GNSS Receivers, Introduction

DANISH GPS CENTER

GNSS Receivers, Introduction

Kai Borre, Head of DGC

Darius Plaušinaitis

Danish GPS Center, Aalborg, Denmark

Recap on GNSS

2013 Danish GPS Center 2

Time tags

Ephemerides

Ionosphere data

Almanac etc.

GNSS control

station

GNSS satellites

Uplink

GNSS monitor

stations

L1, E1, L2, L5, E5,…

GNSS:

GPS

Galileo

GLONASS

Compass

…

GPS Block

2R,

Lockheed

Martin

Global Navigation Satellite Systems

• GPS (II – 1980)

• GLONASS (1993)

• COMPASS

• Galileo


2006

~1981

• 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, ST-

Ericsson, u-blox and others)

GNSS Receiver Structure


GNSS application

GNSS receiver

User

Coordinates transformation

Coord. transformation (optional)

Mapping

Position computation

Acquisition

Visualization

NMEA 0183 or binary interface

Tracking

Other

applications

TTFF – Stand Alone Case


From seconds to minutes From 18 to 30 seconds 0 – 6s

* May require multiple reception of the same data when BER is high (for weak signals)

Signal

acquisition

Tracking

lock

Bit

sync.

Collection of

ephemeris data*

Frame

sync.*

Receiver activation

Code phases of received signals

are known: relative

measurements are available

TOW is known: pseudoranges can

be computed (receiver clock

error is unknown)

Satellite positions can be computed: receiver position can be computed

time

Receiver Functional Blocks


GNSS Receiver

RF front-end(s)

Amplifier

GNSS

antenna(s)

A/D

Correlators

(channels)

Code

generators

Carrier

generators

Receiver,

navigation

processor

Display,

keyboard

Battery

powered

clock and

memory

Power

supply

Receiver

clock

Frequency

synthesizer

Mixer

Other sensors

(IMU, baro etc.)

GNSS Antenna Types

Consumer Professional


Professional for very high precission

applications

Antenna arrays are used for special

applications (military, airports and other)

The RF Front-end


GNSS Receiver

RF front-end(s)

Amplifier

GNSS

antenna(s)

A/D

Correlators

(channels)

Code

generators

Carrier

generators

Receiver,

navigation

processor

Display,

keyboard

Battery

powered

clock and

memory

Power

supply

Receiver

clock

Frequency

synthesizer

Mixer

Other sensors

(IMU, Baro etc.)

• Converts RF

GNSS signals to

a lower frequency

digital signals

• Generates clock

signals that are

used in all

aspects of a

GNSS receiver

• Varying

possibilities for

configuration of

the signal

reception chain

GNSS Receiver Clock

• Receiver clock is used for local signal

creation and for time keeping (receiver time)

• Crystal oscillators have good short term

stability, low phase noise

• There is some sensitivity to temperature

changes

• Various types of crystal oscillators exist, but

the most popular for GNSS applications are – TCXO – temperature-compensated crystal oscillator

(0.5-3ppm typically are used for GPS)

– OCXO – oven-controlled crystal oscillator (~0.001ppm)

• Chip scale atomic clock is a new product in

the market with the best precision in this

size class (<0.00015ppm)


Consumer Receivers

• Primary concerns

– Low cost

– Low power

– Small size

– That it works everywhere

– Easy integration

– Precision – typically <3-10m


Receivers For Surveying

• Typically these are dual

frequency devices with

dedicated antennas

• Always used in DGPS

mode

• Today typically a

network of receivers is

employed for DGPS

• Precision is about 1cm,

but can be also <1mm

• Price is about 10000$


An Example From AAU Station


Aviation

• The main information of interest is horizontal position

• GNSS vertical position is not so good and an airplane

has more precise altitude instruments

• The main difference from an ordinary GNSS receiver –

a certified aviation grade GNSS receiver must reliably

indicate when it is not reliable

• Airports may have their own

monitoring receivers


https://buy.garmin.com/shop/shop.do?cID=154&fKeys=FILTER_CUSTOM_GPSNAVCOMM

Aalborg EGNOS Station


Receivers for Space Applications

• Receiver architecture: – Flexible configuration of up to 24 tracking channels

– L1 and L2 frequencies, C/A code, P(Y) code

• Interfaces: – UART (RS-422) TC/TM interface

– MIL-STD-1553B TC/TM interface extension possible

– 1 PPS output (RS-422)

– Secondary power interface

– 2 antenna inputs

• Navigation Solution Accuracy: – Position (1σ, 3d): <5 m

– Velocity (1σ): <1 cm/s

– time offset 1PPS (1σ)?: < 50 ns

• Time to first fix: – Hot start < 5 s

– Warm Start < 90 s

– Cold start < 20 min.

