TRACKING WILDLIFE BY SATELLITE 



NOAA operates a network of satellites for providing 

 global data on the earth's environment on a daily basis. 

 The primary mission of Tiros-N satellites is to obtain data 

 for weather forecasting. Satellites are launched at an ap- 

 proximate rate of one per year to maintain continuous 

 operation. Additional satellites will enable the program to 

 continue into the future. 



The near-polar, sun-synchronous orbit of the Tiros-N 

 series allows images of a particular area to be acquired at 

 approximately the same local solar time each day. To 

 maintain sun-synchronous operation, the orbital plane of 

 the satellite must revolve, or precess, about the earth's 

 polar axis in the same direction and at the same average 

 rate as the earth's annual revolution around the sun. Differ- 

 ences in the altitudes of the two orbits ensure that the same 

 location on earth is not viewed simultaneously by both 

 satellites. Because of the earth's rotation during the ap- 

 proximately 102 min of each orbit, two successive satellite 

 ground tracks are separated by 25 longitude at the equa- 

 tor, the second ground track being to the west of the first. 

 The satellite orbits are inclined approximately 98 to the 

 equatorial plane (8 to the polar axis), so the ground tracks 

 of two successive passes cross each other at 82 latitude, 

 and both poles are "seen" by the satellite during each 

 overpass. Therefore, the number of passes over a given 

 location each day is a function of latitude, ranging from 6 

 per day over a site on the equator to 28 per day at latitudes 

 higher than 82 (Fig. 1). 



The location of a transmitter is estimated from the Dop- 

 pler shift in its carrier frequency. The Doppler effect is the 

 perceived change in frequency resulting from the relative 



movement of the source and receiver. The frequency re- 

 ceived by instruments on the satellite is higher than the 

 transmitted frequency (401.650 MHz) as the satellite ap- 

 proaches the transmitter, but becomes lower as it moves 

 away from the transmitter (Fig. 2). When the received and 

 transmitted frequencies are equal (the inflection point of 

 the Doppler curve), the position of the transmitter is per- 

 pendicular to the satellite ground track. Normal process- 

 ing by Argos requires four transmissions during an over- 

 pass to estimate a location. 



Each Doppler measurement produces two possible po- 

 sitions for the transmitter that are symmetrical with re- 

 spect to the satellite ground track. The more likely of the 

 two positions is determined from previous locations, 

 transmitter velocity, and the earth's rotation. For slow- 

 moving transmitters, the ambiguity can be resolved in 

 95% of the cases (Argos 1978). 



Location accuracy is influenced by several factors in- 

 cluding the stability of a transmitter's oscillator, the 

 elevation of the transmitter, ionospheric propagation er- 

 rors, and errors in satellite orbital data (Le Traon 1987). 

 Errors resulting from differences in actual transmitter ele- 

 vation and assumed transmitter elevation occur primarily 

 in the longitudinal plane. The magnitude of transmitter- 

 elevation error also depends on the maximum elevation of 

 the satellite during the pass (French 1986; Table 1). 



In April 1987, Argos began categorizing locations 

 by location quality (LQ) indices. Indices range from 

 to 3, with 3 being the highest-quality location. Table 

 2 shows the expected standard deviation of a cluster of 

 locations for LQ 1 to 3 as well as the criteria used in 



8 



40 60 

 Latitude 



Fig. 1. Relation between latitude of a study 

 area and the degree of coverage by the 

 two satellites (from Argos 1984). 



