrier with a frequency of 9.84MHz and the speed of light is approximately 300,000,000 m/s ( a more accurate value is 299,792,458 m/s), then: 44 22  1/29/2016 Phase Shift ⚫ In an EDM the fractional part is determined by comparing the returning signal to that of a replica of the transmitted signal to determine the phase shift. ⚫ It is important to remember that points on a modulated carrier are defined by phase angles 0⁰, 90⁰,180⁰, 270⁰, etc. ⚫ When two waves reach exactly the same phase angle they said to be in phase, coherent or phase locked. When two waves reach the same angle at different times, they are out of phase or phase shifted. In GPS, the process is called carrier phase ranging and the measurement is done on the carrier itself. 45 The Cycle Ambiguity Problem ⚫ While the determination of the fractional part of a wavelength can be solved, the number of full wavelengths is still a problem. ⚫ GPS ranging applies a different technique for solving the cycle ambiguity problem because the satellites broadcast only 2 carriers of constant wavelengths, in one direction from the satellites to the receivers. 46 23 2/17/2016 1 2/17/2016 2 2/17/2016 3 2/17/2016 4 2/17/2016 5 2/17/2016 6 2/17/2016 7 2/17/2016 8 2/17/2016 9 2/17/2016 10 2/17/2016 11 2/17/2016 12 2/17/2016 13 Global Navigation Satellite Systems SATELLITE GEODESY Objectives At the end of the lecture the students should be able to: Explain how GPS/GNSS works Identify errors present in GPS. Enumerate some applications of using GPS. Perform a simple exercise in way-finding and navigation. Outline Definition How does GPS Work? Parts of the System Errors in GPS Differential GPS Applications  Where am I? The basic idea of GPS navigation can be traced back centuries to the first explorer who asked the question, "Where am I?"  The Navigation Problem The ancient question: Where am I? Earth coordinates: latitude and longitude Manila: N14º/E121º Latitude can be determined by Sun angle What about longitude? Latitude and Longitude Latitude and Longitude GNSS Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few meters using time signals transmitted along a line-of-sight by radio from satellites Initial Operational Capability - December 8,1993 Full Operational Capability declared by the US Secretary of Defense at 00:01 hours on July 17, 1995 GNSS derived information Time Location – Latitude – Longitude – Elevation Derived – Speed – Direction – Stored Locations – Routes GNSS around the world GPS is a “brand name”. Current Functional GNSS: e.g. – GPS GNSS = – GLONASS Toothpaste GPS = Colgate – Galileo – COMPASS GPS Official name is NAVSTAR (Navigation System with Timing And Ranging). The Global Positioning System (GPS) was developed by the U. S. Department of Defense (DOD), Ivan Getting, and the Massachusetts Institute of Technology (MIT). Originally consisting of 11 orbiting satellites, the GPS was launched by DOD in 1978 strictly for military use. ГЛОНА СС GLONASS (Russian: ГЛОНАСС, abbreviation of ГЛОбальная НАвигационная Спутниковая Система; tr.: GLObal'naya NAvigatsionnaya Sputnikovaya Sistema; "GLObal NAvigation Satellite System" in English) is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces. Galileo Galileo is a GNSS currently being built by the European Union (EU) and European Space Agency (ESA). The €3.4 billion project is named after the famous Italian astronomer Galileo Galilei. One of the political aims with Galileo is to provide a high-accuracy positioning system upon which European nations can rely independent from the Russian GLONASS and US GPS systems which can be disabled for commercial users in times of war or conflict. COMPASS The COMPASS system (also known as Beidou-2, BD2) is a project by China to develop an independent global satellite navigation system. The new system will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites and 30 medium Earth orbit (MEO) satellites, that will offer complete coverage of the globe. The general designer of Compass navigation system is Sun Jiadong, who is also the general designer of its predecessor, Beidou navigation system. GPS Segments Space Segment: the constellation of satellites Control Segment: control the satellites User Segment: users with receivers Space Segment System consists of 24 satellites in the operational mode Altitude: 20,200 km with periods of 12 hr. Satelliete Generations/Blocks: I (testing), II/IIA, IIR (current), IIF (for replenishment) Block IIR- $25,000,000; 2000 kg Hydrogen Maser Atomic Clocks  Control Segment Track the satellites for orbit and clock determination Time synchronization Upload the Navigation Message Manage DOA  Control Segment Master Control Station is located at the