• Physical / Environment: – Size: 300x240x50 mm

– Weight: < 1,3kg

– Operating temperature: -25º C to +60º C

– Radiation: Cumulative dose >20 kRad (Si)

– Power Consumption: < 8W (TBC)


Receivers for Space Applications

• ”Low cost” 12 Channel L1 C/A

code space GPS receiver for

small satellites for $17,900

• Mass: 20g (40g with screens)

• Design lifetime (LEO): 7 Years

• Radiation Tolerance: >10kRad

(Si) Total Dose

• Interfaces : Serial data

interface; Pulse-per-second

• Position to 10m (95%)

• Velocity to 15cm/s (95%)

• Typical TTFF (warm) 50s

• Typical TTFF (cold) 550s

• 5V Supply, 0.8W

• 70 x 45 x 10mm


Marine Applications

• Used for position, heading, rate of turn, speed

and other measurements in the sea and in the

ports

• Usually part of a total navigation system of the

ship (INS, compass, autopilot, AIS etc.)

• DGPS, and Kalman filters are often used


Timing Receivers

• For science

• For network synchronization

– Internet, mobile and other

• Can synchronize

– Time

– Frequency

• Can be combined with additional

clocks in the same package


Alternative Systems

• Research and development continues how to

adapt or modify existing systems to provide

positioning services

– Legacy ground based systems (no perspectives)

– WiFi (very limited precision capabilities)

– Mobile Networks (does not meet today’s GNSS

precision level; new protocol versions are under

development)

– TV (DVB) signals based

– Proprietary, local (for example LOCATA)

– New methods based on GPS+LEOS (for example

Boeing Timing & Location)


GNSS Measurements


Tasks To Perform For Position

Computations In a GNSS Receiver

• Acquire GNSS signals

• Track all acquired satellite signals

• Decode the navigation messages from all satellites

• Do measurements of transmission time (in other

words take “snapshots” of all signal, time tracking

counters)

• Correct transmission time

• Compute satellite position at transmission time

• Compute pseudoranges to all satellites

• Compute the receiver position based on

pseudoranges and satellite positions (include

compensations for various signal delays)


Range And Pseudorange

• The true (geometrical) range between satellite

k and receiver i is denoted as ρik

• The range can be expressed through satellite

signal travel time (in GPS time)

• The receiver can measure only the sum of the

true range and the signal delay


k

i

k

i

k

i

k

i

k

i

k

i eITdtdtcP )(

Geometrical

range

Clock

errors

Troposphere

delay

Ionosphere

delay

Other errors

(including

multipath)

)( k

i

k

i ttc

GNSS Signal


Data frames and sub-frames

Contained data:

Satellite status (health etc.)

Satellite clock corrections

Satellite coordinates (ephemerides)

Ionosphere correction

GNSS to UTC time conversion

Almanac

Subframe start mark (preamble) and a time tag

The GNSS signal is also a ruler

For GPS case:

1 sub-frame = 300 bits = 6 sec

1 bit = 20 spreading codes

1 spr. Code = 1023 chips = 1 ms

1 spr. Code ≈ 300 km

1 chip ≈ 1μs (at 1.023 Mc/s) ≈ 300 m

1 carrier wave at 1.5 GHz = 0.1903 m

The Speed of Light

299 792 458 m/s

Data For Pseudorange

Measurements

• The pseudorange measurement procedure

must know when a GNSS signal was

transmitted

• Therefore navigation data processing must

provide information at which code start a

subframe was detected and what is TOW of

that subframe


tk = 500.095s GPS time

Number of

complete bits

Number of

complete codes

Number

of chips

TLM HOW Navigation data of the sub-frame

Time of Transmission

• All satellites transmit signals at the same time

• Due to different distances between receiver

and satellites the GPS (GNSS) signals will

arrive at receiver at different time instances


t1 = 500.067s

t2 = 500.068s t3 = 500.077s

t4 = 500.095s GPS time

CH: 1

CH: 2

CH: 3

CH: 4

Position Basic Computation


LS or

Kalman

filter

Solution

X, Y, Z

Velocity

dti

Pseudorange

construction

Pki = (ti - (t

k + dtk))•c

(may also include DGPS

and other corrections)

