Philips AP Navigator Mk9

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
floehopper
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Philips AP Navigator Mk9

Postby floehopper » Sun Dec 16, 2018 1:35 pm

I have a Philips AP Navigator Mk9 purchased in the late 1980s or early 1990s. It still works to some extent, but, although the time is accurate, the date is out by a number of months. Also it never seems to receive more than the bare minimum number of satellites, that is, its [warning]-light system always shows amber or at best amber-green, never just green, which presumably means the position is not always very accurate.

Since it was manufactured prior to the end of the first GPS epoch (21 August 1999), I assume the date problem is occurring because of the GPS week rollover concern [1]. Is it likely that the incorrect date is having any negative effects on picking up satellites? That is, will it be predicting the position of satellites incorrectly and therefore looking for the wrong satellites?

Also is there any way I can update the software to fix the GPS week rollover problem?

[1]: https://www.spirent.com/blogs/positioni ... lover-week

jimh
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Re: Philips AP Navigator Mk9

Postby jimh » Mon Dec 17, 2018 3:35 pm

To speculate on exactly how the GPS receiver in the particular device was designed is going to be difficult.

Typically the state in which a GPS receiver is powered on and has no idea of the date, time, or its own location is called a COLD START. This means the GPS receiver begins to look for satellites without any hints about which satellites ought to be in view. In contrast, a GPS receiver that was turned to ON after only having been OFF for a short time still has some stored data, and it will know:

--the date
--the time
--the position of the receiver
--and a satellite orbit ephemeris for each satellite

In a COLD START situation, it can take a long time for the GPS receiver to find the first satellite and begin to copy the navigation message from the satellite. The GPS satellites send information about their orbit and the orbits of other satellites, but the data rate is very low. It takes about 15-minutes of continuous copy without errors for the receiver to get the entire navigation message, containing the ephemeris for the current state of all satellite orbits. If there is an error, the receiver has to wait 15-minutes more for data to come around again. By that time, the satellite may be out of view.

Older receivers typically had a limited number of correlators that could be used to search for a GPS signal, so their ability to find satellites without knowing which satellites to look for and what their orbital relationship was to the location of the receiver--which is important to know to account for Doppler shift in the frequency of the received signal--is limited. As a result, it could be several hours before a COLD START receiver obtains its first fix. Modern GNSS receivers have perhaps 70 or more correlators, and can throw all of them at the task of finding the first satellite, which will reduce the time to first position fix to a few minutes.

ASIDE: it was typical in older receivers when beginning at a COLD START condition that the user would be asked to input the date, the time, and the approximate location of the receiver, so that the receiver would be able to make better guesses about what satellites might be in view.

Considering that in 2018 the device you are asking about is about 30-years-old, I don't think it is reasonable to expect that there would be a way to update it.

Modern GNSS receiver that can receive GPS, GLONASS, GALILEO, and BEIDOU are now being introduced and at very modest cost. It may be time to upgrade.

Jefecinco
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Re: Philips AP Navigator Mk9

Postby Jefecinco » Tue Dec 18, 2018 8:14 am

Older GPS units were usually sold with an external antenna and cabling to connect [the external antenna] to the GPS [receiver]. Newer units usually do not need an external antenna.

If you have not connected the antenna to the GPS [receiver], [the GPS receiver] could need quite a lot of time to find any satellite. Older GPS antennas sometimes need a clear view of the sky.

If you have an antenna consider some experimentation to find a location that improves performance. If you lack the antenna try to find one IF your GPS [receiver] has a cable connection for an external antenna.
Butch

jimh
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Re: Philips AP Navigator Mk9

Postby jimh » Tue Dec 18, 2018 7:02 pm

Jefecinco wrote:Older GPS antennas sometimes need a clear view of the sky.


All GPS receiving antennas need a clear view of the sky, that is, a clear view of the sky for radio signals. Radio signals will pass through non-conductive materials like plastic that block optical signals. But no matter the age of a GPS receiver, giving its antenna an open and unobstructed view of the sky will permit the GPS receiver to work as best as it can. Any conductive surface that obscures a portion of the sky from a GPS antenna will prevent that receiver from getting signals from satellites in that direction.