RADNAV 2. ZH
If the PRI of a radar is 2100 microseconds, the maximum range of the radar is: a. 170 NM b. 315 NM c. 340 NM d. 630 NM
a. 170 NM
The period of validity of the navigational database is: a. 28 days b. 1 month c. determined by the national authority and may be from 28 days to 91 days d. 91 days
a. 28 days
A primary radar has a pulse repetition frequency of 275 pps. The time interval between the leading edges of successive pulses is: a. 3.64 milliseconds b. 36.4 milliseconds c. 3.64 microseconds d. 36.4 microseconds
a. 3.64 milliseconds
Refer to Appendix A. What are the correct selections to insert the most accurate position into the IRS? a. 3R then 4R b. 2R then 4R c. 4R then 3R d. 3L then 4R
a. 3R then 4R
An airborne weather radar is required to detect targets up to a maximum range of 200 NM. Ignoring pulse length and flyback in the CRT calculate the maximum PRR. a. 405 pps b. 810 pps c. 1500 pps d. 750 pps
a. 405 pps
The navigation and system data message is transmitted through the: a. 50 Hz modulation b. the C/A and P PRN codes c. the C/A code d. the P code
a. 50 Hz modulation
An echo is received from a target 900 microseconds after the pulse was transmitted. The range to the target is: a. 73 NM b. 270 NM c. 135 NM d. 146 NM
a. 73 NM
The special SSR codes are as follows: emergency ............... , radio failure ................ , unlawful interference with the conduct of the flight ................ a. 7700 7600 7500 b. 7700 7600 7500 c. 7600 7500 7700 d. 7500 7600 7700
a. 7700 7600 7500
On which displays will the range markers be displayed regardless of the weather selection? a. MAP b. EXP ILS/VOR, MAP c. MAP, FULL ILS d. PLAN, EXP ILS/VOR, MAP
a. MAP
The most accurate satellite fixing information will be obtained from: a. NAVSTAR/GPS & GLONASS b. TRANSIT & NAVSTAR/GPS c. COSPAS/SARSAT & GLONASS d. NAVSTAR/GPS & COSPAS/SARSAT
a. NAVSTAR/GPS & GLONASS
The frequency band and rate of scan of Airfield Surface Movement radars are: a. SHF; 60 rpm b. SHF; 200 rpm c. EHF; 100 rpm d. EHF; 10 rpm
a. SHF; 60 rpm
NAVSTAR/GPS operates in the ....... band the receiver determines position by .......: a. UHF range position lines b. UHF secondary radar principles c. SHF secondary radar principles d. SHF range position lines
a. UHF range position lines
To improve the resolution of a radar display requires: a. a narrow pulse width and a narrow beamwidth b. a high frequency and a large reflector c. a wide beamwidth and a wide pulse width d. a low frequency and a narrow pulse width
a. a narrow pulse width and a narrow beamwidth
The mapping mode of Airborne Weather Radar utilizes: a. a pencil/weather beam from 70 NM to 150 NM b. a cosecant²/fan-shaped beam which is effective to 150 NM c. a pencil/weather beam with a maximum range of 70 NM d. a cosecant²/fan-shaped beam effective 50 NM to 70 NM
a. a pencil/weather beam from 70 NM to 150 NM
A ground based radar with a scanner rotation of 60 rpm, a beamwidth in the order of 0.5° and a PRF of 10 000 pps would be: a. an Airfield Surface Movement Indicator with a theoretical range of 8 NM b. a Precision Approach Radar c. an Airfield Surface Movement Indicator with a theoretical range of 16 NM d. a high resolution Surveillance Approach Radar
a. an Airfield Surface Movement Indicator with a theoretical range of 8 NM
When using GNSS to carry out a non-precision approach the MDA will be determined using: a. barometric altitude b. GPS altitude c. radio altimeter height d. either barometric or radio altimeter altitude
a. barometric altitude
The PRN codes are used to: a. determine the time interval between the satellite transmission and receipt of the signal at the receiver b. pass ephemeris and clock data to the receivers c. synchronize the receiver clocks with the satellites clocks d. determine the range of the satellites from the receiver
a. determine the time interval between the satellite transmission and receipt of the signal at the receiver
The purpose of the pseudo-random noise codes in NAVSTAR/GPS is to: a. identify the satellites b. pass the almanac data c. pass the navigation and system data d. pass the ephemeris and time information
a. identify the satellites
The IRS position can be updated: a. on the ground only b. at designated positions en route and on the ground c. on the ground and overhead VOR/DME d. at selected waypoints and on the ground
a. on the ground only
The factor which determines the maximum range of a radar is: a. pulse repetition rate b. pulse width c. power d. beamwidth
a. pulse repetition rate
