NAVIGATIONAL DATA

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8/28/2012 LICE-05 GPS Navigational DataDr. J.K.Ghosh, CED

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Navigation Data

GPS satellites transmit datastreams, called navigation

message

Contains data about the

satellite orbit, clock, health etc.

Both with L1 and L2 signals

Rate of transmission is 50 bps. Get modulated to both the

codes using modulo-2 addition


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Data modulated with code


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Navigation Data.

Data organized in frames and sub-frames


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Navigation Data.

Consists of 25 frames, each containing 1500

bits.

Frame consists of FIVE 300 bit sub-frames

Sub-frame consists of 10 words of 30 bits

each

At 50-bps rate it takes 6 sec to transmit a

sub-frame, 30 sec to complete a frame, and

12.5 min for one complete transmission of

the navigation message.


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Navigation Data.

The sub-frames 1, 2, and 3 are transmitted

with each frame thus these three frames

repeat every 30 sec.

Sub-frames 4 and 5 are each sub-

commutated 25 times.

The 25 versions of sub-frames 4 and 5 are

referred to as pages 1 through 25. Thus,

each of these pages repeats every 750 sec orevery 12.5 min.


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Navigation Data (Sub-frame 1)

Sub-frame begins with the telemetry word (TLM)

The telemetry word begins with a preamble (8bit), and contain information for authorized user(22 bits).

Handover word (HOW) follows TLM in a sub-

frame. HOW represents GPS time of week (TOW).

As soon as the receivers gets locked to the C/A-code, the HOW word gets extracted.

Thus, receiver knows exactly which part of thelong P(Y)-code is being transmitted which leads toidentification of SV.


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Nav.. Data (Sub-frame 1)

The Z count is a 29-bit number that contains several pieces

of timing information. One part of the Z count is the HOW.

HOW multiplied by provides Xl count at the start of asub-frame

GPS time is directly related to the X1 counts of the P(Y)-code.

A full week equals 403,199 X1 counts.

Other parts of the Z count can be converted to time ofweek (TOW), which represents the number of seconds

since the beginning of the GPS week. Yet another part of the Z count gives the current GPS week

number (modulo 1024).


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Nav.. Data (Sub-frame 1)

First sub-frame further contains

satellite clock correction terms

clock reference time toc

differential group delay, TGD

IODC (issue of date, clock) term

IODC indicates the issue number of theclock data set to alert users of changes in

the clock parameters. Messages are updated usually every 4

hours.


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Keplerian parameters `


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Navigation Data (Sub-frame 2 & 3)

8/28/2012 LICE-04 GPS SignalDr. J.K.Ghosh, CED

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Navigation Data (Sub-frame 2 & 3)

Elements are least- squares curve fitting of the

predicted ephemeris over a well-specifiedinterval of time (not valid for the total orbit).

lODE (issue of data, ephemeris) to detect

changes in the ephemeris parameters.The broadcast elements should be

used with the ephemeris algorithm

resulting in coordinates in theWGS 84 coordinate system.


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N i ti D t (S b f 4)


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25 pages

contain special messages

ionospheric correction terms (8 coefficients)

coefficients to convert GPS time to UTC

almanac for satellites with SVN number 25 andhigher.

For deriving UTC as accurately as possible from GPS

time, the message provides a constant offset term, a linearpolynomial term, the reference time tot, and the currentvalue of the leap second. With these data a correction can

be computed and added to the GPS time.

N i ti D t (S b f 5)


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25 pages

Containsalmanac of satellites 1 through 24.

(

N D t C l i


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Nav.. Data Conclusion

Mean anomaly, the longitude of the

ascending node, the inclination, and UTC(if desired) are formulated as a polynomial

in time; the time argument is GPS time.

The polynomial coefficients are, of course,a function of the epoch of expansion. The

respective epochs are toc, toe, too and tot.

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Clock reference epoch toc and the

ephemeris reference epoch toe are chosen 2-h after the first valid transmission for a 4h

fit interval data set, and 3 h for a 6h fit

interval data set.

Almanac and UTC reference epochs too and

tot are 3.5 days after the first valid

transmission.

N D t C l i


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The navigation message contains

User Range Error (URE) [projection of the

ephemeris curve fit errors onto the user range,

and includes effects of satellite timing errors]

Flags that indicate the health status of thesatellite

Etc.


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8/28/2012 LICE-04 GPS SignalDr. J.K.Ghosh, CED 19

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