Add old positions to APRS.fi

[Stefan Biereigel]

Hi!
Is there currently an out-of-the-box functionality to correctly insert "old" positions into the APRS.fi ground-path of a station? I'm thinking about scenarios with long durations in areas without APRS coverage (for example balloons over oceans, deserts, ...). These payloads could transmit "old" positions from time to time to have them inserted along the APRS.fi path when having APRS coverage again. Is the APRS specified timestamp of any use here (i.e. if I transmit with an old timestamp of, say, 12 hours ago) - will the position be "sorted in" the right way?

Or is there no way of doing it currently?

73s
Stefan

[Heikki Hannikainen]

Hi,

On Fri, 17 Jul 2015, Stefan Biereigel wrote:

> Is there currently an out-of-the-box functionality to correctly insert
> "old" positions into the APRS.fi ground-path of a station?

Not really. The balloon folks in the UK have implemented a custom solution
of their own, where past positions are encoded as additional data in the
comment section of the packet, I think.

aprs.fi does not support that, though, it's just available at
http://tracker.habhub.org/ which is a very nice site targeted for HAB use
specifically.

> along the APRS.fi path when having APRS coverage again. Is the APRS
> specified timestamp of any use here (i.e. if I transmit with an old
> timestamp of, say, 12 hours ago) - will the position be "sorted in" the
> right way?

No. Last time I checked the timestamps sent by stations had a surprisingly
bad tendency to be incorrect, so they could not be used for sorting by
aprs.fi. A lot of stations transmit local timestamps as UTC timestamps (so
they're off by their local timexone offset), or the other way around, or
they were simply off due to no GPS or NTP synchronisation being used. And
some other oddities.

Anyways, aprs.fi only uses the timestamps as a substitute for a packet
sequence number, so that old / out-of-order packets from the same station
can be discarded when the timestamp suddenly jumps back.

- Hessu

[Stefan Biereigel]

Hi,
thank you very much for the detailed information! I was speaking to the nice folks from tracker.habhub.org and implemented as you told me. This scheme of sending old /-reports seemed to work beautiful, the old packets were just ignored by APRS.fi as expected. This was on my table, but here comes reality:

Yesterday I launched a balloon doing just that: transmitting the last 24 hours of positions interleaved with the current position. Problem is: APRS.fi now drops the out-of-order timestamps correctly, but also ignores the following packet, because the location changes too fast. .. Which means, that I currently only sporadically can see new positions on aprs.fi (namely in the period where no backlog is transmitted .. which will only be there before the first 24 hours are over...

It seems, APRS.fi updates the last seen position even when a packet is ignored? Am I correct with this assumption and what would be the solution to still be compatible to APRS.fi? With my own APRS-IS-application I can sort my packets by timestamp and everything looks good there..

Thanks,
Stefan!

[Heikki Hannikainen]

Hi,

As I tried to describe in my previous message, aprs.fi does not use the
timestamps for sorting, since they're not correct for a lot of the
stations transmitting timestamps.

So, no, you can not add old positions to aprs.fi, aprs.fi treats APRS_IS
as a real-time feed of current events. Sorry.

On Sat, 1 Aug 2015, Stefan Biereigel wrote:

> thank you very much for the detailed information! I was speaking to the nice folks from tracker.habhub.org and implemented as you told me. This scheme of sending old
> /-reports seemed to work beautiful, the old packets were just ignored by APRS.fi as expected. This was on my table, but here comes reality:
>
> Yesterday I launched a balloon doing just that: transmitting the last 24 hours of positions interleaved with the current position. Problem is: APRS.fi now drops the
> out-of-order timestamps correctly, but also ignores the following packet, because the location changes too fast. .. Which means, that I currently only sporadically can
> see new positions on aprs.fi (namely in the period where no backlog is transmitted .. which will only be there before the first 24 hours are over...
>
> It seems, APRS.fi updates the last seen position even when a packet is ignored? Am I correct with this assumption and what would be the solution to still be compatible
> to APRS.fi? With my own APRS-IS-application I can sort my packets by timestamp and everything looks good there..

It only looks good because it's your station, and your timestamps. aprs.fi
is not able to trust the timestamps because it needs to have timestamps
which are in linear order for the real-time display to update correctly,
and a lot of stations transmit timestamps which are not in any way aligned
with reality, since their clocks are not synchronized.

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- Hessu