Consolidated Space Operations Center (CSOC) at Schriever Air Force Base near Colorado Springs  User Segment Users - Receivers  GPS Position By knowing how far one is from three satellites one can ideally find their 3D coordinates To correct for clock errors one needs to receive at least four satellites  GPS Signals GPS Satellites transmit microwave carrier signals: – L1 frequency (1575.42 MHz) carries the navigation message and the Standard Positioning Service /SPS code signals. – L2 frequency (1227.60 MHz) is used by Precise Positioning Service /PPS receivers. Three binary codes shift the L1 and/or L2 carrier phase. – C/A Code modulates the L1 carrier phase, provides the basis for the civilian SPS. – P-Code modulates both the L1 and L2 carrier phases, provides the basis for the PPS. – The Navigation Message also modulates the L1-C/A code signal. It is a 50 Hz signal consisting of data bits describing GPS satellite orbits, clock corrections, and other parameters. GPS Signals  GPS Clock Signals Two types of clock signals are transmitted C/A Code - Coarse/Acquisition Code available for civilian use on L1 provides 300 m resolution P Code - Precise Code on L1 and L2 used by the military provides 3m resolution  How GPS works Video  GPS: How does it work? Typical receiver: one channel C/A code on L1 During the “acquisition” time you are receiving the navigation message also on L1 The receiver then reads the timing information and computes the “pseudo-ranges” The pseudo-ranges are then corrected Corrected ranges are used to compute the position This is a very complicated iterative nonlinear equation  GPS Contributory Errors Atmosphere Multi-path Error Signal Obstruction Selective Availability Receiver Clock Satellite Calibration Error Clock Ephemeris Error Atmospheric Delay GPS signals are delayed as they pass through the atmosphere Ion osp Trohpere osp here < 10 km > 10 km  Satellite Mask Angle Atmospheric Refraction is greater for satellites at angles that are low to the receiver because the signal must pass through more atmosphere. There is a trade off between mask angle and atmospheric refraction. Setting high angles will decrease atmospheric refraction, but it will also decrease the possibility of tracking the necessary four satellites.  Multi-Path Errors Occurs when GPS signals are reflected and the receiver detects two signal instead of one at different times. This causes confusion in some low-end GPS units, but is generally easy to correct. High-end receivers compensate for multipath Mapping and Survey units use a hardware solution: a special semi-directional antenna  Multi-path Errorsa signal from a satellite, due to the A delay in receiving signal bouncing off of other objects, such as trees or buildings. Multi-path in the Environment  Signal Obstruction When something blocks the GPS signal. Areas of Great Elevation Differences – Canyons – Mountain Obstruction – Urban Environments Indoors Obstruction  Using Off-Set Positioning ►Used to compensate for obstruction ►Uses compass bearing and an offset distance to calculate the position of the obstructed target. ►Usually makes use of another instrument such as theodolite and tape, or a total station. Denial of Accuracy (DOA) The US military uses two approaches to prohibit use of the full resolution of the system Selective Availability (SA) - noise is added to the clock signal and the navigation message has “lies” in it Anti-Spoofing (AS) - P-code is encrypted with Y code  Selective Availability (S/A) Government introduces artificial errors to reduce GPS position accuracy Discourages hostile forces from using GPS Largest source of error Eventually removed in May 2000 How S/A Works Off-setting satellite clocks. Introduction of ephemeris error by the Space Command control center Only the military has the correction information. S/A Error Error introduced by S/A is up to 70 meters With S/A on you can expect know better than 100 meter accuracy With S/A off you can expect accuracies from 20 to 40 meters Selective Availability Removed! On May 2, 2000, Selective Availability was eventually removed  Differential GPS Differential GPS is an effective way to correct various inaccuracies in the GPS system. Differential GPS or "DGPS" can yield measurements good to a couple of meters in moving applications and even better in stationary situations. That improved accuracy has a profound effect on the importance of GPS as a resource. With it, GPS becomes a universal measurement system capable of positioning things on a very precise scale.  