Workshop on Fundamentals of GNSS/IRNSS
and Applications to Atmospheric Science
26th Feb, 2016
ATUL P. SHUKLA Group head, DCTG/SNAA
Space Applications Centre, ISRO Ahmedabad
E-mail: [email protected] 1
GNSS : System & User Receivers
Outline of Presentation
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• GNSS Scenario • Indian Regional Satellite Navigation
System • GAGAN (GPS Aided GEO Augmented Navigation) • User receivers
How GNSS Works ? - outlook Receiver captures in-band
RF signals. It extracts the desired
signal by unique code. It decodes navigation data
and compute satellite position & correction parameters.
Receiver measures the range of each satellite.
Compute the user position, velocity and time using satellites position, ranges & nav. data. 3
BAND
GNSS is like a “Super Lighthouse & Super Time Piece” in the sky provide PVT services to all mankind, all the time in all weather conditions
GNSS Scenario
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• 4 Global Constellations: • GPS (24/31) & GLONASS (29) fully operational • Galileo (30) – 10 satellites are operational • Beidou (35) – 20 satellites are operational
• 2 Regional Constellations: • IRNSS (7) – 4 satellites are operational • QZSS (7) – 1 satellite is operational
• GNSS (SBAS) Augmentations: • WAAS (2), EGNOS (2), GAGAN (2), MSAS (1) are operational • SDCM (Russia) under development
Comparison of IRNSS with other Navigation Systems
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GNSS Scenario (contd.)
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• The trend is towards Multi-Constellation and Multi- Frequency User Receiver development
• Improved Availability, Accuracy • Dual / Triple frequency for Civilian Users • Improved resistance to Jamming and Spoofing • Modernized Signals from GPS/Galileo (by 2020)
• Faster TTFF • Weak Signal Tracking and Acquisition • Indoor positioning • Improved Multipath performance • Search & Rescue capability
GNSS Civil Sig. : Combine & Conquer
Compatibility
Do no harm
Interoperability provides users a PNT solution using signals from different GNSS systems
No additional receiver cost or complexity
No degradation in performance
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GPS Constellation
• 24 GPS Satellites in MEO (Medium Earth Orbit) • Satellites in 6 orbital planes, 4 satellites per orbital plane • Satellites at a height of of 20200 Km above the surface of the earth • Orbital planes inclined at 55o with respect to the equatorial plane • Orbital period is about 11 hours 58 minutes
L1 1575.42MHz L2 1227.60MHz Coarse Acquisition (C/A) Precision (P-Code)
(GLObal’naya NAvigatsionnaya Sputnikovaya Sistema) - GLONASS 19100 km orbit with 64.8 degree inclination Orbital period 11.25 hrs. (Slightly shorter than GPS) Antenna on each satellite have wide beam width of
30 deg. So navigation services is also available to users at altitudes of 2000 km, ideal for space vehicles
Two Service. SPS for civilian use & High Precision (HP) service for military use
GLONASS uses same code for each satellite but different frequencies
Code Rate different than GPS L1 Freq. : 1602 + n*0.5625 MHz L2 Freq. : 1246 + n*0.4375 MHz
IRNSS System : The Indian GPS
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• Currently 5 satellites are in operation – IRNSS only based positioning demonstrated. • IRNSS Constellation to be completed by March 2016.
IRNSS - Service Area Definition
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IRNSS service area is divided into three regions: Indian Land Mass : The area encompasses the Indian Geo-Political boundary. Primary Service Area: The area covered by 1500 km contour from Indian geopolitical boundary inclusive of the Indian Land Mass. Extended Service Area: The area between primary service area and area enclosed by the rectangle of Lat 300S to 500N, Long 300E to 1300E.
