Contenu connexe Similaire à Septentrio en GLONASS receiver design (20) Plus de Hydrographic Society Benelux (20) Septentrio en GLONASS receiver design1. Impact of GLONASS
on receiver design
FDMA versus CDMA
Frank Boon
NiN Workshop 27/03/09
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
March 27, 2009 © Septentrio GLONASS workshop Amsterdam 2
3. Contents
Background
FDMA in detail
GLONASS Impact on:
RF Design
Satellite Acquisition
Tracking
Positioning Algorithms
Receiver Compatibility
Summary
Outlook
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4. FDMA in detail
Satellite identification division multiple access
GPS uses digital identification : one central carrier
GLONASS uses frequency identification
k=-2 k=-1 k=0 k=+1 k=+2
one PRN-code for all GLONASS satellites
12 individual frequency carriers in-view
k : Frequency Number, Fn
more k-values defined
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5. 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
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6. Impact on RF design
Current GNSS spectrum
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7. Impact on RF design
band-pass filter choices : individual filters
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8. Impact on RF design
band-pass filter choices : combined GPS+GLO filters
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9. Impact on RF design
Individual carriers per SV
each GLONASS satellite excites different part of RF
phase delay [rad]
frequency [Hz]
Filter Frequency Response
Individual group delay and phase shift per SV
pseudoranges and carrier phases will be biased
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10. Impact on RF design
RF quality determines measurement quality
linearize group delay : simplifies calibration
reduce slope of filter response
ensure temperature stability
phase delay [rad]
bias around cm-level
Combined GPS+GLO RF frequency [Hz]
equal mean RF group delay for GPS+GLONASS
GPS enables calibrate GLONASS hardware delays
improved temperature stability & coherency
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11. 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...
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12. 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)
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13. 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
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14. 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
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15. 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
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16. 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...
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17. Outlook
Thanks for listening.
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