Differential GPS: What errors can be corrected? ✓ Satellite/Receiver clock error ✓ Satellite Ephemeris error ✓ Atmospheric Refraction – Ionospheric Refraction – Tropospheric Refraction Receiver Noise Multipath ✓ Selective Availability  DGPS: How does it Two GPS receivers are used for work? DGPS. A high precision “Base” GPS receiver (Base Receiver or Base Station) is placed at a known “controlled” point of reference such as a National Geodetic Survey marker. This receiver collects GPS signals and compares the results to the actual known coordinate of the Base. A “rover” receiver collects autonomous information in the field.  DGPS: How does it work? Software/Hardware at the base station calculates the difference (differential) between the known position and the GPS position. This differential is an effective measurement of positional offset, in both direction and distance. The differential data can be used to correct the positional errors in the data collected from the Rover GPS receivers in either real-time or after the fact. GPS Error Sources and DGPS Correction GPS Community Base Stations A way to make the GPS even more accurate Works by canceling out most of the natural and man made (S/A) errors  “Public” GPS Stations Public GPS stations: Continuous observation and dissemination of the received signals. By combining the signals from the Electronic GCP with those received by user’s GPS, positional accuracy with several cm order can be achieved on real-time basis. GPS Community Base Station in UP Located on rooftop of Melchor Hall Beneath therooftop Located on base station antenna of Melchor Hall is a (College of Engineering) Coast and Geodetic Survey control point Beneath the base station antenna is a Coast d an Geodetic Survey control point GPS Community Base Station in UP The antenna is connected to receiver -> computer UP GPS Community Base Station  GPS Equipment - Antenna Mounted on a roof, a pole, a truck, or a person.  GPS Equipment - Receivers Each satellite that is tracked requires a "channel" in the receiver. Receivers generally have 6, 8, or 12 channels available. Recreation-grade receivers: designed for casual users (recreationists). Typically uses CA code, and has the least accurate positioning ability. Navigating receivers: same as recreation receivers, designed for portability and long battery life. Mapping-grade receivers: Designed to include more features, allow higher accuracy, and to store more data, as well as attribute data. Typically use L1, L2 code. Survey-grade receivers: Designed for extreme accuracy. Usually larger and heavier than other receiver types. Very expensive. Typically use L1, L2 code. GPS Receivers - Cost How much? Recreation Grade: PhP4200- PhP33,600 dramatic drop in prices over last years Mapping Grade: PhP33,600 - PhP560K Survey Grade: PhP500K - PhP3M GPS Receivers - Performance What kind of positional accuracy can I get? (Horizontal accuracy) Recreation Grade: 1 - 3 meters possible, typically requires post processing or realtime correction. Mapping Grade: 1m - sub-meter possible Survey Grade: 1-5 cm (yes, centimeters!)  Uses of GPS Airplane and Boat Navigation Continental Drift Surveying Precise Timing Iceberg Tracking Archaeological Expeditions Mobile Applications  GPS Applications In Surveying Proper establishment of horizontal/vertical control and their densification Structural deformation studies Airborne photogrammetry Dynamic positioning Navigation for hydrographic survey vessels and dredges Hydraulic study River or floodplain cross-section location Core drilling location Creation of different maps  GPS Applications In Aviation Controlling an unmanned helicopter Enroute navigation Improving safety in air traffic control by allowing air traffic controllers to strategically manage aircraft flight routes in real time according to the current volume of air traffic and collision avoidance GPS Applications In Shipping Oceanic navigation Voyage planning Monitoring and control Coastal navigation Inland waterways navigation Harbor and port entry Vessel traffic management system Fleet management Fishing Zoning Offshore exploration GPS Applications In Land GPS is being used in vehicle tracking and monitoring Vehicle guidance Intelligent road transport Rail monitoring and control Fleet management Road profile recording Law enforcement Search and rescue operations  GPS Applications In Space Satellites and space craft launchings Satellite attitude and orbital determination Re-entry landing In Science Time measurement Atmospheric sounding Seismic study Glacier and crustal movements Astronomical observations GPS Applications In Environmental Management Monitoring and tracking different animal species Locating vegetation transects for the preservation of rain forest biodiversity GPS Applications Integration of GPS with other technologies is also valuable: Data collection systems Telecommunications Photogrammetry Remote sensing Geographic information systems (GIS) GPS Applications GIS  incorporates different tools for input, storage, analysis and display of geographic information.  