An advanced CFAR technique for SSR reply detection G. Galati*, S. Gelli**, F. Fiori*, E. G. Piracci* * “Tor Vergata” University of Rome Via del Politecnico, 1 - 00133 Roma, ITALY email:
[email protected],
[email protected] ** SELEX Sistemi Integrati SpA Via Tiburtina Km.12.400 - 00131 Roma, ITALY email:
[email protected] Abstract: In a network of SSR (Secondary Surveillance Radar) stations FRUIT (False Replies Unsynchronized in Time) and garbling limit the detection/decoding performance. The use of matched filter allows us to improve the conventional and the Mode S replies detection. In order to reduce the false alarms it is useful to insert a CFAR circuit to set the detection threshold. The CFAR technique is often used in Primary radar signal processing; we propose a novel approach to introduce CFAR in SSR signal processing.
1. Introduction This paper deals with the problem of detecting Conventional (i.e. Mode A/C) and Mode S SSR Replies in a Fruit environment with a high interference rate. The growing deployment and installation of Mode S Stations in the last years has brought to a constant reduction of the so-called FRUIT (False Replies Unsynchronized in Time) phenomenon but has created an increasing interference environment characterized by a high number of Mode S All Call and Roll Call replies [1]. Moreover, the detection process of SSR conventional replies is affected by the fact that Mode S-like Communication channel is exploited by a large number of systems, such as, for example, TCAS, ADS-B, Multilateration, and, in the near future, TIS-B and FIS-B. The Mode S reply detection process generally inhibits the conventional reply processing when a Mode S Preamble is detected. This is because it is possible to recognize the F1-F2 pattern (the “brackets”) of the conventional replies in the typical bit sequence of the Mode S replies PPM modulated data block, thereby leading to the wrong detection of several SSR replies. Experimental observations in operational sites show that this approach is valid only for high Signal to Noise Ratios, i.e. when the Mode S preamble can be clearly detected. Since the typical Mode S-Fruit power is between -40 dBm and -70 dBm, it often occurs that the preamble is missed and that the data block of a Mode S Fruit is processed by the Conventional Reply processor. In addition to this, a general design requirement for modern Mode S Radars is to exploit a few (4 or 5) parallel conventional SSR processors, so that, a Mode S data block can saturate the radar processing capabilities and affect the general detection performance on conventional targets. This is true, in particular, in high density areas like airport aprons or for targets very close to each other, that is typical for military aircrafts flying in a group (mass raid). In this paper we present a matched filter approach [2] to SSR reply processing in conjunction with an evaluation of how many SSR replies are typically detected inside a Mode S data block. The matched filter processes the input samples calculating the correlation with the reference “SSR F1 F2 bracket” signal of the conventional reply. In this environment the “False alarm probability” has to be meant as the “Probability of detecting more than N-1 SSR replies inside a Mode S data block”, where N is the number of available SSR reply processors (tipically, 4 or 5). The remaining reply processor is the one in charge for processing and decoding of valid SSR reply, [3].
It will be shown that the typical “Matched Filter” approach to F1-F2 pattern brings to the detection of a large number of SSR replies (from 25 to 45) with a strong dependance on the length of the Mode S data block. In order to reduce the effect of this phenomenon on the conventional target detection, a CA (Cell Average) CFAR-like (Constant False Alarm Rate) technique is proposed [4]. Under the hypothesis that a SSR reply is overlapped to a Mode S Fruit and that the Mode S preamble has been lost, the SSR detection threshold is raised adaptively to maintain a Constant False Alarm Rate. The typical scheme of the CA CFAR Processing used in Primary Radar is maintained and statistical analysis of the input and output distributions is carried out. The other aspect considered is the Mode S preamble detection. We propose a novel method based on the joint use of the preamble matched filter and of the CA-CFAR threshold. This technique permits a detection probability improvement. Finally, experimental results will be shown by using the RASS© System [5], a dedicated tool for radar performance analysis and Radio Frequency scenario simulation. The proposed techniques will be applied to samples of Mode S / SSR Mixed environments with Mode S Fruit underlying scenarios as generated by the RASS tool and received by a Monopulse-SSR (Selex-SI SIR-S Radar) receiver.
2. SSR matched filters and CFAR The matched filter can be implemented as a correlator, in which the input signal is multiplied by a replica of the signal expected at a particular time td. In SSR systems (conventional and Mode S) it is possible to introduce a matched filter for replies detection, as the brackets and Mode S preamble shapes are known, see figure 1.
Fig. 1a: Mode S preamble matched filter impulse response
Fig. 1b: Brackets (conventional Mode) matched filter
CFAR techniques are used in primary radar, while in SSR the noise can be considered a nonlimiting factor, being generally several dB below the received signal. In SSR it is possible to consider as “clutter” the interferences by non-detected replies (FRUIT and garbling), and the signals present in the 1090 MHz channel (ADS, TCAS, TIS, DME). In conventional (Mode A/C) processing, to satisfy the surveillance requirements it is suggested to use more than one parallel processor. In the previous section it has been explained that an undetected Mode S reply detection missing may produce more than 30 conventional false alarms, saturating all the conventional processors. If a conventional reply is present along a Mode S FRUIT it will be lost, this is the first problem mitigated by the introduction of CFAR technique. In Mode S processing the introduction of preamble matched filter + CFAR improves the replies probability detection also, in a FRUIT environment, as shown in par. 3.
