A.4 Assistance Data

04.313GPPLocation Services (LCS)Mobile Station (MS) - Serving Mobile Location Centre (SMLC) Radio Resource LCS Protocol (RRLP)TS

A.4.1 General

The Assistance Data is a RRLP component from the network to the MS. It is used by the network to provide assistance data to enable MS-based E-OTD or MS-based Assisted GPS capabilities in the MS. It contains the following elements.

Table A.11: Assistance Data component content

Element

Type/Reference

Presence

E-OTD Reference BTS for Assistance Data

E-OTD Reference BTS for Assistance Data 2.2.3

C

E-OTD Measurement Assistance Data

E-OTD Measurement Assistance Data 2.2.4

C

E-OTD Measurement Assistance Data for System Information List

E-OTD Measurement Assistance Data for System Information List 2.2.5

C

GPS Assistance Data

GPS Assistance Data 4.2.4

C

GPS Time Assistance Measurement Request

GPS Time Assistance Measurement Request 4.2.4a

O

GPS Reference Time Uncertainty

GPS Reference Time Uncertainty 4.2.4b

O

A.4.2 Elements

A.4.2.1 E-OTD Reference BTS for Assistance Data Element

This element is conditional. It is as described in 2.2.3. If the network can provide assistance data, and data for E-OTD has been requested, this element is included.

A.4.2.2 E-OTD Measurement Assistance Data Element

This element is conditional. It is as described in 2.2.4. If the network can provide assistance data, and data for E-OTD has been requested, this element is included.

A.4.2.3 E-OTD Measurement Assistance Data for System Information List Element

This element is conditional. It is as described in 2.2.5. If the network can provide assistance data, and data for E-OTD has been requested, this element is included.

A.4.2.4 GPS Assistance Data Element

The GPS Assistance Data element contains a single GPS assistance message that supports both MS-assisted and MS-based GPS methods. This element can contain one or more of the fields listed in table A.12 below, which support both MS-assisted and MS-based GPS methods. As table A.12 shows, all fields are optional.

Note that certain types of GPS Assistance data may be derived, wholly or partially, from other types of GPS Assistance data.

In addition, an Integrity Monitor (IM) shall detect unhealthy (e.g., failed/failing) satellites and also shall inform users of measurement quality in DGPS modes when satellites are healthy. Excessively large pseudo range errors, as evidenced by the magnitude of the corresponding DGPS correction, shall be used to detect failed satellites. Unhealthy satellites should be detected within 10 seconds of the occurrence of the satellite failure. When unhealthy (e.g., failed/failing) satellites are detected, the assistance and/or DGPS correction data shall not be supplied for these satellites. When the error in the IM computed position is excessive for solutions based upon healthy satellites only, DGPS users shall be informed of measurement quality through the supplied UDRE values. After the Bad Satellite Present flag has been set, if the satellites return to healthy condition for some period of time, the flag shall be reset.

Table A.12: Fields in the GPS Assistance Data element

Parameter

Presence

Repetition

Reference Time

O

Yes

Reference Location

O

No

DGPS Corrections

O

Yes

Navigation Model

O

Yes

Ionospheric Model

O

No

UTC Model

O

No

Almanac

O

Yes

Acquisition Assistance

O

Yes

Real-Time Integrity

O

Yes

When RRLP pseudo-segmentation is used, Table A.12 indicates which parameters may be repeated in more than one RRLP segment in order to provide data for multiple satellites. When any such parameter appears in more than one segment, the following rules shall apply.

1. There shall be no repetition of data for the same satellite.

2. Optional and conditional elements in the parameter not associated with a particular satellite shall each appear in at most one RRLP segment.

3. Any mandatory element not associated with a particular satellite shall assume consistent values in the case of an element related to current GPS time and the same value otherwise.

4. The maximum number of satellites defined in subclause 5.1 for which data can be included for any parameter in one RRLP segment shall apply also when counted over all RRLP segments.

Reference Time

These fields specify the relationship between GPS time and air-interface timing of the BTS transmission in the reference cell. These fields occur once per message; some are mandatory and some are conditional, as shown in table A.14. Note that Reference Time may also be present within the Acquisition Assistance parameter. In such a case, the SMLC shall ensure consistency.

