Specification for WAAS/EGNOS decoder module: - GetACoder

Specification for WAAS/EGNOS decoder module:
Introduction:
The required software module will written in Visual Basic and accept information in
the form of 250 bit data packets, at a rate of at least 1Hz, identify the message type
and decode the contained information appropriately, applying the extracted data if
relevant to an internally stored database of recently received information.
The module will provide several functions (specified below) which apply this
received and stored data to calculating positional corrections and accuracy
information for GPS satellites, and outputting this information in suitable number
formats.
The required equations and procedures, and the data formats of the received packets
are all fully specified in either:
U.S. Department of Transportation ‘WIDE AREA AUGMENTATION SYSTEM
(WAAS) [FAA-E-2892B, Change 1, DTFA01-96-C-00025 Modification No:0051]
GLOBAL POSITIONING SYSTEM STANDARD POSITIONING SERVICE
SIGNAL SPECIFICATION 2nd Edition June 2, 1995
Both of which will be provided along with other documents of an explanatory or
background nature.
Of particular importance is the correct application of the ‘Issue of Data’ and ‘Time of
applicability’ information associated with the received data in ensuring only currently
relevant information is used.
Most of the stored information will be seconds or minutes old, and to calculate the
corrections to the relevant precision, it will be necessary ‘calculate forward’ from the
data time to the requested time, though the equations necessary are provided in the
GPS and WAAS specifications.
Tropospheric corrections will be calculated elsewhere (it is a ‘static’ model and
tropospheric information is not transmitted) and can be ignored.
Message Type 12 (WAAS network time parameters) are currently not being
broadcast, therefore it is assumed that WAAS network time = GPS network time.

Initialisation:
Input:
Receiver location in WGS-84 ECEF for determining IGPs and WAAS satellite (GEO)
suitability.
Function 1: Information collection and database maintenance.
Input:
WAAS/EGNOS 250 bit Data packet.
GPS time (millisecond of current GPS week)
Function Description:
In order to process the received WAAS information and ensure the relevancy of the
received data, an internal WAAS database of recently received information will have
to be created and maintained. This will be necessary to implement the error
protection protocols inherent in the WAAS system.
This function will have to examine the incoming WAAS/EGNOS messages and
update the internal database.
The CRC of the incoming packet will have to be checked (the packet being rejected
on failure, and some indication of packet failure given), and the Issue of Data
messages contained within the WAAS packets analysed in accordance with the
procedure given in the WAAS specification (FAA-E-2892B) to ensure the validity of
the data before entry into the database.
Output:
None
Waas/Egnos Packet
GPS Time
Check Packet CRC
If the packet valid,
and the contained information satisfies all of the ‘Issue of
Data’ and ‘Time of Applicability’ conditions, update
internal Waas database.

Function 2: Generation of Satellite Corrections for a given SV
Inputs:
GPS time (millisecond of current GPS week)
Satellite (SV) PRN Number
Function Description:
This function will output information on a specified satellite, calculated from the
received WAAS information. Again, received information will have to be stored
internally for later use, as the transmission of certain message types is irregular.
Each message type, or section of message may have a ‘Time of Applicability’, after
which the information should not be used, and these will have to be checked before
application of the data.
For example:
In order to calculate the ionospheric corrections for a specified satellite, firstly the
location of the receiver must be known in order to select store and process the
Ionospheric information. The nearest ionospheric grid points (IGP’s) must be
determined, the pierce point of the ionosphere by the satellite signal and the
Ionospheric Correction (IC i ) calculated. (Specified in FAA-E-2892B)
Output:
If corrections not available for PRN:
Message indicating ‘Do not use’, ‘Not Monitored’ , ‘Corrections not available’ etc.
If WAAS/EGNOS corrections available for given PRN:
Equivalent to a line of the table below, giving the following calculated to the
requested GPS Time:
Positional corrections with respect toWGS-84 ECEF co-ordinate system for the
named satellite, taking into account the long term and fast corrections, as specified in
FAA-E-2892B Sections 4.4.3, 4.4.7, and 4.48.
Clock time error estimate for the named satellite.
UDRE (Integrity) of the fast and long term error corrections (combined)
Pseudorange (RMS i ) error estimate for the named satellite.
Ionospheric corrections (IC i ), using techniques described in FAA-E-2892B sections
4.4.9 to 4.4.10
Integrity of Ionospheric corrections (σ 2 UIVE)
Residual error (σ 2 flt) as specified in FAA-E-2892B sections 4.5.1 to 4.5.2.

Function2: Calculation of Satellite Corrections.
SV PRN No.
GPS Time in
‘milliseconds of week’
Calculate WAAS corrections for given SV
PRN, for given GPS Time, and output
corrections table:
Position
Corrections
(WGS-84
ECEF)
Clock
Time
Error
Estimate.
Fast
Corr
Integrity
PseudoRange
RMS Error
Estimate
Ionospheric
Corrections
Integrity
Residual
Errors
m s m m m s s 2 m 2
δx δy δz δ∆t SV (t k PRC i UDRE RMS i IC i σ 2 UIVE σ 2 flt

Function 3:
Inputs:
GPS time (millisecond of current GPS week)
GEO (WAAS) PRN Number
Function Description:
This function should accept the PRN number of a GEO (Geostationary
WAAS/EGNOS satellite) and a GPS time, and calculate the position of the specified
satellite at the requested time using all of the GEO satellite information previously
decoded and stored from GEO navigation messages.
Output:
GEO satellite position in WGS-84 ECEF coordinate system, using stored GEO
navigation and almanac information to calculate position using error corrections to be
correct at the requested time as specified in FAA-E-2892B sections 4.4.11
GEO PRN
No.
GPS Time in
‘milliseconds of week’
Calculate WAAS positions for given GEO
PRN and GPS Time, and output GEO Position
at requested GPS time.
GEO
PRN
GEO Position
(WGS-84 ECEF)
User
Range
Accuracy
m m m m
PRN X G Y G Z G URA