The initial range calculation at the receiver is known as a pseudo-range, because it is not yet corrected for: a. receiver clock errors b. receiver and satellite clock errors c. receiver and satellite clock errors and propagation errors d. receiver and satellite clock errors and ephemeris errors
a. receiver clock errors
Above latitudes of 84° a twin FMS/triple IRS system will go to de-coupled operations. The reason for this is: a. to prevent error messages as the IRS longitudes show large differences b. to ease the pilot's workload c. to improve the system accuracy d. because the magnetic variation changes rapidly in high latitudes
a. to prevent error messages as the IRS longitudes show large differences
If a receiver has to download the almanac, the time to do this will be: a. 2.5 minutes b. 12.5 minutes c. 25 minutes d. 15 minutes
b. 12.5 minutes
The frequency available to non-authorized users of NAVSTAR/GPS is: a. 1227.6 MHz b. 1575.42 MHz c. 1602 MHz d. 1246 MHz
b. 1575.42 MHz
A primary radar system has a pulse repetition frequency of 450 pps. Ignoring pulse width and flyback at the CRT, the maximum range of the radar would be: a. 333 NM b. 180 NM c. 666 NM d. 360 NM
b. 180 NM
The orbital height and inclination of the NAVSTAR/GPS constellation are: a. 20 180 km, 65° b. 20 180 km, 55° c. 19 099 km, 65° d. 19 099 km, 55°
b. 20 180 km, 55°
An aircraft using a basic 2D RNAV system is on a section between WP1 and WP2, a distance of 45 NM. The aircraft is 20 NM from the phantom station, which is 270°/30 NM from the VOR/DME. The aircraft is 15 NM from the VOR/DME. The range read-out will show: a. 15 NM b. 20 NM c. 25 NM d. 30 NM
b. 20 NM
The maximum range at which VOR bearing information will be used by the B737-400 FMC for fixing is: a. 10 NM b. 25 NM c. 50 NM d. 60 NM
b. 25 NM
The time between the transmission of a pulse and the reception of the echo from a target is 1720 microseconds. What is the range of the target? a. 139 km b. 258 km c. 278 km d. 516 km
b. 258 km
When using SSR the ground controller will ask the pilot to cancel mode C if there is a discrepancy of more than ............... between the altitude detected by the radar from the reply pulses and the altitude reported by the pilot read from an altitude with the subscale set to ............... a. 100 ft Regional QNH b. 300 ft 1013 HPa c. 50 ft 1013 HPa d. 300 ft Regional QNH
b. 300 ft 1013 HPa
The minimum number of satellites required for a 3D fix is: a. 3 b. 4 c. 5 d. 6
b. 4
Using airborne weather radar the weather beam should be used in preference to the fan-shaped beam for mapping in excess of ................ NM: a. 20 to 25 b. 60 to 70 c. 100 to 150 d. 150 to 200
b. 60 to 70
A frequency used by airborne weather radar is: a. 8800 MHz b. 9.375 GHz c. 93.75 GHz d. 1213 MHz
b. 9.375 GHz
The NAVSTAR/GPS reference system is: a. A geo-centred 3D Cartesian coordinate system fixed with reference to the sun b. A geo-centred 3D Cartesian coordinate system fixed with reference to the prime meridian, equator and pole c. A geo-centred 3D Cartesian coordinate system fixed with reference to space d. A geo-centred 3D system based on latitude, longitude and altitude
b. A geo-centred 3D Cartesian coordinate system fixed with reference to the prime meridian, equator and pole
Which positions can be input to the FMC using a maximum of 5 alphanumerics? a. SIDS & STARS, reporting points and airways designators b. Navigation facilities, reporting points and airways designators c. SIDS & STARS and latitude and longitude d. Latitude and longitude, reporting points and airways designators
b. Navigation facilities, reporting points and airways designators
DME operates in the ............... frequency band, it transmits ............... which give it the emission designator of ............... a. SHF double size pulses P01 b. UHF twin pulses P0N c. EHF twin pulses A9F d. UHF double pulses J3E
b. UHF twin pulses P0N
A basic 2D RNAV system will determine tracking information from: a. twin DME b. VOR/DME c. twin VOR d. any of the above
b. VOR/DME
An all in view receiver: a. informs the operator that all the satellites required for fixing and RAIM are in available b. checks all the satellites in view and selects the 4 with the best geometry for fixing c. requires 5 satellites to produce a 4D fix d. uses all the satellites in view for fixing
b. checks all the satellites in view and selects the 4 with the best geometry for fixing
A false indication of water may be given by the AWR display when: a. flying over land with the Land/Sea switch in the Sea position b. flying over mountainous terrain c. there is cloud and precipitation between the aircraft and a cloud target d. attempting to use the mapping beam for mapping in excess of 50 NM