Table A.14: Reference Time (Fields occurring once per message)

Parameter

# Bits

Scale Factor

Range

Units

Incl.

GSM Time Present

1

Boolean

M

GPS Week

10

1

0 – 1023

weeks

M

GPS TOW

23

0.08

0-604799.92

sec

M

BCCH Carrier

10

1

0 – 1023

C

BSIC

6

1

0 – 63

C

FNm

21

1

0 – (221-1)

frames

O

TN

3

1

0 – 7

timeslots

O

BN

8

1

0 – 156

bits

O

GPS TOW Assist

24*N_SAT

—-

O

GSM Time Present

This field indicates whether or not GSM air-interface timing information for the reference cell is present in this message. The MS shall interpret a value of "1" to mean that GSM timing information is present, and "0" to mean that only the GPS Week, GPS TOW and optionally GPS TOW Assist fields are provided. This field is mandatory.

GPS Week

This field specifies the GPS week number of the assistance being provided. GPS Week eliminates one-week ambiguities from the time of the GPS assistance. This field is mandatory.

GPS TOW

The GPS TOW (time-of-week) is a mandatory field and is specified with 80 msec resolution. When GSM Time Present is "1", GPS TOW and BCCH/BSIC/FNm/TN/BN IEs provide a valid relationship between GPS and GSM time, as seen at the approximate location of the MS, ie the propagation delay from BTS to MS shall be compensated for by the SMLC. Depending on implementation, the relation between GPS and GSM time may have varying accuracy. Therefore, the uncertainty of the timing relation may be provided in the optional field GPS Reference Time Uncertainty. If the propagation delay from BTS to MS is not accurately known, the SMLC shall use the best available approximation of the propagation delay and take the corresponding delay uncertainty into account in the calculation of the field GPS Reference Time Uncertainty. When GSM Time Present is "0", GPS TOW is an estimate of current GPS time of week at time of reception of the RRLP segment containing this data by the MS. The SMLC should achieve an accuracy of +/- 3 seconds for this estimate including allowing for the transmission delay between SMLC and MS of the RRLP segment containing GPS TOW. Note that the MS should further compensate GPS TOW for the time between the reception of the segment containing GPS TOW and the time when the GPS TOW field is used.

BCCH Carrier/BSIC/FNm/TN/BN

These fields specify the state of the GSM frame number, timeslot number, and bit number, respectively, of the reference BTS with the specified BCCH carrier and BSIC at the time that correspond to GPS TOW. The SMLC shall use the current serving BTS as the reference BTS. The frame number field is given modulo 221, i.e., the MSB of the GSM frame number is truncated. The MS shall interpret FNm as the most recent of the two possible frame numbers that FNm could represent. These fields are conditionally present when GSM Time Present is "1". The target MS has the option of rejecting a GPS position request or GPS assistance data if the reference BTS is not the serving BTS.

GPS TOW Assist

The GPS TOW (time-of-week) is a mandatory field and is specified with 80 msec resolution. When GSM Time Present is "1", GPS TOW and BCCH/BSIC/FNm/TN/BN IEs provide a valid relationship between GPS and GSM time. When GSM Time Present is "0", GPS TOW is an estimate of current GPS time of week at time of reception of the RRLP segment containing this data by the MS. The SMLC should achieve an accuracy of +/- 3 seconds for this estimate including allowing for the transmission delay between SMLC and MS of the RRLP segment containing GPS TOW. Note that the MS should further compensate GPS TOW for the time between the reception of the segment containing GPS TOW and the time when the GPS TOW field is used.

Table A.15a: GPS TOW Assist (Fields occurring N_SAT times per message)

Parameter

# Bits

Scale Factor

Range

Units

Incl.

SatID

6

0 – 63

M

TLM Message

14

0 – 16383

Bit field

M

Anti-Spoof

1

1

0 –1

Bit field

M

Alert

1

1

0 – 1

Bit field

M

TLM Reserved

2

0 – 3

Bit field

M

SatID

This field identifies the satellite for which the corrections are applicable. The values ranging from 0 to 63 represent satellite PRNs ranging from 1 to 64, respectively.

