Gps, Gagan & Lbs

GPS, GAGAN & LBS

M.R.Sivaraman Retd. Scientist Space Applications Centre Ahmedabad

Introduction •

The major inventions of 20th Century, that has changed our way of life in 21st Century are 1. 2. 3. 4. 5. 6.

Global Positioning System (GPS) Satellite Remote Sensing Internet Cellular Phone Micro Processors VLSI Technology

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They have enabled Hand Held Location Based (LBS) Communication Terminals, that is as much as a Wrist Watch and a Car or a Scooter, for our existence in this modern Competitive World.

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My talk today will be basically on “Principle of GPS, GAGAN and LBS”

•

GPS and GAGAN are basically Satellite Navigation Systems, which enable position determination

What Is Navigation ? •

“Navigation” is the process of Planning, Reading and Controlling the movement of a craft or a vehicle from one place to another.

•

The word “Navigate” is derived from Latin roots “navis” meaning “ship” and “agere” meaning “to move” or “to direct”.

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All navigational techniques involve locating the navigator’s position compared to known locations or patterns.

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It deals with the art of directing any vehicle in Land, Sea or Space.

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It is characterized by four main aspects viz.

1. 2. 3. 4.

Determination of destination Choice of a suitable route Estimation of Course and Speed Regular or continuous monitoring the progress of the vehicle.

What does a Satellite Navigation System Provide • It provides 1. Position – Latitude, Longitude and Height 2. Velocity – Velocity North, East and Up 3. Time – in UTC (Universal Time Coordinated) • To any user Receiver in Land, air or sea • This is often abbreviated as PVT

?

Earth Centered Earth Fixed (ECEF) Coordinate System WGS-84 Parameters a = 6378137 m, 1/f = 298.257223563

Categories of Air Navigation •

There are four categories of Air Navigation :

1. 2. 3.

Enroute : Encompasses Point to point Navigation, say city to city. Departure : It is the portion of the flight from take off to enroute phase. Approach/Landing : During this phase, the pilot locates the runway, aligns itself with the centre of the runway and lands. Terminal : It refers to the phase of transition between the enroute and the approach.

4. •

The RNP requirements during Approach/Landing Phase are most stringent.

•

The Approach/Landing phase is divided into

1. 2.

Precision Approach Non Precision Approach

Standard Airport Landing Pattern

Different Phases of Flight while landing • Landing generally involve five phases of flight. They are 1. Arrival : Where the pilot navigates to the Initial Approach Fix (IAF: a navaid or reporting point), and where holding can take place. 2. Initial Approach : The phase of flight after the IAF, where the pilot commences the navigation of the aircraft to the Final Approach Fix (FAF), a position aligned with the runway, from where a safe controlled descent back towards the airport can be initiated. 3. Intermediate Approach : An additional phase in more complex approaches that may be required to navigate to the FAF.

Different Phases of Flight while landing 4.

Final Approach : Between 4 and 12 Nautical Miles of straight flight descending at a set rate (usually an angle of between 2.5 and 6 degrees).

5.

Missed Approach : An optional phase; should the required visual reference for landing not have been obtained at the end of the final approach, this allows the pilot to climb the aircraft to a safe altitude and navigate to a position to hold for weather improvement or from where another approach can be commenced.

Different stages of Aircraft Landing

Precision & Non Precision Approach • Approaches are classified as either precision or nonprecision, depending on the accuracy and capabilities of the navigation aids (navaids) used. • Precision approaches utilize both lateral (localizer) and vertical (glideslope) information. Nonprecision approaches provide lateral course information only. • Some of the airports, where the air traffic is dense (where, one flight lands or takes off at least once every five minutes), Precision Approach Standards is important to be implemented. • On the other hand, at airports where the air traffic is not so dense (may be a few landings and take offs per day), it is enough to implement Non-Precision Approach Standards. • As defined by ICAO (International Civil Aviation Organisation), responsible for implementing Safe Air Navigation, Precision Landing Systems are separated into three Categories of Operation, Category I, II and III (Cat I, II & III). • Cat III operations further classified into three viz. Cat IIIA, IIIB, IIIC.

Definition of Cat I, Cat II & cat III Landings • Category I operations have decision heights above 200’ with visibility greater than 800 m. • Category II operations have decision heights between 200’ and 100’ and at least 300m of runway visual range. • Category IIIA operations have decision heights between 100’ and 0’with runway visual range of 200m or more. • Category IIIB operations have decision heights between 50’ and 0’, and runway visual ranges between 200 and 50 m. • Finally Category IIIC has no limitations on either decision height (0’) or visibility (0 m).

Dimensions of Tunnels for Cat I, II & III Navigation

NAVIGATION STANDARDS

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Accuracy : Degree of conformance of an aircraft’s measured position with its true position

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Integrity : Ability to provide timely warning of Navigation Aid Failure

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Availability: Probability that the system will meet the Accuracy & Integrity requirements for a specific phase of a flight

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Continuity: Probability that a service will continue to be available for a specified period of time

Required Aircraft Navigation Performance Typical Operation Scenario

Time to Alert Accuracy

Integrity

En Route

3.7 km (H)

1-10-7 /hr

5 min

1-10-4 /hr to 0.99 to 1-10-8 /hr 0.99999

Terminal

0.74 km (H)

1-10-7 /hr

15 sec

1-10-4 /hr to 0.999 to 1-10-8 /hr 0.99999

Initial App., NPA & Dep.

