There seems to be much confusion around using the scan features on FTA receivers.
There are three main scanning methods:
1. satellite scan (f1 on a Viewsat) 2. blind scan (f2 on a Viewsat) 3. transponder scan
Background ----------
An actual DN or B3v subsciber receiver has a built-in satellite list that tells the receiver how to find network channels on each satellite. This list is a mapping of channels to their respective transponder/satellite. In the FTA world, this list resides in the receiver's bin itself (or sometimes on a seperate file). So, when you do a scan, on say your DN receiver, the receiver will scan a predetermined list of transponders on the sats to get the DN package for which the subscriber is paying.
FTA receivers (I'm talking about TRUE FTA receivers - not modified) introduced the blind scan feature. Because true FTA is not a network package that one subscribes to, channel lists aren't sent down to the receivers - the receiver has no idea of what channels are where. This is a problem that blind scan solves.
A blind scan works in a way that ALL frequencies are scanned (i.e. all transponders and their broadcast frequencies) on a particular satellite. In true FTA this populates your receiver's channel list with channels. This method catches EVERYTHING including channels that network subscribers might not get (a good example is the 'Pirate channel').
A transponder scan is like a blind scan, but it will scan only the frequencies on one particular transponder. This type of scan is used because it is much quicker than a blind scan, however, you have to have knowledge of which specific transponder you need to scan (in other words you not only need to know what you are looking for, but also where it is - you can consult lyngsat for that info).
Modified FTA scanning ---------------------
As most viewers don't use their FTA receiver for FTA, but instead modify the receiver's bin file to catch 'other' satellites (e.g. DN and B3v packages), blind scan should not be used in the same way as one would use it with true FTA.
When you first hook up your modified FTA recevier, you configure your antenna set up, put in your keys (or turn autoroll on), and then do an initial scan to get all of your channels. This scan is usually a satellite scan.
After the initial scan, as time passes, the satellite networks will move channels to different transponders, or even to different satellites. They do this for a variety of reasons: thwart piracy, transponder maintenance, etc... But your receiver will re-adjust its channel-to-transponder/satellite map through a feaure known as autoTP ('autoTransPonder'), provided this feature is turned on and is functioning correctly. (Note: you will also need autoTP turned on because at the time of coding of your bin file, channels will have been moved and your bin's channel map will be outdated).
So, if for the most times, you should use the satellite scan method, then when should you use blind (or transponder) scan?
The answer is: it depends... I, personaly, use it for catching all channels on a satellite (dss testing), and, for troubleshooting (with ECMs occuring every now and then, satellite networks playing around with moving channels, possible bugs within the bin for autoTP). Otherwise, you should stick with the satellite scan with autoTP turned on. Plus, blind scan takes more time than does a sat scan.
If you do decide to go with a blind scan, here are some things you should be aware of:
1. You may get duplicate channels. Why does this happen? Remember that blind scan will scan ALL frequencies for a given satellite. With, for example DN, two of its satellite are close to one other (we know this because we can make a poor man's DN dish by tie-wrapping two lnbs directly together) and even though the scan is being performed on one lnb it will pick up the frequencies on both satellites. The scan is truely 'blind' - it just looks at frequencies and it doesn't care where they come from. If you want to avoid duplicate channels: when you are scanning through one lnb then turn off the power to the other lnb through the antenna setup menu while doing the scan.
2. Make sure you scan on all polarities (vertical and horizonal). On some receivers, 'all polarities' is not the default. If you scan on only, say, vertical then you will be missing channels.
3. Blind scan takes longer than does satellite scan. This is because all frequencies are being scanned, as opposed to just the frequencies that are a part of the satellite package.
Summary -------
For the most part, on a modified FTA receiver, you will use satellite scan to populate your channel list. As time goes on, channel moves will be handled (transparently to the viewer) by the autoTP feature. Every so often, something will go wrong, and you're missing channels because they were moved as autoTP did not pick up on it. Then, you have to redo your scans (tip: you might want to delete your channel list first, or rist getting duplicates).
Remember: modified FTA bins are emulations (immitiations) of an actual DN/B3v receiver, and, the immitation is not perfect - so things can go wrong!
If you want to know what satellite, transponder, or frequencies a channel is mapped to, there is a website, lyngsat*com (replace * with a dot), that continuously tracks channels. Bookmark this site - it will be of a great help when the time comes to troubleshoot.