Satellite clock

correction dtk

and position

computation Satellite

coordinates

Measurements

at

ti = tGPS + dti

tk

dtk

Ephemerides k

i

k

i

k

i

k

i

k

i

k

i eITdtdtcP )(

Ext. measurements

)( k

i

k

i ttc

Receiver Measurements

• A GNSS receiver does a number of

measurements

– Signal time of transmission (code) measurements

– “Carrier phase” based measurements

– Doppler based measurements

– Signal to noise ratio measurements

• RF interference measurements

– Some receivers can estimate multipath

• Professional receivers do measurements on

two or more carrier frequencies

• Measurements on all channels are done at the

same time instance (epoch) 2013 Danish GPS Center 27

Ambiguity Resolution

Computational Models (1)

• A one-way code observation on frequency L1 between

receiver 𝒊 and satellite 𝒌 is characterized by:

𝑷𝟏,𝒊𝒌 = 𝝆𝒊

𝒌 + 𝒄 𝒅𝒕𝒊 − 𝒅𝒕𝒌 + 𝑻𝒊𝒌 + 𝑰𝒊

𝒌 + noise.

• A single difference of code observations on L1

between two receivers 𝒊 and 𝒋:

𝑷𝟏,𝒊𝒌 − 𝑷𝟏,𝒋

𝒌 =

𝝆𝒊𝒌 + 𝒄 𝒅𝒕𝒊 − 𝒅𝒕𝒌 + 𝑻𝒊

𝒌 + 𝑰𝒊𝒌

−𝝆𝒋𝒌 − 𝒄 𝒅𝒕𝒋 − 𝒅𝒕𝒌 − 𝑻𝒋

𝒌 − 𝑰𝒋𝒌

+ noise

= 𝝆𝒊𝒌 − 𝝆𝒋

𝒌 + 𝒄 𝒅𝒕𝒊 − 𝒅𝒕𝒋 + 𝑻𝒊𝒌 − 𝑻𝒋

𝒌 + 𝑰𝒊𝒌 − 𝑰𝒋

𝒌 + noise.

• Note that the satellite clock offset term 𝒅𝒕𝒌 cancels!




• Next we calculate the double difference between two

receivers 𝒊 and 𝒋 and two satellites 𝒊 and 𝒍:

𝑷𝟏,𝒊𝒌 − 𝑷𝟏,𝒋

𝒌 − 𝑷𝟏,𝒊𝒍 − 𝑷𝟏,𝒋

𝒍 =

𝝆𝒊𝒌 − 𝝆𝒋

𝒌 + 𝒄 𝒅𝒕𝒊 − 𝒅𝒕𝒋 + 𝑻𝒊𝒌 − 𝑻𝒋

𝒌 + 𝑰𝒊𝒌 − 𝑰𝒋

𝒌

− 𝝆𝒊𝒍 − 𝝆𝒋

𝒍 + 𝒄 𝒅𝒕𝒊 − 𝒅𝒕𝒋 + 𝑻𝒊𝒍 − 𝑻𝒋

𝒍 + 𝑰𝒊𝒍 − 𝑰𝒋

𝒍 + noise =

𝝆𝒊𝒌 − 𝝆𝒋

𝒌 − 𝝆𝒊𝒍 + 𝝆𝒋

𝒍

+𝑻𝒊𝒌 − 𝑻𝒋

𝒌 − 𝑻𝒊𝒍 + 𝑻𝒋

𝒍 + 𝑰𝒊𝒌 − 𝑰𝒋

𝒌 − 𝑰𝒊𝒍 + 𝑰𝒋

𝒍 + noise

• Or with obvious notation

𝑷𝟏,𝒊𝒋𝒌𝒍 = 𝝆𝟏,𝒊𝒋

𝒌𝒍 + 𝑻𝟏,𝒊𝒋𝒌𝒍 + 𝑰𝟏,𝒊𝒋

𝒌𝒍 + noise.

