ephemeris vs almanac

Discussion in 'General GPS Discussion' started by carl, Dec 16, 2003.

  1. carl

    carl Guest

    Hi
    I'd like some clarification on the differences between the ephemeris and
    almanac. I read some details on gpsinformation.net, but then I also read
    some on Trimble's website that is somewhat different.

    gpsinformation.net states:

    *****
    The satellites broadcast two types of data, Almanac and
    Ephemeris. Almanac data is course orbital parameters for all
    SVs. Each SV broadcasts Almanac data for ALL SVs. This Almanac
    data is not very precise and is considered valid for up to
    several months. Ephemeris data by comparison is very precise
    orbital and clock correction for each SV and is necessary for
    precise positioning. EACH SV broadcasts ONLY its own Ephemeris
    data. This data is only considered valid for about 30 minutes.
    The Ephemeris data is broadcast by each SV every 30 seconds.
    *****

    Trimble's website states:

    *****
    Ephemeris (or orbital) data is constantly being transmitted by the
    satellites.
    Receivers maintain an "almanac" of this data for all satellites and they
    update these almanacs as new data comes in.
    Typically, ephemeris data is updated hourly.
    *****


    I'd like to know who's right, what are the differences between almanac
    and ephemeris, when they are broadcasted, and how long id the data valid.

    Thanks

    Carl
     
    carl, Dec 16, 2003
    #1
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  2. carl

    Sam Wormley Guest

    The GPS Navigation Message
    o Each satellite transmits its own ephemeris data every 30 seconds
    o Each satellite transmits the almanac for the whole constellation ever
    12.5 minutes.

    Interface Control Document ICD-GPS-200C
    http://www.navcen.uscg.gov/pubs/gps/icd200/default.htm
    http://www.edu-observatory.org/gps/ICD-GPS-200C_Fig20-1/

    Also see: http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf
     
    Sam Wormley, Dec 16, 2003
    #2
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  3. Actually they are both wrong. ephemeris is broadcast every 30 seconds
    but it valid for two hours guaranteed and more likely 4 hours (basically
    a full transit for most birds) Trimble is right in that it could be
    updated every hour but this is seldom done. It is only updated on a need
    to basis but if updated it is done at the top of the hour.

    Almanac data is the same as ephemeris but only the upper most
    significant bits with all the detailed position data missing. It can be
    valid for up to 6 mos. Almanac data can be updated from ephemeris data
    or from the almanac download which is broadcast every 12.5 minutes.
    Almanac data is only used to help the receiver determine which birds are
    currently up and where to look for them. It is not used for actual
    position determinination which is done using the ephemeris data. To
    understand how this is done read my article on how a GPS gets a fix. It
    is on gpsinformation.net so I guess you can say that gpsinformation.net
    can have some conflicting data on it depending on who wrote the article
    and how old it is. The quote of 30 minutes for ephemeris is a really old
    quote an not current at all. It was based on the behavior observed on
    old Garmin multiplex units and turned out to be wrong. It should be
    correected.

    Dale
     
    Dale DePriest, Dec 16, 2003
    #3
  4. carl

    carl Guest

    Thanks a lot!

    Carl
     
    carl, Dec 16, 2003
    #4
  5. carl

    Annapress Guest

    Almanac data is only used to help the receiver determine which birds are
    Anybody understand how the new Cobra 18-channel GPS 100 deals with this? They
    claim 50-second time to fix even the first time it is turned on. I assume this
    is because when you are looking for 18 birds instead of just 12, there is a
    good chance at least 4 of the 18 will be above the horizon even if you don't
    have accurate almanac data (in the limit, if you have a 24-channel receiver you
    shouldn't need almanac data at all). Is this right, or am I missing something?

    Steve
     
    Annapress, Dec 17, 2003
    #5
  6. carl

    Bob Guest

    They probably use all 18 channels simultaneously to get the first
    satellite extremely fast, repeating for the second, etc.
     
    Bob, Dec 17, 2003
    #6
  7. carl

    Tim Hogard Guest

    Annapress () wrote:
    : Anybody understand how the new Cobra 18-channel GPS 100 deals with this? They
    : claim 50-second time to fix even the first time it is turned on. I assume this
    : is because when you are looking for 18 birds instead of just 12, there is a
    : good chance at least 4 of the 18 will be above the horizon even if you don't
    : have accurate almanac data (in the limit, if you have a 24-channel receiver you
    : shouldn't need almanac data at all). Is this right, or am I missing something?