220 m (H)

1-10-7 /hr

10 sec

1-10-4 /hr to 0.99 to 1-10-8 /hr 0.99999

APV-I

220 m (H) 20 m (V)

1-2x10-7 per Approach

10 sec

1-8x10-6 /hr 0.99 to in any 15 0.99999 sec

APV-II & CAT I

16 m (H) 8 m (V)

1-2x10-7 per Approach

6 sec

1-8x10-6 /hr 0.99 to in any 15 0.99999 sec

Continuity Availability

Cat I, II & III Requirements

Requirements

Cat I

Cat II

Cat III

Vertical Position Accuracy

7.7 to 4.4 m (200’ to 100’)

2.0 m (110’ to 50’)

2.0 m (100 ’ to 0’)

1-(2x10-7)/App.

10-9/App.

10-9/App.

2s

2s

Integrity Time to Alert Continuity

6s 1-(8x10-6)/15 s

1-(2x10-6)/15 s 1-(2x10-6)/15 s

Major Satellite Navigation Systems •

There are three major Satellite Navigation Systems viz.

1.

The Global Positioning System (GPS) – US Department Of Defense –Declared operational in 1995 – Declared by ICAO, as a possible alternative for civil aviation.

2.

The Global Navigation Satellite System (GLONASS) – The Russian Ministry Of Defense – Not yet operational.

3.

GALILEO – European Civil Navigation Satellite System – Still to be launched

Major Elements of a Satellite Navigation System

GPS Space Segment

• • • • • • • •

Consists of 24 satellites in circular orbits In 6 orbital planes equispaced 600 a part Period of 12 hours siderial time Semimajor axis of 26500 kms Four satellites in one plane, unevenly placed Eccentricity < 0.02 Inclination 550 This constellation allows atleast 4 satellites to be visible to a user on the earth, anywhere anytime (typically 6-8 satellites are visible at a time).

Orbital Configuration of GPS

Block IIR Satellite

GPS Satellite Payload • The satellite carries an Atomic Clock that has a stability of 10-13 and it provides precise time and a reference frequency to generate all signals • It has a CDMA Transmitter at 157.42 MHz and 1227.6 MHz (Commonly known as L1 and L2) • It has a PRN (Pseudorandom Noise Generator) Code generator that generates two codes viz 1.023 MHz C/A Code and 10.23 MHz P Code • It contains a Memory, that stores Orbital Parameters (Commonly called Navigation Data), uploaded from Control Segment

Space Segment •

The Navigation Data is computed and transmitted to the satellites from a ground station, which is a part of the Control Segment, whenever the satellite passes over the ground station.

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This data is stored by the satellite in its memory and the satellite retransmits the data valid at the time of transmission on two frequencies, derived from the atomic clock.

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The satellite also transmits accurate time, derived from the satellite clock.

GPS SIGNAL STRUCTURE Details of signal

Primary

Secondary

Signal Designation

L1

L2

Carrier Frequency (Hz)

1575.42 x 106

1227.60 x 106

PRN Codes (Chips/Sec)

P(Y) =10.23 x 106 & C/A=1.023 x 106

P(Y)=10.23 x 106

Navigation message data modulation

50 Hz BPSK

50 Hz BPSK

Output Transmitter Power

+11.3 dbW & +14.3 dbW

+8.1 dbW

Received Power

-160.0 & -163.0 dbW

-166.0 dbW

GPS Signal Structure • GPS transmits at two frequencies viz. 1575.42 & 1227.6 MHz (Commonly known as L1 & L2) • They are coherently selected multiples of a 10.23 MHz master clock, derived from an atomic standard. • L1 is bi-phase modulated with a 10.23 MHz P code & 1.023 MHz C/A code. • L2 is biphase modulated by 10.23 MHz P code only. • In fact P code & C/A code are modulo 2 added to navigation data, D, to form P⊕D & C/A⊕D before biphase modulating the carriers. • At times, P code is converted to a secure antispoof Y code, P(y) code, before modulo 2 addition with D.

GPS Signal Structure C/A Code • Meant for civil users • Short code with a period of 210 -1 = 1023 bits or 1 ms duration at a 1.023 MHz chip rate • Short code permits rapid acquisition, since there are only 1023 chip time bins to search • Taken from a family of codes known as gold codes • They are formed by the products of two equal period 1023 bits PN codes. • The product code is also 1023 bit period. • It provides good multiple access properties.

GPS Signal Structure

• • •

• • •

P Code Meant for use by DOD authorised users only It is a ± 1 pseudorandom sequence with a clock rate of 10.23 MHz. It is a product of two PN codes, X1 & X2. X1 has a period of 1.5 sec. or 15345000 chips and X2 has a period of 15345037 chips or 37 chips longer. The P code has a period of slightly more than 38 weeks. In GPS, P code is reset every saturday/sunday midnight, so that the period is one week. The P code is difficult to acquire without acquisition aids, because there are 15345000 code chip time bins to search.

GPS Signal Structure • The modulation by C/A & P code are orthogonal to each other. • C/A code is meant for peaceful civil use & popularly known as standard positioning service (SPS). • P code is meant for DoD authorised users only and popularly known as precise positioning service (PPS). • The C/A Code & P code epochs & navigation data are perfectly synchronised.

Space Segment • The PRN code is modulo 2 added to a 50 bit Navigation data (containing the satellite orbital parameters and satellite clock correction data) and then the carrier is biphase modulated by the resultant bit stream. • The carrier, PRN codes and 50 HZ Navigation data are all well synchronized to the satellite time (1 Pulse Per Second, 1 PPS, generated by Satellite Clock). • This signal contains 1. A pure Carrier 2. A PRN Code 3. Navigation Message or Data (Satellite Orbital Parameters). • The Carrier, PRN code and data are perfectly synchronized to the Atomic Clock onboard.

GPS Satellite Signal Generation

GPS NAVIGATION DATA

1. 2. 3. 4. 5. 6. 7. 8. 9.