a vital aspect of the diverse information contained in a GIS is their position GPS is being used to give position to the diverse information contained in a GIS GPS Applications GIS + GPS Utility planning Wastelands Agricultural lands Forest cover Marine life concentrations Wild life and eco-system Disaster management (flood, earthquakes, oil spills, Forest fires) ❑ Plan evacuation routes ❑ Design centers for emergency operations While relief is going on, rescue operations in areas where the communication networks have been destroyed Developed for fire engines, ambulances and other emergency vehicle for monitoring their real time positions before and after crucial situations GPS Position Measurement Systems Land Survey □ A GPS land survey position measurement system usually incorporates three major components: integrated GPS receiver unit rugged data collector data collection software Commercial products available. Developments in GPS Instrumentation Total station equipped with a Global Positioning System (GPS) receiver Able to (re)locate points without need for reference points Agricultural Applications Precision agriculture has made a need to combine GPS and GIS for various applications. These technologies enable real-time data collection with accurate position information and efficient manipulation and analysis of large amounts of spatial data. This information can be used to improve management in farm operation and resources. GPS Position Measurement Systems GPS-based Mapping and GIS Systems are today's most exciting GPS applications Commercial Integrated high-performance GPS receiver and antenna. Commercial software available for planning GPS mapping, differentially processing GPS data, GIS output, plotting, and data dictionary creation.  References ► B. Hofmann-Wellenhof, H. Lichtenegger, and ► J. Collins. GPS: Theory and Practice. Third Edition. Springer-Verlag. 1994. ► T. Logsdon. The Navstar Global Positioning System. Van Nostrand. 1992. ► A. Leick. GPS Satellite Surveying. Second Edition. Wiley. 1995. ► Enrico C. Paringit. Global Positioning Systems Lecture Notes.  References ► T. A. Herring, "The Global Positioning System," Scientific American, pp. 44-50, February 1996. ► N. J. Hotchkiss, A Comprehensive Guide to Land Navigation with GPS, Alexis, 1994. Special Edition on the Global Positioning System, Satellite Times, March/April 1996. ► D. Sobel, Longitude, Walker, 1995.  Web Sites GPS Program Office: http://www.laafb.af.mil/SMC/CZ/homepage/ US Coast Guard Navaigation Center http://www.navcen.uscg.mil/default.htm GPS Precise Orbits http://www.ngs.noaa.gov/GPS/GPS.html GPS World Magazine http://www.gpsworld.com/ 2/17/2016 1 2/17/2016 2 2/17/2016 3 2/17/2016 4 2/17/2016 5 2/17/2016 6 2/17/2016 7 2/17/2016 8 2/17/2016 9 2/17/2016 10 2/17/2016 11 2/17/2016 12 2/17/2016 13 Global Navigation Satellite Systems SATELLITE GEODESY Objectives At the end of the lecture the students should be able to: Explain how GPS/GNSS works Identify errors present in GPS. Enumerate some applications of using GPS. Perform a simple exercise in way-finding and navigation. Outline Definition How does GPS Work? Parts of the System Errors in GPS Differential GPS Applications  Where am I? The basic idea of GPS navigation can be traced back centuries to the first explorer who asked the question, "Where am I?"  The Navigation Problem The ancient question: Where am I? Earth coordinates: latitude and longitude Manila: N14º/E121º Latitude can be determined by Sun angle What about longitude? Latitude and Longitude Latitude and Longitude GNSS Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few meters using time signals transmitted along a line-of-sight by radio from satellites Initial Operational Capability - December 8,1993 Full Operational Capability declared by the US Secretary of Defense at 00:01 hours on July 17, 1995 GNSS derived information Time Location – Latitude – Longitude – Elevation Derived – Speed – Direction – Stored Locations – Routes GNSS around the world GPS is a “brand name”. Current Functional GNSS: e.g. – GPS GNSS = – GLONASS Toothpaste GPS = Colgate – Galileo – COMPASS GPS Official name is NAVSTAR (Navigation System with Timing And Ranging). The Global Positioning System (GPS) was developed by the U. S. Department of Defense (DOD), Ivan Getting, and the Massachusetts Institute of Technology (MIT). Originally consisting of 11 orbiting satellites, the GPS was launched by DOD in 