ephemeris vs almanac

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

 

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

 

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

 

Thanks a lot!

Carl

 

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

 

They probably use all 18 channels simultaneously to get the first
satellite extremely fast, repeating for the second, etc.

 

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

 

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.

 

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)

 

That's true. I had missed that part of the almanac was sent in subframe 4
but now I know.