This document discusses the impact of GLONASS on GNSS receiver design compared to GPS. It notes that GLONASS uses frequency division multiple access (FDMA) rather than the code division multiple access (CDMA) used by GPS, requiring receivers to handle multiple frequencies. This affects the RF design, satellite acquisition, tracking algorithms, and positioning calculations. It also discusses issues like inter-receiver compatibility. Overall, GLONASS provides benefits like increased availability but requires more advanced designs to handle the technical challenges of its FDMA approach.

1. 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
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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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