3 satellite GDOP Calculation

Please suggest formula for calculating DOP for three transmitters or 3 known satellites positions. the receiver and transmitters are assumed to be synchronized / no clock error? Thanks for your valuable suggestions and answers in advance

 

Thanks! Nuvi, This I already checked online. Usually I found lot of papers and examples using 4 GPS satellites with 3 variable as 3 coordinates (x,y,z resp..) and 4th is the time variable. I am looking for any paper where we have only 3 variables (x,y,z) and three transmitters.
I am still searching for the answer? don't know where to search.....

 

Google "Mason Geolocation Error Analysis" for a paper on the basics on computing the output error covariance matrix. Then just assume that altitude error is some fixed proportion of the TOA error (eg sigma_alt = sigma_toa) so you can factor out the input error variance as you do when deriving DOP for the pure TOA case.