Septentrio en GLONASS receiver design

Impact of GLONASSon receiver design

FDMA versus CDMA

Frank BoonNiN Workshop 27/03/09

March 27, 2009 © Septentrio 2

Background

Prototyped GPS+GLONASS PolaRx2 based on 4 Proprietary RF Chips

not commercialized : poor GLONASS constellation

served as engineering lessons platform

GLONASS ‘resurrection’ increased demand

AsteRx2 designed as GPS+GLO+GAL from start

AsteRx2 serves as geodetic OEM GPS+GLO receiver in many products

GLONASS workshop Amsterdam


Contents

Background

FDMA in detail

GLONASS Impact on: RF Design

Satellite Acquisition

Tracking

Positioning Algorithms

Receiver Compatibility

Summary

Outlook



FDMA in detail

Satellite identification division multiple access

GPS uses digital identification : one central carrier

GLONASS uses frequency identification

one PRN-code for all GLONASS satellites

12 individual frequency carriers in-view k : Frequency Number, Fn

more k-values defined

k=0 k=+1 k=+2k=-2 k=-1



FDMA in detail

Consequences of FDMA Requires wider band-pass filter

Individual filter response per carrier

Fn-SlotNR relation not defined / guaranteed SlotNR important : antipodal system, Fn reused

users accustomed to PRN, SlotNR feels like PRN

recently 2 SV-combinations changed Fn without announcement.

Individual wavelength for each satellite influence on RTK,PPP algorithms



Impact on RF design

Current GNSS spectrum



Impact on RF design

band-pass filter choices : individual filters



Impact on RF design

band-pass filter choices : combined GPS+GLO filters



Impact on RF design

Individual carriers per SV each GLONASS satellite excites different part of RF

Individual group delay and phase shift per SV pseudoranges and carrier phases will be biased

frequency [Hz]

ph

ase

dela

y [

rad

]

Filter Frequency Response



Impact on RF design

RF quality determines measurement quality linearize group delay : simplifies calibration

reduce slope of filter response

ensure temperature stability

bias around cm-level

Combined GPS+GLO RF equal mean RF group delay for GPS+GLONASS

GPS enables calibrate GLONASS hardware delays

improved temperature stability & coherency

frequency [Hz]

ph

ase d

ela

y [

rad

]



Impact on Satellite Acquisition

Previously cumbersome... GLONASS-M significant improvement

Fn and SlotNR relation not defined upfront

GLONASS : relation only defined in almanac

GLONASS-M : both in almanac & ephemeris

Cold Boot : acquire on Fn first GLONASS : wait for almanac before reporting SLot

GLONASS-M : directly available

RINEX and RTCM data SlotNR based

Still one aging non-M GLONASS satellite...



Impact on Tracking

implementation of receiver Clock Jumps odd Fn can violate integer number wavelengths

1 msec jump shifts c*10-3 / l = f*10-3 wavelenghts

spacing of 0.5625 MHz leads to 562.5 wavelengths

2 msec jumps only, or adjust phase measurements

non FDMA related P-code not encrypted but content unknown

tracking is based on assumptions of content

during past year three changes have happened requires firmware updates (brand indepedent)

use L2CA instead (GLONASS-M only)



Impact on Positioning Algorithms

PZ90-2 solved difference with WGS-84

Cumbersome Ephemeris format

Orbit prediction Quality still lacking

GPS-GLONASS time difference (MOSCOW UTC) varies over time, observed 100 nanosec / month

individual wavelengths adapt RTK and PPP algorithms

use abstract carrier phases : non-integer

pair closely space Fn : large phase residuals

RTCM/CMR generation requires receiver clock bias wrt GLONASS



Inter Receiver Compatibility

RTK based on double differences affected by mixing receiver brands or generations

Different HW design, different group delays RF implementations vary per receiver brand

DD-biases depend on included Fn combinations

Calibrate with Initial GPS only RTK solution determine phase shift and group delay

determination for each Fn single difference

monitor time behavior to compensate T drift



Summary

FDMA has a significant impact on design hardware : RF quality required

firmware : common firmware blocks not possible

Not relevant for end-user if properly designed

GLONASS-M is a step forward direct link between SlotNR and Fn

L2CA is future proof, avoid P accuracy degradation acceptable

GLONASS-K significant leap looking forward to CDMA



Outlook

Not meant as negative towards GLONASS great constellation to support GPS until Galileo

arrives

GPS+GLO improves DOP, height component

GPS+GLO improves availability

GPS+GLO improves integrity (redundancy)

When Galileo arrives GLONASS will modernize to CDMA

L3 overlaps with E5 : great DualFreq GPS+GLO+GAL

fantastic quadruple constellation opportunities next workshop on what will Compass (Beidou-2) bring...



Outlook

Thanks for listening.


Technology

Septentrio en GLONASS receiver design