GPS Signal PDF

1/29/2016 1 The GPS Satellites Block II/IIA/IIR/IIR-M satellite Availability of at least 24 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