b. flying over mountainous terrain
The antenna of an Airborne Weather Radar is stabilized: a. in pitch, roll and yaw b. in pitch and roll c. in pitch and roll whether the stabilization is on or off d. in pitch and roll but only when 0° tilt has been selected
b. in pitch and roll
The use of Doppler techniques to discriminate between aircraft and fixed objects results in second trace returns being generated. These are removed by: a. using a different frequency for transmission and reception b. jittering the PRF c. making regular changes in pulsewidth d. limiting the power output of the radar
b. jittering the PRF
The position derived from NAVSTAR/GPS satellites may be subject to the following errors: a. selective availability, sky wave interference, PDOP b. propagation, selective availability, ephemeris c. PDOP, static interference, instrument d. ephemeris, PDOP, siting
b. propagation, selective availability, ephemeris
Primary radar operates on the principle of: a. transponder interrogation b. pulse technique c. phase comparison d. continuous wave emission
b. pulse technique
The availability of two frequencies in GNSS: a. removes SV ephemeris and clock errors b. reduces propagation errors c. reduces errors caused by PDOP d. removes receiver clock errors
b. reduces propagation errors
The bearing of a primary radar is measured by: a. phase comparison b. searchlight principle c. lobe comparison d. DF techniques
b. searchlight principle
DME is a ............... radar operating in the ............... band and uses ................ in order to obtain range information. The correct words to complete the above statement are: a. primary SHF CW signals b. secondary UHF twin pulses c. secondary SHF "jittered pulses" d. primary UHF pulse pairs
b. secondary UHF twin pulses
The use of LAAS and WAAS remove the errors caused by: a. propagation, selective availability, satellite ephemeris and clock b. selective availability, satellite ephemeris and clock c. PDOP, selective availability and propagation d. receiver clock, PDOP, satellite ephemeris and clock
b. selective availability, satellite ephemeris and clock
If an aircraft manoeuvre puts a satellite being used for fixing into the wing shadow then: a. the accuracy will be unaffected b. the accuracy will be temporarily downgraded c. the receiver will automatically select another satellite with no degradation in positional accuracy d. the receiver will maintain lock using signals reflected from other parts of the aircraft with a small degrading of positional accuracy
b. the accuracy will be temporarily downgraded
For a DME and a VOR to be said to be associated it is necessary for: b. the DME to transmit on the same VHF frequency as the VOR b. the aerial separation not to exceed 100 ft in a TMA or 2000 ft outside a TMA c. the aerial separation not to exceed 100 m in a TMA or 2000 m outside a TMA d. both beacons to have the same first two letters for their ident but the last letter of the DME to be a 'Z'
b. the aerial separation not to exceed 100 ft in a TMA or 2000 ft outside a TMA
The transmission frequency of a DME beacon is 63 MHz removed from the aircraft interrogator frequency to prevent: a. interference from other radars b. the airborne receiver locking on to primary returns from its own transmissions c. static interference d. receiver accepting replies intended for other interrogators
b. the airborne receiver locking on to primary returns from its own transmissions
Referring to DME during the initial stage of the "search" pattern before "lock-on": a. the airborne receiver checks 150 pulses each second b. the airborne transmitter transmits 150 pulses each second c. the ground receiver maintains the ground transmitter pulse transmission rate at no more than 150 per second d. the aircraft transmits 24 pulses per second and the receiver checks a maximum of 150 pulses per second
b. the airborne transmitter transmits 150 pulses each second
A VOR/DME share the same first two letters of their respective identifiers; the last identifying letter of the DME is a Z. This means that: a. they are co-located b. they are more than 600 m apart but serve the same location c. they are widely separated and do not serve the same location d. they are a maximum distance of 30 m apart
b. they are more than 600 m apart but serve the same location
The FMC navigational database can be accessed by the pilots: a. to update the database b. to read information only c. to change information between the 28 day updates d. to change the information to meet the sector requirements