1978 strictly for military use. ГЛОНА СС GLONASS (Russian: ГЛОНАСС, abbreviation of ГЛОбальная НАвигационная Спутниковая Система; tr.: GLObal'naya NAvigatsionnaya Sputnikovaya Sistema; "GLObal NAvigation Satellite System" in English) is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces. Galileo Galileo is a GNSS currently being built by the European Union (EU) and European Space Agency (ESA). The €3.4 billion project is named after the famous Italian astronomer Galileo Galilei. One of the political aims with Galileo is to provide a high-accuracy positioning system upon which European nations can rely independent from the Russian GLONASS and US GPS systems which can be disabled for commercial users in times of war or conflict. COMPASS The COMPASS system (also known as Beidou-2, BD2) is a project by China to develop an independent global satellite navigation system. The new system will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites and 30 medium Earth orbit (MEO) satellites, that will offer complete coverage of the globe. The general designer of Compass navigation system is Sun Jiadong, who is also the general designer of its predecessor, Beidou navigation system. GPS Segments Space Segment: the constellation of satellites Control Segment: control the satellites User Segment: users with receivers Space Segment System consists of 24 satellites in the operational mode Altitude: 20,200 km with periods of 12 hr. Satelliete Generations/Blocks: I (testing), II/IIA, IIR (current), IIF (for replenishment) Block IIR- $25,000,000; 2000 kg Hydrogen Maser Atomic Clocks  Control Segment Track the satellites for orbit and clock determination Time synchronization Upload the Navigation Message Manage DOA  Control Segment Master Control Station is located at the Consolidated Space Operations Center (CSOC) at Schriever Air Force Base near Colorado Springs  User Segment Users - Receivers  GPS Position By knowing how far one is from three satellites one can ideally find their 3D coordinates To correct for clock errors one needs to receive at least four satellites  GPS Signals GPS Satellites transmit microwave carrier signals: – L1 frequency (1575.42 MHz) carries the navigation message and the Standard Positioning Service /SPS code signals. – L2 frequency (1227.60 MHz) is used by Precise Positioning Service /PPS receivers. Three binary codes shift the L1 and/or L2 carrier phase. – C/A Code modulates the L1 carrier phase, provides the basis for the civilian SPS. – P-Code modulates both the L1 and L2 carrier phases, provides the basis for the PPS. – The Navigation Message also modulates the L1-C/A code signal. It is a 50 Hz signal consisting of data bits describing GPS satellite orbits, clock corrections, and other parameters. GPS Signals  GPS Clock Signals Two types of clock signals are transmitted C/A Code - Coarse/Acquisition Code available for civilian use on L1 provides 300 m resolution P Code - Precise Code on L1 and L2 used by the military provides 3m resolution  How GPS works Video  GPS: How does it work? Typical receiver: one channel C/A code on L1 During the “acquisition” time you are receiving the navigation message also on L1 The receiver then reads the timing information and computes the “pseudo-ranges” The pseudo-ranges are then corrected Corrected ranges are used to compute the position This is a very complicated iterative nonlinear equation  GPS Contributory Errors Atmosphere Multi-path Error Signal Obstruction Selective Availability Receiver Clock Satellite Calibration Error Clock Ephemeris Error Atmospheric Delay GPS signals are delayed as they pass through the atmosphere Ion osp Trohpere osp here < 10 km > 10 km  Satellite Mask Angle Atmospheric Refraction is greater for satellites at angles that are low to the receiver because the signal must pass through more atmosphere. There is a trade off between mask angle and atmospheric refraction. Setting high angles will decrease atmospheric refraction, but it will also decrease the possibility of tracking the necessary four satellites.  