u-blox ANTARIS 4 User Manual

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1.6 Interference Issues
A typical GPS receiver has a very low dynamic range. This is because the antenna should only detect thermal
noiseintheGPSfrequencyband,giventhatthepeakpoweroftheGPSsignalis15dBbelowthethermalnoise
floor. This thermal noise floor is usually very constant over
 time. Most receiver architectures use an automatic
ndjammingsignalsdecreasesmoreor
esignificantly.
 in desensitizing of the whole receiver. One such particularly difficult scenario is the
tegrationoftheantennawithotherdigitalsystems,oneshouldmakesure

zan
On thereceiver side there’snotmuch that can be done toimprove the situationwithout significanteffort.Of
course, high price military receivers have integrated counter-measures against intentional jamming. But the
the scope of thisdocument and might evenconflict withexportrestrictions for
e specific applications. In
situ n ectronics,
inte e

rtolimitdisruptive
fthereceiver.Inthatcase,onlyspecialfilteringbetweentheGPSantenna
andreceiverinputwillhelptoreducesignallevelstotheleveloflinearoperationatthefront-end.
o rlyprovestobehardtosolve.Here,the
emittingsourceis
gain control(AGC) circuitryto automatically adjustto theinput levels presentedbydifferentantennaand pre-
amplifiercombinations.ThecontrolrangeoftheseAGC’scanbeaslargeas50dB.However,thedynamicrange
for a jamming signal exceeding the thermal noise
 floor is typically only 6 to 12dB, due to the one or two bit
quantizationschemescommonlyusedinGPSreceivers.Iftherearejammingsignalspresentattheantennaand
the levels of these signals exceed the thermal noise power, the AGC will regulate the jamming signal,
suppressing the
GPS signal buried in thermal noise even further. Depending on thefiltercharacteristicsof the
antennaandthefrontendoftheGPSreceiver,thesensitivitytosuchin-ba
lessrapidlyifthefrequencyofthejammingsignalmovesawayfromGPSsignalfrequency.Wecan
concludethat
a jamming signal exceeding thermal noise floor within a reasonable bandwidth (e.g. 100 MHz) around GPS
signalfrequencywilldegradetheperformanc
Evenout-of-bandsignalscanaffectGPSreceiverperformance.Ifthesejammingsignalsarestrongenoughthat
evenantenna andfront-endfilter attenuation arenotsufficient,
theAGCwill stillregulatethejamming signal.
Moreover, very high jamming signal levels can result in non-linear effects in the pre-amplifier stages of the
receiver, resulting
transmitting antenna of a DCS handset (max. 30dBm at 1710
MHz) in close proximity to the GPS antenna.
WhenintegratingGPSwithotherRFtransmittersspecialcareisnecessary.
Iftheparticularapplicationrequiresin
thatjammingsignallevelsarekepttoanabsoluteminimum.Evenharmonicsofa
CPUclock canreachashighas
1.5GH dstillexceedthermalnoisefloor.
methods employed are out
 of
dual-usegoods.
The recommendations and concepts in this section are completely dependent on th
atio s where an active antenna is used in a remote position, e.g. >1m away from other el
rfer nce
shouldnotbeanissue.
Note If antenna and electronics are to be integrated tightly, the following sections should be read very
carefully.
1.6.1 Sources of Noise
BasicallytwosourcesareresponsibleformostoftheinterferencewithGPSreceivers:
1. StrongRFtransmittersclosetoGPSfrequency,e.g.DCSat1710MHzorradarsat1300MHz.
2. Harmonicsoftheclockfrequencyemittedfromdigitalcircuitry.
Thefirstproblemcanbeverydifficulttosolve,but
ifGPSandRFtransmitteraretobeintegratedclosetoeach
other, there’s a good chance that there is an engineer at hand who knows the specifications of the RF
transmitter.Inmostcases,countermeasuressuchasfilterswillberequiredforthetransmitte
emissions
belowthenoisefloorneartheGPSfrequency.
Even if the transmitter is quiet in the GPS band, a very strong emission close to it can cause saturation in the
front-endofthereceiver.Typically,thereceiver'sfront-endstagewillreachitscompressionpoint,which willin
turnincreasetheoverall
noisefigureo
Thesec ndproblemismorecommonbutalsoregula
notwellspecifiedandtheemissioncanbeofbroadbandnature,makingspecificcountermeasuresverydifficult.
GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals
GPS.G4-MS4-05007-A1
  Page 22

u-blox ANTARIS 4 Specifications

General

Brand	u-blox
Model	ANTARIS 4
Category	GPS
Language	English