b. to read information only
The horizontal deviation on the expanded ILS display represented by one dot is approximately: a. 1° b. 2° c. 0.5° d. 5°
c. 0.5°
The accuracy required of a precision area navigation system is: a. 0.25 NM b. 2 NM c. 1 NM d. 0.5 NM
c. 1 NM
A DME beacon having a transmit frequency of 962 MHz would have a receive frequency of: a. 1030 Mhz b. 902 Mhz c. 1025 Mhz d. 962 Mhz
c. 1025 Mhz
A radar is designed to have a maximum range of 12 km. The maximum PRF that would permit this is: a. 25 000 pps b. 6700 pps c. 12 500 pps d. 13 400 pps
c. 12 500 pps
If the PRF of a primary radar is 500 pulses per second, the maximum range will be: a. 324 NM b. 300 NM c. 162 NM d. 600 NM
c. 162 NM
The NAVSTAR/GPS operational constellation comprises how many satellites? a. 12 b. 21 c. 24 d. 30
c. 24
For a VOR and a DME beacon to be said to be associated the aerial separation must not exceed ............... in a terminal area and ................ outside a terminal area. a. 100 m 2000 m b. 50 ft 200 ft c. 30 m 600 m d. 50 m 200 m
c. 30 m 600 m
The minimum number of satellites required for receiver autonomous integrity monitoring is: a. 3 b. 4 c. 5 d. 6
c. 5
Airborne weather radar operates on a frequency of: a. 8 800 MHz because gives the best returns from all types of precipitation b. 13 300 MHz c. 9 375 MHz because it gives the best returns from rainfall associated with Cb d. 9.375 GHz because this frequency is best for detecting aircraft in flight
c. 9 375 MHz because it gives the best returns from rainfall associated with Cb
The sequence of displays accessed on initialization is: a. POS INIT, IDENT, RTE b. IDENT, RTE, POS INIT c. IDENT, POS INIT, RTE d. POS INIT, RTE, IDENT
c. IDENT, POS INIT, RTE
The correct format for the input of position 50N 00527E to the CDU is: a. 5000.0N00527.0E b. N50E00527 c. N5000.0E00527.0 d. N5000E00527
c. N5000.0E00527.0
Which of the following statements concerning NAVSTAR/GPS time is correct? a. Satellite time is the same as UTC b. The satellite runs its own time based on seconds and weeks which is independent of UTC c. The satellite runs its own time based on seconds and weeks which is correlated with UTC d. Satellite time is based on sidereal time
c. The satellite runs its own time based on seconds and weeks which is correlated with UTC
The model of the earth used for NAVSTAR/GPS is: a. WGS90 b. PZ90 c. WGS84 d. PZ84
c. WGS84
EGNOS is: a. the proposed European satellite navigation system b. a LAAS c. a WAAS d. a system to remove errors caused by the difference between the model of the earth and the actual shape of the earth
c. a WAAS
The NAVSTAR/ GPS control segment comprises: a. the space segment, the user segment and the ground segment b. a ground segment and the INMARSAT geostationary satellites c. a master control station, a back-up control station and five monitoring stations d. a master control station, a back-up control station, five monitoring stations and the INMARSAT geostationary satellites
c. a master control station, a back-up control station and five monitoring stations
Secondary radars require: a. a target which will respond to the interrogation, and this target will always be an aircraft b. a target which will respond to the interrogation, and this target will always be ground based c. a target which will respond to the interrogation, and this target may be either an aircraft or a ground based transponder d. a quiescent target
c. a target which will respond to the interrogation, and this target may be either an aircraft or a ground based transponder
Airborne DME equipment is able to discriminate between pulses intended for itself and pulses intended for other aircraft because: a. aircraft transmit and receive on different frequencies b. aircraft will only accept unique twin pulses c. aircraft reject pulses not synchronized with its own random pulse recurrence rate d. each aircraft has its own frequency allocation
c. aircraft reject pulses not synchronized with its own random pulse recurrence rate
The FMC position will be at its most inaccurate: a. on take-off b. at TOC c. at TOD d. on final approach
c. at TOD
DME and VOR are "frequency paired" because: a. the same receiver can be used for both aids b. the VOR transmitter is easily converted to the required DME frequency c. cockpit workload is reduced d. both ground transmitter aerials can be placed on the same site if required
c. cockpit workload is reduced
