75766RE: Re: [softrock40] RE: GPS Disciplined Reference for Ensemble
- Nov 11, 2013Good link but it makes my case .....
When ephemeris and almanac data are stored in the GPS receiver, it depends on their actuality how long the GPS needs for the first position determination. If the receiver has not had any contact to the satellites for long time, the first position determination will take longer. If the contact has only been interrupted for a short time (e.g. when driving through a tunnel), the position determination is restarted instantly and we speak of reacquisition.
If position and time are known and the almanac and ephemeris data are up-to-date, we speak of a hot start. This is the case when the receiver is turned on at approximately the same position within 2 – 6 hours after the last position determination. In this case a position fix can be obtained within approximately 15 seconds.
If the almanac data are available and the time of the receiver is correct but the ephemeris data are outdated, this is called a warm start. In this case it takes about 45 seconds to actualize the ephemeris data and obtain a position fix. Ephemeris data are outdated when more than 2 – 6 hours have elapsed since the last data reception from the satellites in view. The more new satellites have come into view since the last position determination, the longer the warm start takes.
If neither ephemeris nor almanac data and the last position are known, we talk of a cold start. Then in the first step all almanac data have to be collected from the satellites, this procedure takes up to 12.5 minutes. This happens when the receiver was switched off for several weeks, was stored without batteries or has travelled approximately 300 km or more since the last position fix.
In the last case no almanac data have to be collected, but as the “wrong” satellites are in view, the receiver has to screen all satellites till it finds the ones in view. For a lot of receivers the duration of a cold start can be shortened when the date and approximate position are entered manually.
---In firstname.lastname@example.org, <eb4apl@...> wrote:No, they are not geosynchronous, even they are not in the equatorial plane, they are inclined about 55º. They orbit the earth every 12 sideral hours (about 11 hours 58 min).
The results are so precise because the orbits are monitored continuously and the corrections are transmitted to the receivers by all satellites in the constellation in the navigation message.
A good and easy introduction to the GPS is here:
73 de Ignacio EB4APL
On 09/11/2013 23:28, see_how_deep_da_rabbithole_goes@... wrote:
If they didn't move with the earth's rotation then they wouldn't be geosync
For GPS the errors in the satellites' orbits are sent to the receivers as an 'almanac', parts of it good for several days (Coarse corrections), parts of it good for a few hours(Fine corrections). That's why if you leave a GPS off for a month it takes a long time for it to hook up because it's waiting to download the main part of the almanac (The several day data or rough correction) that comes around every few minutes. To get the accuracy down (RDOP, HDOP, etc) to <1meter resolution can take as long as an hour until it gets the full almanac with the fine correction data
I do a lot of mapping with GPS along with a depth finder with NMEA Depth serial output to make 3-D depth maps of structure for fishing and I don't even begin doing any mapping until the GPS has been running for at least an hour which maintains repeatability between mapping sessions which may be several days to several months apart
---In email@example.com, <alan.r.hill@...> wrote:
While I agree with atmospheric interference I cannot accept that the satellites move. The signals are so precise that to me it makes no sense for them to be moving. Remember that they are 23000 miles above the earth in geosynchronous orbits.
Alan – W6ARH
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