BASICS ON GNSS
RECEIVERS

Gabriella PoveroNavigation Technologies

GNSS Training
AIT, Bangkok
14 -18 January 2019©2019 LINKS

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THE HARD WORK OF GNSS RECEIVERS
Control system errors:• Clocks• Ephemeris
DopplerAtmosphericerrors:• Ionosphere• Troposphere

Low
SNR

Urban
canyons
IndoorMultipath

Bluetooth
WLAN

Interferenceand jamming©2019 LINKS

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THE RECEIVER CHAIN
Let us consider the SIS of a single SV (space vehicle)
𝑦𝑇𝑋 𝑡 =𝑦𝑅𝐹 𝑡 =

2𝑃𝑅 c 𝑡 d 𝑡 cos 2𝜋 𝑓𝑅𝐹 𝑡 + 𝜗𝑅𝐹

2𝑃𝑅 c 𝑡 − 𝜏 d 𝑡 − 𝜏 cos 2𝜋 𝑓𝑅𝐹 + 𝑓𝑑 𝑡 + 𝜗𝑅𝐹 + 𝑤(𝑡)y RF (t )Antenna

RFFront-end©2019 LINKS

ADC

GNSSDigitalreceiver3THE RECEIVER CHAIN
Let us consider the SIS of a single SV (space vehicle)
𝑦𝑇𝑋 𝑡 =

2𝑃𝑅 c 𝑡 d 𝑡 cos 2𝜋 𝑓𝑅𝐹 𝑡 + 𝜗𝑅𝐹noise𝑦𝑅𝐹 𝑡 =

2𝑃𝑅 c 𝑡 − 𝜏 d 𝑡 − 𝜏 cos 2𝜋 𝑓𝑅𝐹 + 𝑓𝑑 𝑡 + 𝜗𝑅𝐹 + 𝑤(𝑡)y RF (t )Antennadelay
RF
Front-end©2019 LINKS

Dopplerfrequency
ADC

GNSSDigital
receiver4

GNSS ANTENNAS➢ Antennas receive, amplify and band-pass filters GNSS signals➢ GNSS antennas are Right Hand Circularized Polarized (RHCP)➢ Important parameters used to characterize GPS antennas are:
➢ Central frequency (e.g.: GPS L1 = 1575.42 MHz) and bandwidth➢ Single or multi-frequency➢ Radiation pattern (directivity). Antenna GPS antennas are usually
hemispherical➢ Impedance (typically 50 ohm)
➢ Voltage Standing Wave Ratio (VSWR)➢ Noise factor➢ Gain➢ Multipath rejection capability➢ Jamming mitigation capability©2019 LINKS

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THE RECEIVER CHAIN
Let us consider the SIS of a single SV (space vehicle)
𝑦𝑇𝑋 𝑡 =𝑦𝑅𝐹 𝑡 =2𝑃𝑅 c 𝑡 d 𝑡 cos 2𝜋 𝑓𝑅𝐹 𝑡 + 𝜗𝑅𝐹

2𝑃𝑅 c 𝑡 − 𝜏 d 𝑡 − 𝜏 cos 2𝜋 𝑓𝑅𝐹 + 𝑓𝑑 𝑡 + 𝜗𝑅𝐹 + 𝑤(𝑡)y RF (t )Antennay IF [n ]  2 PR c ( b ) ( nTs   )d ( nTsRFDown-convertor

𝑦𝐼𝐹 𝑡 =
©2019 LINKS

y IF (t )ADC

GNSSDigital
receiver2𝑃𝑅 c 𝑡 − 𝜏 d 𝑡 − 𝜏 cos 2𝜋 𝑓𝐼𝐹 + 𝑓𝑑 𝑡 + 𝜗𝐼𝐹 + 𝑤(𝑡)6GNSS FRONT-END
RF down convertor
It down-converts and filters RF signals to an intermediate frequency (IF)compatible with analogue-to-digital converter (ADC) acceptable input.
Key parameters:➢ Input frequency/ies➢ Noise➢ Linearity➢ Automatic Gain Control (AGC)➢ Isolationy RF (t )Antenna

RFDown-convertor

𝑦𝐼𝐹 𝑡 =
©2019 LINKS

y IF (t )ADC

GNSSDigitalreceiver

2𝑃𝑅 c 𝑡 − 𝜏 d 𝑡 − 𝜏 cos 2𝜋 𝑓𝐼𝐹 + 𝑓𝑑 𝑡 + 𝜗𝐼𝐹 + 𝑤(𝑡)7ANALOG TO DIGITAL CONVERTER (ADC)
Important parameters used to characterize the ADC are:➢ # of bits: GNSS receivers works well with a low number of bits per samples (1,2, 1.5, 4bits). Higher number of bits (8 bits) used for specific applications (e.g.: interferencedetection)➢ Analog input range: defines the voltage range in input. Used in the design of thefront end gain
➢ Maximum sampling frequency: depends on the device (e.g.: 60, 80, 100 Msps)➢ Analog input bandwidth: Determine the maximum frequency of the signal that canbe processed by the device

