what constitutes the nav msg of GPS?

[D driver]

another question...

Which of the following data, in addition to the Pseudo Random Noise (PRN) code, forms part of the so called Navigation Message transmitted by NAVSTAR/GPS satellites?

a)Time; data to impair the accuracy of the position fix (Selective Availability SA)
b)Almanac data; satellite status information
c)Data to correct receiver clock error; almanac data
d)Time; position of the satellites

answer given is B I wonder what is wrong with D? an excerpt from oxford ATPL says:-
The information contained in the nav and system data message is:
SV position
SV clock time
SV clock error
Information on ionospheric conditions
Supplementary information, including the almanac (orbital parameters for the SVs), SV
health (P-code only) correlation of GPS time with UTC and other command and control
functions.

so how does it not give time(or SV clock time)and position of sattelites(almanac) i.e. D?

[Groundloop]

Depends on what is meant by "position" of the satellite.

The message includes the satellite's ephemeris which is information about the satellite's predicted orbit. The RECEIVER uses this ephemeris and time signal from the satellite to calculate the satellite's position. So, being pedantic, the satellite does not broadcast its actual position - only its orbital data.

[Oktas8]

GPS satellites do not transmit their position. They transmit a mathematical equation that, when time is input as an independent variable, can be solved to resolve position. This is the almanac.

In theory the equation need never change, so the satellite would only need to transmit a time signal. However, in practice the satellites' orbits do change slightly on a regular basis, so the current (updated) almanac is continuously re-broadcast.

With a current orbital equation (almanac) and an accurate time, you can generate a pseudo-range between the receiver and the satellite. Given at least four pseudo-ranges, you can solve for position in 3D.

It is important to note that the satellites do not transmit position. They transmit an equation and a time signal.

An example would be a B747 travelling on a great circle path between the Brisbane VOR and the SFO VOR at precisely 400kts GS.
- Can you find its position? Yes, if you know the time it set heading from BNE VOR.
- Did the B747 transmit its position? No, never. Just the equation of motion and the time of set heading.

Hope this helps.

[D driver]

I beg to differ @Groundloop and @Octas8 .
@Groundloop - by position I and I guess anybody who has delved even a little into the subject would think of ephemeris, which by definition is position of a satellite along its ecliptic and that is corroborated by the excerpt I posted from oxford ATPL. Though I concur the option could have been more specific stating ephemeris instead. But so could it be in the case of option B stating "satellite almanac" instead of just "almanac". So basically comes down to phrasing of the question if am not wrong.

@Octas8- I think you have just jumbled up the whole concept. In that, firstly Its not the satellites that transmit any equation. Equations are setup at the receiver. 4 such equations are setup which result in the final derived position of the receiver, which is not the almanac as you said.
secondly, you do not generate the pseudo ranges after setting up the equations, in fact its the other way round. 4 linear simultaneous equations each with 4 unknown quantities i.e. X,Y,Z coordinates and T (time) are solved by iteration to give the final 3D fix and accurate time reference.

[Amadis of Gaul]

Holy Guacamole, there are people with serious smarticles out there!

[A Squared]

Originally Posted by D driver

you do not generate the pseudo ranges after setting up the equations, in fact its the other way round. 4 linear simultaneous equations each with 4 unknown quantities i.e. X,Y,Z coordinates and T (time) are solved by iteration to give the final 3D fix and accurate time reference.

Driver, Oktas' statment is correct. To wit:

Originally Posted by Oktas8

With a current orbital equation (almanac) and an accurate time, you can generate a pseudo-range between the receiver and the satellite.

As stated, there is nothing incorrect there. Now as a practical matter, accurate time is usually not a known, and therefore is solved for in the system of equations as you pointed out.

At any rate, you have your answer, whether you like it or not. D is incorrect because the satellites do not broadcast their *position*, they broadcast a set of orbital parameters which allow the position at any time to be calculated. That isn't the same thing.

Incidentally, a satellite does not have an ecliptic. There is only one Ecliptic and it belongs to the sun. Satellites have orbits, which are elliptical, but that isn't an ecliptic. "Ecliptic" is not a word for "elliptical orbit"

[A Squared]

As a side note, this kind of question is an example of what is wrong with aviation testing. I believe that the question comes from an ATPL exam someplace. I think it has no place as such. Yes, there is a pedantic distinction between broadcasting orbital parameters and broadcasting "position". Is the distinction important to a pilot? No, not even a little bit.

[twistedenginestarter]

It's very interesting but you're right

Is the distinction important to a pilot?

I must admit I cannot think of one good reason why a pilot needs to know how GPS works.

[Groundloop]

by position I and I guess anybody who has delved even a little into the subject would think of ephemeris, which by definition is position of a satellite along its ecliptic and that is corroborated by the excerpt I posted from oxford ATPL.

Wrong. You obviously have not delved deep enough! The ephemeris of a satellite consists of a series of variables which when combined with the satellite time signal is input into an equation in the RECEIVER which enables the receiver to CALCULATE the satellites estimated position. I am afraid that this is not the only error within Oxford's ATPL notes about GPS!