• We observe that all clock offsets cancel in a double

difference! That was exactly the purpose of making this

linear combination of the four observed one-ways.




• The standard deviation of a C/A-code observation is

𝛔C/A−code = 3 m and that one of a P-code observation

𝝈P−code = 0.3 m.

• Geodetic GPS receivers additionally observe the

phase of the carrier wave. A phase observation 𝚽𝒊𝒌 𝒕

is the difference in phase from a signal generated at

the same frequency as pseudorange and multiplied by

the wave length 𝝀. The basic equation is

𝜱𝒊𝒌 𝒕 = 𝝆𝒊

𝒌 − 𝑰𝒊𝒌 + 𝑻𝒊

𝒌 + 𝒄 𝒅𝒕𝒊 𝒕 − 𝒅𝒕𝒌 𝒕 − 𝝉𝒊𝒌

+ 𝝀 𝝋𝒊 𝒕𝟎 − 𝝋𝒌 𝒕𝟎 + 𝝀𝑵𝒊𝒌 + noise.




• The new terms are the ambiguities𝑵𝒊𝒌 between satellite

𝒌 and receiver 𝒊, and the nonzero initial phases 𝝋𝒌 𝒕𝟎

and 𝝋𝒊 𝒕𝟎 . Finally the double difference for the phase

is

𝚽𝟏,𝒊𝒋𝒌𝒍 = 𝝆𝟏,𝒊𝒋

𝒌𝒍 − 𝑰𝒊𝒋𝒌𝒍 + 𝝀𝟏 𝑵𝟏,𝒊

𝒌 − 𝑵𝟏,𝒊𝒍 − 𝑵𝟏,𝒋

𝒌 − 𝑵𝟏,𝒋𝒍 + noise.

• The standard deviation of a phase observation is

𝝈𝒑𝒉𝒂𝒔𝒆 = 𝟑 mm.

• The important observation to make is that in double

differences 𝑵𝒊𝒋𝒌𝒍 is an integer. Knowing the correct

integer and with phase 𝝈𝒑𝒉𝒂𝒔𝒆 = 𝟑 mm it becomes

possible to estimate the baseline between 𝒊 and 𝒋 at

centimeter level.


GNSS Distance Measurement

Errors


GNSS Signal Propagation

2013 33 Danish GPS Center

GNSS Receiver

GNSS Satellite

Signal

generation Amplifier

Amplifier

Atmosphere

...

Antenna

• Signal changes

during propagation:

• Attenuations

• Frequency and

phase offsets

• Signal delays

• Reflections

Troposphere

Free space

Ionosphere

Antenna

Measurement Corrections

• Models are used in standalone case

• Differential GPS is using measurements from

a second GPS receiver (called the base

station) to correct the distance

measurements. Professional user can use a

network of GPS base stations (monitoring

stations)

• WAAS provides Ionosphere correction data

and also DGPS type corrections

– WAAS monitoring stations use also other types of

atmosphere measurements

• A-GPS can provide DGPS corrections 2013 Danish GPS Center 34

The Menu of Future GNSS Signals

• Originally GPS and

GLONASS had one

signal on one carrier

for civil applications

• Future GNSS offer

system diversity and

frequency diversity

System Signal

Carrier

frequency

[MHz]

Component Type Data rate

[sps/bps] Modulation

Chipping

rate

[Mcps]

Code length

[chips]

Full

length

[ms]

GLONASS L1 OF 1605.375-

1609.3125

standard Data -/50 BPSK

0.511 511 1

SF high accur. Military 5.11

COMPASS B1 1575.42

B1-CD Open

100/50 MBOC(6,1,1/11) 1.023

B1-CP -/-

B1 Authorized 100/50

BOC(14,2) 2.046

-/-

Galileo E1 1575.42

A PRS cosBOC(15,2.5) 2.5575

B Data, SOL 250/125 CBOC(6,1,1/11) 1.023

4092 4

C Pilot, SOL -/- 4092 * 25 100

GLONASS L1 OC/SC 1575.42

GPS L1 1575.42

C/A Data -/50 BPSK 1.023 1023 1

P(Y) Military

BPSK 10.23 7 days 7 days

M BOC(10,5) 5.115

Galileo E6 1278.75

A PRS cosBOC(10,5)