From
Analog Device
AD9288©2019 LINKS

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THE RECEIVER CHAIN
Let us consider the SIS of a single SV (space vehicle)
𝑦𝑇𝑋 𝑡 =𝑦𝑅𝐹 𝑡 =

2𝑃𝑅 c 𝑡 d 𝑡 cos 2𝜋 𝑓𝑅𝐹 𝑡 + 𝜗𝑅𝐹2𝑃𝑅 c 𝑡 − 𝜏 d 𝑡 − 𝜏 cos 2𝜋 𝑓𝑅𝐹 + 𝑓𝑑 𝑡 + 𝜗𝑅𝐹 + 𝑤(𝑡)y RF (t )AntennaRFy IF [n ]  GNSS
2 PR c ( b ) ( nTs   )d ( nTsy IF (t )ADC

Front-endDigital
receivery IF [n ]  2 PR c ( b ) ( nTs   )d ( nTs   ) cos( 2 ( f IF  f d )nTs  IF )  wIF [n ]delay©2019 LINKS

Dopplerfrequency
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GNSS RECEIVER OPERATIONSy IF [n ]  2 PR c ( b ) ( nTs   )d ( nTs   ) cos( 2 ( f IF  f d )nTs  IF )  wIF [n ]1Sky search

Acquisition

23©2019 LINKS

Search for IDs of visible satellites

Tracking

Code delay and Doppler estimates,
rough alignment of code and carrier
Refines code and carrier alignment

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GNSS RECEIVER FUNCTIONALITIESBaseband processing: Acquires and tracks incoming signals, demodulates signals.

Carrier & Codecorrelator

Channel 1

antenna

Channel 2

RF
Front End

Carrier & Codetracking©2019 LINKS

NAV
Unit

Acquisition
stage

Channel n11

GNSS RECEIVER FUNCTIONALITIESBaseband processing: Acquires and tracks incoming signals, demodulates signals.

Key parameters:➢ Number of channels➢ Measurement rate
➢ Carrier-to-Noise ratio (C/N0)➢ Multipath immunity➢ Signals/modulationsCarrier
processed& Codecorrelator➢ Dynamics➢ Interference cancellation
antenna➢ Jamming mitigation
RFFront End

Channel 2

NAV
Unit

Acquisition
stageCarrier & Codetracking©2019 LINKS

Channel 1

Channel n

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GNSS RECEIVER OPERATIONS
4

Measurements

5Computation

6

Usually the PVT

Integration
with external
info
7©2019 LINKS

Pseudorange and data demodulation

HMI

Not present in all receivers

Not present in all receivers

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GNSS RECEIVER FUNCTIONALITIESPVT (& Application) processing:Computes the estimated position and receiver time offset relative to the constellation’sreference time.
Evaluation of user position
Channel 1
antenna

Channel 2

RF
Front End

Key parameters:➢ Solution type (GNSS, Differential GNSS)➢ Single or Multi constellation➢ Update rate

NAV
Unit

Acquisition
stage

Position
VelocityTime

Channel nExternal aiding➢ Latency©2019 LINKS

14RECEIVERS CLASSES
Description

Device Price[€]

Handheld receivers for hikers andsailors. Small size with latitude-longitudedisplays and maps.

100 - 600

Integrated GPS in mobile phones. Low
cost and mostly single frequency.

50-600Maritime navigators. Fixed mount, large
screens with electronics chart

100-3000

In-car navigation systems. Detailed
street maps and turn-by-turn directions.
These systems can be also handheld(e.g. PDA)

100-2000

Price differences are due to reason independent from the embedded
GNSS chip©2019 LINKS

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RECEIVERS CLASSES
Description
Aviation receivers. FAA in US andEASA in Europe certified, panel
mounted with maps.

Approx. Price[€]>3000INTEGRITY REQUIRED !

Survey and mapping professional
receivers. Multi-frequency anddifferential GPS, centimeter
accuracy

1500 – 30000

Price differences are due to reason independent from the embedded
GNSS chip©2019 LINKS

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GNSS MODULES
Description

Plug-in modules.
Integrated receivers and antenna.
Employed in tracking systems
Original Equipment Manufactureboards.
Employed for integration in othercomplex systems.
Chip sets.
Employed for integration, but all
the circuitry is needed©2019 LINKS

Approx. Price[€]
30 – 700

100 – 5000

1 – 3017

PROFESSIONAL VS MASS-MARKET RECEIVERS

Carrier PhasevsCode Phase?
Raw measurements
availabilityand configurability

Configurability©2019 LINKSDGNSS … RTK18

RECEIVERS CLASSIFICATION: MARKET SEGMENT
Category

Receiver Characteristics

ConsumerSingle frequency (few also double frequency), cost driven,high volume, moderate performance, also multi
constellation