Also, there appears to be some confusion between empheris and almanac. From the European Space Agency's Navipedia:

"Every satellite receives from the ground antennas the navigation data which is sent back to the users through the navigation message. The Navigation Message provides all the necessary information to allow the user to perform the positioning service. It includes the Ephemeris parameters, needed to compute the satellite coordinates with enough accuracy, the Time parameters and Clock Corrections, to compute satellite clock offsets and time conversions, the Service Parameters with satellite health information (used to identify the navigation data set), Ionospheric parameters model needed for single frequency receivers, and the Almanacs, allowing the computation of the position of ”all satellites in the constellation”, with a reduced accuracy (1 - 2 km of 1-sigma error), which is needed for the acquisition of the signal by the receiver. The ephemeris and clocks parameters are usually updated every two hours, while the almanac is updated at least every six days."

And from the "horse's mouth", WWW.GPS.GOV

"Navigation (NAV) Message Data. The data provided to a GPS receiver via each satellite's SIS containing the satellite's predicted clock correction polynomial ("clock"), the satellite's predicted orbital elements ("ephemeris"), a reduced-precision subset of the clock and ephemeris data for all operational satellites in the constellation ("almanac"), pseudorange correction data, parameters relating GPS time to UTC, single-frequency ionospheric correction model parameters, and satellite status information."

I think it is fair to say that the European Space Agency and the US Government know a bit more about GPS than Oxford!

[msbbarratt]

Of course the vast majority of GPS receivers (i.e. mobile phones) these days don't get the ephemeris or almanac from the satellites themselves. They ask their friendly local cell tower for it, a set up called Assisted GPS (AGPS).

In a mobile this is very useful - it can take up to 20 minutes to receive the whole data set via the GPS signal itself. It's a very slow data rate. Getting it immediately from the cell stations means getting a fix very quickly, as the receiver can be primed on exactly the right channels at pretty much the right point in the PRN code for the satellites that are known to be in view. Saves a lot of power at the same too.

Presumably a proper GPS receiver in an avionics suite doesn't rely on AGPS.

AGPS is quite interesting. Having got a GPS fix before embarking on a long haul flight I was impressed when the mobile (in flight mode) was perfectly capable of getting both a GPS and GLONASS fix within a few seconds somewhere well north of Russia. Clearly the cached AGPS data is fairly global (I was a long, long way from the take off point by then) and it was still valid. Modern electronics!

[D driver]

happy new year

Thanx for the inputs guys. but its too early to shoot down oxford and my understanding of the subject...

for one it is from that very "Oxford" that i drew this question and the accompanying answer.

secondly,the orbital parameters or in other words the "EPHEMERIS" that you all very readily accept as being provided by the satellite is indeed the position I was talking about, though I never said sat transmits the position related to earth referenced coordinates but i guess you anyways assumed.

This Ephemeris is the information about the orbit and the position of the satellite along this orbit. And it is to this position that i have been referring to. To quote oxford Pg 338:
"Using keplerian laws and given a starting point, the satellite - space vehicles(SV's) calculate their positions at all points in their orbits.The SV's orbital position is known as ephemeris"

now since mr @groundloop has pronounced it to be an absolute error or shear ignorance on the part of oxford, here's something from GSP radio aids by R.B. underdown and david cockburn Pg.181 to quote:

"The transmission from every satellite includes the orbit that each satellite is following, and its position in the orbit, as an "almanac". The receiving computer uses that almanac to calculate where each satellite should be in its orbit..." also, Pg 182:

"The satellite transmits its own exact position and path, called the "ephemeris", to provide the exact spot from which the ranges must be measured."

And finally from the horse's mouth(Welcome to GPS.gov), one of the elements of the orbital parameters constituting the ephemeris signal: Mean anomaly at epoch (Mo) defines the position of the orbiting body along the ellipse at a specific time (the "epoch").

In summary the ephemeris is the detail of the orbital alignment and the position of the satellite along that orbit. and
"The almanac data are a reduced-precision subset of the clock
and ephemeris parameters." again giving positions of the satellites.

and so why not D?

[WeeJeem]

and so why not D?

Because it's not how GPS works.

Due to relativistic effects, "position of the satellites" is a meaningless concept in this context.
It would be a useless bit of "information" to transmit; one might as well transmit old episodes of Bleep and Booster.

Some of the relativistic corrections required to make the system accurate enough to be worth using can be made on the SV, but others can only be performed by the receiver, because the relative movements of both the observed (SV) and the observer (receiver) must be taken into account.
("relative movements", "relativity", geddit? ).

[A Squared]

Originally Posted by D driver

and so why not D?

Again, because the satellites do not broadcast their position. You can argure that the orbital parameters broadcast in the Ephemeris is "position". It is not. Yes, you've managed to dig up some preferences which say that the position is broadcast. They are incorrect, the satellites do not broadcast their position.

[D driver]

what I could think was that the receiver is required to combine all the different orbital elements transmitted and thus orbital position alone would not be of much use without orbit alignment etc. and thus would not be entirely correct to term it as position ?
@WeeJeem is that what you mean? If not mind explaining a bit preferably with references?