TLM Message

This field contains a 14-bit value representing the Telemetry Message (TLM) being broadcast by the GPS satellite identified by the particular SatID, with the MSB occurring first in the satellite transmission.

Anti-Spoof/Alert

These fields contain the Anti-Spoof and Alert flags that are being broadcast by the GPS satellite identified by SatID.

TLM Reserved

These fields contain the two reserved bits in the TLM Word being broadcast by the GPS satellite identified by SatID, with the MSB occurring first in the satellite transmission.

Reference Location

The Reference Location field contains a 3-D location (with uncertainty) specified as per 3GPP TS 03.32. The purpose of this field is to provide the MS with a priori knowledge of its location in order to improve GPS receiver performance. The allowed shape is:

– 3-D location with uncertainty (ellipsoid point with altitude and uncertainty ellipsoid);

DGPS Corrections

These fields specify the DGPS corrections to be used by the MS. All fields are mandatory when DGPS Corrections are present in the GPS Assistance Data.

Table A.15: DGPS Corrections

Parameter

# Bits

Scale Factor

Range

Units

Incl.

The following fields occur once per message

GPS TOW

20

1

0 – 604799

sec

M

Status/Health

3

1

0-7

M

N_SAT

4

1

1-16

M

The following fields occur once per satellite (N_SAT times)

SatID

6

0 – 63

M

IODE

8

0 – 255

M

UDRE

2

0 – 3

M

PRC

12

0.32

655.34

meters

M

RRC

8

0.032

4.064

meters/sec

M

Delta PRC2

8

M

Delta RRC2

4

M

Delta PRC3

8

M

Delta RRC3

4

M

GPS TOW

This field indicates the baseline time for which the corrections are valid.

Status/Health

This field indicates the status of the differential corrections contained in the broadcast message. The values of this field and their respective meanings are shown below in table A.16.

Table A.16: Values of Status/Health IE

Code

Indication

000

UDRE Scale Factor = 1.0

001

UDRE Scale Factor = 0.75

010

UDRE Scale Factor = 0.5

011

UDRE Scale Factor = 0.3

100

UDRE Scale Factor = 0.2

101

UDRE Scale Factor = 0.1

110

Reference Station Transmission Not Monitored

111

Data is invalid – disregard

The first six values in this field indicate valid differential corrections. When using the values described below, the "UDRE Scale Factor" value is applied to the UDRE values contained in the message. The purpose is to indicate an estimate in the amount of error in the corrections.

The value "110" indicates that the source of the differential corrections (e.g., reference station or external DGPS network) is currently not being monitored. The value "111" indicates that the corrections provided by the source are invalid, as judged by the source. In the later case, the message shall contain no corrections for individual satellites. Any MS that receives DGPS Corrections in a GPS Assistance Data IE shall contain the appropriate logic to properly interpret this condition and look for the next IE.

N_SAT

This field indicates the number of satellites for which differential corrections are available. Corrections for up to 16 satellites.

SatID

This field identifies the satellite for which the corrections are applicable. The values ranging from 0 to 63 represent satellite PRNs ranging from 1 to 64, respectively.

IODE

This IE is the sequence number for the ephemeris for the particular satellite. The MS can use this IE to determine if new ephemeris is used for calculating the corrections that are provided in the broadcast message. This eight-bit IE identifies a particular set of ephemeris data for a GPS satellite and may occupy the numerical range of [0, 255]. The transmitted IODE value will be different from any value transmitted by the GPS satellite during the preceding six hours. For more information about this field can be found from RTCM-SC104.

User Differential Range Error (UDRE)

This field provides an estimate of the uncertainty (1-) in the corrections for the particular satellite. The value in this field shall be multiplied by the UDRE Scale Factor in the common Corrections Status/Health field to determine the final UDRE estimate for the particular satellite. The meanings of the values for this field are described in table A.18.

Table A.18: Values of UDRE IE

Value

Indication

00

UDRE  1.0 m

01

1.0 m < UDRE  4.0 m

10

4.0 m < UDRE  8.0 m

11

8.0 m < UDRE

Each UDRE value shall be adjusted based on the operation of an Integrity Monitor (IM) function which exists at the network (SMLC, GPS server, or reference GPS receiver itself). Positioning errors derived at the IM which are excessive relative to DGPS expected accuracy levels shall be used to scale the UDRE values to produce consistency.

