NDB lament.

[IO540]

Am I correct in understanding that the GPS satellites can be moved around to suit the requirements of the people who own them i.e. the US military?

No..........................

GPS

[BackPacker]

Am I correct in understanding that the GPS satellites can be moved around to suit the requirements of the people who own them i.e. the US military? So if they have operations somewhere they may move the satellites to improve coverage there and potentially leave other areas uncovered?

The GPS satellites are not in geostationary orbit so they have to move about (as seen from the earth) otherwise they'd just fall from the sky. So the military cannot simply move a few satellites from one location to the other, and then keep them there. The only thing they can do is alter their orbits.

There are six intersecting orbits defined, each containing four satellites spread evenly throughout this orbit. The net result of this is that between 6 and 12 satellites are visible from any point on earth simultaneously, assuming that you have a full view of the sky in all directions. But you only need to see four satellites for a reasonably accurate position fix (measured in meters). In that sense, there is no need for the military to move satellites about at random to improve coverage somewhere.

On the other hand, due to the earths uneven field of gravity and all sorts of other cosmic influences, satellites do deviate from their designed orbits. Plus, there are some satellites stacked as 'spares' in higher or lower orbits, ready to take the place of any failed satellite. And if a satellite fails, it has to be moved to a graveyard orbit. For this purpose all satellites will have little booster rockets on board so that the orbit can be adjusted.

GPS satellites regularly broadcast their designed orbit (the "Almanac") and even their current deviation from their defined orbit (that's part of WAAS I think). Your GPS receiver uses this orbit information plus the precise timing signals to determine your position.

So as far as the satellites are concerned, GPS coverage across the globe, including the poles, is fairly consistent and there's no need to adjust the orbits to improve accuracy somewhere.

But there may be atmospheric or terrain issues that cause a degradation in signal. Plus, it's possible for the US military to disable or scramble the GPS signal when the satellites are over a certain area (active warzones for instance) although this will probably limit the Americans ability to wage war too. And it's possible to install local transmitters on the ground which broadcast on the same frequencies as the satellites, leading to local jamming of the signal.

[IRRenewal]

Personally, I wouln't want to fly over the North Atlantic without a non-GPS backup to GPS - e.g. Loran (for lack of an affordable INS).

LORAN was shut down in the US late last year.

[Ultranomad]

IRRenewal, not last year, but a month and half ago, and only in the US; Canada will decommission its network in October. European countries reconfirmed their intent to proceed with further development of eLoran (enhanced Loran with built-in low-speed datalink to transmit corrections). Russia does also maintain its Loran-compatible Chaika network. Furthermore, International Loran Association claims that elements of Loran-C infrastructure with a potential for use in eLoran will be retained in the US as well.

[A and C]

Adam

To go back to the origanal point of the thread, I have to agree with IO540 in the fact that not enough money has been spent on maintaning the aircraft you are flying for years.

The problem is that a lot of aircrafr have the King KR85 ADF fitted, in it's day it was a good unit but with out product support from King it is getting very hard to maintain these units and with the end of ADF not far away the £4000+ bill for fitting a reliable King KR87 ADF it is simply not worth doing.

I have found myself in this position as I have an aircraft with a KR85, A pole of my customers indicated that they would rather have an IFR GPS fitted at much the same cost.
The other aircraft that I rent out has a KR87 that works just fine by ADF standards.

[India Four Two]

GPS satellites regularly broadcast their designed orbit (the "Almanac") and even their current deviation from their defined orbit (that's part of WAAS I think).

BP, that's not quite right. Ground stations track all the GPS satellites and the information is used to predict the orbits (ephemerides) of the satellites. This information is transferred to the satellites and broadcast to GPS receivers.

More information here: http://is.gd/b7l7k

GPS Division personnel process daily satellite data from NGA, USAF, and International GPS Service GPS monitor stations. This data is used to produce daily precise (cm level to WGS 84) ephemerides (orbit), predicted orbits, and clock solutions for all GPS satellites.

WAAS (Wide Area Augmentation System) was developed by the FAA to provide corrections to GPS signals to improve the accuracy of the signal to allow Cat 1 precision approaches. Ground reference stations are used to calculate correction factors which are broadcast from a geo-stationary satellite. WAAS-enabled GPS receivers use this information to refine the calculated location to an accuracy of typically 1 m horizontally and 1.5 m vertically.

[englishal]

A bit more about WAAS and Differential GPS....

The ground reference station is located at a very accurately known location (surveyed), say on an airport for sake of argument. This reference station also receives the same GPS signal as is received in the cockpit by the GPS. The ground reference station then compares the position derived from the raw GPS with its known location and calculates any error, then re-broadcasts this error (via RF, satellite, Spot beam etc....) so that the GPS in the cockpit can "correct" its position by applying the same error correction. This can actually achieve millimetric accuracy if the GRS is relatively close to the GPS it is correcting.

You probably won't have a reference station right in your vicinity so when using differential GPS then a number of GRSs are normally used which may be located hundreds of miles apart and the results of all of them are averaged to give an overall correction to apply to the GPS. This was particularly useful in the old days when the US military used top apply selective availability to the GPS signal to deliberately introduce errors to keep accuracy above say 100m. Using reference stations one could pretty much "undo" a these errors to give 5m or so accuracy. Back in 2000 the errors were turned off and Clinton signed a memorandum stating that the US would not turn off or scramble GPS signals again.

Rather than invest in their own system, I think Europe should have offered to invest in GPS and possibly shared some of the control over it, and maybe even have a second control centre in somewhere like Geneva, which would ensure its integrity, rather than re-inventing the wheel which is what seems to be the Euro way....GPS is a standard, and is always going to be better than any new system, so why not just help improve it rather than waste billions on a new project.

[IO540]

The European version of WAAS is called EGNOS.

The EGNOS signal has been receivable by handheld GPSs for a few years but IFR (panel mounted) units have bizzarely been prohibited from using it.

However, this has just appeared on some other forums

Dear EGNOS user,

The certification of the ESSP SAS as an Air Navigation Service Provider is progressing according to plan and is expected to be completed by the second half of this year. Once the ESSP SAS is certified as an ANSP, the message type 0 will be removed from the EGNOS Signal in Space. After an initial observation period (around 1 month), the EGNOS Safety of Life service will be declared ready for use. After this milestone, the corresponding ANSPs will begin the publication of GNSS RNAV procedures based on EGNOS, many of them currently under development. Therefore it is expected that several GNSS RNAV procedures will be available for use in Europe before the end of 2010 and many others will follow during early 2011.

Best regards,

The EGNOS Helpdesk