semread

Read data from SEM almanac file

Since R2022b

Syntax

[data,title] = semread(filename)

[data,title] = semread(filename,GPSWeekEpoch=refdate)

Description

[data,title] = semread(filename) reads data from the SEM (System Effectiveness Model) almanac file for the date specified by filename, and returns the parameters of each associated satellite as a timetable, as well as the almanac title.

[data,title] = semread(filename,GPSWeekEpoch=refdate) specifies the reference date from which the SEM almanac file counts the GPS (Global Positioning System) week number.

Examples

Read GPS Navigation Message Data from SEM Almanac File

Download the SEM almanac file from NAVCEN website and parse it. Specify the date for which the file is to be downloaded.

d = datetime("today") - 2;

Create the URL.

baseURL = "https://www.navcen.uscg.gov/sites/default/files/gps/almanac/";
almanacType = "/sem/";
almanacExtension = ".al3";
url = baseURL + d.Year + almanacType + day(d,"dayofyear") + ...
      almanacExtension;

Specify the file name.

filename = "semalmanac" + "_" + d.Year + "-" + ...
           d.Month + "-" + d.Day + ".al3";

Save the file.

websave(filename,url);

Get the orbital parameters from the downloaded SEM almanac file.

[data,title] = semread(filename)

data=31×16 timetable
    17-Jul-2022 17:03:42    2219    61440    1    63    0    0.0120391845703125    0.0144615173339844    -2.48837750405073e-09    5153.61865234375    0.51555073261261    0.297461271286011    0.881654858589172    0.000319480895996094    -7.27595761418343e-12    0    11
    17-Jul-2022 17:03:42    2219    61440    2    61    1    0.0199451446533203    0.007476806640625    -2.53567122854292e-09    5154.81982421875    0.486109972000122    -0.446799159049988    0.925528883934021    -0.000651359558105469    0    0    9
    17-Jul-2022 17:03:42    2219    61440    3    69    0    0.00439071655273438    0.0102882385253906    -2.56841303780675e-09    5153.53857421875    0.844612836837769    0.320030331611633    0.529847264289856    -0.000309944152832031    -1.09139364212751e-11    0    11
    17-Jul-2022 17:03:42    2219    61440    4    74    0    0.00190067291259766    0.00612449645996094    -2.46654963120818e-09    5153.54736328125    -0.811291694641113    -0.953091979026794    -0.536049604415894    -0.00014495849609375    3.63797880709171e-12    0    12
    17-Jul-2022 17:03:42    2219    61440    5    50    0    0.00610446929931641    0.00614738464355469    -2.61570676229894e-09    5153.70166015625    0.831113338470459    0.338927388191223    -0.287315726280212    -8.96453857421875e-05    0    0    10
    17-Jul-2022 17:03:42    2219    61440    6    67    0    0.00243806838989258    0.0142459869384766    -2.48473952524364e-09    5153.71875    0.512914299964905    -0.275499105453491    0.955711841583252    0.00038909912109375    1.45519152283669e-11    0    11
    17-Jul-2022 17:03:42    2219    61440    7    48    0    0.0163321495056152    0.00248146057128906    -2.53567122854292e-09    5153.59375    -0.487354755401611    -0.716959476470947    0.676499962806702    0.000318527221679688    0    0    10
    17-Jul-2022 17:03:42    2219    61440    8    72    0    0.00738525390625    0.00658035278320312    -2.48110154643655e-09    5153.515625    0.170664310455322    0.0531282424926758    -0.420778274536133    -7.62939453125e-05    0    0    11
    17-Jul-2022 17:03:42    2219    61440    9    68    0    0.00224971771240234    0.00393486022949219    -2.49565346166492e-09    5153.74755859375    -0.828534364700317    0.607595086097717    -0.25430428981781    -0.000318527221679688    3.63797880709171e-12    0    11
    17-Jul-2022 17:03:42    2219    61440    10    73    0    0.00771760940551758    0.0102214813232422    -2.56841303780675e-09    5153.63232421875    0.843742370605469    -0.797283291816711    0.24565315246582    -0.000494956970214844    -1.45519152283669e-11    0    11
    17-Jul-2022 17:03:42    2219    61440    11    78    0    0.000565528869628906    0.0065765380859375    -2.57205101661384e-09    5153.5556640625    0.528360724449158    0.913978695869446    -0.423171401023865    -1.04904174804688e-05    0    0    12
    17-Jul-2022 17:03:42    2219    61440    12    58    0    0.00853252410888672    0.00836372375488281    -2.53567122854292e-09    5153.7099609375    -0.132869362831116    0.417471408843994    0.468230128288269    -0.000247955322265625    -7.27595761418343e-12    0    10
    17-Jul-2022 17:03:42    2219    61440    13    43    0    0.00612878799438477    0.00843238830566406    -2.43380782194436e-09    5153.63427734375    -0.779114127159119    0.291950225830078    -0.548626184463501    0.000349998474121094    7.27595761418343e-12    0    9
    17-Jul-2022 17:03:42    2219    61440    14    77    0    0.00212907791137695    0.0030670166015625    -2.58660293184221e-09    5153.5986328125    -0.143590927124023    -0.983529925346375    0.45069146156311    -0.000111579895019531    0    0    12
      ⋮

