Here are some answers to the most frequently asked questions.
My question is not answered here. Where can I post questions?
How can I see my car's past positions (track history)?
You can also enter the tracking mode by clicking on 开始跟踪 in the info balloon after clicking on an icon on the map.
The date browsing menu only shows the days when aprs.fi has received a new position for the station.
Where is the displayed data obtained from?
The positions of the amateur radio operators and their vehicles are received from the APRS-IS network on the Internet. Most of them are originally transmitted on amateur radio frequencies (typically around 145 MHz), and received and forwarded to the APRS-IS by igate stations run by clubs and individuals around the world.
The positions of ships are received from the AIS frequencies by receiving stations around the world. If you wish to receive AIS transmissions and provide them to the service, you're more than welcome to do so.
How accurate is the position or speed data on aprs.fi?
Data accuracy varies greatly. Usually the data is fairly accurate, but quite often it is not, and there are no guarantees that it would be valid at all. It's trivial, on the AIS and amateur radio APRS networks, to impersonate someone or send invalid data, it can happen intentionally, and it often happens accidentally. A lot of data is corrupted before it reaches the service - there is no end-to-end error detection or real authentication in the APRS network.
For technical reasons (digipeater, igate, APRS-IS or aprs.fi issues) position packets sometimes take minutes to reach the service. Very frequently some GPS receivers give out incorrect position fixes, and cars jump tens of meters or hundreds of kilometers away from their real position. These jumps cause very strange speed readings. GPS receivers are also pretty inaccurate when reporting altitude.
Specifically, the data is so inaccurate that it is in no way useful for giving speed tickets. The service can be used for entertainment and hobby purposes, it is not good for professional, governmental or mission-critical use. Although the aprs.fi service is relatively stable and usually accurate, the global APRS network is run by individual users on their spare time, and it's availability, accuracy or performance is not guaranteed.
Could you please delete my data from the service?
The data will be deleted automatically after 2 years. Some data will be deleted earlier.
Email communications can be easily forged, and I have no way of making sure you're really asking me to delete your data. Also, I do not have much time to manually respond to requests by individual users - there are tens of thousands of users who each have different kind of unique needs. Instead, I prefer to use my time to develop software features which benefit hundreds or thousands of users.
I won't be able to implement a software feature which would allow you to delete your APRS data, since that would allow anyone to delete data transmitted by someone else. There is no easy way to make sure you really are who you claim you are.
Amateur radio transmissions are defined to be in the public domain (by FCC rules in the US, and by respective legislation in most other countries, and I suppose, by international regulations). Anyone can receive them, and retransmit, publish or store them as they wish. If you do not wish your position to be published on the Internet, the only guaranteed way is not to transmit it.
How can I prevent my data from being displayed on this site?
If you wish to prevent your APRS data from entering the Internet, you can append either NOGATE or RFONLY in your digipeater path.
If your current digipeater path is WIDE1-1,WIDE2-1 you need to configure WIDE1-1,WIDE2-1,NOGATE as the path. If your current path is WIDE2-2 you can use WIDE2-2,RFONLY.
Please note that there is no guarantee that everyone would obey the NOGATE or RFONLY tags. If you really, really wish to protect your privacy and avoid publishing your position on the Internet by someone, do not transmit it at all.
Could you please implement feature X and fix bug Y?
Probably. Please submit your ideas on the aprs.fi discussion group, or by email (the address can be found in the blog's profile page). I'm very happy to receive suggestions and I've implemented many of them. A lot of them are in the "things to do" queue - things which I think I should do, but haven't found the time to do yet. I always write the good ones down.
I try to first implement features which benefit as many users as possible. Sometimes I implement features which I find the most interesting for me personally. The selection is not entirely scientific or democratic.
Can I write software to download data from aprs.fi and display it on my site?
If the data is available through the API and the terms are OK for your application, then the answer is yes. Otherwise, no. If the data is not available through the API, please get in touch – maybe it can be added easily.
Downloading data for application use using the user interface (for example, fetching the /info/ page just to get the current coordinates of a station) wastes both human and computer resources. First, you need to write a parser to extract the data from the HTML (and then fix your parser every time I change my HTML layout). Second, aprs.fi needs to do the user interface magic (language and timezone selection, user-friendly template formatting, session set-up) for every request, all of which is unnecessary. Third, the user interface often generates much more information than you actually need. The position API responds in a few milliseconds since the current positions are generally available in memory, while the corresponding /info/ page can take tens or hundreds of milliseconds to generate because it goes and finds the nearby stations, and the digipeaters and igates used, which often requires disk access. All of this extra overhead consumes CPU time, which in turn heats up the computer room, consumes electricity, destroys tropical rain forests, accelerates global warming, and kills kittens. And baby seals.
Photo by Heikki Siltala, Turkish van kittens from Cesmes cattery, 2006. License of photo: Creative Commons Attribution 3.0 Unported.