    An almanac will say things like SV3 is at about N43,W22 and thats
    its using PRN21. Based on that info, you can figure out how far
    away it is and how its frequencies are jumping around and what its
    doppler shift is to speed up locking on to it. If you don't know
    thouse things, you have to do far more scaning but modern DPS
    hardware is good at that.

    They could use several channels in parallel once they find a specifc
    sat and use the other channels to help track the pseudorandom jumps till
    it gets enough info to help lock downs its position.

    I have feed a year+ old alamanc to my Garmin GPS12 (with the date
    codes fixed so it wouldn't throw it out), and I was supprised at
    how well it worked. I'm guessing you could create a backup almanac
    to help speed up inital searches.

    Back to the original questions about alamanc vs empheris:
    The empheris data is much more precise, its more like the sat saying
    its at N43.123123,W22.23123123 moving at 55.00202 degrees at a speed
    of 18,000.00023 mi/hr and is being pulled towards jupiter at an
    angle of .000000000000000443 arcseconds per second squared. The
    empheris is just factors for a great big polynomial. For thouse
    that have unpleaset memories of these, this one has about 12 factors
    and tends to be done in polar coorinates.

    -tim
    http://web.abnormal.com
     
    Tim Hogard, Dec 17, 2003
    #7
  8. carl

    Stefan Kvist Guest

    There is almanac data for one satellite in each 30 second frame. In 12.5
    minutes there are 25 such frames but there are currently 28 active
    satellites so I guess you can't get the whole constellation from one
    satellite in 12.5 minutes.
     
    Stefan Kvist, Dec 17, 2003
    #8
  9. carl

    Sam Wormley Guest

    You didn't read the reference--Almanac covers 32 satellites!

    REFERENCES
    1. GPS Interface Control Document ICD-GPS-200 - NAVSTAR GPS Space Segment and
    Navigation User Interfaces
    2. Wells D, Guide To GPS Positioning, 2nd ed, Canadian GPS Associates, Frederiction,
    New Brunswick, Canada 1987
    3. Email - Bill Straka


    GPS MESSAGE CONTENT
    see: http://www.cnde.iastate.edu/staff/swormley/gps/Wells.et.al.6.06.gif

    SUBFRAME 1

    Flags (L2 code & data; week #; satellite accuracy and health)
    Age of Data
    Satellite clock correction coefficients

    Telemetry Word (TLM) - Each TLM word is 30 bits long, occurs every six
    seconds in the data frame, and is the first word in each subframe/page.
    The format shall be as partially shown below. Bit 1 is transmitted
    first. Each TLM word shall begin with a preamble (10001011) followed by
    the TLM message, two reserved bits, and six parity bits. The TLM
    message contains information needed by the authorized user.

    Handover Word (HOW) - The HOW shall be 30 bits long and shall be the
    second word in each subframe/page, immediately following the TML word.
    A HOW occurs every 6 seconds in the data frame. The format and content
    of the HOW shall be as partially shown below. The MSB is transmitted
    first. The HOW begins with the 17 MSBs of the time-of-week (TOW) count.
    (The full TOW count consists of the 19 LSBs of the 29-bit Z-count.
    These 17 bits correspond to the TOW-count at the X1 epoch which occurs
    at the start (leading edge of the next following subframe.

    Word 3 in subframe 1 is the week number (WN), which gives you the date.
    Combine the week number with the TOW, plus GPS origin date, and you
    have GPS time. Actually, you need to look a little further on at the
    SV's clock correction, drift rate and IODC (Issue of Data, Clock - in
    other words, when the correction parameter was inserted into the
    message), but this is pretty small.

    In the present format of the message, there is no provision for giving
    which 1024 week cycle you are in. There is some discussion of providing
    a couple extra bits in the reserved parts of the message. For a receiver
    built today, the way around the 1024 week cycle limitation is to have
    the user's receiver provide that information (are you between 1980 and
    1999? Are you between 1999 and .... etc, or simpler, are you in the
    1980's, 1990's, 2000's, or whatever decade? or tie it closer to the
    year.) It can be done as a user input, just like the time zone/daylight
    time offset. Or, just use the ROM issue date or software version date.

