global positioning systems (gps): timesitotaw/site/fall2007_files/lecture15.pdf · global...

ENGI 3703Surveying and Geomatics

TopicInstructor: Prof. Ken Snelgrove

Lect 15 - Oct 22/07 Slide 1 of 10

Global Positioning System(GPS)

Introduction and Course Measurement

Global Positioning Systems (GPS): (Chapter 13 &14). The advancement of computing and spacetechnology is changing the role of the traditional surveyor. Today, the discipline is known as Geomaticsrather than Surveying in part to recognize these technology changes. GPS is certainly changing the waythat point positions are located and set out on and above earth’s surface. Rather than measuring anglesand distances using tapes, theodolities and levels, we measure time. The essence of GPS is themeasurement of precise timing changes between pulses of electro-magnetic (EM) waves. These time lagsare multiplied by the speed of light to determine the distance from the GPS receiver to a satellite vehicle(SV). Distance measurement from three separate SVs is sufficient to determine geometrically your locationby resection.

“Rudolph needs a GPS”(or a hearing aid)






GPS History: (TRANSIT) Our modern day GPS system was developed and put into place by the USMilitary. Prior to modern day GPS a US Navy system called TRANSIT was developed to allow ballisticmissile submarines to determine their position anywhere in the world, in any weather, day or night in a‘receive-only’ mode of communication (so as not to give away the submarines position). This satellitesystem was developed in the early 1960’s and was retired in 1996.

The system operated with 5 satellites (together with spares) that orbited at 1100 km. Observation of aconstant frequency radio message from a single satellite was required for a 2 minute period. Over that timea unique Doppler pattern trace would be recorded that could be matched to the theoretical pattern given aSV traveling at 17,000 mph moving N-S in a polar orbit. Once matched the pattern provided the range anddirection to the SV. This combined with the precise location of the SV (monitored from ground stations andbroadcast via the satellite) allowed the receiver to be located to within 100m. Ship motion presented asource of error since that added to or subtracted from the satellite based Doppler shift.

Sate

llite M

otio

n (N

-S)

Earth Motion (E-W)

SV

First Transit SatellitePolaris Submarine






GPS History: (LORAN) LORAN (LOng RAnge Navigation) is a radio navigation system developed duringWWII. It is a ground based system that relies on the transmission of long-wave radio pulses (~100 kHz)from land based stations. Controlled by a master station, a LORAN station in a chain of stations sends apulse of radio energy after a precise time lag from receipt of the master signal. The signals received fromtwo stations (A&B below) can be used to produce lines of constant time difference between distantstations. Over lap of these lines from another station provides grid intersections from which absolutepositions can be determined to within 500 m.

LORAN suffers from weather and sea conditions which affect the speed of wave travel. It also suffersfrom a lack of global coverage.

LORAN-C Coverage Map

LORAN lines of constant time differencebetween two stations A & B






LORAN Tower Cape Race Newfoundland: The original LORAN-Ctower was located near Cape Race and was built in 1965. At that timeand until the construction of the CN Tower (1815 ft high) in 1976 in wasthe tallest structure in Canada at 1350 ft. It collapsed in Feb, 1993 dueto a fatigue failure of a guy wire eye bolt.

It was replaced with the present day tower 854 ft tower later in 1993.

LORAN, while still in operation, is being targeted for phase out after2007 due to the availability superior GPS technology. Critics of GPScite that LORAN should remain as a back-up to GPS since the USmilitary can choose to block GPS signals at any time.

700 ft Loran Tower(Not the Cape Race Tower)

Loran-C Tower Location near Cape Race






GPS Overview: (Section 13.2). The GPS system became operational in December, 1993 afterdevelopment and testing that saw the first satellite launch in 1978. NAVSTART GPS was meant initially formilitary use only. However, the shooting down of Korean Airline (Flight 007) in 1983 after it wandered intoSoviet Union airspace pointed to the need to allow civilian use. Initial civilian use was hampered by theinsertion of a seemly random error known as Selective Availability (SA) that introduced a 100 m horizontalerror. This was turned-off briefly during the First Gulf War between 1990-1991 because insufficientnumbers of military grade GPS receivers were available for field troops. SA was reintroduced on July 1,1991 but was removed permanently on May 1, 2000 to allow civilian users 20 m positioning accuracy.

