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messages sent by the satellites are time-tagged with the time of
transmission;
by comparing the time the message is received and the time the
message was sent, the aircraft can measure the time taken by the
message to travel from the satellite to the aircraft;
knowing the speed at which messages travel (the speed of light), and
the time taken, the aircraft can compute the distance travelled by the
message (or “range”), as follows:
speed = distance/time, hence > distance= speed x time
ATMB–CAAC Workshop on GNSS Beijing, 16-17 April 2007
Basic technical principles (3)
Some complications:
the simplified geometrical explanation assumes that time reference used
by the satellites and by the aircraft are the same
however, this is not the case – the satellites carry “precise” clocks
(atomic clocks), whereas the aircraft typically carries a relatively
imprecise (and less expensive) quartz clock
hence, the “range” computed by the aircraft based on the equation
shown above is not the “true” range – it is a “pseudorange”
Example: a 1 μs (microsecond) synchronization error between clocks corresponds to a 300
m error in range measurement
Solution: the clock error is resolved by using a fourth additional satellite
to provide additional information to estimate aircraft clock error and thus
derive “true range” information
Instead of three equations in three unknowns (the three position
coordinates of the aircraft), the aircraft receiver solves four equations in
four unknowns (the three position coordinates and the clock error)
ATMB–CAAC Workshop on GNSS Beijing, 16-17 April 2007
GNSS elements: GPS
Nominal constellation: 24 satellites (30 active as of March
2007)
Six orbital planes
Near-circular, 20,200 km altitude (26,600 km radius) 12-hour
orbits
First experimental satellite launched in 1978, operational in
1995
Managed by the US National Space-Based Positioning,
Navigation, and Timing (PNT) Executive Committee
Standard positioning service (SPS) frequency: 1 575.42 MHz
Selective availability (SA) discontinued in 2000
ICAO Annex 10, Volume I, section 3.7.3.1
ATMB–CAAC Workshop on GNSS Beijing, 16-17 April 2007
GNSS elements: GLONASS
Nominal constellation: 24 satellites (fewer active as of March
2007)
Three orbital planes
Near-circular, 19,100 km altitude (25,500 radius) 11:15-hour
orbits
First experimental satellite launched in 1982, operational in
1995, subsequent decline (plans to restore full operational
capability by 2010)
Operated by the Ministry of Defence of the Russian
Federation
Channel of standard accuracy (CSA) frequencies: 1602 MHz
± 0.5625n MHz
ICAO Annex 10, Volume I, section 3.7.3.2
ATMB–CAAC Workshop on GNSS Beijing, 16-17 April 2007
GNSS elements:
augmentation systems
Three (and a half) ICAO GNSS augmentation
systems:
aircraft-based augmentation system (ABAS)
satellite-based augmentation system (SBAS)
augmentation system (GBAS)
>ground-based regional augmentation system (GRAS)
Purpose: to overcome inherent limitations in the
service provided by the core constellations
ground-based
ATMB–CAAC Workshop on GNSS Beijing, 16-17 April 2007
GNSS elements: ABAS
ABAS: aircraft-based augmentation system
The basic element of ICAO GNSS
Purpose: to augment/integrate GNSS information with on-board
aircraft information
Required to ensure that performance meets Annex 10 requirements
(Volume I, Table 3.7.2.4-1)
Uses redundant satellite range measurements (and/or barometric
information) to detect faulty signals and alert the pilot
Receiver-autonomous integrity monitoring (RAIM) – five satellites
required (or four + baro)
Fault detection and exclusion (FDE) – six satellites required (or five
+ baro)
RAIM/FDE availability: are sufficient redundant measurements
available?
ICAO Annex 10, Volume I, section 3.7.3.3
ATMB–CAAC Workshop on GNSS Beijing, 16-17 April 2007
GNSS elements: SBAS (1)
SBAS: satellite-based augmentation system
Augments core satellite constellations by providing ranging, integrity and
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