[wanabee777]

Interesting information regarding GPS but way, way above my pay grade.

Do you fellas have any opinion as to whether or not it would be advantageous for aircraft to use GPS generated altitudes as opposed to the long standard use of flying atmospheric pressure patterns (ie barometric based altitudes)? How about low orbit spaceflight?

Thanks for the primer!!

[WeeJeem]

Originally Posted by D driver

what I could think was that the receiver is required to combine all the different orbital elements transmitted and thus orbital position alone would not be of much use without orbit alignment etc. and thus would not be entirely correct to term it as position ?

That's gitting closer to the principles, but there's more to consider also.

Originally Posted by D driver

@WeeJeem is that what you mean? If not mind explaining a bit preferably with references?

The most relevant document is probably this one here, specifically Appendix II, but whether it's a help, well, YMMV...

Soooo, bearing this in mind, i'll give it my best shot to explain in "Brian Cox" terms, but I'd ask others who also know the system to help out here when I flag a bit

Firstly, the SVs are not geostationary. They are tazzing about in various directions at an orbital altitude of about 20,000km, at an orbital speed of about 14,000km/hour.

Secondly, here's the tricky bit that "just is": according to Special Relativity, time is relative to an observer. So, for example, two events that occur simultaneously for one observer may not be simultaneous for another observer, i.e the concept of "now" is relative.

Why is this so important? Well, because it means, for example, that if an SV the broadcasts a message saying "I am at point X,Y,Z and the time is HH:MM:SS", unfortunately the only observers who can use that information are i) the SV itself and ii) any other observer who happens to be in the same inertial reference frame.

If we recall what we said earlier about the SVs " tazzing about in various directions", then clearly any (all!) SVs cannot be sharing the same inertial reference frame with our GPS receiver, so "now" is not the same for the SV and the receiver.

Which makes it kinda tricky to calculate "time of flight" when we can't agree on the start time to use to calculate the time of flight.

What most certainly can be done is to measure time differences between arrivals of multiple signals ("time-difference of arrivals", aka TDOA) and then iteratively solve a simultaneous set of multilateral hyperboloid equations to derive the receiver's position - not triangles, not even spheres, but intersecting hyperboloids

The other little sting in the tail is that seemingly innocuous little word "iteratively" - what needs to be done is to make a first iteration of the equations that provides an initial solution, then use that first solution to refine the calculation to derive a second, more accurate solution, and use that to refine the calculation, and so on, and so on.

Take a look at Figure 20-3 in the linked document, and you'll at least get a feel for a) what a gps receiver is "responsible" for, and b) you'll see a couple of the refining iteration loops on the right hand side (clock correction, tropospheric model, etc).


Hope this helps a bit - time for a cuppa, methinks

[dClbydalpha]

Originally Posted by D driver

and so why not D?

Ephemeris data is not position data. As an analogy, if I were to say that I was an hour south of Birmingham on the M5 with an average speed of 60mph, I have not given you position. What I have given you is the ability to calculate my position if you have knowledge of the route of the M5. You need more than just what's in the message, even if in the satellite's case it is an understanding of standard orbital mechanics.

As for time, well then we are talking abstracts.

[Groundloop]

and so why not D?

Only one answer is correct.

a)Time; data to impair the accuracy of the position fix (Selective Availability SA)
b)Almanac data; satellite status information
c)Data to correct receiver clock error; almanac data
d)Time; position of the satellites

a) Incorrect, no information on Selective Availability is ever broadcast.

b) Correct.

c) Incorrect. There is no way the satellites know what is wrong with the clocks in every receiver in the world.

d) Incorrect. Time is broadcast, position is not as discussed as nauseum above.

You just refuse to accept that Oxford has got it wrong!

[WeeJeem]

Originally Posted by wanabee777

Do you fellas have any opinion as to whether or not it would be advantageous for aircraft to use GPS generated altitudes as opposed to the long standard use of flying atmospheric pressure patterns (ie barometric based altitudes)?

Call me old-fashioned, but I personally would be more than a tad nervous about using a traffic separation system where the inherent error associated with my estimated ("probable") altitude can vary significantly, depending on a) what satellites my GPS receiver happens to be using at the time, and b) what lattitude I happen to be flying at (and ditto for all other aircraft in the vicinity).

How about low orbit spaceflight?

Take a stopwatch, time your orbital period, plug that time into Kepler's 3rd Law. Simples.

[A Squared]

Originally Posted by WeeJeem

Call me old-fashioned, but I personally would be more than a tad nervous about using a traffic separation system where the inherent error associated with my estimated ("probable") altitude can vary significantly, depending on a) what satellites my GPS receiver happens to be using at the time, and b) what lattitude I happen to be flying at (and ditto for all other aircraft in the vicinity).

Without intending to advocate for GPS derived altitudes, b) shouldn't be much of a concern. Altitude errors which are a function of latitude would affect all aircraft at the same latitude. And you have to be at the same latitude to collide.