Light
Professional

Single frequency, multi constellation, cost driven, lowvolume, good performance, integration with externaldevices, professional features

Professional

Multi frequency, multi constellation, cost/requirements
driven, low volume, high performance, advanced
processing algorithms

Double/ Multi frequency, multi constellation, requirements
Safety of Life driven, low volume, high performance, high reliability,integrity, certification

PRS©2019 LINKS

Double frequency, low volume, high performance, highreliability, requirements driven, integrity, advanced
processing algorithms
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GNSS RX FEATURES➢➢➢➢➢
➢
➢
➢
➢
➢
➢

Constellation(s) exploited
Military or civil receiver
PVT update rate
Indoor operations or highmultipath environment
Interference mitigation
Dynamic conditions (fromstatic to high dynamics)
DGPS or WAAS/EGNOS
capability (RTK input/output)
Storage of log data
Shock and vibration tolerance
Cartographic support
INS integration or dead-©2019 LINKS

➢
➢➢➢
➢

reckoning systems
Integration with COMsystems
PortabilityUsability
Power consumption
Cost20EXAMPLE OF TECHNICAL SPECIFICATION (1)Septentrio PolaRx4 PRO•

264 hardware channels•

TRACK+: Septentrio’s low-noise tracking algorithms•

GPS L1/L2/L2C/L5,•

GLONASS L1/L2•

Galileo E1, E5a, E5b, E5 AltBOC and GLONASS CDMA L3, Beidou
signals (initially experimental)•AIM+: Advanced Interference Monitoring and Mitigation•

APME+: extends Septentrio’s patented A Posteriori Multipath
Estimator to GLONASS, Galileo and Beidou signals•ATrack+: is Septentrio’s patented Galileo AltBOC tracking.

©2019 LINKS

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EXAMPLE OF TECHNICAL SPECIFICATION (2)Septentrio PolaRx4 PRO
Pseudorange noise (not smoothed)Carrier Phase

GPS L1 C/A

16 cm

L1/E1

<1 mm

GLONASS L1 open

25 cm

L21 mmGalileo E1 B/C

8 cm

L5/E5

1.3 mmGalileo E5 A/B

6 cm

Galileo E5 AltBOC

1.5 cmGPS L2 P(Y)

10 cm

GLONASS L2 (mil)

10 cm©2019 LINKSDoppler
L1/L2/L50.1 Hz22

EXAMPLE OF TECHNICAL SPECIFICATION (3)NovAtel 628•

120 hardware channels•GPS L1 L2 L2C L5•

GLONASS L1 L2•

Galileo E5a E5b E5 AltBOC•Beidou B1 B2

•

QZSS

•

L-Band•RT-2 (RTK algorithm)•

Pulse Aperture Correlator (PAC) multipath mitigationtechnology•

SPAN INS integration technology•…

©2019 LINKS

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EXAMPLE OF TECHNICAL SPECIFICATION (4)NovAtel 628
Pseudorange noise (not smoothed)Carrier Phase

GPS L1 C/A

4 cm

L1 GPS

0.5 mmGLONASS L1open

8 cm

L1 GLONASS

1 mm

GPS L2 P(Y)

8 cmL21 mm

GPS L2C

8 cm

L2C

0.5 mmGPS L53 cm

L50.5 mm

GLONASS L2open8cm

GLONASS L2 mil

8 cm©2019 LINKS

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EXAMPLE OF TECHNICAL SPECIFICATION (5)NovAtel 628
Position Accuracy (RMS)

Signal Reacquisition

Single point L1

1.5 m

L1

<0.5 s (typical)

Single point L1/L2

1.2 m

L2

<1.0 s (typical)

SBAS (GPS)0.6 m

Maximum Data Rate

DGPS

0.4 m

Measurements

100 Hz (20 SV)

L-band VBS

0.6 m

Positions

100 Hz (20 SV)

L-band XS

15 cm

L-band HP

10 cm

Randomvibe

MIL-STD 810G
(Cat 24, 7.7 g RMS)

1 cm + 1ppm
(BL)

Sine vibe

IEC 60068-2-6

RT-2©2019 LINKSVibration

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GNSS RECEIVERS CAPABILITYGNSS Technology Report2018 (GSA)
GNSS Market Report 2015
(GSA)

©2019 LINKS

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GNSS RECEIVERS CAPABILITYGNSS Technology Report2018 (GSA)

GNSS Market Report 2015(GSA)©2019 LINKS

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https://www.gsa.europa.eu/system/files/reports/gnss_user_tech_report_2018.pdf©2019 LINKS

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CONTACTGABRIELLA POVERO
gabriella.povero@linksfoundation.com
www.navsas.eu
www.ismb.it

LINKS FOUNDATION
Via Pier Carlo Boggio 61 | 10138 Torino+(039) 011 2276 150©2019 LINKS@linksfoundation

linksfoundation@linksfoundation

linksfoundation.com

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