SYMBOL DEFINITION M0 MEAN ANOMALY AT REFERENCE TIME ∆n MEAN MOTION DIFFERENCE FROM COMPUTED VALUE e ECCENTRICITY OF THE ORBIT A SQUARE ROOT OF THE SEMIMAJOR AXIS Ω0 LONGITUDE OF ASCENDING NODE OF THE ORBIT PLANE AT REFERENCE TIME I0 INCLINATION AT REFERENCE TIME ω ARGUMENT OF PERIGEE Ω DOT RATE OF RIGHT ASCENSION IDOT RATE OF INCLINATION

GPS NAVIGATION DATA SYMBOL DEFINITION 10. Cuc AMPLITUDE OF THE COSINE HARMONIC CORRECTION TERM TO THE ARGUMENT OF LATITUDE 11. Cus AMPLITUDE OF THE SINE HARMONIC CORRECTION TERM TO THE ARGUMENT OF LATITUDE 12. Crc AMPLITUDE OF THE COSINE HARMONIC CORRECTION TERM TO THE ORBIT RADIUS 13. Crs AMPLITUDE OF THE SINE HARMONIC CORRECTION TERM TO THE ORBIT RADIUS 14. Cic AMPLITUDE OF THE COSINE HARMONIC CORRECTION TERM TO THE INCLINATION 15. Cis AMPLITUDE OF THE SINE HARMONIC CORRECTION TERM TO THE INCLINATION 16. Toe REFERENCE TIME FOR EPHEMERIS 17. IODE ISSUE OF DATA EPHEMERIS

Control Segment

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The Control Segment consists of

1. 2.

A worldwide network of satellite tracking stations, A Master Control Station which receives the satellite tracking data from the tracking stations in real time, A Computer Centre collocated with the Master Control Station that computes the future orbital parameters, including their perturbations for next 12 hours (satellite ephemeris data).

3.

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The Master Station also has a precise atomic clock (maintaining a Reference Time), which is used to compare the satellite clocks and determine their offsets and correction parameters.

•

This data is stored and transmitted to the respective satellites by the Control Segment, whenever that satellite passes over.

GPS Control Segment

User Segment • The user Receiver generates a replica of the PRN code, synchronized to its local clock, which is offset with respect to the satellite Atomic Clock by say, Δtu. • The User Receiver matches the PRN Codes received from each satellite (correlates), with its locally generated replica and measures the shift in time required by shifting the locally generated replica to match exactly with the received code. • Assuming that Radio waves travel with velocity of light (which is not true due to the presence of Ionosphere and Troposphere) and by multiplication of time delay by the velocity of light, the range to the satellite can be determined. • Since 4 satellites transmit different PRN codes, the 4 ranges can be determined simultaneously by the Receiver (R1’, R2’, R3’ and R4’).

PRINCIPLE OF GPS

Principle of Pseudorange Measurements

Principle of GPS • Since all the four satellite clocks and the Receiver clock are offset with respect to a Standard Time Reference and there is also propagation delay of the signal due to Ionosphere and Troposphere, this range measured is not the true Geometric range between the satellite and the user. • It is more than that, due to additional delay of the signal while passing through Ionosphere and troposphere. • Hence this is called Pseudorange.

GPS

GPS

C∆ TS2

C∆ TS1

GPS

C∆ TS3

R2

R1

C∆ TS4

R3

C∆ Tu

USER C∆ TA4

C∆ TA3

GPS

C∆ TA1 C∆ TA2

PRINCIPLE OF GPS

R4

Principle of GPS • Pseudorange (Ri’, I = 1,4), can be written as Ri’ = Ri + C Δtai + C (Δtu – Δtsi ) (2) Where Ri’ = Pseudorange measurement to the ith satellite Ri = Geometric range to the ith satellite Δtai = Atmospheric propagation delay (Ionospheric and Tropospheric Propagation delay) Δtu = Receiver clock offset with respect to a Standard Time Reference Δtsi = Satellite clock offset with respect to a Standard Time Reference C = 3 x 108 m/sec • The atmospheric Correction Δtai can be applied using Models (as explained later). • The Satellite Clock Correction Δtsi can be applied using the Navigation data received. • The user clock offset Δtu, is an unknown and thus there are four unknowns viz. Xu, Yu, Zu and Δtu. • So range measurements to four satellites are required to solve for four unknowns.

Principle of GPS • If Xsi , Ysi & Zsi are the ith satellite coordinates (i=1,4) & Xu, Yu, Zu are the user position, then we can write the four observation equations for four satellites at any time t, as recorded by Receiver Clock as (the satellite coordinates should be calculated at corrected receiver clock time t, corrected for signal transit time delay, estimated using the most recent satellite and receiver coordinates known). _____________________ • R1’ = √ (Xs1 -Xu)2+(Ys1 -Yu)2+(Zs1 -Zu)2+CΔtu ______________________ • R2’ = √ (Xs2 -Xu)2+(Ys2 -Yu)2+(Zs2 -Zu)2+CΔtu ______________________ (3) • R3’ = √ (Xs3 -Xu)2+(Ys3 -Yu)2+(Zs3 -Zu)2+CΔtu ______________________ • R4’ = √ (Xs4 -Xu)2+(Ys4 -Yu)2+(Zs4 -Zu)2+CΔtu • Each one of the four equations above is an equation for a Sphere, with radius equal to the Pseudorange measurements. • The four spheres intersect at user location as shown.

Navigation Solution

Principle of Carrier Phase Measurements •

The ranging accuracy achieved using code correlation is limited.

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Typically the accuracy achieved using C/A code is about 30 m and with P code is about 3 m.

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The ranging accuracy can be improved to better than 0.5 m, using Carrier Phase measurements.

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The receiver also has a Phase lock loop, where a locally generated carrier is locked to the received carrier.

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As soon as the satellite acquisition is done, (the received carrier is locked to the locally generated carrier), the instantaneous phase (Φ0) is measured.