Pseudo-Range Correction (PRC)

This field indicates the correction to the pseudorange for the particular satellite at the GPS Reference Time, t0. The value of this field is given in meters (m) and the resolution is 1, as shown in table A.15 above. The method of calculating this field is described in T1P1.5/98-440r0.

If the SMLC has received a request for GPS assistance data from an MS, which included a request for the navigation models and DGPS (i.e., bit D and E are set to one in ‘Requested GPS Assistance Data, see 3GPP TS 09.31), the SMLC shall determine, for each satellite, if the navigation model stored by the MS is still suitable for use with DGPS corrections (also see navigation model update conditions right before Table A.19) and if so and if DGPS corrections are supported the SMLC should send DGPS corrections without including the navigation model.

The IODE value sent for a satellite shall always be the IODE value that corresponds to the navigation model for which the pseudo-range corrections are applicable.

The pseudo-range correction shall correspond to the available navigation model (the one already stored in and identified by the MS or the one included in the same procedure as the pseudo-range correction). The MS shall only use the PRC value when the IODE value received matches its available navigation model.

Pseudo-Range Rate Correction (RRC)

This field indicates the rate-of-change of the pseudorange correction for the particular satellite, using the satellite ephemeris identified by the IODE IE.. The value of this field is given in meters per second (m/sec) and the resolution is 0,032, as shown in table A.15 above. For some time t1 > t0, the corrections for IODE are estimated by

PRC(t1, IODE) = PRC(t0, IODE) + RRC(t0, IODE)(t1 t0) ,

and the MS uses this to correct the pseudorange it measures at t1, PRm(t1, IODE), by

PR(t1, IODE) = PRm(t1, IODE) + PRC(t1, IODE).

The SMLC shall always send the RRC value that corresponds to the PRC value that it sends (see above for details). The MS shall only use the RRC value when the IODE value received matches its available navigation model.

Delta Pseudo-Range Correction 2 (Delta PRC2)

This IE indicates the difference in the pseudorange correction between the satellite’s ephemeris identified by IODE and the previous ephemeris two issues ago IODE – 2. The value of this IE is given in meters (m) and the resolution is 0.32. The method of calculating this IE are described in RTCM-SC104.

This IE is not used. The sender shall set it to zero and the receiver shall ignore it.

Delta Pseudo-Range Rate Correction 2 (Delta RRC2)

This IE is not used. The sender shall set it to zero and the receiver shall ignore it.

Delta Pseudo-Range Correction 3 (Delta PRC3)

This IE is not used. The sender shall set it to zero and the receiver shall ignore it.

Delta Pseudo-Range Rate Correction 3 (Delta RRC3)

This IE is not used. The sender shall set it to zero and the receiver shall ignore it.

Navigation Model

This set of fields contains information required to manage the transfer of precise navigation data to the GPS-capable MS. In response to a request from an MS for GPS assistance data, the SMLC shall determine whether to send the navigation model for a particular satellite to an MS based upon several factors like the T-Toe limit specified by the MS and any request from the MS for DGPS (also see above). . This information includes control bit fields as well as satellite ephemeris and clock corrections. This field is present when Navigation Model Present bit in Field Types Present is "1". The individual fields are given in Table A.19 below, and the conditions for their presence is discussed below.

Table A.19: Navigation Model (per-satellite fields – (1) = Positive range only)

Parameter

# Bits

Scale Factor

Units

Incl.

