Page 414 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Global positioning technology C HAPTER 14.1

receiver, preceded by two 30-bit words with SV-specific
Table 14.1-1 Original navigation signal accuracy targets
telemetry and handover information. The first three
for SPS and PPS
subframes, containing clock correction and ephemeris
Horizontal Vertical Timing data relevant to the specific SV, are refreshed as neces-
accuracy accuracy accuracy sary for each data frame transmitted during the naviga-
tion message broadcast. The almanac and other data
Standard 100 metres 156 metres 340 ns transmitted in the final two subframes are longer data
positioning service
segments, relevant to the entire GPS, requiring the full
Precise positioning 22 metres 27.7 metres 200 ns 25 data frames to be broadcast completely. Below is
service a brief description of the contents of each subframe. For
an illustration of the complete Navigation Message, refer
Note: By design, all accuracies are statistically achievable 95% of the time. to Fig. 14.1-1.

14.1.2.2.1 Clock correction subframe
greatly increased positioning accuracy for non-military The clock correction subframe, the first subframe
GPS applications. The cessation of S/A should allow transmitted in the navigation message data frame, con-
users of the SPS a level of accuracy similar to those using tains the GPS week number, accuracy and health in-
the PPS. Within the first week of the discontinuation formation specific to the transmitting SV, and various
of S/A, positioning accuracies within 10 metres were clock correction parameters relating to overall system
already being reported, without any upgrade to the GPS time, such as clock offset and drift.
receivers being used.
14.1.2.2.2 SV ephemeris subframes
14.1.2.2 The navigation message The second and third subframes of the navigation mes-
sage contain ephemeris data. This data provides the
The navigation message broadcast by every GPS satellite GPS receiver with precise orbit information and cor-
contains a variety of information used by each GPS rection parameters about the transmitting SV that the
receiver to calculate a PVT solution. The information in receiver uses accurately to calculate the satellite’s cur-
this message includes time of signal transmission, clock rent position in space. This information, in turn, is used
correction and ephemeris data for the specific SV, and an with the clock information to calculate the range to the
extensive amount of almanac and additional status and SV. Included in the ephemeris subframes are telemetry
health information on all of the satellites in the GPS. parameters specific to the transmitting SV, such as cor-
Each SV repeatedly broadcasts a navigation message rection factors to the radius of orbit, angle of inclination,
that is 12.5 minutes in length, and consists of 25 1500-bit and argument of latitude, as well as the square root of the
data frames transmitted at 50 bits per second. A single semi-major axis of rotation, the eccentricity of the orbit
data frame is composed of five 300-bit subframes, each of the SV, and the reference time that the ephemeris data
containing different status or data information for the was uploaded to the SV.

Fig. 14.1-1 Navigation message. TLM, telemetry word; HOW, handover word.

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