A DME receiver is able to distinguish between replies to its own interrogations and replies to other aircraft because: a. DME is secondary radar and each aircraft transmits and receives on a different frequency b. DME transponders reply to interrogations with twin pulses and the airborne equipment ejects all other pulses c. each aircraft transmits pulses at a random rate and will only accept synchronized replies d. when DME is in the search mode it will only accept pulses giving the correct range
c. each aircraft transmits pulses at a random rate and will only accept synchronized replies
A DME receiver is able to distinguish between replies to its own interrogation pulses and those intended for other aircraft using the same transponder because: a. DME is a secondary radar and each aircraft transmits and receives on a different frequency b. DME transponders reply to interrogations by means of twin pulses and the airborne equipment rejects all single pulses c. each aircraft transmits pulses at a random rate("jittering") and will only accept replies that match this randomization d. when DME is in the range search mode it will accept only pulses separated by + or - 63 MHz from the interrogation frequency
c. each aircraft transmits pulses at a random rate("jittering") and will only accept replies that match this randomization
The correct sequence of colours of a colour Airborne Weather Radar as returns get stronger is: a. red yellow green b. yellow green red c. green yellow red d. red green yellow
c. green yellow red
The most significant error of GNSS is: a. PDOP b. receiver clock c. ionospheric propagation d. ephemeris
c. ionospheric propagation
When a DME transponder becomes saturated: a. it reverts to standby b. it increases the number of pulse pairs to meet the demand c. it increases the receiver threshold to remove weaker signals d. it goes into a selective response mode of operation
c. it increases the receiver threshold to remove weaker signals
A DME transponder does not respond to pulses received from radars other than DME because: a. each aircraft transmits pulses at a random rate b. DME transmits and receives on different frequencies c. it will only accept the unique twin DME pulses d. DME only responds to the strongest 100 interrogators
c. it will only accept the unique twin DME pulses
The main advantage of continuous wave radars is: a. no maximum range limitation b. better range resolution c. no minimum range limitation d. better range resolution
c. no minimum range limitation
The factor which limits the minimum detection range of a radar is: a. pulse repetition interval b. transmitter power c. pulse width d. pulse repetition frequency
c. pulse width
Distance Measuring Equipment is an example of ............... radar operating on a frequency of ............... in the ................ band. a. primary 8800 MHz SHF b. secondary 1030 MHz UHF c. secondary 962 MHz UHF d. primary 9375 MHz SHF
c. secondary 962 MHz UHF
The position used by the FMC in the B737-400 is: a. an average of the two IRS positions b. an average of the two IRS positions, smoothed by the Kalman filtering process c. taken from the selected IRS, smoothed by Kalman filtering and updated to the external reference d. generated from the external reference and updated by the IRS as part of the Kalman filtering process
c. taken from the selected IRS, smoothed by Kalman filtering and updated to the external reference
The heading inputs to the EHSI are from: a. the IRS b. the FMC c. the IRS through the symbol generator d. the FMC through the symbol generator
c. the IRS through the symbol generator
The receiver of airborne DME equipment is able to "lock on" to its own "reply pulses" because: a. each aircraft has its own unique transmitter frequency and the receiver only accepts reply pulses having this frequency b. the reply pulses from the ground transmitter have the same frequency as the incoming interrogation pulses from the aircraft c. the aircraft receiver only accepts reply pulses which have the same time interval between successive pulses as the pulses being transmitted by its own transmitter d. the aircraft receiver only accepts reply pulses which arrive at a constant time interval
c. the aircraft receiver only accepts reply pulses which have the same time interval between successive pulses as the pulses being transmitted by its own transmitter
An Airborne Weather Radar system uses a frequency of 9 GHz because: a. it has a short wavelength so producing higher frequency returns b. the short wavelength allows signals to be reflected from cloud water droplets of all sizes c. the wavelength is such that reflections are obtained only from the larger water droplets d. the frequency penetrates clouds quite easily enabling good mapping of ground features in the mapping mode