envelope
Preamble matched filter
Threshold computation
Comparison with the threshold
Differentiator + Zero-crossing
Fig. 2 a): Mode S preamble matched filter scheme
Time references
Fig. 2 b): CFAR thresholding block diagram
The CFAR system, suited to our application, performs an estimation of the noise + interference level by averaging signal samples in range windows at the matched filter output. There are two versions, for the conventional and for the Mode S processing chains. Conventional SSR CFAR The bracket matched filter (fig. 1b) output is recorded in a n range cells buffer, (sliding window type). For a first implementation we chose 18 cells, 9 leading and 9 lagging, over the guard cells and the cell under test. As the conventional pulse width is 0.45 µs, the correlation peak from two pulses is about 1 µs wide, that is the chosen cell duration chosen. Then, the window length is 21 µs. As shown in figure 2b, averaging the leading and lagging cells samples, we obtain U and V, then we obtain Z averaging U and V (other logics are possible, e.g. taking Max(U,V)). Then Z is multiplied by an empirical constant T (1.2), and a constant detection threshold, (equal to 6 @ 16 MHz sampling) is added. The constant threshold is necessary to guarantee the detection in interferences-less cases (in which Z should be a value, on the average, small). Mode S SSR CFAR The preamble matched filter output is the input to the CFAR block. For Mode S SSR the cells number is 10, as the Mode S preamble is 8 µs long. By the same considerations about the correlation peak length, we chose a 1 µs cell length. The block diagram is the same as in conventional (Mode A/C) processing but there isn’t a constant detection threshold addition. An enhanced scheme derives Z as the max of U and V. In par. 3 it is shown better performance with this enhanced scheme than with the starting, averaging scheme.
3. Experimental results Figure 3 shows a conventional and a Mode S replies garbled, they are recorded signal by the RASS tool. In the lower graph it is shown the matched filter output and the CFAR threshold. Only three threshold crossings are present, and only the central one corresponds to a conventional reply. Therefore using 4 parallel conventional processors it is possible to detect conventional replies in Mode S FRUIT situations like this.
Fig. 3: a) Mode S and conventional replies garbled. b) bracket matched filter output and CFAR threshold
The bracket matched filter + CFAR was evaluated using a RASS generated scenario with conventional replies in a Mode S FRUIT environment. The FRUIT rate is 10000 s-1, that is 70% conventional, 21% short Mode S and 9% long Mode S. The target signal power is equal to -50 dBm and the FRUIT power is distributed in -70 : -40 dBm. Figure 4 shows the results, by comparing matched filter + CFAR to conventional processing. Using matched filter + CFAR it is possible to obtain the same Pd in a +15 dB (or more) stronger FRUIT environment. 0.96 Matched Filter + CFAR Conventional Processing
0.94 0.92 0.9
P
d
0.88 0.86 0.84 0.82 0.8 0.78 -70
-65
-60
-55 -50 FRUIT power [dBm]
-45
-40
Fig. 4: Conventional replies detection probability in a FRUIT environment. Comparison between matched filter + CFAR (-----) and conventional processing (——)
Using a preamble matched filter + CFAR in Mode S processing it is possible to obtain a Mode S replies probability detection improvement as shown in figure 5. We compare today’s standard detection techniques with the novel matched filter and matched filter + CFAR techniques, varying the SNR of a single Mode S reply. At a 0.9 detection probability there is a 5 dB gain. In the SSR usual operational range (SNR > 10 dB), there is a 100% constant detection probability.
1 0.9 0.8 0.7
P
d
0.6 0.5 0.4 0.3 Conventional preamble detection Matched filter + CFAR Matched filter + CFAR enhanced
0.2 0.1 0 0
5
10
15 SNR [dB]
20
25
30
Fig. 5: Preamble detection techniques comparison, varying the signal to noise ratio (SNR)
The preamble matched filter + CFAR was also evaluated in a FRUIT environment. The FRUIT rate is at 10000 s-1, which 70% from conventional replies, 21% from short Mode S and 9% from long Mode S, the FRUIT power is in -70 -40 dBm interval. Figure 6 shows the pertaining results, versus the FRUIT power: there is a stable and significant detection probability improvement, corresponding to + 20 dB interference power. 0.96 Matched filter + CFAR Conventional processing
0.94 0.92 0.9
P
d
0.88 0.86 0.84 0.82 0.8 0.78 -70
-65
-60
-55 -50 FRUIT power [dBm]
-45
-40
Fig. 6: Mode S replies detection probability in a FRUIT environment. Comparison between: matched filter + CFAR and conventional detection technique
4. Conclusions A novel approach, i.e. matched filter + CFAR, was proposed for Secondary Surveillance Radar in order to reduce conventional and Mode S replies detection problems. Earlier results demonstrate better performance than the today’s processing, in a FRUIT environment (and in noise only). The tests were performed using RASS, an ATC radar systems validation tool, and a commercial monopulse-SSR equipment (SIR-S by Selex SI).
References [1] [2] [3] [4] [5]
M.C. Stevens, Secondary Surveillance Radar, Artech House 1988. M. Barkat, Signal detection and estimation, Artech House, 2005. F. Fiori, Simulazione completa della catena ricevente SIR-S e valutazione dei suoi possibili miglioramenti, Tesi di Laurea Università degli Studi Tor Vergata, Roma, a.a. 2005-2006. David K. Barton, Modern radar system analysis, Artech House 1988 RASS© web site, http://www.intersoft-electronics.com/