Navigation Model Flow Control (once per message)

Num_Sats_Total

4(1)

1

M

Satellite and Format Identification (once per satellite)

SatID

6(1)

M

Satellite Status

2

Boolean

M

Satellite Navigation Model and Associated Bits (once per satellite)

C/A or P on L2

2

Boolean

C

URA Index

4

Boolean

C

SV Health

6

Boolean

C

IODC

10(1)

C

L2 P Data Flag

1

Boolean

C

SF 1 Reserved

87

C

TGD

8

2-31

sec

C

toc

16(1)

24

sec

C

af2

8

2-55

sec/sec2

C

af1

16

2-43

sec/sec

C

af0

22

2-31

sec

C

Crs

16

2-5

meters

C

n

16

2-43

semi-circles/sec

C

M0

32

2-31

semi-circles

C

Cuc

16

2-29

meters

C

e

32(1)

2-33

C

Cus

16

2-29

radians

C

(A)1/2

32(1)

2-19

meters1/2

C

toe

16(1)

24

sec

C

Fit Interval Flag

1

Boolean

C

AODO

5

900

sec

C

Cic

16

2-29

radians

C

OMEGA0

32

2-31

semi-circles

C

Cis

16

2-29

radians

C

i0

32

2-31

semi-circles

C

Crc

16

2-5

radians

C

32

2-31

semi-circles

C

OMEGAdot

24

2-43

semi-circles/sec

C

Idot

14

2-43

semi-circles/sec

C

Num_Sats_Total

This field specifies the number of satellites that are included in the provided Navigation Model. A range of 1-16 is available. This field is mandatory when the Navigation Model field is included in the message.

SatID

This field identifies the satellite for which the assistance is applicable. This value is the same as the PRN number provided in the navigation message transmitted by the particular satellite. The range is 0 to 63, with 0-31 indicating GPS satellites 1-32, respectively, and 32-63 indicating satellites in future augmentation systems (e.g., WAAS or EGNOS). This field is mandatory for each included satellite.

Satellite Status

This field is a two-bit value that indicates the status of the Navigation Model for the particular satellite specified by SatID. This field is mandatory for each included satellite. The MS shall interpret the combinations of the two bits as follows:

Table A.20: Satellite Status (per-satellite field)

MSB

LSB

Interpretation

0

0

New satellite, new Navigation Model

0

1

Existing satellite, same Navigation Model

1

0

Existing satellite, new Navigation Model

1

1

Reserved

This Satellite Navigation Model and associated bit fields include the parameters that accurately model the orbit and clock state of the particular satellite. For the particular satellite, these fields are conditional based on the value of Satellite Status for that satellite. The fields are absent when Satellite Status is "01", and present for all other values. The format for the ephemeris, clock corrections, and associate bits are specified in ICD-GPS-200.

Ionospheric Model

The Ionospheric Model contains fields needed to model the propagation delays of the GPS signals through the ionosphere. The information elements in this field are shown in table A.21. Proper use of these fields allows a single‑frequency GPS receiver to remove approximately 50 % of the ionospheric delay from the range measurements. The Ionospheric Model is valid for the entire constellation and changes slowly relative to the Navigation Model. All of the fields must be included when Ionospheric Model is present.

Table A.21: Ionospheric Model (occurs once per message, when present)

Parameter

# Bits

Scale Factor

Units

Incl.

0

8

2-30

seconds

C

1

8

2-27

sec/semi-circle

C

2

8

2-24

sec/(semi-circle)2

C

3

8

2-24

sec/(semi-circle)3

C

0

8

211

seconds

C

1

8

214

sec/semi-circle

C

2

8

216

sec/(semi-circle)2

C

3

8

216

sec/(semi-circle)3

C

UTC Model

The UTC Model field contains a set of parameters needed to relate GPS time to Universal Time Coordinate (UTC). All of the fields in the UTC Model are mandatory when the field is present.

Table A.22: UTC Model (occurs once per message,
when present per-satellite fields – (1) = Positive range only)

Parameter

# Bits

Scale Factor

Units

Incl.

A1

24

2-50

sec/sec

C

A0

32

2-30

seconds

C

tot(1)

8

212

seconds

C

WNt(1)

8

1

weeks

C

tLS

8

1

seconds

C

WNLSF(1)

8

1

weeks

C

DN

8

1

days

C

tLSF

8

1

seconds

C

Almanac

These fields specify the coarse, long-term model of the satellite positions and clocks. These fields are given in table A.23 below. With one exception (i), these parameters are a subset of the ephemeris and clock correction parameters in the Navigation Model, although with reduced resolution and accuracy. The almanac model is useful for receiver tasks that require coarse accuracy, such as determining satellite visibility. The model is valid for up to one year, typically. Since it is a long-term model, the field should be provided for all satellites in the GPS constellation. All fields in the Almanac are mandatory when the Almanac is present. The fields toa and WNa specify the GPS time-of-week and week number, respectively, that are the reference points for the Almanac parameters.