B Data 1000/500 BPSK(5) 5.115

5115 1

C Pilot -/- 5115 * 100 100

COMPASS B3 1268.52

B3

Authorized

-/500 QPSK(10) 10.23

B3-AD 100/50 BOC(15,2.5) 2.5575

B3-AP -/-

GLONASS L2 OF 1242.9375-

1248.1875

standard Data -/50 BPSK

0.511 511 1

SF high accur. Military 5.11

GPS L2 1227.6

L2 CM Data 50/25

or -/50 TM and BPSK 0.5115 10230 20

L2 CL Pilot -/- 767250 1500

P(Y) Military

BPSK 10.23 7 days 7 days

M BOC(10,5) 5.115

GLONASS L3 OC 1207.14 QBSK(10)

GLONASS L3 OF/SF 1201.743-

1208.088

COMPASS B2 1191.795

B2aD

Open

50/25

AltBOC(15,10) 10.23

B2aP -/-

B2bD 100/50

B2bP -/-

Galileo E5

(1191.795)

E5a 1176.45 a-I Data 50/25

AltBOC(15,10) 10.23

10230 * 20 20

a-Q Pilot -/- 10230 * 100 100

E5b 1207.14 b-I Data, SOL 250/150 10230 * 4 4

b-Q Pilot, SOL -/- 10230 * 100 100

GPS L5 1176.45 I Data 100/50

QPSK 10.23 10230 1

Q Pilot -/- 1


Figure source – “GPS World”

GNSS L1 (carrier) spectrum

Space Based Augmentation Systems

• Wide Area Augmentation System (WAAS), USA

• European Geostationary Navigation Overlay Service

(EGNOS)

• System for Differential Correction and Monitoring

(SDCM), Russia

• GPS And Geo-Augmented Navigation (GAGAN)

system, India

• Quasi-Zenith Satellite System (QZSS), Japan

• Multi-functional Satellite Augmentation System

(MSAS), Japan


Receiver Changes I

• More frequency bands (radio front-ends), but

not all may be needed for all applications

• Extra channels required

• More complex channels than for GPS

– Galileo memory codes need extra resources

• Legacy GLONASS is using FDMA and a

different approach to satellite position data

• GPS and Galileo have compatible satellite

ephemerides representation and computation

• More complex navigation data processing

(FEC, interleaving etc.)


Receiver Changes II

• More navigation information will be

transmitted

– More detailed inter-signal delay information

– Open signals authentication for Galileo was under

consideration

– Differential corrections

• More powerful CPU is needed due to extra

channels and more complex PVT computation

• A likely consequence of increased complexity

– the receiver will require more power

– This is somehow alleviated by other receiver

technology improvements


Low Cost GNSS Receivers

• Device cost, size, power consumption and

integration price reduction are the primary

drivers

• Part of the receiver market are solutions for

car navigation – some receivers come with

INS integration features

• INS integration is likely to spread also in other

markets


Professional Receivers

• More added features, communication

possibilities, service integration

• Receivers exploit the system and frequency

diversity already today

• Continuation (relatively slow) of size and cost

reduction

• Tendency to contain a full set of channels per

GNSS system for maximum performance


GNSS Development Schedule

• In the first

decades GPS

and GLONASS

evolved slowly

• In the last

decade the

development

of space and

ground control

segments is

intense


Test & deployment GPS III

GPS II

2020 2012 2010 2015

COMPASS

COMPASS 1 (end date unknown)

Test & deployment of L5

GPS III FOC

L1C FOC

L5 FOC

L2C Full Operational Capability (FOC)

GLONASS Full Operational Capability (FOC)

COMPASS 2 test & depl. COMPASS 2/3, regional service; global service depl. COMPASS 3 FOC

New

signals

FOC

SDCM design/tests

GLONASS-M (launched until

2012)

GLONASS-K2 (KM after 2015)

New: L1OC, L3OC, L1SC, L2SC (CDMA), SAR

GLONASS-K1 New: L3OC (CDMA),

SAR

Galileo launch

Sys. testbed v1/v2 IOV Deployment

Galileo operational

SDCM fully deployed

18 SV OC

Test & deployment of L1C

Test & deploym. of L2C, staged roll-out of CNAV

Thank You For Your Attention

http://gps.aau.dk

2013 42 Danish GPS Center

DANISH GPS CENTER

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Documents

GNSS Receivers, Introduction