Multi-Path Errors Occurs when GPS signals are reflected and the receiver detects two signal instead of one at different times. This causes confusion in some low-end GPS units, but is generally easy to correct. High-end receivers compensate for multipath Mapping and Survey units use a hardware solution: a special semi-directional antenna  Multi-path Errorsa signal from a satellite, due to the A delay in receiving signal bouncing off of other objects, such as trees or buildings. Multi-path in the Environment  Signal Obstruction When something blocks the GPS signal. Areas of Great Elevation Differences – Canyons – Mountain Obstruction – Urban Environments Indoors Obstruction  Using Off-Set Positioning ►Used to compensate for obstruction ►Uses compass bearing and an offset distance to calculate the position of the obstructed target. ►Usually makes use of another instrument such as theodolite and tape, or a total station. Denial of Accuracy (DOA) The US military uses two approaches to prohibit use of the full resolution of the system Selective Availability (SA) - noise is added to the clock signal and the navigation message has “lies” in it Anti-Spoofing (AS) - P-code is encrypted with Y code  Selective Availability (S/A) Government introduces artificial errors to reduce GPS position accuracy Discourages hostile forces from using GPS Largest source of error Eventually removed in May 2000 How S/A Works Off-setting satellite clocks. Introduction of ephemeris error by the Space Command control center Only the military has the correction information. S/A Error Error introduced by S/A is up to 70 meters With S/A on you can expect know better than 100 meter accuracy With S/A off you can expect accuracies from 20 to 40 meters Selective Availability Removed! On May 2, 2000, Selective Availability was eventually removed  Differential GPS Differential GPS is an effective way to correct various inaccuracies in the GPS system. Differential GPS or "DGPS" can yield measurements good to a couple of meters in moving applications and even better in stationary situations. That improved accuracy has a profound effect on the importance of GPS as a resource. With it, GPS becomes a universal measurement system capable of positioning things on a very precise scale.  Differential GPS: What errors can be corrected? ✓ Satellite/Receiver clock error ✓ Satellite Ephemeris error ✓ Atmospheric Refraction – Ionospheric Refraction – Tropospheric Refraction Receiver Noise Multipath ✓ Selective Availability  DGPS: How does it Two GPS receivers are used for work? DGPS. A high precision “Base” GPS receiver (Base Receiver or Base Station) is placed at a known “controlled” point of reference such as a National Geodetic Survey marker. This receiver collects GPS signals and compares the results to the actual known coordinate of the Base. A “rover” receiver collects autonomous information in the field.  DGPS: How does it work? Software/Hardware at the base station calculates the difference (differential) between the known position and the GPS position. This differential is an effective measurement of positional offset, in both direction and distance. The differential data can be used to correct the positional errors in the data collected from the Rover GPS receivers in either real-time or after the fact. GPS Error Sources and DGPS Correction GPS Community Base Stations A way to make the GPS even more accurate Works by canceling out most of the natural and man made (S/A) errors  “Public” GPS Stations Public GPS stations: Continuous observation and dissemination of the received signals. By combining the signals from the Electronic GCP with those received by user’s GPS, positional accuracy with several cm order can be achieved on real-time basis. GPS Community Base Station in UP Located on rooftop of Melchor Hall Beneath therooftop Located on base station antenna of Melchor Hall is a (College of Engineering) Coast and Geodetic Survey control point Beneath the base station antenna is a Coast d an Geodetic Survey control point GPS Community Base Station in UP The antenna is connected to receiver -> computer UP GPS Community Base Station  GPS Equipment - Antenna Mounted on a roof, a pole, a truck, or a person.  GPS Equipment - Receivers Each satellite that is tracked requires a "channel" in the receiver. Receivers generally have 6, 8, or 12 channels available. Recreation-grade receivers: designed for casual users (recreationists). Typically uses CA code, and has the least accurate positioning ability. Navigating receivers: same as recreation receivers, designed for portability and long battery life. Mapping-grade receivers: Designed to include more features, allow higher accuracy, and to store more data, as well as attribute data. Typically use L1, L2 code. Survey-grade receivers: Designed for extreme accuracy. Usually larger and heavier than other receiver types. Very expensive. Typically use L1, L2 code. GPS Receivers - Cost How much? Recreation Grade: PhP4200- PhP33,600 dramatic drop in prices over last years Mapping Grade: PhP33,600 - PhP560K Survey Grade: PhP500K - PhP3M GPS Receivers - Performance What kind of positional accuracy can I get? (Horizontal accuracy) Recreation Grade: 1 - 3 meters possible, typically requires post processing or realtime correction. Mapping Grade: 1m - sub-meter possible Survey Grade: 1-5 cm (yes, centimeters!)  