The Almanac also is useful as an acquisition aid for network-based GPS methods. Given a recent Almanac (< 3-4 weeks old), the MS only needs Reference Time and Reference Location information to quickly acquire the signals and return measurements to the network.

The Almanac also contains information about the health of that satellite as described in ICD-GPS-200. If this Alamanc has been captured from the satellite signal, the SV Health field represents the predicted satellite health at the time the GPS control segment uploaded the Almanac to the satellite. According to ICD-GPS-200, this health information may differ from the SV Health field in the Navigation Model (table A.19) due to different upload times.

The parameters Num_Sats_Total and SatID shall be interpreted in the same manner as described under table A19.

Table A.23: Almanac (per-satellite fields – (1) = Positive range only)

Parameter

# Bits

Scale Factor

Units

Incl.

The following fields occur once per message

Num_Sats_Total

6(1)

1

M

WNa

8(1)

1

weeks

M

The following fields occur once per satellite

SatID

6(1)

M

e(1)

16

2-21

dimensionless

M

toa(1)

8

212

sec

M

i

16

2-19

semi-circles

M

OMEGADOT

16

2-38

semi-circles/sec

M

SV Health

8

Boolean

M

A1/2(1)

24

2-11

meters1/2

M

OMEGA0

24

2-23

semi-circles

M

24

2-23

semi-circles

M

M0

24

2-23

semi-circles

M

af0

11

2-20

seconds

M

af1

11

2-38

sec/sec

M

Acquisition Assistance

The Acquisition Assistance field of the GPS Assistance Data Information Element contains parameters that enable fast acquisition of the GPS signals in network-based GPS positioning. Essentially, these parameters describe the range and derivatives from respective satellites to the Reference Location at the Reference Time. Table A.24 illustrates the assistance data occurring once per message and table A.25 illustrates the assistance data occurring per number of satellites for which acquisition assistance is being provided. Figure A.2 illustrates the relation between some of the fields.

This field is optional. The field would probably appear when the Method Type field of the Positioning Instructions IE is set to 0 (MS-Assisted) and the Positioning Methods field of the Position Instructions IE is set to 1 (GPS).

Table A.24: GPS Acquisition Assist – Parameters appearing once per message

Parameter

Range

Bits

Resolution

Incl.

Notes

Number of Satellites

0 – 15

4

M

Reference Time

GPS TOW

0 –604799.92sec

23

0.08 sec

M

BCCH Carrier

0 – 1023

10

O1

BSIC

0 – 63

6

O1

Frame #

0 – 2097151

21

O1

Timeslots #

0 – 7

3

O1

Bit #

0 – 156

8

O1

NOTE 1: All of these field shall be present together, or none of them shall be present.

Table A.25: GPS Acquisition Assist – Parameters appearing [number of satellites] times per message

Parameter

Range

Bits

Resolution

Incl.

Notes

SVID/PRNID

1 – 64 (0 – 63 )

6

M

Doppler (0th order term)

-5,120 Hz to 5,117.5 Hz

12

2.5 Hz

M

Doppler (1st order term)

-1.0 – 0.5 Hz/sec.

6

1/42 Hz/sec.

O1

Doppler Uncertainty

12.5 Hz – 200 Hz
[2-n(200) Hz, n = 0 – 4]

3

O1

Code Phase

0 – 1022 chips

10

1 chip

M

Integer Code Phase

0-19

5

1 C/A period

M

GPS Bit number

0 – 3

2

M

Code Phase Search Window

1 – 192 chips

4

M

Azimuth

0 – 348.75 deg

5

11.25 deg

O2

Elevation

0 – 78.75 deg

3

11.25 deg

O2

NOTE 1: Both of these fields shall be present together, or none of them shall be present.

NOTE 2: Both of these fields shall be present together, or none of them shall be present.