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Subsequently, the accumulated carrier (the number of full additional cycles + fraction of the cycle), N1 and Φ1, N2 and Φ2, etc are measured.

Principle of Carrier Phase measurements

Principle of Carrier Phase Measurements GPS

N3+ø3 N2+ø2

Receiver Measures phase of received signal

x

x

N1+ø1

Range (L) = (N2+ø2/2π)λ + No No – Initial Ambiguity

x

ø0 ah Pr ei rr a C

x

to

t1

t2

t3

t4

Time

Receiver

Principle of Carrier Phase Measurements •

At the first instant of measurement, the number of full cycles between the satellite and receiver, N0 cannot be measured and it is known as Integer Ambiguity.

•

Then at each subsequent time of measurements, the satellite ranges are given by (N0+(Φ0/2π))λ, (N1+(Φ1/2π))λ + N0λ, (N2+(Φ2/2π))λ + N0λ, etc, where λ is the wavelength of the received satellite signal.

•

Since the phase measurements can be done to a fraction of a wavelength of the received satellite signal (the wavelength at 1575.42 MHz is about 19 cm), cm level accuracy in ranging can be achieved using Carrier Phase measurements, if Integer Ambiguity can be solved.

•

The Integer Ambiguity N0, can be determined using different techniques.

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The Navigation equations using Carrier Phase measurements can be written as L(Φ) = (N +(Φ/2π))λ + N0 λ = Ri + C Δtai + C (Δtu – Δtsi )

•

where N and Φ are the number of full cycles and fraction of the cycle respectively at any time.

Principle of P Code Receiver • In a P code Receiver, a Dual frequency measurement of Pseudorange measurement can be made. • The dual frequency measurements (in a P Code Dual Frequency Receiver), can be used to remove Ionospheric effects. • Pseudorange measurements at L1 and L2 are given below. Pi1 = Ri1 + C(dtu –dtis)+dtiiono,1 + dtitropo + dqi1 + dqr,1 (1) • Where, Pi1 – Pseudorange measurement at L1(1575.42 MHz) from ith • satellite Ri1 – Geometric distance between ith satellite and Receiver • • C – Velocity of light dtis – Clock offset of ith satellite, from GPS Standard Time • dtu – Receiver clock offset, from GPS reference time • dtiiono,1 – Ionospheric delay at frequency L1 • dtitropo – Tropospheric delay • dqi1 – ith satellite transmitter delay at L1 • dqr,1 – Receiver delay at L1 •

Principle of P Code Receiver •

Similarly for L2 (1227.6 MHz), Pi2 = Ri2 + C(dtu –dtis)+dtiiono,2 + dtitropo + dqi2 + dqr,2 (2)

• • • • • • • • •

Where Pi2 – Pseudorange measurement at L1(1575.42 MHz) from Ith satellite Ri2 – Geometric distance between ith satellite and Receiver C – Velocity of light dtis – Clock offset of ith satellite, from GPS Standard Time dtu – Receiver clock offset, from GPS reference time dtiiono,1 – Ionospheric delay at frequency L2 dtitropo – Tropospheric delay dqi1 – ith satellite transmitter delay at L2

Principle of P Code Receiver •

It can be shown using Appleton-Hartree Equation for Refractive Index that at GPS Frequencies, The Ionospheric delay at L1 = dtiiono,1 = 40.3 TEC / L12 and The Ionospheric delay at L2 = dtiiono,2 = 40.3 TEC / L22

• where TEC is the Total Electron Content in a column of one Sq. m. along the satellite direction from the Receiver. L1 and L2 are the transmitter frequencies, 1575.42 and 1227.6 MHz. • Substituting in equations 1 and 2, we get Pi1 = Ri1 + C(dtu –dtis) + 40.3 TEC/L12 + dtitropo + dqi1 + dqr,1 (3) Pi2 = Ri2 + C(dtu –dtis) + 40.3 TEC/L22 + dtitropo + dqi2 + dqr,2 (4)

Principle of P Code Receiver •

Substracting one equation from the other, we get

Pi1 - Pi2 = 40.3 TEC ((1/L12) – (1/L22)) + (dqi1 + dqr,1 - dqi2 - dqr,2) (5) •

The equation (6) above can be used to determine TEC, Total Electron Content in a column of one Sq. m. along the satellite direction from the Receiver, If dual frequency Pseudorange measurements can be made and the satellite transmitter and Receiver delays at L1 and L2 are known.

•

A Kalman Filter can be used to determine TEC and the Receiver and Transmitter delays and TEC can be determined to an accuracy to about 3 TECU (One TECU (TEC Unit) is equal to 1016 Electrons / m2).

•

1 TECU causes about 0.163 m of delay at L1.

Principle of P Code Receiver •

Once TEC is determined, using the relation dtiiono,1 = 40.3 TEC / L12

•

the Ionospheric correction at L1 can be applied.

•

A similar set of two equations for L1 and L2 can be written for Carrier Phase measurements and used for improved Ionospheric correction. Thus dual frequency measurements can be used to correct Ionospheric delay to an accuracy of about 0.5 m.

•

But in practice, a Kalman Filter cannot be implemented in Dual frequency receivers, to apply Ionospheric delay correction in real time and typical accuracies, achieved without a Kalman Filter are only about 2m.

Error in GPS Solution (Error in GPS Solution) = (Geometry Factor) x (Pseudorange error Factor) = GDOP X σUERE

GPS Error Sources 1.

Selective Availability :

•

The signals transmitted by the GPS satellites can be degraded to limit the accuracy of the system to 100-300 m, to the Civilian users (C/A Code Receivers).