Extended Service area
Indian Landmass
Primary Service Area
IRNSS – Indian Regional Navigation Satellite System
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IRNSS System
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What is unique about IRNSS? • IRNSS is an indigenous system – full control • Uses Dual Frequency (L5/S) for Civilian Users • Third satellite navigation system (worldwide) to be fully
Operational by mid 2016 • Uses Grid based model for ionosphere delay correction
(accurate for single frequency users at L5/S band) • S-band for navigation – first time being used (low ionospheric
delay to benefit single frequency (S band) users • IRNSS can be used to broadcast short messages (potential to
be used also as a Disaster Warning Dissemination System) • All satellites to be visible over Indian region for almost all the
time • RS (Restricted Services) signal for strategic users
IRNSS Ground Segment Architecture
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IRNSS Satellite
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IRNSS Satellite Payload Functions Reception of navigation uplink data through TC . Generation of navigation message, SV time, code generation,
code encryption, Spreading codes, modulation, up-conversion, amplification, filtering and transmission
Three signals in each L5 and S Band (SPS, RS-D, and RS-pilot signals, Interplex signal is added to maintain the constant envelope characteristic of the composite signal
The IRNSS Payload nominally transmits signal with SPS (22.2%), RS-D (44.4%), RS-Pilot (22.2%) and
Interplex (11.1%) power distribution SPS signal is BPSK (1) Modulation RS-D and RS-Pilot uses BOC (5,2) Modulation Onboard Rb Atomic Clocks for Highly Frequency Stability
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IRNSS Signals
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IRNSS Signals, Services & Accuracy
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Service Type
Signal
Frequency
Accuracy
Standard Positioning Services (SPS)
BPSK (1) L5 (1176.45 MHz) S (2492.028 MHz)
Single Frequency < 20 meters
Restricted Positioning Services (RS)
BOC (5,2) L5 (1176.45 MHz) S (2492.028 MHz)
Dual Frequency < 10 meters
GPS – Accuracy
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Typical GPS Accuracies:
GAGAN System
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Main Components of GAGAN: • GPS Satellites • A network of Ground Reference Stations over Indian air-space known as INRES • Indian Master Control Station (INMCC) • Indian Land Uplink Station (INLUS) • Geo-stationary Satellites (GSAT-8, GSAT-10 main; and GSAT-15 in-orbit spare)
GAGAN Services
Certified RNP 0.1 Service over Indian FIR
Certified APV 1 Service over Indian Landmass.
• GAGAN is certified by DGCA for RNP 0.1 services over Indian FIR (30th Dec, 2013) • GAGAN is certified by DGCA for APV 1.0 services over 76 % of Indian landmass (21st April, 2015)
GPS Augmentation Systems in the World
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• GAGAN System Certified by DGCA for En-route Navigation (RNP 0.1) • GAGAN System Certified by DGCA for Aircraft Navigation (APV 1.0)
GAGAN Performance : Ahmedabad (accuracy)
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SBAS/GBAS
GAGAN - Benefits to Aviation Primary Means of Navigation - Take-Off, En Route, Approach and Landing
More Direct Routes - Not Restricted By Location of Ground-Based Equipment
Precision Approach Capability - At Any Qualified Airport
Decommission of Older, Expensive Ground-Based Navigation Equipment
Reduced/Simplified Equipment On Board Aircraft
Increased Capacity - Reduced Separation Due to Improved Accuracy
Increase safety by using 3D approach operations
RADIO NAVIGATION TECHNIQUES
Hyperbolic positioning Doppler positioning Trilateration
HYPERBOLIC POSITIONING Based on measurement of difference in time of
arrival of signal from a set of two transmitters Observer clock is not required to be
synchronized with transmitter
System based on this principle is called Time Difference Of Arrival (TDOA) system
LORAN and OMEGA were based on this
principle
HYPERBOLIC POSITIONING METHOD
DOPPLER POSITIONING Due to relative motion between transmitter and
observer there is shift in received frequency called Doppler shift
Navigation approach based on this principle is called Doppler positioning.
The transmitted and received frequency are related
fR = fT (1 – ŕ/vs) Where ŕ is distance (changing) vs = relative velocity of transmitter w.r.t. source fT & fR are the transmit and receive frequencies
respectively TRANSIT is an example
TRILATERATION
Transit time of radio waves from transmitter is measured..
Distance between transmitter and observer is estimated.
Observer position can be estimated from three pseudo range measurements.
Navigation based on this is known as Time-of-Arrival (TOA).
GPS is an example of TOA System.
TRILATERATION POSITIONG METHOD
What a Navigation Rx will see ?
Block Diagram of Navi. Rx
Building Blocks Of GNSS Receiver
What Is the Role Of RF Front End ?