Figure A.2. Exemplary calculations of Acquisition Assistance fields.

This field indicates whether or not angle information is present in this message. The MS shall interpret a value of "1" to mean that angle (Azimuth and Elevation) information is present, and "0" to mean that it is not provided. This field is mandatory.

Number of Satellites

This field contains the number of satellites identified in this information element. This field is mandatory.

Range: 0 – 15

Reference Time

The Reference Time field of the GPS Acquisition Assistance Data IE specifies the relationship between GPS time and air-interface timing of the BTS transmission in the reference cell.

GPS TOW subfield specifies the GPS TOW for which the location estimate is valid. When the parameters BCCH Carrier/BSIC/Frame #/Timeslots #/Bit # are present, together with GPSTOW they provide a valid relationship between GPS and GSM time, as seen at the approximate location of the MS, ie the propagation delay from BTS to MS shall be compensated for by the SMLC. Depending on implementation, the relation between GPS and GSM time may have varying accuracy. The uncertainty of the timing relation may be provided in the optional field GPS Reference Time Uncertainty. If the propagation delay from BTS to MS is not accurately known, the SMLC shall use the best available approximation of the propagation delay and take the corresponding delay uncertainty into account in the calculation of the field GPS Reference Time Uncertainty. GPS TOW is mandatory when the GPS Acquisition Assistance Data Information Element is included.. When the GSM time parameters are not present the GPS TOW is an estimate of current GPS time of week at time of reception of the RRLP segment containing the GPS TOW by the MS. The SMLC should achieve an accuracy of +/- 3 seconds for this estimate including allowing for the transmission delay between SMLC and MS of the RRLP segment containing GPS TOW. Note that the MS should further compensate GPS TOW for the time between the reception of the segment containing GPS TOW and the time when the GPS TOW field is used.

Range: 0 – 604799.92 sec

The BCCH Carrier # and BSIC subfields specify the reference cell for which GSM timing is provided. These subfields are optional when the GPS Acquisition Assistance Data Information Element is included. If included, the SMLC shall set the reference cell to the current serving cell. A target MS has the option of rejecting a GPS position request or GPS assistance data if the reference cell is not the serving cell.

The Frame # subfield specifies the GSM frame number of the BTS transmissions for the reference cell that occur at the given GPS TOW. This subfield is optional when the GPS Acquisition Assistance Data Information Element is included.

Range: 0 – 2097151

The Timeslots # subfield specifies the GSM timeslot of the BTS transmissions for the reference cell that occur at the given GPS TOW. This subfield is optional when the GPS Acquisition Assistance Data Information Element is included.

Range: 0 – 7

The Bit # subfield specifies the GSM and bit number of the BTS transmissions for the reference cell that occur at the given GPS TOW. This subfield is optional when the GPS Acquisition Assistance Data Information Element is included.

Range: 0 – 156

SVID/PRNID

This field identifies the particular satellite for which the measurement data is supplied. This value is the same as the PRN number provided in the navigation message transmitted by the particular satellite.

The range is 0 to 63, where SVID = PRNID – 1

Doppler (0th order term)

This field contains the Doppler (0th order term) value. A positive value defines the increase in satellite signal frequency due to velocity towards the MS. A negative value defines the decrease in satellite signal frequency due to velocity away from the MS. This field is mandatory.

Range: 5,120 Hz to 5,117.5 Hz

Doppler (1st order term)

This field contains the Doppler (1st order term) value. A positive value defines the rate of increase in satellite signal frequency due to acceleration towards the MS. A negative value defines the rate of decrease in satellite signal frequency due to acceleration away from the MS. This field is optional.

Range: -1,0 Hz to 0,5 Hz / s

Doppler Uncertainty

This field contains the Doppler uncertainty value. It is defined such that the Doppler experienced by a stationary MS is in the range “Doppler  Doppler Uncertainty” to “Doppler  Doppler Uncertainty”. This field is optional.

Range: 12,5 Hz to 200 Hz

Code Phase

This field contains code phase, in units of 1 GPS chip, in the range from 0 to 1022 GPS chips, where increasing binary values of the field signify increasing predicted pseudoranges, as seen by a receiver at the Reference Location at the time GPS TOW. The Reference Location would typically be an apriori estimate of the MS location. This field is mandatory.