•

This mode of degraded operation of GPS is called the Selective Availability (SA). The accuracy degradation is accomplished through manipulation of the Broadcast Ephemeris data (Orbital error Component) and through dithering of the Satellite Clock (clock error component). The combined magnitude of the two errors is of the order of 30 m (1σ) random. This Selective Availability operation of GPS has been withdrawn now and no more exist in GPS.

•

• •

GPS Error Sources 2.

Satellite Broadcast Ephemeris Error :

•

Satellite Ephemeris errors are differences between the actual satellite location and the location provided by the Satellite Broadcast Ephemeris data (Transmitted Navigation Data), transmitted by the satellite. Normally these errors are about 30-50 m and contribute about 3- 4.5 m, as bias error in UERE.

•

3.

Satellite Clock Errors :

•

Satellite Clock errors are differences between the actual satellite clock time and that predicted by the satellite data. The magnitude of this error is of the order of 3 m and is usually a bias error in UERE

•

GPS Error Sources 4.

Ionospheric Propagation Delay :

•

These are the signal propagation delays caused by electrons present in the region 80 to 1000 kms above earth’s surface known a Ionosphere. The Ionosphere can delay the satellite signals by as much as 30 - 50 m, under the following worst case conditions : during Solar Maximum Period and Magnetic Storms, at low elevation angles and in the afternoon. An algorithm has been suggested by Klobuchar and the values of the coefficients used in the algorithms are transmitted by GPS as part of the Navigation data. Single Frequency C/A code GPS receiver correct for Ionospheric delay using these coefficients and the algorithm suggested by Klobuchar. But this algorithm can cause residual errors in correction of the order of 50 % and so even after this, Ionospheric error of the order of 25 m maximum can still remain as bias errors in UERE (User Equivalent Range Error). Dual Frequency transmission can be used to reduce these errors to less than 2 m

•

• • •

•

GPS Error Sources 5.

Tropospheric Propagation Delay :

•

These are the signal propagation delays caused by the Neutral atmosphere in the height region 0 to 8 kms above earth, known as Troposphere.

•

While these delays are sometimes as much as 10-15 m, at low elevation angles, they are quite consistent and modellable.

•

Various models to correct these errors exist but at about 50 elevation angles, the unmodelled actual error will be around 2-3 m as bias error in UERE.

GPS Error Sources 6.

Multipath Propagation Delay :

•

Multipath errors are introduced at the Receiver from reflected GPS signals received by the antenna.

•

These vary depending on the types of reflective surfaces around the Receiver (for example water, Concrete Buildings etc) and the type of antennas used.

•

Typically a C/A code Receiver will experience a Multipath error of the order of 3 m and this error is usually Random.

•

The effect of Multipath can be reduced significantly through the use of an antenna that will reject reflected signals from directions below the horizon.

GPS Error Sources 7.

Receiver Noise :

•

Modern GPS Receivers are capable of both Pseudorange and carrier Phase measurements.

•

Hence typical errors in Pseudorange measurements are of the order of 0.5 m.

GPS ERROR BUDGET Sr. No.Error Source

1

P code

Satellite Clock stability

3.0

3.0

Selective Availability

0.0

0.0

Others

1.0

1.0

Ephemeris Error

4.5

4.5

Others

1.0

1.0

Ionospheric Delay

25.0

2.0

Tropospheric Delay

3.0

3.0

Multipath

3.0

3.0

Receiver Noise

0.5

0.5

UERE

26.0

7.5

Horizontal Accuracy (95%)

80

22.5

Vertical Accuracy (95%)

105

30.0

2 3

Space

C/A code (1 σ Error in m)

Control User

Types of Receiver and Techniques • Single Frequency SPS Navigation Rx • Dual Frequency PPS Rx • DGPS Rx • WAAS Rx • Land Survey Rx • Timing Rx • Ionospheric Rx • Occultation Rx

Land Survey Receiver

Upper left: a sunken concrete monument (not visible on photo); Upper right: a reused triangulation pillar; lower left: survey rivet installed near manhole; lower right: reused fundamental benchmark •

Benchmarks

GPS Surveying Receivers

GPS Surveying – Relative Positioning

GPS Land Survey Techniques 1.

Rapid Static GPS Surveying :

•

Static positioning with short observation times of 5-20 minutes (vs 1-2 hours) ... giving centimetre accuracies! Such a technique is well suited for short range applications such as control densification and engineering surveys, or any job where many points need to be surveyed (see Figure below). Unlike the "kinematic" and "stop & go" techniques there is no need to maintain lock on the satellites when moving from one station setup to another

•

GPS Land Survey Techniques 2. REOCCUPATION GPS SURVEYING TECHNIQUES : • Centimetre positioning accuracy with two occupations per site, each for a short static observation period (few minutes) ... • Also known variously as pseudo-kinematic and pseudo-static • The field procedure is otherwise similar to the "rapid static" or conventional static techniques. • One receiver is located at a known point (the "reference" receiver) while the second "roving" receiver moves from point to point. • For example, the roving receiver stops at a site, where it is static for a short period, and then moves on to the next point.

GPS Land Survey Techniques • The roving receiver must revisit the same point one or more hours later (see Figure below). • The second occupation is the same as the first: the receiver stops on the point, is static for 10 or so minutes, and then is off again. (To increase redundancy, the points may be revisited more than twice.) • The receiver need not be tracking satellites between the sessions (it can in fact be switched off), however continuous tracking (no cycle slips) should be maintained during the on-site observation period.

Field procedure for the "reoccupation" surveying technique.

GPS Land Survey Techniques 3. •

Stop & Go GPS Surveying Techniques : This is a true kinematic technique because the receiver continues to track satellites while it is in motion.

