Determining ground based conjunctions via database query.

Example: Poker Flat, local midnight, noon-midnight orbit

Assume: poker_lng is the longitude of Poker Flats, Alaska. Then GMT at midnight in Poker is at poker_lng/15.0. So we select all times approximately within an hour of midnight by checking that the hour GMT is either int(poker_lng/15.0 - 1 hr) or int(poker_lng/15). Then we check that FAST is in the north by requiring that lat > 0.

To make things simple, we limit our search to times when the spacecraft is at 70 degrees invariant latitude by reqiring that cross70 = 1. Finally we requre that the spacecraft position is within dLat degrees of poker_lng - so of poker_lng is 165 degrees and dLat is 15 degrees, poker midnight would be at 11 GMT and our query would be:

select time, alt, lat, lng, mlt, ilng from ephemeris_events, events_data where data = ident

and (datepart(hh,time) = 10 or datepart(hh,time) = 11)

and cross70 = 1 and lat > 0 and lng > 150 and lng < 180

You could further restrict the query to only get times when the apogee is in the

northern hemisphere by requiring that alt > 2000

Determining ground based conjunctions using IDL Routine.

An IDL routine was created to finds times when FAST has a conjunction with a ground point near MLT=0000. To use this routine take the following steps:

• Start IDL

• At IDL prompt (idl>), type:

  idl>fairb_fast_conj,','1998-01-01/00:00:00','1998-02-01/00:00:00',FILE='JAN98'

This will find conjunctions for the given period and save them to a file called JAN98.

• Check accuracy of times using idl routine plot_fa_crossing. At idl prompt, type:

idl>plot_fa_crossing,/fill,xmark='yyyy-mm-dd/hh:mm:ss'

This will display a graphical view of the earth from space, with a large white X designating the ground position of FAST. This can be compared to the desired conjunction site (the default used in the example above, is Poker Flat/Fairbanks, Alaska).