Range. 0-1022 chips

Integer Code Phase

This field contains integer code phase, i.e. the number of the code periods that have elapsed since the latest GPS bit boundary, in units of C/A code period, as seen by a receiver at the Reference Location at the time GPS TOW. This field is mandatory.

Range: 0-19

GPS Bit Number

This field contains GPS bit number (expressed modulo 4) currently being transmitted at the time GPS TOW, as seen by a receiver at the Reference Location. This field is mandatory.

Range: 0-3

Code Phase Search Window

This field contains the code phase search window. The code phase search window accounts for the uncertainty in the estimated MS location but not any uncertainty in GPS TOW. It is defined such that the expected code phase is in the range “Code Phase – Code Phase Search Window” to “Code Phase + Code Phase Search Window”. This field is mandatory.

Range: 0-15 (i.e. 1-192 chips according to following table)

Table A.26: Code Phase Search Window Parameter Format

CODE_PHASE_WIN

Code Phase Search Window (GPS chips)

‘0000’

1023

‘0001’

1

‘0010’

2

‘0011’

3

‘0100’

4

‘0101’

6

‘0110’

8

‘0111’

12

‘1000’

16

‘1001’

24

‘1010’

32

‘1011’

48

‘1100’

64

‘1101’

96

‘1110’

128

‘1111’

192

Azimuth

This field contains the azimuth angle. An angle of x degrees means the satellite azimuth a is in the range (x  a < x+11.25) degrees. This field is optional.

Range: 0 – 348.75 degrees.

Elevation

This field contains the elevation angle. An angle of y degrees means the satellite elevation e is in the range (y  e < y+11.25) degrees except for y = 78.75 where the range is extended to include 90 degrees. This field is optional.

Range: 0 – 78.75 degrees

Real-Time Integrity

The Real-Time Integrity field of the GPS Assistance Data Information Element contains parameters that describe the real-time status of the GPS constellation. Primarily intended for non-differential applications, the real-time integrity of the satellite constellation is of importance as there is no differential correction data by which the mobile can determine the soundness of each satellite signal. The Real-Time GPS Satellite Integrity data communicates the health of the constellation to the mobile in real-time. The format is shown in tables A.27 to A.29.

Table A.27: Real-Time Integrity (Fields occurring once per message)

Parameter

# Bits

Scale Factor

Range

Units

Incl.

Bad Satellite Present

1

1

Boolean

M

Table A.28: Real-Time Integrity – Parameters appearing when Bad Satellite Present is set

Parameter

# Bits

Scale Factor

Range

Units

Incl.

NBS

4

1

1 – 16

C

Table A.29: Real-Time Integrity – Parameters appearing NBS times

Parameter

# Bits

Scale Factor

Range

Units

Incl.

Bad_SVID

6

1

1 – 64 (0-63)

C

Bad Satellite Present

This single bit parameter indicates that (0) all GPS satellites in the constellation are OK to use at this time, or (1) that there is at least one satellite that should be ignored by the MS-based GPS application.

NBS (Number of Bad Satellites)

This four bit field is dependent on Bad Satellite Present being set to one. In this case, the NBS field indicates the number of satellite ID’s that follow that the user should not use at this time in a fix. This field is conditional.

Bad_SVID

This six bit field appears NBS times, and indicates the SVID of satellites that should not be used for fix by the user at this time. This field is conditional.

A.4.2.4a GPS Time Assistance Measurement Request Element

This element is optional and controls if the MS shold return GPS time assistance measurements or not to the SMLC. The inclusion of this parameter implies use of measure Position Request The description is found in sub-chapter 2.2.4a.

A.4.2.4b GPS Reference Time Uncertainty Element

This element is conditional and provides the accuracy of the relation GPS and GSM time in the Acquisition Assistance in GPS Assistance Data Element. The interval, range and treatment is as described in sub-clause 2.2.4b.

A.4.2.5 More Assistance Data To Be Sent Element

This element is set by the SMLC to indicate to the MS if more Assistance Data components will be sent in the current procedure in order to deliver the entire set of assistance data.