c. the wavelength is such that reflections are obtained only from the larger water droplets
The accuracy associated with DME is: a. + or - 3% of range, or 0.5 NM, whichever is greater b. + or - 1.25% of range c. + or - 3% of range d. +/-0.25 NM +/-1.25% of range
d. +/-0.25 NM +/-1.25% of range
The range indicated by DME is considered to be accurate to within: a. 3% of range b. 1.25 % of range c. 0.5 NM d. +/-0.25 NM +/-1.25% of range
d. +/-0.25 NM +/-1.25% of range
To double the range of a primary radar would require the power to be increased by a factor of: a. 2 b. 4 c. 8 d. 16
d. 16
An aircraft flying at FL250 wishes to interrogate a DME beacon situated 400 ft AMSL. What is the maximum range likely to be achieved? a. 210 NM b. 198 NM c. 175 NM d. 222 NM
d. 222 NM
A radar is required to have a maximum range of 100 NM. What is the maximum PRF that will achieve this? a. 1620 pulses per second (pps) b. 1234 pps c. 617 pps d. 810 pps
d. 810 pps
An advantage of a phased array (slotted antenna) is: a. better resolution b. less power required c. reduced side lobes and clutter d. all of the above
d. all of the above
A LAAS requires: a. an accurately surveyed site on the aerodrome and a link through the INMARSAT geostationary satellites to pass corrections to X, Y & Z coordinates to aircraft b. an accurately surveyed site on the aerodrome and a link through the INMARSAT geostationary satellites to pass satellite range corrections to aircraft c. an accurately surveyed site on the aerodrome and a system known as a pseudolite to pass satellite range corrections to aircraft d. an accurately surveyed site on the aerodrome and system known as a pseudolite to pass corrections to X, Y & Z coordinates to aircraft
d. an accurately surveyed site on the aerodrome and system known as a pseudolite to pass corrections to X, Y & Z coordinates to aircraft
If the SSR transponder IDENT button is pressed: a. it causes a momentary distinctive display to appear on the controller's screen b. an identification pulse is automatically and continuously transmitted for 20 seconds, 4.35 μs before the last framing pulse c. an identification pulse is automatically and continuously transmitted for 10 seconds, 4.35 μs after the last framing pulse d. an identification pulse is automatically and continuously transmitted for 20 seconds, 4.35 μs after the last framing pulse
d. an identification pulse is automatically and continuously transmitted for 20 seconds, 4.35 μs after the last framing pulse
The colours used to denote variations in rainfall rate on an Airborne Weather Radar screen are ........... for very light or no returns, ............... for light returns, ........... for medium returns and ............ for strong returns. a. black yellow green magenta b. black green yellow magenta c. grey green yellow red d. black green yellow red
d. black green yellow red
The most accurate fixing information will be obtained from: a. four satellites spaced 90° apart at 30° above the visual horizon b. one satellite close to the horizon and 3 equally at 60° above the horizon c. one satellite directly overhead and 3 equally spaced at 60° above the horizon d. one satellite directly overhead and 3 spaced 120° apart close to the horizon
d. one satellite directly overhead and 3 spaced 120° apart close to the horizon
Airborne Weather Radar is an example ................ of radar operating on a frequency of .............. in the ............... band. a. primary 8800 MHz SHF b. secondary 9.375 MHz UHF c. secondary 9375 MHz SHF d. primary 9375 MHz SHF
d. primary 9375 MHz SHF
Secondary Surveillance Radar is a form of .............. radar with ..............type emissions operating in the .............. band. a. primary pulse SHF b. primary pulse UHF c. secondary FM SHF d. secondary pulse UHF
d. secondary pulse UHF
Concerning FMC operation, which of the following is true: a. the FMC combines the long term accuracy of the IRS with the short term accuracy of the external reference b. the FMC combines the long term accuracy of the IRS with the long term accuracy of the external reference c. the FMC combines the short term accuracy of the IRS with the short term accuracy of the external reference d. the FMC combines the short term accuracy of the IRS with the long term accuracy of the external reference
d. the FMC combines the short term accuracy of the IRS with the long term accuracy of the external reference