The basic rule for articles in English is that you use “a” for words that begin with consonants and “an” for words that begin with vowels.
However, if you are talking about abbreviations, especially in science papers, this basic rule doesn’t work. In fact, the choice between “a” and “and” is based on the pronunciation of the word, not the spelling. For instance, “UFO” (which stands for unidentified flying object) begins with the vowel “U” but is pronounced “you-eff-oh.” Hence, the pronunciation starts with the consonant sound “y” and “a UFO” is correct.
an AMD chip because it is “an ay-em-dee chip”
a GPS device because it is “a gee-pee-ess device”
an SRT because it is “an ess-are-tee”
There are exceptions! This happens when abbreviations are easy to pronounce as words. For instance, “a MATLAB program” or “a SIFT feature” are correct because these abbreviations are pronounced as if they were words.
How do you know if this is the case? For instance, is it “a HOG feature” or “an HOG feature”? There is no easy answer unless you spend time with researchers in this speciality and hear them talk. The other option is to google both phrases (surrounded by quotes) and look to see which is more commonly used.
Another notable exception to the basic rule occurs in fields such as physics and semiconductors, where chemical names are used. For instance, Si, which stands for silicon, is actually read out as “silicon,” not “ess-i,” so it is “a Si-wafer chip,” not “an Si-wafer chip.”