•

It is known as the "stop & go" (or semi-kinematic) technique because the coordinates of the receiver are only of interest when it is stationary (the "stop" part), but the receiver continues to function while it is being moved (the "go" part) from one stationary setup to the next

•

The technique is well suited when many points close together have to be surveyed, and the terrain poses no significant problems in terms of signal disruption

Stop & Go Technique

GPS Land Survey Techniques 4.

KINEMATIC GPS SURVEYING TECHNIQUES :

•

Determine the antenna position while in motion. Very similar to Stop & Go Technique

Handheld GPS Receivers •

Several companies now offer small handheld GPS receivers built with a small number of integrated circuits.

• Some of these ICs are generic ones used in different kinds of electronic devices, but some of them have been specially developed for GPS use. • These application-specific integrated circuits (ASICs) have significantly reduced the number of components needed to build GPS receivers and hence their size and power consumption. • In fact, the chip manufacturers offer the ASICs to other manufacturers for building standalone GPS receivers or for embedding a receiver into another product.

Handheld GPS Receivers

GPS/GIS Data Collectors

Hand held GPS Receivers from Trimble, Garmin and Leica

Garmin Forerunner GPS Receiver

Phones with GPS

A Typtcal OEM GPS Receiver Module, measuring 15X17 mm

Applications of GNSS in INDIA

1. Remote Sensing • Establishment of Ground Control Points for Satellite imagery and aerial photography corrections • Preparation of Digital Terrain Maps from Satellite Based data (Cartography)

Applications of GNSS in INDIA

2. Satellite Tracking LEO (like IRS) & GEO satellites • Precise satellite tracking of LEO satellites used in Remote Sensing • Satellite Altimetry • GPS Occultation Studies etc • Tracking Geo Satellites: NASA’s navigator

Applications of GNSS in INDIA 3. Time Synchronization for Telecommunication Network

• Precise Time transmission by atomic clocks can be used for wireless telecommunication networks for network management, time tagging and for synchronization of the many frequency references • Maintenance and development of International time standards • Time and Frequency calibration of Atomic clocks • Electronic Banking, Traffic Light control etc

Applications of GNSS in INDIA 4. Aerial and Land Survey, Natural Resources Survey

• Real Time Kinematics (RTK) Techniques • Establishment of Geodetic Control Points • Navigation for aerial photography • Survey for Construction of Dams, Bridges, Roads, Rails. • Geographic Information System (GIS)

Applications of GNSS in INDIA 5. Space Research - TEC, Scintillation, Meteorology

• Monitoring Ionospheric behavior (Total Electron Content and Ionospheric Scintillation) • Meteorology (determination of vertical structure of temperature and water vapor distribution up to 45-50 kms) • Monitoring large scale Tectonic movements for studies in Geodynamics, glaciology.

Applications of GNSS in INDIA

6. Disaster Management • Detection of Natural Disaster like Land Slides, storms, forest fires etc and Issue Warning

Applications of GNSS in INDIA

7.

Defense

• Guidance and Control of Cruise Missiles • Position location service for Soldiers, armored vehicles • Tracking enemy bases and camp movements • Nuclear Detonation Detection • Aircraft and Ship Navigation

Applications of GNSS in INDIA

8. Civil Aviation •Cat I, II and III Precision Landing •Enroute Navigation •Air Traffic Control •Safety Improvement •Distress Alert

Applications of GNSS in INDIA

9. Maritime Navigation •Automatic Identification System (AIS) •Deep Sea Navigation •Harbor Navigation •Inland Waterway Navigation •Safety Improvement •Distress Alert

Applications of GNSS in INDIA

10. Road Transport •Route Guidance • Automatic Transmission of Location • Fleet Management • ATM: Automatic Traffic Management • Safety Improvement •Breakdown Assistance

Applications of GNSS in INDIA

11. Rail Transport • Rail Traffic management • Wagon and Cargo Control • Passenger Information Service • Rail Track Survey • Safety Enhancement • Collision Avoidance • Maintenance of Rail Networks

Applications of GNSS in INDIA

12. Search & Rescue •Rescue of People in Distress like Accidents •Coordinating Emergency Services

Applications of GNSS in INDIA

13.

Location Based Services

•Position Navigation Timing (PNT) enabled Mobile Telephones •Distress Alert •Using mobile phones to find nearest hospitals, restaurants, petrol stations, parking lot and other points of interest

Applications of GNSS in INDIA

14.

Marine Exploration

•Marine Survey & Engineering •Marine Seismic Exploration

Applications of GNSS in INDIA 15.

Time Synchronization for Electricity, Oil and Gas Distribution System

•Design Construction and Operation requires Accurate Positioning system. •Integration of Energy Distribution Networks require Time Synchronization •During power outage or failures, precise information of infra structure required can be easily located. •Oil & Natural Gas Supply - Identification of location for Rigs, water, waste water and gas distribution facilities

Principle of Differential GPS (DGPS) • It Can be observed from the Table above that, GPS by itself cannot provide Cat I Precision Service. • An alternative is a Technique called Differential GPS (DGPS). • In DGPS, a Reference GPS Receiver is kept at well surveyed point, within about 50 kms of the user who may be in Land, Sea or Air. • The Reference Receiver Coordinates are determined to an accuracy better than 1m, in WGS-84 Spheroid. • The Reference receiver calculates the difference between the measured Pseudoranges, for all visible GPS satellites and calculated Pseudorange from the known coordinates and Navigation Data. • This difference is the error in Pseudorange measurements, ΔR which includes all the errors like Ephemeris, clock, Ionopshere and Troposphere lumped together.