title = 
'CURRENT.ALM'

View Satellite Positions from SEM Almanac File for Receiver Position

Read GPS navigation message data from a SEM almanac file.

gpsData = semread("semalmanac_2022-4-10.al3");

Get the satellite positions, velocities, and IDs at the first time step.

t = gpsData.Time(1);
[satPos,satVel,satID] = gnssconstellation(t,gpsData,GNSSFileType="SEM");

Specify a GNSS receiver position in geodetic coordinates (latitude, longitude, and altitude).

recPos = [42 -71 50]; % Natick, MA

Get the azimuth and elevation look angles of the satellite positions for the specified receiver position. The isVis output indicates which satellites are visible. Find the total number of visible satellites by using nnz.

[az,el,isVis] = lookangles(recPos,satPos);
fprintf('%d satellites visible at %s.\n',nnz(isVis),t);

9 satellites visible at 12-Apr-2022 16:50:54.

Visualize all the visible satellites.

skyplot(az(isVis),el(isVis),satID(isVis))

Plot Trajectories of Satellites over Time from SEM Almanac File

Read GPS navigation message data from a SEM almanac file.

gpsData = semread("semalmanac_2022-4-10.al3");

Specify the relevant time-stepping information.

startTime = gpsData.Time(1);
numHours = 12;
secondsPerHour = 3600;
dt = 60;
timeElapsed = 0:dt:(numHours*secondsPerHour);
t = startTime + seconds(timeElapsed);

Specify a GNSS receiver position in geodetic coordinates (latitude, longitude, and altitude).

recPos = [42 -71 50]; % Natick, MA

Get the azimuth and elevation look angles for the all the positions of the satellites for the specified receiver position.

numSats = numel(gpsData.PRNNumber); % Number of satellites
[allAz,allEl] = deal(NaN(numel(t),numSats));
for i = 1:numel(t)
    [satPos,~,satID] = gnssconstellation(t(i),gpsData,GNSSFileType="SEM");
    [az,el,vis] = lookangles(recPos,satPos);
    allAz(i,:) = az;
    allEl(i,:) = el;
end

Mark all satellites below the horizon as NaN for no visibility.

allEl(allEl < 0) = NaN;

Visualize the trajectories of the satellites.

figure
skyplot(allAz,allEl,satID)

Animate the trajectories of the satellites.

figure
sp = skyplot(allAz(1,:),allEl(1,:),satID);
for i = 1:size(allAz, 1)
    set(sp,AzimuthData=allAz(1:i,:),ElevationData=allEl(1:i,:));
    drawnow
end

Input Arguments

filename — SEM almanac file name

string scalar | character vector

SEM almanac file name, specified as a string scalar or character vector. The file name can include an absolute path, relative path, or no path. The file name can also include the file extensions.