    This allows your GPS receiver to sync with GPS time.... Take a look
    at subframe 4 page 18 for the relationship of GPS time with UTC.



    |<--------- TLM Word -------->|<----------- HOW ----------->|
    (word 1) (word 2) (word 3)

    |1 |31 |61 |91 300
    |---------------------|-|-----|----------------|-|--|-|-----|---------|-|---|-----|-|-----|----/ /---|
    | | | | | | | | | | | | | | | |
    | TLM |C| P | TOW | | |t| P | WN | | | | | P | |
    | | | | MS 17-BITS | | | | | 10-BITS | | | | | | |
    |10001011 | | |MSB LSB| | | | | | | | | | | |
    |---------------------|-|-----|----------------|-|--|-|-----|---------|-|---|-----|-|-----|----/ /---|



    SUBFRAMES 2 & 3
    Orbit Parameters

    |<--------- TLM Word -------->|<----------- HOW ----------->|
    (word 1) (word 2) (word 3)

    |1 |31 |61 |91 300
    |---------------------|-|-----|---------------------|-|-----|-------|---------------|-----|----/ /---|
    | | | | | | | | | | |
    | TLM |C| P | HOW |t| P | IODE | C(rs) | P | |
    | 22 BITS | | | 22 BITS | | |8 BITS | 16 BITS | | |
    |10001011 | | | | | | | | | |
    |---------------------|-|-----|---------------------|-|-----|-------|---------------|-----|----/ /---|
    |1 |31 |61 |91 300
    |---------------------|-|-----|---------------------|-|-----|---------------|-------|-----|----/ /---|
    | | | | | | | | | | |
    | TLM |C| P | HOW |t| P | C(ic) | | P | |
    | 22 BITS | | | 22 BITS | | | 16 BITS |8 BITS | | |
    |10001011 | | | | | | | | | |
    |---------------------|-|-----|---------------------|-|-----|---------------|-------|-----|----/ /---|


    SUBFRAME 4
    Almanac for satellites 25-32 (pages 2-5, 7-10)
    Ionospheric model, and UTC data (page 18) partially shown
    Antispoof flag - 32 satellites (page 25)
    Satellite configuration - 32 satellites (page 25)
    Health of satellites 25-32 (page 25)

    Reserved (pages 1,6,11,12,16,19,20-24)
    Spares (pages 13-15)
    Special messages (page 17)


    |1 |211 219 227 |241 249 257 |271 279 300
    |----/ /---|-------|-------|-------|-----|-------|-------|-------|-----|-------|-------------|-|-----|
    | | | | | | | | | | | | | |
    | | | t(ot) | WN(t) | P |dt(LS) |WN(LSF)| DN | P |dt(LSF)| SPARE |t| P |
    | |8 BITS |8 BITS |8 BITS | |8 BITS |8 BITS |8 BITS | |8 BITS | 14 BITS | | |
    | | | | | | | | | | | | | |
    |----/ /---|-------|-------|-------|-----|-------|-------|-------|-----|-------|-------------|-|-----|
    | | | |
    (word 8) (word 9) (word 10)

    Universal Coordinated Time (UTC) - Page 18 of subframe 4 includes (1)
    the parameters needed to relate GPS time to UTC, and (2) notice to the
    user regarding the scheduled future or recent past (relative to NAV
    message upload) value of the delta time to to leap seconds dt(LSF),
    together with the week number WN(LSF) and the day number (DN) at the end
    of which the leap second become effective. "Day one" is the forst day
    relative to the end/start of week and the WN(LSF) value consists of
    eight LSBs of the full week number. The user must account for the
    truncated nature of this parameter as well as truncation of WN, WN(t),
    and W(LSF) due to rollover of the full week number. The Command Segment
    (CS) shall manage these parameters such that the absolute value of the
    difference between the untruncated WN and WN(LSF) value shall not exceed
    127. For more detail see section 20.3.3.5.2.4 of ICD-GPS-200.


    SUBFRAME 5
    Almanac for satellites 1-24 (pages 1-24)
    Health of satellites 1-24 (page 25)
     
    Sam Wormley, Dec 17, 2003
    #9
  10. carl

    Stefan Kvist Guest

    That's true. I had missed that part of the almanac was sent in subframe 4
    but now I know.
     
    Stefan Kvist, Dec 17, 2003
    #10
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