Receiver Processing for the GPS Waveform
)cos()(/)(2)( θω +⋅⋅⋅= ttACtdSts
C/A code waveform Signal power S(nW)
50 Bits/sec data stream(±1) 1 Mbits/sec pseudo-random code (±1)
1575.42MHz L1 carrier frequency Time(sec)
Carrier phase(rad)
Track the carrier to get delta pseudorange(doppler)
Track C/A code to get pseudorange
Demodulate d(t) to strip off data
Amplify S for subsequent processing
Antenna picks up signals(pico-watts) and filters reject sidelobes
SATELLITE SIGNAL
RECEIVER PROCESSING
Only L1 C/A code from single satellite
shown here
Typical Task partitions
The Search Problem
Serial Search Acquisition Frequency sweep and Code phase sweep Total number of cells to search: (1023x2)x(2x5000/400) = 51150 (Half chip shift and ±5kHz in steps of 400Hz) Higher Accuracy Very robust in Noise conditions Easy to Implement Less efficient (maximum search time)
Serial Search Acquisition contd…
Parallel Frequency Space Search Uses Fourier transform to detect carrier in one step 1023 cells (Full chip shift) Accuracy depends on length of DFT
(Frequency Resolution = fs/2 / N/2 = fs/N)
Efficiency depends on speed of used Fourier algorithm
Implementation is complex IRNSS doppler is not an issue, so not useful
Parallel Frequency Space Search contd…
Complex conjugate of Fourier Transform of local code replica is multiplied with Fourier Transform of down-converted signal (0 IF)
41 cells (for ±10kHz in steps of 500Hz) Efficiency depends on speed of used Fourier
algorithm Implementation is complex Acq. Time Minimum but Noise performance
not good
Parallel Code Phase Search (FFT Circular Correlation)
Parallel Code Phase Search contd…
FFT
IFFT
I2 +Q2
Tracking Code Tracking
Delay Locked Loop (DLL)
Carrier Tracking Costas Loop (PLL)
Code Tracking (DLL)
Early-Late Correlation
Code Tracking Loop Discriminators
Coherent IE – IL Noncoherent (IE
2 + QE2) – (IL
2 + QL2)
(IE2 + QE
2) – (IL2 + QL
2)/ (IE2 + QE
2) + (IL2 + QL
2) IP(IE – IL) + QP(QE - QL)
Carrier Tracking (PLL)
Carrier Tracking Loop Discriminators
Sign(I)·Q – output is proportional to sin(θ) I·Q – output is proportional to sin(2θ) Tan-1(Q/I)- output is phase error – known
as arctangent discriminator. Optimal but also the most computationally intensive.
Optimized BPSK Correlator
Sequence of events upto PVT
What follows Tracking Loops? Navigation data as output of demodulator Bit synchronization, frame Synchronization
follows. Position Velocity and Time is estimated to
form position Fix But there are some imperfections. What are the sources of such errors?
Multi path
Satellite clock bias
Receiver Clock bias
Ephemeris errors
Sources of Error
Ionospheric delay
Tropospheric delay
SOURCES OF ERROR :
Ephemeris ~ 1 m Satellite clock ~ 1 m Receiver error ~ 1.5 m Multi-path ~ 1.5 m Troposphere delay ~ 1 m Ionosphere delay ~ 6 m UERE ( RSS of all above errors ) ~ 6.5 GDOP ~ 3 Position Accuracy = 6.5 x 3 = 19.5 m
16-Mar-16
Technology of Navigation GPS – first operational global navigation system
Glonass followed and Compass, Galileo are in line Multiplicity of Constellation (System diversity),
Multiplicity of Carriers poses (frequency diversity) new challenges for navigation Processing
80’s-90’s – the first professional, GPS + GLONASS receivers
2011 – “launch year” for the first consumer, mobile phone GPS + GLONASS receiver chips (from Qualcomm, Broadcom, STEricsson, u-blox and others)
GNSS Antenna Types
GNSS Receiver Clocks
Technology trends : Terms Digital Radio. After the front-end stage the incoming
signal is converted into the digital domain to be processed by a digital signal processor (DSP). The processing is often done in a defined, non reconfigurable way, using hardware and ASICs.
Software Radio (SR). As technology progresses, the
digitization is at (or very near to) the antenna and all of the processing required for the digital radio is performed by software residing in high-speed digital signal processing elements.
Terms (Contd.) Software-Defined Radio (SDR)—The RF to
IF is done prior to ADC, again followed by a digital radio that is performed by software residing on a programmable platform. Thus, the analog frontend remains unchanged, although all processing is fully reconfigurable. Therefore, the SDR solution represents a more realistic implementation of a digital radio.
SDR Architecture
Processing : SDR way (GNSS SW Rx)
Where we stand ?
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