Uses of GPS Airplane and Boat Navigation Continental Drift Surveying Precise Timing Iceberg Tracking Archaeological Expeditions Mobile Applications  GPS Applications In Surveying Proper establishment of horizontal/vertical control and their densification Structural deformation studies Airborne photogrammetry Dynamic positioning Navigation for hydrographic survey vessels and dredges Hydraulic study River or floodplain cross-section location Core drilling location Creation of different maps  GPS Applications In Aviation Controlling an unmanned helicopter Enroute navigation Improving safety in air traffic control by allowing air traffic controllers to strategically manage aircraft flight routes in real time according to the current volume of air traffic and collision avoidance GPS Applications In Shipping Oceanic navigation Voyage planning Monitoring and control Coastal navigation Inland waterways navigation Harbor and port entry Vessel traffic management system Fleet management Fishing Zoning Offshore exploration GPS Applications In Land GPS is being used in vehicle tracking and monitoring Vehicle guidance Intelligent road transport Rail monitoring and control Fleet management Road profile recording Law enforcement Search and rescue operations  GPS Applications In Space Satellites and space craft launchings Satellite attitude and orbital determination Re-entry landing In Science Time measurement Atmospheric sounding Seismic study Glacier and crustal movements Astronomical observations GPS Applications In Environmental Management Monitoring and tracking different animal species Locating vegetation transects for the preservation of rain forest biodiversity GPS Applications Integration of GPS with other technologies is also valuable: Data collection systems Telecommunications Photogrammetry Remote sensing Geographic information systems (GIS) GPS Applications GIS  incorporates different tools for input, storage, analysis and display of geographic information.  a vital aspect of the diverse information contained in a GIS is their position GPS is being used to give position to the diverse information contained in a GIS GPS Applications GIS + GPS Utility planning Wastelands Agricultural lands Forest cover Marine life concentrations Wild life and eco-system Disaster management (flood, earthquakes, oil spills, Forest fires) ❑ Plan evacuation routes ❑ Design centers for emergency operations While relief is going on, rescue operations in areas where the communication networks have been destroyed Developed for fire engines, ambulances and other emergency vehicle for monitoring their real time positions before and after crucial situations GPS Position Measurement Systems Land Survey □ A GPS land survey position measurement system usually incorporates three major components: integrated GPS receiver unit rugged data collector data collection software Commercial products available. Developments in GPS Instrumentation Total station equipped with a Global Positioning System (GPS) receiver Able to (re)locate points without need for reference points Agricultural Applications Precision agriculture has made a need to combine GPS and GIS for various applications. These technologies enable real-time data collection with accurate position information and efficient manipulation and analysis of large amounts of spatial data. This information can be used to improve management in farm operation and resources. GPS Position Measurement Systems GPS-based Mapping and GIS Systems are today's most exciting GPS applications Commercial Integrated high-performance GPS receiver and antenna. Commercial software available for planning GPS mapping, differentially processing GPS data, GIS output, plotting, and data dictionary creation.  References ► B. Hofmann-Wellenhof, H. Lichtenegger, and ► J. Collins. GPS: Theory and Practice. Third Edition. Springer-Verlag. 1994. ► T. Logsdon. The Navstar Global Positioning System. Van Nostrand. 1992. ► A. Leick. GPS Satellite Surveying. Second Edition. Wiley. 1995. ► Enrico C. Paringit. Global Positioning Systems Lecture Notes.  References ► T. A. Herring, "The Global Positioning System," Scientific American, pp. 44-50, February 1996. ► N. J. Hotchkiss, A Comprehensive Guide to Land Navigation with GPS, Alexis, 1994. Special Edition on the Global Positioning System, Satellite Times, March/April 1996. ► D. Sobel, Longitude, Walker, 1995.  Web Sites GPS Program Office: http://www.laafb.af.mil/SMC/CZ/homepage/ US Coast Guard Navaigation Center http://www.navcen.uscg.mil/default.htm GPS Precise Orbits http://www.ngs.noaa.gov/GPS/GPS.html GPS World Magazine http://www.gpsworld.com/