Satgen 147 RS12 Advanced User by GM4IHJ 19 Jan 92 Have had several enquirers asking the following questions :- 1. How can you accurately predict RS12 location ? 2. How can you tell that the the signal path from RS12 is coming to you by a NON GREAT CIRCLE route ? Both answers rely on accurate measurement of RS12 doppler shift. Take RS12 on 16 Jan 92 . Computed Keplerian elements predicted Acquisition time, Time of Closest Approach to my station, and, Loss of satellite time. Of these 3 predictions the first Acquisition , and the last, Loss time are useless because Atmospheric refraction of the HF signal can give errors of up to two minutes. By contrast Time of Closest Approach is often clearly evident provided propagation conditions near you, are undisturbed. To measure Closest Approach CPA, simply plot the times at which the RS signal dopplers down a succession of 100Hz steps. Acquire the signal , note the time and tune for zero beat ( when signal audio goes so low you lose it ). Then quickly retune 100Hz lower and time passage of zero beat at this lower frequency. Repeat 100Hz zero beat checks to produce table :- Frequency Time ut Secs/100Hz Frequency Time Ut Secs/100Hz 29.40938 1221:30 - 29.40878 1228:40 36 29.40928 1225:00 210 29.40868 1229:17 37 29.40918 1226:05 65 29.40858 1229:57 40 29.40908 1226:53 48 29.40848 1230:52 55 29.40898 1227:30 37 29.40838 1232:06 74 29.40888 1228:04 34 Signal faded 1234:40 NOTE Doppler rate figure refers to time half way between each reading Eg 34 sec/100Hz was the doppler rate between 1227:30 and 1228:04. So this reading refers to 1227:47 ut. When plotting Doprate use this 1/2 way time. From curve plotted CPA was very close to the computer prediction of 1228 ut. So no change was necessary in this case. CPA frequency for future reference was 29.40895 MHz . So for future CPA checks with this exact receiving system , it is only necessary to time signal doppler passage through this frequency. It is not necessary to obtain a new doppler curve provided CPA is checked on similar near overhead passes. Given accurate RS location data , you can check for Non Great Circle prop. Slant range change per minute allows accurate doppler computation once your computer model checks OK at CPA. So you can predict Doppler shift at every point in the pass. When Doppler heard is at variance with doppler predicted,you probably have a Non Great Circle propagation situation. NGC situations at HF occur frequently particularly when RS is Sub Horizon. Typical situations met in UK are :- 1. Orbit coming S to N up the Atlantic west of UK. RS signal very low doppler Eg 29.4084 instead of predicted 29.4092. You are hearing a signal leaving the rear of RS travelling south to then backscatter off an Ionospheric anomaly and then come north to UK. 2. Orbit coming from Canada , heading N to S down the Atlantic west of UK. Doppler anomalously high say 29.4095 MHz . RS signal coming to UK via a scatterer south of the satellite and south west of UK. So you hear the high doppler signal from the front of RS. Both the above situations are typified by signals which do not exhibit gradual doppler shift, but show fixed Hi or Low. 73 de GM4IHJ@GB7SAN