GNSS, INS/IRS & FMS

A. 24 satellites in 6 orbits

The NAVSTAR constellation is 24 satellites in 6 orbital planes.

D. acceleration

The accelerometers of an INS measure acceleration; the computer integrates it to velocity and then to distance.

A. 12 hours

GPS satellites orbit at about 20,200 km with a period of about 12 hours (one orbit per half sidereal day).

A. ionospheric propagation; using two frequencies (L1/L2)

The ionosphere delays the signal by an amount that depends on frequency, so comparing two frequencies largely removes it.

The question was recalled without its options; these options were written to it.

A. UHF, range position lines

NAVSTAR/GPS transmits in the UHF band (L1 1575.42 MHz) and the receiver fixes position from simultaneous ranges (range position lines) to the satellites.

C. traffic advisories and resolution advisories in the vertical plane

TCAS II: TA ("Traffic, traffic") plus vertical RAs (climb/descend). It never gives turn instructions; TCAS I gives TAs only.

The question was recalled without its options; these options were written to it.

C. strapdown

Strapdown (IRS): no stabilised platform; the computer resolves the body-axis accelerations using the laser-gyro rates. A gimballed INS keeps its platform level and aligned.

The question was recalled without its options; these options were written to it.

B. 55 degree to the plane of equator

GPS orbits are inclined 55 deg to the equatorial plane.

B. 4

Four unknowns (three position co-ordinates and the receiver clock error) need four pseudo-ranges. RAIM needs a fifth; fault exclusion a sixth.

The question was recalled without its options; these options were written to it.

C. Simultaneous range from a number of satellites

GNSS position is found from simultaneous ranges (pseudo-ranges) to several satellites, not from bearings.

A. 28 days

The FMS/FMC navigation database is valid for 28 days (one AIRAC cycle).

A. quickly and steadily descent or climb

A TCAS resolution advisory gives vertical avoidance: climb or descend.

A. excessive descent rate

Mode 1 excessive descent rate ("Sink rate / Pull up"); Mode 2 excessive terrain closure; Mode 3 altitude loss after take-off or go-around ("Don't sink"); Mode 4 unsafe terrain clearance not in landing configuration ("Too low gear/flaps"); Mode 5 below glide slope; Mode 6 callouts.

The question was recalled without its options; these options were written to it.

A. 1 & 2

An altitude alerting system warns on approaching the selected altitude and on deviation from it.

B. stays engaged, manoeuvres the aircraft in response to pressure on the control wheel and holds the attitude when released

CWS: the pilot steers through the autopilot servos; release the wheel and the attitude is held.

The question was recalled without its options; these options were written to it.

B. it includes receiver clock error

Pseudo-range includes the receiver clock error because the receiver clock is not perfectly synchronised.

C. by integrating measured acceleration

Accelerometers measure acceleration; the first integration gives velocity (groundspeed), the second distance.

The question was recalled without its options; these options were written to it.

B. attitude only (heading must be entered manually), no navigation

ATT is a reversionary mode after loss of alignment in flight: pitch and roll, and heading if set by the crew - no present position.

The question was recalled without its options; these options were written to it.

D. NAV and ALIGN

Alignment runs in ALIGN, and also if NAV is selected directly from OFF (the system aligns, then enters NAV). ATT gives attitude only.

The question was recalled without its options; these options were written to it.

B. multi-function control display unit (MCDU/CDU)

The pilot types the route and performance data on the CDU keyboard; the FMC stores and computes; the IRS supplies position.

The question was recalled without its options; these options were written to it.

B. a table of variation stored in its memory

The IRS computes true heading; it converts to magnetic using a world magnetic variation model stored in memory.

The question was recalled without its options; these options were written to it.

A. before the aircraft is moved

Alignment needs the aircraft stationary (it senses earth rate and gravity); moving it during alignment spoils it, so NAV is selected before taxi.

The question was recalled without its options; these options were written to it.

B. altitude loss after take-off or missed approach

Mode 3 is active after take-off/go-around until a safe height is gained: any significant loss of altitude gives "Don't sink".

The question was recalled without its options; these options were written to it.

A. a TA only, no RA

Without altitude (Mode C/S) TCAS cannot judge the vertical conflict, so it can only show traffic (TA), never resolve it (RA).

The question was recalled without its options; these options were written to it.

A. no moving parts, so long life, no spin-up time and no precession or gimbal errors

A ring laser gyro measures rotation from the phase shift between two laser beams travelling opposite ways round a closed path. With no rotor it is reliable and long-lived, and ready almost immediately.

B. overcome laser lock-in at low rates of rotation

At very low rotation rates the two beams lock together and show no rotation; mechanically vibrating (dithering) the block keeps it out of the lock-in zone.

The question was recalled without its options; these options were written to it.

A. on the primary display, in red

Warnings (red) are given the highest priority on the primary engine/warning display.