Principle of Differential GPS (DGPS)

Principle of Differential GPS (DGPS) • This correction, ΔR for all the visible satellites are transmitted via VHF Link to the user. • The user carries out Pseudorange measurements to all the visible GPS satellites. • It applies correction, ΔR to the pseudorange observations and then computes its position. • Since the Reference station and the user are in the neighbourhood, the pseudorange errors are same for both receivers • Thus User Pseudorange observations are free from errors and the position computed will be more accurate

Principle of Differential GPS Used IN Land Survey

DGPS for Land Survey

Hand Held Differential GPS Receiver

DGPS ERROR BUDGET Sr. No. Error Source

1

2 3

Space

C/A code (1 σ Error in m) GPS 40 KM 100 Km 500 Km Satellite Clock stability

3.0

0.0

0.0

1.5

Selective Availability

0.0

0.0

0.0

0.0

Others

1.0

0.0

0.0

1.0

Ephemeris Error

4.5

0.0

1.0

2.0

Others

1.0

0.0

0.0

1.0

Ionospheric Delay

25.0

0.0

5.0

20.0

Tropospheric Delay

3.0

0.0

0.0

2.0

Multipath

0.5

0.5

0.5

0.5

Receiver Noise

0.5

0.5

0.5

0.5

UERE

26.0

1.2

5.2

20.0

Horizontal Accuracy (95%)

80

3.6

15

60

Vertical Accuracy (95%)

105

4.8

20

80

Control User

Planned Modernization of GPS Signals Current frequency Plan

Capabilities (additional) -----------------------------------------------------------------------------------------------------------------Carrier frequencies Additional civilian frequency 6 dB higher power relative to L1 L1 : 1575.42 MHz L2 : 1227.60 MHz

Planned Frequency

L5 : 1176.45 MHz (safety-of-life service frequency protection (ARNS-band))

Code frequencies (pseudorandom)

ME code (L1/L2)

20 MHz broadcast bandwidth Improved signal cross correlation M-code designed to enhance system security

P-code: 10.23 MHZ (on L1/L2)

to improve anti-jamming

Code frequencies (gold code) C/A-code:1.023MHz(onL1) C/A code on L2(1127.60MHz)

Dual freq. ionosphere correction (improved) UERE and better accuracy)

Navigation message Ephemeris, SV clock parameters On L1, L2 and L5 ionospheric parameters, SV health On L1 and L2

Basic Details on GPS, Glonass, Galileo

Constellation

GPS

GLONASS

GALILEO

Total Satellites Orbital Period Orbital planes Orbital height (km) Sat. In each plane Inclination Plane Separation Frequency

24+3 12 hrs 6 20200 4 55 deg 60 deg 1575.42MHz 1227.6MHz

24 (4 Opr) 11hrs 15min 3 19100 8 64.8 deg 120 deg 1246 - 1257 MHz 1602 - 1616 MHz

27+3 14Hrs 22min 3 23616 10 56 deg 120 deg 1164 - 1300 MHz 1559 - 1591 MHz

Modulation

CDMA

FDMA

CDMA

Beidou ( Chinese Satellite Navigation System) •

Beidou system consists of two geo-synchronous satellites in space and a third used as back up, a control centre located at Beijing and number of monitoring and calibration stations on ground distributed through out China and the Beidou positioning receivers.

•

Beidou system is fully operational in early 2004.

•

Similar to that of the Geostar regional navigation system. - Radio determination satellite service (RDSS)

•

Besides positioning, the system can perform two way data communication.

•

Users can determine their position and also transmit messages to each other.

•

Accuracy

(H – about 100 meters, T accuracy < 100 ns)

Principle of Beidou

COMPASS (Future Chinese Satellite Navigation System) •

Compass is the Chinese own Satellite Navigation System planned

•

5 GEO + 30 MEO satellites

•

Aims to provide two navigation services viz.

1. Open service with 10 m position, 0.2 m/sec velocity and 50 ns timing accuracies 2. Authorised service, which will offer safer better position, velocity and timing accuracy to authorised users only. •

Already four test satellites in Geo orbits called Beidou launched

Principle of IRNSS GEOs at 32,83,134 GSOs at 111

GSOs at 55

IRNSS User IRNSS Ranging & Monitoring Station IRNSS Ranging & Monitoring Station IRNSS MCC

IRNSS Telemetry & Command stattion

IRNSS Satellite Constellation

IRNSS Error Budget SYSTEM

IRNSS(D)

Error

(1 sigma)

EPH

5.0

Clock

2.0

Ionosphere

2.2

Troposphere

0.2

Rx. Noise

0.6

Multipath

1.5

UERE(m)

6.1

HDOP

3.0

VDOP

3.0

Pos. Accu.-H(m)

~18.3

Pos. Accu.-V(m)

~18.3

“GAGAN” – FUTURE AIRCRAFT NAVIGATION IN INDIA

GPS & WAAS ERROR BUDGET FOR C/A CODE RECEIVERS Error Source

GPS

WAAS

LAAS

EGNOS

GAGAN

1 σ Error in m Total SIS URE (Space & Control segment Errors

3.0

0.5

0.20

0.65

0.65

Ionospheric Delay

25

0.2

0.02

0.50

0.50

Tropospheric Delay

3.0

0.2

0.02

0.20

0.20

Receiver Noise + Multipath

0.5

0.5

0.50

0.50

0.50

UERE

26

0.7

0.50

1.00

1.00

Horizontal Accuracy (95%)

80

2.1

1.50

3.00

3.00

PRINCIPLE OF WAAS

Present STATUS OF US WAAS •

INMARSAT has launched four INMARSAT III satellites with CXL and CXC payload for WAAS implementation

•

INMARSAT has launched advanced INMARSAT IV satellites, carrying CxL1 and CxL5 payloads

•

US has tested WAAS over US airspace using 2 INMARSAT III satellites over AOR-E & AOR-W

•

The performance meets Cat I Accuracy, Integrity, Availability and Continuity requirements under magnetically quiet Ionospheric conditions

•

It is not yet Operational over US.