The system is described with three components: a) space segment, b) control segment, c) user segment.






Space Segment: (Section 13.2). At least 24 satellites orbit the earth on 6 orbital planes (4 on each plane)inclined at 55º to the equator at an altitude of 20,200 km. This guarantees that at least 4 SV can be seen atany point on the earths surface. The 55º incline avoids over concentration of SVs at the poles as wouldpolar orbiting satellites. Each SV carries a precise atomic clock that effectively allows the entire system tooperate with a common time to the nearest 3 nanoseconds. A number of spare SVs also orbit with 31satellites broadcasting today. See: ftp://tycho.usno.navy.mil/pub/gps/gpsb2.txt

GPS Block IIR SVGPS Constellation (to scale)

GPS Atomic Clock






GPS Monitoring Location

Hawaii Monitoring Station

Control Segment: (Section 13.2). The SVs are controlled and monitored from the grounded stations onthe map below. Here the precise location of the SVs are tracked by 5 original monitoring stations andreported to the Master Control Station in Colorado Springs. Here the SVs receive information on theirorbital parameters and clock corrections (to with 10-9 sec) which are then rebroadcast to users. 6 additionalstations were added in September, 2005 to improve tracking and 5 more are due to be added so that eachsatellite can be monitored from 3 independent locations. The total operating budget for the US NAVSTARGPS system is ~$750M annually. The Russian government operate separate system called GLONASS andthe EU has much delayed plans to introduce a third civilian system called Galileo.

Since these corrections are only broadcast to the satellites twice per day, small errors can occur.Retrospective corrections are released after 13 hours with final orbit corrections released after 8-10 days.These are used to post-process very accurate GPS collected survey data. See: http://www.ngs.noaa.gov/orbits/






User Segment: (Section 13.2). The user segment refers to GPS receivers which monitor the SVs andreport receiver positions. Two general services are available: i) Standard Position Services (SPS) whichwas intended to provide 100 m horizontal accuracy and ii) Precise Positioning Service (PPS) which wasintended for military use only to provide a horizontal accuracy of 18 m.

Many other receivers, beyond those originally intended, have been developed which monitor satellitesignals and rebroadcast error data. These offer horizontal accuracy as low as the centimeter level.






GPS Signals: (Section 13.3). GPS send two signals from satellite: ii) the L1 signal containing theNavigation Message, the C/A code and the P-code and ii) the L2 signal which carries all but the C/A code.All of these are based on a fundamental frequency of 10.23 Mhz (~30 m) and are modulated onto highfrequency the L1 (19 cm wavelength) and L2 (25 cm wavelength) carrier waves.

Navigation Message - contain the SV number, orbital (aka ephemeris) and time (aka almanac) data andcorrections as well as SV health and ionosphere corrections.

C/A Code (Coarse/Aquistion Code) - known as a pseudorandom noise (PRN) code. These codes areunique to each satellite and are the essence of distance measurement. C/A codes are 1023 bits long andrepeat once per millsecond. The wavelength of the C/A is ~300 m.

P-Code - (Precise Code) are only available to military users. They are also PRN but are 10 times shorterthan C/A code at ~30 m to allow move accurate matching. These repeat every 22 days.

Phase Modulation of Binary Data(interrupt and 180° phase shift)






Distance Determination: (Section 13.3). The PRN codes are square waves modulated onto the carrierwaves. The PRN codes look essentially random but have a known pattern unique to each satellite that isstored in all GPS receivers. The precise clock in the GPS satellite generate these PRN codes which aretransmitted to GPS receivers on earth. The receiver simultaneously generates the same code as thatgenerated by the satellite. However, a time lag occurs due to the the transmission of the code over the +/-20,200 km distance to the receiver. This lag is precisely determined by matching the satellite code to thereceiver code. The amount of the shift is measured as a time shift and converted to a distance bymultiplying by the speed of light. Hence the distance between the SV and receiver is known.

Matching process

Receiver PRN Code

Satellite PRN Code

Time Delay

global positioning systems (gps): timesitotaw/site/fall2007_files/lecture15.pdf · global...

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