The question was recalled without its options; these options were written to it.

A. 55 deg

A satellite's ground track reaches the latitude equal to its orbit inclination. Higher up they are always seen at an angle - though still in view.

B. the short-term accuracy of the IRS with the long-term accuracy of radio aids

IRS positions are smooth and precise over minutes but drift with time; radio (DME/DME, GPS) fixes do not drift but are noisy. The FMC blends them.

The question was recalled without its options; these options were written to it.

B. UHF

GPS L1 and L2 are in the UHF band.

B. monitor the satellites' ephemeris and clocks (and upload corrections)

Monitor stations track the satellites; the master control station computes orbit and clock corrections and uploads them via ground antennas.

The question was recalled without its options; these options were written to it.

C. IDENT - POS INIT —RTE

The first three CDU pages at start-up are IDENT, POS INIT and RTE.

B. will not accept a 10 deg error in initial latitude but will accept a 10 deg error in initial longitude

During alignment the INS checks the entered latitude against the earth-rate measurement and rejects a gross latitude error, but a longitude error does not affect alignment.

C. it can only be used for attitude reference

After an INS has lost power in flight it cannot re-align; it can be used only as an attitude reference.

B. 60 05.7 N

The INS flies the great circle, which bows towards the pole: at the mid-longitude, tan lat = tan 60 / cos 5 = 1.739, i.e. 60 deg 05.7 N.

C. Red full square.

On TCAS a resolution advisory is a red filled square; a traffic advisory is an amber filled circle.

B. 28days

The navigation database in an INS/IRS/FMS is updated every 28 days (AIRAC cycle).

A. three automatic display modes plus one manual mode, showing system synoptics and the actions to take

ECAM shows warnings with corrective actions and the affected system page automatically (flight phase, advisory, failure modes). Boeing's EICAS concentrates on engine indications and crew alerts.

D. VOR/DME (and DME/DME, GPS)

Radio updates (DME/DME, VOR/DME, GPS) correct the IRS drift; altitude comes from the ADC and heading from the IRS.

The question was recalled without its options; these options were written to it.

A. must not be moved

An INS must be stationary during alignment; any movement corrupts the gyro-compassing and level alignment.

A. receiver clock errors

The first range is not yet corrected for receiver clock error.

C. tracks all satellites in view and uses the best ones for the fix

It has enough channels to track every visible satellite at once, choosing the best geometry (lowest DOP) - unlike a sequential receiver.

The question was recalled without its options; these options were written to it.

C. TAS

The ADC supplies TAS, Mach, altitude and temperature; heading and position come from the IRS, groundspeed from the IRS/FMC.

The question was recalled without its options; these options were written to it.

B. is done within the receiver, which needs at least 5 satellites to detect a faulty one

The receiver checks the consistency of redundant ranges: 5 satellites to detect a failure, 6 to exclude it.

The question was recalled without its options; these options were written to it.

A. 081 deg

Track = heading + drift (right drift is added). So heading = track - drift = 090 - 9 = 081. An IRS shows drift directly from its groundspeed vector, but the relationship is the same as on the CX-3.

A. 3; 19,100 km; 64.8 deg

With altitude supplied, three ranges solve two positions plus clock. GLONASS: about 19,100 km, inclination 64.8 deg (GPS: 20,180 km, 55 deg).

D. one satellite directly overhead and 3 spaced 120° apart close to the horizon

The best geometry (lowest DOP) is one satellite overhead plus three spread 120 deg apart low on the horizon.

C. computer-generated from the IRS and radio navigation positions

The FMC filters the IRS positions with radio updates to produce its own best estimate - not a simple average.

The question was recalled without its options; these options were written to it.

A. the time taken for each satellite signal to arrive (ranges)

The receiver measures the signal travel time from each satellite, converts it to range, and fixes position from the ranges.

The question was recalled without its options; these options were written to it.

C. the actual track over the ground

The track line runs from the aircraft symbol along the track made good (heading +/- drift); the lubber line/top of the rose is heading.

The question was recalled without its options; these options were written to it.

B. keep the vertical speed out of the restricted (red) band - no change is needed if already outside it ("Monitor vertical speed")

A preventive RA tells you what NOT to do - avoid the vertical speeds shown in red. A corrective RA requires a change of vertical speed ("Climb, climb" / "Adjust vertical speed"). TCAS II never commands turns.

B. velocity along the local meridian

acceleration -> (1st integration) velocity -> (2nd integration) distance / change of position.

The question was recalled without its options; these options were written to it.

A. the geodetic datum (reference ellipsoid and coordinate system) used by GPS and adopted by ICAO

WGS-84 (World Geodetic System 1984) is the reference ellipsoid and coordinate frame in which GPS computes positions; ICAO requires published aeronautical coordinates to be referred to WGS-84.