•

It is used by Civilians for other applications

Present Status of WADGPS in other countries • Canada is planning to implement CWAAS, on lines similar to US WAAS, over Canadian Airspace • In Europe, the European Tripartite Group (ETG), Consisting of European Space Agency (ESA), Commission of European Union (CEU) & Eurocontrol is testing WADPGS, known as EGNOS, over European airspace, using INMARSAT III satellites AOR-E & IOR. • It is not yet Operational over Europe • Japan is testing MTSAT Satellite based Augmentation System (MSAS), with two GSOs over Japan.

What is GAGAN ? •

GAGAN stands for GPS Aided Geostationary satellite Augmented Navigation

•

It is a Joint program between ISRO and AAI (Airport Authority of India) to implement WADGPS over Indian Airspace.

•

It has two Phases viz.

1.

Technology Demonstration System (GAGAN TDS)

2.

Final Operational System (GAGAN FOP)

GEO

GPS

C1

L2 L1

L1

(GEO)

GEO

GEO Ranging +Integrity message +WAD correction

C2

GPS

L1

L1/L2 (GPS)

L1/C2 (GEO)

L1

L2

INRES GEO C1

L GEO C2

INLUS 1

INMCC

Elements of GAGAN

INLUS 2

PRESENT SERVICE COVERAGE FOR WAAS, EGNOS & PROPOSED MSAS

Present Service Coverage for WAAS, EGNOS, MSAS And Proposed INSAT Nav Payload Coverage INSAT Coverage 74 & 93.5 E

3 Satellite Coverage

CONFIGURATION OF TECHNOLOGY DEMONSTRATION SYSTEM

•

8 INRES (Indian Reference Stations)

•

1 INMCC (Indian Master Control Centre)

•

1 INLUS (Indian Navigation Land Uplink Station)

•

Navigation Payload (L1 & L5) for GEO

•

Aircraft WAAS Receivers for demonstration

•

Communication links between INRES & INMCC

•

TEC Network (18 Stations) for IONO Correction

INRES • INRES (Indian Reference Station) are GPS Tracking stations, set up at well surveyed points, whoso coordinates in WGS-84 are well known. • They have mainly an Atomic Clock and redundant (atleast three) GPS Dual Frequency Receivers. • The Receivers collect Pseudorange and Carrier Phase measurements at Dual frequencies and transmit the data in real time to INMCC (Indian Master Control Centre) • Either a Fibre Optic cable or a Vsat terminal is used for Data Transmission

Bangalore INRES Facility

INMCC • •

INMCC collects GPS data transmitted from all the INRES in the network in real time. They process the data to compute

1. 2. 3.

GPS Ephemeris and Clock Errors Intergrity Parameters GIVD (Grid Ionospheric Vertical Delay) and GIVE (Grid Ionospheric Vertical Error) in a 50 by 50 over Indian region to correct Ionospheric delay

•

Sends the data to a Navigation Land Earth Station (NLES)

INLUS • INLUS received the WADGPS Correction data from INMCC • It suitable formats the data and transmits to a Geostationary Satellite (GSAT-4 in GAGAN) • It transmits the data in C Band and receives back in C Band the stored data in the Satellite • It verifies the Data for its validity

11 Mtr Antenna with Prime Focus Feed

GSAT-4 Satellite • GSAT-4 Satellite, to be yet launched will be placed at 840 E • It will carry two CXL and one CXC bent pipe Transponder • The two CXL Transponders are CXL1 and CXL5

Major Challenge in GAGAN • The major Challenge in GAGAN is to bring down the error due to Ionosphere to less than 0.5 m, using Grid Based Models.

Worldwide TEC during Near-Solar Maximum Conditions 21 August 2001 (from JPL)

Atmospheric Effects on L Band Signals Transmitted from Navigation Satellites Ionosphere causes 1. Additional Group Delay or Absolute Range Error 2. Carrier Phase Advance or Relative Range Error 3. Amplitude Scintillations 4. Phase Scintillations 5. Doppler Shift or Range rate Error 6. Faraday Rotation 7. Refraction or Bending 8. Distortion of Pulse waveforms Troposphere causes 1. Tropospheric Refraction or Absolute Range Error 2. Amplitude Scintillations

Major Features of Equatorial Ionosphere over Indian Region •

India lies in Equatorial Anomaly region

•

Ionospheric delay at L band are very high (50-60 m)

•

It is highly variable with time and latitude (Dynamic)

•

Behaviour is unpredictable

•

Ionospheric scintillation is high

•

Ionospheric Bubbles occur more frequently over the region

•

Ionosphere influences accuracy, integrity, availability & Continuity of service

IS TE C SL AN T

DI R M ECT EA I SU ON RE TO D G BY PS R E IN CE WH IV IC ER H

GPS

0 35

Km ’S TH R EA

IONOSPHERIC PIERCE POINTS VERTICAL DIRECTION IN WHICH TEC IS FIRST COMPUTED FROM SLANT TEC

CE FA R SU

50 BY 50 GRID POINT NODES WHERE VERTICAL TEC IS COMPUTED FROM MEASUREMENTS

TEC RECEIVER

50 BY 50 GRID

USER AIRCRAFT RECEIVER

SLANT DIRECTION TO GPS FROM USER IN WHICH TEC IS REQUIRED BY USER AIRCRAFT

NOT TO SCALE

PRINCIPLE OF GRID BASED IONOSPHERIC MODEL

Present Status of GAGAN • TDS has been completed suddessfully. • Under benign Ionospheric conditions, the performance of GAGAN is good enough to meet APV 1 service • Ionospheric Scintillations that occur after sunset and till two hours after midnight is still a problem over indian region • FOP is being planned, to improve the performance of GAGAN

Principle of Local Area Augmentation System (LAAS)