Example: "semalmanac_2022-1-18.al3"

Example: "mydir/semalmanac_2022-1-18.al3"

Example: "C:/mydir/semalmanac_2022-1-18.al3"

Data Types: char | string

refdate — Reference date

"06-Jan-1980" | "21-Aug-1999" | "06-Apr-2019"

Reference date, specified as one of these valid datetime strings that coincide with the GPS week number rollover dates:

• "06-Jan-1980"

• "21-Aug-1999"

• "06-Apr-2019"

These dates occur every 1024 weeks, starting from January 6, 1980 at 00:00 (UTC). The default value is a datetime string that coincides with the most recent GPS week number rollover date before the current day.

Example: GPSWeekEpoch="21-Aug-1999"

Data Types: char | string

Output Arguments

data — Parameters of each satellite

timetable

Parameters of each satellite, returned as a timetable with a row for each record and a column for each parameter in that record. Find more information on the SEM almanac parameters definition, see the Table 40-II in ICD-GPS-240D [2].

Parameters	Data Type	Description
Time	datetime	GPS clock time, calculated using GPSWeekNumber and GPSTimeOfApplicability.
GPSWeekNumber	double	GPS week number, continuous, not mod(1024).
GPSTimeOfApplicability	double	Number of seconds since the beginning of the GPS week number.
PRNNumber	double	Satellite pseudorandom noise number.
SVN	double	Space vehicle reference number of the satellite.
AverageURANumber	double	Average URA number of the satellite.
Eccentricity	double	Eccentricity of the orbit.
InclinationOffset	double	Inclination angle offset from 54 degrees, in semicircles.
RateOfRightAscension	double	Rate of change in the measurement of the angle of right ascension, in semicircles per second.
SqrtOfSemiMajorAxis	double	Square root of the semimajor axis, in meters1/2.
GeographicLongitudeOfOrbitalPlane	double	Geographic longitude of the orbital plane at the weekly epoch, in semicircles.
ArgumentOfPerigee	double	Angle from the equator to perigee, in semicircles.
MeanAnomaly	double	Angle of the position of the satellite in its orbit relative to perigee, in semicircles.
ZerothOrderClockCorrection	double	Satellite almanac zeroth-order clock correction term, in seconds.
FirstOrderClockCorrection	double	Satellite almanac first-order clock correction term, in seconds per second.
SatelliteHealth	double	Satellite vehicle health data code.
SatelliteConfiguration	double	Satellite vehicle configuration code.

title — Almanac title

character vector

Almanac title, returned as a character vector.

Tips

• To download SEM almanac files from the NAVCEN website for the current date, you must specify a date 2 days before the current date because the GPS time of applicability is approximately 70 hours later than the transmission time of the almanac data set. See the Read GPS Navigation Message Data from SEM Almanac File example for more details.

References

[1] Science Applications International Corporation. NAVSTAR GPS Space Segment/Navigation User Interfaces. IS-GPS-200M. Los Angeles, CA: United States Space Force Space Systems Command, approved May 21, 2021. https://www.navcen.uscg.gov/sites/default/files/pdf/gps/IS_GPS_200M.pdf.

[2] Science Applications International Corporation. NAVSTAR GPS Space Segment/Navigation User Interfaces. ICD-GPS-240D. Los Angeles, CA: United States Space Force Space Systems Command, approved May 21, 2021. https://www.navcen.uscg.gov/sites/default/files/pdf/gps/ICD_GPS_240D.pdf.

[3] United States Coast Guard. "GPS Almanacs, NANUs, and OPS Advisories Archives." US Coast Guard Navigation Center. Accessed May 6, 2022. https://www.navcen.uscg.gov/archives.

Version History

Introduced in R2022b